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Organic matter and nitrogen dynamics in conventional versus integrated arable farming
Agriculture Ecosystems & Enwronment ELSEVIER
Agriculture, Ecosystems and Environment 51 (1994) 209-226
Organic matter and nitrogen dynamics in conventional versus integrated arable farming H.G. van Faassen*, G. Lebbink DLO Research Institute for Agrobiology and Soil Fertility, P.O. Box 129, 9750 AA Haren, Netherlands
Accepted 6 August 1993
Abstract
Carbon and nitrogen cycling and crop yields in integrated (INT) arable farming, with 35% of N fertilisation by organic manures, were compared with those in conventional (CONV) management, using only mineral-N fertiliser, in a long-term field trim on a calcareous silt loam soil. In fields with a relatively high initial soil organic matter (SOM) content, crops, especially potatoes, seemed to benefit from the increased soil N supply. Under INT management, original SOM levels were maintained in fields with 2.2% and 2.8% SOM in the 0-25 cm layer; under CONV management these levels decreased to 2.15% and 2.6% SOM in 6 years. In situ, periods o f N immobilisation as well as of N mineralisation were found, while in vitro only net N mineralisation occurred. Model calculations of C and N turnover in soil were used to estimate the N mineralisation pattern and the pool sizes of microbial biomass and young humus. Mineral-N balance calculations for potatoes suggested N losses of 0-100 kg h a - 1 over the 1989 growing season. For the rotation cycle ( 1988-1991 ), calculated N losses to the environment were less than 300 kg h a - t and 170 kg ha -t, at the low and the high SOM level, respectively. High losses may be partly explained by high levels of rainfall in 1988 and 1990, with denitrification as a likely cause of N loss. The highest (risk of) N loss in spring is for potatoes and sugar beet, where high nitrate levels are present in soil and N uptake starts relatively late. Calculated average efficiencies of N inputs were 88% and 72% for CONV management started on soil with 2.8% and 2.2% SOM, respectively; for INT management these values were 82% and 65%. Values of N efficiencies and N losses are still preliminary because steady state had not yet been attained. Keywords: Carbon cycling; Farming system, conventional; Farming system, integrated; Nitrogen cycling; Simulation model; Soil
organic matter
1. Introduction Environmental and economic problems with high-input arable farming in the Netherlands led to the start of the Dutch programme on Soil Ecology of Arable Farming Systems in 1985 * Corresponding author. 1 Communication No. 64 of the Dutch Programme on Soil Ecology of Arable Farming Systems.
(Brussaard et al., 1988), directed to the development of integrated forms of arable farming. A hypothesis of this programme was that a change from conventional (CONV) to integrated ( INT ) farm management could improve nutrient use efficiencies and thereby reduce nitrogen (N) losses to the environment. Integrated management differed from conventional management in the partial replacement of mineral-N by processed organic manure and
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H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
compost, less use of pesticides (no soil fumigation) and by reduced tillage (ploughing depth 12-15 cm instead of 20-25 cm). We tested the above hypothesis by comparing organic matter and nitrogen turnover in the soilplant ecosystem under CONV and INT management, mainly based on results over the period 1988-1991 from a field experiment started in the autumn of 1985 (Lebbink et al., 1994). Integrated N fertilisation means a shift from directly 'feeding the plant' with mineral-N to 'feeding the soil organisms' with organic forms of N and, thereby indirectly feeding the plant through N mineralisation by soil organisms (Lopez-Real, 1986). Practically, this means that the use of organic manure, compost and mineral-N has to be coherent with crop (residue) management. For its N supply, the crop becomes more dependent on soil mineralisation and an interesting question is how this N supply from the soil can be synchronised with crop N demand. An extra input of stabilised organic matter in INT is meant to increase or conserve a large pool of 'young humus' in the soil, which might act as a 'slow-biorelease fertiliser'. To prevent nitrate leaching during the winter period, green manures should be grown after the early harvest of the cereal crops. On a national scale, integrated N fertilisation contributes to waste recycling by the use of spent mushroom compost and processed pig manure solids. Since organic matter dynamics and N turnover in the soil plant ecosystem are intimately linked, they have to be studied together. Turnover in agro-ecosystems consists of continuous transformations of organic forms of carbon (C) and nitrogen (N) into inorganic forms of C and N and vice versa, often called mineralisation immobilisation turnover (MIT). Primary production of organic matter is by photosynthesis of plants and mineralisation of part of this production in soil is by soil organisms (primary and higher order decomposers) and by root respiration. Mass balance calculations for organic matter and N in agro-ecosystems can provide information about their turnover in relation to crop N demand and potential N losses to the environ-
ment. To show the dynamics of organic matter and N, information is required on input and output flow rates of the different pools of organic matter and N in the plant-soil ecosystem. To obtain insight in MIT we used a simulation model in which we described the turnover processes (flows between pools) as a function of time. In this way we analysed SOM dynamics and the pattern of N mineralisation/immobilisation throughout a complete crop rotation. This modelling also provided an estimate of the steady state pool sizes and fluxes that will eventually be attained by the system after several crop rotation periods. This paper presents the calculated mineral-N balances over the growing season of a potato crop in 1989 and the total-N balance calculations over a crop rotation period of 4 years. Field data obtained during 1988-1991 were used as input to simulate the pattern of N mineralisation over a growing season and the fluctuations in the organic N pools. The output of the model was compared with some independent field determinations of N pools and flows.
2. Materials and methods
Because changes in soil organic matter (SOM) show up only after several years, we started both CONV and INT management on the plots of a previous field experiment, where two levels of SOM of 2.8°/o (Block A) and 2.2% (Block B) had been attained as a result of different organic inputs since 1953. Thus, four variants were created: CONV and INT management on both Blocks A and B (CONVA, CONVB, INTA, INTB). Previous management on Block A had been to some degree comparable with 'integrated' and that on Block B more or less comparable with 'conventional'. Therefore, only slight changes in SOM turnover were expected in INTA and CONVB, on which research was concentrated, and much larger changes in CONVA and INTB. As a fifth variant, INT management with minimal tillage (MTC) was started on a separate Block C, where by only 7 cm ploughing the organic inputs were concentrated in the top 7 cm.
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
Previous management on Block C differed from that on Block B only by the use of green manures, and had resulted in 0.1% higher SOM content of 2.3%. A 4 year crop rotation was practised of winter wheat, sugar beet, spring barley and ware potatoes in all variants. Mineral fertilisers in CONV were partly replaced by the use of organic manure in INT. Crops on Block B were given additional mineral-N (30 kg ha- ~) to compensate for the higher N supply from mineralisation on Block A. Compost (30 t h a - 1) was mixed with the soil, as part of INT fertilisation, together with crop residues after winter wheat harvest, but this made growth of a green manure crop impossible; prevention of nitrate leaching then depended on N immobilisation by microbes. Stabilised organic granules produced from pig slurry (since 1990 enriched with nitrate) were applied in INT to potatoes in spring. Mineral-N present in the soil profile (known from sampling the 0-60 or 0-100 cm layer, depending on the crop) in February/ March was used to calculate the amount of mineral-N that had to be applied to the crops. For a detailed description of the experimental site and the management systems used see Lebbink et al. (1994).
2. I. Weather conditions and atmospheric deposition In view of the effect of weather conditions on crop development and on soil processes, rainfall, air and soil temperature were measured daily at the experimental site (De Vos et al., 1994). For the atmospheric deposition of mineral-N, data of the chemical composition of precipitation were used from a nearby regional weather station (KNMI/RIVM, 1988).
2.2. Chemical analyses of crops, soil and organic manures The following parameters were determined: (1) crop yields and N uptake every year for winter wheat, sugar beet, spring barley and ware potatoes for each of the five variants;
211
(2) each year, for one of the crops, dry matter production and N uptake was followed by harvesting two or three subplots (3-16 m 2), four or five times during the growing season; (3) in 1989, crop residues and green manure production were quantified and analysed for their content of organic matter by oxidation with dichromate/sulphuric acid and for their total-N content; (4) mineral-N extractable with 1 M KC1, layerwise in the soil profile down to 1 m, at the experimental harvesting dates and in spring; extracts were analysed for ammonium and nitrate using colorimetric autoanalyser methods; (5) SOM content by oxidation with dichromate/sulphuric acid and total-N contents, annually in the 0-10, 10-25 and 25-40 cm soil layers; (6) compost and organic manure granules were analysed for dry matter, crude ash, total-N, NHa-N, NO3-N, P and K content; (7) C and N inputs by roots of the main crops and of green manures were quantified by Van Noordwijk et al. (1994).
2.3. N mineralisation/immobilisation Potential N mineralisation was measured each year for one of the crops by incubating homogenised and sieved soil, sampled at the experimental crop harvest dates, for 6 weeks at 20 ° C. The increase in mineral-N between Weeks 1 and 6 was used to calculate N mineralisation rates. In 1990 we started to determine N mineralisation also in the field, by sequential soil coring and exposure of largely undisturbed soil columns confined within PVC tubes (Raison et al., 1987; Debosz and Vinther, 1989 ). On each sampling date, two series of ten PVC tubes (length 50 cm, internal diameter 4.5 cm) were driven into the soil, in subplots of 3 m 2 in fields 12B (CONVB) and 16A (INTA), where winter wheat was grown in 1990; bulk soil samples were taken at the same time to determine mineral-N in the 0-40 cm layers. The tubes were covered with thin, airpermeable, plastic sheet to prevent N leaching by rain. After 6 weeks of exposure in the field, the tubes were removed and mineral-N in the 0-40
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H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
cm layers was determined; bulk soil samples were again taken and analysed for mineral-N. Every 3 weeks, two new series of tubes were inserted into the soil; thus, 17 overlapping exposure periods of 6 weeks were investigated during the year. Net N mineralisation was calculated as the difference in mineral-N in the covered tubes at the end of the exposure period and mineral-N in the bulk soil 6 weeks before.
2.4. N balance calculations Previously, mineral-N balance calculations over one growing season have been published for winter wheat (1986), sugar beet (1987) and spring barley (1988) (Van Faassen and Lebbink, 1990). In this paper a mineral-N balance is given for potatoes, the fourth crop of the rotation, based on measurements during the growing season of 1989. Inputs for this balance were mineral-N in the 0-60 cm soil layer at the start (No), atmospheric deposition, 'seed potato-N', fertiliser N applied, and the calculated N mineralisation (in 1989 still based on potential mineralisation, corrected for field soil temperature). Outputs for this balance were mineral-N in the 0-60 cm soil layer at crop harvest (Are) and total-N present in harvested tubers and in crop residues. Total-N balances were calculated for a crop rotation using experimental data over the period 1988-1991. For each crop, average values of N uptake, N removal and N inputs were used from the years 1988-1991. Inputs used in the balance were total amounts of fertiliser N and of organic N applied to the four crops, and total atmospheric N deposition. Outputs used in the balance were total amounts of N removed in harvested products (kernels, straw, beet and tubers). Changes in the amounts of total-N in the 0-25 cm soil layer were estimated using average totalN contents (1988-1991 data) and average volumetric masses (1990 data) for three fields per variant. At steady state, the difference between total-N input and N removal with the crop indicates the potential N loss to the environment. Some pools and flows of N within the soil-plant ecosystem were also calculated: the total average
amount of N taken up by the crops and the green manures, the total amount of N recycled in crop residues (roots, stubble, sugar beet tops and leaves, harvest loss of small potato tubers, dead dried potato leaves and total production of green manures), gross N immobilisation based on model calculations, and gross N mineralisation (from the N balance of the young humus pool ), N in young humus (total-N in SOM minus N in old organic matter). The amount of N present in supposedly inert old humus was estimated by subtracting N in young humus, estimated by model calculations, from total-N in SOM, based on soil analyses of variant CONVB; the same amount of old humus was assumed to be present in the other variants.
2.5. Modelling C and N dynamics in soil Our model is a modification of the model by Jenkinson and Rayner (1977) on the turnover of soil organic matter (Van Faassen and Smilde, 1985 ). C and N turnover in soil is described by coupling the biodegradation of organic matter to the production of microbial biomass. The model distinguishes four pools of organic matter, each with its own first-order rate of degradation and efficiency ofbiomass formation (Fig. 1, Table 1 ). A large pool of old SOM is considered as inert, thus the model is restricted to 'young SOM'. Crop residues added to the soil are distributed among two pools: easily decomposable plant material (DPM) and resistant plant material (RPM), depending on their N content. Compost and processed organic manures also contain a fraction of humic materials, formed during previous biological turnover, and this fraction is directly added to the pool of young humus (POM) in soil. Microbial biomass in soil (BIOM) is the fourth pool. Soil fauna is not represented as a separate pool, because its pool size is generally small and its predating role on microbial biomass is included in the value of the rate constant k3. This implies that the model does not include any effect of soil fauna on the rate constants kl, k2 and
k4. Each organic matter pool is assigned its own C / N ratio. Since microbial biomass in this soil
H. G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (I 994) 209-226
213
..~_
C02 DPM kl = 40% =- BIOM
y I INPUT _ I - -t - , -
RPM k2 = ~~2°°/~ B POM O IM 90 %,~ 002
POM
k4 - 10% -
=
so % f i
"-Z
BIOM ka = ~2o%
BIOM C02
=
POM
'young' soil organic matter 'old' soil organic matter
COM
inert
Fig. 1. Structure of the model used to calculate C and N turnover in soil; each organic matter pool is given its own C/N ratio; k~, k2, k3 and k4 are first-order, temperature-dependent rate constants. For further details, see Table 1.
consisted mainly of bacteria, we used a C / N ratio of 4 for the BIOM pool (Bloem et al., 1994). The C / N ratio of 12, used for the POM pool, equals the average C / N ratio determined for total SOM of Block A over the period 1966-1984. The same C / N ratios are used by De Ruiter et al. (1994), simulating N mineralisation based on food web interactions. N turnover is derived from C turnover: N released from decomposed organic matter inputs minus N immobilised in products formed equals net N mineralisation or net immobilisation of mineral-N. Biodegradation of RPM and BIOM was assumed to produce not only CO2 but also POM (young humus). Degradation rate constants are based on empirical humification coefficients (PAGV, 1989) that give the mass fractions of
organic matter inputs still present in soil 1 year after its addition and include turnover products formed from this input. Each crop residue or organic manure has been distributed among the model fractions of DPM, RPM and POM, so that their summed turnover calculated by the model fits with the given humification coefficients of 0.2 for young plant materials, 0.35 for straw, stubble and old roots and 0.50-0.85 for very persistent organic matter. Crop residues added to the soil were distributed among the model pools of DPM and RPM. The RPM fraction of crop residues, mainly structural material from (older) roots and stubble was estimated first and was assigned a C / N ratio of 80; then the remainder of the residue C and N was put into the DPM pool, resulting in a
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H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
Table 1 Description of the model pools of organic matter with their C/N ratios and first-order, temperature-dependent rate constants (k~, k2, k3, k4) of turnover; soil temperature distribution within I year as used in the model, based on data from 1986 Pool
C/N ratio
k (month -l ) at 10° C
DPM Rapidly decomposable plant material RPM Resistant, slowly decomposable plant material POM Processed organic matter (input) POM Physically protected organic matter (in soil) BIOM Microbial biomass in soil COM Chemically stabilised soil organic matter The COM pool is assumed to be inert and is not included in the model
15-22 80 12 12 4 12
kt=0.35 k2=0.15 k3=0.005 k3=0.005 k4=0.15 k5=0
Temperature dependence: k(T) = k(10) × T/10, at T> 0 k(T) =0, atT~<0 T is the monthly average soil temperature at about 15 cm depth Temperature distribution used in the model Month J F M A Temp. (°C) 3 0 2 6
M 12.5
J 15
variable C/N ratio generally between 15 and 20. The model used a time step of I month in calculating the sizes of the pools throughout a 4 year rotation. A pool containing the total amount of CO2 released was calculated to check the carbon balance. A pool of mineral-N was calculated that contained the total-N mineralised from the organic matter pools. According to the model equations used and the C/N ratios chosen, degradation of DPM and RPM always led to immobilisation of mineral-N, whereas degradation of POM and BIOM always led to net N mineralisation. The model took temperature fluctuations over the course of 1 year into account by linearly relating rate constants with monthly average soil temperature. The soil temperature distribution of 1986 was used for each year (Table 1 ). The effects of fluctuations in soil moisture content were not yet accounted for in the model. Decreased rates during a dry period were expected to be compensated by increased rates shortly after rewetting. The main result of the model was the pattern of N mineralisation-immobilisation turnover over a rotation, showing the amounts ofmineral-N made available during and after the growing season. By repeating the model calculations several times, using the pool sizes at the end of each rotation as the starting
J 17.5
A 16
S 12.5
O 12
N 9
D 5.5
Av. 9.3
point for the next rotation, the model also gave an estimate of the steady-state pool sizes that would eventually be attained.
3. Results
3.1. Weather conditions and atmospheric deposition Of the 6 years studied, 1987, 1988 and 1990 were exceptionally wet, with 920 mm, 1020 mm and 920 mm of rainfall, respectively, compared with the 1943-1990 average of 740 ram. Atmospheric deposition of mineral-N was fairly constant over the years 1978-1987 (about 19 kg ha- 1 year- ~), and roughly equally distributed among the seasons. (In Fig. 6 a slightly lower estimate of 70 kg ha- 1has been used for the period 1988-1991.)
3.2. Crop yields and N efficiency Table 2 presents the average yields of grain, sugar and potato tubers over the period 19881991. INT crop yields were on average 90% (range 83-102%) of CONV crop yields; crop
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
215
Table 2 Average crop yields (t ha - ~) for the rotation 1988-1991 Crop
Winter wheat grain ~ Sugar beet Spring barley grain ~ Potato tubers (fresh wt.)
Variant CONVA
CONVB
INTA
INTB
MTC
6.7 (0.7) 14.3(4.4) 5.0(0.5) 60.1 (6.6)
6.7 (0.8) 14.5(1.6) 5.3(0.6) 57.4(7.5)
5.6(0.8 ) 12.9(1.2) 4.8(0.7) 58.4(7.1 )
5.6 (0.8) 13.3(1.2) 4.7(0.6) 50.6(9.1 )
5.0(0.9) 11.5(2.3) 3.9(0.9) 51.4(5.7)
~Dry matter basis. Figures in parentheses are standard deviation (n = 4). Table 3 Average values of N inputs, crop N uptake at harvest and N removal with products (kg ha-~ ) for the period 1988-1991 Variant CONVA
CONVB
INTA
INTB
MTC
118 244 138 95 70
153 274 165 120 70
70 142 71 47 25
89 177 108 71 25
71 161 79 61 25
665
782
355
470
397
0 0 70 10
0 0 70 10
200 60 70 10
200 60 70 10
200 60 70 10
Total input
745
862
695
810
737
N uptake Sugar beet Potatoes Wheat Barley Green manures
228 245 146 112 150
232 225 149 109 150
198 232 122 99 84
182 211 120 88 83
163 200 106 68 57
Total uptake
881
864
735
684
594
N removal Beet Tubers Wheat t Barley~
100 234 146 112
102 215 149 109
80 222 113 99
84 201 111 88
69 190 97 68
N inputs Mineral fertilizer Sugar beet Potatoes Wheat Barley Green manures Subtotal Compost Organic manure Deposition Seeds
Total output
593
574
514
484
423
Recycling2 N efficiency3 (%)
289 88
291 72
222 82
200 65
171 63
~Grain + straw. 2Recycling = uptake - removal. 3N efficiency = (N removal) / (total N i n p u t - deposition ) × 100%.
216
H.G. van Faassen, G. Lebbink /Agriculture, Ecosystems and Environment 51 (1994) 209-226
40
N mineralization in 6 weeks (kg/ha)
30 20 10 0 -10 -20 5
0
10
15
20
25
30
35
40
45
50
week number (1990) INTA in-situ
i
CONVB in-situ
~ INTA lab
~ CONVB lab
Fig. 2. In situ N mineralisation in 1990 in winter wheat fields 12B (conventional management on Block B) and 16A (integrated management on Block A) compared with potential N mineralisation, corrected for actual soil temperature.
N (kg/ha) 400
350
+106
300 250 200~ 150 100 50 0 90
IL 120
150
. . . . CONVB
180 210 day number 1989 --~-- INTA
-~
INTB
240
270
---~ MTC
300
© CONVA
Fig. 3. Mineral-N in the soil (0-60 cm layer) and total-N taken up by the potato crop during the 1989 growing season, under conventional and integrated management; the arrow indicates the second conventional gift of mineral-N; results for CONVA only at harvest (Day 285). Variants: CONVA/CONVB, conventional management on Blocks A and B respectively; INTA/ INTB, integrated management on Blocks A and B respectively; MTC, integrated management with minimum tillage on Block C.
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
90
precipitation (total 679 mm), mm/decade
217
oc 20
80 70
1,o
,~J"
I 197
60
1152
/ laa
50
/~ /
15
I lo6
I I
10
40
L_
30 20 10 0
,I,
I
6
11
16
21
26
31
36
decade x
average air temp.
Fig. 4. Distribution of rainfall in 1989 and monthly average air temperature; arrows indicate the dates and amounts (top to bottom) of mineral fertiliser N dressings of potatoes, in variants CONVA,CONVB,INTA, INTB and MTC (for explanation of variants see legend to Fig. 3 ). yields in MTC were on average lower than in the other four variants, except for potatoes. Table 3 gives the average N inputs to the four crops of a rotation and to the green manures, average N uptake by the crops at harvest, average N removal in harvested products and average amounts of N recycled to the soil in crop residues and green manures. Total-N inputs by compost and organic manure and by atmospheric deposition are given for the total period because as a result of their slow mineralisation these inputs cannot be ascribed to a single crop. In INT, intentionally, less mineral-N was given to the crops than the N uptake by the crops; thus in INT the crops were more dependent for their N supply on N mineralisation from applied compost, organic manure and SOM. Total-N input followed the order INTA < MTC < CONVA < INTB < CONVB, while total-N uptake and total-N removal followed the order MTC < INTB < INTA < CONVB < CONVA. More N was recycled to the soil in CONV than in INT crop residues, mainly because in CONV two green
manure crops were grown and in INT only one. Efficiencies of the total applied N inputs followed the order MTC < INTB < CONVB < INTA < CONVA. Atmospheric deposition, a rather uncertain input not made by the farmer, was excluded from the calculation of the N efficiencies. 3.3. N m i n e r a l i s a t i o n / i m m o b i l i s a t i o n
Figure 2 shows, for two winter wheat fields in 1990, the in situ rates of N mineralisation and the potential rates determined by incubation of soil samples at 20 °C in the laboratory, corrected for actual soil temperatures. In the field, periods of N immobilisation as well as periods of N mineralisation were observed, whereas in the laboratory, net N mineralisation was always found. In spring especially, the potential rates gave an overestimate of the field rates. In summer (around Week 32), both potential and field mineralisation rates were low. Potential rates of N mineralisation were almost always lower for field
218
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (I 994) 209-226 4O5 372 N mineralization
fertilizer
seed+deposition soil mineral N
72
255
1~ 27 input
36
deficit
253
tubers
]u
crop residues
73 soil mineral N output
72
18 1~ input
321
321
95
79
95
18 29
~ ,
input
245
lh 44 output
CONVB
297
152
106
303
CONVA
294
4O5
372
309
309
115
104
254
I
188
206
162
178
33
I
18 20
111 26
18 14
~0 17
input
output
input
output
output
INTA
INTB
MTC
Fig. 5. Nitrogen balance sheets for potatoes, grown under conventional and integrated management in 1989; soil mineral-N has been calculated for the 0-60 cm layer (for explanation of variants see legend to Fig. 3 ).
12B (CONVB) than for field 16A (INTA). The cumulative in situ N mineralisation until harvest of the winter wheat crop was 44 kg h a - 1and 56 kg h a - ' for CONVB and INTA, respectively; from potential laboratory rates we calculated for the same period 62 kg ha-~ and 75 kg ha-~ for CONVB and INTA, respectively.
3.4. N balance and N dynamics for potatoes Figure 3 shows that 150-200 kg of mineral-N was present in soil between Days 90 (after fertilisation) and 150, which meant a substantial risk of N loss by denitrification for this crop. Conditions conducive to denitrification--heavy rainfall, soil temperature over 8 °C and air-filled soil
volumes of less than 12%--have been shown very often in this soil in spring (Lebbink et al., 1994; Vos and Kooistra, 1994). The second application of fertiliser N to CONVB was only partially recovered in the crop, indicating that part of it had been lost or immobilised. Figure 4 shows the rainfall distribution and temperatures over 1989 as well as the dates of fertiliser N applications. The second application of fertiliser N in CONV might thus have been particularly at risk to loss by denitrification. Although N uptake was most rapid in CONVB, where fertiliser N exceeded N uptake, a higher mineralisation in INTA might explain the same N uptake at harvest in both cases. The mineral-N balances in Fig. 5 show that a
H. G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
219
70 depolitlon
7O deposition
¢ONVB
CONVA
ATMOSPHERE crol~
hervwtld product=
leed
minQml rtilizer
J
3500
~
I
A~ERE crops
~
- 4 ~ 573
i
I
old orglnlo n~,tter
SOIL (~891 ~ ~ s ~
(152 + X) losses to the environment
70 depomtlon
70 deposition I
mineral Rillzer
tNTB
INTA ATMOSPHERE crops
seed
;5
t o t~ e n v i r o n m e n t
--m" 514" "-" i partial . ~ recycling
~
I I260
i M= 1542- X') I
~x~
k
_~_~
o2a6Onie ~ .cycling
oo-
L young ofganlo
J rnltblr I
a..,oo
I
I o d organlc matter
..
....
SOIL (181 - X) losses to the environme~
(326- X)losses to the environment
Fig. 6. Total-N balance ofa 4 year crop rotation ( 1988-1991 ), for conventional (CONV) and integrated (INT) management, each started at two levels of soil organic matter on Blocks A and B: U, N uptake by crops and green manures; M, gross N mineralization;I, gross N immobilisation;X, decrease in storage of N in young organic matter.
substantial part of the large amounts of mineralN applied to potatoes and supplied by N mineralisation was not recovered from crop plus soil at harvest in CONVA, CONVB, INTB and MTC. N input from mineralisation was calculated from laboratory incubations for the period from 31 May, when sampling started, until harvest; thus, this input might be underestimated. To explain the N uptake by the crop in INTA, N mineralisation must have been at least 98 kg ha- ~or about equal to the experimental estimate of 95 kg ha- 1. Only a negligible contribution to N supply of potatoes was expected from the mineralisation of the applied organic manure, in INTA, INTB and MTC, based on incubation of soil with this material. 3.5. N balance for a crop rotation
Figure 6, which is mainly based on data from Table 3, shows for four variants the total average amounts of N circulated in a 4 year crop rotation. The calculated total-N losses to the environment assume steady-state conditions. When
the amount of N stored in SOM had increased or decreased over the period 1988-1991, the actual N loss would have been lower or higher, respectively. Even relatively small errors in the determination of total-N in soil give large uncertainties in terms of absolute amounts of N; thus, differences in total-N between 2 years ( 1988 and 1991 ) are too uncertain to make reliable estimates of actual total-N losses. The pool of N in old SOM was assumed to be constant and equal in all variants. N in old SOM was estimated to be about 3500 kg h a - 1, the difference between total-N, from soil analyses of CONVB, and N in young SOM, from model calculations for CONV. The pool of N in young SOM was calculated from average amounts of total-N, determined during 1988-199 l, minus 3500. Changes in the amount of mineral-N were assumed to be negligible. Gross N immobilisation (I) was first estimated by model calculations, and then gross N mineralisation (M) was calculated, assuming no overall change in the pool of young SOM.
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Table 4 Experimental carbon and organic nitrogen inputs (kg h a - ~) with crop residues, green manures, compost and organic manure during a crop rotation, and their estimated distribution among model pools of organic matter Year
Date of input
Model component DPM C/N
RPM C/N
Total C / N ratio
Description
35 20 19 22
Residue Residue Residue Residue
POM C/N
Conventional 1 2 3 4
1 Oct. 1No~ 1 Nov. 1Nov.
Total
120/7 800/54 2200/125 1315/83
230/3 360/4 160/2 735/9
-
4435/269+1485/18+0
potatoes wheat + mustard sugar beet barley + grass
20.6 (5920/287)
Integrated 1 2 2 3 4 4 Total
1 Oct. 1 Sept. 1 Sept. 1No~ 1 Nov. 1 Apr.
120/7 280/14 1000/72 1970/113 1260/68 -
245/3 320/4
680/8 160/2 1260/16
4630/274+2660/33+2070/172
1320/110 750/62
36 33 16 19 30 12
Residue potatoes Residue wheat Spent mushroom compost Residue sugar beet Residue barley + grass Organic manure
19.6 (9360/479)
Year I of the model starts on 1 September.
3.6. Modelling C and N dynamics in soil Table 4 presents the average carbon and organic N inputs into the soil with crop residues, green manures and organic manures, based on field data, and the estimated distribution of these inputs over the model pools. Organic matter from compost was distributed among the DPM, RPM and POM pools in such a way that 1 year after its application about 55% of its C would still be present in the soil, according to our model. Processed organic manure was totally added to the POM pool, because it showed a negligible mineralisation during incubation. Figure 7 shows, for the simulated steady state of the crop rotation, the fluctuations in the organic N pools: N in easily decomposable (DPMN) and resistant (RPMN) plant material, microbial biomass (BIOMN) and stabilised organic matter (POMN). For the mineral-N pool, the resulting accumulation pattern is shown because plant uptake and possible mineral-N losses are not taken into account; the accumulated mineral-N after 4 years equals the input of organic N during the
rotation because steady state has been attained. The pattern of mineral-N accumulation shows a (desirable) immobilisation (decrease) of mineral-N during three winter periods; if this also occurs in the field it may prevent N loss. Modelled N supply from mineralisation during the growing season was highest for spring barley and grass green manure and lowest for winter wheat. In fact, the model shows that about half of the N mineralised in the first year, after potatoes, becomes available in autumn and may be lost during winter. The steady-state calculations indicated that in CONV, on average 89 kg N ha-1 would be present in microbial biomass, 277 kg in young humus and about 35 kg in crop residues; in INT there would be on average 109 kg, 1103 kg and 25 kg N ha-1 in microbial biomass, young humus and crop residues, respectively. Thus, the main difference between CONV and INT management would be the size of the young humus (POM) pool and the contribution of this pool to N mineralisation. Only under INT management would this POM pool serve as a substantial source of'slow-biorelease fertiliser'. Soil
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
Conventional Management
kg N per ha
250 -
221
wheat +
luglt bltt
blrlly + grill
150
,oo
/
50 -
~
1200 ,-
.Integrated Management
= .................
............
looo~ wheat
badey + grass
sugar beet
potatoes
400 300 200 -
O~ ...........
I
9
I
I I I I I I
1
5
ii I I I I i
5
/
I I i i i I iI I I I iI
9
1
5
9
I I I i g I I I t
1
5
9
month DPMN
-=-
RPMN
~
BIOMN
---
POMN
~
Nmin
cumulative
Fig, 7. Modelled fluctuations in crop residue and soil organic N pools and the resulting cumulative mineral-N for a 4 year crop rotation under conventionaland integrated management, with the assumption of steady state.
temperature distribution is mainly responsible for the slowing down ofN mineralisation during winter. The general effect of microbial turnover of organic matter and N in soil is to transform the large differences in yearly inputs into much smaller differences in yearly N mineralisation (cf. Table 4 and Fig. 7 ). 3. 7. Trends in soil organic matter and N contents
Figure 8 shows that the initial two levels of SOM and soil total-N diverged into four levels.
The SOM content decreased rapidly in variant CONVA, slightly in variant CONVB, remained constant in variant INTA and increased in variant INTB. Total-N in soil was maintained under CONV management and increased under INT management. The top 25 cm soil of the variants INTA, CONVA, INTB and CONVB weighed about 3.44 Gg, 3.53 Gg, 3.62 Gg and 3.68 Gg ha-1, respectively, which we calculated to contain about 5100 kg, 4700 kg, 4300 kg and 3900 kg N, respectively. Subtraction of 400 kg N hafor variant CONVB, i.e. the steady-state amount calculated to be present in microbial biomass,
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3.5
organic matter (%)
66
0.18
i
i
i
i
70
74
78
82
86
90
total N (%)
0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 66
i
L
i
i
i
70
74
78
82
86
90
year CONVA
--+-- INTA
- 4 ~ CONVB
~
INTB
Fig. 8. Trends in soil organic matter and total-N contents in the 0-25 cm layer, 1966-1991. Each point represents the average valuefor three fieldsthat havebeenpart of the variantsCONVA,CONVB,INTAand INTBsinceautumn 1985.For explanation of variants see Fig. 3. The arrowindicatesthe start of conventionaland integratedmanagement. young humus and crop residues, resulted in 3500 kg N ha - l , assumed to be present in stable, old humus,
4. Discussion 4.1. Weather conditions and atmospheric deposition
Weather conditions directly affect crop development, for instance the early senescence of a
crop in a dry period. However, indirect effects on crop development, by limiting the availability of mineral-N in soil, can also be important. Dry soil can limit N uptake. Heavy rainfall may cause N losses by denitrification, especially in spring when there is a high level of mineral-N in the topsoil and soil temperature is already above 5 ° C; much rainfall over the winter period may cause substantial leaching of nitrate (Van Faassen and Lebbink, 1990). Because the large variation in rainfall distribution will certainly affect turnover as well as N losses, a first step to im-
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
prove the model should be to include effects of differences in soil moisture content.
4.2. Crop yields and N efficiency Whereas CONV crop yields were on average higher than INT crop yields, except for potatoes in INTA, the average N efficiencies showed a greater difference between Blocks A and B than between CONV and INT (Tables 2 and 3 ). Block A had a higher content of SOM than Block B and the crops on Block A got less mineral fertiliser N than the crop on Block B. Thus it might be coneluded that the extra amount of mineral-N given to the crops on Block B, to compensate for the higher N mineralisation on Block A, had been less efficient than the N supplied by mineralisation on Block A.
4.3. N mineralisation/immobilisation The results indicated that potential laboratory rates of N mineralisation gave an overestimate for net in situ N mineralisation. N immobilisation was never found in the laboratory, which might be explained by the sieving out of visible crop residues, which in the field might cause N immobilisation (Van Noordwijk et al., 1993). The in situ measurements had some advantages: temperature correction was no longer needed and periods where N loss occurred could be identified. However, some artefacts might also have occurred in the field: for instance soil compaction, by inserting the tubes into the soil, and differences caused by the exclusion of crop N uptake and nitrate leaching from the tubes.
4.4. N balance and N dynamics for potatoes N losses by denitrification seemed to be the most likely explanation for the large deficit in the mineral-N balance of potatoes (Fig. 5) since leaching losses during the growing season were unlikely. During a period of more than 2 months, when crop uptake of N was still low, the mineralN content of the soil was high in all the variants and periods of heavy rainfall may then have caused denitrification (Figs. 3 and 4). When
223
substantial loss of mineral-N occurs in a critical period of rapid crop development, the crop becomes more dependent on actual N mineralisation. N uptake by the potatoes suggested that N mineralisation eventually compensated possible N loss in variant INTA, but not in variants INTB and MTC, where N availability might have limited crop development. The higher input of fertiliser N on Block B than on Block A might have contributed to the higher deficits in the mineralN balances of CONVB and INTB, compared with CONVA and INTA. The high deficit in the N balance of MTC might be explained by a large N loss through denitrification, because of a relatively low gas-filled porosity; previous experience with minimal tillage at the Lovinkhoeve has shown that it took at least 10 years before the formation of biopores countered this negative effect of reduced tillage (C. Van Ouwerkerk, personal communication, 1993 ).
4.5. N balance for a crop rotation The higher deficits in the N balance for the variants INTB and CONVB than for the variants INTA and CONVA (Fig. 6) may be accounted for by the higher input of mineral-N on Block B than on Block A. High inputs of mineral-N in spring, when N uptake by the crop is still low, increase the risk of high N losses by denitrification. Although more N was returned to the soil by crop residues in CONV, the total input of organic N in CONV was much lower than in INT. Mineralisation supplied the crop with mineral-N throughout the growing season, without much accumulation of mineral-N, thus with a low risk of large losses. This might partly explain the higher N efficiencies on Block A, where input of fertiliser N was lower and N mineralisation was higher than on Block B. Loss of mineralised N might occur after crop harvest. Therefore, in such periods either a green manure crop should be present or N immobilisation should be promoted by the addition of decomposable organic matter with a high C / N ratio. Since a steady state had not yet been attained, the high N efficiency in CONVA may have occurred at the expense of the decreasing level of
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soil organic matter and may not be sustainable. On the same block, the lower overall N efficiencies in INT in comparison with CONV, may be due to the fact that part of the organic manure and compost applied in INT served to increase the pool of young humus in soil. The N efficiencies of the INT variants were, therefore, expected to increase with time until a new steady state would be attained. Hence, N efficiencies of CONV variants were probably overestimated, whereas those oflNT variants were conservative estimates. It should be noted that the calculated N losses to the environment (Fig. 6) were rather uncertain because of the large uncertainty of changes in the total amounts of N stored in SOM (1% difference in the latter was equal to 40-50 kg ha -1 ). 4.6. Modelling C and N dynamics in soil Microbial biomass and young soil humus, the latter especially in case of INT management, act as storage pools from which significant amounts of N can be mineralised, mainly during the growing season. The strong N immobilisation during turnover of crop residues by the microbial biomass, which consists mainly of bacteria in the soil used, results from their low C / N ratio of 4 and the C / N ratio of the organic inputs of about 20 (Fig. 7, Table 4). The simulation shows a substantial accumulation of mineral-N after the harvest of potatoes, which might be lost for the following winter wheat crop. To prevent possible loss of mineral-N after harvest of potatoes, the application of degradable organic manure with a relatively high C / N ratio should be considered to promote immobilisation of this mineral-N. Since this N will be mineralised eventually, less mineral-N should be applied to following crops to prevent the problem being shifted in time. Another possibility might be to harvest the potatoes earlier, to allow a subsequent green manure crop to be grown. The model shows that during the growing season of spring barley and undersown grass more N is mineralised than during the growth of the other crops. Therefore, the low amounts of fertiliser N applied to spring barley
in INT (Table 3 ) may be quite realistic. Generally, it is clear that in INT less mineral-N has to be applied to supplement soil N supply than under CONV management, because of higher steady-state N mineralisation in INT management, as a result of the application of organic manures. Simulated N mineralisation during the growth period of winter wheat is 40 kg ha-1 and 70 kg ha- l for CONV and INT, respectively; these values agree best with the in situ determined value of 44 kg ha- 1 (CONVB) and with the potential laboratory incubation value of 75 kg ha -1 (INTA) for winter wheat in 1990. We have to await further results of in situ measurements to see how good the model results will fit for the other crops. Since soil temperature has such a large effect on turnover rates, the exact timing of organic matter applications will greatly determine their fate. Working crop residues and green manures into the soil 1 month later may shift their main turnover from autumn to spring, and may cause quite a different N supply to the crop. In view of the measured rates of N mineralisation (Fig. 2 ), it seems necessary to improve the model by taking into account the slowing down of N mineralisation in summer, which may be the result of drying out of the topsoil. 4. 7. Trends in soil organic matter and N contents Long-term data of soil organic matter and N contents, as shown in Fig. 8, and knowledge about the agricultural history of the fields are necessary to identify significant trends. Significant differences in SOM and total-N content developed as a result of the change in management in 1985. These differences were consistent when compared with the fluctuating differences that were found between duplicates before 1985. However, differences in total-N contents were not always representative for differences in absolute amounts oftotal-N in soil, because differences in soil volumetric mass were also present. Thus, we compare varying amounts of soil in the 0-25 cm layer of each variant in Fig. 8. Complicating fac-
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tors were that over the period shown, not only were the inputs of organic matter into the soil varied, but also the depth of ploughing and the weather conditions from year to year were different. A decrease in SOM content, but not in totalN content (CONVA and CONVB), indicates a decrease in the C / N ratio of SOM; this might be explained by the relatively high amounts of fertiliser N used, resulting in crop residues with a relatively low C / N ratio and generally sufficient mineral-N available for N immobilisation. During the first years of a transition from a high level of SOM to a low level, an extra amount of N may become available through mineralisation, which may considerably increase N losses, if the N input is not decreased at the same time. The calculated range of 400-1600 kg N ha-1, in young humus, indicated that a large decrease in soil organic N might strongly decrease the capacity of the soil to supply the crop with mineral-N, because the relative decrease in young SOM N was much larger than the relative decrease in total-N. However, at steady state, differences in N mineralisation will be proportional to differences in inputs of organic N, and not to differences in the amount of N in young SOM.
5. Conclusions Six years of comparison between conventional and integrated management allow the following conclusions. ( 1 ) Integrated crop yields were on average 90% (83-102%) and average crop N uptakes 85% (74-103%) of those under conventional management. (2) Replacement of 35% of mineral fertiliser N by stabilised organic N, in the form of compost and processed animal manure, resulted in increased amounts of N in the pool of young humus in the soil and concomitantly increased the N supply to the crop from soil N mineralisation. (3) Overall efficiencies of applied mineral-N plus organic N increased in the order INTB < CONVB < INTA < CONVA (65%, 72%, 82% and 88%, respectively); these efficiencies are expected to increase in INT variants and to de-
225
crease in CONV variants, when in the long-term new steady states of SOM and N will be attained. Extra mineral-N applied to Block B, to compensate for the higher N supply from mineralisation on Block A, might explain the lower N efficiency on Block B than on Block A. (4) The highest (risk of) N losses might be expected from potatoes and sugar beet, where high levels of nitrate are present in the soil in spring and crop N uptake starts relatively late. For potatoes N loss might also be important after crop harvest, if the crop is not followed by a green manure. (5) N loss by denitrification might be more important in this silt loam soil than nitrate leaching. Conditions conducive to denitrification often prevailed in spring, whereas the N uptake by green manures reduced nitrate leaching. (6) The lowest N losses might be expected from INT management of winter wheat and especially spring barley, where most of the N taken up by the crop and by the green manure would be supplied by N mineralisation from SOM, more or less in synchrony with crop growth. (7) The higher N efficiencies on Block A, with a higher content of SOM than on Block B, indicated that it might be important to maintain a relatively high level of SOM. Only under INT management could the SOM level of Block A be maintained by applying stabilised organic matter in addition to the crop residues and green manures. However, the increase of SOM content by INT management on Block B was shown to be rather slow. 6. Acknowledgement The authors thank Dr. P. de Willigen for his constructive criticism of this paper. This work was supported by the Netherlands Integrated Soil Research Programme.
References Bloem, J., Lebbink, G., Zwart, K.B., Bouwman, L.A., Burgers, S.L.G.E., de Vos, J.A. and de Ruiter, P.C., 1994. Dynamics of microorganisms, microbivores and nitrogen mineralization in winter wheat fields under conventional
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and integrated management. Agric. Ecosyst. Environ., 5 l: 129-143. Brussaard, L., Kooistra, M.J., Lebbink, G. and van Veen, J.A., 1988. The Dutch programme on arable farming systems. I. Objectives, approaches and some preliminary results. Ecol. Bull., 39: 35-40. Debosz, K.K. and Vinther, F.P., 1989. An in situ technique for simultaneous measurements of mineralization, leaching and plant uptake of nitrogen applied to agricultural soils. In: J.A. Hansen and K. Henriksen (Editors), Nitrogen in Organic Wastes Applied to Soils. Academic Press, London, pp. 3-10. De Ruiter, P.C., Bloem, J., Bouwman, L.A., Didden, W.A.M., Lebbink, G., Marinissen, J.C.Y., de Vos, J.A., VreekenBuijs, M.J., Zwart, K.B. and Brussaard, L., 1994. Simulation of dynamics in nitrogen mineralization in the belowground food webs of two amble farming systems. Agric. Ecosyst. Environ., 51: 199-208. De Vos, J.A., Raats, P.A.C. and Vos, E.C., 1994. Macroscopic soil physical processes considered within an agronomical and a soil biological context. Agric. Ecosyst. Environ., 51: 43-73. Jenkinson, D.S. and Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Sci., 123: 298-305. KNMI/RIVM, 1988. Chemische samenstelling van de neerslag over Nededand. Jaarrapport 1987. KNMI, De Bilt, Netherlands. Lebbink, G., van Faassen, H.G., van Ouwerkerk, C. and Brnssaard, L., 1994. The Dutch programme on soil ecology of arable farming systems: farm management, monitoring programme and general results. Agric. Ecosyst. Environ., 51: 7-20.
Lopez-Real, J.M., 1986. Sustainable agriculture: the microbial potential--the microbiologist's challenge. In: J.M. Lopez-Real and R.D. Hodges (Editors), The Role of Microorganisms in a Sustainable Agriculture. AB Academic Publishers, Berkhamstead, UK, pp. 1-8. PAGV, 1989. Handbook for Arable Farming and Outdoor Vegetable Growing. PAGV, Lelystad, Netherlands, pp. 177-179. (In Dutch.) Raison, R.J., Connell, M.J. and Khanna, P.K., 1987. Methodology for studying fluxes of soil mineral-N in situ. Soil Biol. Biochem., 19: 521-530. Van Faassen, H.G. and Lebbink, G., 1990. Nitrogen cycling in high-input versus reduced-input arable farming. Neth. J. Agric. Sci., 38: 265-282. Van Faassen, H.G. and Smilde, K.W., 1985. Organic matter and nitrogen turnover in soils. In: B.T. Kang and J. van der Heide (Editors), Nitrogen Management for Farming Systems in Humid and Subhumid Tropics. Institute for Soil Fertility/International Institute of Tropical Agriculture, Haren/Ibadan, pp. 39-55. Van Noordwijk et al., 1993. Synlocation of biological activity, roots, cracks and recent organic inputs in a sugar beet field. Geoderma, 56: 265-276. Van Noordwijk, M., Brouwer, G., Honing, M., Meijboom, F. and Grzebisz, W., 1994. Production and decay of structural root material of winter wheat and sugar beet in conventional and integrated cropping systems. Agric. Ecosyst. Environ., 51:99-113. Vos, E.C. and Kooistra, M.J., 1994. The effect of soil structure differences in a silt loam soil under various farm management systems on soil physical properties and simulated land qualities. Agric. Ecosyst. Environ., 51: 227238.
Agriculture, Ecosystems and Environment 51 (1994) 209-226
Organic matter and nitrogen dynamics in conventional versus integrated arable farming H.G. van Faassen*, G. Lebbink DLO Research Institute for Agrobiology and Soil Fertility, P.O. Box 129, 9750 AA Haren, Netherlands
Accepted 6 August 1993
Abstract
Carbon and nitrogen cycling and crop yields in integrated (INT) arable farming, with 35% of N fertilisation by organic manures, were compared with those in conventional (CONV) management, using only mineral-N fertiliser, in a long-term field trim on a calcareous silt loam soil. In fields with a relatively high initial soil organic matter (SOM) content, crops, especially potatoes, seemed to benefit from the increased soil N supply. Under INT management, original SOM levels were maintained in fields with 2.2% and 2.8% SOM in the 0-25 cm layer; under CONV management these levels decreased to 2.15% and 2.6% SOM in 6 years. In situ, periods o f N immobilisation as well as of N mineralisation were found, while in vitro only net N mineralisation occurred. Model calculations of C and N turnover in soil were used to estimate the N mineralisation pattern and the pool sizes of microbial biomass and young humus. Mineral-N balance calculations for potatoes suggested N losses of 0-100 kg h a - 1 over the 1989 growing season. For the rotation cycle ( 1988-1991 ), calculated N losses to the environment were less than 300 kg h a - t and 170 kg ha -t, at the low and the high SOM level, respectively. High losses may be partly explained by high levels of rainfall in 1988 and 1990, with denitrification as a likely cause of N loss. The highest (risk of) N loss in spring is for potatoes and sugar beet, where high nitrate levels are present in soil and N uptake starts relatively late. Calculated average efficiencies of N inputs were 88% and 72% for CONV management started on soil with 2.8% and 2.2% SOM, respectively; for INT management these values were 82% and 65%. Values of N efficiencies and N losses are still preliminary because steady state had not yet been attained. Keywords: Carbon cycling; Farming system, conventional; Farming system, integrated; Nitrogen cycling; Simulation model; Soil
organic matter
1. Introduction Environmental and economic problems with high-input arable farming in the Netherlands led to the start of the Dutch programme on Soil Ecology of Arable Farming Systems in 1985 * Corresponding author. 1 Communication No. 64 of the Dutch Programme on Soil Ecology of Arable Farming Systems.
(Brussaard et al., 1988), directed to the development of integrated forms of arable farming. A hypothesis of this programme was that a change from conventional (CONV) to integrated ( INT ) farm management could improve nutrient use efficiencies and thereby reduce nitrogen (N) losses to the environment. Integrated management differed from conventional management in the partial replacement of mineral-N by processed organic manure and
0167-8809/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0167-8809 (93) 08001-3
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compost, less use of pesticides (no soil fumigation) and by reduced tillage (ploughing depth 12-15 cm instead of 20-25 cm). We tested the above hypothesis by comparing organic matter and nitrogen turnover in the soilplant ecosystem under CONV and INT management, mainly based on results over the period 1988-1991 from a field experiment started in the autumn of 1985 (Lebbink et al., 1994). Integrated N fertilisation means a shift from directly 'feeding the plant' with mineral-N to 'feeding the soil organisms' with organic forms of N and, thereby indirectly feeding the plant through N mineralisation by soil organisms (Lopez-Real, 1986). Practically, this means that the use of organic manure, compost and mineral-N has to be coherent with crop (residue) management. For its N supply, the crop becomes more dependent on soil mineralisation and an interesting question is how this N supply from the soil can be synchronised with crop N demand. An extra input of stabilised organic matter in INT is meant to increase or conserve a large pool of 'young humus' in the soil, which might act as a 'slow-biorelease fertiliser'. To prevent nitrate leaching during the winter period, green manures should be grown after the early harvest of the cereal crops. On a national scale, integrated N fertilisation contributes to waste recycling by the use of spent mushroom compost and processed pig manure solids. Since organic matter dynamics and N turnover in the soil plant ecosystem are intimately linked, they have to be studied together. Turnover in agro-ecosystems consists of continuous transformations of organic forms of carbon (C) and nitrogen (N) into inorganic forms of C and N and vice versa, often called mineralisation immobilisation turnover (MIT). Primary production of organic matter is by photosynthesis of plants and mineralisation of part of this production in soil is by soil organisms (primary and higher order decomposers) and by root respiration. Mass balance calculations for organic matter and N in agro-ecosystems can provide information about their turnover in relation to crop N demand and potential N losses to the environ-
ment. To show the dynamics of organic matter and N, information is required on input and output flow rates of the different pools of organic matter and N in the plant-soil ecosystem. To obtain insight in MIT we used a simulation model in which we described the turnover processes (flows between pools) as a function of time. In this way we analysed SOM dynamics and the pattern of N mineralisation/immobilisation throughout a complete crop rotation. This modelling also provided an estimate of the steady state pool sizes and fluxes that will eventually be attained by the system after several crop rotation periods. This paper presents the calculated mineral-N balances over the growing season of a potato crop in 1989 and the total-N balance calculations over a crop rotation period of 4 years. Field data obtained during 1988-1991 were used as input to simulate the pattern of N mineralisation over a growing season and the fluctuations in the organic N pools. The output of the model was compared with some independent field determinations of N pools and flows.
2. Materials and methods
Because changes in soil organic matter (SOM) show up only after several years, we started both CONV and INT management on the plots of a previous field experiment, where two levels of SOM of 2.8°/o (Block A) and 2.2% (Block B) had been attained as a result of different organic inputs since 1953. Thus, four variants were created: CONV and INT management on both Blocks A and B (CONVA, CONVB, INTA, INTB). Previous management on Block A had been to some degree comparable with 'integrated' and that on Block B more or less comparable with 'conventional'. Therefore, only slight changes in SOM turnover were expected in INTA and CONVB, on which research was concentrated, and much larger changes in CONVA and INTB. As a fifth variant, INT management with minimal tillage (MTC) was started on a separate Block C, where by only 7 cm ploughing the organic inputs were concentrated in the top 7 cm.
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Previous management on Block C differed from that on Block B only by the use of green manures, and had resulted in 0.1% higher SOM content of 2.3%. A 4 year crop rotation was practised of winter wheat, sugar beet, spring barley and ware potatoes in all variants. Mineral fertilisers in CONV were partly replaced by the use of organic manure in INT. Crops on Block B were given additional mineral-N (30 kg ha- ~) to compensate for the higher N supply from mineralisation on Block A. Compost (30 t h a - 1) was mixed with the soil, as part of INT fertilisation, together with crop residues after winter wheat harvest, but this made growth of a green manure crop impossible; prevention of nitrate leaching then depended on N immobilisation by microbes. Stabilised organic granules produced from pig slurry (since 1990 enriched with nitrate) were applied in INT to potatoes in spring. Mineral-N present in the soil profile (known from sampling the 0-60 or 0-100 cm layer, depending on the crop) in February/ March was used to calculate the amount of mineral-N that had to be applied to the crops. For a detailed description of the experimental site and the management systems used see Lebbink et al. (1994).
2. I. Weather conditions and atmospheric deposition In view of the effect of weather conditions on crop development and on soil processes, rainfall, air and soil temperature were measured daily at the experimental site (De Vos et al., 1994). For the atmospheric deposition of mineral-N, data of the chemical composition of precipitation were used from a nearby regional weather station (KNMI/RIVM, 1988).
2.2. Chemical analyses of crops, soil and organic manures The following parameters were determined: (1) crop yields and N uptake every year for winter wheat, sugar beet, spring barley and ware potatoes for each of the five variants;
211
(2) each year, for one of the crops, dry matter production and N uptake was followed by harvesting two or three subplots (3-16 m 2), four or five times during the growing season; (3) in 1989, crop residues and green manure production were quantified and analysed for their content of organic matter by oxidation with dichromate/sulphuric acid and for their total-N content; (4) mineral-N extractable with 1 M KC1, layerwise in the soil profile down to 1 m, at the experimental harvesting dates and in spring; extracts were analysed for ammonium and nitrate using colorimetric autoanalyser methods; (5) SOM content by oxidation with dichromate/sulphuric acid and total-N contents, annually in the 0-10, 10-25 and 25-40 cm soil layers; (6) compost and organic manure granules were analysed for dry matter, crude ash, total-N, NHa-N, NO3-N, P and K content; (7) C and N inputs by roots of the main crops and of green manures were quantified by Van Noordwijk et al. (1994).
2.3. N mineralisation/immobilisation Potential N mineralisation was measured each year for one of the crops by incubating homogenised and sieved soil, sampled at the experimental crop harvest dates, for 6 weeks at 20 ° C. The increase in mineral-N between Weeks 1 and 6 was used to calculate N mineralisation rates. In 1990 we started to determine N mineralisation also in the field, by sequential soil coring and exposure of largely undisturbed soil columns confined within PVC tubes (Raison et al., 1987; Debosz and Vinther, 1989 ). On each sampling date, two series of ten PVC tubes (length 50 cm, internal diameter 4.5 cm) were driven into the soil, in subplots of 3 m 2 in fields 12B (CONVB) and 16A (INTA), where winter wheat was grown in 1990; bulk soil samples were taken at the same time to determine mineral-N in the 0-40 cm layers. The tubes were covered with thin, airpermeable, plastic sheet to prevent N leaching by rain. After 6 weeks of exposure in the field, the tubes were removed and mineral-N in the 0-40
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H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
cm layers was determined; bulk soil samples were again taken and analysed for mineral-N. Every 3 weeks, two new series of tubes were inserted into the soil; thus, 17 overlapping exposure periods of 6 weeks were investigated during the year. Net N mineralisation was calculated as the difference in mineral-N in the covered tubes at the end of the exposure period and mineral-N in the bulk soil 6 weeks before.
2.4. N balance calculations Previously, mineral-N balance calculations over one growing season have been published for winter wheat (1986), sugar beet (1987) and spring barley (1988) (Van Faassen and Lebbink, 1990). In this paper a mineral-N balance is given for potatoes, the fourth crop of the rotation, based on measurements during the growing season of 1989. Inputs for this balance were mineral-N in the 0-60 cm soil layer at the start (No), atmospheric deposition, 'seed potato-N', fertiliser N applied, and the calculated N mineralisation (in 1989 still based on potential mineralisation, corrected for field soil temperature). Outputs for this balance were mineral-N in the 0-60 cm soil layer at crop harvest (Are) and total-N present in harvested tubers and in crop residues. Total-N balances were calculated for a crop rotation using experimental data over the period 1988-1991. For each crop, average values of N uptake, N removal and N inputs were used from the years 1988-1991. Inputs used in the balance were total amounts of fertiliser N and of organic N applied to the four crops, and total atmospheric N deposition. Outputs used in the balance were total amounts of N removed in harvested products (kernels, straw, beet and tubers). Changes in the amounts of total-N in the 0-25 cm soil layer were estimated using average totalN contents (1988-1991 data) and average volumetric masses (1990 data) for three fields per variant. At steady state, the difference between total-N input and N removal with the crop indicates the potential N loss to the environment. Some pools and flows of N within the soil-plant ecosystem were also calculated: the total average
amount of N taken up by the crops and the green manures, the total amount of N recycled in crop residues (roots, stubble, sugar beet tops and leaves, harvest loss of small potato tubers, dead dried potato leaves and total production of green manures), gross N immobilisation based on model calculations, and gross N mineralisation (from the N balance of the young humus pool ), N in young humus (total-N in SOM minus N in old organic matter). The amount of N present in supposedly inert old humus was estimated by subtracting N in young humus, estimated by model calculations, from total-N in SOM, based on soil analyses of variant CONVB; the same amount of old humus was assumed to be present in the other variants.
2.5. Modelling C and N dynamics in soil Our model is a modification of the model by Jenkinson and Rayner (1977) on the turnover of soil organic matter (Van Faassen and Smilde, 1985 ). C and N turnover in soil is described by coupling the biodegradation of organic matter to the production of microbial biomass. The model distinguishes four pools of organic matter, each with its own first-order rate of degradation and efficiency ofbiomass formation (Fig. 1, Table 1 ). A large pool of old SOM is considered as inert, thus the model is restricted to 'young SOM'. Crop residues added to the soil are distributed among two pools: easily decomposable plant material (DPM) and resistant plant material (RPM), depending on their N content. Compost and processed organic manures also contain a fraction of humic materials, formed during previous biological turnover, and this fraction is directly added to the pool of young humus (POM) in soil. Microbial biomass in soil (BIOM) is the fourth pool. Soil fauna is not represented as a separate pool, because its pool size is generally small and its predating role on microbial biomass is included in the value of the rate constant k3. This implies that the model does not include any effect of soil fauna on the rate constants kl, k2 and
k4. Each organic matter pool is assigned its own C / N ratio. Since microbial biomass in this soil
H. G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (I 994) 209-226
213
..~_
C02 DPM kl = 40% =- BIOM
y I INPUT _ I - -t - , -
RPM k2 = ~~2°°/~ B POM O IM 90 %,~ 002
POM
k4 - 10% -
=
so % f i
"-Z
BIOM ka = ~2o%
BIOM C02
=
POM
'young' soil organic matter 'old' soil organic matter
COM
inert
Fig. 1. Structure of the model used to calculate C and N turnover in soil; each organic matter pool is given its own C/N ratio; k~, k2, k3 and k4 are first-order, temperature-dependent rate constants. For further details, see Table 1.
consisted mainly of bacteria, we used a C / N ratio of 4 for the BIOM pool (Bloem et al., 1994). The C / N ratio of 12, used for the POM pool, equals the average C / N ratio determined for total SOM of Block A over the period 1966-1984. The same C / N ratios are used by De Ruiter et al. (1994), simulating N mineralisation based on food web interactions. N turnover is derived from C turnover: N released from decomposed organic matter inputs minus N immobilised in products formed equals net N mineralisation or net immobilisation of mineral-N. Biodegradation of RPM and BIOM was assumed to produce not only CO2 but also POM (young humus). Degradation rate constants are based on empirical humification coefficients (PAGV, 1989) that give the mass fractions of
organic matter inputs still present in soil 1 year after its addition and include turnover products formed from this input. Each crop residue or organic manure has been distributed among the model fractions of DPM, RPM and POM, so that their summed turnover calculated by the model fits with the given humification coefficients of 0.2 for young plant materials, 0.35 for straw, stubble and old roots and 0.50-0.85 for very persistent organic matter. Crop residues added to the soil were distributed among the model pools of DPM and RPM. The RPM fraction of crop residues, mainly structural material from (older) roots and stubble was estimated first and was assigned a C / N ratio of 80; then the remainder of the residue C and N was put into the DPM pool, resulting in a
214
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
Table 1 Description of the model pools of organic matter with their C/N ratios and first-order, temperature-dependent rate constants (k~, k2, k3, k4) of turnover; soil temperature distribution within I year as used in the model, based on data from 1986 Pool
C/N ratio
k (month -l ) at 10° C
DPM Rapidly decomposable plant material RPM Resistant, slowly decomposable plant material POM Processed organic matter (input) POM Physically protected organic matter (in soil) BIOM Microbial biomass in soil COM Chemically stabilised soil organic matter The COM pool is assumed to be inert and is not included in the model
15-22 80 12 12 4 12
kt=0.35 k2=0.15 k3=0.005 k3=0.005 k4=0.15 k5=0
Temperature dependence: k(T) = k(10) × T/10, at T> 0 k(T) =0, atT~<0 T is the monthly average soil temperature at about 15 cm depth Temperature distribution used in the model Month J F M A Temp. (°C) 3 0 2 6
M 12.5
J 15
variable C/N ratio generally between 15 and 20. The model used a time step of I month in calculating the sizes of the pools throughout a 4 year rotation. A pool containing the total amount of CO2 released was calculated to check the carbon balance. A pool of mineral-N was calculated that contained the total-N mineralised from the organic matter pools. According to the model equations used and the C/N ratios chosen, degradation of DPM and RPM always led to immobilisation of mineral-N, whereas degradation of POM and BIOM always led to net N mineralisation. The model took temperature fluctuations over the course of 1 year into account by linearly relating rate constants with monthly average soil temperature. The soil temperature distribution of 1986 was used for each year (Table 1 ). The effects of fluctuations in soil moisture content were not yet accounted for in the model. Decreased rates during a dry period were expected to be compensated by increased rates shortly after rewetting. The main result of the model was the pattern of N mineralisation-immobilisation turnover over a rotation, showing the amounts ofmineral-N made available during and after the growing season. By repeating the model calculations several times, using the pool sizes at the end of each rotation as the starting
J 17.5
A 16
S 12.5
O 12
N 9
D 5.5
Av. 9.3
point for the next rotation, the model also gave an estimate of the steady-state pool sizes that would eventually be attained.
3. Results
3.1. Weather conditions and atmospheric deposition Of the 6 years studied, 1987, 1988 and 1990 were exceptionally wet, with 920 mm, 1020 mm and 920 mm of rainfall, respectively, compared with the 1943-1990 average of 740 ram. Atmospheric deposition of mineral-N was fairly constant over the years 1978-1987 (about 19 kg ha- 1 year- ~), and roughly equally distributed among the seasons. (In Fig. 6 a slightly lower estimate of 70 kg ha- 1has been used for the period 1988-1991.)
3.2. Crop yields and N efficiency Table 2 presents the average yields of grain, sugar and potato tubers over the period 19881991. INT crop yields were on average 90% (range 83-102%) of CONV crop yields; crop
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
215
Table 2 Average crop yields (t ha - ~) for the rotation 1988-1991 Crop
Winter wheat grain ~ Sugar beet Spring barley grain ~ Potato tubers (fresh wt.)
Variant CONVA
CONVB
INTA
INTB
MTC
6.7 (0.7) 14.3(4.4) 5.0(0.5) 60.1 (6.6)
6.7 (0.8) 14.5(1.6) 5.3(0.6) 57.4(7.5)
5.6(0.8 ) 12.9(1.2) 4.8(0.7) 58.4(7.1 )
5.6 (0.8) 13.3(1.2) 4.7(0.6) 50.6(9.1 )
5.0(0.9) 11.5(2.3) 3.9(0.9) 51.4(5.7)
~Dry matter basis. Figures in parentheses are standard deviation (n = 4). Table 3 Average values of N inputs, crop N uptake at harvest and N removal with products (kg ha-~ ) for the period 1988-1991 Variant CONVA
CONVB
INTA
INTB
MTC
118 244 138 95 70
153 274 165 120 70
70 142 71 47 25
89 177 108 71 25
71 161 79 61 25
665
782
355
470
397
0 0 70 10
0 0 70 10
200 60 70 10
200 60 70 10
200 60 70 10
Total input
745
862
695
810
737
N uptake Sugar beet Potatoes Wheat Barley Green manures
228 245 146 112 150
232 225 149 109 150
198 232 122 99 84
182 211 120 88 83
163 200 106 68 57
Total uptake
881
864
735
684
594
N removal Beet Tubers Wheat t Barley~
100 234 146 112
102 215 149 109
80 222 113 99
84 201 111 88
69 190 97 68
N inputs Mineral fertilizer Sugar beet Potatoes Wheat Barley Green manures Subtotal Compost Organic manure Deposition Seeds
Total output
593
574
514
484
423
Recycling2 N efficiency3 (%)
289 88
291 72
222 82
200 65
171 63
~Grain + straw. 2Recycling = uptake - removal. 3N efficiency = (N removal) / (total N i n p u t - deposition ) × 100%.
216
H.G. van Faassen, G. Lebbink /Agriculture, Ecosystems and Environment 51 (1994) 209-226
40
N mineralization in 6 weeks (kg/ha)
30 20 10 0 -10 -20 5
0
10
15
20
25
30
35
40
45
50
week number (1990) INTA in-situ
i
CONVB in-situ
~ INTA lab
~ CONVB lab
Fig. 2. In situ N mineralisation in 1990 in winter wheat fields 12B (conventional management on Block B) and 16A (integrated management on Block A) compared with potential N mineralisation, corrected for actual soil temperature.
N (kg/ha) 400
350
+106
300 250 200~ 150 100 50 0 90
IL 120
150
. . . . CONVB
180 210 day number 1989 --~-- INTA
-~
INTB
240
270
---~ MTC
300
© CONVA
Fig. 3. Mineral-N in the soil (0-60 cm layer) and total-N taken up by the potato crop during the 1989 growing season, under conventional and integrated management; the arrow indicates the second conventional gift of mineral-N; results for CONVA only at harvest (Day 285). Variants: CONVA/CONVB, conventional management on Blocks A and B respectively; INTA/ INTB, integrated management on Blocks A and B respectively; MTC, integrated management with minimum tillage on Block C.
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
90
precipitation (total 679 mm), mm/decade
217
oc 20
80 70
1,o
,~J"
I 197
60
1152
/ laa
50
/~ /
15
I lo6
I I
10
40
L_
30 20 10 0
,I,
I
6
11
16
21
26
31
36
decade x
average air temp.
Fig. 4. Distribution of rainfall in 1989 and monthly average air temperature; arrows indicate the dates and amounts (top to bottom) of mineral fertiliser N dressings of potatoes, in variants CONVA,CONVB,INTA, INTB and MTC (for explanation of variants see legend to Fig. 3 ). yields in MTC were on average lower than in the other four variants, except for potatoes. Table 3 gives the average N inputs to the four crops of a rotation and to the green manures, average N uptake by the crops at harvest, average N removal in harvested products and average amounts of N recycled to the soil in crop residues and green manures. Total-N inputs by compost and organic manure and by atmospheric deposition are given for the total period because as a result of their slow mineralisation these inputs cannot be ascribed to a single crop. In INT, intentionally, less mineral-N was given to the crops than the N uptake by the crops; thus in INT the crops were more dependent for their N supply on N mineralisation from applied compost, organic manure and SOM. Total-N input followed the order INTA < MTC < CONVA < INTB < CONVB, while total-N uptake and total-N removal followed the order MTC < INTB < INTA < CONVB < CONVA. More N was recycled to the soil in CONV than in INT crop residues, mainly because in CONV two green
manure crops were grown and in INT only one. Efficiencies of the total applied N inputs followed the order MTC < INTB < CONVB < INTA < CONVA. Atmospheric deposition, a rather uncertain input not made by the farmer, was excluded from the calculation of the N efficiencies. 3.3. N m i n e r a l i s a t i o n / i m m o b i l i s a t i o n
Figure 2 shows, for two winter wheat fields in 1990, the in situ rates of N mineralisation and the potential rates determined by incubation of soil samples at 20 °C in the laboratory, corrected for actual soil temperatures. In the field, periods of N immobilisation as well as periods of N mineralisation were observed, whereas in the laboratory, net N mineralisation was always found. In spring especially, the potential rates gave an overestimate of the field rates. In summer (around Week 32), both potential and field mineralisation rates were low. Potential rates of N mineralisation were almost always lower for field
218
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (I 994) 209-226 4O5 372 N mineralization
fertilizer
seed+deposition soil mineral N
72
255
1~ 27 input
36
deficit
253
tubers
]u
crop residues
73 soil mineral N output
72
18 1~ input
321
321
95
79
95
18 29
~ ,
input
245
lh 44 output
CONVB
297
152
106
303
CONVA
294
4O5
372
309
309
115
104
254
I
188
206
162
178
33
I
18 20
111 26
18 14
~0 17
input
output
input
output
output
INTA
INTB
MTC
Fig. 5. Nitrogen balance sheets for potatoes, grown under conventional and integrated management in 1989; soil mineral-N has been calculated for the 0-60 cm layer (for explanation of variants see legend to Fig. 3 ).
12B (CONVB) than for field 16A (INTA). The cumulative in situ N mineralisation until harvest of the winter wheat crop was 44 kg h a - 1and 56 kg h a - ' for CONVB and INTA, respectively; from potential laboratory rates we calculated for the same period 62 kg ha-~ and 75 kg ha-~ for CONVB and INTA, respectively.
3.4. N balance and N dynamics for potatoes Figure 3 shows that 150-200 kg of mineral-N was present in soil between Days 90 (after fertilisation) and 150, which meant a substantial risk of N loss by denitrification for this crop. Conditions conducive to denitrification--heavy rainfall, soil temperature over 8 °C and air-filled soil
volumes of less than 12%--have been shown very often in this soil in spring (Lebbink et al., 1994; Vos and Kooistra, 1994). The second application of fertiliser N to CONVB was only partially recovered in the crop, indicating that part of it had been lost or immobilised. Figure 4 shows the rainfall distribution and temperatures over 1989 as well as the dates of fertiliser N applications. The second application of fertiliser N in CONV might thus have been particularly at risk to loss by denitrification. Although N uptake was most rapid in CONVB, where fertiliser N exceeded N uptake, a higher mineralisation in INTA might explain the same N uptake at harvest in both cases. The mineral-N balances in Fig. 5 show that a
H. G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
219
70 depolitlon
7O deposition
¢ONVB
CONVA
ATMOSPHERE crol~
hervwtld product=
leed
minQml rtilizer
J
3500
~
I
A~ERE crops
~
- 4 ~ 573
i
I
old orglnlo n~,tter
SOIL (~891 ~ ~ s ~
(152 + X) losses to the environment
70 depomtlon
70 deposition I
mineral Rillzer
tNTB
INTA ATMOSPHERE crops
seed
;5
t o t~ e n v i r o n m e n t
--m" 514" "-" i partial . ~ recycling
~
I I260
i M= 1542- X') I
~x~
k
_~_~
o2a6Onie ~ .cycling
oo-
L young ofganlo
J rnltblr I
a..,oo
I
I o d organlc matter
..
....
SOIL (181 - X) losses to the environme~
(326- X)losses to the environment
Fig. 6. Total-N balance ofa 4 year crop rotation ( 1988-1991 ), for conventional (CONV) and integrated (INT) management, each started at two levels of soil organic matter on Blocks A and B: U, N uptake by crops and green manures; M, gross N mineralization;I, gross N immobilisation;X, decrease in storage of N in young organic matter.
substantial part of the large amounts of mineralN applied to potatoes and supplied by N mineralisation was not recovered from crop plus soil at harvest in CONVA, CONVB, INTB and MTC. N input from mineralisation was calculated from laboratory incubations for the period from 31 May, when sampling started, until harvest; thus, this input might be underestimated. To explain the N uptake by the crop in INTA, N mineralisation must have been at least 98 kg ha- ~or about equal to the experimental estimate of 95 kg ha- 1. Only a negligible contribution to N supply of potatoes was expected from the mineralisation of the applied organic manure, in INTA, INTB and MTC, based on incubation of soil with this material. 3.5. N balance for a crop rotation
Figure 6, which is mainly based on data from Table 3, shows for four variants the total average amounts of N circulated in a 4 year crop rotation. The calculated total-N losses to the environment assume steady-state conditions. When
the amount of N stored in SOM had increased or decreased over the period 1988-1991, the actual N loss would have been lower or higher, respectively. Even relatively small errors in the determination of total-N in soil give large uncertainties in terms of absolute amounts of N; thus, differences in total-N between 2 years ( 1988 and 1991 ) are too uncertain to make reliable estimates of actual total-N losses. The pool of N in old SOM was assumed to be constant and equal in all variants. N in old SOM was estimated to be about 3500 kg h a - 1, the difference between total-N, from soil analyses of CONVB, and N in young SOM, from model calculations for CONV. The pool of N in young SOM was calculated from average amounts of total-N, determined during 1988-199 l, minus 3500. Changes in the amount of mineral-N were assumed to be negligible. Gross N immobilisation (I) was first estimated by model calculations, and then gross N mineralisation (M) was calculated, assuming no overall change in the pool of young SOM.
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
220
Table 4 Experimental carbon and organic nitrogen inputs (kg h a - ~) with crop residues, green manures, compost and organic manure during a crop rotation, and their estimated distribution among model pools of organic matter Year
Date of input
Model component DPM C/N
RPM C/N
Total C / N ratio
Description
35 20 19 22
Residue Residue Residue Residue
POM C/N
Conventional 1 2 3 4
1 Oct. 1No~ 1 Nov. 1Nov.
Total
120/7 800/54 2200/125 1315/83
230/3 360/4 160/2 735/9
-
4435/269+1485/18+0
potatoes wheat + mustard sugar beet barley + grass
20.6 (5920/287)
Integrated 1 2 2 3 4 4 Total
1 Oct. 1 Sept. 1 Sept. 1No~ 1 Nov. 1 Apr.
120/7 280/14 1000/72 1970/113 1260/68 -
245/3 320/4
680/8 160/2 1260/16
4630/274+2660/33+2070/172
1320/110 750/62
36 33 16 19 30 12
Residue potatoes Residue wheat Spent mushroom compost Residue sugar beet Residue barley + grass Organic manure
19.6 (9360/479)
Year I of the model starts on 1 September.
3.6. Modelling C and N dynamics in soil Table 4 presents the average carbon and organic N inputs into the soil with crop residues, green manures and organic manures, based on field data, and the estimated distribution of these inputs over the model pools. Organic matter from compost was distributed among the DPM, RPM and POM pools in such a way that 1 year after its application about 55% of its C would still be present in the soil, according to our model. Processed organic manure was totally added to the POM pool, because it showed a negligible mineralisation during incubation. Figure 7 shows, for the simulated steady state of the crop rotation, the fluctuations in the organic N pools: N in easily decomposable (DPMN) and resistant (RPMN) plant material, microbial biomass (BIOMN) and stabilised organic matter (POMN). For the mineral-N pool, the resulting accumulation pattern is shown because plant uptake and possible mineral-N losses are not taken into account; the accumulated mineral-N after 4 years equals the input of organic N during the
rotation because steady state has been attained. The pattern of mineral-N accumulation shows a (desirable) immobilisation (decrease) of mineral-N during three winter periods; if this also occurs in the field it may prevent N loss. Modelled N supply from mineralisation during the growing season was highest for spring barley and grass green manure and lowest for winter wheat. In fact, the model shows that about half of the N mineralised in the first year, after potatoes, becomes available in autumn and may be lost during winter. The steady-state calculations indicated that in CONV, on average 89 kg N ha-1 would be present in microbial biomass, 277 kg in young humus and about 35 kg in crop residues; in INT there would be on average 109 kg, 1103 kg and 25 kg N ha-1 in microbial biomass, young humus and crop residues, respectively. Thus, the main difference between CONV and INT management would be the size of the young humus (POM) pool and the contribution of this pool to N mineralisation. Only under INT management would this POM pool serve as a substantial source of'slow-biorelease fertiliser'. Soil
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
Conventional Management
kg N per ha
250 -
221
wheat +
luglt bltt
blrlly + grill
150
,oo
/
50 -
~
1200 ,-
.Integrated Management
= .................
............
looo~ wheat
badey + grass
sugar beet
potatoes
400 300 200 -
O~ ...........
I
9
I
I I I I I I
1
5
ii I I I I i
5
/
I I i i i I iI I I I iI
9
1
5
9
I I I i g I I I t
1
5
9
month DPMN
-=-
RPMN
~
BIOMN
---
POMN
~
Nmin
cumulative
Fig, 7. Modelled fluctuations in crop residue and soil organic N pools and the resulting cumulative mineral-N for a 4 year crop rotation under conventionaland integrated management, with the assumption of steady state.
temperature distribution is mainly responsible for the slowing down ofN mineralisation during winter. The general effect of microbial turnover of organic matter and N in soil is to transform the large differences in yearly inputs into much smaller differences in yearly N mineralisation (cf. Table 4 and Fig. 7 ). 3. 7. Trends in soil organic matter and N contents
Figure 8 shows that the initial two levels of SOM and soil total-N diverged into four levels.
The SOM content decreased rapidly in variant CONVA, slightly in variant CONVB, remained constant in variant INTA and increased in variant INTB. Total-N in soil was maintained under CONV management and increased under INT management. The top 25 cm soil of the variants INTA, CONVA, INTB and CONVB weighed about 3.44 Gg, 3.53 Gg, 3.62 Gg and 3.68 Gg ha-1, respectively, which we calculated to contain about 5100 kg, 4700 kg, 4300 kg and 3900 kg N, respectively. Subtraction of 400 kg N hafor variant CONVB, i.e. the steady-state amount calculated to be present in microbial biomass,
222
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
3.5
organic matter (%)
66
0.18
i
i
i
i
70
74
78
82
86
90
total N (%)
0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 66
i
L
i
i
i
70
74
78
82
86
90
year CONVA
--+-- INTA
- 4 ~ CONVB
~
INTB
Fig. 8. Trends in soil organic matter and total-N contents in the 0-25 cm layer, 1966-1991. Each point represents the average valuefor three fieldsthat havebeenpart of the variantsCONVA,CONVB,INTAand INTBsinceautumn 1985.For explanation of variants see Fig. 3. The arrowindicatesthe start of conventionaland integratedmanagement. young humus and crop residues, resulted in 3500 kg N ha - l , assumed to be present in stable, old humus,
4. Discussion 4.1. Weather conditions and atmospheric deposition
Weather conditions directly affect crop development, for instance the early senescence of a
crop in a dry period. However, indirect effects on crop development, by limiting the availability of mineral-N in soil, can also be important. Dry soil can limit N uptake. Heavy rainfall may cause N losses by denitrification, especially in spring when there is a high level of mineral-N in the topsoil and soil temperature is already above 5 ° C; much rainfall over the winter period may cause substantial leaching of nitrate (Van Faassen and Lebbink, 1990). Because the large variation in rainfall distribution will certainly affect turnover as well as N losses, a first step to im-
H.G. van Faassen, G. Lebbink / Agriculture, Ecosystems and Environment 51 (1994) 209-226
prove the model should be to include effects of differences in soil moisture content.
4.2. Crop yields and N efficiency Whereas CONV crop yields were on average higher than INT crop yields, except for potatoes in INTA, the average N efficiencies showed a greater difference between Blocks A and B than between CONV and INT (Tables 2 and 3 ). Block A had a higher content of SOM than Block B and the crops on Block A got less mineral fertiliser N than the crop on Block B. Thus it might be coneluded that the extra amount of mineral-N given to the crops on Block B, to compensate for the higher N mineralisation on Block A, had been less efficient than the N supplied by mineralisation on Block A.
4.3. N mineralisation/immobilisation The results indicated that potential laboratory rates of N mineralisation gave an overestimate for net in situ N mineralisation. N immobilisation was never found in the laboratory, which might be explained by the sieving out of visible crop residues, which in the field might cause N immobilisation (Van Noordwijk et al., 1993). The in situ measurements had some advantages: temperature correction was no longer needed and periods where N loss occurred could be identified. However, some artefacts might also have occurred in the field: for instance soil compaction, by inserting the tubes into the soil, and differences caused by the exclusion of crop N uptake and nitrate leaching from the tubes.
4.4. N balance and N dynamics for potatoes N losses by denitrification seemed to be the most likely explanation for the large deficit in the mineral-N balance of potatoes (Fig. 5) since leaching losses during the growing season were unlikely. During a period of more than 2 months, when crop uptake of N was still low, the mineralN content of the soil was high in all the variants and periods of heavy rainfall may then have caused denitrification (Figs. 3 and 4). When
223
substantial loss of mineral-N occurs in a critical period of rapid crop development, the crop becomes more dependent on actual N mineralisation. N uptake by the potatoes suggested that N mineralisation eventually compensated possible N loss in variant INTA, but not in variants INTB and MTC, where N availability might have limited crop development. The higher input of fertiliser N on Block B than on Block A might have contributed to the higher deficits in the mineralN balances of CONVB and INTB, compared with CONVA and INTA. The high deficit in the N balance of MTC might be explained by a large N loss through denitrification, because of a relatively low gas-filled porosity; previous experience with minimal tillage at the Lovinkhoeve has shown that it took at least 10 years before the formation of biopores countered this negative effect of reduced tillage (C. Van Ouwerkerk, personal communication, 1993 ).
4.5. N balance for a crop rotation The higher deficits in the N balance for the variants INTB and CONVB than for the variants INTA and CONVA (Fig. 6) may be accounted for by the higher input of mineral-N on Block B than on Block A. High inputs of mineral-N in spring, when N uptake by the crop is still low, increase the risk of high N losses by denitrification. Although more N was returned to the soil by crop residues in CONV, the total input of organic N in CONV was much lower than in INT. Mineralisation supplied the crop with mineral-N throughout the growing season, without much accumulation of mineral-N, thus with a low risk of large losses. This might partly explain the higher N efficiencies on Block A, where input of fertiliser N was lower and N mineralisation was higher than on Block B. Loss of mineralised N might occur after crop harvest. Therefore, in such periods either a green manure crop should be present or N immobilisation should be promoted by the addition of decomposable organic matter with a high C / N ratio. Since a steady state had not yet been attained, the high N efficiency in CONVA may have occurred at the expense of the decreasing level of
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soil organic matter and may not be sustainable. On the same block, the lower overall N efficiencies in INT in comparison with CONV, may be due to the fact that part of the organic manure and compost applied in INT served to increase the pool of young humus in soil. The N efficiencies of the INT variants were, therefore, expected to increase with time until a new steady state would be attained. Hence, N efficiencies of CONV variants were probably overestimated, whereas those oflNT variants were conservative estimates. It should be noted that the calculated N losses to the environment (Fig. 6) were rather uncertain because of the large uncertainty of changes in the total amounts of N stored in SOM (1% difference in the latter was equal to 40-50 kg ha -1 ). 4.6. Modelling C and N dynamics in soil Microbial biomass and young soil humus, the latter especially in case of INT management, act as storage pools from which significant amounts of N can be mineralised, mainly during the growing season. The strong N immobilisation during turnover of crop residues by the microbial biomass, which consists mainly of bacteria in the soil used, results from their low C / N ratio of 4 and the C / N ratio of the organic inputs of about 20 (Fig. 7, Table 4). The simulation shows a substantial accumulation of mineral-N after the harvest of potatoes, which might be lost for the following winter wheat crop. To prevent possible loss of mineral-N after harvest of potatoes, the application of degradable organic manure with a relatively high C / N ratio should be considered to promote immobilisation of this mineral-N. Since this N will be mineralised eventually, less mineral-N should be applied to following crops to prevent the problem being shifted in time. Another possibility might be to harvest the potatoes earlier, to allow a subsequent green manure crop to be grown. The model shows that during the growing season of spring barley and undersown grass more N is mineralised than during the growth of the other crops. Therefore, the low amounts of fertiliser N applied to spring barley
in INT (Table 3 ) may be quite realistic. Generally, it is clear that in INT less mineral-N has to be applied to supplement soil N supply than under CONV management, because of higher steady-state N mineralisation in INT management, as a result of the application of organic manures. Simulated N mineralisation during the growth period of winter wheat is 40 kg ha-1 and 70 kg ha- l for CONV and INT, respectively; these values agree best with the in situ determined value of 44 kg ha- 1 (CONVB) and with the potential laboratory incubation value of 75 kg ha -1 (INTA) for winter wheat in 1990. We have to await further results of in situ measurements to see how good the model results will fit for the other crops. Since soil temperature has such a large effect on turnover rates, the exact timing of organic matter applications will greatly determine their fate. Working crop residues and green manures into the soil 1 month later may shift their main turnover from autumn to spring, and may cause quite a different N supply to the crop. In view of the measured rates of N mineralisation (Fig. 2 ), it seems necessary to improve the model by taking into account the slowing down of N mineralisation in summer, which may be the result of drying out of the topsoil. 4. 7. Trends in soil organic matter and N contents Long-term data of soil organic matter and N contents, as shown in Fig. 8, and knowledge about the agricultural history of the fields are necessary to identify significant trends. Significant differences in SOM and total-N content developed as a result of the change in management in 1985. These differences were consistent when compared with the fluctuating differences that were found between duplicates before 1985. However, differences in total-N contents were not always representative for differences in absolute amounts oftotal-N in soil, because differences in soil volumetric mass were also present. Thus, we compare varying amounts of soil in the 0-25 cm layer of each variant in Fig. 8. Complicating fac-
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tors were that over the period shown, not only were the inputs of organic matter into the soil varied, but also the depth of ploughing and the weather conditions from year to year were different. A decrease in SOM content, but not in totalN content (CONVA and CONVB), indicates a decrease in the C / N ratio of SOM; this might be explained by the relatively high amounts of fertiliser N used, resulting in crop residues with a relatively low C / N ratio and generally sufficient mineral-N available for N immobilisation. During the first years of a transition from a high level of SOM to a low level, an extra amount of N may become available through mineralisation, which may considerably increase N losses, if the N input is not decreased at the same time. The calculated range of 400-1600 kg N ha-1, in young humus, indicated that a large decrease in soil organic N might strongly decrease the capacity of the soil to supply the crop with mineral-N, because the relative decrease in young SOM N was much larger than the relative decrease in total-N. However, at steady state, differences in N mineralisation will be proportional to differences in inputs of organic N, and not to differences in the amount of N in young SOM.
5. Conclusions Six years of comparison between conventional and integrated management allow the following conclusions. ( 1 ) Integrated crop yields were on average 90% (83-102%) and average crop N uptakes 85% (74-103%) of those under conventional management. (2) Replacement of 35% of mineral fertiliser N by stabilised organic N, in the form of compost and processed animal manure, resulted in increased amounts of N in the pool of young humus in the soil and concomitantly increased the N supply to the crop from soil N mineralisation. (3) Overall efficiencies of applied mineral-N plus organic N increased in the order INTB < CONVB < INTA < CONVA (65%, 72%, 82% and 88%, respectively); these efficiencies are expected to increase in INT variants and to de-
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crease in CONV variants, when in the long-term new steady states of SOM and N will be attained. Extra mineral-N applied to Block B, to compensate for the higher N supply from mineralisation on Block A, might explain the lower N efficiency on Block B than on Block A. (4) The highest (risk of) N losses might be expected from potatoes and sugar beet, where high levels of nitrate are present in the soil in spring and crop N uptake starts relatively late. For potatoes N loss might also be important after crop harvest, if the crop is not followed by a green manure. (5) N loss by denitrification might be more important in this silt loam soil than nitrate leaching. Conditions conducive to denitrification often prevailed in spring, whereas the N uptake by green manures reduced nitrate leaching. (6) The lowest N losses might be expected from INT management of winter wheat and especially spring barley, where most of the N taken up by the crop and by the green manure would be supplied by N mineralisation from SOM, more or less in synchrony with crop growth. (7) The higher N efficiencies on Block A, with a higher content of SOM than on Block B, indicated that it might be important to maintain a relatively high level of SOM. Only under INT management could the SOM level of Block A be maintained by applying stabilised organic matter in addition to the crop residues and green manures. However, the increase of SOM content by INT management on Block B was shown to be rather slow. 6. Acknowledgement The authors thank Dr. P. de Willigen for his constructive criticism of this paper. This work was supported by the Netherlands Integrated Soil Research Programme.
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