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Dry matter and nitrogen accumulation by three leguminous green manure species and the yield of a following wheat crop in an organic production system
Agriculture Ecosystems & Enwonment ELSEVIER
Agriculture,
Ecosystems
and Environment
57
(I 996) I89- 196
Dry matter and nitrogen accumulation by three leguminous green manure species and the yield of a following wheat crop in an organic production system C. Stopes *, S. Millington,
L. Woodward
Elm Farm Research Cenrre. Newhury. RGI5 OHR, l/K Accepted 20 November 1995
Abstract Organic farming systems often include livestock to use the leguminous forages which supply nitrogen (N) to the grain crops in the rotation. An alternative approach (especially relevant to farms with ‘set-aside’) may be to manage leguminous green manure crops by repeatedly cutting and mulching them directly in the field. An experiment (carried out on an organic farm in the UK) compared the dry matter and N accumulation of legumes grown for periods of between 6 months and 2 years, compared with a non-leguminous rye-grass (Z&urn spp.) control. The performance of a subsequent wheat (Triricum aestivum L.) crop was also measured. Red clover (Trififium pratense L.), white clover (Tri’lium repens L.) and trefoil (Medicago lupulina L.) green manures and the rye-grass control were cut to maintain a height of no more than 30-40 cm, and the cut material was left on the soil surface (‘mulching’). Of the legumes, white clover accumulated the most dry matter (12.2 t DM ha-’ year-‘) and red clover the most N above ground (371 kg N ha-’ year-‘) over a 1 year period of green manuring. Both these species accumulated significantly more dry matter and N than trefoil and ryegrass. Yields of following wheat crops varied considerably. Spring wheat (grown after 6 and 18 months of green manuring) did not yield at a commercial level due to poor establishment. Winter wheat generally yielded well following 1 year of green manuring with red (6.0 t ha-‘) and white clover (5.2 t ha-‘) and significantly more than following trefoil and the nil-legume ryegrass control (3.3 t ha-’ and 2.1 t ha- ‘, respectively). The results suggest that red clover is the optimum species for use as green manure. This is relevant in the context of set-aside management. There was no indication that winter wheat yield was improved following a second year of green manuring with either red or white clover. Approximately one third of the total N accumulated by red clover above-ground was lost by leaching (measured using porous ceramic cup samplers) following cultivation of the l-year green manure in September prior to establishing a winter wheat crop. Delaying cultivation until the spring substantially reduced leaching due to uncultivated soil over winter. The environmental risks (and agronomic benefits) associated with farming systems relying on natural N fixation and soil microbial activity should be more fully evaluated. Krywords: Organic
fanning; Green manure; Legume; Nitrogen fixation; Set-aside; Wheat yield
1. Introduction
* Corresponding 1488 548503.
author.
0167-8809/96/$15.00 SSDI 0 167-8809(95)0
Tel.:
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1488 658298;
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0 1996 Elsevier Science B.V. All rights reserved 1002-5
The culture
environmental effects of conventional agrihave been of considerable concern over re-
190
C. Stopes et al./Agriculture,
Ecosystem and Enuironment 57 (1996) 189-196
cent decades, when considering nutrient supply to crops, the increased use of fertilisers has resulted in environmental pollution. Integrated farming systems have been developed exploiting the potential for crop rotation including legumes within a strategy which aims to reduce fertiliser use through carefully targeted applications. Organic or ecological farming systems exclude the use of conventional nitrogen (N) fertilisers, instead requiring a balance between the supply and demand of N through the use of legumebased fertility building phases in the crop rotation (Anonymous, 1991a; Anonymous, 1991b). Legumes are able to accumulate substantial quantities of nitrogen, and the soil’s population of microbes has an enormous capacity to cycle this N in the right conditions (Jarvis et al., 1996). The release of this labile nitrogen can be difficult to control, and may represent an environmental hazard depending on husbandry practices. On most organic farms in the UK a livestock enterprise uses the leguminous forages, and thus the rotation can readily achieve a N balance between fertility building (legume-based) phases and exploitative (arable) phases within the rotation. More than half the farmed area is devoted to legume-based leys (Philipps and Stopes, 1995), and the financial return provided by livestock ensures that the farming system can be economically viable. Such a system is relatively agronomically and environmentally stable from the point of view of maintaining N balance and soil structure whilst suppressing weeds, pests and diseases. If there is no livestock enterprise, leguminous green manures must be used within the rotation, a green manure crop being produced solely for soil incorporation. Traditionally, green manures were used for soil structure improvement and organic matter accumulation (Allison, 1973). The use of leguminous green manures specifically for accumulation of N in organic crop rotations has become increasingly relevant in the context of set-aside management which does not include livestock. The green manure is mulched (‘multiple chop’) several times during the growing season when it reaches an optimum height and the material is left on the surface to desiccate and be incorporated into the surface soil horizons by soil macrofauna. Improvements in crop yield, soil aggregation,
crumb structure and hydraulic conductivity have been observed following green manuring (Allison, 1973; Faris, 1986; Elliot and Papendick, 1986). Loss of N from the system is inevitable, both by volatilisation during the growing season (Whitehead and Lockyer, 1989), and by nitrate leaching following cultivation (Philipps and Stopes, 1995). This paper reports the results of a trial established to determine the effect of green manure legume species and duration of growth on green manure dry matter and N accumulation, the yield of a following cereal crop and the quantity of nitrate leached following cultivation and establishment of the cereal crop. To aid comparison, a non-leguminous control crop of ryegrass was also grown.
2. Materials and methods 2.1. Experimental
site and treatments
A trial was established in the autumn of 1989, on a commercially managed, registered organic farm (United Kingdom of Organic Food Standards; Anonymous, 1991b) in Wiltshire, UK, on a field with an Argillic brown-earth clay-loam soil of the Ardington association. On establishment of the trial, the field had completed the fifth year of organic arable cropping after a 3 year ley (ley: winter wheat, winter wheat, spring peas/barley, winter wheat, spring oats). Table 1 shows the soil analysis at the commencement of the trial. The average annual total rainfall was 800 mm, mean maximum temperature 18°C and minimum temperature 6°C. Rainfall during the period of the experiment (1989- 1992) and the 10 year average rainfall are shown in Fig. 1. Three leguminous green manures and a nonleguminous ryegrass control were grown for between 6 and 25 months in a split plot design (plot size 4 m X 8 m) with four replicates for each treatment. Green manure species treatments were fully randomised within the duration treatments in order to facilitate cultivation and establishment of the test cereal. The period of green manuring was followed by cultivation and sowing of a test cereal crop. A winter wheat crop (WWl) was sown at the start of the experiment to assess yield in the absence of a green manure. Fig. 1 shows the soil cultivation,
C. Stopes et al./Agriculture,
Table I Soil analysis
sowing and harvesting dates for the test cereal and details of the green manure treatments. Before sowing, legumes were inoculated with the Rhizobium species recommended and supplied by the Agricultural Genetics Co. (Cambridge, UK) to facilitate adequate root colonisation. During the growing season the green manure was cut to a height of 5-8 cm when it reached a height of
of trial site (O- 15 cm soil depth) 6.5 3.53% 0.16% 46.9 mg kg-’ 217.0 mg kg-’ 2600.0 mg kg-’ 126.0
PH Organic matter Total N Olsen P K Ca Mg
WWl .... ..... .. . .... ..... x
Test cereal GM growth
x
SW1
Test cereal
SW2 .._._...x ww2 . .......... ..... .. ..._.__..x
ww3
-. x
+ 6month
---------
----___ --_--__--
_____- _______-
- + 13 month --
______
+ 18month
-_-_-____~--
--
+ 25 month
..I989....11990 .............................~1991............................)1992
Year 16OT
191
Ecosystem and Environment 57 (1996) 189-196
2
..._._._... r.
ASONDJFMAMJJASONDJFblAMJJASONDJFMAMJJASOND
-
10 Year Av
- -0 -
Annual rainfall
Incorporation of green manure. X = Harvest cereal SW 1,2 = Spring wheat sown (var. Axona @ 260 kgha-l). WW 1, 2, 3 = Winter wheat sown (var. Mercia @ 220 kgha-I). Green manure treatments. . Code Name Species/variety Seed rate (kg.ha-‘) K.d
+ =
RG
Ryems control
RC WC
Red clover White clover
TR
Trefoil
Italian ryegrass (Lohum var. Atalja) Perennial ryegrass perexn~ var. Condesa) Trifoliumptatense (var. Merviot) Ttiolium rem (var. Aran) Italian + PerenniaI ryegrass Medicaeo (WS) Italian + Perennial ryegrass
Fig. 1. Periods of green manure and crop growth (mulching, year average. Green manure species treatments.
soil cultivation,
sowing, harvesting).
15 15 30 25 5 25 5 Monthly rainfall (mm): 1989-1992
and 10
192
C. Stopes et al./Agriculture,
Ecosystem and Environment 57 (1996) 189-196
30-40 cm using a mulching attachment on a small ‘walking’ tractor which provided a multiple chop. The final mulching of the green manures took place approximately seven days prior to ploughing and preparation of the seed bed for the test cereal crop. During the second year (25 month green manure: 1990-1991) there was incursion of red clover into neighbouring trefoil green manure and ryegrass control plots. Native white clover growth was also observed. In neither case was this growth controllable. 2.2. Assessments Immediately before each mulching occasion, three quadrat (0.25 m2) samples were taken per plot to measure above ground dry matter of the green manure, N content of the dry matter was determined by Kjeldahl digestion and steam distillation (Bremner and Keeney, 1965). The cereal crop after the green manure treatments was harvested using a Wintersteiger plot combine with a 1.3 m cutter width with yield being adjusted to 15% moisture content. Three 0.25-m2 quadrat samples were cut prior to combining for estimation of weed dry matter and grain N content (analysed by Kjeldahl digestion and steam distillation). Damage by sheep of the cereal crop following the 25 month green manure treatment necessitated yield estimates from three 0.25-m2 quadrat samples being taken from each replicate plot. Nitrate leaching following the 13 and 18 month duration red clover and ryegrass control treatments was measured by utilising two porous ceramic cup samplers in each of two of the replicate plots at a depth of 90 cm (four replicate samples per treatment). Soil water was sampled fortnightly by applying a vacuum of - 80 kPa to the ceramic sampler unless the site was snow covered or frozen. Water samples were frozen prior to nitrate analysis by calorimetric methods involving reduction of nitrate to nitrite through a cadmium column (Henriksen and SelmerOlsen, 1970) prior to an azo dye Orange I reaction (Follett and Ratcliff, 1963). Further details of installation and analysis are given in Philipps‘and Stopes (1995). Nitrate leached was calculated from the treatment mean nitrate concentration in the soil-water integrated with cumulative net drainage (Lord and Shepherd, 1993). Cumulative drainage data for the site
were supplied by the Meteorological Office ‘MORECS’ service (Thompson et al., 1981).
3. Results and discussion 3.1. Dry matter and nitrogen accumulation.
Above ground dry matter and N accumulation by the leguminous green manure and control treatments grown for periods of 6-25 months are presented in Table 2. There were no significant differences in dry matter or N accumulation during the first 6 months. Red and white clover green manures both resulted in more dry matter and N accumulation than trefoil and the non-leguminous ryegrass control over 13, 18 and 25 months of growth. Over the second year (13-25 months) red and white clover grew in the trefoil green manure and ryegrass control plots, contributing to the accumulation of dry matter and N over the second year of the experiment, leading to much smaller differences between treatments. In all cases the N content of the mulched red clover was greater than all other treatments, explaining the greater N accumulation in this species compared with the white clover green manure, even where white clover produced more dry matter. Between March and August 1990 (first year of trial) rainfall was approximately half the 10 year average, over the same period in the following year (second year of trial) the 10 year average was slightly exceeded, although rainfall was unevenly distributed (Fig. 1). Dry matter accumulation by red and white clover between March and August in the second year of green manure growth (1991) was similar to that in the first year ( + 1% and - 5%, respectively). In contrast, trefoil accumulated 32% and ryegrass 102% more dry matter in the second year with higher rainfall, thus yield potential of red and white clover was apparently unaffected by low rainfall during the first year. Where the green manure treatment included ryegrass (present in all except the red clover treatment), growth was more vigorous in the second spring, so it may have been advantageous to include a small proportion of ryegrass in the red clover treatment. This would be expected to have increased productiv-
C. Stopes et al./Agriculture.
Table 2 Dry matter and N accumulation
193
Ecosystem and Environment 57 (1996) 189-196
in green manures
Duration (months)
Green manure species a
6
RC WC TR RG cont. Stubble LSD < 5%
No. of mulchings
1 1
I 1 0
Dry matter accumulation (t ha-‘)
N accumulation (kg ha- ‘>
0.8 0.6 0.6 0.7 0.7 NS
21 17 14 I5 15 NS
RC WC TR RG cont. LSD < 5%
6 5 5 5
11.9 12.2 328 9.1 6.4 1.8
371
18
RC WC TR RG cont. LSD <5%
8 7 7 7
14.1 15.2 10.8 7.3 2.0
450 432 265 110 54
25
RC WC TR RG cont. LSD <5%
10 10 10 10
25.4 25.0 20.4 17.5 2.7
741 592 459 346 94
13
211 94 47
a See Fig. I.
ity in the second spring and also reduced weed growth by competitive exclusion. Dry matter and N accumulation by trefoil was low compared with the red and white clover green manures. Under the conditions of the trial this species did not perform well even though satisfactory nodulation was observed and trefoil occurs naturally on this soil type. Trefoil is used extensively in organic farming systems in other European countries, and has the advantage of being from a different genus to the clovers, thus’ providing a break in disease susceptibility. This is an important consideration in organic systems where the disease risk associated with close cropping of identical or related species should be avoided (Millington et al., 1990).
ments were not significant. However, prior to incorporation of the 25 month green manure treatments, soil organic matter content was 3.6%. Over this period total soil N increased from 0.16 to 0.2% (mean of all treatments), with no difference between treatments. Despite the similarity among treatments in soil organic matter and total N content, there were large though non-significant differences in N mineralisation following cultivation, indicated by the quantity of nitrate leached over the winter of 1990-1991 from the 13 and 18 month plots (Table 3). This
Table 3 Nitrate leached (kg NO,-N haDuration (months)
Cultivation date
Nitrate leached (kg NO,-N ha- ‘)
Red clover Ryegrass Red clover Ryegrass
13 13 18 18
9.1990 9.1990 3.1991 3.1991
102 18 26 4
3.2. Soil organic matter and nitrate leaching Organic matter content of the soil prior to incorporation of the green manure increased from 3.5 to 4.3% over 18 months; differences between treat-
’) over winter
Green manure
C. Sropes et al./AgricuNure,
194
Ecosystem and Etwironment
cient for use in bread making (over 1.6%). No other parameters (specific weight, grain size, P and K content) were significantly different between treatments. There was an inverse relationship between weed dry matter at harvest and cereal yield. Where wheat was grown in the first year (198919901, with no green manuring, a lower yield (4.8 t ha-‘) and grain-N content was observed, although it was still higher than that following 1 year of growth of trefoil or ryegrass, both these treatments presumably resulting in net immobilisation of N following incorporation. Following a two year period of green manuring, the mean yield (estimated from quadrat samples) was 5.2 t ha-‘, with no significant differences between treatments. Growth of red and white clover in the trefoil and non-leguminous ryegrass plots during 1991 resulted in substantially increased dry matter and N accumulation over the second year in these treatments, similar to that accumulated over one year by red or white clover (Table 2) and sufficient to mask any treatment effect on crop yield. Spring wheat following both 6 (1990) and 18 month (1991) green manuring failed to yield at a commercial level due to high January-April rainfall, resulting in poor establishment in wet conditions (26% and 19% above 10 year average, Fig. 1). There were, however, significant differences in grain yield after 18 months of green manuring, yields following red clover and white clover being the greatest (2.0 t ha-’ and 1.3 t ha-‘, respectively), compared with only 0.7 t ha-’ for the trefoil-based green manure
supports other leaching studies on organic mixed farms where spring cultivation of legume pastures reduces the total quantity of nitrate leached compared with autumn cultivation over a 2 year period (Philipps and Stopes, 1995). Of the N accumulated above ground by the green manures, 27% and 19% was lost by leaching following September cultivation of the red clover and ryegrass treatments, respectively. The high level of nitrate leaching following incorporation of a legume green manure prior to planting a winter sown cereal represents a potentially important environmental hazard which should be taken into account when implementing green manure based rotations, whether in organic or integrated farming systems. These rotations will typically involve a higher proportion of arable cropping than would be the case on a mixed (livestock based) organic farm, consequently higher levels of nitrate leaching may be expected than are observed on mixed organic farms (Philipps and Stopes, 1995). Where green manures are incorporated into an integrated farming system, including the use of fertilisers, this risk may be even greater. 3.3. Wheat yield following
green manuring
Winter wheat yield was greater following the leguminous green manures than the ryegrass control (Table 4). The highest yield, grain N content and N offtake followed red and white clover, wheat from only these treatments had a grain N content suffi-
Table 4 Wheat yield, grain-N and weed dry matter at cereal harvest following Duration
Green manure species a
57 (1996) 189-196
green manures
Wheat grain yield b
Nitrogen ’
Weed
’
(t DM ha-‘)
(tha-‘1
%
kg ha-
0
Nil
4.8
1.36
55
0.81
13
RC WC TR RG LSDP
6.0 5.2 3.3 2.1 0.9
1.86
95 80 45 30 14
0.89 1.27 1.91 2.05 NS
(months)
<5%
’ See Fig. 1. b Yield expressed at 85% DM. ’ Nitrogen expressed as percentage
DM.
I .80 1.62 I .69 0.12
C. Stops et nl./Agriculture,
and no harvestable leguminous ryegrass.
crop
following
the
Ecosystem and Environment 57 (1996)
non-
4. Conclusion The development of farming systems designed to reduce fertiliser use, provide environmental benefits and save production costs partly depends on optimisation of the supply and cycling of naturally derived N. Fixation by legumes and mobilisation of soil organic matter N through microbial activity are important, however both of these are difficult to predict and control. Soil microbial activity is particularly affected by soil disturbance, moisture and temperature, thus season interacts with husbandry increasing the unpredictability of N supply and limiting the opportunity for control of an environmentally damaging and agronomically wasteful surplus of N. Consequently, although there is much merit in developing cropping systems which reduce the need for fertilisers to supply N, there are associated environmental risks. There is evidence that an organic system has a higher rate of biological activity and improved soil quality compared to a conventional system, a mixed (livestock based) organic system being better in this regard than a stockless (green manure based) system (Wander et al., 1994). This suggests that uncontrollable outputs of nitrogen (particularly important in the context of environmental pollution) may be more likely from an organic compared to a conventional system, however, the overall efficiency of N utilisation has been consistently demonstrated to be higher in organic farming systems (Kristensen and Kristensen, 1992; Van der Werff et al., 1995). This trial has shown that a 1 year period of red or white clover green manuring can lead to increased yield and higher grain N content in a following wheat crop in an organic system. Neither trefoil nor -the ryegrass control accumulated sufficient N to allow net-mineralisation at a rate able to meet the N demand of a commercially viable yield of quality wheat. The effect of green manuring was only studied in the first cereal crop, longer term effects over entire crop rotations are clearly important. However, it would appear that two years of green manuring results in a level of N accumulation in excess of crop
189-196
195
requirements and likely to result in substantial loss of N; the optimum period for green manuring is likely to be 1 year. In a simple two compartment model, mineralisation of organic N in the soil occurs at a rate dependent both on the proportion of readily decomposable and more resistant sources of organic N, and on the rate constant for mineralisation of each type (Jarvis et al., 1996). The large introduction of freshly degradable plant matter following green manuring readily supports mineralisation unlimited by N supply. Green manuring for 2 years accumulates available N in excess of the requirement of the following crop. Much of this is ultimately immobilised, however, the remainder is available for loss by leaching, denitrification and volatilisation. The use of green manures for ‘fertility building’ has wide application in the context of set-aside management, and in those farming systems (including organic) where fertiliser use is more or less reduced. The environmental risks (as well as agronomic benefits) of systems reliant on the rapid N accumulation and cycling potential of natural nitrogen fixation and soil microbial populations should be further evaluated.
Acknowledgements The work reported was carried out under a grant from the Ministry of Agriculture Fisheries and Food (CSA 1466). Acknowledgment and thanks for support are made to M. Marriage, R. Unwin and Prof. Dr H. Vogtmann. We also thank G. Forde and all other EFRC staff who contributed to the project.
References Allison, F.E., 1973. Soil Organic Matter and its Role in Crop F’roduction. Elsevier, New York, pp. 450-456. Anonymous, 1991 a. Council Regulation (EEC) No 2092/9 I of 24 June 1991 on organic production of agriculturaJ products and indications referring thereto on agricultural products and food stuffs. Official Journal of the European Community No. L198:1-15, 22 July 1991. Anonymous, 1991 b. UKROFS Standards for Organic Agricuhute. United Kingdom Register of Organic Food Standards, Ministry of Agriculture, Fisheries and Food, London.
196
C. Stopes et al./Agriculture,
Ecosystem and Environment 57 (1996) 189-196
Bremner, J.M. and Keeney, D.R., 1965. Steam-distillation methods for determination of ammonium, nitrate and nitrite. Anal. Chim. Acta, 32: 485. Elliot, L.F. and Papendick, R.J., 1986. Crop residue management for improved soil productivity. Biol. Agric. Hortic., 3: 131142. Faris, M.A., 1986. Plow down effects of different forage legume species, cultivars, cutting strategies and seeding rates on the yields of subsequent crops. Plant Soil, 95: 419-430. Follett, M.J. and Ratcliff, P.W., 1963. Determination of nitrite and nitrate in meat products. J. Sci. Food Agric., 14: 138-144. Henriksen, A. and Selmer-Olsen, A.R., 1970. Automatic methods for determining nitrate and nitrite in water and soil extracts. Analyst, 95: 514-518. Jatvis, S.C., Stockdale, E.A., Shepherd, M.A. and Powlson, D.S., 19%. Nitrogen mineralisation in temperate agricultural soils: processes and measurement. Adv. Agron., 57, in press. Kristensen, E.S. and Kristensen, IS., 1992. An analysis of nitrogen input, -yield and -loss in organic and conventional dairy farms in Denmark. In: A. Peeters and V. van Bol (Editors), Potential and Limits of Organic Farming. CEC Workshop, Louvain-la-Neuve, Belgium, 17- 18 September 1992, Working document for CEC ref. F.11.3 - SJ/OOO8, pp. 8-13. Lord, E.I. and Shepherd, M.A., 1993. Developments in the use of porous ceramic cups for measuring nitrate leaching. J. Soil Sci., 44: 435-449. Millington, S., Stopes, C., Woodward, L. and Vogtmann, H.,
19%. Rotational design and the limits of organic systems-the stockless organic farm? In: R. Unwin (Editor), Crop Protection in Organic and Low Input Agriculture. BCPC Monogr. No. 45, British Crop Protection Council, Famham, UK, pp. 163173. Philipps, L. and Stopes, C.E., 1995. The impact of rotational practice on nitrate leaching losses in organic farming systems. In: L. Kristensen, C. Stopes, P. Kolster and A. Granstedt (Editors), Nitrate leaching from ecological agriculture. Biol. Agric. Hortic., 11: 123-134. Thompson, N., Barrie, LA. and Ayles, M., 1981. The Meteorological Office Rainfall and Evaporation Calculation System (MORECS). Hydrol. Memo. No. 45, The Meteorological Office, London, UK. Van der Werff, P.A., Baars, A. and Oomen, G.J.M., 1995. Nutrient balances and measurement of nitrogen loss on mixed ecological dairy farms on sandy soils in the Netherlands. In: L. Kristensen, C. Stopes, P. Kolster and A. Granstedt (Editors). Nitrate leaching from ecological agriculture. Biol. Agric. Hortic., 11: 41-50. Wander, M.M., Traina, S.J., Stinner, B.R. and Peters, S.E., 1994. Organic and conventional management effects on biologically active soil organic matter pools. Soil Sci. Sot. Am. J., 58: 1130-l 139. Whitehead, D.C. and Lockyer, D.R., 1989. Decomposing grass herbage as a source of ammonia in the atmosphere. Atmos. Environ., 23: 1867-1869.
Agriculture,
Ecosystems
and Environment
57
(I 996) I89- 196
Dry matter and nitrogen accumulation by three leguminous green manure species and the yield of a following wheat crop in an organic production system C. Stopes *, S. Millington,
L. Woodward
Elm Farm Research Cenrre. Newhury. RGI5 OHR, l/K Accepted 20 November 1995
Abstract Organic farming systems often include livestock to use the leguminous forages which supply nitrogen (N) to the grain crops in the rotation. An alternative approach (especially relevant to farms with ‘set-aside’) may be to manage leguminous green manure crops by repeatedly cutting and mulching them directly in the field. An experiment (carried out on an organic farm in the UK) compared the dry matter and N accumulation of legumes grown for periods of between 6 months and 2 years, compared with a non-leguminous rye-grass (Z&urn spp.) control. The performance of a subsequent wheat (Triricum aestivum L.) crop was also measured. Red clover (Trififium pratense L.), white clover (Tri’lium repens L.) and trefoil (Medicago lupulina L.) green manures and the rye-grass control were cut to maintain a height of no more than 30-40 cm, and the cut material was left on the soil surface (‘mulching’). Of the legumes, white clover accumulated the most dry matter (12.2 t DM ha-’ year-‘) and red clover the most N above ground (371 kg N ha-’ year-‘) over a 1 year period of green manuring. Both these species accumulated significantly more dry matter and N than trefoil and ryegrass. Yields of following wheat crops varied considerably. Spring wheat (grown after 6 and 18 months of green manuring) did not yield at a commercial level due to poor establishment. Winter wheat generally yielded well following 1 year of green manuring with red (6.0 t ha-‘) and white clover (5.2 t ha-‘) and significantly more than following trefoil and the nil-legume ryegrass control (3.3 t ha-’ and 2.1 t ha- ‘, respectively). The results suggest that red clover is the optimum species for use as green manure. This is relevant in the context of set-aside management. There was no indication that winter wheat yield was improved following a second year of green manuring with either red or white clover. Approximately one third of the total N accumulated by red clover above-ground was lost by leaching (measured using porous ceramic cup samplers) following cultivation of the l-year green manure in September prior to establishing a winter wheat crop. Delaying cultivation until the spring substantially reduced leaching due to uncultivated soil over winter. The environmental risks (and agronomic benefits) associated with farming systems relying on natural N fixation and soil microbial activity should be more fully evaluated. Krywords: Organic
fanning; Green manure; Legume; Nitrogen fixation; Set-aside; Wheat yield
1. Introduction
* Corresponding 1488 548503.
author.
0167-8809/96/$15.00 SSDI 0 167-8809(95)0
Tel.:
+44
1488 658298;
fax:
t44
0 1996 Elsevier Science B.V. All rights reserved 1002-5
The culture
environmental effects of conventional agrihave been of considerable concern over re-
190
C. Stopes et al./Agriculture,
Ecosystem and Enuironment 57 (1996) 189-196
cent decades, when considering nutrient supply to crops, the increased use of fertilisers has resulted in environmental pollution. Integrated farming systems have been developed exploiting the potential for crop rotation including legumes within a strategy which aims to reduce fertiliser use through carefully targeted applications. Organic or ecological farming systems exclude the use of conventional nitrogen (N) fertilisers, instead requiring a balance between the supply and demand of N through the use of legumebased fertility building phases in the crop rotation (Anonymous, 1991a; Anonymous, 1991b). Legumes are able to accumulate substantial quantities of nitrogen, and the soil’s population of microbes has an enormous capacity to cycle this N in the right conditions (Jarvis et al., 1996). The release of this labile nitrogen can be difficult to control, and may represent an environmental hazard depending on husbandry practices. On most organic farms in the UK a livestock enterprise uses the leguminous forages, and thus the rotation can readily achieve a N balance between fertility building (legume-based) phases and exploitative (arable) phases within the rotation. More than half the farmed area is devoted to legume-based leys (Philipps and Stopes, 1995), and the financial return provided by livestock ensures that the farming system can be economically viable. Such a system is relatively agronomically and environmentally stable from the point of view of maintaining N balance and soil structure whilst suppressing weeds, pests and diseases. If there is no livestock enterprise, leguminous green manures must be used within the rotation, a green manure crop being produced solely for soil incorporation. Traditionally, green manures were used for soil structure improvement and organic matter accumulation (Allison, 1973). The use of leguminous green manures specifically for accumulation of N in organic crop rotations has become increasingly relevant in the context of set-aside management which does not include livestock. The green manure is mulched (‘multiple chop’) several times during the growing season when it reaches an optimum height and the material is left on the surface to desiccate and be incorporated into the surface soil horizons by soil macrofauna. Improvements in crop yield, soil aggregation,
crumb structure and hydraulic conductivity have been observed following green manuring (Allison, 1973; Faris, 1986; Elliot and Papendick, 1986). Loss of N from the system is inevitable, both by volatilisation during the growing season (Whitehead and Lockyer, 1989), and by nitrate leaching following cultivation (Philipps and Stopes, 1995). This paper reports the results of a trial established to determine the effect of green manure legume species and duration of growth on green manure dry matter and N accumulation, the yield of a following cereal crop and the quantity of nitrate leached following cultivation and establishment of the cereal crop. To aid comparison, a non-leguminous control crop of ryegrass was also grown.
2. Materials and methods 2.1. Experimental
site and treatments
A trial was established in the autumn of 1989, on a commercially managed, registered organic farm (United Kingdom of Organic Food Standards; Anonymous, 1991b) in Wiltshire, UK, on a field with an Argillic brown-earth clay-loam soil of the Ardington association. On establishment of the trial, the field had completed the fifth year of organic arable cropping after a 3 year ley (ley: winter wheat, winter wheat, spring peas/barley, winter wheat, spring oats). Table 1 shows the soil analysis at the commencement of the trial. The average annual total rainfall was 800 mm, mean maximum temperature 18°C and minimum temperature 6°C. Rainfall during the period of the experiment (1989- 1992) and the 10 year average rainfall are shown in Fig. 1. Three leguminous green manures and a nonleguminous ryegrass control were grown for between 6 and 25 months in a split plot design (plot size 4 m X 8 m) with four replicates for each treatment. Green manure species treatments were fully randomised within the duration treatments in order to facilitate cultivation and establishment of the test cereal. The period of green manuring was followed by cultivation and sowing of a test cereal crop. A winter wheat crop (WWl) was sown at the start of the experiment to assess yield in the absence of a green manure. Fig. 1 shows the soil cultivation,
C. Stopes et al./Agriculture,
Table I Soil analysis
sowing and harvesting dates for the test cereal and details of the green manure treatments. Before sowing, legumes were inoculated with the Rhizobium species recommended and supplied by the Agricultural Genetics Co. (Cambridge, UK) to facilitate adequate root colonisation. During the growing season the green manure was cut to a height of 5-8 cm when it reached a height of
of trial site (O- 15 cm soil depth) 6.5 3.53% 0.16% 46.9 mg kg-’ 217.0 mg kg-’ 2600.0 mg kg-’ 126.0
PH Organic matter Total N Olsen P K Ca Mg
WWl .... ..... .. . .... ..... x
Test cereal GM growth
x
SW1
Test cereal
SW2 .._._...x ww2 . .......... ..... .. ..._.__..x
ww3
-. x
+ 6month
---------
----___ --_--__--
_____- _______-
- + 13 month --
______
+ 18month
-_-_-____~--
--
+ 25 month
..I989....11990 .............................~1991............................)1992
Year 16OT
191
Ecosystem and Environment 57 (1996) 189-196
2
..._._._... r.
ASONDJFMAMJJASONDJFblAMJJASONDJFMAMJJASOND
-
10 Year Av
- -0 -
Annual rainfall
Incorporation of green manure. X = Harvest cereal SW 1,2 = Spring wheat sown (var. Axona @ 260 kgha-l). WW 1, 2, 3 = Winter wheat sown (var. Mercia @ 220 kgha-I). Green manure treatments. . Code Name Species/variety Seed rate (kg.ha-‘) K.d
+ =
RG
Ryems control
RC WC
Red clover White clover
TR
Trefoil
Italian ryegrass (Lohum var. Atalja) Perennial ryegrass perexn~ var. Condesa) Trifoliumptatense (var. Merviot) Ttiolium rem (var. Aran) Italian + PerenniaI ryegrass Medicaeo (WS) Italian + Perennial ryegrass
Fig. 1. Periods of green manure and crop growth (mulching, year average. Green manure species treatments.
soil cultivation,
sowing, harvesting).
15 15 30 25 5 25 5 Monthly rainfall (mm): 1989-1992
and 10
192
C. Stopes et al./Agriculture,
Ecosystem and Environment 57 (1996) 189-196
30-40 cm using a mulching attachment on a small ‘walking’ tractor which provided a multiple chop. The final mulching of the green manures took place approximately seven days prior to ploughing and preparation of the seed bed for the test cereal crop. During the second year (25 month green manure: 1990-1991) there was incursion of red clover into neighbouring trefoil green manure and ryegrass control plots. Native white clover growth was also observed. In neither case was this growth controllable. 2.2. Assessments Immediately before each mulching occasion, three quadrat (0.25 m2) samples were taken per plot to measure above ground dry matter of the green manure, N content of the dry matter was determined by Kjeldahl digestion and steam distillation (Bremner and Keeney, 1965). The cereal crop after the green manure treatments was harvested using a Wintersteiger plot combine with a 1.3 m cutter width with yield being adjusted to 15% moisture content. Three 0.25-m2 quadrat samples were cut prior to combining for estimation of weed dry matter and grain N content (analysed by Kjeldahl digestion and steam distillation). Damage by sheep of the cereal crop following the 25 month green manure treatment necessitated yield estimates from three 0.25-m2 quadrat samples being taken from each replicate plot. Nitrate leaching following the 13 and 18 month duration red clover and ryegrass control treatments was measured by utilising two porous ceramic cup samplers in each of two of the replicate plots at a depth of 90 cm (four replicate samples per treatment). Soil water was sampled fortnightly by applying a vacuum of - 80 kPa to the ceramic sampler unless the site was snow covered or frozen. Water samples were frozen prior to nitrate analysis by calorimetric methods involving reduction of nitrate to nitrite through a cadmium column (Henriksen and SelmerOlsen, 1970) prior to an azo dye Orange I reaction (Follett and Ratcliff, 1963). Further details of installation and analysis are given in Philipps‘and Stopes (1995). Nitrate leached was calculated from the treatment mean nitrate concentration in the soil-water integrated with cumulative net drainage (Lord and Shepherd, 1993). Cumulative drainage data for the site
were supplied by the Meteorological Office ‘MORECS’ service (Thompson et al., 1981).
3. Results and discussion 3.1. Dry matter and nitrogen accumulation.
Above ground dry matter and N accumulation by the leguminous green manure and control treatments grown for periods of 6-25 months are presented in Table 2. There were no significant differences in dry matter or N accumulation during the first 6 months. Red and white clover green manures both resulted in more dry matter and N accumulation than trefoil and the non-leguminous ryegrass control over 13, 18 and 25 months of growth. Over the second year (13-25 months) red and white clover grew in the trefoil green manure and ryegrass control plots, contributing to the accumulation of dry matter and N over the second year of the experiment, leading to much smaller differences between treatments. In all cases the N content of the mulched red clover was greater than all other treatments, explaining the greater N accumulation in this species compared with the white clover green manure, even where white clover produced more dry matter. Between March and August 1990 (first year of trial) rainfall was approximately half the 10 year average, over the same period in the following year (second year of trial) the 10 year average was slightly exceeded, although rainfall was unevenly distributed (Fig. 1). Dry matter accumulation by red and white clover between March and August in the second year of green manure growth (1991) was similar to that in the first year ( + 1% and - 5%, respectively). In contrast, trefoil accumulated 32% and ryegrass 102% more dry matter in the second year with higher rainfall, thus yield potential of red and white clover was apparently unaffected by low rainfall during the first year. Where the green manure treatment included ryegrass (present in all except the red clover treatment), growth was more vigorous in the second spring, so it may have been advantageous to include a small proportion of ryegrass in the red clover treatment. This would be expected to have increased productiv-
C. Stopes et al./Agriculture.
Table 2 Dry matter and N accumulation
193
Ecosystem and Environment 57 (1996) 189-196
in green manures
Duration (months)
Green manure species a
6
RC WC TR RG cont. Stubble LSD < 5%
No. of mulchings
1 1
I 1 0
Dry matter accumulation (t ha-‘)
N accumulation (kg ha- ‘>
0.8 0.6 0.6 0.7 0.7 NS
21 17 14 I5 15 NS
RC WC TR RG cont. LSD < 5%
6 5 5 5
11.9 12.2 328 9.1 6.4 1.8
371
18
RC WC TR RG cont. LSD <5%
8 7 7 7
14.1 15.2 10.8 7.3 2.0
450 432 265 110 54
25
RC WC TR RG cont. LSD <5%
10 10 10 10
25.4 25.0 20.4 17.5 2.7
741 592 459 346 94
13
211 94 47
a See Fig. I.
ity in the second spring and also reduced weed growth by competitive exclusion. Dry matter and N accumulation by trefoil was low compared with the red and white clover green manures. Under the conditions of the trial this species did not perform well even though satisfactory nodulation was observed and trefoil occurs naturally on this soil type. Trefoil is used extensively in organic farming systems in other European countries, and has the advantage of being from a different genus to the clovers, thus’ providing a break in disease susceptibility. This is an important consideration in organic systems where the disease risk associated with close cropping of identical or related species should be avoided (Millington et al., 1990).
ments were not significant. However, prior to incorporation of the 25 month green manure treatments, soil organic matter content was 3.6%. Over this period total soil N increased from 0.16 to 0.2% (mean of all treatments), with no difference between treatments. Despite the similarity among treatments in soil organic matter and total N content, there were large though non-significant differences in N mineralisation following cultivation, indicated by the quantity of nitrate leached over the winter of 1990-1991 from the 13 and 18 month plots (Table 3). This
Table 3 Nitrate leached (kg NO,-N haDuration (months)
Cultivation date
Nitrate leached (kg NO,-N ha- ‘)
Red clover Ryegrass Red clover Ryegrass
13 13 18 18
9.1990 9.1990 3.1991 3.1991
102 18 26 4
3.2. Soil organic matter and nitrate leaching Organic matter content of the soil prior to incorporation of the green manure increased from 3.5 to 4.3% over 18 months; differences between treat-
’) over winter
Green manure
C. Sropes et al./AgricuNure,
194
Ecosystem and Etwironment
cient for use in bread making (over 1.6%). No other parameters (specific weight, grain size, P and K content) were significantly different between treatments. There was an inverse relationship between weed dry matter at harvest and cereal yield. Where wheat was grown in the first year (198919901, with no green manuring, a lower yield (4.8 t ha-‘) and grain-N content was observed, although it was still higher than that following 1 year of growth of trefoil or ryegrass, both these treatments presumably resulting in net immobilisation of N following incorporation. Following a two year period of green manuring, the mean yield (estimated from quadrat samples) was 5.2 t ha-‘, with no significant differences between treatments. Growth of red and white clover in the trefoil and non-leguminous ryegrass plots during 1991 resulted in substantially increased dry matter and N accumulation over the second year in these treatments, similar to that accumulated over one year by red or white clover (Table 2) and sufficient to mask any treatment effect on crop yield. Spring wheat following both 6 (1990) and 18 month (1991) green manuring failed to yield at a commercial level due to high January-April rainfall, resulting in poor establishment in wet conditions (26% and 19% above 10 year average, Fig. 1). There were, however, significant differences in grain yield after 18 months of green manuring, yields following red clover and white clover being the greatest (2.0 t ha-’ and 1.3 t ha-‘, respectively), compared with only 0.7 t ha-’ for the trefoil-based green manure
supports other leaching studies on organic mixed farms where spring cultivation of legume pastures reduces the total quantity of nitrate leached compared with autumn cultivation over a 2 year period (Philipps and Stopes, 1995). Of the N accumulated above ground by the green manures, 27% and 19% was lost by leaching following September cultivation of the red clover and ryegrass treatments, respectively. The high level of nitrate leaching following incorporation of a legume green manure prior to planting a winter sown cereal represents a potentially important environmental hazard which should be taken into account when implementing green manure based rotations, whether in organic or integrated farming systems. These rotations will typically involve a higher proportion of arable cropping than would be the case on a mixed (livestock based) organic farm, consequently higher levels of nitrate leaching may be expected than are observed on mixed organic farms (Philipps and Stopes, 1995). Where green manures are incorporated into an integrated farming system, including the use of fertilisers, this risk may be even greater. 3.3. Wheat yield following
green manuring
Winter wheat yield was greater following the leguminous green manures than the ryegrass control (Table 4). The highest yield, grain N content and N offtake followed red and white clover, wheat from only these treatments had a grain N content suffi-
Table 4 Wheat yield, grain-N and weed dry matter at cereal harvest following Duration
Green manure species a
57 (1996) 189-196
green manures
Wheat grain yield b
Nitrogen ’
Weed
’
(t DM ha-‘)
(tha-‘1
%
kg ha-
0
Nil
4.8
1.36
55
0.81
13
RC WC TR RG LSDP
6.0 5.2 3.3 2.1 0.9
1.86
95 80 45 30 14
0.89 1.27 1.91 2.05 NS
(months)
<5%
’ See Fig. 1. b Yield expressed at 85% DM. ’ Nitrogen expressed as percentage
DM.
I .80 1.62 I .69 0.12
C. Stops et nl./Agriculture,
and no harvestable leguminous ryegrass.
crop
following
the
Ecosystem and Environment 57 (1996)
non-
4. Conclusion The development of farming systems designed to reduce fertiliser use, provide environmental benefits and save production costs partly depends on optimisation of the supply and cycling of naturally derived N. Fixation by legumes and mobilisation of soil organic matter N through microbial activity are important, however both of these are difficult to predict and control. Soil microbial activity is particularly affected by soil disturbance, moisture and temperature, thus season interacts with husbandry increasing the unpredictability of N supply and limiting the opportunity for control of an environmentally damaging and agronomically wasteful surplus of N. Consequently, although there is much merit in developing cropping systems which reduce the need for fertilisers to supply N, there are associated environmental risks. There is evidence that an organic system has a higher rate of biological activity and improved soil quality compared to a conventional system, a mixed (livestock based) organic system being better in this regard than a stockless (green manure based) system (Wander et al., 1994). This suggests that uncontrollable outputs of nitrogen (particularly important in the context of environmental pollution) may be more likely from an organic compared to a conventional system, however, the overall efficiency of N utilisation has been consistently demonstrated to be higher in organic farming systems (Kristensen and Kristensen, 1992; Van der Werff et al., 1995). This trial has shown that a 1 year period of red or white clover green manuring can lead to increased yield and higher grain N content in a following wheat crop in an organic system. Neither trefoil nor -the ryegrass control accumulated sufficient N to allow net-mineralisation at a rate able to meet the N demand of a commercially viable yield of quality wheat. The effect of green manuring was only studied in the first cereal crop, longer term effects over entire crop rotations are clearly important. However, it would appear that two years of green manuring results in a level of N accumulation in excess of crop
189-196
195
requirements and likely to result in substantial loss of N; the optimum period for green manuring is likely to be 1 year. In a simple two compartment model, mineralisation of organic N in the soil occurs at a rate dependent both on the proportion of readily decomposable and more resistant sources of organic N, and on the rate constant for mineralisation of each type (Jarvis et al., 1996). The large introduction of freshly degradable plant matter following green manuring readily supports mineralisation unlimited by N supply. Green manuring for 2 years accumulates available N in excess of the requirement of the following crop. Much of this is ultimately immobilised, however, the remainder is available for loss by leaching, denitrification and volatilisation. The use of green manures for ‘fertility building’ has wide application in the context of set-aside management, and in those farming systems (including organic) where fertiliser use is more or less reduced. The environmental risks (as well as agronomic benefits) of systems reliant on the rapid N accumulation and cycling potential of natural nitrogen fixation and soil microbial populations should be further evaluated.
Acknowledgements The work reported was carried out under a grant from the Ministry of Agriculture Fisheries and Food (CSA 1466). Acknowledgment and thanks for support are made to M. Marriage, R. Unwin and Prof. Dr H. Vogtmann. We also thank G. Forde and all other EFRC staff who contributed to the project.
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19%. Rotational design and the limits of organic systems-the stockless organic farm? In: R. Unwin (Editor), Crop Protection in Organic and Low Input Agriculture. BCPC Monogr. No. 45, British Crop Protection Council, Famham, UK, pp. 163173. Philipps, L. and Stopes, C.E., 1995. The impact of rotational practice on nitrate leaching losses in organic farming systems. In: L. Kristensen, C. Stopes, P. Kolster and A. Granstedt (Editors), Nitrate leaching from ecological agriculture. Biol. Agric. Hortic., 11: 123-134. Thompson, N., Barrie, LA. and Ayles, M., 1981. The Meteorological Office Rainfall and Evaporation Calculation System (MORECS). Hydrol. Memo. No. 45, The Meteorological Office, London, UK. Van der Werff, P.A., Baars, A. and Oomen, G.J.M., 1995. Nutrient balances and measurement of nitrogen loss on mixed ecological dairy farms on sandy soils in the Netherlands. In: L. Kristensen, C. Stopes, P. Kolster and A. Granstedt (Editors). Nitrate leaching from ecological agriculture. Biol. Agric. Hortic., 11: 41-50. Wander, M.M., Traina, S.J., Stinner, B.R. and Peters, S.E., 1994. Organic and conventional management effects on biologically active soil organic matter pools. Soil Sci. Sot. Am. J., 58: 1130-l 139. Whitehead, D.C. and Lockyer, D.R., 1989. Decomposing grass herbage as a source of ammonia in the atmosphere. Atmos. Environ., 23: 1867-1869.