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Soil mineral nitrogen and nitrate leaching losses in soil tillage systems combined with a catch crop
Soil & Tillage Research 50 (1999) 115±125
Soil mineral nitrogen and nitrate leaching losses in soil tillage systems combined with a catch crop Maria Stenberga,*, Helena Aronssonb, BoÈrje LindeÂnc, Tomas Rydberga, Arne Gustafsonb a
Department of Soil Sciences, Division of Soil Management, Swedish University of Agricultural Sciences, PO Box 7014, S-750 07, Uppsala, Sweden b Department of Soil Sciences, Division of Water Quality Management, Swedish University of Agricultural Sciences, PO Box 7072, S-750 07, Uppsala, Sweden c Department of Agricultural Research Skara, Swedish University of Agricultural Sciences, PO Box 234, S-532 23, Skara, Sweden Received 8 October 1998; accepted 15 December 1998
Abstract Annual nitrogen leaching losses from arable land in south Sweden usually amount to 15±45 kg haÿ1. The objective of this three-year study was to investigate the timing effect of mouldboard ploughing (early autumn, late autumn or spring) on soil mineral nitrogen content and nitrate leaching in a cropping system with spring-sown small grain crops (barley, oats and wheat). Late autumn ploughing was studied with and without perennial ryegrass (Lolium perenne L.) as a catch crop, and with and without preceding stubble cultivation. The effects of removal compared to incorporation of straw were also studied. Soil mineral nitrogen in the 0±90 cm layer was determined on 4±5 occasions each year. Nitrate was determined in soil water sampled with ceramic suction cups at 60 and 90 cm. Total nitrogen in above-ground catch crops, weeds and volunteer plants was determined at ripeness of the main crop and before tillage operations. Time of tillage, as opposed to catch crop or crop residue management, in¯uenced nitrogen leaching. After early tillage, soil mineral nitrogen increased during autumn. In November, the nitrogen content was on an average 68 kg N haÿ1 in 0±90 cm in early ploughed soil and 39 kg N haÿ1 when ploughing was delayed to spring. Also nitrate leaching was greater in treatments with early than with late tillage. This was probably because of increased nitrogen mineralization. However, when tillage was delayed there was also a substantial growth of weeds and volunteer plants during autumn, particularly couch-grass (Elymus repens L.). This caused grain yield to decrease by up to 40% when tillage operations were delayed until late autumn or spring. Delaying tillage operations to late autumn or spring seems to be necessary to reduce the risk of nitrogen leaching. Swedish regulations for farmers to control nitrogen losses have been changed as a consequence of these results. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Soil tillage; Soil mineral nitrogen; Nitrate leaching; Lolium perenne; Sandy loam
1. Introduction
*Corresponding author. Tel.: +46-18-671213; fax: +46-18672795; e-mail: [email protected]
Nitrogen leaching through drainage water from arable land has been identi®ed as a problem during recent decades in regions such as Scandinavia. Annual
0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0167-1987(98)00197-4
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M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
nitrogen losses usually amount to 15±45 kg haÿ1 in southern Sweden (SNV, 1997), with the largest losses on easily permeable sandy soils with low water holding capacity and restricted rooting depths. Most losses occur as nitrate during autumn, winter and early spring when evaporation is low, precipitation is high and no crop is growing. Worldwide, nitrate as a contaminant in drinking water can be a problem. Nitrogen in surface waters leading to eutrophication is also a problem. In Sweden, nitrate concentrations >11.3 mg NO3±N lÿ1, which is the recommended limit for nitrate in drinking water (WHO, 1993), have been found in several drinking water wells (LoÈfgren, 1992). Keeping the soil covered with a growing crop during autumn and winter is considered to be an ef®cient measure to reduce nitrogen leaching from arable land both in Sweden and elsewhere (e.g. Martinez and Guiraud, 1990; Lewan, 1994; Wyland et al., 1996). During part of the autumn and milder winter periods when no main crop is established but when drainage normally occurs, a catch crop may be grown to take up nitrogen from the soil pro®le. Perennial ryegrass under-sown in the main crop has proved to be a useful catch crop (Hansen and Djurhuus, 1997; Aronsson and Torstensson, 1998). In most studies on catch crops the reference treatments have been conventionally managed cropping systems with tillage operations in early autumn soon after harvest of the main crop. However, not only the catch crop, but probably also the tillage treatment may greatly in¯uence nitrogen transformations and ¯ows in the soil. In Denmark on a sandy loam, nitrate leaching was reduced when ploughing was delayed until spring (Hansen and Djurhuus, 1997), but on a coarse sandy soil, spring ploughing did not reduce leaching compared with autumn ploughing while direct drilling did. In the UK, annual losses through mole and tile drainage systems were smaller from direct-drilled than from ploughed plots in a ®eld experiment on a clay soil (Goss et al., 1993). In New Zealand, Francis et al. (1995) found that nitrate leaching losses were greater during winter from a fallow soil ploughed to 20 cm depth in the previous March compared with soil ploughed in May. The nitrogen content and C:N ratio of crop residues are important for nitrogen immobilization processes after incorporation (e.g. Jawson and Elliot, 1986; Aulakh et al., 1991; Bhogal et al., 1997), where soil
microbial biomass acts as a sink for nitrogen (Paul and Juma, 1981). Thus, incorporation instead of removal of straw residues with high C:N ratios might be a way to reduce the risk of nitrogen leaching, due to immobilization of nitrogen in the soil. In studies made by Jarvis et al. (1989) and Nicholson et al. (1997), incorporation of straw decreased N leaching signi®cantly compared with soils where straw was burned. In contrast, several authors (e.g. Schjùnning, 1983; Davies et al., 1996) have reported very small or insigni®cant effects on leaching by straw incorporation. The objective of this study was to investigate the effect on soil mineral nitrogen content and nitrogen leaching both of an under-sown catch crop and of different soil tillage treatments, with or without removal of crop residues. 2. Materials and methods 2.1. Field site The ®eld experiment was established in 1992 on a sandy loam (Table 1) at Mellby (568290 N, 13800 E, alt. 10 m) in south-west Sweden. The soil consists of sand deposits with a thickness of 90±130 cm, covering glaci¯uvial clay. The soil was classi®ed as a Haplic Phaeozem (FAO soil classi®cation) (Johnsson, 1991). The mean annual temperature was 7.28C and the mean annual precipitation 773 mm (data from Genevad 1961±1990, 10 km north of Mellby) (Alexandersson et al., 1991). The whole experimental ®eld was tiledrained at about 90 cm depth. 2.2. Treatments The experiment consisted of ®ve tillage/catch crop systems as main treatments, and two crop residue Table 1 Soil particle size distribution and organic matter content (g kgÿ1) at the site Depth (cm)
Particle size (mm) <2
2±100
100±2000
0±30 30±60 60±90
103 47 43
131 90 145
709 846 803
Organic matter 57 17 9
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Table 2 Treatments in the field experiment (the catch crop was perennial ryegrass under-sown in the main crop in spring) Main treatment (tillage)
Sub-treatment (crop residues)
MB MB MB MB MB MB MB MB
Incorporated Removed Incorporated Removed Incorporated Removed Incorporated Incorporated
first week in September (early autumn) first week in September first week in November (late autumn) first week in November first week in November and catch crop first week in November and catch crop first week in November preceded by stubble cultivation once first week in September in early April (spring)
MBmouldboard ploughing.
treatments as sub-treatments (Table 2) compared in an incomplete split-plot design with three blocks and a plot size of 9 m18 m. The main crops, all of them spring sown, were oats (Avena sativa L.) in 1994, barley (Hordeum vulgare L.) in 1995 and wheat (Triticum aestivum L.) in 1996. Different times for mouldboard ploughing to 20±25 cm depth (early September, early November and early April) were compared. Late autumn ploughing (early November) was studied both without a catch crop and preceding stubble cultivation and with perennial ryegrass (Lolium perenne L.) as a catch crop, as well as with stubble cultivation once in early September. Crop residues were incorporated by the tillage operation, or removed from the plots. First experimental treatments were carried out in the autumn of 1993 and the ®rst harvest in 1994. Seedbed preparation and sowing were carried out uniformly in the experiment. In spring each year, the catch crop was under-sown on the same day as the main crop was sown or one day later, and in the same direction with a conventional seed drill. Nitrogen fertilizer was applied during seedbed preparation in spring at rates normal for the region (90 kg haÿ1 to barley and oats and 110 kg haÿ1 to spring wheat). The rates of potassium and phosphorus fertilizers applied in each experimental plot were based on the amounts removed with crop yields and straw. Chemical weed and pest control during main crop growth was performed according to local recommendations. 2.3. Measurements Above-ground biomass of catch crop, weeds and volunteer plants were determined in one randomly
located micro-plot (0.5 m2.4 m) in each experimental plot. Sampling was carried out in early August at yellow ripeness of the main crop, stage 87 (Zadoks et al., 1974), in early November (not in the early ploughed plots) and in early April before spring ploughing. Grain yield of the main crop was determined in 22 m2 in each plot at harvest in years 1994± 1996. The occurrence of couch-grass (Elymus repens L.) in the plots was determined in September 1996 as the percentage of the area covered. Due to an error, no sampling of plant material was carried out in November 1993. Determinations of total nitrogen and carbon contents in plant material were performed with a LECO CNS-2000 analyser. Nitrogen and carbon were converted into gases in a dry combustion process and the amount of carbon (CO2) was then determined in infrared cells and nitrogen (N2) in thermal conductivity cells. Soil was sampled each year for mineral nitrogen content at yellow ripeness in August, before tillage operations in early September and early November and before ploughing in early April. In 1993 and 1994, soil was also sampled in early December, after the late autumn ploughing. Soil cores with a diameter of 20 mm were taken to a depth of 90 cm. Eight cores per plot from 0 to 30 cm and six cores per plot from 30±60 to 60±90 cm were pooled. Due to high costs for analyses of nitrogen, only pooled samples for each treatment were generally analysed. However, the samples were analysed plotwise in December 1993, in April 1994 and in August, September and November 1995. Before analysis, soil samples were stored deepfrozen (ÿ208C). The samples were extracted with 2 M KCl and analyses of NO3±N and NH4±N were made
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with a TRAACS 800. The concentrations of NO3±N were determined with the colorimetric cadmium reduction method (Grasshoff, 1964; Wagner, 1974), and NH4±N with a spectrophotometric method by which a coloured complex of NH4±N and sodiumsalicylate is formed in the presence of prussidic acid and chloride (Benesch and Mangelsdorf, 1972). The analytical values were transformed to kg haÿ1 using measured average bulk densities (0±30 cm: 1.35 g cmÿ3, 30±60 cm: 1.56 g cmÿ3 and 60±90 cm: 1.68 g cmÿ3) and actual gravimetric water contents. Statistical analyses of differences between treatments were only performed when samples were analysed plotwise. Soil water samples were taken every second week with suction cups at 60 and 90 cm depth according to Djurhuus (1990) and Hansen (1991). Three suction cups per plot were installed at 90 cm in autumn 1992 and three cups per plot at 60 cm in autumn 1994. A suction of about 0.7 bar was applied for 24 h before extracting the samples. Samples were treated sepa-
rately. The concentration of NO3±N was determined as described above for soil samples. Nitrate leaching was calculated from soil water nitrate concentrations at 90 cm in the experiment and drainage measured daily in an adjoining experiment in the same ®eld, established in 1982, with individually tile-drained plots with similar treatments and soil type (Torstensson and Johnsson, 1996). The experiment reported here was carried out as a complement to the existing experiment. According to earlier studies in the adjoining experiment (Torstensson and Aronsson, 1999), drainage from each treatment could be assumed not to differ, and the average drainage in that experiment was therefore used here. Linear interpolation between measured NO3±N concentration values was used to obtain daily concentration values. Concentrations were then multiplied by daily discharge. Mean monthly air temperature and monthly precipitation and drainage as measured at the experimental site from July 1993 to June 1996 are shown in Fig. 1.
Fig. 1. Mean monthly air temperature (8C) 1.5 m above ground and monthly precipitation (white bars) and drainage (solid bars) at Mellby from July 1993 to June 1996. The figures above the bars refer to total annual precipitation and drainage, respectively (mm).
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
119
Table 3 Grain yields (kg haÿ1) and relative yields in 1994±1996 in the field experiment Treatment
Oats 1994
Barley 1995
Wheat 1996
Mean 1994±1996
Early plough., res. incorp. Early plough., res. removed Late plough., res. incorp. Late plough., res. removed Late plough., cc, res. incorp. Late plough., cc, res. remov. Late plough., cult., res inc. Spring plough., res. incorp.
3120 3130 2938 2561 2723 3101 3130 2921
4783 4365 4746 4767 4803 5106 4968 4958
4714 4420 2573 2604 3464 3823 3874 3063
4210 3970 3420 3310 3690 4010 3990 3650
Early ploughing in autumn Late ploughing in autumn Late ploughing in autumn and catch crop Early stubble cultivation and late ploughing in autumn Spring ploughing
3130100 88 93 100 93
4570100 104 108 109 108
4570100a 57b 81ab 85ab 67ab
4090100 82 94 98 89
Plant residues incorporated Plant residues removed
2970100 99
4850100 98
3540100 102
3790100 99
Statistical significance
n.s.
n.s.
Tillagea
n.s.
b
Values followed by different letters are significantly different (P<0.05). a Significance levels: P<0.05; n.s.not significant. b res.residues, incorp.incorporated, cccatch crop, cult.cultivation.
3. Results 3.1. Grain yields of main crops No signi®cant differences in grain yields between three-year treatment means for 1994±1996 were detected (Table 3) but there was a tendency for delayed tillage operations to result in lower yields than early autumn ploughing or stubble cultivation. In 1996 (wheat), grain yields differed signi®cantly between treatments but not in 1994 (oats) and 1995 (barley). In particular, late autumn ploughing without a preceding stubble cultivation or a catch crop resulted in lower yield than the early ploughing, most probably due to increased amounts of couch-grass with the years, which was a severe problem at the site (Fig. 2). Ryegrass as a catch crop signi®cantly reduced the amount of couch-grass in spite of late autumn ploughing in this treatment (Fig. 2). 3.2. Dry matter and nitrogen content of catch crop, weeds and volunteer plants Average above-ground biomass and nitrogen content, from August 1993 to August 1996, of catch crop,
Fig. 2. Couch-grass cover (% of plot area) after harvest in September 1996. Treatments not sharing the same letter are significantly different (P<0.05).
weeds and volunteer plants in August at ripeness of the main crop and in early November before ploughing are shown in Table 4. In 1993, catch crop establishment in the summer and growth during autumn was good, while in 1994 establishment was poor due to a very dry summer, which in turn reduced growth in the
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M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
Table 4 Average nitrogen content in harvested grain and above-ground biomass dry matter (DM) and nitrogen (kg haÿ1) of catch crop, weeds and volunteer plants at ripeness of the main crop in early August and before ploughing in early November from August 1993 to September 1996 Treatment
Nitrogen in grain
DM in August
Nitrogen in August
DM in November
Nitrogen in November
Early plough., res. incorp.c Early plough., res. removed Late plough., res. incorp. Late plough., res. removed Late plough., cc, res. incorp. Late plough., cc, res. remov. Late plough., cult., res inc. Spring plough., res. incorp.
69.5 66.5 56.7 56.4 60.4 69.4 66.8 58.4
179 115 447 375 191 369 96 422
2.7 2.2 4.4 4.1 3.6 5.9 1.4 4.7
± ± 407 469 670 729 177 415
± ± 11.4 13.9 16.1 19.6 6.3 11.5
Early ploughing in autumn Late ploughing in autumn Late ploughing in autumn and catch crop Early stubble cultivation and late ploughing in autumn Spring ploughing
68.0 56.5 64.9 66.8 58.4
147 411 280 96 422
2.5 4.2 4.8 1.4 4.7
± 438a 699b 177c 415ad
± 12.7a 17.9b 6.3c 11.5ac
Plant residues incorporated Plant residues removed
62.4 64.1
267 286
3.4 4.1
417 599
11.3 16.8
Statistical significance
n.s.
n.s.
Tillageb
Tillagea
n.s.
Values followed by different letters are significantly different (P<0.05). a Significance level: P0.01. b Significance level: P0.001; n.s.not significant. c res.residues, incorp.incorporated, cccatch crop, cult.cultivation.
autumn. During that year the above-ground nitrogen content of the catch crop in November was similar to that in weeds in treatments with late autumn ploughing and spring ploughing. In 1995, establishment of the catch crop was good, and despite a dry summer, growth of the catch crop was good during the autumn. Weeds also grew vigorously in the autumn. In August 1996, the nitrogen content of weeds (dominated by couch-grass) in the treatments with late autumn ploughing without a catch crop and spring ploughing clearly exceeded the nitrogen content in catch crop and weeds in the catch crop treatments. Before ploughing in November, the average nitrogen content in above-ground plant material was almost 18 kg N haÿ1 in the catch crop treatments (Table 4). This was more than three times the content at yellow ripeness. In November 1993, the catch crop was well developed but due to an error, no sampling was carried out. The overall average of nitrogen in the catch crop in November would have been 21 kg N haÿ1 if the nitrogen content in that catch crop was estimated from catch crop harvest in the adjoining ®eld experiment and included in the average for this experiment.
3.3. Soil mineral nitrogen Soil mineral nitrogen (nitrate and ammonium) content in the 0±90 cm layer in August at ripeness of the main crop was similar in all tillage treatments both in individual years and as a mean of all three years (Table 5 and Fig. 3). At sampling before ploughing in early September, nitrogen contents had increased in 0±90 cm but were still similar in the different tillage treatments. The treatments with a catch crop or spring ploughing, however, showed slightly lower values than the other treatments. Measurements in late autumn (November and December) showed a large increase in the content of mineral nitrogen in early ploughed as well as in early stubble cultivated plots. On these occasions, differences between tillage treatments were strongly signi®cant (Table 5). In November, the nitrogen content was on an average 68 kg N haÿ1 in 0±90 cm in early ploughed plots and 39 kg N haÿ1 in spring ploughed plots. In the spring of 1996, the contents were higher than in November 1995 in all treatments. The winter of 1995/1996 was long, cold and dry with very small
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121
Table 5 Soil mineral nitrogen (kg N haÿ1) in 0±90 cm depth at all sampling occasions (statistical analysis of variance was performed when sampling was carried out plotwise) Treatmentb
1993
1994
1995
1996
Ac
S
N
D
Ap
A
S
N
D
Ap
A
S
N
Ap
A
S
1 2 3 4 5
64 55 47 49 44
64 63 34 52 50
91 42 22 58 40
87a 46b 25c 68d 39bce
43 46 40 49 35
50 41 33 41 41
44 47 44 39 37
65 50 50 58 43
66 47 47 55 33
33 44 48 40 34
28 31 27 28 25
38 39 37 36 30
66a 40b 38b 46b 34b
82 77 75 78 55
32 31 30 30 34
45 43 46 46 41
10 20
49 60
51 57
49 53
53 53
42 44
44 36
42 44
51 58
48 56
40 41
29 27
35 39
45 48
73 79
32 31
45 44
Significance
±
±
±
Ta,d
n.s
±
±
±
±
±
n.s.
n.s.
Ta
±
±
±
Values followed by different letters are significantly different (P<0.05). Average nitrogen content for 1993-96 is shown in Fig. 3. Significance levels: P0.001; n.s.not significant. b Treatment 1early autumn ploughing, 2late autumn ploughing, 3late autumn ploughing with catch crop, 4early shallow cultivation and late autumn ploughing, 5spring ploughing, 10plant residues incorporated, 20plant residues removed. c AAugust, SSeptember, NNovember, DDecember, ApApril. d Ttillage. a
3.4. Nitrate concentrations in soil water and calculated nitrate leaching
Fig. 3. Average soil mineral nitrogen content in the tillage treatments for each sampling occasion from yellow ripeness in August 1993 to early September 1996. Sampling in December was carried out only in 1993 and 1994.
amounts of drainage water and mineralized nitrogen obviously accumulated in the pro®le (Table 5). In contrast, the winters of 1993/1994 and 1994/1995 were much wetter and nitrogen losses by leaching were greater (Fig. 6). The effects on soil mineral nitrogen of different crop residue managements were comparatively small (Table 5).
At 60 cm, soil water nitrate concentrations differed signi®cantly on several occasions during spring and autumn 1994, autumn 1995 and spring 1996 (Fig. 4). At 90 cm, soil water nitrate concentrations differed signi®cantly between tillage treatments on one occasion, in July 1994 (P<0.05) (Fig. 5). Differences in nitrate concentration between crop residue managements were never signi®cant at any of the depths (data not shown). Nitrate concentrations at 60 cm depth showed distinct peaks when drainage started in autumn, especially in treatments with early ploughing (Fig. 4). At 90 cm depth the peaks were less pronounced and delayed. When the experiment started in 1993 the concentrations were relatively higher in several plots in one of the blocks (data not shown) which caused high variation between plots within treatments. This was most obvious in the catch crop treatments. The high nitrate concentrations were probably a result of large applications of manure in that part of the ®eld previously. However, the growth of the catch crop during autumn 1993 decreased the soil water nitrate concentrations considerably (Figs. 4 and 5). During the course of the experiment, both the soil water
122
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
Fig. 4. Soil water nitrate concentrations at 60 cm depth from July 1993 to June 1996 in the tillage treatments. Significant differences between treatments at P<0.05 are shown with arrows.
Fig. 5. Soil water nitrate concentrations at 90 cm depth from July 1993 to June 1996 in the tillage treatments. Significant differences between treatments at P<0.05 are shown with arrows.
nitrate concentrations and the soil mineral nitrogen content decreased in these plots. The nitrate concentrations at 90 cm soil depth were generally higher than 5 mg NO3±N lÿ1 water, except for a few short periods in the treatments with a catch crop. In general, the concentrations were also higher than or near the WHO limit for drinking water (11.3 mg NO3±N lÿ1, WHO, 1993). Generally, however, the lowest concentrations
were registered in the treatment with spring ploughing. The average nitrate leaching, i.e. the estimated quantity of nitrate that passed the 90 cm depth, was about 50 kg N haÿ1 annually during the ®rst two years of the experiment (Fig. 6), when the precipitation was close to the long-term mean value. In contrast, the autumn of 1995 and spring of 1996 were dry, and
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
Fig. 6. Accumulated nitrate leaching (kg haÿ1), estimated from soil water nitrate concentrations at 90 cm from July 1993 to June 1996.
therefore leaching was only about 11 kg N haÿ1 in 1995/1996. There were no signi®cant differences between treatments in accumulated nitrate leaching due to the high variation between plots, but for the three-year period, losses were 24 kg N haÿ1 smaller from spring ploughed, and 17 kg N haÿ1 smaller from late autumn ploughed than from early autumn ploughed treatments. Presence of a catch crop did not reduce leaching. The effect of incorporation or removal of crop residues was comparatively small and never signi®cant (data not shown). 4. Discussion In plots ploughed or stubble cultivated in early September, mineral nitrogen contents in the soil during autumn increased (Table 5 and Fig. 3), and this led to a tendency for greater nitrate leaching during autumn and winter than in soil ploughed in late autumn or spring (Fig. 6). One reason for large amounts of mineral nitrogen in the soil in November was probably enhanced nitrogen mineralization due to an increase in soil carbon availability brought about by the early tillage operations and the incorporation of crop residues (Shepherd et al., 1992; Stokes et al., 1992). Weather conditions during autumn 1993 were favour-
123
able both for nitrogen mineralization and for plant growth as demonstrated by the small soil mineral nitrogen contents in the catch crop treatments and the large contents in the early tilled treatments (Table 5 and Fig. 3). In 1994 and 1995 the sum of the nitrogen contents in plant material and as soil mineral nitrogen in November were similar in all treatments (Tables 4 and 5), if roots were assumed to contain about the same quantity of nitrogen as the above-ground part of the plants (SjoÈdahl Svensson and Clarholm, 1994). Thus, the growth of weeds and volunteer plants, functioning as a catch crop during autumn, was also an important reason for less accumulation of mineral nitrogen in treatments with postponed ploughing. The results suggest that postponing tillage until late autumn or spring can be a measure to reduce the risk of leaching losses. This was also found in other investigations in Sweden (Wallgren and LindeÂn, 1994; Aronsson and Torstensson, 1998). However, the effect of postponed tillage was not as pronounced as in a similar experiment on a sandy loam in Denmark (Hansen and Djurhuus, 1997). The actual weather conditions largely in¯uenced both the mineral nitrogen content and the drainage and thus also the estimated nitrate leaching. This was obvious after the dry winter of 1995/1996 when mineral nitrogen accumulated, especially in the 0± 30 cm soil layer (data not shown), in the soil until spring in all treatments, but in smaller amounts in the treatment with spring ploughing. During the ®rst two years of the experiment, differences in soil mineral nitrogen between treatments decreased during winter mainly due to nitrogen leaching. Under-sown perennial ryegrass can slightly reduce grain yield of the main crop due to competition (e.g. Andersen and Olsen, 1993; Wallgren and LindeÂn, 1994). Reduced yield after incorporation of a catch crop has also been found in some studies. Martinez and Guiraud (1990) and Jensen (1991) suggested nitrogen immobilization to be the reason, while ThorupKristensen (1993) emphasized that depletion of soil mineral nitrogen by the catch crop before incorporation is probably more important. In this study the catch crop did not reduce grain yields signi®cantly in any of the years (Table 3). During the autumn of 1994, nitrate concentrations at 60 cm were higher than expected in the catch crop treatments (Fig. 4). During that year
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catch crop uptake of nitrogen was small, and this crop developed poorly during autumn. This, in combination with increased nitrogen mineralization due to catch crop incorporation in the previous autumn, could be one of the reasons for accumulation of mineral nitrogen in the soil during autumn. This is supported by results from a similar experiment at Mellby during the same year, where Aronsson and Torstensson (1998) found increased nitrogen mineralization after incorporation of a catch crop. There was no reduction of nitrate leaching in the treatments where a ryegrass catch crop was undersown in spring, which differs from earlier studies in Sweden on the effect of a catch crop on the risk of nitrate leaching (Lewan, 1994; Wallgren and LindeÂn, 1994; Aronsson and Torstensson, 1998). Poor establishment and growth of the catch crop during two out of three years were probably contributing reasons. During the years covered in this particular experiment the tillage treatments seemed to be the major factor in¯uencing the nitrogen leaching and the nitrate concentrations in the soil water. In spite of this, some of the treatments led to reductions in the concentrations to levels clearly below the WHO limit. In this study, the removal or incorporation of straw did not affect crop growth, amounts of soil mineral nitrogen or nitrate leaching (Tables 3±5). Measurements of mineral nitrogen in the soil or soil solution did not indicate that incorporation of straw resulted in substantial net immobilization of nitrogen during autumn. Small or non-signi®cant effects of straw incorporation on nitrogen leaching have also been found in other studies (e.g. Schjùnning, 1983; Davies et al., 1996). However, in the long run, continuous incorporation of crop residues can increase the amount of mineralizable organic nitrogen in the soil. This was found in a long-term experiment (Persson and Kirchmann, 1994) where straw was incorporated every second year, resulting in a slow build-up of soil organic matter. Likewise, repeated incorporation of catch crops would increase the pool of soil organic nitrogen in the long term (Jensen, 1992). 5. Conclusions Soil tillage practices in the autumn had a considerable in¯uence on accumulation of mineral nitrogen in
the soil during autumn and to some extent also on the amount of nitrate leached from the soil. Postponing tillage from early autumn until late autumn or spring reduced the risk of nitrogen leaching, probably due to lower mineralization rates and higher nitrogen uptake by weeds and volunteer plants during autumn. However, delaying primary tillage operations also reduced grain yields. This was the result of severe couch-grass infestation in these treatments. Thus, perennial weeds can be a problem when tillage operations are delayed, but a catch crop such as perennial ryegrass may reduce the problem. Ryegrass as an under-sown catch crop reduced soil mineral nitrogen until ploughing in late autumn, but did not reduce nitrogen leaching compared with late autumn ploughing. This differed from other studies in Sweden where catch crops have been shown to be ef®cient in reducing nitrogen leaching. Acknowledgements The study was ®nancially supported by the Swedish Board of Agriculture and by the Swedish Council for Forestry and Agricultural Research. References Alexandersson, H., KarlstroÈm, C., Larsson-McCann, S., 1991. Temperaturen och nederboÈrden i Sverige 1961±1990. Referensnormaler. (Temperature and precipitation in Sweden 1961± 1990. Reference normals). SMHI. Meteorologi no. 81. NorrkoÈping, 88 pp. Andersen, A., Olsen, C.C., 1993. Rye grass as a catch crop in spring barley. Acta Agric. Scand. Sect. B, Soil and Plant Sci. 43, 218±230. Aronsson, H., Torstensson, G., 1998. Measured and simulated availability and leaching of nitrogen associated with frequent use of catch crops. Soil Use Manage. 14, 6±13. Aulakh, M.S., Doran, J.W., Walters, D.T., Mosier, A.R., Francis, D.D., 1991. Crop residue type and placement effects on denitrification and mineralization. Soil Sci. Soc. Am. J. 55, 1020±1025. Benesch, R., Mangelsdorf, P., 1972. A method for the colorimetric determination of ammonia in seawater. HelgolaÈnder Wiss Meeresunters 23, 365±375. Bhogal, A., Young, S.D., Sylvester-Bradley, R., 1997. Straw incorporation and immobilization of spring-applied nitrogen. Soil Use Manage. 13, 111±116. Davies, D.B., Garwood, T.W.D., Rochford, A.D.H., 1996. Factors affecting nitrate leaching from a calcareous loam in East Anglia. J. Agric. Sci. 126, 75±86.
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125 Djurhuus, J., 1990. Sammenligning af nitrat i jordvand udtaget med sugkopper og ekstrahert fra jordprùver. (A comparison of soil water nitrate determined by coring and solution extraction techniques). Landbrugsministeriet, Statens Planteavlsforsùg, Saertryck af Tidsskrift for Planteavl, vol. 94, pp. 487±495. Francis, G.S., Haynes, R.J., Williams, P.H., 1995. Effects of the timing of ploughing-in temporary leguminous pastures and two winter cover crops on nitrogen mineralization, nitrate leaching and spring wheat growth. J. Agric. Sci. 124, 1±9. Goss, M.J., Howse, K.R., Lane, P.W., Christian, D.G., Harris, G.L., 1993. Losses of nitrate±nitrogen in water draining from under autumn-sown crops established by direct drilling or mouldboard ploughing. J. Soil Sci. 44, 35±48. Grasshoff, K., 1964. Determination of nitrate in sea and drinking water. Kieler Meeresforsch. 20, 5±11. Hansen, E.M., 1991. Sammenligning af keramiske sugekopper og lysimetre med hensyn til udtagning af jordvñske til bestemmelse af NO3-N-koncentration. (Comparison of porous ceramic cups and drainage lysimeters for sampling soil water). Statens Planteavlsforsùg, Beretning no. 2119. Lyngby, Denmark. Hansen, E.M., Djurhuus, J., 1997. Nitrate leaching as influenced by soil tillage and catch crop. Soil Till. Res. 41, 203±219. Jarvis, S.C., Barraclough, D., Unwin, R.J., Royle, S.M., 1989. Nitrate leaching from grazed grassland and after straw incorporation in arable soils. In: Germon, J.C. (Ed.), Management Systems to Reduce Impact of Nitrates. Elsevier, London, pp. 110±125. Jawson, M.D., Elliot, L.F., 1986. Carbon and nitrogen transformations during wheat straw and root decomposition. Soil Biol. Biochem. 18, 15±22. Jensen, E.S., 1991. Nitrogen accumulation and residual effects of nitrogen catch crops. Acta Agric. Scand. 41, 334±344. Jensen, E.S., 1992. The release and fate of nitrogen from catchcrop materials decomposing under field conditions. J. Soil Sci. 43, 335±345. Johnsson, H., 1991. The soil at Mellby experimental field. Internal report. Swedish University of Agricultural Sciences, Division of Water Quality Management, Uppsala, 5 pp. Lewan, E., 1994. Effects of a catch crop on leaching of nitrogen from a sandy soil: simulations and measurements. Plant and Soil 166, 137±152. LoÈfgren, S., 1992. Jordbrukets inverkan paÊ yt- och grundvatten. (Influence of agriculture on surface and ground water). SNV, Rapport 4150, Stockholm, Sweden. Martinez, J., Guiraud, G., 1990. A lysimeter study of the effects of a ryegrass catch crop, during a winter wheat/maize rotation, on nitrate leaching and on the following crop. J. Soil Sci. 41, 5±16. Nicholson, F.A., Chambers, B.J., Mills, A.R., Strachan, P.J., 1997. Effects of repeated straw incorporation on crop fertlizer nitrogen requirements, soil mineral nitrogen and nitrate leaching losses. Soil Use Manage. 13, 136±142.
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Paul, E.A., Juma, N.G., 1981. Mineralization and immobilization of soil nitrogen by microorganisms. In: Clark, F.W., Rosswall, T. (Eds.), Terrestrial Nitrogen Cycles. Ecol. Bull., Stockholm, Sweden 33 (1981) 179±195. Persson, J., Kirchmann, H., 1994. Carbon nitrogen in arable soils as affected by supply of N fertilizers and organic manures. Agric. Ecosyst. Environ. 51, 249±255. Schjùnning, P., 1983. Halmnedmuldning. (Straw incorporation). In: Hansen, J., Kyllingsbñk, A. (Eds.), Kvñlstof og Planteproduktion. Tidsskrift for Planteavls Specialserie, Bereitning S 1669, Copenhagen, Denmark, pp. 71±76. Shepherd, M.A., Johnson, P.A., Smith, P., 1992. The effect of crop rotation on nitrate leaching losses; sandland and limestone aquifers. Aspects Appl. Biol. 30, 183±190. SjoÈdahl Svensson, K., Clarholm, M., 1994. Dynamics of biomass, nitrogen and carbon in shoots and roots of a ryegrass catch crop during winter. Swedish J. Agric. Res. 24, 77±84. SNV (Swedish Environmental Protection Agency), 1997. KvaÈve fraÊn land till hav. (Nitrogen from land to sea). Rapport 4735, Stockholm, Sweden. Stokes, D.T., Scott, R.K., Tilston, C.H., Cowie, G., SylvesterBradley, R., 1992. The effect of time of soil disturbance on nitrate mineralisation. Aspects Appl. Biol. 30, 279±282. Thorup-Kristensen, K., 1993. The effect of nitrogen catch crops on the nitrogen nutrition of a succeeding crop. I. Effects through mineralization and pre-emptive competition. Acta Agric. Scand., Sect. B, Soil and Plant Sci. 43, 74±81. Torstensson, G., Aronsson, H., 1999. Nitrogen leaching and availability in cropping systems with recurrent use of liquid manure and catch crops. Nutrient Cycling in Agroecosystems, submitted. Torstensson, G., Johnsson, H., 1996. Simulation of Water and Nitrogen Dynamics in a Five Year Leaching Experiment with Varying Fertilisation and Manure Treatments. Reports and Dissertations 29. Department of Soil Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden. Wagner, R., 1974. A new method for automated nitrate determination in sea water using the autoAnalyzer. Technicon Symposium, Frankfurt a. M. Wallgren, B., LindeÂn, B., 1994. Effects of catch crops and ploughing times on soil mineral nitrogen. Swedish J. Agric. Res. 24, 67±75. World Health Organization, 1993. Guidelines for drinking-water quality, vol. 1. Recommendations. Geneva, Switzerland. Wyland, L.J., Jackson, L.E., Chaney, W.E., Klonsky, K., Koike, S.T., Kimple, B., 1996. Winter cover crops in a vegetable cropping system: impacts on nitrate leaching, soil water, crop yield, pests and management costs. Agric. Ecosyst. Environ. 59, 1±17. Zadoks, J.C., Chang, T.T., Konzak, C.F., 1974. A decimal code for the growth stages of cereals. Weed Res. 14, 415±421.
Soil mineral nitrogen and nitrate leaching losses in soil tillage systems combined with a catch crop Maria Stenberga,*, Helena Aronssonb, BoÈrje LindeÂnc, Tomas Rydberga, Arne Gustafsonb a
Department of Soil Sciences, Division of Soil Management, Swedish University of Agricultural Sciences, PO Box 7014, S-750 07, Uppsala, Sweden b Department of Soil Sciences, Division of Water Quality Management, Swedish University of Agricultural Sciences, PO Box 7072, S-750 07, Uppsala, Sweden c Department of Agricultural Research Skara, Swedish University of Agricultural Sciences, PO Box 234, S-532 23, Skara, Sweden Received 8 October 1998; accepted 15 December 1998
Abstract Annual nitrogen leaching losses from arable land in south Sweden usually amount to 15±45 kg haÿ1. The objective of this three-year study was to investigate the timing effect of mouldboard ploughing (early autumn, late autumn or spring) on soil mineral nitrogen content and nitrate leaching in a cropping system with spring-sown small grain crops (barley, oats and wheat). Late autumn ploughing was studied with and without perennial ryegrass (Lolium perenne L.) as a catch crop, and with and without preceding stubble cultivation. The effects of removal compared to incorporation of straw were also studied. Soil mineral nitrogen in the 0±90 cm layer was determined on 4±5 occasions each year. Nitrate was determined in soil water sampled with ceramic suction cups at 60 and 90 cm. Total nitrogen in above-ground catch crops, weeds and volunteer plants was determined at ripeness of the main crop and before tillage operations. Time of tillage, as opposed to catch crop or crop residue management, in¯uenced nitrogen leaching. After early tillage, soil mineral nitrogen increased during autumn. In November, the nitrogen content was on an average 68 kg N haÿ1 in 0±90 cm in early ploughed soil and 39 kg N haÿ1 when ploughing was delayed to spring. Also nitrate leaching was greater in treatments with early than with late tillage. This was probably because of increased nitrogen mineralization. However, when tillage was delayed there was also a substantial growth of weeds and volunteer plants during autumn, particularly couch-grass (Elymus repens L.). This caused grain yield to decrease by up to 40% when tillage operations were delayed until late autumn or spring. Delaying tillage operations to late autumn or spring seems to be necessary to reduce the risk of nitrogen leaching. Swedish regulations for farmers to control nitrogen losses have been changed as a consequence of these results. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Soil tillage; Soil mineral nitrogen; Nitrate leaching; Lolium perenne; Sandy loam
1. Introduction
*Corresponding author. Tel.: +46-18-671213; fax: +46-18672795; e-mail: [email protected]
Nitrogen leaching through drainage water from arable land has been identi®ed as a problem during recent decades in regions such as Scandinavia. Annual
0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0167-1987(98)00197-4
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M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
nitrogen losses usually amount to 15±45 kg haÿ1 in southern Sweden (SNV, 1997), with the largest losses on easily permeable sandy soils with low water holding capacity and restricted rooting depths. Most losses occur as nitrate during autumn, winter and early spring when evaporation is low, precipitation is high and no crop is growing. Worldwide, nitrate as a contaminant in drinking water can be a problem. Nitrogen in surface waters leading to eutrophication is also a problem. In Sweden, nitrate concentrations >11.3 mg NO3±N lÿ1, which is the recommended limit for nitrate in drinking water (WHO, 1993), have been found in several drinking water wells (LoÈfgren, 1992). Keeping the soil covered with a growing crop during autumn and winter is considered to be an ef®cient measure to reduce nitrogen leaching from arable land both in Sweden and elsewhere (e.g. Martinez and Guiraud, 1990; Lewan, 1994; Wyland et al., 1996). During part of the autumn and milder winter periods when no main crop is established but when drainage normally occurs, a catch crop may be grown to take up nitrogen from the soil pro®le. Perennial ryegrass under-sown in the main crop has proved to be a useful catch crop (Hansen and Djurhuus, 1997; Aronsson and Torstensson, 1998). In most studies on catch crops the reference treatments have been conventionally managed cropping systems with tillage operations in early autumn soon after harvest of the main crop. However, not only the catch crop, but probably also the tillage treatment may greatly in¯uence nitrogen transformations and ¯ows in the soil. In Denmark on a sandy loam, nitrate leaching was reduced when ploughing was delayed until spring (Hansen and Djurhuus, 1997), but on a coarse sandy soil, spring ploughing did not reduce leaching compared with autumn ploughing while direct drilling did. In the UK, annual losses through mole and tile drainage systems were smaller from direct-drilled than from ploughed plots in a ®eld experiment on a clay soil (Goss et al., 1993). In New Zealand, Francis et al. (1995) found that nitrate leaching losses were greater during winter from a fallow soil ploughed to 20 cm depth in the previous March compared with soil ploughed in May. The nitrogen content and C:N ratio of crop residues are important for nitrogen immobilization processes after incorporation (e.g. Jawson and Elliot, 1986; Aulakh et al., 1991; Bhogal et al., 1997), where soil
microbial biomass acts as a sink for nitrogen (Paul and Juma, 1981). Thus, incorporation instead of removal of straw residues with high C:N ratios might be a way to reduce the risk of nitrogen leaching, due to immobilization of nitrogen in the soil. In studies made by Jarvis et al. (1989) and Nicholson et al. (1997), incorporation of straw decreased N leaching signi®cantly compared with soils where straw was burned. In contrast, several authors (e.g. Schjùnning, 1983; Davies et al., 1996) have reported very small or insigni®cant effects on leaching by straw incorporation. The objective of this study was to investigate the effect on soil mineral nitrogen content and nitrogen leaching both of an under-sown catch crop and of different soil tillage treatments, with or without removal of crop residues. 2. Materials and methods 2.1. Field site The ®eld experiment was established in 1992 on a sandy loam (Table 1) at Mellby (568290 N, 13800 E, alt. 10 m) in south-west Sweden. The soil consists of sand deposits with a thickness of 90±130 cm, covering glaci¯uvial clay. The soil was classi®ed as a Haplic Phaeozem (FAO soil classi®cation) (Johnsson, 1991). The mean annual temperature was 7.28C and the mean annual precipitation 773 mm (data from Genevad 1961±1990, 10 km north of Mellby) (Alexandersson et al., 1991). The whole experimental ®eld was tiledrained at about 90 cm depth. 2.2. Treatments The experiment consisted of ®ve tillage/catch crop systems as main treatments, and two crop residue Table 1 Soil particle size distribution and organic matter content (g kgÿ1) at the site Depth (cm)
Particle size (mm) <2
2±100
100±2000
0±30 30±60 60±90
103 47 43
131 90 145
709 846 803
Organic matter 57 17 9
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
117
Table 2 Treatments in the field experiment (the catch crop was perennial ryegrass under-sown in the main crop in spring) Main treatment (tillage)
Sub-treatment (crop residues)
MB MB MB MB MB MB MB MB
Incorporated Removed Incorporated Removed Incorporated Removed Incorporated Incorporated
first week in September (early autumn) first week in September first week in November (late autumn) first week in November first week in November and catch crop first week in November and catch crop first week in November preceded by stubble cultivation once first week in September in early April (spring)
MBmouldboard ploughing.
treatments as sub-treatments (Table 2) compared in an incomplete split-plot design with three blocks and a plot size of 9 m18 m. The main crops, all of them spring sown, were oats (Avena sativa L.) in 1994, barley (Hordeum vulgare L.) in 1995 and wheat (Triticum aestivum L.) in 1996. Different times for mouldboard ploughing to 20±25 cm depth (early September, early November and early April) were compared. Late autumn ploughing (early November) was studied both without a catch crop and preceding stubble cultivation and with perennial ryegrass (Lolium perenne L.) as a catch crop, as well as with stubble cultivation once in early September. Crop residues were incorporated by the tillage operation, or removed from the plots. First experimental treatments were carried out in the autumn of 1993 and the ®rst harvest in 1994. Seedbed preparation and sowing were carried out uniformly in the experiment. In spring each year, the catch crop was under-sown on the same day as the main crop was sown or one day later, and in the same direction with a conventional seed drill. Nitrogen fertilizer was applied during seedbed preparation in spring at rates normal for the region (90 kg haÿ1 to barley and oats and 110 kg haÿ1 to spring wheat). The rates of potassium and phosphorus fertilizers applied in each experimental plot were based on the amounts removed with crop yields and straw. Chemical weed and pest control during main crop growth was performed according to local recommendations. 2.3. Measurements Above-ground biomass of catch crop, weeds and volunteer plants were determined in one randomly
located micro-plot (0.5 m2.4 m) in each experimental plot. Sampling was carried out in early August at yellow ripeness of the main crop, stage 87 (Zadoks et al., 1974), in early November (not in the early ploughed plots) and in early April before spring ploughing. Grain yield of the main crop was determined in 22 m2 in each plot at harvest in years 1994± 1996. The occurrence of couch-grass (Elymus repens L.) in the plots was determined in September 1996 as the percentage of the area covered. Due to an error, no sampling of plant material was carried out in November 1993. Determinations of total nitrogen and carbon contents in plant material were performed with a LECO CNS-2000 analyser. Nitrogen and carbon were converted into gases in a dry combustion process and the amount of carbon (CO2) was then determined in infrared cells and nitrogen (N2) in thermal conductivity cells. Soil was sampled each year for mineral nitrogen content at yellow ripeness in August, before tillage operations in early September and early November and before ploughing in early April. In 1993 and 1994, soil was also sampled in early December, after the late autumn ploughing. Soil cores with a diameter of 20 mm were taken to a depth of 90 cm. Eight cores per plot from 0 to 30 cm and six cores per plot from 30±60 to 60±90 cm were pooled. Due to high costs for analyses of nitrogen, only pooled samples for each treatment were generally analysed. However, the samples were analysed plotwise in December 1993, in April 1994 and in August, September and November 1995. Before analysis, soil samples were stored deepfrozen (ÿ208C). The samples were extracted with 2 M KCl and analyses of NO3±N and NH4±N were made
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M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
with a TRAACS 800. The concentrations of NO3±N were determined with the colorimetric cadmium reduction method (Grasshoff, 1964; Wagner, 1974), and NH4±N with a spectrophotometric method by which a coloured complex of NH4±N and sodiumsalicylate is formed in the presence of prussidic acid and chloride (Benesch and Mangelsdorf, 1972). The analytical values were transformed to kg haÿ1 using measured average bulk densities (0±30 cm: 1.35 g cmÿ3, 30±60 cm: 1.56 g cmÿ3 and 60±90 cm: 1.68 g cmÿ3) and actual gravimetric water contents. Statistical analyses of differences between treatments were only performed when samples were analysed plotwise. Soil water samples were taken every second week with suction cups at 60 and 90 cm depth according to Djurhuus (1990) and Hansen (1991). Three suction cups per plot were installed at 90 cm in autumn 1992 and three cups per plot at 60 cm in autumn 1994. A suction of about 0.7 bar was applied for 24 h before extracting the samples. Samples were treated sepa-
rately. The concentration of NO3±N was determined as described above for soil samples. Nitrate leaching was calculated from soil water nitrate concentrations at 90 cm in the experiment and drainage measured daily in an adjoining experiment in the same ®eld, established in 1982, with individually tile-drained plots with similar treatments and soil type (Torstensson and Johnsson, 1996). The experiment reported here was carried out as a complement to the existing experiment. According to earlier studies in the adjoining experiment (Torstensson and Aronsson, 1999), drainage from each treatment could be assumed not to differ, and the average drainage in that experiment was therefore used here. Linear interpolation between measured NO3±N concentration values was used to obtain daily concentration values. Concentrations were then multiplied by daily discharge. Mean monthly air temperature and monthly precipitation and drainage as measured at the experimental site from July 1993 to June 1996 are shown in Fig. 1.
Fig. 1. Mean monthly air temperature (8C) 1.5 m above ground and monthly precipitation (white bars) and drainage (solid bars) at Mellby from July 1993 to June 1996. The figures above the bars refer to total annual precipitation and drainage, respectively (mm).
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
119
Table 3 Grain yields (kg haÿ1) and relative yields in 1994±1996 in the field experiment Treatment
Oats 1994
Barley 1995
Wheat 1996
Mean 1994±1996
Early plough., res. incorp. Early plough., res. removed Late plough., res. incorp. Late plough., res. removed Late plough., cc, res. incorp. Late plough., cc, res. remov. Late plough., cult., res inc. Spring plough., res. incorp.
3120 3130 2938 2561 2723 3101 3130 2921
4783 4365 4746 4767 4803 5106 4968 4958
4714 4420 2573 2604 3464 3823 3874 3063
4210 3970 3420 3310 3690 4010 3990 3650
Early ploughing in autumn Late ploughing in autumn Late ploughing in autumn and catch crop Early stubble cultivation and late ploughing in autumn Spring ploughing
3130100 88 93 100 93
4570100 104 108 109 108
4570100a 57b 81ab 85ab 67ab
4090100 82 94 98 89
Plant residues incorporated Plant residues removed
2970100 99
4850100 98
3540100 102
3790100 99
Statistical significance
n.s.
n.s.
Tillagea
n.s.
b
Values followed by different letters are significantly different (P<0.05). a Significance levels: P<0.05; n.s.not significant. b res.residues, incorp.incorporated, cccatch crop, cult.cultivation.
3. Results 3.1. Grain yields of main crops No signi®cant differences in grain yields between three-year treatment means for 1994±1996 were detected (Table 3) but there was a tendency for delayed tillage operations to result in lower yields than early autumn ploughing or stubble cultivation. In 1996 (wheat), grain yields differed signi®cantly between treatments but not in 1994 (oats) and 1995 (barley). In particular, late autumn ploughing without a preceding stubble cultivation or a catch crop resulted in lower yield than the early ploughing, most probably due to increased amounts of couch-grass with the years, which was a severe problem at the site (Fig. 2). Ryegrass as a catch crop signi®cantly reduced the amount of couch-grass in spite of late autumn ploughing in this treatment (Fig. 2). 3.2. Dry matter and nitrogen content of catch crop, weeds and volunteer plants Average above-ground biomass and nitrogen content, from August 1993 to August 1996, of catch crop,
Fig. 2. Couch-grass cover (% of plot area) after harvest in September 1996. Treatments not sharing the same letter are significantly different (P<0.05).
weeds and volunteer plants in August at ripeness of the main crop and in early November before ploughing are shown in Table 4. In 1993, catch crop establishment in the summer and growth during autumn was good, while in 1994 establishment was poor due to a very dry summer, which in turn reduced growth in the
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M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
Table 4 Average nitrogen content in harvested grain and above-ground biomass dry matter (DM) and nitrogen (kg haÿ1) of catch crop, weeds and volunteer plants at ripeness of the main crop in early August and before ploughing in early November from August 1993 to September 1996 Treatment
Nitrogen in grain
DM in August
Nitrogen in August
DM in November
Nitrogen in November
Early plough., res. incorp.c Early plough., res. removed Late plough., res. incorp. Late plough., res. removed Late plough., cc, res. incorp. Late plough., cc, res. remov. Late plough., cult., res inc. Spring plough., res. incorp.
69.5 66.5 56.7 56.4 60.4 69.4 66.8 58.4
179 115 447 375 191 369 96 422
2.7 2.2 4.4 4.1 3.6 5.9 1.4 4.7
± ± 407 469 670 729 177 415
± ± 11.4 13.9 16.1 19.6 6.3 11.5
Early ploughing in autumn Late ploughing in autumn Late ploughing in autumn and catch crop Early stubble cultivation and late ploughing in autumn Spring ploughing
68.0 56.5 64.9 66.8 58.4
147 411 280 96 422
2.5 4.2 4.8 1.4 4.7
± 438a 699b 177c 415ad
± 12.7a 17.9b 6.3c 11.5ac
Plant residues incorporated Plant residues removed
62.4 64.1
267 286
3.4 4.1
417 599
11.3 16.8
Statistical significance
n.s.
n.s.
Tillageb
Tillagea
n.s.
Values followed by different letters are significantly different (P<0.05). a Significance level: P0.01. b Significance level: P0.001; n.s.not significant. c res.residues, incorp.incorporated, cccatch crop, cult.cultivation.
autumn. During that year the above-ground nitrogen content of the catch crop in November was similar to that in weeds in treatments with late autumn ploughing and spring ploughing. In 1995, establishment of the catch crop was good, and despite a dry summer, growth of the catch crop was good during the autumn. Weeds also grew vigorously in the autumn. In August 1996, the nitrogen content of weeds (dominated by couch-grass) in the treatments with late autumn ploughing without a catch crop and spring ploughing clearly exceeded the nitrogen content in catch crop and weeds in the catch crop treatments. Before ploughing in November, the average nitrogen content in above-ground plant material was almost 18 kg N haÿ1 in the catch crop treatments (Table 4). This was more than three times the content at yellow ripeness. In November 1993, the catch crop was well developed but due to an error, no sampling was carried out. The overall average of nitrogen in the catch crop in November would have been 21 kg N haÿ1 if the nitrogen content in that catch crop was estimated from catch crop harvest in the adjoining ®eld experiment and included in the average for this experiment.
3.3. Soil mineral nitrogen Soil mineral nitrogen (nitrate and ammonium) content in the 0±90 cm layer in August at ripeness of the main crop was similar in all tillage treatments both in individual years and as a mean of all three years (Table 5 and Fig. 3). At sampling before ploughing in early September, nitrogen contents had increased in 0±90 cm but were still similar in the different tillage treatments. The treatments with a catch crop or spring ploughing, however, showed slightly lower values than the other treatments. Measurements in late autumn (November and December) showed a large increase in the content of mineral nitrogen in early ploughed as well as in early stubble cultivated plots. On these occasions, differences between tillage treatments were strongly signi®cant (Table 5). In November, the nitrogen content was on an average 68 kg N haÿ1 in 0±90 cm in early ploughed plots and 39 kg N haÿ1 in spring ploughed plots. In the spring of 1996, the contents were higher than in November 1995 in all treatments. The winter of 1995/1996 was long, cold and dry with very small
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
121
Table 5 Soil mineral nitrogen (kg N haÿ1) in 0±90 cm depth at all sampling occasions (statistical analysis of variance was performed when sampling was carried out plotwise) Treatmentb
1993
1994
1995
1996
Ac
S
N
D
Ap
A
S
N
D
Ap
A
S
N
Ap
A
S
1 2 3 4 5
64 55 47 49 44
64 63 34 52 50
91 42 22 58 40
87a 46b 25c 68d 39bce
43 46 40 49 35
50 41 33 41 41
44 47 44 39 37
65 50 50 58 43
66 47 47 55 33
33 44 48 40 34
28 31 27 28 25
38 39 37 36 30
66a 40b 38b 46b 34b
82 77 75 78 55
32 31 30 30 34
45 43 46 46 41
10 20
49 60
51 57
49 53
53 53
42 44
44 36
42 44
51 58
48 56
40 41
29 27
35 39
45 48
73 79
32 31
45 44
Significance
±
±
±
Ta,d
n.s
±
±
±
±
±
n.s.
n.s.
Ta
±
±
±
Values followed by different letters are significantly different (P<0.05). Average nitrogen content for 1993-96 is shown in Fig. 3. Significance levels: P0.001; n.s.not significant. b Treatment 1early autumn ploughing, 2late autumn ploughing, 3late autumn ploughing with catch crop, 4early shallow cultivation and late autumn ploughing, 5spring ploughing, 10plant residues incorporated, 20plant residues removed. c AAugust, SSeptember, NNovember, DDecember, ApApril. d Ttillage. a
3.4. Nitrate concentrations in soil water and calculated nitrate leaching
Fig. 3. Average soil mineral nitrogen content in the tillage treatments for each sampling occasion from yellow ripeness in August 1993 to early September 1996. Sampling in December was carried out only in 1993 and 1994.
amounts of drainage water and mineralized nitrogen obviously accumulated in the pro®le (Table 5). In contrast, the winters of 1993/1994 and 1994/1995 were much wetter and nitrogen losses by leaching were greater (Fig. 6). The effects on soil mineral nitrogen of different crop residue managements were comparatively small (Table 5).
At 60 cm, soil water nitrate concentrations differed signi®cantly on several occasions during spring and autumn 1994, autumn 1995 and spring 1996 (Fig. 4). At 90 cm, soil water nitrate concentrations differed signi®cantly between tillage treatments on one occasion, in July 1994 (P<0.05) (Fig. 5). Differences in nitrate concentration between crop residue managements were never signi®cant at any of the depths (data not shown). Nitrate concentrations at 60 cm depth showed distinct peaks when drainage started in autumn, especially in treatments with early ploughing (Fig. 4). At 90 cm depth the peaks were less pronounced and delayed. When the experiment started in 1993 the concentrations were relatively higher in several plots in one of the blocks (data not shown) which caused high variation between plots within treatments. This was most obvious in the catch crop treatments. The high nitrate concentrations were probably a result of large applications of manure in that part of the ®eld previously. However, the growth of the catch crop during autumn 1993 decreased the soil water nitrate concentrations considerably (Figs. 4 and 5). During the course of the experiment, both the soil water
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Fig. 4. Soil water nitrate concentrations at 60 cm depth from July 1993 to June 1996 in the tillage treatments. Significant differences between treatments at P<0.05 are shown with arrows.
Fig. 5. Soil water nitrate concentrations at 90 cm depth from July 1993 to June 1996 in the tillage treatments. Significant differences between treatments at P<0.05 are shown with arrows.
nitrate concentrations and the soil mineral nitrogen content decreased in these plots. The nitrate concentrations at 90 cm soil depth were generally higher than 5 mg NO3±N lÿ1 water, except for a few short periods in the treatments with a catch crop. In general, the concentrations were also higher than or near the WHO limit for drinking water (11.3 mg NO3±N lÿ1, WHO, 1993). Generally, however, the lowest concentrations
were registered in the treatment with spring ploughing. The average nitrate leaching, i.e. the estimated quantity of nitrate that passed the 90 cm depth, was about 50 kg N haÿ1 annually during the ®rst two years of the experiment (Fig. 6), when the precipitation was close to the long-term mean value. In contrast, the autumn of 1995 and spring of 1996 were dry, and
M. Stenberg et al. / Soil & Tillage Research 50 (1999) 115±125
Fig. 6. Accumulated nitrate leaching (kg haÿ1), estimated from soil water nitrate concentrations at 90 cm from July 1993 to June 1996.
therefore leaching was only about 11 kg N haÿ1 in 1995/1996. There were no signi®cant differences between treatments in accumulated nitrate leaching due to the high variation between plots, but for the three-year period, losses were 24 kg N haÿ1 smaller from spring ploughed, and 17 kg N haÿ1 smaller from late autumn ploughed than from early autumn ploughed treatments. Presence of a catch crop did not reduce leaching. The effect of incorporation or removal of crop residues was comparatively small and never signi®cant (data not shown). 4. Discussion In plots ploughed or stubble cultivated in early September, mineral nitrogen contents in the soil during autumn increased (Table 5 and Fig. 3), and this led to a tendency for greater nitrate leaching during autumn and winter than in soil ploughed in late autumn or spring (Fig. 6). One reason for large amounts of mineral nitrogen in the soil in November was probably enhanced nitrogen mineralization due to an increase in soil carbon availability brought about by the early tillage operations and the incorporation of crop residues (Shepherd et al., 1992; Stokes et al., 1992). Weather conditions during autumn 1993 were favour-
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able both for nitrogen mineralization and for plant growth as demonstrated by the small soil mineral nitrogen contents in the catch crop treatments and the large contents in the early tilled treatments (Table 5 and Fig. 3). In 1994 and 1995 the sum of the nitrogen contents in plant material and as soil mineral nitrogen in November were similar in all treatments (Tables 4 and 5), if roots were assumed to contain about the same quantity of nitrogen as the above-ground part of the plants (SjoÈdahl Svensson and Clarholm, 1994). Thus, the growth of weeds and volunteer plants, functioning as a catch crop during autumn, was also an important reason for less accumulation of mineral nitrogen in treatments with postponed ploughing. The results suggest that postponing tillage until late autumn or spring can be a measure to reduce the risk of leaching losses. This was also found in other investigations in Sweden (Wallgren and LindeÂn, 1994; Aronsson and Torstensson, 1998). However, the effect of postponed tillage was not as pronounced as in a similar experiment on a sandy loam in Denmark (Hansen and Djurhuus, 1997). The actual weather conditions largely in¯uenced both the mineral nitrogen content and the drainage and thus also the estimated nitrate leaching. This was obvious after the dry winter of 1995/1996 when mineral nitrogen accumulated, especially in the 0± 30 cm soil layer (data not shown), in the soil until spring in all treatments, but in smaller amounts in the treatment with spring ploughing. During the ®rst two years of the experiment, differences in soil mineral nitrogen between treatments decreased during winter mainly due to nitrogen leaching. Under-sown perennial ryegrass can slightly reduce grain yield of the main crop due to competition (e.g. Andersen and Olsen, 1993; Wallgren and LindeÂn, 1994). Reduced yield after incorporation of a catch crop has also been found in some studies. Martinez and Guiraud (1990) and Jensen (1991) suggested nitrogen immobilization to be the reason, while ThorupKristensen (1993) emphasized that depletion of soil mineral nitrogen by the catch crop before incorporation is probably more important. In this study the catch crop did not reduce grain yields signi®cantly in any of the years (Table 3). During the autumn of 1994, nitrate concentrations at 60 cm were higher than expected in the catch crop treatments (Fig. 4). During that year
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catch crop uptake of nitrogen was small, and this crop developed poorly during autumn. This, in combination with increased nitrogen mineralization due to catch crop incorporation in the previous autumn, could be one of the reasons for accumulation of mineral nitrogen in the soil during autumn. This is supported by results from a similar experiment at Mellby during the same year, where Aronsson and Torstensson (1998) found increased nitrogen mineralization after incorporation of a catch crop. There was no reduction of nitrate leaching in the treatments where a ryegrass catch crop was undersown in spring, which differs from earlier studies in Sweden on the effect of a catch crop on the risk of nitrate leaching (Lewan, 1994; Wallgren and LindeÂn, 1994; Aronsson and Torstensson, 1998). Poor establishment and growth of the catch crop during two out of three years were probably contributing reasons. During the years covered in this particular experiment the tillage treatments seemed to be the major factor in¯uencing the nitrogen leaching and the nitrate concentrations in the soil water. In spite of this, some of the treatments led to reductions in the concentrations to levels clearly below the WHO limit. In this study, the removal or incorporation of straw did not affect crop growth, amounts of soil mineral nitrogen or nitrate leaching (Tables 3±5). Measurements of mineral nitrogen in the soil or soil solution did not indicate that incorporation of straw resulted in substantial net immobilization of nitrogen during autumn. Small or non-signi®cant effects of straw incorporation on nitrogen leaching have also been found in other studies (e.g. Schjùnning, 1983; Davies et al., 1996). However, in the long run, continuous incorporation of crop residues can increase the amount of mineralizable organic nitrogen in the soil. This was found in a long-term experiment (Persson and Kirchmann, 1994) where straw was incorporated every second year, resulting in a slow build-up of soil organic matter. Likewise, repeated incorporation of catch crops would increase the pool of soil organic nitrogen in the long term (Jensen, 1992). 5. Conclusions Soil tillage practices in the autumn had a considerable in¯uence on accumulation of mineral nitrogen in
the soil during autumn and to some extent also on the amount of nitrate leached from the soil. Postponing tillage from early autumn until late autumn or spring reduced the risk of nitrogen leaching, probably due to lower mineralization rates and higher nitrogen uptake by weeds and volunteer plants during autumn. However, delaying primary tillage operations also reduced grain yields. This was the result of severe couch-grass infestation in these treatments. Thus, perennial weeds can be a problem when tillage operations are delayed, but a catch crop such as perennial ryegrass may reduce the problem. Ryegrass as an under-sown catch crop reduced soil mineral nitrogen until ploughing in late autumn, but did not reduce nitrogen leaching compared with late autumn ploughing. This differed from other studies in Sweden where catch crops have been shown to be ef®cient in reducing nitrogen leaching. Acknowledgements The study was ®nancially supported by the Swedish Board of Agriculture and by the Swedish Council for Forestry and Agricultural Research. References Alexandersson, H., KarlstroÈm, C., Larsson-McCann, S., 1991. Temperaturen och nederboÈrden i Sverige 1961±1990. Referensnormaler. (Temperature and precipitation in Sweden 1961± 1990. Reference normals). SMHI. Meteorologi no. 81. NorrkoÈping, 88 pp. Andersen, A., Olsen, C.C., 1993. Rye grass as a catch crop in spring barley. Acta Agric. Scand. Sect. B, Soil and Plant Sci. 43, 218±230. Aronsson, H., Torstensson, G., 1998. Measured and simulated availability and leaching of nitrogen associated with frequent use of catch crops. Soil Use Manage. 14, 6±13. Aulakh, M.S., Doran, J.W., Walters, D.T., Mosier, A.R., Francis, D.D., 1991. Crop residue type and placement effects on denitrification and mineralization. Soil Sci. Soc. Am. J. 55, 1020±1025. Benesch, R., Mangelsdorf, P., 1972. A method for the colorimetric determination of ammonia in seawater. HelgolaÈnder Wiss Meeresunters 23, 365±375. Bhogal, A., Young, S.D., Sylvester-Bradley, R., 1997. Straw incorporation and immobilization of spring-applied nitrogen. Soil Use Manage. 13, 111±116. Davies, D.B., Garwood, T.W.D., Rochford, A.D.H., 1996. Factors affecting nitrate leaching from a calcareous loam in East Anglia. J. Agric. Sci. 126, 75±86.
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