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Agronomy Journal 93:842-849 (2001)
© 2001 American Society of Agronomy

SOIL MANAGEMENT

Tillage and Previous Crop Effects on Dynamics of Nitrogen in a Wheat–Soil System

Yoong Kee Soon*,a, George W. Claytonb and Wendell A. Ricea

a Agric. and Agri-Food Canada, Res. Branch, P.O. Box 29, Beaverlodge, AB, T0H 0C0, Canada
b Agric. and Agri-Food Canada, Res. Cent., 6000 C & E Trail, Lacombe, AB, T4L 1W1, Canada

* Corresponding author (soony{at}em.agr.ca)

Received for publication February 9, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The effects of tillage and preceding legume crops on N flux in the soil–plant system require quantification for developing sustainable cropping systems. We measured changes in soil and plant N under the influence of tillage [no till (NT) vs. conventional tillage (CT)] and previous crops [spring wheat (Triticum aestivum L.), red clover (Trifolium pratense L.) green manure, and field pea (Pisum sativum L.)]. The study was conducted from 1994 through 1996 on a well-drained sandy loam soil (coarse-loamy, mixed, frigid, Typic Cryoboralf) near Fort Vermilion, Alberta (58°23'N, 116°2'W). Nitrogen uptake by wheat was increased by NT and legume crops. At seeding, CT soil had 28 kg ha-1 more NO3–N to 100-cm depth than NT soil. Apparent net N mineralization in the growing season was 71 and 22 kg N ha-1, respectively, for the NT and CT systems. Previous crop effect on net N mineralization (kg N ha-1) was red clover (56) > field pea (51) > wheat (34). Approximately 18 kg N ha-1 was net-mineralized from red clover residues compared with insignificant amounts from pea and wheat residues. Microbial biomass turnover's contribution to net N mineralization (28 to 40 kg N ha-1) was increased by NT and previous legume crop. Soluble organic N decreased by 7 kg ha-1 between seeding and maturity for all experimental treatments. The results indicate that N fertilizer recommendations should allow for greater mineralization of organic N under NT than CT and following a legume green manure.

Abbreviations: ANM, apparent net N mineralization • CT, conventional tillage • MBN, microbial biomass N • NT, no till • SON, soluble organic N


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TILLAGE AND CROP ROTATION with legume crops are two management practices that can influence the N dynamics of soil–plant systems. Legume crop residue, particularly green manure, is an effective source of N (Bremer and van Kessel, 1992a; Haynes et al., 1993). When released in synchrony with crop N demand, crop residue N is a particularly desirable source of N as losses to the environment are minimized (Stute and Posner, 1995). However, Bremer and van Kessel (1992a) showed that the influence of crop residues on plant-available N depends on how they effect net mineralization of other soil N sources. Thus, they reported that while lentil (Lens culinaris Medik.) green manure (C/N = 10) increased mineralization of indigenous soil N, lentil straw (C/N = 31) increased net immobilization of indigenous and fertilizer N. Groffman et al. (1987) reported that although N derived from a legume cover crop (Trifolium incarnatum L.) residue was sufficient for the following cereal crop, it was available at a slower rate than fertilizer N. Thus, the availability of legume residue N may be lower than that of chemical fertilizers because of the dependence of legume N mineralization on residue decomposition (Varco et al., 1993).

Reports of tillage effects on soil mineral N are mixed. This may be partly because these effects can be influenced by local soil and climatic conditions. For example, Grant and Lafond (1992) reported that soil NO3 in the 15- to 60-cm depth was higher under conventional tillage (CT) compared with no till (NT) or minimum tillage in autumn in the eastern Canadian Prairie while Malhi et al. (1992) found no difference in soil NO3 among CT, minimum tillage, and NT treatments in the spring and autumn in the western Canadian Prairie. Rice et al. (1987) reported more N mineralization in a ploughed, well-drained soil than in a poorly drained soil compared with the corresponding NT treatment. Tillage, by mixing and incorporating crop residues in the soil, tends to promote faster release of residue N than surface-placed (NT) residue (House et al., 1984; Varco et al., 1993). Groffman et al. (1987) found that CT increased the availability of N from legume residues compared with NT, but they attributed this to the greater biomass and N content of the legume cover crop under CT management. They and others (e.g., House et al., 1984; Varco et al., 1993) also reported that NT reduced the availability of N from chemical fertilizer, presumably due to its greater immobilization under NT (Carter and Rennie, 1987). In contrast, Haugen-Kozyra et al. (1993) reported that tillage did not affect the recovery of fertilizer N under a barley (Hordeum vulgare L.) monoculture. Gilliam and Hoyt (1987) reviewed the effects of tillage on N cycling and concluded that although NT tended to increase N immobilization, crop N uptake was no different than that observed for CT.

While green manuring with legumes typically results in a net gain of soil N, soil N balance after pulse crops, such as field pea, can be positive or negative (Haynes et al., 1993; Armstrong et al., 1994). The effects of tillage systems, green manure, and pulse crops on the dynamics and availability of soil N in a cool, subhumid climate are not well documented. Therefore, the objective of this study was to quantify and compare the growing-season dynamics of soil mineral N, soil microbial biomass N (MBN), soluble organic N (SON), and crop N uptake as influenced by previous crop and tillage.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site and Experimental Treatments Description
The study, situated near Fort Vermilion, Alberta, was part of a larger experiment comparing legume-based cropping systems. The first crop was planted in spring of 1992. The data for the N dynamics study are presented for 1994 through 1996. The soil, a Leith sandy loam (coarse-loamy, mixed, frigid, Typic Cryoboralf) with an organic C content of 25 g kg-1 and a pH of 5.65 (1:2 ratio, 0.01 M CaCl2) in the top 15-cm depth, had developed on moderately coarse-textured alluvial material and is well drained. The site receives an average of 155 mm of precipitation during the growing season (June–August). The main experiment was a split-plot factorial arranged in a randomized complete block design with three replications. Two tillage treatments (CT and NT) were applied to the main plots and crop rotations to the subplots, which were 3.6 m wide and 25 m long. The four rotations were: (i) field pea–wheat–canola (Brassica rapa L.)–wheat, (ii) red clover green manure–wheat–canola–wheat underseeded to red clover, (iii) fallow–wheat–canola–wheat, and (iv) continuous wheat. Each phase of the rotation was present every year. For this study, we selected three crop sequences: field pea–wheat from rotation (i), red clover green manure–wheat from rotation (ii), and continuous wheat from rotation (iv). The legume crops were inoculated with commercial inoculants for N fixation. The same set of plots was used for continuous wheat cropping, whereas plots grown under rotation were sampled according to cropping sequences.

It is possible that the observations made in each of the experimental years were influenced not only by the immediately preceding crops, but also by other previous crops in the rotations. We believe, however, that the latter effect was negligible compared with the former and with growing conditions during the year. The observations were repeated over 3 yr mainly to obtain trends typically associated with an average growing season.

Field Operations
Conventional tillage comprised an autumn cultivation with a heavy-duty cultivator or disc to a depth of 10 to 15 cm (depending on amount of residues and soil condition) and two spring tillage operations with a field cultivator to incorporate crop residues followed by harrowing and packing. Weeds were controlled with glyphosate [N-(phosphono-methyl) glycine] before seeding for NT treatments. Red clover growth in the NT green manure treatment was killed in the summer (bloom stage of growth) by glyphosate burn off, and subsequent weed growth was controlled with appropriate herbicides. Red clover growth in the CT treatment was terminated by discing, and subsequent weed growth suppressed by mechanical tillage during the remainder of the fallow period.

A Valcon DS 100 ConservaPak drill with 23-cm row spacings was used to seed all crops. Fertilizers were banded to one side of the seed row at seeding. Nitrogen was applied mostly as urea [(NH2)2CO]-N. Phosphorus was applied as monoammonium phosphate (NH4H2PO4), which also supplied the balance of the required N. Fertilizer rates followed soil test recommendations based on autumn soil sampling. For N, this was 90 kg N ha-1 minus soil test NO3–N to 60-cm depth, except in 1994 when the target was 105 kg N ha-1 instead. No allowance was made for N in incorporated crop residues. Phosphorus fertilizer rates varied between 10 and 15 kg P ha-1.

Sampling and Chemical Analysis
Soils cropped to wheat were sampled four times each year: (i) immediately before sowing, i.e., before fertilizer application; (ii) at stem elongation; (iii) at soft dough; and (iv) at maturity. Four 38-mm-diam. cores were taken to 100-cm depth per subplot at each sampling. The extracted cores were sectioned into 0- to 15-, 15- to 30-, 30- to 60-, and 60- to 100-cm sections (with the surface being the 0-cm reference point), and the sections for each depth were bulked and mixed. Two additional cores were taken from each subplot at stem elongation and maturity in 1994 for bulk density determination. In subsequent years, bulk density measurements were taken only at stem elongation because paired t-test showed no significant differences in the bulk density of the experimental soil between time of sampling.

Wheat plant samples were cut 2 to 3 cm above ground level at stem elongation, soft dough, and maturity from a 2-m2 area of each subplot and then dried and weighed. After grinding, the samples were analyzed for total N using a Leco N analyser (Model FP428). Samples of the preceding wheat and field pea were taken at maturity for dry matter and total N determinations. At termination of the red clover, plant samples were taken from three 1-m2 areas for dry matter and total N determinations; soil samples from those plots were taken at that time and in the autumn for inorganic N analysis.

The soil samples were transported to the laboratory in ice-cooled insulated containers and extracted in the field-moist condition with 1 M KCl (using a 1:10 dry soil equivalent/extractant ratio) within 48 h. Extracts were filtered and analysed for NH+4 and NO-3 by autoanalyser using the indophenol and Cd reduction methods, respectively. Nitrite-N was assumed to be negligible for NO3 determination. In 1995 and 1996, soils from the 0- to 15-cm depth were also analyzed for MBN using the fumigation–extraction procedure of Brookes et al. (1985) with the following modifications: (i) fumigation with chloroform (CHCl3) was for a period of 96 h; and (ii) NH4 and organic N in the extracts were oxidized to NO3 by persulfate oxidation in an autoclave (Cabrera and Beare, 1993), and the NO3–N was determined by automated colorimetry. Soluble organic N was calculated in unfumigated soil extracts as total extractable N minus the sum of NO3 plus NH4–N. In addition, samples were taken in late autumn (early October) from the top 15 cm of soil for MBN and SON determination. Soil N values were converted to a kg ha-1 basis using soil bulk density values measured for that growing season. The mean bulk densities for the three experimental years were 1.25, 1.48, 1.41, and 1.43 Mg m-3 for the 0- to 15-, 15- to 30-, 30- to 60-, and 60- to 100-cm depths, respectively. Coefficients of variation between subplots in any one year and between years were about 10% or less. In contrast, soil NO3 concentrations between subplots can vary by more than one order of magnitude.

Statistical Analysis
The data were analysed by general linear model using SAS programs (SAS Inst., 1990). The data for NO3 were highly variable, and statistical analysis was preceded by a log10 transformation to obtain more homogeneous variances. Nitrate-N data were back-transformed for data presentation. Here data variability will be presented as coefficients of variation instead of standard errors of the mean because it would be inappropriate to use back-transformed standard errors of the mean. Data were analysed separately by year as well as combined over years subject to homogeneity of variances (McIntosh, 1983). We considered tillage (the main plot effect) to be significant at P <= 0.10 because it was based on only six plots (2 error df). The previous crop and previous crop x tillage interaction (subplot effects) were considered to be significant at P <= 0.05. Changes in soil NO3 between any two sequent samplings were analyzed for significant differences at P <= 0.05 by paired t-test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen Returned in Residues of Preceding Crops
The data sets for residue dry matter and N content were small (n = 6 for each crop type each year) and, because variances were not homogeneous, could not be combined for analysis across year and crop type (Table 1). Conventional tillage increased wheat straw production; however, the effect was significant only in one year (1995, when growing-season moisture was average). Tillage had no effect on the amount of residues or N returned by field pea. No-till management of red clover tended to result in greater amounts of residues and N returned to the soil than CT; however, the differences were significant only in 1993. Although field pea returned slightly more straw residues to the soil than wheat, its N contribution was only 3 to 4 kg ha-1 more than that of wheat straw. Jensen (1989) and Armstrong et al. (1994) reported that N contained in mature pea straw can vary from 16 to 92 kg N ha-1, with N concentrations of 8.5 to 23 g kg-1 dry matter. The N input from red clover in this experiment would be within the lower half of the reported range for this crop in the experimental soil zone (44 to 290 kg N ha-1 with a mean of 110 kg N ha-1) (Rice 1980) and was most likely associated with moisture limitation.


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  		Table 1. Dry matter and N returned to the soil in previous crop residues as affected by tillage in 1993, 1994, and 1995.

 
Nitrogen Uptake and Dry Matter Production by Wheat
At maturity, the mean dry matter and N uptake of wheat following red clover was higher than that of continuous wheat (Table 2). Wheat following field pea had intermediate values of biomass and N uptake. However, previous crop effects on dry matter production and N uptake of wheat varied with the tillage system used. Conventional tillage resulted in higher biomass and N uptake of continuous wheat than NT up to the soft dough stage, after which the trend was reversed with respect to tillage effects although the differences were not significant. Continuous wheat under CT attained maximum N uptake at soft dough, whereas under NT, the maximum was attained only at maturity, indicating a difference in N dynamics. Tillage had little effect on dry matter and N uptake of wheat following field pea. At maturity, N uptake of wheat following red clover was higher under NT compared with CT even though shoot biomass was essentially similar. This observation is in accord with the greater amount of N returned in red clover residues under NT (Table 1). Mean N uptake of wheat across previous crops was higher with NT compared with CT only at maturity, indicating that soil N became more available under NT later in the growing season, i.e., there was a tillage-induced difference in seasonal N dynamics.


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  		Table 2. Dry matter production and N uptake of wheat at different growth stages as influenced by tillage (T) and previous crop (PC).{dagger}

 
Soil Mineral Nitrogen
There were only a few significant interactions between year and previous crop, tillage, or both. Therefore, because we were mainly interested in an average trend over time in the N pools of a wheat–soil system, the data presented are mostly the means of 3 yr (1994–1996 inclusive). Tillage and previous crops had little effect on exchangeable NH4 content of the experimental soil (data not shown). There were small but significant increases in exchangeable NH4 in the soil below the 30-cm depth from soft dough to maturity of wheat (Table 3). Exchangeable NH4 was low relative to NO3 levels and, except for 1994, seldom exceeded 3 kg N ha-1 in the various soil depths sampled. The relatively high mean exchangeable NH4 in the top 15 cm of soil at stem elongation was probably due to inhibition of fertilizer N nitrification associated with an early summer drought in 1994.


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  		Table 3. Profile distribution of NH4–N, NO3–N, and soil moisture, by soil layer, at four sampling times during the growing season. Data are means of tillage and crop sequences, 1994 through 1996 (n = 54), unless otherwise indicated.

 
Soil NO3 in different layers of soil sampled tended to vary more than exchangeable NH4 over time and depth (Table 3). Throughout the growing season, especially through stem elongation, there was more NO3 in the 60- to 100- than 15- to 60-cm depth. Few tillage x previous crop interactions were significant, indicating that the tillage effect was mostly independent of cropping sequences. This contrasts with the strong presence of a tillage x previous crop interaction effect on crop N uptake (Table 2). Therefore, only the main effects of tillage and previous crops on soil N will be emphasized. Soil moisture profiles suggest that little or no downward movement of soil water occurred during crop growth (Table 3).

Nitrate in the top 15 cm of soil for the two tillage treatments varied between 10 and 28 kg N ha-1 during the growing season and was significantly higher in NT plots than in CT plots only at the soft dough stage of wheat development (Fig. 1). At sowing, subsurface soil NO3 (15–60 cm depth) was higher under CT compared with NT. Although NO3 in the 60- to 100-cm depth was also greater under CT than under NT, the difference was not significant. There was no other significant effect of tillage on soil NO3 due to its high variability. Soil NO3 to the 100-cm depth (especially 15–100 cm depth) decreased rapidly between stem elongation and soft dough under both tillage systems (P <= 0.05) and tended to increase (P = 0.07) between soft dough and maturity under NT only.



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  		Fig. 1. Effect of tillage on NO3 in surface soil (0–15 cm) and top 100 cm of soil during the growing season. Nitrate in the 15- to 100-cm depth (subsurface soil) is the difference between the two graphs depicted. Data are averages over 3 yr (1994–1996) and previous crops (n = 27). * indicates significant difference at P <= 0.05; *a indicates significant difference at P <= 0.05 for 15- to 60-cm increment only.

 
The only significant previous crop effect on NO3 was in the top 15 cm of soil at maturity of wheat: Plots in continuous wheat had more NO3 than plots that were previously under legume crops (Fig. 2). Nitrate, especially that below the 30-cm depth, decreased considerably between stem elongation and soft dough of wheat in all plots, presumably due to crop uptake, and tended to increase (P = 0.09) between soft dough and maturity where red clover and field pea were the previous crops (Fig. 2). In contrast, NO3 in the corresponding soil depth following wheat decreased 13 kg N ha-1 (but not significantly) in the same time period.



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  		Fig. 2. Effect of previous crop on NO3 in surface soil (0–15 cm) and top 100 m of soil during the growing season. Nitrate in the 15- to 100-cm depth is the difference between the two graphs depicted. Data are averages over 3 yr (1994–1996) and tillage treatments (n = 18). * indicates significant difference at P <= 0.05.

 
The method of red clover termination (mechanical tillage in CT plots and chemical desiccation in NT plots) had mostly no measurable effect on soil mineral N in the year of termination (data not shown). The data were, therefore, averaged across tillage treatments. In 1994, exchangeable NH4 was higher in autumn approximately 10 wk after termination of red clover, but NO3 was not, probably because of nitrification inhibition by drought (Table 4). However, in 1995, a more normal year for soil moisture, NO3 increased significantly in three of the four soil layers in the autumn sampling, 10 wk after red clover termination accumulation; the increase in NH4 was small but mostly significant. Data are not presented for 1996 because those for the autumn sampling were lost due to contamination.


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  		Table 4. Mineral N in soil at different depths at termination of red clover in the summer and autumn of 1994 and 1995. Data are means of tillage (n = 6).

 
Labile Organic Nitrogen in the Surface Soil
Microbial biomass N decreased with time during the growing season (Fig. 3). Tillage treatments were significantly different only in the late autumn sampling, and previous crop effect was significant at maturity and late autumn. The seasonal trend in MBN was similar to that of SON, which in turn correlated well (r = 0.91) with soil moisture content (Fig. 4). However, SON was affected by neither tillage nor previous crop.



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  		Fig. 3. Effect of (a) tillage and (b) previous crop on soil microbial biomass N (MBN) at 0- to 15-cm soil depth at five sampling times. Data are averages over 2 yr (1995 and 1996). *indicates significant difference at P <= 0.05. Pooled SE = 1.93.

 


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  		Fig. 4. Variation in soluble organic N (SON) and soil moisture in surface soil (0–15 cm soil depth) with time. Data are averages over 2 yr (1995 and 1996) and across tillage and previous crop treatments.

 
Nitrogen Budget
Apparent net N mineralization (ANM) and net changes in labile N pools between sowing and maturity of wheat were calculated for the 1995 and 1996 growing seasons only because in 1994, data on MBN and SON were not collected (Table 5). Negative values of net change indicate lower values at maturity than at sowing, i.e., loss from that N pool. The calculation for ANM assumed that N loss from the soil–plant system was negligible and because a leak-proof system is unlikely, the estimate is probably on the low side. It was also assumed that 50% of fertilizer N was assimilated by the wheat crop and the remainder immobilized in the soil or lost. This was based on reported recovery of 15N-labelled fertilizer by spring cereal crops of between 33 to 67% (Glendining et al., 1997; Haugen-Kozyra et al., 1993; Nielsen et al., 1988). No adjustment was made for tillage treatments because Haugen-Kozyra et al. (1993) found that tillage did not affect crop recovery of fertilizer N. Nitrogen may be mineralized from MBN, SON, and soil organic matter, which in cropping systems, is derived from crop residues and other organic materials. Because there was no other input of organic materials in the experiment, the portion of net N mineralized from crop residues may be estimated as ANM less the combined change in MBN and SON between sowing and maturity. It indicated that approximately 18 kg N ha-1 was derived from red clover residue decomposition. However, the net change in pea and wheat residue N was essentially nil after allowing for estimation error.


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  		Table 5. Effect of previous crop and tillage on N uptake by wheat at maturity, apparent net N mineralization (ANM), and net changes in labile soil N pools and residue N between sowing and maturity of wheat.{dagger}

 
Accumulated soil NO3 at seeding was a bigger potential source of available N for the CT system than the NT system and was similar among previous crops. Net changes in soil NO3 between seeding and maturity differed by 41 kg N ha-1 between tillage practices compared with practically no difference between previous crops. Growing-season ANM was 17 to 22 kg N ha-1 greater after a legume crop than after wheat and nearly 50 kg N ha-1 greater under NT compared with CT management. Growing-season ANM in plots previously cropped to wheat or field pea appeared to be derived mostly from SON and MBN mineralization. In plots following red clover, net N mineralization from SON and MBN accounted for only 68% of ANM, i.e., the other 32% (18 kg N ha-1) was derived from crop residue decomposition. Residue N mineralization was not estimated for tillage treatments because N was probably lost from the CT system. This loss is likely when the net decrease in labile N pools (excluding fertilizer N) exceeds or equals crop uptake. Soil NO3 at seeding and turnover of MBN were, apparently, the two major sources of N for CT wheat while N mineralization (including MBN turnover) during the growing season was the main source of N for NT wheat.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effect of Tillage
Approximately 50 kg ha-1 more N was mineralized during crop growth under NT compared with CT and because of this synchrony between supply and demand, crop N uptake was also higher under NT (Table 5). Our results on tillage effects on N mineralization and crop N uptake agree with those reported by Stein et al. (1987) but contrast with crop uptake results reported by Gilliam and Hoyt (1987). The greater turnover of MBN under NT (Table 5) is consistent with findings that showed that NT surface soils have higher levels of microbial biomass, organic N, and mineralizable N and recycle N more efficiently than ploughed soils (Doran, 1987; House et al., 1984; Salinas-Garcia et al., 1997). However, more NO3 was found in the 15- to 60-cm depth of soil at sowing under CT compared with NT even though soil NO3 tended to be higher under NT at the prior harvest. This may be due to (i) stimulation of mineralization of soil and crop residue N in CT soil by autumn tillage (Dowdell and Cannell, 1975), (ii) greater loss of N by denitrification during spring snowmelt under NT (Aulakh et al., 1984), and/or (iii) greater immobilization of N in microbial biomass between autumn and spring with NT management (Fig. 3).

As the wheat crop approached maturity, more N was recovered from the NT soil–plant system than from the CT system (Table 2 and Fig. 1), suggesting that utilization of N was less efficient in the CT system, more N may have leaked from the CT system, or both. The NT wheat gained an average of 23 kg N ha-1 between soft dough and maturity compared with a gain of only 5 kg N ha-1 for the CT wheat (Table 2). The CT continuous wheat had 3.7 kg ha-1 less N at maturity than at soft dough. Nitrogen loss from plants between anthesis and maturity have been reported either as gases from aboveground growth or through rhizodeposition (Wetselaar and Farquhar, 1980; Janzen, 1990).

For the current study, N fertilizer recommendations were based on soil NO3 in autumn samples without regard to tillage practices. The results suggest that tillage practices must be reckoned with as growing-season mineralization and crop utilization of soil N under NT were considerably greater than those under a CT system. Conversely, N mineralization between autumn and spring was greater under CT (Fig. 1). Also, N fertilizer rates were based on NO3 in the 0- to 60-cm depth only, whereas substantial amounts of NO3 were found in the 60- to 100-cm depth, especially under NT management. Therefore, these rates were likely overestimated. As part of a better N management package, soil testing for NO3 should extend to the 100-cm depth, especially for coarse-textured soil.

Previous Crop Effect
The higher NO3 content in the surface soil layer under continuous wheat at maturity (Fig. 2) is attributable to accumulation of excess NO3 in 1994, which the wheat crop could not utilize because of a summer drought, and was not a previous crop effect. This accumulation reduced the average amount of fertilizer N added subsequently in 1995 and 1996 (Table 5) because the continuous wheat treatment was not rotated and used only a single set of plots.

Both field pea and red clover resulted in greater N uptake by the sequent wheat crop than continuous wheat throughout the growing period. Crop uptake of N from various labile pools was nearly similar among the previous crop treatments (Table 5); the exceptions were higher turnover of MBN where the previous crop was field pea and of residue N where the previous crop was red clover. This suggests that more N was recycled from biotic stocks in legume-based cropping systems compared with continuous wheat.

The red clover green manure returned, on average, 74 kg N ha-1 to the soil in aboveground growth (Table 1). Red clover managed under NT returned approximately 20 kg more N ha-1 than CT red clover, and N uptake by the sequent wheat crop fully reflected this advantage (Table 2). In a previous study on a silty clay loam soil, significantly more mineral N was found in the top 30 cm of soil after red clover growth had been incorporated (Soon, 1998). The tendency for soil mineral N to be higher following a legume is well documented (Evans et al., 1991). Therefore, it is surprising that red clover in the current study had no measurable effect on soil mineral N the following spring (Fig. 2). This may be due to the high variability of NO3 in the experimental soil, which even a log transformation did not reduce sufficiently. It is also possible that a portion of green manure N was mineralized and denitrified and/or leached below the 100-cm sampling depth following percolation of spring snowmelt. At stem elongation of wheat, however, NO3 in the rooting depth of soil following red clover increased by more than 20 kg N ha-1, presumably due to mineralization of residues, compared with no measurable increase in the other crop sequences (Fig. 2).

During the growing season, approximately 18 kg N ha-1 was mineralized from red clover residues compared with none or insignificant amounts from wheat and field pea residues (Table 5). The N mineralized from red clover represented 24% of N in the aboveground biomass, a proportion within the 11 to 27% recovery range reported by Ladd et al. (1981)(1983) for 15N-labelled legume material (Medicago littoralis Rohde ex Lois.) that included roots. A significant portion of the N mineralized from crop residues before the spring sampling was probably immobilized in soil microbial biomass, which increased substantially (averaging approximately 27 kg N ha-1 in the top 15 cm of soil) between autumn and spring. However, microbial N in the spring was not significantly different among previous crop treatments. Bremer and van Kessel (1992b) also found that biomass C and N were at their maximum levels at the time of sowing in May, 50% higher than at experiment initiation the previous autumn.

Total aboveground N uptake by wheat following pea was nearly 10 kg ha-1 more than that of continuous wheat, whereas pea straw returned only 3 to 4 kg ha-1 more N to the soil that wheat straw. However, with a C/N ratio of 48 (assuming a C content of 400 mg g-1), it is unlikely that pea straw contributed significantly to the increased N uptake by wheat. Bremer and van Kessel (1992a) showed that lentil straw was no different from wheat straw as a source of N while lentil grown as green manure more than doubled the soil supply of mineral N. The additional available N following pea may be attributed to its "soil N conservation effect" (Evans et al., 1991), i.e., conservation of soil N due to the pea crop meeting its N requirement mainly by symbiotic N fixation, N contained in root residues and rhizodeposits, or both (Jensen, 1996). Jensen (1996) reported that mineral N derived from root and rhizodeposits 1 mo after crop maturity was twice as high after pea as after barley.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
1. Nitrogen uptake by wheat was higher under NT than under CT. Tillage tended to decrease MBN and had no effect on exchangeable NH4 and SON. Soil NO3 at sowing was higher under CT than under NT, presumably due to stimulation of N mineralization by autumn cultivation. Apparent net N mineralization during the growing season was notably higher for NT compared with CT.

2. Differences in N dynamics between previous crop treatments were smaller than those between tillage treatments. Previous legume crops significantly increased N uptake by the sequent wheat crop due mainly to greater mineralization of N from organic matter and microbial biomass during crop growth compared with wheat monoculture. Red clover green manure returned substantially more N to the soil than field pea; however, N uptake by the sequent wheat crop was only slightly higher.

3. The considerable influence of tillage practices and previous crops on crop and soil N dynamics and availability should be factored in when formulating fertilizer N requirements of sequent cereal crops. Synchrony of N supply and demand is improved under NT.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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