Published in Agron J 91:643-649 (1999)
© 1999 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Agronomy Journal 91:643-649 (1999)
© 1999 American Society of Agronomy
SOIL MANAGEMENT
Winter Rye Cover Crop Following Soybean Under Conservation Tillage
Residual Soil Nitrate
Anabayan Kessavaloua and
Daniel T. Waltersa
a Dep. of Agronomy, Univ. of Nebraska–Lincoln, Lincoln, NE 68583-0915 USA
dwalters1{at}unl.edu
Received for publication July 31, 1997.
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ABSTRACT
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Use of a winter rye (Secale cereale L.) cover crop following soybean [Glycine max (L.) Merr.] has been shown to reduce the soil erosion potential in a corn (Zea mays L.)–soybean rotation system, but little is known about the effect of rye on residual soil NO3–N (RSN). An irrigated field study was conducted for 4 yr on a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll) to compare crop rotation and winter rye cover crop following soybean effects on RSN under several tillage practices and N fertilization rates. Treatments each year were (i) tillage: no-till or disk; (ii) rotation: corn following soybean/rye (Cbr) or soybean/rye following corn (BRc), corn following soybean (Cb) or soybean following corn (Bc), and corn following corn (Cc); and (iii) N rate: 0, 100, and 300 kg N ha-1 (applied to corn). Rye in the Cbr/BRc rotation was planted in the fall following soybean harvest and chemically killed in the spring of the following year prior to corn planting. Each spring, before tillage and N application, RSN was determined to a depth of 1.5 m, at 30-cm intervals. The net spring-to-spring change in RSN between subsequent spring seasons was computed for each plot, and annual aboveground N uptake for rye, corn, and soybean were determined. Rye, rotation, N rate, and tillage significantly influenced RSN in the top 1.5 m of soil. The presence of rye (BRc) reduced total spring RSN between 18 and 33% prior to corn planting in 2 of the 3 yr, compared with the no-rye system (Bc), as rye immobilized from 42 to 48 kg N ha-1 in aboveground dry matter. Recycling of N in high-yielding rye cover crop residues led to an increase in RSN accumulation after corn in the succeeding spring. Up to 277 kg RSN ha-1 accumulated at high rates of N following corn in the Cbr rotation, compared with 67 kg RSN ha-1 in the no-rye system (Cb) in 1992. Regardless of the presence of rye, significant accumulation of RSN occurred following corn in the rotation sequence, while RSN declined following soybean. Less RSN was found in the top 1.5 m of soil under continuous than rotation corn, and disking tended to increase NO-3 accumulation in rotation systems at high rates of N application. Although RSN declines following a rye cover crop, the ready release of this immobilized N suggests that some N credit should be given, reducing N recommendation for corn following winter rye cover, to minimize potential NO-3 leaching under corn–soybean/rye rotations.
Abbreviations: RSN, residual soil nitrate. Rotation sequences: Bc, soybean following corn • BRc, soybean/rye following corn • Cb, corn following soybean • Cbr, corn following soybean/rye • Cc, corn following corn. Uppercase letters denote the crop grown prior to spring soil sampling. Underscoring indicates the crop between soil sampling dates
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INTRODUCTION
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NITRATE ACCUMULATION in the rhizosphere and its leaching loss to ground water are influenced by cropping pattern, tillage, N fertilization rate, and irrigation management practices. Generally, yield potential of corn is increased when it is grown in rotation with soybean (Peterson and Varvel, 1989; Franzleubbers et al., 1994). However, low residue production by soybean and the rapid decomposition of that residue causes 35% more soil loss following soybean than corn (Laflen and Moldenhauer, 1979; Miller et al., 1988). Kessavalou and Walters (1997) examined the feasibility of using a winter rye cover crop following soybean to increase the amount of winter and spring surface residue cover prior to planting and during the seedbed and establishment phase of the corn year. They demonstrated that regardless of tillage management or rate of N application, planting a winter rye cover crop following soybean increased the total surface residue cover by 30% over soybean alone. The combined surface cover obtained from soybean and rye residues was equivalent in cover and persistence to that provided by corn residue. Increased residue cover reduced the risk of soil erosion until development of a protective corn canopy, without adversely affecting corn grain yield.
Research indicates that in addition to affecting surface residue cover, a winter rye cover crop can also influence the dynamics of N cycling (Ditsch and Alley, 1991). Planting a winter rye cover crop following corn in a no-till system reduced spring NO3–N accumulation in soil due to N uptake by rye (Ditsch et al., 1993; Shipley et al., 1992). Early-spring rye is usually killed and its residues returned to the soil before planting corn. When these residues decompose, significant changes in soil NO3–N can occur due to mineralization of N from rye residue. This may cause a build-up of NO3–N in soil and increase the risk of NO3–N leaching at the end of the growing season.
Residual soil nitrate (RSN) testing is also a principal determinant used across the north-central region of the United States to adjust fertilizer N application rates applied to corn (Hergert, 1987). If RSN testing is used to adjust corn fertilization rate, immobilized N in rye residue will not be accounted for in corn N rate adjustments. Little is known about rye residue effects on residual NO3–N when rye follows soybean in rotation. Our objective was to compare RSN accumulation in a continuous corn and corn–soybean rotation with and without a winter rye cover crop after soybean under disk and no-till management with different N application rates.
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Materials and methods
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An irrigated field study was conducted at the Agricultural Research and Development Center at Mead, NE, from fall 1990 to 1993 on a corn–soybean rotation plot established in 1989. The soil type was Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll). Basic soil physical and chemical properties are given in Table 1
, and general climatic conditions are given in Table 2
. The experimental design was a split-split-plot arrangement of a randomized complete block design, with three replications. Main plot treatments were two tillage systems: spring disk or no-till, with one cultivation in both systems for weed control. Subplot treatments included three crop rotations: (i) corn following soybean with a winter rye cover crop (Cbr or BRc), (ii) corn following soybean (Cb or Bc), and (iii) corn following corn (Cc or cC). The study was designed so that each crop in the rotation was growing in every year. The sub-subplot treatment consisted of three N rates applied to corn: 0, 100, and 300 kg N ha-1. The size of a sub-subplot for rotation corn with and without a winter rye cover crop was 12.2 by 3.1 m (4 rows wide); that for continuous corn was 12.2 by 6.1 m (8 rows wide). All the plots were sprinkler irrigated as needed with a linear-move irrigation system, based on rainfall and evapotranspiration rates.
Corn (Pioneer1
hybrid `3189') was planted during the second week of May each year, in 76-cm rows at a rate of 72000 seed ha-1. In 1991, tillage and corn planting were delayed until 21 May, due to heavy spring rainfall. Soybean (`Century 84') was planted each year before 25 May at a seeding rate of 500000 plants ha-1. Each spring, before tillage, N fertilizer was surface-applied to corn as NH4NO3 prills. In 1991, N fertilizer was mistakenly applied to soybean. Cultivation was done at V8 stage for corn and V5–V6 stage for soybean, using a Buffalo till cultivator. Corn and soybean were harvested in the fall of each year. Data collected included corn and soybean grain yield, dry weight of corn, and stover and N content of grain and stover (Kessavalou and Walters, 1997).
In the fall of 1990, 1991, and 1992, immediately after the harvest of soybean, winter rye (`Aroostook') was drilled into soybean stubble in 18-cm rows at a rate of 62 kg seed ha-1. In the spring of each year (7 or 8 May), prior to tillage and N application, rye was hand-harvested at the vegetative stage from five randomly selected 0.09-m2 areas per plot in all tillage x N rate treatments (sub-subplot) for the determination of aboveground dry matter yield and N uptake. Rye was then killed by spraying glyphosate [N-(phosphonomethyl)glycine] at 3 kg a.i. ha-1.
After rye harvest, and before tillage and N application, soil was sampled to a depth of 1.5-m at 30-cm intervals in each replicate of each sub-subplot. Two, 5-cm-diam. soil cores per treatment were collected, composited, and subsampled. Soil samples were air-dried, crushed, and sieved through a 2-mm (10-mesh) sieve. A soil–water (1:2.5) extract was obtained by shaking 10 g of soil with 25 mL of deionized water and 0.2 to 0.3 g of CaCO3 (to obtain turbid-free extract) in a mechanical shaker for 5 min and filtering through a Whatman no. 2 filter paper. Nitrate-N concentration in the extract was determined by the cadmium reduction technique using a Lachat NO3–N analyzer (Keeney and Nelson, 1982). Amount of NO3–N in the soil was then estimated on a volumetric basis, using an average bulk density of 1.3 Mg m-3 for all depths, and expressed as kg NO3–N ha-1.
Net Change in Residual Soil NO3–N
Net change in RSN in each plot from spring of 1991 to 1992, and 1992 to 1993 was computed for each depth: net change in residual soil NO3–N = (RSNi - RSNi-1), where RSNi is residual soil NO3–N, in kg ha-1, of the ith year and RSNi-1 is RSN of the previous year.
Analysis of variance was computed using the general linear model (GLM) procedure (SAS Inst., 1992) to detect differences among treatment means. A probability level of 0.05 or lower was used to declare statistical significance of treatment effects. Since N rates were unequally spaced, orthogonal contrasts for unequally spaced treatment levels were used to test N effects. Also, appropriate contrasts were computed to test the effects of previous crop, N rate, and tillage on RSN.
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Results and discussion
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Rye Dry Matter Yield and N Uptake
Amounts of N removed from and returned to the soil by rye, corn, and soybean at different N rates are given in Table 3
. Rye aboveground N uptake averaged 9 to 60 kg N ha-1 across N rates (Table 3). Rye dry matter yield and N uptake were low in 1992, due to diminished growth and development resulting from cold spring weather conditions. Further details on winter rye cover crop and corn performances are given by Kessavalou and Walters (1997). In general, aboveground N uptake by corn in rotation with soybean (with or without rye) was approximately 25% greater than that by continuous corn. Soybean grain removed 165 to 215 kg N ha-1 per year, across N rates. Amounts of N returned in soybean stover and leaf were determined only in 1990, at 0 and 100 kg N ha-1, and averaged 72 and 95 kg N ha-1, respectively.
Residual Soil Nitrate-N (RSN): Cover Crop Effects on RSN
Winter rye cover crop played a key role in reducing RSN following soybean (Table 4)
. Within-year analysis indicates that planting a winter rye cover crop following soybean led to a reduction in RSN in 2 of the 3 yr, compared with the no-rye rotation system (Fig. 1)
. In spring 1991, a 33% reduction in total RSN occurred, especially in the top 60 cm of the soil, following the winter rye cover crop (BRc), compared with the soybean without rye (Bc). Low residual soil NO3–N following rye was due mainly to N uptake by rye (42–48 kg N ha-1), which accounted for 60 to 95% of the differences in the total RSN (0–1.5 m) observed between the rye and no rye system. Ditsch et al. (1993) reported similar reductions in residual soil NO3–N in a silt loam soil in Virginia due to winter rye in a no-till continuous corn system. No appreciable reduction in RSN was observed following rye in spring 1992, due to poor rye dry matter production. Total RSN in the top 1.5-m soil profile for rye (BRc) and no-rye (Bc) systems (averaged across N rates) was 60 and 90 kg N ha-1 in 1991, 131 and 146 kg N ha-1 in 1992, and 153 and 187 kg N ha-1 in 1993, respectively (Table 5)
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Table 4 Analysis of variance for residual soil NO3–N distribution in the top 1.5 m of soil, at 30-cm intervals, as influenced by rye, rotation, N rate, and tillage in May (1991–1993) at Mead, NE*
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Fig. 1 Distribution of residual soil NO3–N following soybean in the top 1.5 m of soil as influenced by the presence (BRc) or absence (Bc) of a winter rye cover crop in May (1991–1993) at Mead, NE. See Table 4 for means separation statistics
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Table 5 Distribution of residual soil NO3–N in the top 1.5 m of soil, at 30-cm intervals, as influenced by rye, rotation, N rate, and tillage in May (1991–1993) at Mead, NE
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In rotation studies, within-year comparison among treatments generally provides only apparent treatment effects on RSN, because of the inherent differences in initial NO3–N status of individual experimental plots. A more realistic evaluation of the effects of rye and crop rotation on RSN may be obtained by determining the net change in RSN due to treatment in each plot between two growing seasons. For example, in spring 1993, although the apparent difference in total RSN (0–1.5 m) averaged 34 kg N ha-1 between the BRc and Bc system (Table 5), the actual decline in total RSN due to 1993 rye in the rotation plot (difference between Cbr in spring 1992 and BRc in spring 1993) was estimated at 25 kg NO3–N ha-1 (Table 5 and 6)
. A major portion of this reduction (18–26 kg N ha-1) was observed within the 60- to 120-cm soil depth (Fig. 2)
; however, no actual decline in RSN occurred from the 1992 rye crop because of its poor growth and development (Table 5).
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Table 6 Analysis of variance for the net change from May to May in residual soil NO3–N (RSN) in the top 1.5 m of soil, at 30-cm intervals, as influenced by rye, rotation, N rate, and tillage (1991–1993) at Mead, NE*
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Fig. 2 Net annual change from May to May in residual soil NO3–N in the top 1.5 m of soil due to the presence (BRc) or absence (Bc) of a winter rye cover crop following soybean (1991–1993) at Mead, NE. Underscoring indicates the crop between soil sampling dates; e.g., cBR–BRc for 1992–1991 = change in residual soil NO3–N (RSN) due to 1991 soybean/rye from spring 1991 to spring 1992 in the corn–soybean/rye rotation. See Table 6 for means separation statistics
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Although spring soil NO3–N levels declined due to rye N uptake, large accumulations of residual NO3–N in soil occurred in the succeeding spring season. For instance, in spring 1992, significant accumulation of residual NO3–N occurred in soil, especially at the high rate of N application when corn followed rye (Cbr) (Table 4 and Fig. 3)
. From spring 1991 to 1992, RSN accumulations varied from 6 to 277 kg N ha-1 across N rates in soil planted to corn in 1991 following soybean/rye (brC–Cbr), compared with 5 to 67 kg N ha-1 (bC–Cb) without rye (Fig. 4
and Table 5). Presumably, increased RSN was due, in part, to added mineralization of N from rye residues and less N immobilization in the absence of corn residues. A residue decomposition study conducted in the same field during 1991 showed that N was rapidly mineralized from rye residues after desiccation and that net mineralization of N from soybean residue was enhanced in the presence of rye residues (Kessavalou, 1994). Indeed, the average C:N ratio of rye was 15:1, compared with 36:1 for soybean residue and 70:1 for corn residue. Soluble C in rye residue was also three times greater than in corn residue (57 vs. 19%) and more than twice the concentration of soybean residue (27%). It is also probable that early N availability from both rye and soybean residues diminished the fertilizer N requirement of corn and contributed to the increase in RSN. Time of desiccation of rye in spring may play an important role in NO3–N dynamics since the C:N ratio and lignin, cellulose, and hemicellulose content of rye increase with aging (Wagger, 1989). In North Carolina, Wagger (1989) found that delaying desiccation time of winter rye cover crop at flowering stage for 12 d (until the first week of May) led to a significant reduction in the rate of decomposition and mineralization of rye residue. Under our climatic and experimental conditions, rye was at its early boot stage during the first week of May with a chemical composition favorable for rapid residue decomposition. Since corn in this region should be seeded before 15 May to avoid risk of yield loss, little opportunity exists for producing mature residues to reduce the rate of residue decomposition. No post-rye effects on RSN were observed for the 1992 to 1993 period (Fig. 4), due to poor spring rye growth.
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Fig. 3 Residual soil NO3–N distribution in the top 1.5 m of soil as influenced by crop sequence and N application rate to corn in May (1991–1993) at Mead, NE: circles, 0 kg N ha-1; squares, 100 kg N ha-1; triangles, 300 kg N ha-1. The first crop in the sequence (uppercase letters) represents the crop grown prior to soil sampling. See Table 4 for means separation statistics
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Fig. 4 Net annual change from May to May in residual soil NO3–N in the top 1.5 m of soil as influenced by crop sequence and N application rate to corn (1991–1993) at Mead, NE: circles, 0 kg N ha-1; squares, 100 kg N ha-1; triangles, 300 kg N ha-1. Underscoring indicates the crop between soil sampling dates; e.g., bC–Cb for 1992–1991 = change in residual soil NO3–N (RSN) due to 1991 corn from spring 1991 to spring 1992 in the corn–soybean rotation. See Table 6 for means separation statistics
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Effects of Soybean vs. Corn on Residual Soil NO3–N
Soils under corn–soybean rotation (with or without rye) exhibited a general pattern of RSN accumulation following corn and RSN depletion following soybean (Fig. 4). Within rotation systems in general, total RSN in the top 1.5-m of soil declined following soybean (Bc and BRc), but increased significantly following corn (Cb and Cbr). Between spring 1991 and 1992, a maximum of 67 kg N ha-1 of RSN accumulated in the top 1.5 of soil following soybean (with or without rye, cBR–BRc and cB–Bc) at the 300 kg N ha-1 rate (Fig. 4). During the same period, as much as 277 kg N ha-1 of RSN accumulated in the soil following corn in the rotation sequence (brC–Cbr and bC–Cb). Disking also tended to promote NO-3 accumulation in the system (Table 5). Between spring 1992 and 1993, when soybean was in the rotation sequence, the amount of RSN in the top 1.5 m of soil was greatly reduced, especially at the 300 kg ha-1 N rate, where the reduction amounted to 160 kg N ha-1 (Fig. 4). Decline in RSN following soybean in the rotation sequence was due, in part, to the lack of N application to soybean and removal of large amounts of N in soybean grain (Table 2). Higher quantities of RSN accumulation following soybean for the period between spring 1991 and 1992 vs. spring 1992 and 1993 may be attributed to the accidental N fertilization of soybean plots in 1991 (Fig. 3 and 4).
Soils cropped to continuous corn accumulated lower amounts of RSN than soils under corn after soybean (with and without rye), even though the corn was fertilized every year and removed 25% less N in aboveground yield than corn in rotation with soybean. (Fig. 3 and 4). We also noted that in a year of high N demand, such as 1992, RSN was depleted at deeper soil depths under Cb and Cbr than under Cc. In 1991 and 1992, soils under rotation corn (Cb or Cbr) receiving 300 kg N ha-1 built up large amounts of RSN, mostly in the 30- to 120-cm soil depth, compared with monoculture corn (Cc) (Fig. 3). Presumably, a greater degree of N immobilization resulting in a smaller labile N pool under continuous corn was the major reasons for lower RSN values in this treatment. Absence of this effect during spring 1993 (Fig. 3) was probably due to high corn grain yield and N demand which resulted in approximately 28% more N removal by corn during the 1992 growing season.
Tillage Effects on Residual Soil NO3–N
Generally, more NO3–N accumulated in disked than in no-till planted soil, as influenced by crop rotation during spring 1991 and 1992 (Table 5). In spring 1991, corn following soybean accumulated about 100% (79 kg ha-1) more RSN under disk tillage than no-till (Table 5). In spring 1992, corn–following soybean with rye (Cbr) and continuous corn (Cc) accumulated 54% (76 kg ha-1) and 100% (79 kg N ha-1), respectively, more RSN under disk tillage than no-tillage management. A build-up of RSN under the tilled soil may be due to a favorable soil microenvironment created in the surface soil (0–15 cm), which led to an increased net N mineralization of crop residues and soil organic N (Doran and Linn, 1994). In addition, rye dry matter persisted in an undecomposed state for a much longer period of time under no-till (Kessavalou and Walters, 1997)
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Conclusions
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Lower RSN quantity was found in following spring soil sampling when rye was included as a winter cover crop after soybean due to temporary immobilization (uptake) of NO3–N by winter rye. Rye N uptake was nearly equivalent to the observed reduction in spring RSN. As the season progressed, however, N was rapidly mineralized from rye residue. Consequently, greater amounts of RSN accumulated in the soil profile following the next year's N-fertilized corn crop, compared with corn following soybean without a rye cover crop. No-till planting tended to reduce the accumulation of residual soil NO3–N in the rotation system. Spring RSN sampling is a principal determinant used to adjust fertilizer N recommendations to corn in the North Central region of the United States. As RSN sampling precedes the flush of N mineralization following rye desiccation, we conclude that some N credit (
40 kg N per tonne of rye dry matter) should be given to a winter rye cover crop in formulating corn N recommendations much the same as is given when soybean is the previous crop. This practice may lower the risk of RSN accumulation, improve fertilizer N use efficiency and reduce the potential for NO3–N leaching.SAS Institute 1992
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ACKNOWLEDGMENTS
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The authors thank Mr. Gregory J. Teichmeier for his technical assistance in conducting this study and Dr. John Doran and Dr. Dennis Francis for their constructive suggestions on the manuscript. This research was supported in part, by a grant from the National Fertilizer Development Center of the Tennessee Valley Authority (Grant no. TV-71916A), and The Rotary International.
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NOTES
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Contribution of the Univ. of Nebraska Agric. Res. Div. Journal Series no. 11979.
1 Brand names are mentioned for the reader's convenience, and do not imply endorsement over other products that may also be suitable. 
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
