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SOIL AND CROP MANAGEMENT

Grass and Legume Cover Crop Effects on Dry Matter and Nitrogen Accumulation

^a Dep. of Soil Sci., Univ. of Nairobi, P.O. Box 30197, Nairobi, Kenya
^b Faculty of Agric. Sci., Univ. of Br. Columbia, 2357 Main Mall, Vancouver, BC, Canada V6T 1Z4

Corresponding author (biofix{at}arcc.or.ke)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Careful cover crop management during the spring growth period may allow farmers to maximize dry matter (DM) yield and N accumulation for the subsequent crop. A 2-yr study was conducted to determine the effect of grass and legume cover crops on spring DM production and N accumulation. Each year, cover crops were planted in late August and late September on a loamy, mixed, mesic Humaquept in the Fraser River Delta. Wheat (Triticum aestivum L.), rye (Secale cereale L.), and ryegrass (Lolium multiflorum L.) were planted in monoculture and in mixtures with crimson clover (Trifolium incarnatum L.). Other treatments included pure stand of crimson clover and wheat–hairy vetch (Vicia villosa Roth.) mixture. Cover crop biomass was sampled three times in 1995 and four times in 1996 during the spring growth period. Dry matter accumulation of early planted cover crops increased by 26 to 269% during the spring growth period, ranging between 0.6 Mg ha^-1 for clover and 10 Mg ha^-1 for wheat, wheat–clover, and wheat–vetch treatments. Late-planted cover crops produced between 15 and 75% lower DM yield compared with early planted cover crops. Nitrogen accumulation increased by 3 to 74 kg ha^-1 for early planted crops and by 3 to 47 kg ha^-1 for late-planted crops. Nitrogen accumulation at final spring sampling ranged from 44 to 144 kg ha^-1 for early planted crops and from 10 to 99 kg ha^-1 for late-planted crops. The low C/N ratio of wheat–vetch treatment compared with wheat monoculture at final sampling indicated the potential for vetch to increase the N content of the mixture.

Abbreviations: DM, dry matter

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
COVER CROPS CAN HAVE AN IMPORTANT ROLE in N management, especially in areas that have high levels of winter precipitation, which can leach mobile nutrients such as NO^-₃ from soils during the winter season (Kowalenko, 1987). Out of an average annual precipitation of 1000 mm in south coastal British Columbia, nearly three-fourths of it occurs from October to April. Grass cover crops can use a significant amount of residual soil and fertilizer N in the fall during the period of rapid establishment (Meisinger et al., 1991). However, grass cover crops pose a risk of short-term N immobilization because of their wide C/N ratio (Ranells and Wagger, 1996). Legumes begin active growth in the following spring and can benefit a grass-based cover crop system during the spring growth period by increasing the N concentration of the cover crop mixture through biological N fixation and minimizing the potential for short-term N immobilization (Ranells and Wagger, 1997).

In cool northern climates such as Ontario (Tollenaar et al., 1993), Washington (Kuo et al., 1996, 1997), and Maine (Griffin et al., 2000), rye can produce a substantial amount of DM as a winter cover crop, generally from 2.5 to 5.5 Mg DM ha^-1. However, the tissue N concentration is commonly 10 to 15 g kg^-1 or less, especially after seed head emergence, so the total N accumulation may be low. Griffin et al. (2000) reported that the total DM for rye and rye–vetch mixture ranged from approximately 3.7 to 6.9 Mg DM ha^-1 just before incorporation in late May. The total DM for rye and rye–vetch treatments was similar to that found in more southern locations, including Maryland (Clark et al., 1994; Shipley et al., 1992), North Carolina (Ranells and Wagger, 1996), and Georgia (McVay et al., 1989), demonstrating that these cover crops are well adapted to a moderately cool growing environment.

Traditionally, nonlegume cover crops have been allowed to grow until the summer crop is planted, often resulting in residues with wide C/N ratios >25:1 (Ditsch and Alley, 1991). The C/N ratio of the residue has been shown to affect N availability to the following crop (Ranells and Wagger, 1997). The authors reported that the N recovery values for 2-yr corn (Zea mays L.) ranged from 14 to 21% of ¹⁵N labeled crimson clover compared with only 4% from ¹⁵N rye monoculture residue. The low corn N recovery from the rye monoculture was attributed to the relatively high C/N value of this residue. The date of termination of cover crop growth affects the cover crop DM yield, N content, and C/N ratio. Wagger (1989a) reported an increase in the DM yield of 2000 kg ha^-1 by hairy vetch from late April to early May in North Carolina. In a study to determine the optimum spring kill date in Maryland, Clark et al. (1995) reported that the biomass of hairy vetch increased significantly from 2800 to 4630 kg ha^-1 between early April to mid-May, and the associated N content increased from 96 to 149 kg ha^-1. Most spring growth of hairy vetch occurs between 15 March and 15 May, and corn is usually planted during the last half of this period. Therefore, the period of maximum cover crop growth and normal corn planting dates overlap, providing management options to producers using legume cover crops. Although hairy vetch and cereal rye accumulate more aboveground DM and N when the spring kill date is delayed, the C/N ratio of rye increases rapidly during this period, often exceeding 25:1 (Wagger, 1989b; Sullivan et al., 1991). Clark et al. (1997) reported that the C/N ratio of hairy vetch was constant (9:1–11:1) across spring kill dates from late March to early May in Maryland. The C/N ratio of the vetch–rye mixture ranged from 11 to 51. By early April, the rye C/N ratio exceeded 25:1, which is a threshold between net mineralization and immobilization (Allison, 1966). In North Carolina, the DM production of crimson clover, averaged over 2 yr, increased from 2.3 to 5.6 Mg ha^-1, and the N concentration declined from 30.2 to 21.2 g kg^-1 as crimson clover matured from the late vegetative to early seed set growth stages (Ranells and Wagger, 1992). Therefore, knowledge of the cover crop DM yield and N accumulation patterns in the spring is important to maximize residue N availability to the following summer crop. This information may be useful in cover crop management during the early spring to better synchronize cover crop N release and N demand by the summer crop.

There is little information on the best management practices for integrating various types of cover crops into the common crop rotations in the Fraser River Delta that include pea (Pisum sativum L.), bean (Phaseolus vulgaris L.), potato (Solanum tuberosum L.), corn, and small grains. In particular, cover crop species need to be identified that provide satisfactory biomass production for green manuring while supplying N to the succeeding crops. Because the DM yield and N accumulation of cover crops vary from one region to another depending on the climate and crop and soil management conditions, there is a need to determine the DM yield and N accumulation of cover crops under the soil and climatic conditions of the Fraser River Delta. The objective of this study was to determine the effect of grass and legume cover crops, grown alone and in mixtures, on spring DM and N accumulation.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Sites
A 2-yr experiment was conducted on farmers fields in Westham Island in the Fraser River Delta, 30 km south of Vancouver, British Columbia. Different sites were used each year but all were located on westham silty clay loams (loamy, mixed, mesic Humaquept) classified as Rego humic gleysols in the Canadian System of Classification. Selected physical and chemical properties of the surface 20 cm of the soil at the beginning of each experiment are reported in Table 1. Air temperature and precipitation measurements were obtained from the Environment Canada station at Vancouver International Airport, approximately 10 km from the study sites.

Aboveground cover crop samples were hand-clipped at the soil surface from two 0.25 m² quadrats per plot. At each sampling date, subsampling locations were randomly selected, and each location was only sampled once during the season. The biomass sample was hand-separated into component species to estimate the proportion of legumes in the species mixtures. The samples were dried at 65°C, weighed, ground to 1-mm, and analyzed for total N and total C.

Laboratory Analysis
The total N of the plant tissue was determined by the method described by Parkinson and Allen (1975). The total C was determined by combustion using a LECO CR analyzer (Leco Corp., St. Joseph, MI). The particle size was determined by the hydrometer method and the pH and electrical conductivity were determined in 1:1 soil/water suspension and 1:1 soil/water extract, respectively. The cation exchange capacity was measured using neutral 1 M ammonium acetate (NH₄OAc) (Rhoades, 1982). The available P was determined using Bray P-1 extract (Olsen and Sommers, 1982). The exchangeable K was determined according to the procedure by Thomas (1982). The total inorganic N was determined on moist samples by extraction with 2 M KCl at an extraction ratio of 1:10 wt./vol. (Keeney and Nelson, 1982). Soil extracts were analyzed on a Lachat autoanalyzer (Lachat Instruments, Milwaukee, WI).

Statistical Analysis
Analysis of variance was performed using the SAS General Linear Models (GLM) procedure (SAS Inst., 1988). Because data in the spring were collected from the same experimental units, the data for DM, N concentration, N content, and C/N ratio were analyzed as a split-plot with the cover crop treatment as the main plot and the sample date as the subplot (Snedecor and Cochran, 1980; Zar, 1984). Appropriate LSD values were calculated for the cover crop, sampling date, or their interaction when the F-value was significant at 0.05 level of probability.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The mean monthly air temperature and precipitation recorded for the 1994–1995 and 1995–1996 seasons are shown in Fig. 1. The mean monthly temperature in November 1995 was 3°C higher than in November 1994 and 2°C above normal. The mean monthly temperature in December 1995 was 1.2°C above normal. In 1995, the mean monthly temperature in January and February was 1.2°C above normal while in January and February 1996, it was 1.2°C below normal.

View larger version (33K): [in this window] [in a new window]   Fig. 1. (a) Monthly mean air temperature and (b) monthly precipitation at Vancouver International Airport (source: Environment Canada, Climate Services Vancouver, and Canadian Climate Normals for British Columbia)

Dry Matter Yield
There were significant differences in the DM yield among cover crops at each sampling date for both early and late-planted cover crops in 1995 and 1996 (Table 4). On 30 Mar. 1995, the early planted cover crops had accumulated DM yields greater than 4 Mg ha^-1, with the exception of crimson clover, which had a significantly lower DM yield (2.3 Mg ha^-1) (Table 4). The DM yields increased by 2 to 43% from the 30 March to 13 April dates. At the third sampling on 27 April, the cover crop DM yields had increased by 26 to 113% compared with the first sampling date. Wheat, wheat–clover, and wheat–vetch treatments had accumulated >9 Mg DM ha^-1 at the third sampling. The late-planted cover crops exhibited a similar trend in DM production although the amount of DM produced was 15 to 59% lower than for the early planted cover crops. The increase in DM yield between the first and third sampling ranged from 31 to 71%. Crimson clover accumulated less DM than the other cover crops at all sampling dates. In 1996, the DM yield of all treatments increased with each subsequent sampling, and the early planted cover crops accumulated more DM than the late-planted treatments. At the fourth sampling on 7 May, the cover crop DM yields had increased by 114 to 269% compared with the first sampling date. The mean DM yield of the late-planted cover crops at the fourth sampling was 37 to 75% lower than for the early planted cover crops. Late-planted clover accumulated an insufficient amount of DM to be sampled between the first and third sampling. By the fourth sampling on 7 May, both early and late-planted clover had accumulated significantly less DM than all of the other cover crops.

Nitrogen Concentration
Differences in N concentration were observed among cover crops at each sampling date for early and late-planted cover crops in 1995 and 1996 (Table 5). In 1995, early planted clover had a significantly greater N concentration than all of the other cover crops at all sampling dates; values ranged between 20 and 25 g N kg^-1 (Table 5). Late-planted clover had a greater N concentration than all of the other cover crops at the second and third sampling dates; values ranged between 21 and 28 g N kg^-1. The N concentration of the early and late-planted clover and the late-planted wheat–vetch mixture increased from the first to the third sampling date, probably due to increased N fixation by the legume. The N concentration in all of the other cover crops decreased with each subsequent sampling, possibly due to the dilution effect of greater DM (Wagger, 1989b). In 1996, the N concentration of early planted cover crops decreased from the first to the fourth sampling; values ranged from 25 to 36 g N kg^-1 at first sampling and from 10 to 18 g N kg^-1 at the fourth sampling (Table 5). Crimson clover exhibited a slower rate of decreasing N concentration compared with all of the other cover crops, especially between the first and third sampling. All of the late-planted cover crops, with the exception of ryegrass and wheat–clover and wheat–vetch mixtures, depicted a trend of an increase in the N concentration between the first and second sampling and a subsequent decrease in N concentration between the second and fourth sampling. There is a possibility that the N concentration may have been underestimated at the 26 March sampling date because the cover crops were just recovering from partial winterkill, and the samples collected contained some amounts of winter-killed plant tissue. Ryegrass and wheat–clover mixture showed a continuous decrease in the N concentration with each subsequent sampling; values declined from 16 and 21 g N kg^-1 at the first sampling to 8 and 11 g N kg^-1 at the fourth sampling. The N concentration of the wheat–vetch mixture remained unchanged between the first and second sampling, but it decreased between the second and fourth sampling from 19 to 15 g N kg^-1. Generally, in 1996, the early planted cover crops had higher N concentrations at the beginning of spring sampling, probably due to greater N uptake in the fall, and then exhibited a rapid decline compared with 1995.

The C/N ratios were highest for the grass monocultures and lowest for the crimson clover monoculture in both years. High C/N ratios were associated with higher DM accumulation, perhaps due to a greater N dilution effect of greater DM. The C/N ratio of plant residues has frequently been used as a tool for predicting the rate of decomposition. Carbon/N ratios of 25 to 30 have been suggested as the threshold between net mineralization and immobilization of N (Allison, 1966). The clover, rye–clover, and wheat–vetch treatments in 1995 and the clover treatment in 1996 had C/N ratios <30:1 by the last sampling date, indicating that net mineralization was likely to occur upon their decomposition. The consistently lower C/N ratio of the wheat–vetch treatment at the final sampling dates compared with the wheat monoculture indicates the potential of vetch to increase the N content of the mixture.

Legume Proportions in Cover Crop Mixtures
Legume proportions (%) in the mixtures fluctuated considerably throughout the sampling periods in 1995 and 1996 (Table 8). In 1995, the legume proportions in the mixtures ranged from 4.8% at the first sampling to 12.4% at the third sampling for the early planted crops and from 9.1 to 21.2% at the third sampling for the late-planted crops (Table 8). In 1996, the legume proportions were low compared with 1995, perhaps due to the poor growth of crimson clover. The legume proportions ranged from 0.8% at the first sampling to 11.7% at the third sampling for the early planted crops from and 5.5% at the first sampling to 35.6% at the third sampling for the late-planted crops. Generally, the late-planted cover crop mixtures had a greater legume proportion than the early planted cover crop mixtures. This could be attributed to the relative growth pattern of grass and legumes as well as the competition within the mixtures. The rapid establishment of winter annual grasses in the fall when soil N is still adequate can favor grass dominance of the mixture by wintertime. In the spring, when legumes begin rapid growth, the well-established grass can outcompete the legume, resulting in a low legume DM yield. Ranells and Wagger (1997) reported that the more rapid fall establishment and early winter growth of rye resulted in greater spring DM accumulation in rye and rye–crimson clover treatments than in crimson clover and fallow treatments (3.35 vs. 0.74 Mg ha^-1). In contrast, legume species in late-planted mixtures will likely compete better with grasses because the grass species is not as well established as in the early planted treatments. Early planting provides an opportunity for more rapid root extension, resulting in greater soil exploration and increased plant N uptake. This implies that late-planted grasses may not be as effective as early planted grasses for scavenging N. The consistent increase in the vetch proportion in the wheat–vetch treatment in the spring of 1995 and 1996 for both early and late-planting dates indicated that vetch may have grown more vigorously than crimson clover. Ranells and Wagger (1996) made similar observations with rye–crimson clover and rye–hairy vetch mixtures. This may relate to differences in the growth habits of the two legume species when grown as mixtures, and also in this study, adaptation to the soil and climatic conditions in the Fraser River Delta. Hairy vetch has a viney growth habit. Consequently, the winter wheat canopy structure in the wheat–vetch treatment provided an excellent scaffold for hairy vetch to grow up into the canopy and intercept a greater percentage of light compared with the more limited exposure to light of crimson clover.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	ACKNOWLEDGMENTS

 
The authors thank the members of the Delta Farmers Conservation Group, specifically Gordon Ellis and Rod Swenson for their cooperation during this study.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This study forms part of a research project conducted by the University of British Columbia Soil and Water Conservation Group and the Delta Farmers Soil Conservation Group. Funding was provided by the Canada–British Columbia Soil Conservation Agreement, and the cost was shared equally by the governments of Canada and British Columbia Ministry of Agriculture, Fisheries, and Food.

Received for publication January 4, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Odhiambo, J. J.O. Articles by Bomke, A. A. Search for Related Content PubMed Articles by Odhiambo, J. J.O. Articles by Bomke, A. A. Agricola Articles by Odhiambo, J. J.O. Articles by Bomke, A. A. Related Collections Crop Rotation Systems Other Crop Management Crop Physiology & Metabolism Other Cropping Systems Other Soil Management