- Agronomic: corn, oats, rye, wheat
- Crop Production: cover crops, nutrient cycling
- Soil Management: nutrient mineralization, organic matter
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The inherent inefficiency of fertilizer N utilization by corn can lead to a relatively large pool of residual soil N subject to leaching and possible contamination of groundwater supplies, particularly on sandy soils in the southeastern Coastal Plain. The objectives of this research were to: (1) evaluate the potential of several cover crops to capture residual fertilizer N from a corn production system, and (2) study the field and laboratory decomposition of cover crops for the purpose of developing a simulation model to describe N release from cover crops over a wide range of soil and climatic environments.
Field experiments on a Coastal Plain soil (Norfolk loamy sand) served as a basis for characterizing NO3 leaching potential and the subsequent potential of winter annual cover crops (crimson clover, rye, spring oat, wheat, and native weeds) to recover and recycle residual fertilizer N. Two approaches were used to evaluate these N dynamics. The first method established two levels of residual soil N via fertilizer N applied to the previous corn crop, while the second method employed 15N- enriched potassium nitrate applied to microplots immediately prior to planting rye and crimson clover cover crops in early fall. Field decomposition of cover crop residues was monitored by placing air-dried plant material on the soil surface in nylon mesh bags for 1,2,4,8, and 16 weeks. Following retrieval, mesh bag contents were analyzed for dry weight and N concentration. As a corollary to the field decomposition study, laboratory experiments were conducted to determine decomposition rate constants under non-limiting conditions for carbohydrates and cellulose pools of the CERES-N model for cover crops decomposing on the soil surface.
Prior N fertilizer rate for corn profoundly influenced subsequent cover crop DM accumulation both years, indicating the ability of cover crops to increase N accumulation under conditions favoring carryover of fertilizer N. At both residual soil N levels, rye demonstrated the ability to reduce profile soil inorganic N compared to legume cover crops. Soil inorganic N to a 90-cm depth was greater under hairy vetch compared to crimson clover in April (prior to corn planting) and September of both years, reflecting differences in total N content and decomposition dynamics between the two legumes.
Estimates of the subsequent cover crop N pool potentially available to corn by 16 weeks was in the order of crimson clover>rye>spring oat>wheat. Based on a recommended fertilizer N rate of 150 kg/ha for corn grown in the North Carolina Coastal Plain, the percentage of this N requirement met by cover crop N release ranged from 12% with wheat to 51% with crimson clover.
In general, results from the 15N experiment confirmed findings from the unlabeled N experiment. Rye recovery of fall-applied 15N-enriched fertilizer was 35% by the following April compared to 10% by native weeds and only 5% by crimson clover. As a percentage of the total residue 15N, nearly 23% of the crimson clover N was released by corn maturity compared to 14% of the native weed N and 7% of the rye N. Low total N accumulation by rye and crimson clover, however, severely limited actual N contributions to corn growth and yield.
Under controlled environment conditions, the dynamics on C and N mineralization of a mix of leaves and stems was different from the patterns predicted from isolated leaves and stems. Laboratory results indicated a strong interaction between stems and leaves during early stages of decomposition, which may be relevant for predicting N mineralization from cover crop residues. The best predictors for N mineralization were residue C/N ratio and the reciprocal of residue N concentration.
In summary, there are some clear patterns that emerged in these experiments. Results of studies using two approaches to assess NO3 leaching potential and the subsequent cover crop recovery of residual fertilizer N in soil indicated the ability of rye over that of spring oat, wheat, crimson, clover, and native weeds to fill this niche. In situations (environmental stress or pest-related pressures) where low yields of a high-N requirement crop such as corn result in relatively high levels of residual soil inorganic N, rye should be the cover crop of choice for remedial action. With respect to cover crop release of N, crimson clover would serve better in this capacity compared to the grasses evaluated. Therefore, in order to optimize the inherent capabilities of grasses and legumes, a grass-legume biculture may be more appropriate in cover-crop based production systems. Further investigation is warranted on the role of bicultures in soil water and nutrient dynamics.
Finally, if cover crops are to be widely used as a soil management tool, the potential scope of application should be expanded. This expansion might include the evaluation of rye cultivars with respect to nutrient recovery and the use of cover crops to recycle nutrients contained in animal wastes.
With the aformentioned factors in mind, the objectives of this research were to:
1. Evaluate the potential of several cover crops to capture residual fertilizer N from a corn production system.
2. Study the field and laboratory decomposition of cover crops for the purpose of developing a simulation model to describe N release from cover crops over a wide range of soil and climatic environments.