Improved Nitrogen Use-Efficiency in Cover Crop Based Production Systems
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.
Field experiments were conducted over a two-year period in three physiographic regions (Coastal Plain, Piedmont, and Mountains) of the southeastern USA. Four cover crops (rye, wheat, spring oat, and crimson clover), along with a fallow or no cover treatment, were evaluated for their ability to recover soil N following corn harvest. Cover crop measurements included patterns of dry matter and N accumulation from early winter until row crop planting in the spring. Soil sampling for inorganic N determinations coincided with the schedule for cover crop sampling. Cover crop growth was terminated in April/May and corn planted via no-tillage. Corn yield was measured in each of the cover crop based production systems.
Cover crop decomposition was monitored with nylon mesh bags containing plant residue from the respective cover crop treatments. Mesh bags were placed on the surface of corresponding cover crop plots one week after corn planting and retrieved at various intervals during the growing season. The dry weight and residue N remaining in each bag was then used to develop N release curves over time for each cover crop residue. As a corollary to the field decomposition studies, laboratory experiments were conducted to describe cover crop N release under varying moisture and temperature environments.
In general, dry matter and N accumulation by crimson clover lagged behind the small grain cover crops (rye, wheat, and oat) from late fall to early spring at all locations. Relative growth rates among the grass cover crops during this same period were in the order rye > wheat > oat on the Piedmont soil and rye > oat > wheat on the Coastal Plain soil. Wheat accumulated the greatest amount of biomass at the Mountain location. The N content in above-ground cover crop biomass, across all locations, ranged from 22 to 156 lb N/A. Crimson clover typically had the highest N content in above-ground biomass by corn planting. In contrast, soil inorganic N concentrations prior to corn planting were generally lower under grass compared to legume cover crops, suggesting that grasses were more efficient scavengers for residual N in soil.
Field decomposition of cover crop residues indicated that crimson clover released N faster to the subsequent corn crop than the grass cover crops. This faster N release by crimson clover was more evident during the early part of the growing season (4 to 6 weeks).
Laboratory decomposition studies were conducted to assess the dynamics of C and N mineralization from leaves and stems of crimson clover, rye, oat, and wheat. In general, CO2 emission rates were highest for leaves and lowest for stems. The total CO2-C evolved and remaining C after 160 days for leaves and stems of all residues was similar to the amounts predicted from isolated leaves and stems. In contrast, net N mineralized from leaves and stems of oat and wheat was higher than the amount predicted from isolated plant parts. Data from these and other studies will be used to adjust a simulation model of N mineralization. Results from this research illustrate the role winter annual cover crops can play in conserving N within the soil-plant-system while maintaining soil productivity. A relatively simple model describing N release from cover crop residues will enable further streamlining of summer crop N requirements. Finally, this work suggests that a grass legume/ biculture may incorporate the nutrient scavenging abilities of grasses and the N contribution of legume, thereby fostering more efficient nutrient management in cropping systems.