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Short-term Nitrogen Mineralization and Soil Microbial Response to the Incorporation of Warm-season Cover Crops in Organic Farming Systems

O'Connell, Suzanne. 2013. North Carolina State University, PhD

Abstract

A survey was developed and distributed to more than 200 self-identified sustainable farmers in the Southern region to determine the level of utilization, current practices, and perceptions related to cover cropping. Eighty-nine percent of participants reported having a crop rotation plan that included cover crops indicating that their use was wide-spread. Soil quality improvements, erosion control, weed management and nitrogen contributions were the most highly rated cover crop attributes. Next, a series of field and laboratory studies were conducted to assess short-term nitrogen mineralization and soil microbial response to the incorporation of warm-season cover crops. We found that all warm-season cover crops resulted in net N mineralization for ~3 months after incorporation and legume-dominated crops had greater potential for N mineralization compared to grasses. Cover-cropped soils demonstrated increased ability to retain soil inorganic N, moderated its release and reduced leaching after an intense precipitation event. Short-term N mineralization from warm-season cover crops was affected by both plant tissue quality and environmental conditions. The soil microbial community appeared to reducet its carbon use efficiency (CUE) during a dry season, resulting in lower microbial N demand and net N mineralization from cover crops with C:N >40:1. The incorporation of cover crop residues stimulated an increase in both soil microbial biomass N and cellulase enzyme activity. In terms of distinguishing among cover crop quality, the most sensitive enzyme was %u03B2-1,4-glucosidase (EC 3.2.1.21) which was also positively correlated with both potential C and N mineralization. Further analysis of residue biochemical composition revealed that percent neutral detergent fiber (NDF), percent hemicellulose and lignin:N ratio in addition to shoot C:N could be used to distinguish between legume-dominated and grass cover crops suggesting their utility to predict differences among mineralization rates of relatively high quality plant residues. The C:N ratio of the free, particulate organic matter (F-POM) soil fraction was different between legume-dominated and grass cover crops and positively correlated with shoot C:N. Overall, we did not find F-POM to be a good indicator of potential N mineralization but it did appear to reflect the properties of the cover crop residues in the weeks to months after soil incorporation.