Decomposition and Nutrient Release of Different Cover Crops in Organic Farm Systems

Cover crops act as green manure adding organic matter to agricultural-soils. For legume green manures to be an effective nitrogen (N) source for organic farming systems, their N release must be in synchrony with crop N demand. The objectives of this study were 1) determine the decomposition rates of three common cover crops (white clover, (Trifolium repens,L) red clover (Trifolium pratense L.) and soybean (Glycine max L) in order to determine when most N was released and its synchrony with subsequent corn crop uptake; 2) we focused on the effect of cover crops on soil N levels. This study was conducted in a certified organic field, near Mead, NE. We assessed the decomposition rates and chemical composition of three cover crops with different incorporation time (fall and spring). Cover crop samples were taken and air dried in fall and spring. Litterbags containing plant samples were buried at a depth of treatment, samples were dug up every four weeks. Soil samples from each experimental unit were taken at the same time. Extracted litterbags were oven-dried and samples were analyzed for biomass fractions (soluble, hemicellulose, cellulose, and lignin) and total C and N content. As conclusion, decomposition rates of five treatments follow the order: white clover incorporated in spring> red clover incorporated in spring> white clover incorporated in fall> red clover incorporated in fall> soybean incorporated in fall. Mass loss, nutrient content, and litter quality were all changed mostly in the first 0-3 months, which indicates that early stage of cover crops incorporation is critical to cover crop management. To describe the decomposition process, asymptotic models are more appropriate. Cover crops killed in spring have a better synchrony with corn uptake curve. After growing season, soil following red clover had greater level of soil nitrate-N. i Acknowledgements I would like to express my deepest appreciation to all those who provided me the possibility to complete this thesis. A special gratitude I give to my committee, Dr. their support and guidance. Thank you to their patient help. The good advice, support and friendship of my colleague, Katja Koehler-Cole, has been invaluable on both an academic and a personal level, for which I am extremely grateful. I would also like to acknowledge with much appreciation staff of our department. Including Jeremy Hiller, who helped me with the technical problems; Karen and Charles Messenger, who drove me to Mead before I got the driver license; …