Linked Crop Production and Soil Organic Matter Impacts of Winter Annual Legumes in Upper Midwest Organic Agroecosystems.

Our experimental objectives were: 1) assess viability of fall-planted, winter annual legume cover crops in the upper Midwest, 2) quantify contribution to labile soil C pools, and 3) improve understanding of C decomposition and N mineralization in organic agroecosystems. The experiment compared two hairy vetch genotypes (V1 and V2), winter rye (RYE), red clover (CLO), vetch/rye biculture (MIX), and bare-ground control (noCC), in randomized 4-block design at two sites - southwest (SWROC) and north-central (NCROC) Minnesota. Cover crops were established in fall and terminated in spring prior to sweet corn planting. Aboveground biomass and soils were sampled at termination. Soils were sampled again two weeks after termination. We determined microbial biomass C, permanganate-oxidizable C (POX-C), size-fractionated particulate organic matter (POM-C), total C, and extractable soil N via 7-day anaerobic potentially mineralizable N (PMN) assays. We hypothesized 1) legumes would increase labile soil C and N levels compared to noCC and RYE treatments, 2) cover crop biomass N would provide significant N for crop production, and 3) increased labile soil C would be correlated with organic N mineralization. V1 and V2 biomass ranged from 1300 Ð 2000 kg dry matter ha-1, resulting in 60-90 kg N ha-1. KCl extractable N increased after termination at both sites, approaching 100 mg N kg soil-1 in vetch at NCROC. At NCROC, post-termination PMN in all cover-crop treatments was > 200 mg N kg soil-1, increasing from ~ 50 mg N kg soil -1 pre-termination. Soil C indicates time effects yet no treatment differences for POX-C nor microbial biomass C. Microbial biomass C was correlated with PMN (PearsonÕs correlation = 0.24, p = 0.002), indicating labile C may be effective indicator of N dynamics. V1 and V2 corn yield was about 15 mg ha-1, approaching state conventional average (16 mg ha-1).