Management Impacts On GHG Emissions and Yield Performance From Organic Soybean.

Agricultural land has been recognized as a significant source of greenhouse gas (GHG) emissions, however GHG release from soils varies as crop production strategies vary.  This study was conducted to determine the effects of tillage, cover crop, and compost rate on the soil release of two GHG, nitrous oxide and carbon dioxide, under an organic soybean cropping system.  A corn-soybean-wheat organic cropping system was initiated in 2012 in an organic transition field at the University of Missouri Bradford Research Center using a randomized complete block, split plot design with four replications. Each crop treatment variable included tillage/cover crop for the main plot treatment and compost rate for the split-plot treatment. Tillage/cover crop treatments included tilled without cover crop, tilled with cover crop, and no-till with cover crop.  Compost treatments were 0, low, medium, and high rates, based on soil-test phosphorus. GHG were collected in the field and analyzed according to USDA-ARS GRACEnet project protocols.  Gas samples were collected at least once a week during the growing season, and several times in succession after significant events such as rain, irrigation, and cultivation. Preliminary results show no significant correlation between GHG and treatment effects in 2012.  Soybean yields were highest in the tilled plots without cover crop at 3173 kg/ha, and lowest in the no-till plots without cover crop at 2287 kg/ha. Cover crop had an intermediate level of yield reduction compared to the tilled plots.  A medium compost application rate (100 kg P/ha) produced the greatest soybean yield over other compost treatments.  Precipitation in 2012 was 33% less than the average of the previous three years, causing overall yield reduction.  Excessive heat exacerbated soybean water stress. Cover crops seemed to have suppressed yield as well.  However, adjusting compost application rate based on P content appears to benefit adequate nutrition and yields of organically grown soybeans.