Organic production reduces subsurface nitrate leaching and maintains crop yields in a US Mollisol

Organic production aims to diversify crop rotation and use organic fertilizer sources to build soil fertility and improve soil health indicators. The impact of organic farming on water quality in artificially drained midwestern Mollisols has received limited attention. This 7-year study compares tile nitrate loss and yields under three cropping systems: (1) conventional corn (Zea mays L)-soybean (Glycine max L.), (2) organic corn-soybean-oat (Avena sativa L.)/alfalfa (Medicago sativa L)-alfalfa, and (3) organic perennial pasture. Nitrogen (N) fertilization consisted of sidedress urea ammonium nitrate for conventional corn (168 kg N ha^-1) and spring-applied composted manure for organic corn (168 kg N ha^-1) and oats (56 kg N ha^-1). Overall, the 4-year organic rotation reduced N loads by 50% and pasture reduced loads by 93% compared with the conventional 2-year corn-soybean rotation. Reductions in N loads were related to the diversified cropping system as no difference in N losses was detected when only corn-soybean phases of the organic rotation were compared with the conventional system. Annual variations in N loads were explained by precipitation and varied by crop. Soil health indicators sampled in the fall showed minimal influence on N losses. Organic corn (4 of 7 years) and soybean (6 of 7 years) yields were equivalent to or higher than conventional in most years. Results of this study suggest that organic farming practices that combine use of animal manure and inclusion of small grains, forage legumes, and green manures can improve water quality in artificially drained landscapes while maintaining crop yields.