Compost Legacy Down-Regulates Biological Nitrogen Fixation in a Long-Term Field Experiment

Biological nitrogen fixation (BNF) is a fundamental process relied on in agriculture, yet few field studies have examined regulation through soil inorganic N feedbacks or considered seasonal effects. In a Michigan long-term field experiment we examined soil labile C and N pools and impact on BNF in two species, over multiple years. The N-15 natural abundance method was used to quantify BNF, with nodulated and non-nodulated soybean [Glycine max (L.) Merr.] isolines and red clover (Trifolium pratense L.). Soil organic C status of plots was consistent with a gradient established through historical inputs: 0.8% (fertilizer management) to 0.9% (+ cover crop) to 1.2% (+ compost + cover crop). The fraction of nitrogen derived from atmosphere (fNdfa) in red clover and soybean over 3 yr was 61 to 58% with compost management, and 71 to 72% with fertilizer. This represented a downregulation of 19% (red clover) and 15% (soybean) in compost and cover crop managed plots, relative to inorganic fertilizer. In addition to management effects, we found that weather markedly influenced the total amount of N fixation over the 3 yr of the study. A mesic season supported vigorous soybean growth and 195 kg N ha(-1) BNF, compared to 95 and 90 kg N ha(-1) BNF in a dry and an excessively wet season. This study found that compost-based management increased pools of labile C and N that internally downregulated BNF, while enhancing soybean yield compared to conventional management.