Snapp, S. S., B. Wilke, L. Gentry, and D. Zoellner. 2017. Compost legacy down-regulates biological nitrogen fixation in a long-term field experiment. Agronomy Journal 109:2662-2669.

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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 15N 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.

DOI: 10.2134/agronj2017.03.0152

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Living Field Lab

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