Nongrowing season soil nitrous oxide emissions as influenced by cover crops and fall tillage termination

Abstract

Cropland soil is a major driver of global nitrous oxide (N2O) emissions. In cold climates, nongrowing season (NGS) emissions can be significant due to high fluxes during freeze–thaw (FT) cycles. Cover crops can alter key soil conditions that govern N2O-producing microbial processes, with multiple potential pathways to either increase or decrease N2O production during FT cycles. Cultivating cover crops in the fall to terminate may further disrupt these processes and the overall impact of cover crops on N2O emissions. Yet, few studies have touched on how termination practices of cover crops impact FT emissions over the NGS. Using the flux gradient method to continuously measure N2O emissions from a conventional corn–soybean rotation, we investigated the effects of summer-established cover crops (perennial ryegrass and crimson clover) (with cover crops, +CC; without cover crops, −CC) when terminated by fall cultivation (with fall cultivation, +FC; without fall cultivation, −FC) over a six-month NGS that was characterized by several freezing and thawing periods. Crimson clover cover crop was completely winterkilled, while the ryegrass survived on the +CC−FC field. Total NGS (Nov–Apr) emissions varied nearly 2.5-fold among treatments from 395.1 (−CC−FC) to 978.1 (+CC+FC) g N2O-N ha−1. Compared with the control treatment (−CC−FC), fall cultivation alone (−CC+FC) and cover crops alone (+CC−FC) increased total NGS N2O emissions, and fall cultivation with cover crops (+CC+FC) increased N2O fluxes even more. Careful CC species selection and management are important to avoid elevated NGS emissions.