Nitrogen fertilization offsets the N2O mitigating effects of cover‐crops and double‐crop soybean in a wheat–sorghum system

Abstract

AbstractCover crops (CCs) have been promoted as a key strategy to reduce nitrous oxide (N2O) emissions by scavenging excess mineral nitrogen (N) after cash crop harvest. Optimum N fertilizer application rates are also critical to minimize N2O emissions. Integrating these strategies requires research to understand how they interact to drive N2O emissions. The objective was to evaluate N2O emissions from winter wheat (Triticum aestivum L.) harvest through the subsequent fallow period and grain sorghum [Sorghum bicolor (L.) Moench] production phase of a long‐term, no‐till winter wheat–grain sorghum–soybean [Glycine max (L.) Merr] cropping system. Fallow management treatments included chemical fallow (CF), double‐crop soybean (DSB), and two CCs: sorghum‐sudangrass (Sorghum bicolor × sorghum bicolor var. sudanese) and daikon radish (Raphanus sativus L.). Nitrogen fertilizer rates (0, 90, and 180 kg ha−1) were applied within 1 wk after sorghum planting. Soil nitrate‐N was consistently greater in CF than in the other treatments during the fallow period. Although, on average, CCs and DSB decreased cumulative N2O emissions (cN2O) by 1.75‐fold, during the fallow period compared to CF, they significantly increased cN2O in the following sorghum crop. Emissions increased with greater applications of N regardless of how the fallow was managed, indicating that N fertilizer application in the following sorghum tended to offset any potential mitigating effects of CCs and DSB. These results highlight the importance of evaluating N2O emissions for the system (fallow + following cash crop) to account for potential tradeoffs that could offset cN2O mitigation when N fertilizer is applied.