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

Cover 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.