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

Abstract

Cover crops (CCs) have been promoted as a key strategy to reduce nitrous oxide (N 2 O) emissions by scavenging excess mineral nitrogen (N) after cash crop harvest. Optimum N fertilizer application rates are also critical to minimize N 2 O emissions. Integrating these strategies requires research to understand how they interact to drive N 2 O emissions. The objective was to evaluate N 2 O 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, notill 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 N 2 O emissions (cN 2 O) by 1.75-fold, during the fallow period compared to CF, they significantly increased cN 2 O 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 N 2 O emissions for the system (fallow + following cash crop) to account for potential tradeoffs that could offset cN 2 O mitigation when N fertilizer is applied.

Cover crops support the climate change mitigation potential of agroecosystems

“…The overall conclusion of the meta-analysis is that cover cropping increases N 2 O emissions only to a very small extent and that the variability is very large. For example, compared to a chemical fallow system, Olofsson and Ernfors [ 39 ] identified the highest value with 1.80 kg N 2 O-N ha -1 for oilseed radish, whereas Preza-Fontes et al [ 40 ] identified the lowest emissions with -0.54 kg N 2 O-N ha -1 for a sorghum-sudan-grass-mixture. The result of our calculation based on a systematic literature search also shows only a very small increase in N 2 O emissions from cover cropping.…”