N2O and CO2 emissions, nitrogen use efficiency under biogas slurry irrigation: A field study of two consecutive wheat-maize rotation cycles in the North China Plain

Reuse of biogas slurry (BGS) could reduce fertilizer application rate because this material is an alternative source of nutrients and irrigation that could replace some of applied fertilizer. However, the environmental impacts of BGS irrigation, such as greenhouse gas emissions (GHG) are poorly understood. We investigated the global warming potential (GWP) and greenhouse gas intensity (GHGI) of BGS irrigation by quantifying the nitrous oxide (N2O) and carbon dioxide (CO2) emissions under different BGS irrigation rates in a two-year wheat-maize rotation field in the North China Plain. We found BGS irrigation did not change the patterns and fluxes of CO2 and N2O emission during the entire wheat-maize cropping cycle. In addition, there were no significant difference between BGS treatments (BSL, BSM, and BSH) and conventional fertilizer treatment (CF) in GWP. Further, no significant difference occurred in GHGI between BSM (33% BGS and 67% groundwater were mixed and irrigated during the wintering period and joining stage of wheat and after maize sowing, 315 kg·N·hm−1), BSH (50% BGS and 50% groundwater were mixed and irrigated during the wintering period and joining stage of wheat and after maize sowing, 477 kg·N·hm−1) and CF (conventional NPK fertilizer, 420 kg·N·ha−1), despite a higher value BSL (33% BGS and 67% groundwater were mixed and irrigated during the wintering period of wheat and after maize sowing, 210 kg·N·hm−1).The BSL and BSM treatments significantly increased the nitrogen use efficiency (NUE) compared to the CF treatment (11.3% for BSL and 15.3% for BSM). Despite the lower NUE in the BSH treatment relative to CF, it was not statistically significant. Therefore, BGS reuse can increase NUE and does not cause environmental impacts. According to the results of GHGI and NUE, it can be concluded that BSM should be recommended as an improved recycling management practice for the wheat-maize rotation systems.