Delayed nitrogen application after straw and charred straw addition altered the hot moment of soil N₂O emissions

The combination of nitrogen (N) fertilization and straw incorporation has complex influences on soil N transformations and derived nitrous oxide (N₂O) emissions. This study aimed to reveal the coupled effects of straw returning forms and N fertilizer management on soil N₂O emissions. Here, carbon (C) sources with varied availabilities were supplied by different straw returning forms, including straw, charred straw, and combined application of straw and charred straw. The time gaps between additions of exogenous N and C sources were carried out by managing the timing of N fertilization, including N application simultaneous with or delayed after straw return. Soil N₂O emissions under different straw returning forms and their response relationships to soil C, N, microbial biomass, as well as soil pH and oxygen (O₂) were explored. Co-application of straw and N fertilizer provided sufficient C and N sources, enhanced the microbial biomass, and consequently increased N₂O emissions. Delayed N fertilization could decouple the response of N₂O emissions to straw addition, because of constrained N availability and limited pH decline, which decreased the cumulative N₂O emissions significantly. Possibly due to straw induced N immobilization coupled with charred straw induced pH elevation, the combined application of straw and charred straw constrained soil N₂O emissions compared with straw only application, regardless of N fertilizer management. By shaping the microbial biomass and soil C, N, and O₂ dynamics, delayed application of N fertilizer could further enhance the inhibition effect of the charred straw and straw combination on soil N₂O emissions. Therefore, the varied straw returning forms and the timing of N fertilization could affect the supply of available C and N, influencing the hot moments of N₂O emissions, and the joint addition of straw and charred straw with a few weeks delayed N application could possibly reduce the risk of soil N₂O emissions from straw return systems. Such mitigation potential should be evaluated further under field conditions. Highlights: Delayed N fertilization attenuated the magnitude of N₂O emissions after straw additions. Combination of charred straw with straw mitigated N₂O emissions, further enhanced by delayed N fertilization. Soil pH was the chief regulator for N₂O emissions under different straw returning forms and N application.