The divergent role of straw return in soil O2 dynamics elucidates its confounding effect on soil N2O emission

The divergent effects of straw returns on nitrous oxide (N₂O) emissions from soil require elucidation of the underlying mechanisms and factors that explain the inconsistency in in-situ conditions. We conducted a field experiment based on a long-term trial under different regimes of nitrogen (N) fertilization and straw management, complemented by laboratory incubation experiments involving visualized O₂ dynamics imaging. In the field trial, we performed hourly basis high-time-resolution measurements of soil matrix oxygen (O₂), N₂O concentrations and fluxes during N₂O “hot moment” events. We found that straw return increased cumulative N₂O emissions by 32.7% under conventional high N input (N_con), but showed no effect on N₂O emission under optimized N input (N_opt). In situ O₂ content and further microcosm experiments with visualized O₂ spatiotemporal distribution suggested that long-term straw return increases porosity and soil O₂ content, which reduced N₂O emission under low N substrate conditions by improving soil pore structure and aeration during “hot moment” events. By contrast, straw return increased N₂O emission via creating short-term O₂ depletion zone and triggering denitrification in anoxic microsites when excess N substrate was available. Although straw return showed inconsistent effects on N₂O emission under different N application rates, it consistently decreased N₂O concentration in the soil matrix during the "hot moment" events, suggesting that straw return increases the transport of the produced N₂O in soil matrix to the soil surface. Our study underscores the multifaceted role of straw return in soil O₂ dynamics, i.e., stimulating O₂ consumption in a short-term microscale of soil, but increasing soil porosity in a long-term mesoscale of soil. This explains the confounding effects of straw management on the production and transportation of soil N₂O in situ and emphasizes the importance of optimized N fertilization for reducing the “hot moment” N₂O emissions when straw is incorporated.