Microbial process-oriented understanding of stimulation of soil NO emission following the input of organic materials.

Although crop residue return increases upland soil emissions of nitrous oxide (NO), a potent greenhouse gas, the mechanisms responsible for the increase remain unclear. Here, we investigate NO emission pathways, gross nitrogen (N)-cycling rates, and associated N-cycling gene abundances in an upland soil following the addition of various organic material under aerobic incubation using a combination of N tracing technique, acetylene (CH) inhibition, and real-time PCR (qPCR) methods. Increased total NO emissions following organic material amendment was attributed to both increased nitrification-derived NO emissions, following increased ammonia-oxidizing bacteria (AOB)-amoA abundance, and denitrification-derived NO emissions, following increased nirS and decreased nosZ abundance. Increasing plant residue carbon (C)/N ratio decreased total NO emissions by decreasing the contribution of denitrification to NO emissions, potentially due to higher proportions of denitrified N emitted as NO than nitrified N emitted as NO. We further propose a novel conceptual framework for organic material input effects on denitrification-derived NO emissions based on the decomposable characteristics of the added organic material. For slowly decomposing organic materials (e.g., plant residue) with insufficient available C, NO-N immobilization surpassed denitrification, resulting in gradual decrease in denitrification-derived NO emissions with an increase in mineralization of plant residue C losses. In contrast, available C provided by readily available C sources (e.g., glucose) seemed sufficient to support the co-occurrence of NO-N immobilization and denitrification. Overall, for the first time, we offer a microbial process perspective of NO emissions following organic material input. The findings could facilitate the improvement of process-orientated models of NO emissions and the formulation of appropriate NO mitigation strategies for crop residue-amended soils.