Microbial drivers of N₂O mitigation in soils: Role of nosZ II subclades under conservation tillage and stover mulching

Conservation tillage is widely promoted to enhance soil health and mitigate greenhouse gas emissions, yet the microbial mechanisms driving nitrous oxide (N₂O) reduction in black soils remain poorly understood. This study investigates the functional role of nos Z II and its subclades in comparison to conventional nosZ I in soils subjected to three distinct tillage regimes. Utilising data from a long-term conservation tillage experiment in China, initiated in 2007, we integrated nosZ II, nos Z I and 16S rRNA amplicon sequencing of soils with microcosm incubation and physicochemical/genetic analyses to unravel the regulatory mechanisms of nos Z II and its subclades in relation to N 2 O emissions and the soil nitrogenome under no tillage (NT) with stover mulching. The results indicate that NT without stover mulching (NT0) resulted in the highest N 2 O emissions, whereas NT with full stover mulching (NT100) exhibited lower emissions than CT. Tillage practices, microcosm treatments and their interactions significantly influenced greenhouse gas emissions and the soil nitrogenome. The gene copy number of nosZ II generally increased in response to NT, stover mulching, higher soil moisture and nitrogen/carbon (N/C) fertilization, although shifts in nosZ II subclades varied considerably. The nosZ II B/E/G and other subclades appear to be key players in N₂O consumption, despite challenges in determining their individual contributions. Path analysis revealed that NT100 fostered deterministic selection of nosZ II taxa via enhanced soil moisture and organic carbon, thereby decoupling N₂O emissions from nitrification activity. This indicates that the enriched nosZ II community acts as an efficient sink, immediately consuming the N₂O produced from nitrification, thereby preventing its escape to the atmosphere. Our findings demonstrate that residue retention—not merely tillage cessation—is critical for steering microbial N₂O sinks, providing scientific evidence to guide conservation tillage and residue-management strategies in the black soil region. • No-till with full stover mulching (NT100) significantly reduces N₂O emissions. • nosZ IIB, IIE, IIG are key microbial drivers of N₂O mitigation under conservation tillage. • NT100 promotes deterministic selection of nosZ II taxa via enhanced soil moisture/organic carbon. • Stochastic processes dominate nosZ II community assembly in CT and NT0. • Residue retention is critical for enriching N₂O-reducing microbes and sustaining soil health.