Divergent impacts of partial manure substitution on soil nitrous oxide emissions dependent on phosphorus availability

Manure substitution is increasingly advocated as a sustainable fertilization strategy to reduce synthetic nitrogen inputs in agriculture, although its implications for greenhouse gas emissions, particularly nitrous oxide (N₂O), remain uncertain. In subtropical regions, where phosphorus (P) deficiency is common, the effectiveness of manure substitution in mitigating N₂O emissions under nutrient-imbalanced conditions is poorly understood. Here, we used soils collected from a 21-year fertilization experiment and conducted controlled laboratory incubations with ¹⁵N tracing techniques to examine the effects of partial manure substitution (replacing 30 % of synthetic nitrogen with manure-nitrogen) on N₂O emissions under two fertilization regimes: nitrogen-only (N, 110 kg N ha⁻¹ yr⁻¹) and nitrogen-plus-phosphorus (NP, 110 kg N ha⁻¹ yr⁻¹ + 30 kg P ha⁻¹ yr⁻¹). Our results revealed that manure substitution increased N₂O emissions by 30 % under the N-only regime, but decreased emissions by 22 % under the NP regime. These contrasting responses were closely associated with changes in gross nitrogen transformation processes. Under N-only conditions, manure substitution stimulated autotrophic nitrification and denitrification rates, leading to increased N₂O production. Conversely, under NP conditions, manure substitution reduced N₂O emissions primarily by lowering the contribution of heterotrophic nitrification to total N₂O flux, despite unchanged heterotrophic nitrification rates. This reduction coincided with significant enrichment of nosZ clade II genes. Overall, these findings suggest that the effect of manure substitution on N₂O emissions is highly context dependent, with P availability acting as a key regulator of microbial nitrogen transformation pathways and thereby modulating the direction and magnitude of N₂O responses to organic fertilization in subtropical Ultisols.