Organic substitution stimulates ammonia oxidation-driven NO emissions by distinctively enriching keystone species of ammonia-oxidizing archaea and bacteria in tropical arable soils.

Partial organic substitution (POS) is pivotal in enhancing soil productivity and changing nitrous oxide (NO) emissions by profoundly altering soil nitrogen (N) cycling, where ammonia oxidation is a fundamental core process. However, the regulatory mechanisms of NO production by ammonia oxidizers at the microbial community level under POS regimes remain unclear. This study explored soil ammonia oxidation and related NO production, further building an understanding of the correlations between ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) activity and community structure in tropical arable soils under four-year field management regimes (CK, without fertilizer N; N, with only inorganic N; M1N1, with 1/2 organic N + 1/2 inorganic N; M1N2, with 1/3 organic N + 2/3 inorganic N). AOA contributed more to potential ammonia oxidation (PAO) than AOB across all treatments. In comparison with CK, N treatment had no obvious effects on PAO and lowered related NO emissions by decreasing soil pH and downregulating the abundance of AOA- and AOB-amoA. POS regimes significantly enhanced PAO and NO emissions relative to N treatment by promoting the abundances and contributions of AOA and AOB. The stimulated AOA-dominated NO production under M1N1 was correlated with promoted development of Nitrososphaera. By contrast, the increased AOB-dominated NO production under M1N2 was linked to the enhanced development of Nitrosospira multiformis. Our study suggests organic substitutions with different proportions of inorganic and organic N distinctively regulate the development of specific species of ammonia oxidizers to increase associated NO emissions. Accordingly, appropriate options should be adopted to reduce environmental risks under POS regimes in tropical croplands.