Contrasting Land-Use Legacies Reorganize Soil Resource–Function Balance and Alter Potential N2O Emissions Following Tropical Forest Conversion

Loss of soil organic resources after forest conversion is often assumed to constrain microbial functioning; however, resource status and functional potential may respond differently to land-use legacies. We compared soils under natural secondary forest, six-year poultry-integrated agroforestry, and 60-year paddy cultivation derived from the same regional forest type. Soil properties, microbial biomass, hydrolyzable organic N fractions, extracellular enzyme activities, nitrogen (N)-cycling genes, and potential N2O emissions were measured using three independent field replicates per land use. To quantify the alignment between resources and microbial functioning, we separately standardized indicators of retained organic resources (soil organic carbon (SOC), total N, microbial biomass C, and four hydrolyzable organic N fractions) and microbial functional potential (four extracellular enzymes and five N-cycling genes). We then calculated their difference as a study-specific organic resource–microbial activation imbalance index (ORMAI), for which positive values indicate high functional potential relative to retained resources. Relative to the forest, SOC declined by 29.5% under agroforestry and 61.1% under paddy cultivation, while total N declined by 16.6% and 59.7%, respectively. Paddy soil also contained the lowest concentrations of all hydrolyzable organic N fractions, with reductions of 28.3–81.5% relative to the forest. Despite this depleted resource status, paddy soil exhibited the highest activities of all measured C-, N-, and phosphorus-acquiring enzymes and the greatest abundances of AOA-amoA, AOB-amoA, nirK, nirS, and nosZ. It also had a higher (nirK + nirS)/nosZ ratio than the other soils. Paddy soil produced 3.91 and 4.69 times as much cumulative N2O as forest and agroforestry soils, respectively. The resulting ORMAI was strongly positive in paddy soil (2.34) but negative in the forest and agroforestry soils (approximately −1.17). The three systems therefore exhibited distinct configurations: high resource status with intermediate functional potential in the forest, partial resource retention with restrained functional activation under agroforestry, and depleted resource status coupled with high functional and emission potentials in paddy soil. These findings identify resource–function imbalance as an informative dimension of land-use legacy, revealing that soil organic-resource depletion can coincide with enhanced microbial functional and N2O emission potentials rather than constraining them. These results highlight the importance of considering microbial functional potential alongside soil organic-resource status when assessing potential N2O emission risks and developing land-management strategies following tropical forest conversion.