Nutrient availability mediates organic carbon turnover in paddy soils through regulating microbial metabolism

Nutrient availability significantly influences soil microbial communities and the soil organic carbon (SOC) cycle. Yet, the precise response of microbial metabolism, particularly carbon use efficiency (CUE), to varying nutrient levels, and the subsequent impact on microbial respiration in paddy soils, remains a key area of investigation. In this study, we utilized six paddy soils collected across a latitudinal gradient from Northeast to Southeast China to examine the effects of nutrient availability on CUE, employing the ¹⁸O water labeling approach. We also assessed associated microbial metabolic parameters, including growth, biomass, turnover rate, and extracellular enzyme activity, under different nitrogen, phosphorus, and potassium (NPK) fertilization regimes. Our results revealed that the highest NPK amendment rate (NPK_H) generally led to the greatest microbial growth and biomass, coupled with the slowest microbial biomass turnover rates, especially in the southeastern paddy soils. This enhanced growth and biomass accumulation likely resulted from increased CUE following the alleviation of C and nutrient limitations through fertilization. Furthermore, we observed that reducing these limitations led to decreased extracellular enzyme activity and a corresponding reduction in microbial respiration. This suggests a critical balance between microbial investment in anabolic processes (biomass production) and catabolic processes (decomposition of soil organic matter) in determining CO₂ loss from paddy soils. Overall, our findings indicate that soil microorganisms exhibiting higher CUE, faster growth, larger biomass, but lower CO₂, turnover and extracellular enzyme activity contribute to reduced SOC mineralization and, consequently, limited release. These results suggest that strategic nutrient management in paddy soils has the potential to mitigate C emissions by effectively regulating key aspects of microbial metabolism, specifically by promoting a shift towards biomass production over decomposition.