Microbial metabolic capacity regulates the accrual of mineral-associated organic carbon in subtropical paddy soils

In general, the accrual of mineral-associated organic C (MAOC) level is limited by the finite area of reactive minerals. However, microbial metabolism probably affects MAOC accumulation and saturation because microbial necromass C constitutes the primary component of MAOC. To address this gap in current understanding, we used 18O-H2O incubation and biomarker analyses to quantify microbial C metabolism and necromass accumulation respectively, and quantify their relationship with MAOC and saturation levels across gradients of soil organic C (SOC) that ranged from 4.0 to 40.1 g kg−1 in subtropical paddy soils. Results showed that particulate organic C (POC) increased linearly as SOC increased whilst MAOC tended to saturation. Notably, MAOC accumulation showed a significant positive relationship to increases in microbial necromass C indicating that MAOC accumulation prior to saturation was closely linked to the activity of microbial C metabolism. Compared with low C soils, lower microbial growth efficiencies (qgrowth) and slower biomass turnover rates were observed in high C soils. Moreover, the necromass accumulation coefficient (i.e. necromass produced per unit of microbial biomass) was also lower in high C soils than in low C soils. The lower microbial growth efficiency in the high C soils was attributed to greater C/P and higher clay proportion, which decreased microbial biosynthesis and substrate accessibility. Microbial communities showed adaptive shifts in composition to modulate bacterial metabolic activity, with increases in the relative abundance of r-strategists and decreases in the K-strategists. Management strategies for actively increasing the size of the MAOC pool, by bolstering microbial anabolism and necromass accumulation, are essential for increasing the MAOC content in paddy soils where the MAOC is unsaturated. For high C soils, where the MAOC is already saturated, the use of structural and recalcitrant organic materials to increase the size of the POC pool is needed to enhance carbon sequestration efforts.