Distinct Allocations of Microbial Versus Plant Residues in Mineral Particles Counterbalance the Accumulation and Stabilization of Soil Organic Carbon

Increasing evidence suggests that the interactions between heterogeneous organic compounds and soil minerals play a critical role in controlling soil organic carbon (SOC) sequestration. However, the influence of long-term fertilization on the allocation of microbial- and plant-derived components to physically separated SOC fractions remains largely unknown. As indicated by the biomarker of amino sugars and lignin phenols, the retention of microbial necromass and plant residues in four soil particle size fractions (2000–250 μm, 250–53 μm, 53–2 μm, and < 2 μm) were assessed under four treatments: no fertilizer application (CK), chemical fertilizers (NPK), and synthetic fertilizers combined with manure applied at low (M1NPK) and high rate (M2NPK) over 30 years. The inherent allocation patterns of microbial necromass and plant residues are responsible for the dual enrichment of SOC in the corresponding fractions, contributing to SOC stability and availability, respectively. Compared with CK, long-term NPK application enhanced the accumulation of amino sugars in each fraction to the same extent, but did not alter SOC and lignin despite the increased plant input, indicating that NPK application initially improved SOC stability. This was primarily attributed to the retention of microbial necromass. In comparison, the significant accumulation of SOC after manure application was mainly associated with the enhanced allocation of lignin in sand fractions and the hierarchical migration of microbial necromass from clay to sand fractions. A higher rate of manure application caused a microbial saturation effect in clay fraction and promoted the preferential retention of lignin in sand fraction more than in the microbial nocromass. Thus, the mineral-associated protection of components was weakened during the SOC build-up. Once the protective capacity of clay minerals against microbial necromass reaches a critical value, the inherent biochemical properties of components, particularly the decomposability of plant residues, primarily control the long-term accumulation and turnover potential of SOC.