Biochemical heterogeneity of soil components manipulating long-term organic carbon pool buildup over mineral protection in the mollisol

Increasing evidence shows that the interaction of heterogeneous constituents with soil minerals critically controls soil organic carbon (SOC) sequestration. However, the impact of long-term fertilization on the distinct allocation of microbial- and plant-derived components in physically separated fractions is still uncertain. Using amino sugars and lignin phenols, the retention of microbial necromass and plant debris in soil particle size fractions was evaluated under 30-year chemical fertilization (NPK) application and NPK combined with manure at both low and high application rates. Amino sugars were inherently enriched in the clay, whereas lignin was preferentially accumulated in fine and coarse sand fractions, regardless of the fertilization regime. Compared with unfertilized plot, long-term NPK application enhanced amino sugar accumulation in all the particle size fractions to the same extent (ca. 23 %) but did not alter SOC and lignin concentrations, implying that the improved SOC stability after NPK application was primarily attributable to the preservation of microbial necromass rather than changes in mineral protection. Comparatively, manure applications increased SOC accumulation by 51.4–89.4 %, which was mainly associated with the enhanced allocation of lignin in sand and the hierarchical migration of microbial necromass from clay to sand fraction. High manure rates caused microbial saturation in clay fraction and, simultaneously, the preferential retention of lignin in coarse sand fraction. In conclusion, mineral-associated protection of soil heterogeneous components was attenuated during the SOC pool buildup. The inherent biochemical properties of microbial- and plant-derived components, particularly the decomposability of plant debris, primarily control long-term accumulation and turnover potential of SOC.