Biochemical Heterogeneity of Soil Components Manipulating Long-Term Organic Carbon Pool Buildup Over Mineral Protection in the Mollisol

Abstract

Improving soil organic carbon (SOC) storage is pivotal to mitigating climate change. Increasing evidence shows that the interaction of heterogeneous constituents with soil minerals critically controls SOC sequestration. However, the influence of long-term fertilization on the distinct allocation of microbial- and plant-derived components in physically separated fractions remains unclear. Indicated by the dynamics of amino sugars and lignin phenols, the retention of microbial necromass and plant residues in four soil particle size fractions were assessed under 30-year chemical fertilization (NPK) and NPK combined with manure applied at low or high rates, respectively.Amino sugars were inherently enriched in the clay fraction, whereas lignin was preferentially accumulated in the sand fractions, regardless of the fertilization regime. Such an allocation pattern was responsible for the dual enrichment of SOC in the corresponding fractions. Fertilization management influences the translocation of heterogeneous components into particle size fractions in different ways. Compared with the unfertilized plot, long-term NPK application enhanced amino sugar accumulation in the separated fractions to the same extent but did not alter SOC and lignin concentrations, implying that the initially improved SOC stability after chemical fertilizer application was primarily attributable to the retention of microbial necromass rather than changes in mineral protection. Comparatively, substantial SOC accumulation after manure application was mainly associated with the enhanced allocation of plant lignin in the sand fractions and the hierarchical migration of microbial necromass from clay to sand. High manure application rates caused microbial saturation in the clay fraction. This was accompanied by the preferential retention of plant lignin in the coarse sand fraction. Thus, mineral-associated protection of soil components was attenuated during the buildup of the SOC pool. The inherent biochemical properties of microbial- and plant-derived components, particularly the decomposability of plant residues, primarily control the long-term accumulation and turnover potential of SOC.

Keywords: soil organic carbon accumulation and stabilization, fertilization regime, particle size fractionation, Microbial necromass, plant lignin, amino sugar