Dominant Microbial, Aggregate, and Mineral Pathways of SOC Quantity and Lability: Insights From SOC Functional Groups

Abstract

Organic amendments are widely used to restore soil carbon (C) in Mollisols of croplands, yet how microbial-, aggregate-, and mineral-associated processes are related to soil C quantity and lability remains unclear. Here, we conducted a 3-year field experiment in Mollisols of Northeast China to evaluate manure, biochar, and their co-application effects on soil organic carbon (SOC), easily oxidizable organic C (EOC), SOC functional group composition, microbial network properties, aggregate-associated C, and mineral-associated C. Across the 3 years, biochar increased SOC, manure enhanced EOC, and co-application increased both. These divergent C responses were accompanied by changes in SOC functional groups, with manure enriching carbohydrate-derived labile fractions and biochar increasing SOC hydrophobicity. Random forest analysis identified hydrophobicity, Aryl C, and Carboxyl C as key predictors of SOC, whereas O-alkyl and Di-O-alkyl C best explained EOC variation. Microbial co-occurrence network properties were statistically associated with SOC hydrophobicity and Di-O-alkyl C, while macroaggregate-associated C and mineral-associated C fractions were positively linked to SOC hydrophobicity. Partial least squares path modeling further suggested that SOC accumulation was associated with increases in Aryl C and Carboxyl C, with stronger positive contributions from macroaggregate formation than microbial network properties. In contrast, microbial co-occurrence network complexity showed a negative correlation with O-alkyl and Di-O-alkyl C, indicating a potential linkage between network structure and labile C dynamics. Overall, our findings show that manure-biochar co-application achieved concurrent increases in SOC and EOC, accompanied by greater aggregate- and mineral-associated C fractions and lower microbial co-occurrence network size. This integrated evidence highlights manure-biochar co-application as a practical strategy for improving both soil C storage and labile C availability in Mollisols.