Biochar modulates microbial- and plant-derived carbon allocation in soil aggregates to enhance organic carbon sequestration in paddy fields under different water management practices

Microbial- and plant-derived carbon (C) constitutes a substantial portion of soil organic carbon (SOC) pools. However, their dynamics under biochar amendment in soil aggregates and roles in SOC sequestration remain unclear, especially across different water management practices. To address these knowledge gaps, we examined the bulk SOC content and the proportions of soil aggregate fractions (large macroaggregates (LM), macroaggregates (MA), microaggregates (MI), and silt + clay (SC)) in paddy fields under four treatments: flooded irrigation without biochar amendment (FI), flooded irrigation with biochar amendment (FIB), controlled irrigation without biochar amendment (CI), and controlled irrigation with biochar amendment (CIB). The distributions of microbial- and plant-derived C in soil aggregate fractions were measured through the analysis of amino sugars and lignin phenols, along with their contributions to SOC. Biochar amendment increased bulk SOC by 17.06 % under CIB and 18.10 % under FIB compared to non-biochar treatments, with divergent mechanisms operating across water management practices. Biochar amendment enhanced plant-derived C contents in the LM, MA, and MI fractions regardless of water management practice, in addition to increased microbial-derived C content in the MI and SC fractions of the CI treatment. However, no significant amendment effect was observed on microbial-derived C contents in the FI treatment. Biochar amendment increased the contribution of plant-derived C to SOC in the MA fraction under different water management practices, while it enhanced the contribution of microbial-derived C to SOC in the MI fraction only under CI. Additionally, linear regression analysis revealed that SOC sequestration was strongly linked to microbial-derived C in MI fractions and plant-derived C in MA fractions. These findings demonstrate that biochar amendment promotes SOC sequestration through water management-dependent mechanisms: preferential accumulation of plant-derived C in MA fractions under both FI and CI, alongside enhanced microbial-derived C in MI fractions specifically under CI. This study provides novel insights into the mechanisms by which biochar amendment enhances SOC sequestration in paddy soils.