Macroaggregates magnify the positive feedback of trophic cascades on soil organic carbon accrual

The interactions between nematodes and fungi are important for soil carbon cycling. However, their cascading effects on soil organic carbon (SOC) accrual remain unclear, particularly the role of soil aggregates and manure amendments in mediating this trophic cascade. Using a 19-year fertilization experiment, we examined how nematode predation influences fungal necromass carbon (FNC) and glomalin-related soil proteins (GRSPs), and quantified their contributions to SOC across soil aggregates under different manure amendments. Our findings showed that nematode predation significantly enhanced fungal biomass and promoted deterministic assembly of fungal communities. These effects were strongly dependent on aggregate size, with the most pronounced responses observed in the large macroaggregate (LA) fraction. A complementary microcosm experiment confirmed that nematode predation increased fungal biomass by over 6%, particularly in the LA fraction. Manure amendments further stimulated fungal growth and reinforced deterministic community assembly, thereby enhancing trophic cascade-driven accrual of FNC and GRSPs. Of the two fungal-derived carbon sources, FNC contributed more substantially to SOC (40%) than GRSPs (17%), with the greatest contribution found in the LA fraction. Path analysis further revealed that nematode-induced changes in fungal communities mediated the positive effects of manure amendments on fungal-derived carbon accrual. Overall, these findings underscore the pivotal role of nematodes in driving positive trophic cascade impact on SOC accrual. Our study offers new insights into aggregate-scale carbon dynamics and biologically mediated strategies for soil carbon management.