Tree mycorrhizal types differentially mediate soil organic carbon and its fractions at the aggregate scale: key roles of metal elements and microbial traits.

Aggregates, fundamental structural units of soils, significantly affect soil organic carbon (SOC) dynamics. Yet, how tree mycorrhizal types drive SOC dynamics at the aggregate scale remains unclear. We investigated the impacts of three arbuscular mycorrhizal (AM) and three ectomycorrhizal (ECM) tree species on SOC and its particulate fractions, and the roles of metal elements and microbial communities across aggregate size classes in a temperate forest. AM tree soils exhibited higher macroaggregate proportions and aggregate stability, as well as increased contents of SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) across all aggregate size fractions than those under ECM trees. AM tree soils were richer in Ca, Mg, Fe, and Al, whereas ECM tree soils had higher K and Na. Mycorrhizal types and aggregate size (macroaggregates vs microaggregates) directly or indirectly affected SOC fractions through soil metal elements, total nitrogen (TN), fungal richness, and Bacteroidota abundance. Our study demonstrates that AM and ECM trees influence SOC dynamics through aggregate-size-dependent changes in soil TN, metal elements, and microbial traits, and that Ca, fungal richness, and Bacteroidota are particularly influential mediators. These findings underscore the importance of incorporating tree mycorrhizal traits into SOC dynamics under global change.