Pyrogenic organic matter decreases while fresh organic matter increases soil heterotrophic respiration through modifying microbial activity in a subtropical forest

As the carbon (C) credit market evolves, incorporating organic matter into soils has emerged as a key strategy in C farming. Soil heterotrophic respiration (RH) plays a pivotal role in maintaining the C balance in terrestrial ecosystems, yet the contrasting impacts of fresh and pyrogenic organic matter applications on soil RH, and associated underlying mechanisms, have not been fully investigated. Through a 2-year field experiment, we investigated how applying maize straw and its derived biochar affect the physical, chemical, and microbial properties of soil in a subtropical Moso bamboo forest. Results showed that straw application increased soil RH, while biochar application suppressed it. Soil RH was correlated positively with β-glucosidase and cellobiohydrolase activities but negatively with RubisCO enzyme activity. Increased soil RH under straw application was linked to the increased β-glucosidase/cellobiohydrolase activities driven by elevated water-soluble organic C and O-alkyl C levels as well as GH48 and cbhI gene abundances, and the decreased RubisCO enzyme activity caused by reduced cbbL gene abundance. Conversely, reduced soil RH under biochar application was linked to reductions in β-glucosidase and cellobiohydrolase activities induced by increased aromatic C and decreased GH48 and cbhI gene levels, and increases in RubisCO enzyme activity driven by higher cbbL gene abundance. More importantly, changes in soil RH were clearly linked to microbial dynamics. Specifically, increases in the relative abundances of Alphaproteobacteria and Sordariomycetes and decreases in AD3 and Tremellomycetes contributed to the enhanced soil RH under straw application. With biochar application, the reverse effect occurred, ultimately contributing to the reduced soil RH. Our study demonstrates that maize straw application increases while biochar application decreases soil RH in the subtropical forest. These findings reveal that biochar reduced soil RH through changing microbial activity in subtropical forests, providing insight into complex dynamics of soil C cycling in response to diverse interventions.