Roots have greater effects on the accumulation of soil microbial residue carbon in microaggregate fractions than leaf litter in a subtropical forest

Abstract

Microbial residues as the key component of the stable soil organic carbon (SOC) play a critical role in stabilizing SOC, and their accumulation is influenced by plant litter. However, how the accumulation of microbial residues in different aggregate fractions of forest soils responds to changes in the inputs of leaf and root litter remains poorly understood. Here, we test the hypothesis that root exclusion has greater effects on microbial residues than leaf-litter exclusion among soil aggregate fractions based on a litter-input manipulation experiment in a subtropical coniferous forest. The concentrations of bacterial, fungal, and total microbial residues were higher in large macroaggregates (> 2000 μm) but lower in small macroaggregates (250–2000 μm). Root exclusion decreased the concentrations of bacterial, fungal, and total microbial residues in microaggregates (< 250 μm), and leaf-litter exclusion decreased fungal residues in microaggregates. Fungal residues had higher proportions to SOC (20.7–34.9%) than bacterial residues (10.8–18.6%) in soil aggregate fractions, and their proportions were higher in large macroaggregates than other aggregate fractions. Root exclusion reduced the higher proportions of bacterial, fungal, and total microbial residues to SOC in microaggregates (14.5%, 15.7%, and 15.3%, respectively), compared to the reduction observed in large macroaggregates (10.8%, 7.1%, and 8.4%, respectively). And leaf-litter exclusion reduced the proportions of fungal and total microbial residues to SOC in microaggregates. Soil microbial residue-carbon accumulation coefficient was higher in large macroaggregates and lower in small macroaggregates. The coefficient was increased by root exclusion in macroaggregates but decreased in microaggregates, and it was also decreased by leaf-litter exclusion in large macroaggregates and microaggregates. Proportions of fungal residues to SOC were more strongly related to the live microbial biomass and nutrients than those of bacterial residues. Our results suggest that root exclusion exerts stronger effects on soil microbial residues than leaf-litter exclusion but these effects vary among soil aggregate sizes.