Root-Litter Interactions and Nutrient Addition Modulate Soil Microbial Biomass and Labile Organic Carbon Dynamics in a Subtropical Moist Evergreen Broad-Leaved Forest

Soil microbial biomass and labile organic carbon (LOC) are pivotal regulators of carbon and nutrient cycling in forest ecosystems. However, the interactive effects of plant roots and forest litter on the seasonal dynamics of soil microbial biomass and LOC still remain poorly quantified, particularly in nutrient-poor subtropical forest ecosystems under monsoon climates. We performed a three-year (2022–2025) factorial experiment in a montane subtropical moist evergreen broad-leaved forest in central Yunnan, southwestern China. We manipulated plant roots (present vs. removed) and forest litter (present vs. removed) in a fully crossed design within the control and fertilized plots, and measured soil moisture, microbial biomass carbon (MBC) and nitrogen (MBN), MBC/MBN ratio, and LOC and its turnover time at monthly intervals. The study results showed that litter presence exerted a stronger positive influence on soil MBC, MBN, and LOC than root presence. The combined presence of roots and litter (i.e., R+L+) produced the highest soil microbial biomass and LOC, indicating an interactive root-litter effect. Fertilization significantly elevated soil moisture, MBC, MBN, and LOC across all experimental treatments, and shortened turnover time of soil LOC. However, fertilization did not significantly alter the monthly dynamics of any measured variable. Soil MBC/MBN ratio exhibited greater temporal variability in R−L+ treatment than the other three treatments, highlighting the important role of forest litter in structuring the composition of soil microbial community. These results reveal a “quantity-rhythm decoupling” in soil microbial responses to nutrient enrichment: fertilization regulates the magnitude of microbial biomass and LOC but not alters their seasonal periodicity. The seasonal rhythm of soil microbial biomass and LOC appears governed by plant phenology and monsoon-driven hydrothermal cycles. Our findings provide mechanistic insights into plant-microbe intense competition for nutrients in nutrient-poor subtropical forests and have positive implications for modeling soil carbon and nutrient dynamics under global climate change.