Effects of forest-floor litter manipulations on soil organic carbon pools in a temperate mixed forest: a stoichiometric perspective

Litter traits are closely associated with soil organic carbon (SOC) persistence. However, quantified effects of litter quality and quantity on SOC formation and loss are still debated, as they depend on complex biotic and abiotic interactions. Specifically, it remains unclear how the elemental (e.g., carbon [C] and nitrogen [N]) stoichiometry impacts the SOC pool through its control over nutrient cycling and energy flow. Here, we quantified the variations in bulk SOC and its fractionations (particulate organic carbon [POC], mineral-associated organic carbon, dissolved organic carbon and microbial biomass carbon [MBC]) under different litter treatments varying in quality and quantity, using a 13-year detrital manipulation experiment in a temperate mixed forest. We found that double mixed litter input increased bulk SOC pool by 58.5%, with a 67.2% increase in POC at 0–10 cm depth. Litter removal reduced POC by 40.4% (0–10 cm) and 49.8% (10–20 cm). Notably, litter removal and double woody litter input reduced the carbon to nitrogen ratio (C:N) of bulk soil, particulate and mineral-associated fractions, but had no effect on the microbial biomass C:N. The MBC was positively correlated with POC and soil moisture at 0–10 cm depth. Our findings indicate that litter quantity dominates SOC dynamics by regulating POC. Double mixed litter exhibited non-additive effects on SOC formation, likely due to trade-offs between fresh C inputs and priming-induced C losses. While litter removal reduced the soil C:N, microbial biomass C:N was unchanged, suggesting the need for longer-term studies to understand these decoupled responses.