Physical rather than chemical protection determines soil organic carbon accumulation in a subtropical Chinese fir plantation treated by litter manipulation

Background and aim Soil organic carbon (SOC) is crucial for soil fertility and combating climate change, which may be regulated by aboveground litter input. However, how SOC accumulation responds to litter manipulation remains unclear. Methods In this study, we conducted a 7-year litter manipulation experiment, including litter addition, litter removal, and control treatments. We used an elemental analyzer, along with measurements of bulk density, to estimate SOC stock, the wet-sieving method to analyze soil aggregate distribution, amino sugar content to calculate microbial necromass carbon (C) in aggregates, and a Fourier transform midinfrared spectrometer (FIRT) to determine the SOC chemical functional groups along a 60 cm profile in a Chinese fir (Cunninghamia lanceolata) plantation. Results We found that litter addition significantly increased SOC stock by 44.59% in the 0-10 cm layer, while litter removal had no effect on SOC stock. Litter addition increased the proportion of macroaggregates (> 2 mm) and the geometric mean diameter (GMD). Moreover, litter addition increased the bacterial necromass C in macroaggregates, which was positively correlated with SOC. SOC stock in topsoil was explained by GMD and fractal dimension, which might increase the protection for bacterial necromass C. However, litter addition decreased the chemical stability of SOC. Conclusion Long-term C input increased topsoil SOC accumulation by strengthening the physical protection of aggregates rather than by the chemical protection of SOC functional groups in this Chinese fir plantation, particularly through the accumulation of bacterial necromass C in macroaggregates.