Forest soils play a key role in the global carbon (C) pool and in mitigating climate change. The mechanisms by which understory and litter management affect soil organic C (SOC) concentrations are unclear in subtropical forests. We collected soils along a 60 cm profile in a Chinese fir (Cunninghamia lanceolata) plantation treated by only aboveground litter removal and understory vegetation preservation (Only-ALR), both aboveground litter and understory vegetation removal (ALR+UVR), and both aboveground litter and understory vegetation preservation (control) for 7 consecutive years. Five SOC fractions, physico-chemical properties, the biomass of microbial communities and the activities of C-acquiring enzymes were measured, and their correlations were analyzed for each of four soil layers (0–10, 10–20, 20–40 and 40–60 cm). Compared with control, Only-ALR decreased labile C pool I (LP-C I), labile C pool II (LP-C II) and dissolved organic C (DOC) in topsoil (0–20 cm) but had no effect on soil C fractions in subsoil (20–60 cm). A higher fungi and bacteria biomass in LP-C II and microbial biomass C (MBC) stock was observed in Only-ALR compared to ALR+UVR treatment. Soil pH and Gram-positive bacteria generally had impact on the variation of soil C fractions in topsoil and subsoil, respectively. Understory vegetation preservation offsets the declines of SOC and recalcitrant C but not the decreases in labile C caused by aboveground litter removal. Understory vegetation helps sustain SOC stock mainly via decreased C input and elevated soil pH which would change microbial biomass and activities when litter is removed. Our findings highlight the potential influence of long-term understory manipulation practices on C pool within a soil profile in subtropical plantation forests.

Despite the fact that rhizosphere soil microorganisms play a critical role in nutrient cycling, the effects of phosphorus (P) addition on microbial diversity in subtropical evergreen forests remain insufficiently understood. In this study, we examined how P fertilization influences rhizosphere microbial communities and enzyme activities in two dominant tree species, Schima superba and Castanopsis fargesii, using colorimetric analyses and high-throughput sequencing. The results showed that P addition significantly increased microbial diversity in the rhizosphere of S. superba but not C. fargesii. This response in S. superba was associated with shifts in saprotrophic microorganisms. Although P addition increased the abundance of Acidobacteria and Zygomycota in both species, the mechanisms underlying the microbial responses differed, reflecting the contrasting nutrient acquisition strategies of their mycorrhizal types. Soil-available P primarily shaped bacterial community composition in S. superba, whereas total nitrogen and soil pH were key drivers in C. fargesii. P fertilization also altered several C- and N-cycling enzyme activities, which were driven by differential regulation of the soil microbial community in two species. Overall, our findings demonstrate that the effects of P addition on rhizosphere microbial diversity and function vary substantially between tree species, highlighting the importance of plant–microbe interactions in regulating nutrient cycling in subtropical forests.