关键词: 林窗扰动, 土层深度, 胞外酶活性, 土壤微生物代谢策略, 酶化学计量学, 马尾松次生林

Abstract:

Objective: This study aims to explore the interactive effects of forest gap size and soil depth on soil physicochemical properties, soil extracellular enzyme activities and microbial metabolic processes, clarify the soil layer differentiation mechanism of soil nutrient cycling in secondary Pinus massoniana forests under forest gap disturbance, and identify the key environmental factors underlying changes in the activity of soil extracellular enzymes as well as microbial metabolic strategies. Method: The degraded P. massoniana forest at Tianxinge Forest Farm, Cili County, Hunan Province was targeted, and four forest gap treatments, including an uncut control (CK), small gap (S, 68–75 m²), medium gap (M, 180–190 m²), and large gap (L, 445–480 m²), were established in a randomized block design in December 2018. In May 2024, five 2 m×2 m quadrats were randomly set up in each treatment plot, and soil samples were collected from litter and four mineral soil layers (0–2, 2–5, 5–10, and 10–20 cm). Litter indices, soil physicochemical properties, and the activities of four soil extracellular enzymes of β-1,4-glucosidase (BG), cellobiohydrolase (CBH), N-acetyl-β-D-glucosaminidase (NAG), and acid phosphatase (ACP) were measured. Enzyme stoichiometry was used for vector analysis (vector length, VL; vector angle, VA). Two-way ANOVA was used to examine the interactive effects of forest gap size and soil depth on soil physicochemical properties, soil extracellular enzyme activities and microbial metabolic processes. Redundancy analysis (RDA) and partial least squares structural equation modeling (PLS-SEM) were applied to identify key environmental drivers of enzyme activity and stoichiometric variation. Result: 1) Compared to the control, the medium gap (M) significantly increased soil pH, water content, and inorganic nitrogen concentrations. There was significant interaction between gap size and soil depth on soil organic carbon content total nitrogen content and mineral-associated organic matter carbon content. The interaction decreased with increasing soil depth. 2) The responses of soil enzyme activities to forest gaps declined with increasing soil depth. The forest gap effects were predominantly concentrated in the 0–10 cm layer, where the medium gap significantly enhanced CBH and ACP activities. The stimulatory effect of gaps on enzyme activity became negligible in the 10–20 cm layer. 3) Soil microbial metabolism in the study area was generally constrained by phosphorus (P) (VA>45°). The large gap (L) significantly exacerbated the carbon resource limitation of microorganisms, manifested as a significant increase in the enzyme stoichiometric C∶N ratio (E_C∶N) and vector length (VL). Moreover, P limitation became more severe with increasing soil depth. 4) Soil water content was the primary factor driving the spatial variation of enzyme activities, while the soil C∶N ratio was the key factor controlling enzyme stoichiometry and microbial resource allocation strategies. 5) Structural equation modeling revealed that forest gap size and soil depth indirectly modulated soil enzyme activities and stoichiometry by altering the litter properties and vertical nutrient distribution, which in turn affected the intensity of microbial carbon and phosphorus limitation. Conclusion: Forest gaps significantly improve the soil microenvironment in 0–10 cm layer in secondary P. massoniana forests, thereby affecting extracellular enzyme activity and microbial metabolic strategies. The effects gradually decline with increasing depth. A medium-sized forest gap is most effective in improving soil water content and nitrogen availability, while large gaps exacerbate microbial resource limitation. Soil microbial metabolism in the study area is consistently limited by phosphorus, with stronger limitation in deeper soil layers. Soil water content and soil C∶N ratio are important environmental factors that influence extracellular enzyme activity and changes in microbial metabolic strategies. These findings highlight that optimized gap-size manipulation in forest management and restoration can effectively alleviate microbial nutrient limitation and promote belowground ecosystem function in degraded P. massoniana forests.

Key words: forest gap disturbance, soil depth, extracellular enzyme activity, soil microbial metabolic strategy, enzymatic stoichiometry, Pinus massoniana secondary forest