Objective Investigating the effects of forest gap size on soil quality provides a scientific basis for optimizing soil nutrient management and promoting forest regeneration.

Method This study examined forest gaps formed by pine wood nematode disturbance in Pinus massoniana plantations in Xingguo County, Jiangxi Province, China. Gaps were classified into five classes (I-V, small to large) based on diameter-to-height ratio, with adjacent closed-canopy stands as controls. According to the crown width of edge trees, each gap was further divided into a central zone, a transitional zone, and an edge zone. We analyzed physicochemical properties and stoichiometric ratios in surface (0-10 cm) and subsurface (10-20 cm) soils for differences among gap classes and interrelationships, and calculated the soil quality index for comprehensive evaluation.

Result (1) Soil water content in both the surface and sub-surface layers of Class V gaps was significantly higher than that in Class II, III, and IV gaps (P < 0.05). Non-capillary porosity in Class II gaps was significantly higher than in Class I gaps in the surface layer and significantly higher than in Class V gaps in the sub-surface layer (P < 0.05). (2) Total phosphorus content in both layers of Class IV gaps was significantly higher than in Class V gaps, and total potassium content in the sub-surface layer of Class IV gaps was significantly higher than in Class V gaps (P < 0.05). Total potassium content in the surface layer of Class II ~ V gaps was significantly lower than in the control (P < 0.05). (3) In the surface layer, C:P and N:P ratios in the edge and transition zones were significantly higher than in the central zone (P < 0.05). In Class IV gaps, the sub-surface C:N ratio and surface N:P ratio were significantly higher than in the control (P < 0.05). (4) As gap size class increased, the overall number of significantly correlated index combinations between soil C, N, and P stoichiometric ratios and soil physical and chemical properties showed a decreasing trend. (5) Soil quality index values in the surface and sub-surface layers of the gaps ranged from 0.39 ~ 0.45 and 0.27 ~ 0.36, respectively. There were no significant differences in soil quality index among different gap size classes (P > 0.05).

Conclusion Class IV gaps performed relatively better in improving soil phosphorus and potassium content, and represent a more suitable gap scale for optimizing soil nutrients in P. massoniana plantations in the study area.