Objective The aims were to investigate the impact of different forest types in subtropical regions on soil microbial nutrient limitation mechanisms, in order to provide theoretical support for forest ecological restoration.

Method Two typical forest types were selected in the subtropical region, secondary forests and Eucalyptus plantations, and soil nutrients, enzyme activities and their stoichiometric ratios at various depths were systematically measured to reveal differences in microbial nutrient limitations in forest soils.

Result ① Significant differences in soil properties were observed between the two forest types (P < 0.05). The carbon-to-phosphorus (C:P) and nitrogen-to-phosphorus (N:P) ratios in secondary forest soils were significantly higher than those in Eucalyptus plantations. Specifically, they were 99.1% and 109% higher in the 0 ~ 20 cm soil layer, and 111.2% and 200.9% higher in the 20 ~ 40 cm layer. This suggests that P limitation in the study area may be alleviated and transition to N limitation. ② In both soil layers, the activities of phosphorus-acquiring enzymes (acid phosphatase) in secondary forests were significantly higher than those in Eucalyptus plantations, exceeding by 129.9% and 124.9%, respectively. However, the activities of C-acquiring enzymes (β-glucosidase) and N-acquiring enzymes (N-acetylglucosaminidase, leucine aminopeptidase) showed no significant differences between the two forest types. These findings indicate that soil microbes in both forest types have relatively consistent strategies for acquiring C and N, while secondary forests exhibit higher P utilization efficiency. ③ Enzyme vector trait analysis revealed that vector angles in both forest types were below 45°, and compared to eucalyptus plantations, soil microbial communities in the secondary forest experienced greater relative C and N limitation. ④ Redundancy analysis indicated that soil water content, dissolved organic C and total N were the main factors influencing enzyme activities and their stoichiometric ratios in both soil layers.

Conclusion The results reveal that secondary forests in subtropical regions are more effective in alleviating soil nutrient limitations compared to Eucalyptus plantations. Their abundant soil nutrient reserves and complex ecosystem structures provide a more suitable environment for microbial communities, thereby optimizing nutrient cycling and ecological processes, promoting healthy plant growth and enhancing ecosystem stability.