Objective This study aims to reveal the mechanism underlying the effects of soil aggregate size differentiation on enzyme activities and stoichiometric ratios under the background of broadleaved transformation of Chinese fir (Cunninghamia lanceolata) plantations. It provides scientific support for enhancing the productivity and sustainable management of subtropical plantations in China.

Method Four stand types, including pure Chinese fir plantation (CL), pure Mytilaria laosensis plantation (PM), pure Castanopsis hystrix plantation (PC), and Mytilaria laosensis / Castanopsis hystrix mixed plantation (MP), were selected as research objects. Soil physicochemical properties, microbial biomass, and activities of carbon (C), nitrogen (N), and phosphorus (P) cycling-related enzymes in bulk soil and different aggregate fractions at 0 ~ 10 cm soil layer were determined. The main driving factors for variations in soil enzyme activities and stoichiometric ratios after conversion from Chinese fir plantations to native broadleaved forests were analyzed.

Result (1) Compared with CL, SOC in PM, PC, and MP increased by 42.00%, 85.25%, and 136.53%, soil C∶P ratio increased significantly by 30.80%, 49.77%, and 73.13%, MBN increased significantly by 25.98%, 32.88%, and 54.18%, and MBP increased significantly by 12.99%, 28.31%, and 49.62%, respectively. Activities of C-, N-, and P-cycling enzymes were significantly enhanced in bulk soil and all aggregate sizes, with the highest values in 0.25 ~ 1 mm and < 0.25 mm fractions in MP. In particular, C- and N-cycling enzyme activities in MP were significantly higher than those in other stands; (2) The soil enzyme C∶N∶P ratio was about 1∶1.06∶1.15, deviating from the global meta-analysis average of 1∶1∶1, indicating higher microbial enzyme activities for N and P acquisition than for C acquisition, and higher P-acquiring enzyme activity than N-acquiring enzyme activity; (3) Enzyme vector analysis showed that CL was P-limited (VA > 45°), while PM, PC, and MP were N-limited (VA < 45°). Vector length (VL) was greater than 1 in all stands but decreased after broadleaved transformation, indicating that C limitation was gradually alleviated; (4) SOC, total phosphorus (TP), soil N∶P, soil C∶P and total nitrogen (TN) were the dominant factors affecting soil enzyme activities, while SOC, soil C∶N and MBC mainly controlled the variation of enzyme stoichiometric ratios.

Conclusion Broad-leaved tree species (especially mixed forests) can effectively improve soil enzyme activities and promote soil nutrient cycling and quality. Nevertheless, N limitation still exists in the converted stands. Therefore, appropriate nitrogen application or introduction of nitrogen-fixing tree species is recommended to optimize nutrient supply and maintain high ecosystem productivity.