Abstract: Land use change (LUC) is a key driver of various ecosystem processes, eventually influencing ecosystem function and stability. Hence, the knowledge of the impact of various popular LUCs on ecosystem processes and functions is vital for land management. We investigated the responses of soil enzyme activities and microbial nutrient limitations to a typical tropical LUC sequence: tropical rain forest, rubber monoculture, and rubber and Amomum villosum agroforestry system. Activities of six soil enzymes related to carbon (C), nitrogen (N), and phosphorus (P) cycle were investigated, including β-glucosidase (BG), β-xylosidase (BX), cellobiohydrolase (CBH), β-1,4-N-acetyglucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (AP). The relationships between soil enzyme activities, enzymatic stoichiometry, soil nutrients, and microbial biomass C (MBC) and N (MBN) among different forest types were analyzed. The results showed that: (1) Soil physicochemical properties differed greatly among the three forest types (P<0.05). Soil total nitrogen (TN) and total phosphorus (TP) concentrations of the rubber monoculture were higher than those of the other two forest types. Available phosphorus (AP), ammonium nitrogen (NH₄⁺-N), organic carbon (SOC) and dissolved organic carbon (DOC) concentrations were the highest in the agroforestry system, while the other soil physicochemical indices were the highest in tropical rain forest. (2) Forest conversion significantly influenced BG, CBH, NAG, and AP activities. CEs:NEs (C/N ratio of extracellular enzymatic activities) and CEs:PEs (C/P ratio of extracellular enzymatic activities) of the rubber monoculture were significantly higher than that of the other two forest types. (3) Enzymatic stoichiometry showed that soil microorganisms were limited by C and N across the three forest types. Specifically, microbial C limitation in rubber monoculture and N limitation in tropical rainforest were the highest. Microbial C and N limitations were mitigated in the agroforestry system. The redundancy analysis showed that soil NH₄⁺-N, SOC, MBN, MBC, NO₃^--N, and DON were the main environmental factors affecting soil enzyme activity and stoichiometry. In conclusion, the conversion of tropical rainforest to rubber monoculture significantly aggravated microbial C limitation, while the intercropping practice of A. villosum alleviated microbial C and N limitations caused by rubber plantation. Therefore, A. villosum shows promise as an intercropping species for establishing sustainable rubber-based agroforestry systems and improving soil nutrient balance.

Key words: forest transformation, extracellular enzyme activity, enzyme stoichiometry, soil physicochemical property, rubber agroforestry system