Background Soil microbial biomass is the most active part of soil organic matter, which can be used to characterize soil property, and plays an important role in the process of biogeochemical cycling of carbon and nitrogen. Investigating the character of soil microbial biomass under different water conservation forests could be helpful in the construction of water conservation forest in Dahuofang Watershed.

Methods In order to clarify the effect of vegetation type on soil microbial biomass, a field experiment was conducted in 4 different vegetation types (Larix gmelinii, Pinus koraiensis, Pinus tabuliformis, and Robinia pseudoacacia) of water conservation forests in Dahuofang Watershed. We measured soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) and analyzed the response of soil microbial biomass to soil physical-chemical properties and litter nutrient content by path analysis model.

Results 1) The content of soil organic carbon (SOC), total nitrogen (TN), ammonium nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃^--N) were significantly higher in R. pseudoacacia natural secondary forests than other three vegetation types (P < 0.05), which decreased gradually with increasing soil depth. The range of C/N was 11.17-16.42, and the content of soil NH₄⁺-N accounted for 77% of soil total inorganic nitrogen. 2) The content of MBC and MBN were 92.69~562.55 mg/kg and 64.10-193.42 mg/kg, respectively, which were in the order of R. pseudoacacia natural secondary forests > P. tabuliformis plantation > L. gmelinii plantation > P. koraiensis plantation. The content of MBC and MBN in 0-10 cm soil layer were 1.83-2.55、1.18-1.62 times more than those in 10~20 cm soil layer, respectively. 3) The ration of MBC/MBN was 1.40-3.15, and soil microbial biomass carbon entropy (qMBC) and nitrogen entropy (qMBN) were 0.38%-0.81%、3.26%-3.59%, respectively. 4) There was a significant positive correlation between microbial biomass and soil physical-chemical properties and litter nutrient content. Path analysis showed that the TN, NH₄⁺-N, NO₃^--N, SOC and SWC (soil water content) were the most important factors affecting soil microbial biomass in different vegetation types of water conservation forests in Dahuofang Watershed.

Conclusions The content of MBC, MBN and litter nutrient content in R. pseudoacacia natural secondary forests was higher than other three vegetation types, which suggests that R. pseudoacacia natural secondary forests better improves soil quality than coniferous plantations. Our results impliy that we should pay more attention to the protection of R. pseudoacacia natural secondary forests in the Dahuofang Watershed in order to improve soil quality.