Effects of tree species transition on soil microbial biomass and community structure in subtropical China

Abstract Large-scale land use changes have remarkably influenced the global carbon (C) and nitrogen (N) cycling. Soil microorganisms are known to be the key drivers of these processes and act as susceptive indicators of changes in ecosystem functioning due to land use changes. In forest ecosystems, differences in the stability and turnover of soil C and N pools are mainly associated with the variation in the above- and belowground litter/root inputs to the soil from the tree species. However, the impact of soil C and N pool differences caused by tree species on soil microbial community structure has not been fully investigated in subtropical China. This study aimed to assess the effects of tree species conversion from a coniferous to broad-leaved plantation on the soil microbial biomass and community structure associated with C and N transformations within the plant-soil system. The microbial biomass and composition (reflected by 28 phospholipid fatty acid profiles), soil C and N pools in the top soils, and C and N contents of certain litter and fine root profiles were measured 19 years after the reforestation of Chinese fir ( Cunninghamia lanceolata ) woodland with the same species or a native broadleaf species Mytilaria laosensis . The results suggested that soil microbial biomass was significantly higher in the M. laosensis than in the C. lanceolata plantations, and non-metric multidimensional scaling ordination plots showed distinct patterns of soil microbial community structure between these two species. Soil microbial biomass showed negative correlations with litter N or mineral N content, i.e., ammonium N (NH 4 + -N) and nitrate N (NO 3 − -N), but was positively correlated with soil C content and litter C: N ratio. Further, there were negative correlations between soil microbial biomass C and mineral N pools. These results indicated that tree species transition from M. laosensis to C. lanceolata might have improved the soil labile C and N pools and their availability, leading to an increase in the soil microbial biomass. Redundancy analysis conducted to elucidate the relationships between the microbial community and C or N parameters also showed that the soil C: N ratio, soil total N, and NH 4 + -N might be the major factors influencing the soil microbial community. However, soil microbial diversity and richness were not significantly altered by the tree species transition. These results suggested that the potential process rates mediated by litter-derived C and N availabilities might not always be accompanied by a remarkable response from community diversity, but might affect microbial biomass. In conclusion, long-term tree species transition from coniferous to broad-leaved plantations significantly improved soil C and N pools and their availabilities, thereby increasing the soil microbial biomass and changing the composition of in situ soil microbial community. Previous events (e.g., land use history) might have considerable long-lasting impacts on soil microbial diversity and richness than the contemporary environment variables caused by the tree species transition 19 years after reforestation.