Soil microbial biomass and community composition along a latitudinal gradient in the arid valleys of southwest China

Identifying the latitudinal patterns and drivers of soil microbial biomass and community composition helps to better understand the biogeochemical cycling of carbon and nutrients. However, little is known about how and to what extent biotic and abiotic factors influence soil microbial biomass and specific microbial groups along latitudinal gradients, especially in arid and semi-arid regions. In this study, we sampled the topsoil (0–10 cm) and subsoil (10–20 cm) from 105 natural shrubland sites across a latitudinal gradient (23.2 °N to 32.3 °N) in the arid valleys of southwest China. We measured the soil microbial biomass and community composition (using phospholipid fatty acid [PLFA] analysis) and their possible environmental drivers. The results showed that soil microbial biomass (expressed as total PLFAs) increased significantly with increasing latitude, independent of the soil layer. More specifically, in the two soil layers, the PLFAs of Gram-negative bacteria and fungi, and the fungi/bacteria (F/B) ratio increased significantly along with the latitude, while the PLFAs of Actinobacteria did not significantly change with latitude. Bacterial and Gram-positive bacterial PLFAs increased significantly with latitude only in the subsoil, but not in the topsoil. Soil microbial biomass and the PLFAs of specific groups were closely related to vegetation properties (fine root biomass and plant species richness) and soil nutrients (e.g., soil organic carbon), while the F/B ratio was mainly related to climate (e.g., mean annual precipitation) and soil pH. The relationships between soil microbial biomass and community composition and environmental factors were similar in the topsoil and subsoil. Overall, our study elucidated the latitudinal patterns in soil microbial biomass and community composition, and our findings highlight that climate, vegetation, and soil properties jointly drive changes in soil microbial communities with latitude in the arid valleys of southwest China.