Influences of rock fragment content and vegetation on soil microbial communities

Soil microbial communities are key drivers of ecosystem processes and can be strongly affected by soil properties. While the effects of soil texture and aggregate have been well studied, the effect of rock fragment content (RFC) on soil microbial communities is still poorly understood, particularly in arid ecosystem. We examined the re-sponses of soil microbial biomass and communities to RFC gradients using a randomized block experiment with four RFCs (0 %, 25 %, 50 % and 75 %, V/V) and four planting treatments (Artemisia vestita, Bauhinia brachycarpa, Sophora davidii and no plants) in Minjiang arid valley. We explored soil community composition identified by total phospholipid fatty acids, and edaphic properties after two growth seasons. In most of current planting treatments, total phospholipid fatty acids, bacteria, saprotrophic fungi and arbuscular mycorrhizal fungi significantly decreased under 50-75 % RFCs versus 0-25 % RFCs. Compared to non-planted treatment, plants prominently increased soil microbial biomass and altered microbial communities only in 75% RFC. The ratio of gram-positive bacteria to gram-negative bacteria remarkably reduced in 75 % RFC of non-planted treatment, while significantly decreased in soil with 0 % RFC beneath A. vestita and S. davidii. These variations in soil microbial communities could be explained by the changes in soil water content, available nitrogen, total phosphorus and plant biomass allocation to fine root. These results indicated that both moderate and high RFC conditions inhibited soil microbial development, while plant can promote microbial biomass in high RFC by improving edaphic physiochemical properties and alleviating environmental stresses. Moreover, soil water content and nutrient levels explained much of the variation in soil microbial communities in these stony soils.