Effects of disturbance on soil microbial abundance in biological soil crusts on the Loess Plateau, China

Biological soil crusts (biocrusts) are important surface cover in drylands, but they are vulnerable to disturbance. To date, research has mostly specialized in changes in the soil microorganisms between disturbed and undisturbed biocrusts, and it is unclear whether disturbance intensity drives the soil microorganisms in biocrusts toward distinct responses. Thus, we investigated the changes in biocrust characteristics, soil properties and microbial abundance at six simulated grazing disturbance gradients (based on the coverage of the broken biocrusts) in the Loess Plateau region of China. The results showed that moderate disturbance, in which the biocrust coverage experienced a 20-30% breakage, was beneficial for improving the abundances of bacteria, actinomycetes and total microorganisms, but severe disturbance (i.e., 40-50% disturbance) may result in a significant reduction in abundance. The increased fungal abundance with the disturbance gradients was related to the aerobic feature of fungi. Additionally, a varied proportion of actinomycetes and bacterial abundance was found under disturbance. Disturbance induced the alteration of soil aeration, moisture and nutrient status (total N or ratio of C/N) that driving the changes of microbial abundance. These results will be meaningful for evaluating the ecosystem services of biocrusts under a disturbance event in dryland ecosystems.