Gravel mulching effects on soil physicochemical properties and microbial community composition in the Loess Plateau, northwestern China

Gravel mulch has been used as an agricultural water conservation measure for over three hundred years in China's semiarid regions. In this study, a 3-year gravel mulch experiment on slopes in the northwestern Loess Plateau was conducted to evaluate the impact that different sized gravel mulch and natural rainfall has on surface runoff, soil physicochemical properties and microbial communities. The following three treatments were investigated: small-sized gravel mulch (AG, 2–5 mm particle size), large-sized gravel mulch (BG, 40–60 mm particle size), and no gravel mulch as a control (CK). We found that gravel mulch could effectively control surface rainwater runoff, especially in the AG treatment. AG treated soil was typically oligotrophic compared to BG treated soil with higher soil carbon and nitrogen content. PCR-amplified 16S rRNA genes and internal transcribed spacer (ITS) sequencing showed that soil bacterial and fungal communities in the AG treatment differed from the BG treatment or the CK treatment, and soil bacterial and fungal richness and bacterial diversity changed significantly after mulching. The relative abundance of Acidobacteria, Actinobacteria, Ascomycota and Bacillus significantly increased and the relative abundance of Proteobacteria and Bacteroidetes significantly decreased in the AG treatment compared to the CK treatment. Furthermore, the relative abundance of Bacillus was significantly increased in the AG treatment and was slightly increased (not significant) in the BG treatment compared to the control. Redundancy analysis (RDA) revealed that soil carbon and available nutrients were important driving factors for changes in microbial communities. Overall, the capacity of runoff production suppression and soil bacterial diversity and richness were higher in the AG treatment than the other treatments. Therefore, small-sized gravel mulching is recommended for arid and semiarid areas.