Long-term effects of gravel mulching and straw mulching on soil physicochemical properties and bacterial and fungal community composition in the Loess Plateau of China

Gravel and straw mulching are the most effective and widely used surface coverage practices used to mitigate drought stress and soil erosion in Loess Plateau of northwestern China. Unlike for straw mulching, however, benefits of gravel mulching fluctuate on a long-term scale, possibly leading to soil degradation. Here we investigated variation in soil physicochemical properties and the bacterial and fungal communities in soil after 10 years of applied gravel mulching (GM), straw mulching (SM), and non-mulching (CK). Proteobacteria (30.46-31.39%) was the most abundant phylum in all three treatments. We found Actinobacteria (19.10-24.92%) enriched in both GM and SM soils, while Acidobacteria (13.53-19.42%) had a higher relative abundance in gravel mulching soil only. A heatmap and principal component analysis (PCA) revealed that gravel or straw mulching significantly changed the soil bacterial and fungal community structure. Soil fungal community is more sensitive to gravel than straw mulch applications, under SM, the relative abundance of Mortierellomycota and Glomeromycota phyla increased while that of Ascomycota was decreased. In-depth taxonomic analysis of Proteobacteria demonstrated that the relative abundance of the class Alphaproteobacteria in straw mulching soil significantly exceeded that in the non-mulching soil. Comparing the three treatments, relative abundances of Nitrosomonadaceae family members were significantly higher in gravel mulching soil, while those of Steroidobacteraceae were significantly higher in the non-mulching soil. Redundancy analysis (RDA) suggested pH and soil water content were key factors driving these long-term changes in soil bacterial community structure.