Slope gradient altered microbial community composition in the sloping cropland in black soil

Soil microorganisms are the main driving force in ecological and biogeochemical processes, such as carbon mineralization, humus formation, and nutrient cycles. Soil erosion may dramatically alter microbial properties. In hilly areas, soil erosion is highly attributed to topographic properties such as slope position and gradient. However, it is still unclear how slope position and gradient affects soil microbial characteristics. Herein, we compared the soil microbial characteristics in the sloping cropland between different slope gradients (0 & DEG;-1.5 & DEG; , 3 & DEG;-5 & DEG; , >5 & DEG;) and positions (top, middle, and bottom) in the typical black soil area of Northeast China. The results indicated that slope gradient significantly influenced soil clay content, water stable aggregate, organic matter, available potassium, total dissolved nitrogen and pH. In comparison, slope position had no noticeable impact on the physical or chemical properties. Results showed that Proteobacteria, Acidobacteria, Gemmatimonadetes, Planctomycetes, and Actinobacteria comprise 92 % of all phyla, whereas the top five bacterial families and genera account for 72 % and 50 % of the bacterial sequences, respectively. Ascomycota, Basidiomycota, and Chytridiomycota made up 83 % of the phyla, and the top five families and genera made up 43 % and 31 %, respectively, of the entire fungi sequence. Microbial alpha diversity (bacteria and fungi) was similar among the slope gradients and slope positions. The bacterial and fungal community composition was only affected by the slope gradient, with the highest slope gradient having a significantly lower value than other levels. Based on RDA analysis, water stable aggregate, clay content, organic matter, available nitrogen and cation exchange capacity concentration appeared to be the most critical factors in shaping bacterial and fungal communities. This study suggests that slope gradient but not slope position might be the main topographic factor decreasing microbial community composition in the sloped cropland area.