Long-term cultivation drives soil carbon, nitrogen, and bacterial community changes in the black soil region of northeastern China

The deteriorating soil carbon, nutrients, and microbial habitat pose a serious threat to crop production. However, the effects of long-term cultivation with soil erosion on soil carbon, nitrogen, and bacterial communities remain poorly understood. Consequently, this study aims to explore the changes in these soil indicators in the last 110 years and their responses to soil erosion. Luvic Phaeozems in hillslope and Haplic Phaeozems in flat were sampled along 113- and 67-year cultivation chronosequences. The soil organic carbon (SOC), total nitrogen (TN), soil loss rate, the soil bacterial community, and so on were respectively determined using the Walkley-Black, the Kjeldahl, nuclide tracing method, and 16S rRNA amplicon sequencing methods. The comparisons of SOC, TN, and soil bacterial community between Luvic Phaeozems and Haplic Phaeozems were used to differentiate their responses to the soil erosion intensity. The results indicated that SOC and TN decreased negatively and exponentially during long-term cultivation and took longer to regain stability in Luvic Phaeozems than in Haplic Phaeozems (e.g., SOC: from 55.77 to 23.11 g kg-1 after 152 years vs. from 91.83 to 34.24 g kg-1 after 32 years). The bacterial community transitioned into Proteobacteria-dominant communities and the quantity of carbon-fixing and nitrogen-fixing functional groups decreased. These shifts were mainly driven by the SOC and TN contents (40%) and their loss ratios (44%). In conclusion, soil erosion leads to more severe SOC and TN loss on hillslope than flat under long-term improper cultivation, ultimately endangering soil bacterial ecological functions, which has important implications for effective soil erosion control and encouraging conservation tillage in this food-producing region.