Effects of land-use type and soil depth on soil physicochemical properties, enzyme activities and bacterial community composition in black soil region of Northeast China

Abstract

Soil bacteria play a crucial role in maintaining ecosystem functions and regulating carbon (C) and nitrogen (N) cycles. However, soil bacterial communities in Northeast China’s black soil region remain understudied regarding land use and depth effects. We used high-throughput sequencing and FAPROTAX to analyze impacts of land use (forest: FL, grassland: GL, 20/100-year arable lands TY/HY) and soil depth (0–100 cm, 5 layers) on soil properties, enzymes, bacteria, and function. Changes from natural ecosystems (GL/FL) to arable land (HY/TY) reduced enzyme activity but increased alpha diversity (Shannon: GL:5.185, FL:5.78, HY:5.914, TY:6.267), attributed to high organic inputs with low disturbance. Agricultural intensification enriched eutrophic taxa (Proteobacteria, Acidobacteria) and enhanced chemoheterotrophic (TY vs GL: +77 %; TY vs FL: +13 %) and nitrate reduction functions (TY vs GL: +13 %; TY vs FL: +9%), but suppressed nitrogen fixation (TY vs GL: –32 %; TY vs FL: −55 %). Soil organic carbon (SOC), total phosphorus (TP), and available potassium (AK) drove community composition (p < 0.01). Over-cultivation decreased SOC (HY vs TY: −24 %), TN (−38 %), enzyme activity, and alpha diversity. Vertically, SOC, TN, and enzymes declined > 50 % from surface to 100 cm depth. Alpha diversity decreased with depth except in TY. Chloroflexi increased while other phyla decreased along the profile. Agricultural use thus enhances microbial diversity and C-metabolism plasticity but depletes organic matter and simplifies functionality. Sustainable management must balance productivity with preservation of depth-dependent microbial functions.