Land Use Intensity Modulates Soil C, N and Stoichiometric Ratios Across Profiles: The Mediating Role of pH Buffering Capacity

The intensity of land use is known to alter soil organic carbon (SOC), total nitrogen (TN), and their stoichiometric ratios, yet the underlying biogeochemical mechanisms and the factors governing soil resilience remain unclear. This study investigates the process-based soil response to long-term, divergent land-use intensity changes. We compared adjacent plots converted from cropland to either low-intensity poplar woodland or a high-intensity greenhouse vegetable system, analyzing soil profiles to 100 cm depth across two distinct soil textures. The conversion to woodland initiated a C-driven pathway, where inputs of C-rich litter increased surface SOC and elevated C:N ratios by 4.1%–18.9%. This C accrual was possibly linked to a shift toward a fungal-dominated microbial community, as indicated by high microbial biomass C:N ratios (9.6–14.1). In contrast, the high-intensity greenhouse system triggered an N-driven pathway; heavy organic and inorganic N inputs substantially increased both SOC and TN but decreased C:N ratios by 33.7%–86.6%. This was caused by intensified nitrification, which drove soil acidification and reduced soil pH buffering capacity by 3.4%–41.6%. Crucially, we identify soil pH buffering capacity as a primary control; soils with stronger buffering capacity attenuated these divergent stoichiometric responses by maintaining a more stable microbial biomass composition, which was crucial for stabilizing nutrient availability and buffering soil ecosystems against the impacts of high land-use intensity.