Microbial functional shifts amplify the temperature sensitivity of soil nitrogen across the erosion-deposition continuum

Abstract

Soil erosion drives biogeochemical decoupling of nitrogen (N) transformation pathways via spatial segregation of microbial processing hotspots along toposequences. However, the mechanisms governing this decoupling are poorly understood. Therefore, erosion-mediated reorganization of N cycling processes was investigated using high-resolution quantitative PCR (qPCR)-based functional gene quantification, N fractions analysis, and temperature-gradient incubations. Soil samples were collected in early April from a black soil region in Northeast China. Total N (TN) and mineral N (NO³⁻-N and NH4⁺-N) were determined. Temperature-controlled incubations (15 °C vs. 25 °C) were performed to determine net N mineralization, and qPCR was used to quantify genes involved in N cycling. The results demonstrated that 50.40% of TN and 54.88% of mineral N were depleted in eroding soil compared with non-eroding soil, whereas N accumulated in deposition-enriched subsoil primarily through mineral-associated N accumulation, accounting for 80.22%–93.71% of TN. Functional gene analysis revealed that the denitrification potential was intensified in eroding topsoils, as evidenced by a 3.3-fold upregulation of nirK and a 4.6-fold upregulation of norB. In contrast, depositional sites exhibited preferential activation of nitrification pathways. The temperature sensitivity of N mineralization was spatially divergent; it was 13.3 times higher in eroding topsoil than in depositional sites. Deposition depressed depth-dependent temperature sensitivity. This spatial biogeochemical partitioning establishes a climate-sensitive feedback loopin which erosional hotspots sustain N losses mediated by denitrification, and depositional microsites amplify temperature-contingent nitrification. The functional divergence between nitrification (depositional sites) and denitrification (eroding sites) hotspots is thermally modulated, creating distinct microbial metabolic regimes. These findings demonstrate how erosion–deposition interfaces potentiate soil N-cycling in topsoil and subsoil along a sloping landscape, providing a theoretical basis for preserving soil microbial function and resilience of the soil nitrogen pool in response to erosion and climate warming.