Fungal resilience rewires nitrogen cycling to buffer stoichiometric imbalances in eroded Mollisols landscape

Soil erosion drives topsoil redistribution, resulting in both biotic and abiotic losses that fundamentally alter nitrogen (N) dynamics. Cover crop-based no-tillage could mitigate erosion through enhanced organic matter inputs. However, the incorporation of crop residues into eroded soils may introduce uncertainty regarding microbial-mediated N transformations. Here, we conducted a mesocosm experiment by amending soil sampled from non-eroding, eroding, and depositional sites of a Mollisol landscape with 13 C-labeled maize residues (straw vs. stubble). We quantified N transformation dynamics by coupling soil N fractionation with 13 C-PLFA and functional genes. Results showed that crop residue amendment induced soil C:N imbalance and reduced soil N availability across all sites, with stubble exerting the most severe effects. Microorganisms in non-eroding soils exhibited great active adaptation to stubble-induced N imbalance. Specifically, to maintain soil N availability, microbiomes in non-eroding soils strategically rewired N-cycling pathways, upregulating assimilatory and dissimilatory nitrate reduction genes while downregulating nitrification genes, thereby boosting NH 4 + -N levels by 147–191%. Concurrently, 13 C-PLFA revealed fungi in non-eroding soils actively assimilated stubble, accelerating its decomposition to supply mineral N. These findings highlight the vital role of fungi in mitigating the dual challenges of erosion-induced nutrient depletion and residue-induced stoichiometric imbalances, offering critical insights for sustainable Mollisol management.