Ephemeral Gullies as Engines of Land Degradation: A Cross-Climate, Multi-Indicator Breakdown of Soil Quality Decline

Ephemeral-gully erosion is among the most dynamic yet least monitored drivers of land degradation in agricultural landscapes. Despite its global prevalence, its multidimensional impacts across contrasting climates and management systems remain poorly quantified. Here we present a cross-regional assessment of how ephemeral-gully processes reorganize soil physical, chemical, biological, and nutrient functioning across four agricultural systems spanning semi-arid Kansas (no-till and conventional tillage) and humid-subtropical Mississippi (amended no-till). High-resolution lateral (gully axis to shoulders) and longitudinal (outlet to headcut) sampling in the 0–10 cm layer, supported by depth-resolved observations to 35 cm, revealed robust and spatially coherent degradation patterns. Across all sites, erosional shoulders and mid-gully convergence zones consistently emerged as hotspots of structural breakdown, acidification, nutrient depletion, and biological suppression. In the semi-arid no-till system, degradation was primarily surface-confined and driven by selective removal of fine particles, aggregate destabilization, and desalinization. Conventional tillage amplified these contrasts, producing pronounced carbon and nutrient redistribution and intensified biological stress. In humid amended systems, surface amendments yielded only localized improvements; upslope acidification, organic-carbon loss, and base-cation leaching persisted, with the most humid site exhibiting degradation extending well into the subsoil. Depositional hollows showed partial resilience—higher carbon availability, moisture retention, and microbial activity—but these benefits were spatially limited and insufficient to offset broader hillslope decline. Overall, our results demonstrate that ephemeral gullies function as system-level engines of land degradation rather than transient geomorphic features. By integrating multi-indicator soil responses across climates, managements, and depths, this study provides process-based insights essential for soil monitoring, erosion modelling, and targeted conservation planning, directly informing land-degradation neutrality goals, the UN Decade on Ecosystem Restoration, and the EU Soil Monitoring Law.