Anthropogenic disturbance transforms small agricultural irrigation impoundments into overlooked greenhouse gas hotspots.

Small water bodies are increasingly recognized as disproportionate sources of greenhouse gas (GHG) emissions, yet many anthropogenic aquatic systems embedded within agricultural landscapes remain poorly represented in regional carbon assessments. Small agricultural irrigation impoundments (SAIIs) are widespread across mountainous agricultural regions, but their emission magnitude and underlying controls remain largely unknown. Here, we conducted year-round measurements of CO2, CH4, and N2O concentrations and fluxes across 12 SAIIs characterized by contrasting levels of anthropogenic disturbance. All investigated SAIIs acted as net GHG sources, with area-specific total emissions reaching 8.2 ± 6.9 kg CO2-eq m-2 yr-1. This average emission rate substantially exceeds those reported for most other agricultural aquatic systems, highlighting their potentially important contribution to agricultural landscape GHG budgets. All three GHG fluxes exhibited strong spatial variability along the anthropogenic disturbance gradient, with highly disturbed Village SAIIs exhibiting 3.4-, 4.0-, and 2.5-fold higher CO2, CH4, and N2O fluxes than those observed in weakly disturbed Forested SAIIs, respectively. Increasing anthropogenic influence enhanced carbon and nutrient accumulation in water columns and sediments, thereby stimulating microbial carbon and nitrogen transformation processes and GHG emission potential. Moreover, anthropogenic disturbance reshaped the dominant controls of each GHG. CO2 regulation shifted from primarily sediment carbon mineralization toward stronger coupling with water-column carbon availability and physicochemical conditions, whereas CH4 and N2O remained closely associated with sediment carbon-nitrogen availability and microbial processes. Seasonal variation exerted comparatively limited effects on overall emission magnitude. These findings identify SAIIs as previously overlooked GHG hotspots in agricultural landscapes and highlight the necessity of incorporating these widespread water infrastructures into regional GHG inventories and sustainable agricultural water management strategies.