Regulation of carbon dioxide and methane in small agricultural reservoirs: optimizing potential for greenhouse gas uptake

Small farm reservoirs are abundant in many agricultural regions across the globe and have the potential to be large contributing sources of carbon dioxide (span classCombining double low line"inline-formulaCO2/span) and methane (span classCombining double low lineinline-formulaCH4/span) to agricultural landscapes. Compared to natural ponds, these artificial waterbodies remain overlooked in both agricultural greenhouse gas (GHG) inventories and inland water global carbon (C) budgets. Improved understanding of the environmental controls of C emissions from farm reservoirs is required to address and manage their potential importance in agricultural GHG budgets. Here, we conducted a regional-scale survey (span classCombining double low lineinline-formulag 1/4/span 235 000 kmspan classCombining double low lineinline-formula2/span) to measure span classCombining double low lineinline-formulaCO2/span and span classCombining double low lineinline-formulaCH4/span surface concentrations and diffusive fluxes across 101 small farm reservoirs in Canada's largest agricultural area. A combination of abiotic, biotic, hydromorphologic, and landscape variables were modelled using generalized additive models (GAMs) to identify regulatory mechanisms. We found that span classCombining double low lineinline-formulaCO2/span concentration was estimated by a combination of internal metabolism and groundwater-derived alkalinity (66.5 % deviance explained), while multiple lines of evidence support a positive association between eutrophication and span classCombining double low lineinline-formulaCH4/span production (74.1 % deviance explained). Fluxes ranged from span classCombining double low lineinline-formula-21/span to 466 and 0.14 to 92 mmol mspan classCombining double low lineinline-formula-2/span dspan classCombining double low lineinline-formula-1/span for span classCombining double low lineinline-formulaCO2/span and span classCombining double low lineinline-formulaCH4/span, respectively, with span classCombining double low lineinline-formulaCH4/span contributing an average of 74 % of span classCombining double low lineinline-formulaCO2/span-equivalent (span classCombining double low lineinline-formulaCO2/span-e) emissions based on a 100-year radiative forcing. Approximately 8 % of farm reservoirs were found to be net span classCombining double low lineinline-formulaCO2/span-e sinks. From our models, we show that the GHG impact of farm reservoirs can be greatly minimized with overall improvements in water quality and consideration to position and hydrology within the landscape.