Nitrogen dynamics and greenhouse gas production in Yaqui Valley surface drainage waters

Agricultural runoff is thought to constitute a globally important source of the greenhouse gas nitrous oxide (N₂O), and may also be a significant source of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂). However, production of N₂O, CH₄, and CO₂ in polluted aquatic systems is poorly understood and scarcely reported, especially in low-latitude (0–30°) regions where rapid agricultural intensification is occurring. We measured N₂O, CH ₄, and CO₂ emissions, dissolved N₂O concentrations, and factors likely to control rates of greenhouse gas production in Yaqui Valley drainage canals receiving agricultural and mixed agricultural/urban inputs. Average per-area N₂O flux in both purely agricultural and mixed urban/agricultural drainage systems (16.5 ng N₂O-N cm −2 hr−1) was high compared to other fresh water fluxes, and extreme values ranged up to 244.6 ng N₂O-N cm −2 hr−1. These extremely high N₂O fluxes occurred during green algae blooms, when organic carbon, nitrogen, and oxygen concentrations were high, and only in canals receiving pig-farm and urban inputs, suggesting an important link between land-use and N₂O emissions. N₂O concentrations and fluxes correlated significantly with water column concentrations of nitrate, particulate organic carbon and nitrogen, ammonium, and chlorophyll a. A multiple linear regression model including ammonium, dissolved organic carbon, and particulate organic carbon was the best predictor of [N₂O] (r2 = 52%). Despite high per-area N₂O fluxes, our estimate of regional N ₂O emission from surface drainage (20,869 kg N₂O-N yr −1; 0.046% of N-fertilizer inputs) was low compared to values predicted by algorithms used in global budgets. All canals where we measured CO₂ were net-heterotrophic. CH₄ fluxes, though variable (range: −1372–3990 mg CH₄-C m−2 d−1), were generally quite high (mean: 3208 ng-C cm −2 hr−1) compared with fluxes from similar systems in other locations. C gas evasion from streams was several times greater than stream export of organic C via lateral transport. During algae bloom conditions we observed rapid and complete oxidation and reduction of an entire drainage canal. This has not previously been reported and may have important implications for in-stream transformations and downstream transfer of N, iron, manganese, and sulfur as well as the production of N₂O, CH ₄, and CO₂.