Patterns and controls of nitrous oxide emissions from waters draining a subtropical agricultural valley

摘要

Although nitrous oxide (N_2O) emission from agricultural runoff is thought to constitute a globally important source of this greenhouse gas, N_2O flux from polluted aquatic systems is poorly understood and scarcely reported, especially in low-latitude (0°-30°) regions where rapid agricultural intensification is occurring. We measured N_2O emissions, dissolved N_2O concentrations, and factors likely to control rates of N_2O production in drainage canals receiving agricultural and mixed agricultural/urban inputs from the intensively farmed Yaqui Valley of Sonora, Mexico. Average per-area N_2O flux in both purely agricultural and mixed urban/.agricultural drainage systems (16.5 ng N_2O-N cm~(-2) hr~(-1)) was high compared to other fresh water fluxes, and extreme values ranged up to 244.6 ng N_2O-N cm~(-2) hr~(-1). These extremely high N_2O 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 N_2O emissions. N_2O concentrations and fluxes correlated significantly with water column concentrations of nitrate, particulate organic carbon and nitrogen, ammonium, and chlorophyll a, and a multiple linear regression model including ammonium, dissolved organic carbon, and particulate organic carbon was the best predictor of [N_2O] (r = 52%). Despite high per-area N_2O fluxes, our estimate or regional N_2O emission from surface drainage (20,869 kg N_2O-N yr~(-1); 0.046% of N-fertilizer inputs) was low compared to values predicted by algorithms used in global budgets.