Nitrous oxide and methane production and emission in wetlands receiving nonpoint source nitrate loads

Although wetland restoration is a promising strategy to reduce nonpoint source nitrogen (N) loads, there is some concern over the potential for increased nitrous oxide (N2O) emissions. We examined the production and emission of N2O and methane (CH4) from wetlands designed to intercept and reduce elevated, nonpoint source nitrate (NO3 -) loads. We measured N2O and CH4 flux rates at three wetlands subject to a wide range of NO3 - loading rates. Nitrate, dissolved N2O, and dissolved CH4 associated with inflows and outflows were estimated using measured flows and concentrations, and N2O and CH4 emissions were estimated using floating chambers. Nitrate removal, N2O production, and CH4 production were estimated by mass balance analyses. Methane emission rates averaged 1,010 mg m-2 day-1, similar to rates for restored depressional wetlands, and N2O emission rates averaged 4.49 mg m-2 day-1, similar to rates from cropland. Inflows and outflows contributed little to CH4 fluxes but were significant components of N2O budgets. Dissolved N2O loads to the wetlands from inflow streams ranged from 8.1% to 70% of the total N2O inputs, and dissolved N2O export from the wetlands through outflow to streams ranged from 7.3% to 63% of the total N2O outputs. Nitrous oxide production and emission increased with NO3 - loading; however, these wetlands also exhibited very high NO3 - conversion efficiencies, with N2O-N production and emission averaging approximately 0.5% of NO3 - removal. The fraction of N loading that would be transformed to N2O in these wetlands is much lower than in cropland or downstream riverine systems.