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

Abstract

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.