Greenhouse gas flux and soil carbon in natural, restored and drained Prairie Pothole wetlands

Background/Question/Methods

The Prairie Pothole Region (PPR) in the Northern Great Plains is home to the largest wetland system in North America, consisting of over 5 million wetlands over 750,000 km². Wetland drainage, fertilizer inputs and other land-management activities from extensive agriculture in this region (mostly corn, soybean, wheat and canola crops) have potential to impact the carbon (C) budgets of these wetlands, which collectively can affect continental-scale greenhouse gas emissions. We conducted an extensive study of 120 wetland catchments located in the U.S. portion of the PPR to assess the effects of land use on carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes (rates and cumulative) and soil C. For 2-4 years (2005-2008), flux rates were measured biweekly using static-vented chambers during the growing season at 8 locations within each wetland catchment (from uplands to open waters). The data were then modeled using a modified version of DayCent. Resulting net rates of CO₂, CH₄ and N₂O exchange were utilized to calculate the cumulative change over time in the atmospheric reservoir of each gas, the cumulative radiative forcing of each gas, and the net effect of hydrologic restoration and avoided drainage of PPR wetlands on global climate.

Results/Conclusions

Wetland drainage and associated agricultural land-use changes more than doubled CO₂ and N₂O fluxes compared to natural or restored wetlands, particularly for semipermanent wetlands that typically remain ponded for the duration of the growing season. Wetlands surrounded by agriculture but were not drained also had elevated N₂O emissions, especially for smaller, seasonal wetlands with shorter hydroperiods and greater perimeter to area ratios. Natural wetlands without cropping history had some of the highest cumulative CH₄ fluxes measured in North America (average 1.6 and max 6.5 Mg CH₄ ha^-2 yr^-1). Drained wetlands had minimal CH₄ flux while hydrologically restored wetlands had intermediate values. In general, management of PPR wetlands leads to tradeoffs in CO₂, CH₄, N₂O and soil C fluxes, with the type of management combined with underlying biogeochemistry and year-to-year weather driving the magnitude of these tradeoffs.