Methane emissions and climate from Prairie Pothole Wetlands

Methane is a greenhouse gas that is 30-40 times greater than carbon dioxide in its global warming potential (a relative measure of how much heat a greenhouse gas traps in the atmosphere). Natural emissions from wetlands contribute 20-40% of total global methane emissions, thus having a measurable impact on global climate. However, there is considerable uncertainty regarding how changing land management and climate might affect future emissions because emissions are controlled by complex interactions among abiotic and biotic factors that are not well understood. Recently published research measured methane flux rates, temperature and hydrology over a six-year period to develop models of methane flux rates from depressional, prairie wetlands. These models will assist in projections, planning and management of these wetlands under future climate and land use scenarios.

The northern Great Plains of central North America is home to the Prairie Pothole Region (PPR), a prime location for wetland biogeochemistry research due to the network of over 5 million wetlands that dot the landscape. These wetlands, termed ‘potholes’, are essentially small kettle ponds (generally <0.01 square kilometers) that formed during the last glaciation. The wetlands provide numerous ecosystem services, including breeding habitat for over half of North America’s water fowl, habitat for pollinators, invertebrates, fish and amphibians, flood mitigation, and nutrient retention. In addition, the combination of large inputs of plant biomass from wetland vegetation and anoxic conditions of saturated wetland sediment result in sequestration and long-term storage of carbon belowground, providing a radiative cooling benefit to the atmosphere. However, the same conditions that sequester and store carbon are ideal conditions for methane-producing microorganisms.

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In a broad study covering 119 potholes in the regions, USGS researchers showed that many of these wetlands were hotspots for methane emissions in North America. To identify and model how methane flux rates were affected by temperature and hydrology, the scientists conducted an intensive study of 12 wetlands located at Cottonwood Lake Study Area near Jamestown, North Dakota. At each wetland, methane flux rates were measured every two weeks during the ice-free season from 2009 to 2015. The measurements were made using static chambers located along transects spanning from the upland, grassland zone, through the riparian transition zone and into open waters. The results showed that methane emissions increased exponentially with rising air temperature, meaning that small increases in temperatures (as little as 2°C) could result in 30-60% increases in methane emission rates. The results also indicated that wetlands that are subject to longer periods of inundation have greater methane emission rates. This suggests that changes in winter snow fall, spring melt out, and summer rainfall events would affect methane emissions, as would tile and surface drainage and consolidation of wetlands in crop fields.

Given that methane flux rates are correlated with temperature, and global climate models predict increases in temperature, it is possible that methane emissions from wetlands will increase in the future. However, methane flux is dependent on both temperature and hydrology, and there is considerably more uncertainty regarding the future hydrology of these wetlands, which is influenced both by climate and land use practices. Hydrology models of the PPR suggest that, under a warming climate, the wetlands may become drier because increased evapotranspiration could outweigh expected increases in summer precipitation. Consequently, a warmer and drier scenario would produce considerably lower emissions compared to a warmer and wetter scenario. The research also indicates that not all wetlands are created equal with respect to their methane production potential. Prairie pothole wetlands are chemically unique and have salinity concentrations ranging from fresh to hypersaline. USGS research has shown how a complex interplay of geologic and glacial history, combined with ground water connectivity, results in release of sulfate salts from smaller, ephemeral wetlands into the larger, more permanent wetlands throughout the region. Research in marine settings has shown that high sulfate concentrations inhibit methane production. Therefore, if that process also applies to the PPR, then the larger semi-permanent and permanent wetlands will likely be less affected by changes in climate conditions, while still providing the climate benefit of carbon sequestration and storage.

This analysis of methane flux was the first step towards a much larger effort by the USGS and their partners to develop robust models integrating hydrology, chemistry, biology, and management to predict emissions of methane and the other greenhouse gases as they are affected by changes in climate and land use in the region. Ultimately, the goal is to provide science-based management recommendations to optimize the climate benefits that the prairie potholes and other wetlands provide. 

The paper, "Temperature and hydrology affect methane emissions from Prairie Pothole Wetlands" was published in a special issue of Wetlands.

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