Effects of land use on wetland greenhouse gas fluxes and soil properties

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This article is part of the Spring 2016 issue of the Earth Science Matters Newsletter. 

Wetland and grassland ecosystems throughout the central United States have largely been lost or degraded by anthropogenic activities such as drainage and agricultural production. Wetlands provide multiple environmental benefits to society, including wildlife habitat, floodwater storage, and enhancement of atmospheric carbon sequestration. Projections for future global climate change have led to efforts to identify strategies for mitigating rising concentration of atmospheric greenhouse gases (GHG). Among the strategies being considered is the sequestration of atmospheric carbon in soils through cessation of agricultural activities and subsequent restoration of natural ecosystems such as wetlands. Restoration of croplands also may reduce fluxes of nitrous oxide (N2O), a powerful GHG which can be produced following the application of nitrogen-based fertilizers. However, the same conditions in wetlands that promote carbon sequestration in soils also can be conducive for the production of the GHG methane (CH4). Thus, there are concerns that increased CH4 fluxes may offset the benefits of carbon sequestration.

Aerial photo of the prairie pothole region of North American

Prairie Pothole Region wetlands represent a wide variety of wetland classes and sizes, and can be found in high densities. Photo from U.S. Geological Survey, Northern Prairie Wildlife Research Center.

(Credit: David M. Mushet, USGS. Public domain.)

The Prairie Pothole Region (PPR), located in the northern Great Plains of North America, is characterized by millions of relatively small, mineral-soil wetlands dispersed throughout the agriculture-dominated landscape. Much of the PPR landscape was drained for agriculture during the 20th century, but restoration activities began in the 1980’s to enhance ecosystem services such as GHG sequestration. However, there is a high degree of uncertainty regarding the impact of restoration on the overall GHG balance of these highly variable systems.

A comprehensive, 4-year study funded by the USGS Climate Research & Development Program was conducted on 119 PPR wetland catchments distributed among the major land uses of the region. Samples of GHG fluxes, as well as soil moisture and temperature measurements, were collected every two weeks using static chambers. At the beginning of the study, samples of the soils near each gas-sampling location also were collected and analyzed for nutrients and physical properties. This study was designed to estimate the effects of land use on GHG fluxes and soil properties, and assess the overall efficacy of wetland restoration for the mitigation of atmospheric GHGs.

Results from the study showed that land use had significant effects on GHG fluxes and soil properties, with spatial variability reflecting factors such as landscape position (wetland, upland), wetland classification, geographic location, climate, and agricultural practices. The study also showed that soil organic carbon is lost when relatively undisturbed catchments are converted for agriculture. The rate of soil carbon sequestration after restoration, however, was highly variable, and carbon concentrations of restored and agricultural sites often were similar. In the PPR, some wetlands in a cropland setting are drained, while others are left relatively intact and simply embedded in agricultural fields. Fluxes of CH4 from restored non-drained wetlands were similar to their cropland analogue, while CH4 fluxes of restored drained wetlands were higher than drained cropland sites. This observation indicates that it is important to consider the type of wetland restoration (drained, non-drained) when assessing the overall balance between carbon sequestration and CH4 fluxes. Results also suggest that N2O fluxes were greatest in cropland catchments, and that these fluxes likely would be reduced through restoration.

The overall variability demonstrated by this study was consistent with other wetland investigations and underscores the difficulty in quantifying the GHG balance of wetland systems.

The paper, Effects of land use on greenhouse gas fluxes and soil properties of wetland catchments in the Prairie Pothole Region of North America, was published in Science of the Total Environment. It is available at https://doi.org/10.1016/j.scitotenv.2015.06.148

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