loss and degradation (Pendleton et al., 2012; Roman & Burdick, 2012), and thus there is significant opportunity for GHG management through restoration (Kroeger et al., 2017).
Agriculture and other land use changes are common examples of disturbances to wetland ecosystems. Tidal wetlands are also strongly affected by sea‐level rise and changes in land elevation due to subsidence and glacial rebound. Globally, tidal wetlands are lost at a rate of 0.5–3% per year (Pendelton et al., 2012). Although tidal wetlands have historically survived rising sea levels by accumulating inorganic and organic sediments, this process is threatened by the potential future rate of sea‐level rise and reduced sediment transport (sometimes due to coastal development), which has a range of ecosystem effects including changes to nutrient availability and vegetative growth (Kirwan and Megonigal, 2013). At present, ~27% of CONUS tidal wetlands have some level of impoundment and freshening due to tidal restriction, which can drive enhanced methane and carbon dioxide emission (Kroeger et al., 2017).
2.6. OUTLOOK AND FUTURE RESEARCH NEEDS
Current research on the topic of wetland carbon has major gaps including the study of the impact of climate change on mineral soil wetland carbon stocks, mechanisms controlling wetland dissolved carbon production, vulnerability of wetland carbon to fire, and vulnerability of wetland carbon to other disturbances including agriculture, development, reduction in available sediments, and sea‐level rise. Climate change‐related temperature and precipitation regime changes impact wetland carbon pools and fluxes depending on factors such as wetland and soil types, vertical accretion rates, and interactions with hydrology across different environments. It is clear from findings of the Second State of the Carbon Cycle Report (SOCCR2) (Kolka et al., 2018) that field methodology related to bulk density and percent of organic carbon needs to be improved and standardized to support assessment and modeling needs and to avoid overestimating carbon stocks. As large‐scale soil surveys are expensive, there is potential for geographic analysis and modeling to improve accuracy using available data sets, especially as it relates to the bulk density deficiencies identified by Holmquist et al. (2018). Other important areas of study include hydrology and hydrological cycling, wetland disturbance, and elevation relative to sea level.
There is a need for additional tools and large‐scale models to assess Net Ecosystem Exchange (NEE) of carbon across diverse wetlands. The inherent heterogeneity of wetland soils and sediments needs to be evaluated by interdisciplinary research teams in various wetland types and regions to determine methane flux, carbon cycle components, and carbon sequestration potentials. This work is especially critical because tidal wetland carbon sequestration in the United States may be resilient to sea‐level rise, given adequate horizontal space. The rate at which carbon is accumulated in tidal wetland soils is expected to increase in all climate change scenarios in the next century (Wang et al., 2019b). Wetlands and other negative emissions technologies may play an important role in carbon management, and thus deserve careful attention and study.
There are critical knowledge gaps and areas of research that are needed to support management of GHGs in tidal wetlands. These include the following: mapping of soil and biomass carbon stocks at national to local resolutions; mapping wetland conditions and potential for GHG management from restoration; knowledge and models of methane emissions for different wetland categories and conditions; and understanding how ecological processes, climate change, and sea level rise affect wetland processes. Whether due to management or to climate or weather pattern changes, altered hydrology and vegetation community will affect CO2 and methane emissions, carbon storage and accumulation, and risk of fire and ecosystem disturbances (Kolka et al., 2018).
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15 Holmquist,