Carbon Storage in Wetlands of the United States: Using Data from the 2011 and 2016 National Wetland Condition Assessments

Wetland soils contain some of the densest stores of carbon in the biosphere, but there is little understanding of the quantity and distribution of carbon stored in US wetlands, or how these stocks change over time. Furthermore, wetlands are increasingly threatened by anthropogenic impacts and changing climate patterns, which impacts the ability of wetland soils to hold stored carbon. As a result, wetlands are a potential source of greenhouse gases in addition to being one of our greatest sinks. Currently, the National Wetland Condition Assessment (NWCA) collects the most comprehensive, field-based dataset of wetland soil data in the nation. Providing information about the role of wetlands in carbon storage and how anthropogenic disturbance relates to carbon storage is a key piece of information that connects NWCA to climate change and ecosystem services. Nahlik & Fennessy (2016, NatComm) published the first national estimates of carbon storage in U.S. wetlands using data from the 2011 National Wetland Condition Assessment (NWCA). They showed that, compared to reference sites, wetlands with high amounts of anthropogenic disturbance stored less carbon. Building on this research and additional sets of data from the 2016 and 2021 NWCA, we propose to a) provide improved regional and national wetland carbon storage estimates, b) provide the first national- and regional-scale estimates of carbon storage change, and c) provide insight into how anthropogenic disturbances, particularly physical alterations to the sites and the surrounding landscape collected as part of the NWCA, affect carbon storage. This work will also be incorporated into Lane, Christensen, Golden et al. (ACE 405.4 Effects on Ecosystem Carbon Sequestration and Storage) effort to connect changing precipitation patterns and climate to wetland carbon storage. Results from these efforts will be used to identify wetland regions or types that are vulnerable to carbon loss and to identify management practices that can be applied to help protect carbon storage in wetland soils.

Impact/Purpose

Wetland soils contain some of the highest stores of soil carbon in the biosphere.  However, there is little understanding of the quantity and distribution of carbon stored in US wetlands, or how these stocks change over time.  In this oral presentation at the National Water Quality Monitoring Conference (NWQMC), Amanda Nahlik (EPA/ORD/CPHEA/PESD), Siobhan Fennessy (Kenyon College), Karen Blocksom (EPA/ORD/CPHEA/PESD), and Michael Dumelle (EPA/ORD/CPHEA/PESD) evaluate the change in carbon storage in wetland soils over five years.  To do this, they used data from the 2011 and 2016 US EPA National Wetland Condition Assessment (NWCA).  They evaluated carbon storage and carbon density in approximately 1000 sites in both 2011 and 2016, which represent about 38 million hectares of targeted wetland area across the contiguous US.  Results show that carbon storage and carbon density decreased in wetlands across the US, particularly in deeper soil layers (within 30-60 cm and 60-90 cm) between 2011 and 2016.  The mechanisms driving decreased carbon storage have yet to be explored, but carbon data from 2011 (Nahlik & Fennessy, 2016, NatComm:13835) showed that increased human disturbance was correlated with lower carbon storage in deep soil layers (>60 cm); one hypothesis that will be explored is that the change in carbon storage in resampled sites in 2016 is due to human disturbance.  Other drivers, such as draught or increased ambient temperatures, will also be explored.  These data provide the first empirical, unbiased estimates of carbon storage change in wetlands across the US, especially at soil depths greater than 30 cm.  Because of the probabilistic design from which these data were collected, the consistency used to collect these data in the field, and the repeated data collection on a five-year schedule, this is an important dataset for many efforts being conducted across the US, including efforts by the U.S. Global Change Research Program (Greenhouse Gas Technical Working Group).  This effort exemplifies the power of collecting national data using the NARS approach, and the results of this research further support indicator development efforts by USEPA for future NWCA surveys.  The data we discuss here are necessary to effectively identify patterns of carbon storage and begin to investigate mechanisms that drive change in carbon storage – critical information if we are to implement policies related to climate protection targeted to where they can have the most positive effect.

Citation

Nahlik, A., M. Fennessy, K. Blocksom, AND M. Dumelle. Carbon Storage in Wetlands of the United States: Using Data from the 2011 and 2016 National Wetland Condition Assessments. 13th National Water Quality Monitoring Conference (NWQMC), Virginia Beach, VA, April 24 - 28, 2023.