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

Wetland soils contain some of the densest stores of 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. The 2011 and 2016 National Wetland Condition Assessments (NWCA) provide one of the most comprehensive, field-based wetland soil databases in the nation, if not the world, that can be used to quantify carbon storage in US wetland soils and monitor those stores over time. Soil carbon was measured using the same protocol to a depth of 90 cm in three 30 cm depth increments at approximately 1000 probabilistically-selected wetland sites in each years. Of these, approximately 200 sites were sampled in both the 2011 and 2016 NWCA surveys. These data were used to estimate total carbon storage (Pg) and mean carbon density (tC ha-1) for the NWCA target wetland area across the US (38.7 Mha) and for various subpopulations. We found that there were appreciable changes in the total soil carbon stored from 0-90 cm across the US within the five-year period, with a 12% loss (about 1 PgC) between 2011 and 2016. The loss of soil carbon increased with depth from the soil surface between 2011 and 2016, with mean losses of 8% and 13%  for the 0-30 cm and 30-60 cm depth increments, and a 19% decrease in soil carbon between 60-90 cm.  At the subpopulation level, losses in mean soil carbon density were greatest for inland, organic-dominated wetlands – particularly lacustrine and flats wetland types. The sites sampled in both years (resample sites) showed a similar pattern of soil carbon loss. These data provide the first empirical estimates of soil carbon change for US wetlands and demonstrate the power of probabilistic surveys for upscaling data collected at a limited number of sites to regional and national scales. These data add to our understanding of wetland carbon storage and loss at large scales, providing critical insight for the effective management of carbon stocks for climate regulation.

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 Annual International Meeting of the Society of Wetland Scientists (SWS), Siobhan Fennessy (Kenyon College), Amanda Nahlik (EPA/ORD/CPHEA/PESD), 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.  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: Comparing Data from the 2011 and 2016 National Wetland Condition Assessments. 2023 Society of Wetland Sciences Annual Meeting, Spokane, WA, June 26 - 30, 2023.