Loss in Soil Organic Carbon Storage in Wetlands Across the United States Over a 5-Year Period

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 wetlands of the United States (U.S.), or how these stocks change over time. The 2011 and 2016 National Wetland Condition Assessment (NWCA) provides one of the most comprehensive, field-based national wetland databases in the U.S., if not the world, that can be used to quantify carbon storage in U.S. wetland soils and monitor those stores over time. Soil carbon was measured using the same soil pit sampling protocols to a depth of 90 cm in three 30-cm depth increments at approximately 1,000 probabilistically-selected wetland sites in each year. Of these, approximately 200 sites were sampled in both the 2011 and 2016 surveys. Total carbon storage (Pg) and mean carbon density (tC ha-1) for the NWCA target wetland area across the U.S. (38.73 Mha) and for various subpopulations were estimated. We found that over the 5-year study period, there was a consistent trend of organic carbon loss from each 30-cm depth increment, resulting in an estimated loss of 1.055 PgC, or 12.0% of the carbon stock originally measured in 2011. Mean carbon densities for 0-90 cm decreased significantly between 2011 and 2016 from 225.21±14.84 to 197.97±17.65 tC ha-1 (p=0.016). Carbon density losses increased with depth, ranging from -8.2% (7.9 tC ha-1) from 0-30 cm to -16.4% (9.6 tC ha-1) in the deepest soils (61-90 cm). The sites sampled in both years showed a similar pattern of soil carbon loss. There is strong evidence that changes in soil carbon are driven by losses in organic-dominated soils, especially from 0-30 cm (with a decrease of 36.02±16.63 tC ha-1, p<0.001). While our study was not designed to identify the mechanisms of carbon loss, we show possible relationships between regions of the U.S. in which wetland soil carbon densities declined, initial carbon density, and deviation from the 30-year climate normals. 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), Amanda Nahlik (EPA/ORD/CPHEA/PESD), Siobhan Fennessy (Kenyon College), Karen Blocksom (EPA/ORD/CPHEA/PESD), Michael Dumelle (EPA/ORD/CPHEA/PESD), and Marc Weber 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.  High temperatures, as indicated by the deviation from the 30-year climate normals, in the 5-years preceding the 2016 NWCA survey, may explain some of the large carbon losses measured. 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, M. Dumelle, AND M. Weber. Loss in Soil Organic Carbon Storage in Wetlands Across the United States Over a 5-Year Period. 2024 Society of Wetland Scientists Annual Meeting, Taipei, TAIWAN, PROVINCE OF CHINA, November 11 - 16, 2024.