Carbon Storage in Wetlands of the United States: 2011-2016

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 across US wetlands, or how these stocks change over time. Soil carbon was measured using the same protocol to a maximum depth ranging from 90 to 120 cm at approximately 1000 probabilistically selected wetland sites in each of the 2011 and 2016 National Wetland Condition Assessment (NWCA); 195 of these sites were sampled for carbon in both survey years. Preliminary analyses of the resampled sites show no difference in carbon density in the top 10 cm between years but indicate a decrease in carbon density in deeper layers over the 5-year period. We hypothesize that temporal decreases in soil carbon density deeper in the soil profile are due to human disturbance or climate change, as human disturbance was correlated to lower carbon densities in the deepest (60–90 cm and 90–120 cm) soil layers in the 2011 NWCA data (Nahlik & Fennessy, 2016, NatComm:13835). We explore human disturbance and other mechanisms that may be impacting carbon stores over time. These data provide the first empirical, unbiased estimates of change in soil carbon for wetlands of the United States and demonstrate the power of probabilistic surveys for upscaling data collected at a limited number of sites to regional and national scales. Understanding wetland carbon storage at large scales provides 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 invited talk, Amanda Nahlik (EPA/ORD/CPHEA/PESD), Siobhan Fennessy (Kenyon College), and Karen Blocksom (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 in 195 sites in which carbon densities were measured in both 2011 and 2016 (i.e., resampled over a five-year period) using the same protocols.  Preliminary results suggest that there was no significant change in carbon storage in the top 10 cm of soil over five years.  However, there was a significant decrease in carbon storage in deeper soil layers, particularly 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 USGS to map and quantify blue (oceanic) carbon.  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., S. Fennessy, K. Blocksom, AND M. Dumelle. Carbon Storage in Wetlands of the United States: 2011-2016. 2022 Joint Aquatic Sciences Meeting (JASM), Grand Rapids, Michigan, May 15 - 20, 2022.