Citation:

Uhran, B., L. Windham-Myers, N. Bliss, A. Nahlik, E. Sundquist, AND C. Stagg. Improved Wetland Soil Organic Carbon Stocks of the Conterminous U.S. Through Data Harmonization. Frontiers in Soil Science. Frontiers, Lausanne, Switzerland, 1:706701, (2021). https://doi.org/10.3389/fsoil.2021.706701

Impact/Purpose:

A team of scientists from US Geological Survey (USGS) and Amanda M. Nahlik from the US Environmental Protection Agency (USEPA) produced a harmonized national map of wetland soil organic carbon stocks by 10-cm increments down to 1m depth for inland and tidal wetlands of the conterminous United States.  To do this, the team merged wetland soil data from two national-scale, public datasets: the U.S. Department of Agriculture’s National Cooperative Soil Survey (NCSS) Soil Survey Geographic Database (SSURGO), and the U.S. Environmental Protection Agency (EPA) National Wetland Condition Assessment (NWCA) datasets.  The NWCA datasets were used to assess the representativeness of community-contributed SSURGO dataset, and models were developed to predict the density of soil organic carbon (SOC) in wetlands.  Major findings were: ·       Similar downcore patterns in SOC stocks indicate similar carbon preservation mechanisms with depth below the surface in organic-rich wetland soils ·       SOC stocks show a wide range of SOC densities which were not predictable by region or vegetation type ·       Mean values for SOC density in organic wetland soils were 35% lower in our analysis than reported with SSURGO-based estimates Understanding the amount of SOC retained in wetland soils across the United States is important for predicting carbon vulnerability and climate change modeling.  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.

Description:

Wetland soil stocks are important global repositories of carbon (C) but are difficult to quantify and model due to varying sampling protocols, and geomorphic/spatio-temporal discontinuity. Merging scales of soil-survey spatial extents with wetland-specific point-based data offers an explicit, empirical and updatable improvement for regional and continental scale soil C stock assessments. Agency-collected and community-contributed soil datasets were compared for representativeness and bias, with the goal of producing a harmonized national map of wetland soil C stocks with error quantification for wetland areas of the conterminous United States (CONUS) identified by the USGS National Landcover Change Dataset. This allowed an empirical predictive model of SOC density to be applied across the entire CONUS using relational %OC distribution alone. A broken-stick quantile-regression model identified %OC with its relatively high analytical confidence as a key predictor of SOC density in soil segments; soils <6% OC (hereafter, mineral wetland soils, 85% of the dataset) had a strong linear relationship of %OC to SOC density (RMSE = 0.0059, ~4% mean RMSE) and soils >6% OC (organic wetland soils, 15% of the dataset) had virtually no predictive relationship of %OC to SOC density (RMSE = 0.0348 g C cm−3, ?56% mean RMSE). Disaggregation by vegetation type or region did not alter the breakpoint significantly (6% OC) and did not improve model accuracies for inland and tidal wetlands. Similarly, SOC stocks in tidal wetlands were related to %OC, but without a mappable product for disaggregation to improve accuracy by soil class, region or depth. Our layered harmonized CONUS wetland soil maps revised wetland SOC stock estimates downward by 24% (9.5 vs. 12.5Pg C) with the overestimation being entirely an issue of inland organic wetland soils (35% lower than SSURGO-derived SOC stocks). Further, SSURGO underestimated soil carbon stocks at depth, as modeled wetland SOC stocks for organic-rich soils showed significant preservation downcore in the NWCA dataset (<3% loss between 0 and 30 cm and 30 and 100 cm depths) in contrast to mineral-rich soils (37% downcore stock loss). Future CONUS wetland soil C assessments will benefit from focused attention on improved organic wetland soil measurements, land history, and spatial representativeness.

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