Improved Wetland Soil Organic Carbon Stocks of the Conterminous U.S. Through Data Harmonization

Abstract

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.

Temporal variability in soil organic matter accretion rates in coastal deltaic wetlands under changing depositional environments

“…Our estimate of carbon stock in the Histosols of coastal Louisiana at 0–1 m depth (33.2 ± 0.6 kg C m −2 ) was greater than the values reported by Unran et al. (2021) (27 kg C m −2 ) but smaller than the values reported by Nahlik and Fennessy (2016) (33.2 ± 0.6 kg C m −2 ) for tidal marshes (Histosols) of the conterminous United States. Limited carbon stock data for depths below 1 m is available in the literature indicating the need for deeper (>1 m) carbon profile studies.…”

“…We found increasing wetland soil carbon content with increased depth in the northern part of Barataria Bay, Louisiana. Generally, carbon stored in 1 m soil depth has been considered in the carbon stock of the Histosols in tidal marshes (Nahlik & Fennessy, 2016; Unran et al., 2021). However, the carbon stock in the total 2 m depth of Barataria Bay wetlands was ∼88% greater than the carbon stock in 1 m soil depth alone.…”

Setting a reference for wetland carbon: the importance of accounting for hydrology, topography, and natural variability

“…However, despite their shared vegetation type and hydrogeomorphic class (depressional), as well as their proximity to one another, the 19 forested wetlands we sampled had SOC stocks spanning values characteristic of mineral wetlands (∼20 kg C m −2 ) to those of organic wetlands (>50 kg C m −2 ; table 4). Mean stocks in our forested wetlands were 32.2 ± 8.4 kg m −2 ; in comparison, a recent analysis of 529 inland wetlands, which spanned vegetation types and ecoregions throughout [115] the conterminous US, estimated mean SOC stocks at 19.7 ± 10.6 kg m −2 [90]. We compiled a list of relevant studies encompassing a range of wetland types and found that high within-type variability is common 4).…”

Cryptic carbon: The hidden carbon in forested wetland soils

“…However, many forested wetlands remain mis-classi ed as upland forest due to di culty in con rming soil moisture conditions due to dense canopies that obscure surface features from remote imagery. We suggest that there is signi cant potential to improve on previous work from Nahlik and Fennessey, 11 and Uhran et al, 12 , the latter of which was used for the wetland SOC baseline comparison in this study. 55 and our analysis supports improving SOC estimates with new approaches using probabilistic models that better re ect the spatial distribution of soil moisture conditions.…”

“…In total we used 300 bootstrapped SOC prediction maps where each pixel contained 300 predictions to simulate a distribution from which we extracted the standard deviation to represent the prediction interval uncertainty. We then compared the WIP wetland SOC stocks with 1m SOC stocks from the National Wetland Condition Assessment (NWCA) 12 . Uhran et al, provides the latest mapped wetland SOC stocks at 1 m depths for the continental U.S. modeled from harmonized pedon data from NWCA and mapped using the NLCD wetland extents at a 30 m resolution.…”