Accuracy and Precision of Tidal Wetland Soil Carbon Mapping in the Conterminous United States

James R Holmquist,James T Morris,Lisa Schile-Beers,Lauren N Brown,Joseph M Smoak,Judith Z Drexler ,Kristin Wilson Grimes,William G Reay ,Donald E Weller ,K D Kroeger ,Brandon Boyd ,David J Velinsky ,Karen M Thorne ,Norman B Bliss ,Isa Woo,Nicole Dix,Erik M Smith ,Timothy A Shaw ,Joshua L Breithaupt ,Tiffany G Troxler ,Stephen Crooks,Matthew C Ferner,Kevin J Buffington ,Christopher K Sommerfield ,Glen M Macdonald,E B Watson ,John C Callaway ,Ryan P Moyer,J Patrick Megonigal,Benjamin P Horton,Lisamarie Windham‐Myers ,M E Gonneea ,Lyndie A Hice ,Mark S Woodrey

doi:10.1038/s41598-018-26948-7

Abstract

Tidal wetlands produce long-term soil organic carbon (C) stocks. Thus for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. We assembled and analyzed an unprecedented soil core dataset, and tested three strategies for mapping carbon stocks: applying the average value from the synthesis to mapped tidal wetlands, applying models fit using empirical data and applied using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps. Soil carbon stocks were far lower on average and varied less spatially and with depth than stocks calculated from available soils maps. Further, variation in carbon density was not well-predicted based on climate, salinity, vegetation, or soil classes. Instead, the assembled dataset showed that carbon density across the conterminous united states (CONUS) was normally distributed, with a predictable range of observations. We identified the simplest strategy, applying mean carbon density (27.0 kg C m−3), as the best performing strategy, and conservatively estimated that the top meter of CONUS tidal wetland soil contains 0.72 petagrams C. This strategy could provide standardization in CONUS tidal carbon accounting until such a time as modeling and mapping advancements can quantitatively improve accuracy and precision.

Highlights

Tidal wetlands produce long-term soil organic carbon (C) stocks
We evaluated the efficacy of International Panel on Climate Change (IPCC) Wetlands Supplement guidance for reporting and applying soil carbon stock values based on soil type, climate type, and salinity and vegetation
We assembled a spatially explicit database totaling 1959 soil cores from 49 different studies across conterminous united states (CONUS) (Fig. 1; Supplemental Table 1)

Summary

Introduction

Tidal wetlands produce long-term soil organic carbon (C) stocks. for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. Erosion results in export of particulate and dissolved organic carbon to other aquatic systems, a portion of which is oxidized and returned to the atmosphere[19] In order to both evaluate existing carbon stocks at sub-national to local scales, and estimate emissions from tidal wetlands that are lost during erosion and degradation events, we require accurate and precise soil carbon mapping strategies. We discuss three types of strategies for estimating carbon stocks: applying average carbon stock values from syntheses of soil core data, applying models fit using empirical data and applied spatially using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps that intersect with mapped tidal wetlands. The relative importance of these factors and the efficacy of applying separate estimates have not been evaluated at the scale of the conterminous U.S (CONUS)

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Discussion

Conclusion