The spatial distribution of soil organic carbon in tidal wetland soils of the continental United States

Abstract

Tidal wetlands contain large reservoirs of carbon in their soils and can sequester carbon dioxide (CO ) at a greater rate per unit area than nearly any other ecosystem. The spatial distribution of this carbon influences climate and wetland policy. To assist with international accords such as the Paris Climate Agreement, national-level assessments such as the United States (U.S.) National Greenhouse Gas Inventory, and regional, state, local, and project-level evaluation of CO sequestration credits, we developed a geodatabase (CoBluCarb) and high-resolution maps of soil organic carbon (SOC) distribution by linking National Wetlands Inventory data with the U.S. Soil Survey Geographic Database. For over 600,000 wetlands, the total carbon stock and organic carbon density was calculated at 5-cm vertical resolution from 0 to 300 cm of depth. Across the continental United States, there are 1,153-1,359 Tg of SOC in the upper 0-100 cm of soils across a total of 24 945.9 km of tidal wetland area, twice as much carbon as the most recent national estimate. Approximately 75% of this carbon was found in estuarine emergent wetlands with freshwater tidal wetlands holding about 19%. The greatest pool of SOC was found within the Atchafalaya/Vermilion Bay complex in Louisiana, containing about 10% of the U.S. total. The average density across all tidal wetlands was 0.071 g cm across 0-15 cm, 0.055 g cm across 0-100 cm, and 0.040 g cm at the 100 cm depth. There is inherent variability between and within individual wetlands; however, we conclude that it is possible to use standardized values at a range of 0-100 cm of the soil profile, to provide first-order quantification and to evaluate future changes in carbon stocks in response to environmental perturbations. This Tier 2-oriented carbon stock assessment provides a scientific method that can be copied by other nations in support of international requirements.

Flooding Alters Plant‐Mediated Carbon Cycling Independently of Elevated Atmospheric CO2 Concentrations

“…Heightened decomposition rates are therefore not the only potential route for the C loss we observed. Our overall soil C density across treatments is similar but slightly lower than values reported for Terrebonne Basin, LA (0.049 g * cm À3 [15 cm soil depth] in Hinson et al, 2017; 0.034-0.040 g * cm À3 present study). Our range of soil C pools also compares well to other estimates in salt marsh systems (78 ± 6 Mg * ha À1 [20 cm soil depth] in Chmura et al, 2003;68-84 Mg * ha À1 present study).…”

Tidal Wetland Gross Primary Production Across the Continental United States, 2000–2019

“…In the future, the BC model output could be compared with aboveground biomass data sets, particularly those using calibration‐grade, national level data sets such as Byrd et al (). It could also be compared with belowground carbon burial estimates (see Figure S3 for an example), made for the purposes of the U.S. National Greenhouse Gas Inventory (Crooks et al, ; Hinson et al, ; Holmquist et al, ). Regional studies could also provide fertile ground for cross comparison (Ghosh et al, ).…”

“…We then multiplied this average by the total area of the tidal wetlands, or 24,946 to 26,818 km 2 , and the number of days. The first value for area is from Hinson et al () and is the quantity for all tidal wetlands used by the BC model, and the second is from Windham‐Myers et al (), both of which are similar to Bridgham et al ().…”

“…To address this challenge, the BC model was optimized at a level of “pixel purity” for which these two competing interests were balanced. Pixel purity is defined as the percent of a pixel that was covered by a given tidal wetland class (accomplished by overlaying the 250‐m MODIS footprint with the vector‐based Hinson et al, data set described earlier). All work on the pixel purity portion of the BC model was performed for the two classes separately, woody and herbaceous.…”

Uncertainty in United States coastal wetland greenhouse gas inventorying

“…We quantified the effect of that choice by calculating the inventory using a GWP and median values for all other inputs and then recalculated changing only the GWP to SGW/CP (Neubauer and Megonigal 2015). Also, we tested the assumption of relying on the coastal lands definition for determining how much palustrine wetland area to include in the inventory compared to a tidal wetlands definition from the National Wetlands Inventory (NWI) (Hinson et al 2017. For this alternative analysis, we included all C-CAP palustrine wetlands intersecting an NWI-based tidal wetlands map and treated all palustrine mapped areas as fixed.…”