Investigating Wetland and Nonwetland Soil Methane Emissions and Sinks Across the Contiguous United States Using a Land Surface Model

Abstract

We estimated the distribution of CH 4 emissions and sinks from wetlands (including freshwater and coastal wetlands) and nonwetland (including wet and dry soils) with a newly developed vertically resolved soil CH 4 model, integrated into a global land surface model (ISAM). We calibrated and tested this integrated model with CH 4 observations at test sites in the Contiguous United States (CONUS). ISAM is applied across the CONUS to estimate CH 4 emissions and sinks given both recent past observed climate and wetland extent, and future climate and wetland extent driven by two scenarios, RCP4.5 and RCP8.5. Estimated net CH 4 emissions for the 2000s are 13.8 TgCH 4 yr −1 , mostly from wetland soils. Estimated net emissions under RCP4.5 and RCP8.5 are 30% and 64% higher, respectively, in the 2090s than in the 2000s due to (1) higher temperature and seasonal wetland extent (driven by higher precipitation in the climate scenarios), which increase modeled methanogenic activity more than methanotrophic activity in soils and (2) altered transport in the soil column and exchange with the atmosphere by modeled transport processes (diffusion, ebullition, and aerenchyma transport). Nonwetland soils emit CH 4 (1.4 TgCH 4 yr −1) in some areas and take up CH 4 (−2.9 TgCH 4 yr −1) in other areas, resulting in a net estimated sink for the 2000s; the net nonwetland soil sink increases by 15% and 46% by the 2090s under RCP4.5 and RCP8.5, respectively, mainly due to drier soil conditions, which enhances methanotrophic activity and oxidation of CH 4 diffused into soil from a future atmosphere with higher CH 4 concentration.

Resolving the Carbon‐Climate Feedback Potential of Wetland CO2 and CH4 Fluxes in Alaska

“…Shu et al. (2020) predict CH 4 increase by 30% and 64% at the end of century under RCP4.5 and RCP8.5 in CONUS wetlands, which are the same directional changes of CH 4 emission as predicted in our study.…”

“…We predict the same directional changes in soil CO 2 respiration due to increased soil moisture, but increased CH 4 due to warmer temperature and larger anaerobic fraction in the soil column. Shu et al (2020) predict CH 4 increase by 30% and 64% at the See Table 2 for reference to Scenario 1 and Scenario 2. Each color represents one single site and the contour lines are two-dimensional kernel density derived from 4,000 random samples from posterior estimate constrained by eddy covariance net CO 2 and CH 4 data (50%, 75%, and 95% distributions) derived from CARDAMOM posterior states and parameters.…”

Evaluating the contribution of methanotrophy kinetics to uncertainty in the soil methane sink

“…Conrad 1992, 1993). Accordingly, a subset of soil methane sink models implement Michaelis-Menten kinetics (Shu et al 2020). The Michaelis-Menten model supports quantitative descriptions of cellular and ecosystem-level processes wherein the rate is sensitive to the concentration of available substrate, but reaches some theoretical maximum when the enzyme or community is saturated with substrate (Liu 2007, Alvarez-Ramirez et al 2019.…”

Recent increases in annual, seasonal, and extreme methane fluxes driven by changes in climate and vegetation in boreal and temperate wetland ecosystems

“…In addition to temperature and precipitation selected in previous studies (Delwiche et al, 2021;Knox et al, 2021), we also included LW radiation following Morin et al (2014) and Chen et al (2015), since LW radiation approximates near-surface air temperature and is commonly used in land-surface models (Shu et al, 2020;Zhang et al, 2018;Zhang et al, 2023). We also included the 3-and 30day rolling mean of the mean daily surface air temperature, because long-term average temperatures (especially prior to the growing season) can influence the summer FCH 4 values (Pugh et al, 2018).…”