Improving the accuracy of the gradient method for determining soil carbon dioxide efflux

Abstract

Soil CO2 efflux (Fsoil) represents a significant source of ecosystem CO2 emissions that is rarely quantified with high‐temporal‐resolution data in carbon flux studies. Fsoil estimates can be obtained by the low‐cost gradient method (GM), but the utility of the method is hindered by uncertainties in the application of published models for the diffusion coefficient. Therefore, to address and resolve these uncertainties, we compared Fsoil measured by 2 soil CO2 efflux chambers and Fsoil estimated by 16 gas transport models using the GM across 1 year. We used 14 published empirical gas diffusion models and 2 in situ models: (1) a gas transfer model called “Chamber model” obtained using a calibration between the chamber and the gradient method and (2) a diffusion model called “SF6 model” obtained through an interwell conservative tracer experiment. Most of the published models using the GM underestimated cumulative annual Fsoil by 55% to 361%, while the Chamber model closely approximated cumulative Fsoil (0.6% error). Surprisingly, the SF6 model combined with the GM underestimated Fsoil by 32%. Differences between in situ models could stem from the Chamber model implicitly accounting for production of soil CO2, while the conservative tracer model does not. Therefore, we recommend using the GM only after calibration with chamber measurements to generate reliable long‐term ecosystem Fsoil measurements. Accurate estimates of Fsoil will improve our understanding of soil respiration's contribution to ecosystem fluxes.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

“…Estimates of F soil were obtained using the gradient method through the equation 59 :where F soil (µmol CO 2 m −2 s −1 ), ρ is the air density (mol air m −3 ), is the CO 2 molar fraction gradient (µmol CO 2 mol air −1 ) calculated using the difference between atmospheric CO 2 molar fraction (400 ppm) and the CO 2 value at 10 cm depth, is the vertical gradient (m) and k s is the in situ CO 2 transfer coefficient (m 2 s −1 ) obtained by rearranging Eq. 1:where F chamber was measured by a portable soil CO 2 efflux chamber (Li-8100, Li-Cor, USA) from 18 collars around the instrumented pedons, follow a transect from the south face to the north face going through the valley, every two weeks during the months without snow cover (n=20).…”

Advancing Soil Physics for Securing Food, Water, Soil and Ecosystem Services

“…Measurement of CO 2 emission is important to assessing SOC stability and its impact on climate change. In recent years, new sensors and measurement devices have been developed to enable the use of the gradient method (De Jong and Schappert, 1972; Maier and Schack‐Kirchner, 2014; Sánchez‐Cañete et al, 2017) for continuous gas flux measurements (Flechard et al, 2007; Hirano et al, 2003; Vargas et al, 2010). Also, as a SOC management practice, biochar amendment in soils is being promoted for many agronomic and environmental benefits (Jeffery et al, 2011; Kookana, 2010; Laird, 2008; Lehmann et al, 2006), e.g., improving soil physical and chemical quality (e.g., soil structure, bulk density, water storage, and nutrient retention), decreasing greenhouse gas emission, and immobilizing contaminants in situ (Beesley et al, 2011; Lehmann, 2007; Liu et al, 2016a; Wang et al, 2016).…”

Monitoring Atmospheric, Soil, and Dissolved CO2 Using a Low-Cost, Arduino Monitoring Platform (CO2-LAMP): Theory, Fabrication, and Operation

“…Analysis of ambient CO 2 and soil CO 2 are routinely conducted by discrete sampling and in situ gas analyzers (Jassal et al, 2005;Andrews et al, 2014;Sánchez-Cañete et al, 2017;Jochheim et al, 2018). Discrete sampling is conducted primarily through gas collection into evacuated air-tight or "inert gas flushed" (e.g., helium flushing) containers.…”