A new method for in situ soil gas diffusivity measurement and applications in the monitoring of subsurface CO2 production

Abstract

Subsurface approaches to soil CO2 monitoring are becoming increasingly important for process studies in terrestrial carbon research. When used in conjunction with a diffusion model to determine CO2 production, subsurface methods require good estimates of effective soil gas diffusivity (De). Outlined here is a novel membrane probe and continuous flow system for in situ soil gas diffusivity measurements. Laboratory tests confirm performance across the range of CO2 diffusivities found in natural soils. Field tests were performed across a range of soil moisture contents by artificially hydrating eight soils over a period of three to seven days. These soils were representative of a range of textural classes in eastern Nova Scotia, Canada. The absolute values of diffusivity, and also the rate at which diffusivity decreased with increasing soil moisture content were typically quite different from model predictions. When applied to subsurface CO2 monitoring at two sites, the site-specific diffusivity measurements greatly improved the accuracy of CO2 production estimates. We observed a consistent and close correspondence between calculated profile CO2 production and (independently measured) soil CO2 surface flux. The subsurface CO2 production estimates acquired using in situ gas diffusivity measurements allows detailed vertical profile resolution to be constructed over time. The majority of CO2 was generated at shallow depths, but periodic contributions from deeper depths were important, particularly towards the end of the growing season. Copyright 2008 by the American Geophysical Union.