Lateral diffusion and atmospheric CO2 mixing compromise estimates of rhizosphere respiration in a forest soil

Abstract

Measurements of rhizosphere carbon efflux are critical to the determination of soil carbon balance by CO2 flux measurements. We attempted to measure rhizosphere respiration in a forest ecosystem by transplanting 13C-enriched soils from a tallgrass prairie into a mixed-conifer forest soil but found that atmospheric air mixing and lateral diffusion confounded delta13C-CO2 measurements. Surface CO2 efflux (delta13C [Formula: see text] –20‰) was enriched 6‰ relative to soil CO2 measured at depth because of the presence of atmospheric-derived CO2 (–8‰) near the soil surface. The delta13C-CO2 value of transplanted soil CO2 did not reflect its 13C-enriched carbon source but was within 1‰ of native soil CO2 because of lateral diffusion from the surrounding native soil. A two-component steady-state model of lateral diffusion supported our assertion that this soil was susceptible to atmospheric air mixing and lateral diffusion because of its high effective porosity and relatively low concentration of soil CO2. Percent rhizosphere respiration was estimated at 35 and 45% after applying corrections for atmospheric air mixing and (or) lateral diffusion. These confounding effects may be reduced or eliminated by utilizing a larger transplanted soil pit and by reducing soil CO2 diffusivity, for example, by increasing water content.