Micrometeorological and chamber measurements of CO 2 flux from bare soil

Abstract

Accurate measurements of carbon dioxide (CO 2) flux from soil are important because this flux is an important component of the surface carbon budget, and a good indicator of the level of soil microbial and root activity. Half-hour CO 2 fluxes from bare soil were measured using soil chamber and Bowen ratio/energy balance (BREB) methods for 4 days in December 1992, at the Blackland Research Center, Temple, TX. Soil chamber CO 2 measurements were made sequentially at nine positions in the field. Three CO 2 BREB systems were used. The CO 2 flux was ≈ 0 in the early morning and after sunset and was maximum (slightly less than 0.1 mg m −2 s −1 (2.3 μmol m −2 s −1)) near midday. The coefficient of variation (CV) of chamber CO 2 fluxes across the nine positions averaged 40% throughout the day, indicating the need for a large number of chamber measurements to obtain a representative CO 2 flux measurement. The CV of the three daily BREB CO 2 fluxes was less than 2%, indicating the BREB CO 2 fluxes from the three systems were equal. There was good agreement between fluxes from the two methods. The average chamber and BREB CO 2 fluxes for the entire period of measurements were 0.039 and 0.042 mg m −2 s −1, respectively, while the root mean square difference between half-hour fluxes from the two methods was 0.017 mg m −2 s −1. The methods are complementary and can both be used for soil CO 2 flux measurements. The chamber method is low cost and easy to use, and offers the possibility of replicated measurements over space. The BREB method integrates over a large spatial area and is thus less affected by the high spatial variability of soil CO 2 flux.