Short-term changes in below-ground carbon dioxide concentrations

Soils are an important component of the carbon cycle in many agricultural and forest ecosystems. As part of a study examining the production and storage of CO₂ within the soil, we obtained continuous measurements of below-ground CO₂ concentrations in an agricultural field. Measurements were obtained at 10 and 20 cm below the soil surface of a conventionally-tilled corn field late in the growing season (Sep. 10 - Oct. 20, 2003) as well as during the coldest parts of the winter (Jan 8 - Feb 27, 2004). The sensors used in this study were Vaisala GMP CO₂ probes enclosed in a suitable sampling apparatus to protect against moisture and provide a degree of spatial coverage. Measurements of concentration during the growing season demonstrated a temperature dependence and showed strong diurnal variability. Concentration measurements obtained in the winter, when soil temperatures were near-freezing, revealed much less CO₂ storage and very little relationship with soil temperatures. During both measurement campaigns substantial short-term changes in below-ground CO₂ were observed during periods of high windspeed. These changes in the quantity of CO₂ stored in the soil pores are suspected to be caused by an increase in soil gas diffusivity from pressure fluctuations resulting in enhanced removal of CO₂ from the soil surface. This poster will discuss the magnitude of these short-term changes in below-ground CO₂ storage and their potential effect on micrometeorological measurements of CO₂ flux.