Soil Cracks Intensify Soil-Atmosphere Gas Exchange.

Soil cracks are a transient feature in many landscapes that can significantly affect the rate of gas exchange between soil and atmosphere. Specifically, diurnal temperature variability can drive convective venting within cracks such that gas exchange is enhanced by over two orders of magnitude over diffusive exchange. In situ field measurements show that cracks and fractures can also change the temporal distribution of gas transfer by enhancing nighttime and winter rates. This phenomenon is the converse of most other soil-atmosphere exchange processes, such as vapor exchange that peaks during daytime and summer. The mechanism controlling gas exchange in soil cracks is described and quantified. Three years of continuous field data show diurnal formation of convective venting cells within cracks, with cell morphology and duration exhibiting seasonal variability. Numerical models are presented that explore the relative importance of crack aperture, ambient temperature and soil permeability on the gas exchange process. Through modeling, theory and field data, we quantify the timing, persistence and characteristics of this mechanism.