Assessing magnitudes and directions of CO2 fluxes within a karst landscape

<p>Gas diffusion is a primary driver of carbon dioxide (CO<sub>2</sub>) movement through unsaturated soils. In typical soils, high soil concentrations of CO<sub>2</sub> caused by autotrophic and heterotrophic respiration cause the gas to primarily diffuse upward. However, karst landscapes can have subsurface CO<sub>2</sub> sinks, both due to CaCO<sub>3</sub> weathering and losses via underlying caves and fracture networks. In this study our objective was to quantify the magnitude and direction of CO<sub>2</sub> fluxes in a pastured karst system located in Southwest Virginia (James Cave). Our hypotheses were: 1) the zero-flux plane, or location of maximum CO<sub>2</sub> concentration within the soil profile, is located at deeper depths, ≥60 cm depth during warmer months of the year and located at shallower depths, ≤60 cm, during the colder months of the year, 2) the zero-flux plane will exist ˂60 cm depth at the sinkhole location more often than at the upslope locations, and 3) CO<sub>2</sub> fluxes will be primarily upward during the growing season and primarily downward during the colder months of the year. We installed paired CO<sub>2</sub> and soil moisture sensors at 20 cm, 40 cm, and 60 cm depths, with profiles installed in the shoulder, midslope, and bottom (i.e., sinkhole) of a hillslope adjacent to the cave entrance. The sensors recorded hourly data between 7 February 2017 and 13 September 2019. The depth of the zero-flux plane was identified by the depth of maximum CO<sub>2</sub> concentration for each profile, while the measured concentration gradient from 20 to 60 cm was used to estimate CO<sub>2</sub> flux with Fick’s Law. Our findings support our hypotheses that the relative location of the zero-flux plane was located more often at deeper depths during warmer months of the year and located at shallower depths, i.e. ˂60 cm, during colder months of the year. The zero-flux plane was more frequently shallow (i.e., ˂60 cm) at the sinkhole location compared to the upslope profiles. The CO<sub>2</sub> fluxes reflected upward movement during the growing season and downward movement during the colder months of the year. We speculate that these processes reflect the influence of the underlying cave system, which may serve as a CO<sub>2</sub> sink during colder months, when the cave becomes vented via natural convection. Altogether, these findings suggest that downward diffusion may be an important yet oft-overlooked component of carbon fluxes in karst landscapes.</p>

Soil Physical Methods for Estimating Recharge - Part 3

Measurements in and just below the plant root zone, using principles of soil physics, can be used to estimate recharge. This booklet describes the Zero Flux Plane Method, Methods Based on Darchy's law, and Lysimetry for making such estimates. The work presents the basic concepts of soil water physics that will be referred to in this and other booklets in the series. Another method, the Soil Water Flux Meter, is discussed briefly, but as this is not sufficiently well developed for routine use readers are referred elsewhere for full details. All these methods require that consideration be given to interpolation over time and spatial extrapolation or averaging. A brief discussion of this is given.