Bio-physical competition for pore space and the micro-environment in BSC covered Kalahari Sand soils: Modelling soil thermal, hydraulic and gas efflux behaviour

Soil pore and particle matrices both facilitate and constrain soil-atmosphere exchange of heat, moisture and gas. Further, in soils covered by Biological Soil Crusts (BSCs), soil structure differs in the BSC (<1cm) and near surface layers. Soil-atmosphere gas exchange occurs primarily by diffusion, moisture flow by capillarity and hydrostatic pressure, processes all constrained by soil porosity. Conversely heat flow occurs primarily through the structurally complementary contiguous soil particle matrix and water filled pores.

Pore filling by water and biota also changes the optical properties of the soil, of importance to autotrophic bacteria within the BSC. Additionally, vascular biota, fungal and bacterial structures (e.g. exopolysaccharide sheaths and hyphae) may contribute to liquid and gaseous transport. Soil moisture is important in all of these processes. In sand soils it determines both the thermal conductivity and thermal capacity and hence the thermal diffusivity: the ability for heat to flow through and cause temperature variation within any material.

The soil surface controls heat fluxes to and from deeper soil by alteration of the thermal diffusivity of the surface as well as affecting gaseous diffusional efflux. Thus a number of interdependent bio-physical processes compete for soil pore space and affect soil, hydrology, thermal diffusivity and hence soil temperature profiles, respiration and gas efflux.

Here we model and discuss thermal, hydraulic and diffusional properties of BSC capped Kalahari Sand soils as a function of moisture and surface temperature. Data are presented for seasonal scenarios, surface temperature variations and soil moisture profiles. These data indicate i) the insulating nature of dry surface layers which in turn contribute to the decoupling of and decreases in the variation of subsoil temperature ii) the converse thermal coupling effect of moist soils and decreased gas efflux. The model data assist, for example, in explaining experimental sub-soil temperature and moisture profiles observed at high temporal resolution for Kalahari Sand soil embracing diurnal, monthly and seasonal cycles over a twelve month period.

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