Spatial and temporal variability of soil gas diffusivity, its scaling and relevance for soil respiration under different tillage

Abstract

Soils are an important source and sink for carbon. Soil management such as reduced or no-tillage management has been reported to increase soil organic matter budgets, probably due to a hampered microbial mineralization of organic components. While soil respiration is mainly controlled by temperature and soil moisture, it can be also limited by the soil pore system facilitating diffusive gas fluxes between the soil and the atmosphere. However, soil gas diffusivity as a controlling factor for soil respiration has not been assessed under different soil management. Moreover, no adequate methods have been developed yet that facilitate the description of spatial or temporal variations of the highly non-linear soil gas diffusivity functions. Therefore, the objectives of this study were to deduce and apply a scaling rule for gas diffusivity, and to observe and analyze spatio-temporal variations of soil respiration and gas diffusivity under conventional tillage (CT) and no-tillage (NT). We measured soil respiration rates and gas diffusivities along a transect on an arable field in Hollabrunn (Lower Austria) within the 2014 vegetation period. We also determined the soil hydraulic properties and gas diffusivities as a function of air-filled porosity. By adopting the similar media approach of Miller and Miller we facilitated scaling of spatially variable gas diffusivity model functions. The scaling performed well to derive representative mean parameters while preserving the spatial variability in the scaling factors. The comparison of scaling factors for soil water retention, hydraulic conductivity, and gas diffusivity revealed that flow pathways were not the same for water and gases. This finding was explained by the continuity of pores that are accessible for water or gas movement. Compared to NT, the CT plot was characterized by greater soil respiration rates, gas diffusivities, total porosities, and unsaturated hydraulic conductivities, while soil water retention, observed volumetric water contents, and the spatial variability of these properties were smaller. Soil respiration rates were mainly changing with time as a result of soil temperature and soil water content. However, we also found that the diffusive soil properties slightly influenced CO2 efflux rates.