Electrical conductivity measurements as a proxy for diffusion-limited microbial activity in soils

Abstract. Soils play a highly dynamic role in the carbon cycle, by acting as either a carbon source or sink. Despite their importance in the global carbon cycle, uncertainties surrounding soil-atmosphere interactions remain, due to the many mechanisms that underlie soil carbon dynamics. One of the main mechanisms determining the decomposition of organic C in soil is the access microbial decomposers have to substrates. While not yet formally tested, there is evidence to support the idea that microbial decomposer access to substrates is diffusion-limited. This is underlined by soil respiration rates being strongly dependent on water availability. In recent years, non-destructive geophysical tools, including electrical conductivity measurements, have been used to determine the water content of soils and connectedness of the water phase in the soil pore network. As both respiration and electrical conductivity may depend on water availability and connectivity, our study aimed to determine whether electrical conductivity measurements could be used as a proxy of diffusion-limited microbial activity in soils. This was done by measuring electrical conductivity and respiration rates at different matric potentials. Sieved and undisturbed top and subsoil samples taken from conventional tillage and conservation agriculture management plots were used. Our results revealed an initial increase and consecutive drop in soil respiration associated with a decrease in the matric potential. The electrical conductivity followed a similar decrease throughout the experimental range and these showed a significant non-linear relationship. These results thus suggest that both measured variables depend on the connectedness of the aqueous phase and suggest that they could be used as groundwork for further investigations into soil respiration and electrical conductivity dynamics.