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

Soil respiration is a significant driver of climate change and is anticipated to intensify extreme weather events. This type of respiration is associated with soil microorganisms and is a by-product of the global carbon cycle, known for decomposing organic matter. While several parameters impact the respiration rate, soil moisture content has been identified as the most significant abiotic facto, with one of its negative impacts being the appearance of diffusion-limiting effects. This diffusion of nutrients across the soil profile is believed to be crucial as the bound microorganisms depend on nutrients circling towards them across water-connected pores. However, uncertainties persist regarding the relationship between diffusion and soil moisture content, primarily due to the difficulties of capturing the soil respiration across the entire scale of the soils and the destructive nature of traditional respiration and the destructiveness associated with soil water content analyses. Due to this, geophysical tools, including electrical conductivity measurements, have started to be applied to attempt to capture moisture contents as, similarly to the respiration rates, electrical conductivity (EC) relies on the aqueous phase as both solids and gases are isolators. In the present study, we applied various matric suctions and measured the associated soil respiration flux and electrical conductivity, respectively, to validate our hypothesis that there will be a correlation between both respiration and EC. This fascinating relationship would allow us to find a new methodology to capture the respiration rate without reaching invasive steps. The samples were composed of natural and sieved soils from different types of cultivation, as well as top and subsoils. Our findings revealed a strong positive correlation between respiration rates and EC across varying matric potentials. The optimal matric potential (-250 hPa) demonstrated peak respiration rates, coinciding with the combination of the presence of pore connectedness and oxygen availability. Beyond this threshold, respiration rates and EC declined with decreasing soil water content, particularly in sieved samples, where homogenised pore sizes amplified this effect. These findings suggest EC could serve as a proxy for measuring the optimal conditions of microbial activity, offering a non-invasive tool to study soil respiration across diverse conditions. Future research could further refine this approach, enhancing our understanding of microbial processes and their environmental implications.