Microbe-mediated processes as indicators to establish the normal operating range of soil functioning

Soils are major contributors to global nutrient cycling processes, which are indispensable for the healthy functioning of our ecosystems. In this study, we raise the question whether soil functioning can be captured in a concept denominated normal operating range (NOR), or the normal fluctuations in soil functioning under field conditions. We further examine how this concept could be effectively used to evaluate the impact of disturbances on agricultural ecosystems. We propose the establishment of a NOR on the basis of multiple parameters in the soil. These should include so-called sensitive processes, that is, those processes that are poorly redundant and easily deviate following a stress situation. The model that we built allowed to visualize the interplay of multiple soil parameters, under which the sensitive ones, which would be most indicative of a disturbance. Here we use the initial step of nitrification, i.e. ammonia oxidation, as an example of a sensitive process. By capturing the normal fluctuations in ammonia oxidation-related parameters that take into account population dynamics, and implementing these in a mathematical model, a multidimensional representation of the NOR of soil function is created which is useful in tests of resilience in the context of disturbances.