Figure 1.

A theoretical model of soil microbial growth under press perturbation based on the classic grazing model of Noy-Meyr [5]: (a) growth and removal rates as a function of microbial biomass; (b) different trajectories of microbial biomass over time (the red, green and black solid lines corresponds to the black, red and green dots of panel (a). The dynamics of microbial biomass (state variable) are governed by two factors: the positive contribution of growth (a, black line), which follows a logistic function, and the negative contribution of biomass removal (a, blue line) caused by an abiotic (e.g. drought) or biotic (e.g. grazers) factor held constant (press). We assumed a sigmoidal shape of removal rate (blue curve in a) and searched for parameters that generated two stable states (black and green dots in a and black and green lines in b) and one unstable state (red dot in a, and red horizontal line in b). The unstable state is a critical threshold (red horizontal line): above it, the system settles on stable state 1; below it, the system settles on stable state 2.