Integrating microbial eco-physiological responses to drought and nutrient limitation into ecosystem models

Background/Question/Methods

Microbial activity varies in time in response to changes in environmental and biogeochemical conditions. Describing these responses in mathematical models is complicated by the fact that exogenous environmental forcing and nonlinearities describing the endogenous dynamics result in variability and excitations over many time scales. Experiments tend to highlight the uniqueness of responses observed in each setting, while models are designed to synthetize knowledge into mathematical formulations that effectively capture mechanisms. In this contribution, we focus on responses of microbial activity to changes in moisture and substrate stoichiometry so as to synthetize current understanding of these processes in soil biogeochemical models that can be interfaced with Earth System models. Water availability affects microbes by both requiring physiological adjustments and altering substrate availability. The physiological adjustments include osmoregulation and transition to dormant states, while substrate availability changes due to the reduction in water films around soil particles as soil moisture declines – all these elements are accounted for in a novel mathematical framework. Effects of nutrient availability are instead described by a stoichiometric model of microbial growth and turnover.

Results/Conclusions

Using the proposed model, how physiological adaptations and physical limitations are strongly coupled is assessed, resulting in a continuum of microbial response strategies that ranges from drought-resistant microbes to drought-avoiders. The former synthetize osmolytes in response to drought, although reduced substrate supply may limit this possibility. The latter instead become dormant early during a dry period, thus avoiding the consequence of drought. These strategies are finally compared from the perspective of microbial carbon economy. Low nutrient availability is shown to lower microbial carbon use efficiency as microbes adjust their metabolism by investing more carbon to enzyme production and possibly to ‘futile cycles’, leading to overflow respiration. These enhanced carbon losses may compensate low nutrient availabilities, thus ensuring a balanced diet for the microbial cells.