A Trait-Based Model for Understanding Rates, Patterns, and Ecological Consequences of Microbial Nitrogen Fixation in High-Latitude Terrestrial Ecosystems

Nitrogen limitation constrains primary productivity in high-latitude terrestrial ecosystems. In these ecosystems, Biological Soil Crusts (BSCs) fix the majority of nitrogen and are therefore the dominant nitrogen source to the surrounding soil. Understanding the distribution of nitrogen fixing microorganisms and the constraints on the rate at which nitrogen is fixed is important for reducing uncertainty in predictions of carbon-climate feedbacks. However, few models take into account the environmental, ecological, and energetic constraints on nitrogen fixation. Here we present and discuss a model representing the spatial distribution and activity of nitrogen-fixing and non-fixing microorganisms. We represent a number of functional guilds with different traits associated with resource acquisition (e.g., carbon, nitrogen, phosphorus, and molybdenum) and environmental optima (e.g., light, moisture, and temperature). The model spatially resolves the distribution of nitrogen-fixers or non-nitrogen-fixers across nutrient gradients and demonstrates seasonality in fixation rates. An important aspect of the structure of the model is the inclusion of resource acquisition investments that strongly constrains the distribution of BSCs and impacts the rates of nitrogen fixation at the ecosystem level. Reducing the uncertainty associated with predictions of the fate of high-latitude soil carbon fluxes will require a mechanistic understanding of the nitrogen cycle under a changing climate. This model can significantly contribute to that goal.