Nutrient Dynamics in a Coupled Terrestrial Biosphere and Land Model (ELM-FATES-CNP)

We present a representation of nitrogen and phosphorus cycling in the Functionally Assembled Terrestrial Ecosystem Simulator, a demographic vegetation model within the Energy Exascale Earth System land model. This representation is modular, and designed to allow testing of multiple hypothetical approaches for carbon-nutrient coupling in plants. Novel model hypotheses introduced in this work include, (a) the controls on plant acquisition of aqueous mineralized nutrients in the soil and (b) fairly straight forward methods of allocating nutrients to specific plant organs and their losses through live plant turnover as well as litter fluxes generated through plant mortality. This combines the new with pre-existing hypotheses (such as nitrogen fixation and soil decomposition) into a system that can accommodate plant-soil dynamics for a large number of size- and functional-type-resolved plant cohorts within a time-since-disturbance-resolved ecosystem. Root uptake of nutrients is governed by fine root biomass, and plants vary in their fine root biomass allocation in order to balance carbon and nutrient limitations to growth. We test the sensitivity of the model to a wide range of parameter variations and structural representations, and in the context of observations at Barro Colorado Island, Panama. A key model prediction is that plants in the high-light-availability canopy positions allocate more carbon to fine roots than plants in low-light understory environments, given the widely different carbon versus nutrient constraints of these two niches within a given ecosystem. This model provides a basis for exploring carbon-nutrient coupling with vegetation demography within Earth system models. This work introduces a new set of nutrient cycling hypotheses incorporated into a terrestrial biosphere model. This includes the cycling of carbon, nitrogen and phosphorus, and focuses mainly on plant acquisition, allocation, and turnover. An analysis shows the model offers reasonable responses to perturbations in parameter constants and variability in climate forcing, considering its design balance between process complexity and parameterization burden. The nutrient enabled ELM-FATES model represents expected pattern responses to nutrient availability and parameter perturbations The model has been designed to introduce a reasonably small parameterization burden These hypothesis can capture some but not all elements of carbon-nutrient dynamics and can be further inter-compared with other hypotheses