Hydrologic controls on phosphorus dynamics: A modeling framework

A process-based modeling framework is developed to examine the control that the soil moisture and soil carbon dynamics have on phosphorus (P) availability. The model accounts for soil moisture controls on several key fluxes that regulate P availability, including the decomposition rate of litter and humus, the mortality rate of microbes, the plant P demand and leaching of P from the rooting zone. To examine these dynamics over a wide range of time scales (i.e., from daily to seasonal to annual) we couple a zero-dimensional stochastic soil moisture model with a soil carbon model and a soil P model, all of which are run at the daily time scale. The soil carbon model accounts for the dynamics of the litter pool, humus pool, and microbial biomass pool, while the soil P model includes two inorganic P pools and four organic P pools that are coupled with the soil carbon pools. A mechanism is included in this framework to account for the contribution of enzymatic release to P availability, which has been shown to be an important source of P under conditions of low P availability. The model is applied to a Cerrado ecosystem located in Central Brazil where the phosphorus, carbon and hydrologic cycles have been well documented. Despite this application to a specific system, the modeling framework developed here is general and could be applied to any P-limited ecosystem for studies that seek to examine the controls on P-availability over time scales ranging from weeks to years. Results indicate that the model accurately captures the key driving variables controlling both the short-term (i.e., day to week) and long-term (i.e., year to decade) phosphorus, soil moisture and carbon dynamics. The effect of stochastic rainfall variability on P cycling is examined by running long-term simulations to obtain a probabilistic characterization of the state variables and fluxes under different rainfall regimes. The effect of a change in the precipitation regime suggests that such changes can significantly affect the dynamics of state variables and fluxes controlling P-availability. Moreover, results from the model illustrate the importance of the microbial retention of P, as a P cycling and conserving mechanism for P-limited ecosystems.