Phosphorus Limitation and the Reduction of the Carbon Fertilization Effect in Tropical Ecosystems

Abstract

Predictions for future climate increasingly rely on negative feedbacks of carbon dioxide fertilization in tropical ecosystems to mitigate agricultural and ecosystem changes resulting from higher atmospheric CO2 concentrations. However, this fertilization effect could be reduced if tropical ecosystems become limited by phosphorus. Though phosphorus models exist, they focus on agricultural simulations and are limited by a lack of stoichiometric and allometric data of tropical plants. This thesis adds to those data, which allows for the simulation of a phosphorus cycle in a tropical ecosystem in the Ecosystem Demography model 2 (ED2). Results from greenhouse and field experiments are less than promising, because of a small sample size. They do, however, indicate that supplemental limiting nutrients allow CO2-fertilized plants to grow more, and they give additional allometric constraints to use in a model. Comparing carbon uptake under scenarios with and without a phosphorus cycle and limitation supports previous results that the total uptake of ecosystem carbon will decrease when the system is limited by phosphorus. When phosphorus is in abundance, however, there is no difference between high-CO2 model runs that permit phosphorus limitation and those that do not. On a larger scale, we can expect ecosystem-level shifts as plants that take better advantage of high carbon dioxide concentrations out-compete those that do not. To predict the precise effect on real-life ecosystems, more tropical phosphorus stoichiometric data are necessary.