Rates of plant growth in tropical forests are among the highest on earth. As they grow, tropical plants take up large amounts of carbon dioxide from the atmosphere during photosynthesis, and store a substantial proportion of that carbon in their biomass and ultimately in soil. In addition, there is some evidence that plant growth will accelerate in the future with rising atmospheric CO2 concentrations, taking up and storing more carbon. However, it is unclear whether other plant nutrients are sufficiently plentiful in tropical forest soils to allow plants to grow as much as is predicted. In the tropics, soil phosphorus (P) concentrations are generally very low and such low P concentrations could strongly limit plant growth in the future. This research will explore how plants acquire phosphorus from soil, and which forms of phosphorus are actually accessible to plants. Experiments will explore how plant roots, bacteria and fungi "mine" nutrients from the soil, and will identify the specific microorganisms capable of accessing soil P. This work will be among the first to explore relationships between tree growth and different forms of soil P, it will improve understanding of the controls on biological P availability, and it will shed light on how pervasive P limitation is to plant growth in tropical soils. In addition, the project will enhance our ability to predict future rates of tropical forest productivity and could lead to improvements in how soil fertility is managed in tropical agricultural soils. Finally, the research will involve student training as well as promoting interaction between scientists and environmental journalists.
Knowledge of the true extent of nutrient limitation of plant primary production in the tropics remains incomplete, in large part reflecting our poor understanding of what forms of soil P are actually available to biological organisms. The proposed research will address the following overarching question: Are forms of soil phosphorus (P) that have traditionally been considered biologically unavailable actually available to plants and microorganisms? In the highly-weathered soils that dominate the lowland tropics, only a small fraction of total soil P resides in forms that are traditionally considered biologically available; the vast majority resides in more chemically and/or physically protected P pools that are presumed to be unavailable to plants and soil microbes. However, if plants and soil microbes can effectively overcome low soil P availability by accessing these recalcitrant forms of P that, until now, have been thought to be unavailable, then the extent of P limitation in the tropics could be less than current biogeochemical theory predicts. The proposed research includes a set of field, laboratory and greenhouse experiments conducted in a set of tropical forest sites in Panama. Together, the planned experiments have been designed to test the working hypothesis that P forms that have traditionally been viewed as unavailable are actually biologically available. Building on the experimental data, the research will culminate in a set of synthesis and modeling activities designed to explore how variations in soil P availability may affect future rates of NPP at ecosystem to global scales.
