Ecosystem stoichiometry and individual nutrient dynamics

Elemental or stoichiometric ratios of C:N:P in ecosystems are of interest because they are often remarkably constant across space and through time and they distill great complexity into a simple metric. Purely physical forces, for example mixing in the oceans and aoelian deposition on land, and ecological dynamics, such as photosynthesis, predation and evolution, act in concert to determine specific ratio values, meaning that a given ratio reflects a particular combination of interacting processes. The challenge is to understand how complex interactions between abiotic forcing and biological activity conspire to generate specific ratio values.  Using mathematical models, empirical ecosystem data, and experimental phytoplankton cultures, we are studying how biological processes, such as nutrient recycling, affect whole ecosystem stoichiometry. Models incorporate plant and phytoplankton nutrient uptake physiology as well as growth responses to internal resource concentrations and relate autotroph nutrient requirements to N:P stoichioimetry of ecosystem inputs and losses and recycling.  In collaboration with Josh Weitz at Georgia Tech and Duncan Menge at NCEAS, we are developing models that incorporate coupled, systems-level regulation of inorganic and organic N and P uptake, and in the lab we are performing experiments to parameterize and test our models.

Effects of warming on SOM decomposition

Soils harbor the largest pool of carbon in terrestrial ecosystems.  The carbon found in soils, referred to as soil organic matter (SOM), assumes various compounds with distinct chemical properties, such as recalcitrance to decomposition and C:N ratio.  Old, recalcitrant compounds, often with low C:N ratios comprise the majority of SOM, and are presumed to exhibit the greatest temperature sensitivity of decomposition.  Thus, in order to predict CO2 release from soils in a warmer future, it is necessary to quantify the temperature sensitivity of microbially mediated SOM decomposition. Sharon Billings and I are working to develop theoretical and experimental approaches for studying the temperature sensitivity of SOM decomposition.  We are characterizing the effects of temperature on extracellular enzyme-soil substrate interactions, and will then determine how the temperature sensitivity of such interactions constrain the ability of soil microbes to decompose SOM with differing degrees of recalcitrance.  Our approach uses mathematical models, experiments with extracellular enzymes and substrates in isolation, soil microbes in artificial soils, and incubations of real soils with intact microbial communities.

Joint influence of elevated CO2 and N availability on C fixation in freshwater ecosystems

On land, plants "fix" carbon by removing CO2, the sole carbon source for photosynthesis, from the atmosphere and reducing it to sugars which are used to produce energy.  In aquatic ecosystems, CO2 is much less abundant due to its tendency to dissolve and dissociate into bicarbonate, which poses a challenge for phytoplankton.  However, increases in atmospheric CO2 concentrations will result in increased CO2 concentrations in water, which increases the availabiltiy of C for phytoplankton.  Therefore, primary producers in aquatic ecosystems have the potential to buffer the projected increases in future CO2 concentrations, if their growth isn't limited by something else. N often limits primary production in ecosystems, making it necessary to determine if and when N availability will constrain the uptake of C in aquatic ecosystems.  We are subjecting phytoplankton and bacterial communities harvested from lakes to different CO2 concentrations and N concentrations to determine the potential for aquatic ecosystems to take up additional C in response to elevated CO2 concentrations in the future.

Evolution and ecosystem development in the oceans

Physical mixing and the chemical environment set the stage for biology in the oceans by creating hetergenous habitat and by constraining metabolic pathways used for generating energy or ATP.  Both evolutionary trajectories and trophic interactions are influenced by spatial and temporal variability in environmental conditions.  Diversity, food web structure and ecosystem function associated with microbes to mammals is ultimately determined by redox chemistry and the scales over which ecological interactions occur. We are working to identify the critical spatio-temporal scales associated with the capacity to fully express genomic variability and the mode and tempo of evolution of microbes and phytoplankton in the oceans.   Understanding how primary producers and marine microbes are likely to respond to changing oceanic conditions will be critical for developing solutions to the climate change problem we are currently facing.  We are working with Simon Levin, Paul Falkowski and Oscar Schofield on this project.