Carbon transformations and ecosystem biogeochemistry

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. My research employs theoretical and experimental approaches to study and quantify the temperature sensitivity of SOM decomposition. This ranges from characterizing the effects of temperature and edaphic conditions on extracellular enzyme-soil substrate interactions, to quantifying respiration in the field and in the lab, to analyzing and simulating dynamic models of microbially mediated C transformations. My goal is to link microbial physiology and community structure to biogeochemical fluxes via equations parameterized with empirical data, which can subsequently be used to project ecosystem response to changing environmental conditions.

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.  I experimentally subject phytoplankton and bacterial communities to different CO2 concentrations and N concentrations to determine the potential for aquatic ecosystems to take up and transform additional C into DOC in response to elevated CO2 concentrations in the future.

To close the loop on C dynamics in the oceans, I am working to model DOC transformtions by bacteria using trancriptomic and FTICR-MS data simultaneously.  My goal is develop and parameterize metabolic models that can be used to simulate DOC dynamics under a variety of conditions in the oceans.

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.  I am 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.