I am an ecosystem ecologist who uses a variety of field and laboratory techniques to understand and predict how ecosystems are shaped by climatic change. Specifically, my work investigates how global change alters carbon, nitrogen, or phosphorus cycling in ecosystems, and how the feedbacks between nutrient limitation and plant production will affect ecosystem processes. My research questions broadly encompass ecosystems ranging from nutrient-limited peatlands to a large, forest scale atmospheric carbon dioxide manipulation. The ultimate goal of my research program is to improve our ability to predict ecosystem responses to environmental change and thus better inform policy decisions.

My research is generally focused on answering two questions: (1) How does environmental change alter the balance between ecosystem production and nutrient limitation? (2) How do root production and mortality affect soil carbon storage and nitrogen cycling?

How does environmental change alter the balance between ecosystem production and nutrient limitation?

Scaling plant nutrient use in nutrient-limited peatland ecosystems. A major finding of my research in peatlands was that the efficiency with which plants used nutrients differed at scales ranging from the leaf- to community level, and depended on whether the individual, species, and community were limited more by nitrogen or phosphorus availability.

Nutritional constraints control excess carbon storage under elevated atmospheric [CO2]. In general, forests do not use nitrogen more efficiently for biomass production under elevated [CO2], due in large part to the way that carbon is allocated within the stand. For example, the increased production of ephemeral fine roots in a CO2-enriched sweetgum plantation necessitated increased nitrogen uptake from the soil to produce roots with a high nitrogen concentration. A nitrogen fertilization experiment in an adjacent sweetgum stand indicated that the increased production of ephemeral roots was most likely a mechanism for greater nitrogen acquisition in response to nitrogen limitation within the stand. Two major implications of increased root production at the expense of wood production are: (1) very little extra carbon is stored in forest biomass, and (2) carbon and nitrogen cycle more quickly through the ecosystem.

How do root production and mortality affect soil carbon storage and nitrogen cycling?

Carbon and nitrogen input to the soil from increased root production. Increased belowground allocation may affect soil nutrient cycling and carbon storage because fine root populations turn over quickly in forested ecosystems. I combined a long-term minirhizotron data set with continuous, root-specific measurements of root mass and nitrogen concentration to assess carbon and nitrogen input from root mortality. I found that in a forest growing in an elevated concentration of atmospheric CO2, the flux of carbon and nitrogen into the soil nearly doubled due to stimulated root production and mortality. Moreover, much of the carbon and nitrogen input occurred relatively deep in the soil profile where the dynamics of root decomposition and carbon and nitrogen mineralization are likely to be different from what is commonly observed and modeled in the upper profile.

Root decomposition and carbon storage in soil organic matter. Root decomposition is rarely mechanistically linked with carbon and nitrogen storage in soil organic matter. I previously developed a novel litterbag methodology to examine root decomposition and carbon input to the soil in a CO2-enriched sweetgum plantation and found that root decomposition does not differ under elevated [CO2]. However, the question remains unanswered as to whether the greater amount of root detritus under elevated [CO2] will be incorporated into long-lived soil organic matter. I am currently quantifying the transfer of carbon and nitrogen from decomposing root detritus into different soil organic matter fractions using the unique depleted 13C signature of organic material in plants and soils enriched with elevated [CO2].

Linking fine-root mortality with soil nitrogen cycling. It is currently uncertain whether the input of labile root carbon and nitrogen to the soil profile will stimulate microbial degradation of organic matter and increase soil nitrogen availability (i.e., the “priming effect"), or increase microbial immobilization of available nitrogen and constrain forest production.  To test whether greater root-derived carbon and nitrogen input in a CO2-enriched deciduous forest altered soil nitrogen availability, I used a cutting-edge isotope dilution technique to measure nitrogen cycling rates at several soil depths. My results thus far indicate that nitrogen is available for root uptake deep in the soil profile, which may serve to supplement increasingly limited soil nitrogen availability under elevated [CO2].