Research

The exchange of compounds at the boundary between the atmosphere and terrestrial ecosystems has profound and controlling effects on plant ecophysiology, ecosystem function and the chemical composition of both the biosphere and atmosphere. Further, the magnitudes of the fluxes between the earth and the atmosphere often control the pool sizes of important elements and compounds held in each, the balance of which are intrinsic to the maintenance of life on earth (e.g., carbon dioxide, nitrogen, ozone and other oxidants, and many others). For example, fluxes of various reactive trace gases between terrestrial ecosystems and the atmosphere exert significant influences on plant performance, tropospheric photochemistry, terrestrial biogeochemistry and the maintenance of clean air, water and soil. The study of fluxes between the biosphere and atmosphere requires skills in plant ecophysiology, with its emphasis on the mechanistic controls over flux; biogeochemistry, with its emphasis on the mass balance of biogeochemical cycles; and atmospheric chemistry, with its emphasis on the reactivity and control over turnover times of various atmospheric constituents. My research program at Cornell University focuses on the plant and soil-mediated exchange of compounds at the Earths surface with special emphasis on plant and soil based mechanisms of transfer.

Reactive gas-phase nitrogen compounds [NO_y = NO, NO₂, peroxyacetyl nitrate (PAN), HONO, HNO₃^-, N₂O₅ and various organic nitrates] are present in the atmosphere from both natural processes (soil driven processes and lightning) and industrial sources. In my own research, I am interested in the ecological importance of the interaction between reactive oxidized nitrogen and vegetation at scales from single genes to ecosystem level fluxes. This multiple scale approach is appropriate because the overall effect of reactive nitrogen is very much scale dependent (e.g., a beneficial fertilization effect observed at the leaf level may be offset by changes in litter quality at the ecosystem scale or changes in the oxidative chemistry of the atmosphere at the regional scale). Reactive oxidized nitrogen compounds are of particular interest for three reasons. At high concentrations reactive oxidized nitrogens are known to be phytotoxic to plants causing necrosis, decreased photosynthetic rates and reductions in growth. Interestingly, at lower concentrations reactive oxidized nitrogen compounds have been shown to act as an atmospheric fertilizer increasing plant growth in some cases. Therefore, understanding the basic interactions between vegetation and reactive oxidized nitrogen is of great importance to understanding basic plant ecophysiology.

Further, the input of reactive gaseous nitrogen directly to plants through foliar uptake is a pathway not normally considered in ecosystem nitrogen cycling. Multiple studies have addressed the ramifications of increased nitrogen inputs to ecosystems, but most have been limited to consideration of wet deposition of nitrate or ammonium to the soil system. Additionally, most of the data available for atmospheric nitrogen deposition has been obtained via networks where wet deposition is collected in buckets placed in open areas. Therefore, few studies of nitrogen deposition have considered the role of direct foliar uptake, and no studies have yet been undertaken to fully evaluate the speciation of deposited nitrogen. Additionally, the long-term indirect ramifications of direct foliar uptake of nitrogen on other aspects of ecosystem function, particularly carbon cycling, has yet to be fully explored.

Finally, nitrogen oxides (NO_x) play a central role in controlling the oxidative chemistry of the lower atmosphere-by catalyzing the formation of ozone and therefore influencing the total radical level in the atmosphere. This, in part, regulates the atmospheric concentrations of nitric acid and organic nitrates. Transformations of NO_x into other forms of oxidized nitrogen compounds affect the rate at which ozone and other oxidants are produced. Current atmospheric chemistry models utilize measured soil, industrial and agricultural NO emission rates as a primary input and assume that photochemical transformations of NO to the other components of NO_y occurs well above the influence from plant canopies. This practice ignores the possibility that plants can affect local photochemistry by assimilating, emitting or transforming certain forms of NO_y. Past studies from tropical forests in Brazil and Panama suggest that 30 - 60% of the soil-emitted NO can be transformed and assimilated by the overlying canopy. Therefore, understanding the interactions between NO_y compounds and vegetation is of great importance to regional and global atmospheric chemistry.

Examples of current research projects

The Controls Over the Assimilation and Emission of Atmospheric Reactive Nitrogen by Leaves

The USDA-NRI program has funded our lab group to investigate the physiological and molecular mechanisms of the leaf uptake of NO, NO₂ and peroxyacetyl nitrate (PAN). The research approach has consisted of three experiments and one modeling exercise: (1) quantify, using gas exchange techniques, the leaf uptake and emission of nitric oxide (NO), nitrogen dioxide (NO₂), and peroxyacetyl nitrate (PAN) across four important crop and ornamental plants (corn, wheat, sunflower, and periwinkle) and partition the resistance to leaf uptake among diffusional (stomatal aperture) and mesophyllic (apoplastic chemistry and elimination) processes, (2) use Arabidopsis thaliana genotypes deficient in the expression of nitrate reductase to examine the role of the metabolism of nitrogen on the uptake and emission of NO, NO₂, and PAN, and (3) fumigate wild-type Arabidopsis thaliana with NO₂, ozone, and NO₂ + ozone to examine the up-regulation (or lack thereof) of genes controlling nitrogen metabolism and apoplastic free radical chemistry. The resultant product of the three experiments is then combined into a mechanistic leaf model of gas-phase reactive nitrogen uptake and emission. Thus far, we have examined the leaf gas exchange characteristics for NO and NO₂. Patterns of uptake were similar for both gases with NO being taken up at a rate ten times slower than that observed for NO₂. The uptake of both gases appears to be strongly controlled by the stomatal aperture and the concentration difference between the leaf internal air space and the atmosphere, but significant mesophyllic resistances also exist (Teklemariam and Sparks, in review). The measurements made in this study of the leaf uptake of the organic nitrogen compound peroxyacetyl nitrate (PAN) were some of the first to examine the uptake of this compound at the leaf surface (Sparks et al. 2003).

Ramifications of Exposure to Industrially Derived Air Pollutants for Hardwood Forest Tree Species of New York state

The USDA-Hatch program has funded my lab group to examine the ramifications of the pollution compounds ozone (O₃) and nitrogen dioxide (NO₂) on hardwood tree species in Western New York. In the spring of 2003, we installed three canopy access towers in the Cornell Arnot Experimental Forest. The overarching research objective of this study was to examine the ability of leaves to effectively take up or eliminate pollution compounds. To investigate this, we are examining leaf uptake of ozone and nitrogen dioxide in three species known to exhibited different leaf biochemical properties; Black Locust (a nitrogen fixing tree species with decreased leaf nitrate reductase activity), Red Maple (a deciduous broadleaf with high photosynthetic rates and high levels of leaf nitrate reductase), and white pine (a conifer with decreased leaf nitrogen reductase activity).

Over the course of this project, I anticipate a complete analysis of the three tree species and an ability to predict the resistance of these species to increasing ozone and reactive nitrogen pollution expected in the region.

CAREER: Direct Foliar Uptake of Atmospheric Nitrogen: Molecular to Ecosystem Considerations

The National Science Foundation CAREER program has funded our group and allowed me to implement research examining reactive nitrogen exchange at the molecular, whole plant, and ecosystem level. Thus far, I have started three major research projects. First, the development and deployment of an eddy-covariance based flux instrument for the measurement of reactive nitrogen exchange between terrestrial ecosystems and the atmosphere. In collaboration with the Atmospheric Chemistry Division of the National Center for Atmospheric Research (NCAR), I spent the spring of 2003 developing this instrument. During the summer of 2003, the instrument was deployed in the Duke University Experimental Forest as part of the Chemical Emission, Loss, Transformation and Interactions within Canopies (CELTIC) study (a biosphere-atmosphere measurement study including researchers from three federal agencies and nine universities).

Second, we have started experiments examining the leaf uptake and emission of reactive nitrogen, uptake of ozone, and the emission of volatile organic carbon in the field. This past summer, we measured the emission/uptake of reactive nitrogen (as NO₂) and the uptake of ozone in the two dominant tree species at Duke Forest (sweet gum and loblolly pine). The results of these studies are currently being used (with other information from measurements made at the site) to parameterize a one-dimensional canopy transport and chemistry model for the Duke Forest. In addition, we have made measurements examining the emission of volatile organic carbon (VOC) from leaves of sweet gum under ozone fumigation.

Third, I am in the beginning stages of developing a project aimed at understanding the controls over nitrogen trace gas emissions from desert soils under global climate change. Beginning this summer, we will be measuring the soil emission of NO, NO₂, NH₃, and N₂O from soil collars installed at two sites in Nevada. The first of these sites is the Nevada Desert FACE Facility (NDFF), which uses free-air CO₂ enhancement technology to simulate a future elevated CO₂ environment. We will measure nitrogen trace gas emissions from both elevated CO₂ and control rings to test the hypothesis elevated CO₂ fundamentally changes soil processes and the resultant emissions of trace gases important to atmospheric chemistry. The second site consists of a fully replicated global change experiment simulating summer rain addition, increased nitrogen deposition, and disturbance. Similar to the NDFF site, we will measure nitrogen trace gases from collars to determine if major anticipated perturbations (summer water availability, nitrogen status, and disturbance) will influence soil processes and the emission of nitrogen trace gases important to atmospheric chemistry. This project is enhanced by collaborations with soil scientists from Washington State University and plant ecologists from the University of Nevada-Reno, University of Nevada-Las Vegas, and the Desert Research Institute.

Other Projects in the Lab

1. Relationships between herbivory and the emission of monoterpenes and sesquiterpenes in tropical forests. We have been pursuing a project in collaboration with a scientist from NCAR (Dr. Thomas Karl) examining terpenoid production during herbivory in tropical forest in Costa Rica.

2. The physiological and evolutionary ramifications of yucca moths, cheater yucca moths, and bogus yucca moths on their host Yuccas. This is a side project we have with two researchers from the University of Idaho (Drs. Kari Segraves and David Althoff). Drs. Segraves and Althoff are evolutionary ecologists and we have had several fruitful collaborations combining evolutionary and physiological aspects of this complex insect-plant association.

3. Natural history of slash pine in relation to ephemeral ponds in Central Florida. This is a small project I have pursued in collaboration with Dr. Peter Marks. Dr. Marks and I teach a course in Florida and during the course we have generated a dataset on the community and physiological ecology of this tree species.

4. The effects of CO₂, NO₂, and ozone on plant growth. This is the dissertation research of my graduate student, Ms. Allyson Eller.

5. The use of foliar nitrogen isotope ratios as an indicator of pollution nitrogen in plant biomass. This is the dissertation research of my graduate student, Ms. Dena Vallano.