Biogeochemistry

The study program in terrestrial biogeochemistry examines forest nutrient cycling and its relation to climate and soil type. Our current research activities are focused on A) an improved understanding of nitrogen and phosphorus availability in forested watersheds; B) on vegetation and nutrient dynamics on serpentine-derived soils and C) long-term variations in nutrient cycles after disturbance. Other areas of interest include sulfur cycling as a tracer for methanogenesis in wetlands and the role of nutrient cycling in ancient forests. This research is field-based and experimental, using both traditional mass-balance approaches as well as stable-isotope techniques to analyze mineral fluxes within these ecosystems.

Our research on nutrient availability is focused on the conventional view that nitrogen is limiting to temperate forests and phosphorus is limiting in tropical systems. Unfortunately, these ideas are based on inadequate and sparse measures of nutrient availability. We are examining the role of the plant in determining its nutrient supply via stimulation of rhizosphere activity and through mycorrhizal associations. Our current research suggests that light, temperature and water availability all influence the plant directly, determining the rate at which the plant supplies carbon to the microbial community, in turn affecting mineralization rates and nutrient transfer to the plant. This research is being conducted in Chile and in the Northeastern U.S.

In forests on Serpentine soils, we focus on evaluating the relative roles of nutrients and water on forest development. In these soils, some authors have found that Ca/Mg imbalances and heavy metals such as chromium and nickel are toxic. Serpentine soils develop very slowly and have poor water holding capacity; consequently, water availability may be as much a limit to growth as mineral imbalances. In areas where organic matter has developed on serpentine substrates, full-statured forests develop, apparently unaffected by the mineral relationships. To examine these relationships, we are working in forests in Puerto Rico and in the U.S.

To accomplish this research, we have a broad range of both field and laboratory equipment. Mineral analyses are performed using a Carlo Erba elemental analyzer, Technicon autoanalyzer or a Perkin-Elmer ion-coupled plasma spectrograph, as well as an x-ray diffractometer; isotopic analysis is accomplished with a Delta mass spectrometer equipped with an elemental analyzer and a gas chromatograph.

Current Research

1. Chiloé, Chile forest nutrient cycling project: determining the nutrient fluxes for a cool-temperate, non-polluted forest. This is a large multi-disciplinary study involving several universities.

2. Susúa, Puerto Rico serpentine forest: determining the relationship between forest biomass, growth and species composition relative to the soil characteristics and climate.

3. Chiloé, Chile and Whiteface Mountain, New York ectomycorrhizal project: comparison of nitrogen cycling and transfer from soil organic matter to the overstory vegetation in polluted and non-polluted ecosystems.

4. Chiloé, Chile root priming study: relationships between carbon flow from roots and the stimulation of rhizosphere activity to increase nitrogen mineralization and delivery to the plant.

5. Macapá/Peixe Boi Brazil and Luquillo, Puerto Rico phosphorus availability: soil phosphorus content and plant availability in tropical forests.

6. Luquillo Mountains, Puerto Rico. A 16-year record of watershed scale forest structure, litterfall and aquatic nutrient fluxes is being maintained with the help of the USDA Forest Service International Institute of Forestry and the Luquillo Long-Term Ecological Research program. This record, one of the most detailed available for any ecosystem, is being analyzed and modeled to evaluate the influence of natural disturbances and climate change on forest productivity and composition.