Current Research Projects

Controls on Nitrogen Retention and Loss in Humid Environments Climate Impacts on Productivity and Biogeochemical Cycling Carbon Dynamics is Disturbed and Recovering Tropical

Controls on Nitrogen Retention and Loss in Humid Environments

Tropical forests are the largest natural source of nitrous oxide production, a radiatively important greenhouse gas. The general conditions under which nitrous oxide is produced in humid environments (low soil redox, high carbon and available nitrate) also facilitate a less studied process called dissimilatory nitrate reduction to ammonium (DNRA). Our lab, in collaboration with the Firestone lab group, has documented high rates of DNRA in tropical forests in Puerto Rico and Costa Rica, and has recently found similar patterns in boreal forests in Alaska. We have shown that DNRA effectively conserves nitrogen in the ecosystem, and is likely to limit nitrous oxide production. We are now exploring other ecosystems for DNRA and determining the factors that

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Climate Impacts on Productivity and Biogeochemical Cycling

Systematic climate changes along elevation gradients offer convenient surrogates for climate change. We are currently working along elevation gradients in the Luquillo Experimental Forest (LEF), Puerto Rico, to identify linkages among climate and biogeochemical processes, and to explore direct or indirect plant and soil responses to soil oxygen availability, temperature, and light. In wet tropical forests, we have found that soil oxygen availability is inversely related to rainfall, can reach very low levels. We are measuring the effects of low oxygen levels on biogeochemical processes such as methane production and emission, nitrogen cycling, Fe dynamics, and P availability. Tropical forest productivity appears to be highly sensitive to very small changes in temperature (19-21oC). Our research group is working to identify the mechanisms responsible for this apparent temperature sensitivity.

We have recently begun work in Mediterranean ecosystems in California to explore the effects of climate change on biogeochemical cycling. While most climate models agree that temperature is increasing due to greenhouse gas emissions, these models disagree about future patterns in precipitation. In this study, we are examining the impacts of increased rainfall on annual grassland ecosystems and decreased water inputs to spring-fed wetlands. The grassland-wetland matrix is an important component of the northern California landscape, and our rainfall manipulation study should help us determine patterns in carbon and nitrogen storage and loss in systems that differ in their sensitivity to climate change.

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Carbon Dynamics in Disturbed and Recovering Tropical Forests

The storage of carbon in soils and biomass in tropical forests plays an important role in the global carbon cycle. High rates of tropical deforestation have prompted growing concern about the loss of carbon storage capacity, and increasing rates of carbon emissions to the atmosphere. Considerable recent efforts have focused on documenting the effects of deforestation and land use change on plant and soil carbon pools. Much less research has explored possible mechanisms to help offset carbon losses through reforestation of pasture and agricultural land. In Puerto Rico, we are determining the rate of new carbon accumulation in soils and plants following forest reestablishment, and the mechanisms responsible for patterns of carbon storage and loss.

 

 


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