Biogeochemical Controls on Nitrogen Fixation in a Diverse Neotropical Forest

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Biogeochemical Controls on Nitrogen Fixation in a Diverse Neotropical Forest.Lars Hedin and Anne Kraepiel, Princeton University The ability of tropical forests to neutralize future emissions of CO2 greenhouse gases depends critically upon whether these forests will become increasingly limited by soil nutrients. This project will use state-of-the art measures in Panamanian tropical forests to examine a crucial component of the response to CO2: the ability of soil microbes and certain higher plants in tropical forests to naturally acquire (or 'fix') the essential nutrient nitrogen from the atmosphere. There is considerable controversy among leading scientists as to whether or not nitrogen fixation will decrease as other nutrients become severely limiting to nitrogen fixers in response to climate change. However, there exists virtually no first-hand understanding of the factors that determine nitrogen fixation rates in tropical forests. This project will use direct measures of nitrogen fixation in combination with manipulations of nutrients including nitrogen, phosphorus and molybdenum, to examine and quantify these factors. Recent runs of the NOAA/GFDL earth system model show that solving the carbon and climate problem (i.e., stabilization at 500 ppm atmospheric CO2) is fully twice as difficult over the next fifty years if the tropics become severely limited by nutrients. This means that the role of nutrients in tropical forests constitutes one of the greatest uncertainties in our understanding of how CO2 greenhouse gas emissions will affect the Earth's climate system in the future. This project is an example of how basic science (i.e., the study of fundamental natural processes) has immediate and potentially deep implications for practical issues such as climate change. If the project is successful in understanding controls on nitrogen fixation, it becomes possible to incorporate these processes in the global earth system models, which, in turn, will contribute to reducing a major uncertainty about the existence of future natural sinks for CO2 emissions.