Nitrous oxide (N2O) is one of the three most important biogenic greenhouse gases contributing to the human induced global warming trend over the past 150 years. Consequently, N2O is also an integral component of greenhouse gas accounting practices and included in both European and U.S. carbon markets (i.e. the Chicago Climate Exchange). While recent efforts by the IPCC have balanced the global N2O budget, great uncertainty remains. Nearly all materials on Earth have a consistent relationship between the 17O and 18O isotopes of oxygen that is termed mass dependence. Tropospheric N2O, however, is enriched in 17O relative to oxygen in mass dependence by 0.9 ?. This 17O anomaly in N2O has long been considered to reflect incorporation of 17O from photochemical reactions in the stratosphere and exchange of N2O between the stratosphere and troposphere. Recently it has been proposed that the 17O anomaly in N2O is biologically produced and the result of incorporation of oxygen from water into N2O during its microbial production. If correct, this would be the first observation of a biologically produced 17O anomaly. In this proposal the PIs will test three mechanisms by which the 17O anomaly may be introduced into biologically produced N2O: (1) exchange with water, (2) differential incorporation of oxygen from O2 and water, and (3) denitrification of atmospheric nitrate that carries a 17O anomaly. Further, they will test for the presence of a 17O anomaly during production by inorganic UV photo-oxidation. The PIs will pursue a variety of approaches to evaluate the potential for biological N2O to generate a 17O anomaly that includes production of N2O from purified enzymes, pure microbial cultures and incubation of agricultural soils.
The presence of a 17O anomaly in biologically produced N2O has profound implications for understanding the origin of atmospheric N2O and the global budget of this important greenhouse gas by international organizations such as the IPCC. The instrumentation, methodologies, and knowledge of microbial N2O production that will be developed will become an integral component of a short course at MSU that involves graduate students, professors and professionals from across the country (Stable Isotope Biogeochemistry). The project will also involve a team undergraduate mechanical, electrical and computer engineering students who will develop and deploy an Eddy-covariance Trace Gas Trapping System that will enable collection of sufficient quantities of N2O in a plant canopy to constrain the iso-flux of N2O from soils to the atmosphere.
