| Nitrifying and denitrifying bacteria carry out a vital role in soil systems by regulating supplies of inorganic nitrogen. Nitrifying and denitrifying bacteria also affect the Earth's atmosphere profoundly through the addition of the greenhouse gas, N2O. To better understand the role of nitrification and denitrification in atmospheric greenhouse gas production we need a reliable method to distinguish N2O formed from these two processes. Such information will be particularly valuable in agricultural environments that can be managed to foster one process or the other. The intramolecular distribution of 15N in nitrous oxide (isotopomer) is emerging as a new tool in defining the sources and sinks of this trace gas (Popp et al., in press; Toyoda and Yoshida, 2000; Toyoda et al., in press, Perez et al., 2001) and is commonly expressed in terms of site preference (difference in δ15N between the central and outer N atoms). Using laboratory cultures of whole soil microbes and pure cultures of an N 2O-producing denitrifier, Pseudomonas chlororaphis (ATCC #43928), it was demonstrated that the isotopomer fingerprint of N 2O derived from denitrification is unique from that of nitrification (Christensen and Tiedje, 1988). Furthermore, it was demonstrated that in agricultural soils the consumption of N2O during denitrification has no affect on site preference, but in deciduous forest soils an isotopomer effect is observed. With these results we are now poised to begin to apply isotopomers to apportion the relative contribution of N2O derived from nitrification and denitrification in agricultural soils. |
