Reply to “Differences in Nitrogen‐15 Enrichments of Evolved Nitrous Oxide and Dinitrogen and the Question of a Uniform Nitrate‐15 Pool”

In response to Dr. Focht's comments concerning our paper (Mulvaney and Kurtz, 1984), we would like to point out first of all that the work reported in this paper was designed with two objectives in mind: (i) to establish trends in evolution of N2 and N2O associated with the wetting and drying of soils, and (ii) to assess the relative significance of nitrification and denitrification as sources of the N2O evolved from NHi'-fertilized soils. The procedures we employed in this work for analysis of N2 and N2O provide information concerning the N enrichments of the N2 and N2O evolved from soils treated with N-labeled fertilizer (see Mulvaney and Kurtz, 1982; Siegel et al., 1982), and we included some discussion of this subject in our paper (see Mulvaney and Kurtz [1984], p. 600). We did not emphasize this aspect of our work, however, because we feel that these enrichments have limited significance without further information concerning the isotopic uniformity of the NOf pool undergoing denitrification in N-fertilized soil. We find it difficult to understand why Dr. Focht objects to the statement in our paper referring to the assumption that "N2 and N2O evolved during denitrification in soils treated with N-labeled fertilizer will always have the same N enrichment", because although the reference we cited (Focht et al., 1980) does deal with laboratory studies of denitrification under steady-state conditions, Dr. Focht and his colleagues have employed this same assumption in research with field soils (Ryden et al., 1979). We acknowledge that our procedures for determination of N2 and N2O are based on the assumption that the NOf denitrified can be considered to exist in a single pool having an average N enrichment and that further work is needed to determine whether and when this assumption holds for soil. We also acknowledge that, as Dr. Focht has pointed out with a hypothetical example, these procedures do not permit detection of unlabeled N2 or N2O evolved from soils, hence the title, "A new method for determination of Nlabeled nitrous oxide" (italics added for emphasis), of the paper by Mulvaney and Kurtz (1982). We appreciate the fact that Dr. Focht took the time to derive an equation that allows calculation of the amount of N-labeled N2O without the assumption of a binomial distribution of masses. Use of his equation requires that the ratio difference,