Wednesday, November 7, 2007
293-8

Using 15N-Nitrate Tracer Indicates Incomplete Acetylene Inhibition of Nitrous Oxide Reduction in Denitrification Potential Assay.

Kewei Yu1, Dong Cheol Seo2, and Ronald DeLaune1. (1) Louisiana State University, Wetland Biogeochemistry Inst., Louisiana State Univ., Baton Rouge, LA 70803, (2) Gyeongsang National University, Department of Enviro-Biotechnology, Jinju, South Korea

Acetylene (C2H2) inhibition technique is the most commonly applied method in potential denitrification assay. One lake sediment and three soils for rice production were used in this study to test the effectiveness of inhibition of N2O reduction to N2 by C2H2 using 15N tracer. The three rice soils were pre-incubated under anaerobic conditions to allow time for soil denitrifying enzymes, especially N2O reduction enzyme, to be fully developed. Results show that in presence of C2H2, significant isotopic enrichment of 15N2 was found at end of typical denitrification incubation. The d15N2 value increased from the initial 0‰ to the final 7.8 -19.3‰ and 7.5 -10.6‰ for the treatment with 0.05 and 0.2 mg N labeled nitrate, respectively. Such 15N enrichment can be interpreted as N2 formation on average accounting for 18% and 12% of the total added N in the above two treatments, respectively. Nitrous oxide accumulation in presence of C2H2 could not account for the total added N. Incomplete inhibition of N2O reduction might be largely responsible for this observation. The results also indicated that part of the added nitrate was probably converted to ammonium by dissimilatory nitrate reduction reaction during the incubation. This study presents clear evidence of incomplete inhibition of N2O reduction to N2 by C2H2 in denitrification study.  Nitrogen gas formation in presence of C2H2 challenges validity of any C2H2 based techniques in denitrification study, because the denitrification rate will be underestimated if N2O reduction to N2 is not completely inhibited.