Nitrogen, carbon, and sulfur isotopic change during heterotrophic (Pseudomonas aureofaciens) and autotrophic (Thiobacillus denitrificans) denitrification reactions

In batch culture experiments, we examined the isotopic change of nitrogen in nitrate (delta N-15(NO3)), carbon in dissolved inorganic carbon (delta C-13(DIC)), and sulfur in sulfate (delta S-34(SO4)) during heterotrophic and autotrophic denitrification of two bacterial strains (Pseudomonas aureofaciens and Thiobacillus denitrificans). Heterotrophic denitrification (HD) experiments were conducted with trisodium citrate as electron donor, and autotrophic denitrification (AD) experiments were carried out with iron disulfide (FeS2) as electron donor. For heterotrophic denitrification experiments, a complete nitrate reduction was accomplished, however bacterial denitrification with T. denitrificans is a slow process in which, after seventy days nitrate was reduced to 40% of the initial concentration by denitrification. In the HD experiment, systematic change of delta C-13(DIC) (from -7.7 parts per thousand to -12.2 parts per thousand) with increase of DIC was observed during denitrification (enrichment factor epsilon N was -4.7 parts per thousand), suggesting the contribution of C of trisodium citrate (delta C-13 = -12.4 parts per thousand). No SO42- and delta S-34(SO4) changes were observed. In the AD experiment, clear fractionation of delta C-13(DIC) during DIC consumption (epsilon C = -7.8 parts per thousand) and delta S-34(SO4) during sulfur use of FeS2-S (around 2 parts per thousand), were confirmed through denitrification (epsilon N = -12.5 parts per thousand). Different pattern in isotopic change between HD and AD obtained on laboratory-scale are useful to recognize the type of denitrification occurring in the field. (C) 2015 Elsevier B.V. All rights reserved.