[M-IS26] Biogeochemistry

[MIS26-23] Nitrogen, carbon, and sulfur isotope fractionation during heterotrophic and autotrophic denitrification reactions

*Takahiro HOSONO¹, Kelly ALVAREZ¹, In-tian LIN², Jun SHIMADA² (1.Priority Organization for Innovation and Excellence, Kumamoto University, 2.Graduate School of Science and Technology, Kumamoto University)

Keywords:N-C-S stable isotope ratios, batch culture experiments, groundwater, denitrification

In batch culture experiments, we studied the isotope fractionation of nitrogen in nitrate, carbon in dissolved inorganic carbon, and sulfur in sulfate during heterotrophic and autotrophic denitrification of two bacterial strains (P. aerofaciens and T. denitrificans). Heterotrophic denitrification experiments were conducted with trisodium citrate as electron donor, autotrophic denitrification experiments were carried out with iron disulphide 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 the degree of denitrification achieved in seventy days was 60‰. In the former experiment, systematic change of δ¹³C_DIC with increase of DIC was observed during denitrification (enrichment factor εN was -2.3‰), suggesting the contribution of C of trisodium citrate. No SO₄^2- and δ³⁴S_SO4 changes were observed. In the latter experiment, clear fractionation of δ¹³C_DIC during DIC consumption and δ³⁴S_SO4 during sulfur use of FeS₂-S (around 2‰) were confirmed through denitrification (εN = -12.5‰). The results of this batch experiment study are useful to understand the anaerobic bacterial denitrification processes in contaminated groundwater flow systems where a carbon source and/or pyrite are present. However, in natural aquifers, other anaerobic microbial activities such as sulfate reduction and methanogenesis would take place after or in the middle of the progress of the denitrification reaction, which play a decisive role changing isotope ratios of carbon and sulfur. Nevertheless, obtained results can be applicable in environments where complex simultaneous anaerobic reactions would not occur after, in the middle of the denitrification reaction, or at organic poor land that prevent further heterotrophic bacterial reactions to proceed.