Methanogenic pathways and delta 13 C values from swine manure with a cavity ring-down spectrometer: Ammonia cross-interference and carbon isotope labeling.

Abstract

RATIONALE\nCarbon isotope ratio analysis with cavity ring-down spectroscopy (CRDS) offers insight into the methanogenic activity. In livestock buildings, measuring 13 CH4 /12 CH4 by CRDS is challenging and prone to errors, due to cross-interference from NH3 . Therefore, an interference-corrected and improved approach to monitor methanogenic pathways with CRDS was developed and described in this study.\n\n\nMETHODS\nCross-interference from NH3 and other livestock gases on 13 CH4 /12 CH4 was measured by CRDS. The removal efficiency of livestock gases was quantified by proton transfer reaction mass spectrometry (PTR-MS). The linearity of the cavity ring-down spectrometer was tested by dilution of a 13 CH4 standard up to 15.6 atom % in CH4 . 13 C isotope labeled-acetate was applied to swine manure to quantify the contribution of methanogenic pathways using a stoichiometric model.\n\n\nRESULTS\nSignificant NH3 cross-interference on 13 CH4 /12 CH4 was observed and correction factors were calculated. Cross-interfering gases were effectively removed with a copper scrubber and a Nafion tube. The instrument responded linearly with 13/12 C ratios up to 7.6 atom % in CH4 . Isotope labeled experiments suggested that hydrogenotrophic methanogenesis (HM) was dominant and acetate-derived CH4 was limited in the studied swine manure. The observations were cross-validated and consistent with measurements of the natural isotope signatures.\n\n\nCONCLUSION\nNH3 cross-interference leads to major errors in isotope ratios and hence in the evaluating of the relative methanogenic pathway activity. We present methodological approaches, which are crucial to apply in livestock-related applications where NH3 is abundant. This methodology provides a considerable benefit to advance the understanding and reduction of methane emissions in the livestock sector.