Methanogenic pathways and <i>δ</i> ¹³ C values from swine manure with a cavity ring‐down spectrometer: Ammonia cross‐interference and carbon isotope labeling

Abstract

Rationale Carbon isotope ratio analysis using cavity ring‐down spectroscopy (CRDS) offers insight into methanogenic activity. In livestock buildings, measuring 13 CH 4 / 12 CH 4 by CRDS is challenging and prone to errors, due to cross‐interference from NH 3 . Therefore, an interference‐corrected and improved approach to monitor methanogenic pathways with CRDS was developed and described in this study. Methods Cross‐interference from NH 3 and other livestock gases on 13 CH 4 / 12 CH 4 was measured using CRDS. The removal efficiency of livestock gases was quantified using proton transfer reaction mass spectrometry. The linearity of the cavity ring‐down spectrometer was tested by dilution of a 13 CH 4 standard up to 15.6 atom % in CH 4 . 13 C isotope‐labeled acetate was utilized to elucidate the methanogenic pathway contribution in swine manure using a stoichiometric model. Results Significant NH 3 cross‐interference on 13 CH 4 / 12 CH 4 was observed, and correction factors were calculated. Cross‐interfering gases were effectively removed using a copper scrubber and a Nafion tube. The instrument responded linearly with 13/12 C ratios up to 7.6 atom % in CH 4 . Isotope‐labeled experiments suggested that hydrogenotrophic methanogenesis was dominant and acetate‐derived CH 4 was limited in the studied swine manure. The observations were cross‐validated and consistent with measurements of the natural isotope signatures. Conclusion NH 3 cross‐interference leads to major errors in isotope ratios and therefore in evaluating the relative methanogenic pathway activity. We present methodological approaches, which are crucial to apply in livestock‐related uses where NH 3 is abundant. This methodology provides a considerable benefit to advance the understanding and reduction of methane emissions in the livestock sector.