Oxidation of ¹³C-labeled methane in surface crusts of pig- and cattle slurry

Abstract

Storage tanks for slurry from animal production constitute important point sources for emission of CH₄ into the atmosphere. Recent investigations have demonstrated that surface crust formed on top of animal slurry provides a habitat for CH₄ oxidation activity, a finding which may open for new opportunities to reduce greenhouse gas emissions during storage of animal wastes. In this work, ¹³C-labeled CH₄ was used as a tracer to examine the absolute rates of CH₄ oxidation and production in intact crust materials, collected from six different pig- and cattle slurry tanks in late autumn. Methane concentrations were generally reduced in the presence of surface crust samples, with the exception of a LECA^®-based (light expanded clay aggregates) crust from a pig slurry tank. In four samples, CH₄ consumption was induced following a 2–4 days lag phase, whereas one cattle slurry crust consumed CH₄ immediately and showed a 92% decline in CH₄ concentration within the first week. Consumption of ¹³C-labeled CH₄ was paralleled by the production of ¹³C-labeled CO₂, thus providing direct evidence that microbial oxidation of CH₄ to CO₂ was taking place. Between 23% and 36% of the CH₄–¹³C consumed in the active samples was accounted for in the gas phase CO₂ indicating incomplete conversion of CH₄ to CO₂; however, comparable amounts of ¹³C was immobilized in the crust samples. Overall, the results showed that significant CH₄ oxidation to CO₂ in slurry crust samples occurs immediately or is inducible upon exposure to CH₄.

Keywords:

• Carbon-13
• Crust
• Methane
• Oxidation
• Production
• Slurry storage

Acknowledgements

We are indebted to Anja C. Nielsen for skilled technical assistance with the laboratory work. Financial support to the study was provided by a special grant from the Danish Forest and Nature Agency under the Ministry of the Environment.