Total Solid Levels of Dairy Slurries Can Affect Cumulative CH4 Flux and Abundance of Methanogenic Archaeal Communities.

Stored liquid dairy manures are methane (CH₄) emission hotspots, mainly due to large quantities of substrates, typically as total solids (TS), being stored under anaerobic conditions in the presence of methanogenic communities. Our research has indicated that reduction of TS levels of dairy slurries prior to storage can mitigate CH₄ emissions. In the current study we determine if shifts in the abundance of methanogens is related to decreasing CH₄ emissions.  Mesoscale slurry storage facilities, located at the Bio-Environmental Engineering Center in Truro, Nova Scotia, Canada, have been equipped for continuous gaseous emission monitoring. Fresh dairy slurry having 9.5%TS and two other dilutions (0.3%TS and 5.8%TS) were stored for up to 6 months. Manure samples were taken after 30 and 120 days of the storage (20 May-16 Nov 2010) from the upper and bottom layers of the slurries, stored in lifeguard, and DNA were extracted. Quantitative real-time PCR (qPCR) and PCR-DGGE based determinations of abundance and phylotypes of methanogens, respectively, were studied by targeting the gene encoding the alpha subunit methyl-coenzyme M reductase (mcrA), which catalyzes the final step of methanogenesis. Interestingly, mean abundances of methanogens increased significantly (14.6±3.5×) (p<0.001) as TS decreased from 9.5% to 0.3%. Cumulative CH₄ emissions, however, decreased with TS content (~12.5 and 3.8 kg-CH₄ m^-2 from slurries having 9.5% and 0.3%TS contents, respectively). Nevertheless, compared to the 30 days of storage, mean abundances of methanogens were relatively higher at 120 days, consistent with an increase in the cumulative CH₄ emissions. PCR-DGGE analysis indicated a low methanogen diversity, with most bands sequenced closely related to the genus Methanocorpusculum (>95% amino acid sequence similarity), the hydrogenotrophic methanogens. Results suggest that available carbon substrate and not methanogen abundance may limit cumulative CH₄ emissions at reduced TS levels of dairy slurries.

Keywords: climate change, dairy farming, manure management, total solid