Greenhouse Gas Microbiology in Wet and Dry Straw Crust Covering Pig Slurry
Hansen, Rikke R. | Nielsen, Daniel Aa | Schramm, Andreas | Nielsen, Lars P. | Revsbech, Niels P. | Hansen, Martin N.
Liquid manure (slurry) storages are sources of gases such as ammonia (NH₃) and methane (CH₄). Danish slurry storages are required to be covered to reduce NH₃ emissions and often a floating crust of straw is applied. This study investigated whether physical properties of the crust or crust microbiology had an effect on the emission of the potent greenhouse gases CH₄ and nitrous oxide (N₂O) when crust moisture was manipulated (“dry”, “moderate”, and “wet”). The dry crust had the deepest oxygen penetration (45 mm as compared to 20 mm in the wet treatment) as measured with microsensors, the highest amounts of nitrogen oxides (NO₂ ⁻ and NO₃ ⁻) (up to 36 μmol g⁻¹ wet weight) and the highest emissions of N₂O and CH₄ Fluorescent in situ hybridization and gene-specific polymerase chain reaction (PCR) were used to detect occurrence of bacterial groups. Ammonia-oxidizing bacteria (AOB) were abundant in all three crust types, whereas nitrite-oxidizing bacteria (NOB) were undetectable and methane-oxidizing bacteria (MOB) were only sparsely present in the wet treatment. A change to anoxia did not affect the CH₄ emission indicating the virtual absence of aerobic methane oxidation in the investigated 2-mo old crusts. However, an increase in N₂O emission was observed in all crusted treatments exposed to anoxia, and this was probably a result of denitrification based on NOₓ ⁻ that had accumulated in the crust during oxic conditions. To reduce overall greenhouse gas emissions, floating crust should be managed to optimize conditions for methanotrophs.
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