Enhanced methane oxidation in surface crusts of manure storages with controlled ventilation

Manure storage contributes significantly to agricultural methane (CH₄) emissions. Surface crusts forming during liquid manure (slurry) storage can act as microbial filters that oxidize CH₄. However, in open storage tanks, the variable environment limits growth and activity of methanotrophs. This study investigated a novel automated ventilation control (auto-control) to enhance CH₄ oxidation during storage of cattle slurry with a well-developed crust. Implemented in pilot-scale storage tanks with slurry and crusts transplanted from a practical farm, the auto-control regulates CH₄ and O₂ concentrations above the crust to enhance CH₄ oxidation. Concentration profiles of CH₄, carbon dioxide (CO₂) and nitrous oxide (N₂O) indicated that O₂ penetrated 30-50 mm into the crust. Methane oxidation efficiency was first quantified at fixed ventilation rates, and subsequently using the auto-control ventilation. Isotopic analysis of CH₄ emissions in early summer confirmed a CH₄ oxidation efficiency (f_ox) of 20-50 % at fixed ventilation rates of 0.5-70 m³ h^-1. In late summer, f_ox had increased to 75-80 %, and operation under auto-control reduced ventilation rates and CH₄ emissions compared to a fixed ventilation rate simulating open storage. This was confirmed in additional campaigns during September and October. Microbial analyses confirmed a high abundance and diversity of methanotrophs in the upper 5 cm of the crust. These findings underscore the potential to optimize manure storage conditions for enhanced microbial CH₄ oxidation and reduced emissions.