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 (fox) of 20-50 % at fixed ventilation rates of 0.5-70 m3 h-1. In late summer, fox 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.