Opportunities for reducing greenhouse gas emissions through livestock wastemanagement in Florida

M anagement of livestock wastes can affect greenhouse gas emissions through attenuating both methane and nitrous oxide emissions, as well as by displacing carbon dioxide emissions from fossil fuel use that can be avoided through biogas production and use. Methane is naturally produced from the anaerobic decomposition of livestock manure and is a potent greenhouse gas with 21 times the greenhouse warming potential of CO2, on a mass ratio basis (U.S. EPA 2007). Nitrous oxide is naturally produced as a result of the nitrogen cycle where organic nitrogen in manure and urine undergoes nitrification and denitrification. Nitrous oxide is an even more potent greenhouse gas, with a greenhouse warming potential 310 times that of CO2 on a mass ratio basis (U.S. EPA 2007). Unfortunately, estimates for N2O emissions are uncertain and methods to reduce these emissions are not well developed. In contrast, methods for reducing CH4 emissions have received more attention. Anaerobic digestion in a closed vessel allows microbial degradation of manure to biogas containing CH4. Biogas can be used as a renewable fuel to displace fossil fuel consumption, which not only lessens CH4 emissions from manure management but also lowers fossil CO2 emissions. Methane emissions from livestock include enteric emissions and manure management emissions (IPCC 1996). Enteric emissions of CH4 occur principally from the ruminant activity in cows (dairy and beef) and are a function of feed quality and intake. Other than reducing cow numbers, there is little opportunity to reduce enteric CH4 emissions. In contrast, CH4 emissions from livestock manure management are impacted by chosen management options. EPA (2007) estimates that manure management contributed 41.3 Tg of CO2eq from CH4 emissions and 9.5 Tg of CO2eq from N2O emissions to the U.S. greenhouse gas emissions inventory in 2005. Liquid handling of manure and long-term manure storage increase CH4 emissions. Dry handling systems, dry storage, and short-term storage lower CH4 emissions. Anaerobic digestion of manure with biogas capture can result in lower CH4 emissions, yet some leakage of biogas (estimated to average 1%) prevents complete elimination of CH4 emissions from livestock manure management. IPCC (1996) has developed methods for estimating greenhouse gas emissions from livestock manure management that account for climate, animal type, regional development, and management practices. Generally, the number of animals, the average manure volatile solids (VS) production, and the maximum methane yield (Bo) of the VS are combined with an emission factor to estimate methane emissions. Emission factors have been developed based on both scientific studies and measurements, as well as through model development and calculations. The emission factors vary with region, climate, animal type, and manure handling, thus adding a level of uncertainty to greenhouse gas emissions estimates. For cattle, IPCC (1996) has developed both Tier 1 methods (simple) and Tier 2 methods (more complicated) for estimating emissions.