Greenhouse gas emissions from animal manure

Summary

On-farm emissions from animals and manure must be taken into account when the GHG mitigation potential of grassland management strategies involving grazing are evaluated. Greenhouse gas (GHG) emissions from manure management include direct emissions of CH4 and N2O, as well as indirect emissions of N2O derived from NH3/NOx. Quantification of GHG emissions from manure are typically based on national statistics for manure production and housing systems combined with emission factors which have been defined by the IPCC or nationally. The quality of GHG inventories for manure management is critically dependent on the applicability of these emission factors.
Animal manure is collected as solid manure + urine, as liquid manure (slurry) or as deep litter, or it is deposited outside in drylots or on pastures. These manure categories represent very different potentials for GHG emissions, as also reflected in the methane conversion factors and nitrous oxide emission factors, respectively. However, even within each category the variations in manure composition and storage conditions can lead to highly variable emissions in practice. This variability is a major source of error in the quantification of the GHG balance for a system. To the extent that such variability is influenced by management and/or local climatic conditions, it may be possible to improve the procedures for estimating CH4 and N2O emissions from manure.
Manure (dung and urine) deposited during grazing influences fluxes of both CH4, N2O and NH3 from the pasture. In particular, urine patches are important point sources of NH3 and N2O, whereas the N input may locally reduce CH4 oxidation activity. Ammonia losses from pastures are not specifically represented in the IPCC methodology, which calculates NH3 volatilization as a fixed proportion of total N excreted. However, ammonia losses from excretal returns to the pasture increase with N surplus in the diet since this N is mainly excreted as urea in the urine. Also, several methodologies exist for mitigating NH3 losses from storage facilities. Hence, both optimized feeding and restricted access to grazing with collection of manure on the farm are available as NH3 mitigation options, though not identified by the IPCC methodology. Technical solutions to reduce NH3 volatilization from storages may reduce (slurry) or increase (solid manure) CH4 emissions, an aspect that must also be taken into account.
The N2O emission factor for N deposited on pastures is higher than for N in manure collected during housing, indicating that restricted access to grazing is also a N2O mitigation option. Several studies have suggested that N2O emissions from excreta deposited during grazing interact with factors like feed composition, stock density, N fertilization, soil compaction and climate. However, there is presently little evidence to suggest that emissions of N2O can be consistently changed via management practices.