Methane emissions from livestock manure - effects of storage conditions and climate

Summary

Livestock manure contributes significantly to the global emission of methane (CH4). Methane is emitted during storage of both liquid and solid manure. Part of the solid manure is produced in loose housing systems with solid floors where the manure is stored in a deep litter mat, which is a mixture of straw, urine and faeces. As anaerobic conditions are found in the lower part of the deep litter mat, significant amounts of the carbon stored in the deep litter may be emitted as CH4. It has been estimated that a cattle deep litter mat contributed 11 to 18% of the total CH4 (from cattle digestion and litter) emitted. This source of CH4 does not seem to be included in the IPCC default value for solid manure. During outdoor storage of solid manure, CH4 can be produced at a high rate in the central parts of the heap, and the highest emissions are measured during the thermophilic phase of composting. Methane emissions have been shown to account for 0.01 to 0.2% of the total carbon content, and emissions were positively related to the bulk density of stored solid manure. CH4 may be partly transformed to CO2 during the transport from the inside of a heap towards the surface.
The emission of CH4 from stored anaerobically digested slurry and cattle slurry has been shown to vary between <0.01 and 1.4 g C m-3 h-1. Methane is produced in the bulk of the slurry, and it has been found that log transformed CH4 emissions decrease linearly with the inverse temperature of the slurry. A porous surface cover on the stored liquid manure may reduce CH4 emissions by up to 40%, probably due to CH4 oxidation within the surface cover or at the interface between the cover and liquid in the store. The establishment of a porous cover of slurry stores could be introduced as a mitigation technique and could also be included in the IPCC guidelines for calculating CH4 emissions from animal manure.