8.4.1.6 Manure management

Animal manures can release significant amounts of N₂O and CH₄ during storage, but the magnitude of these emissions varies. Methane emissions from manure stored in lagoons or tanks can be reduced by cooling, use of solid covers, mechanically separating solids from slurry, or by capturing the CH₄ emitted (Amon et al. 2006; Clemens and Ahlgrimm, 2001; Monteny et al. 2001, 2006; Paustian et al., 2004). The manures can also be digested anaerobically to maximize CH₄ retrieval as a renewable energy source (Clemens and Ahlgrimm, 2001; Clemens et al., 2006). Handling manures in solid form (e.g., composting) rather than liquid form can suppress CH₄ emissions, but may increase N₂O formation (Paustian et al., 2004). Preliminary evidence suggests that covering manure heaps can reduce N₂O emissions, but the effect of this practice on CH₄ emissions is variable (Chadwick, 2005). For most animals, worldwide there is limited opportunity for manure management, treatment, or storage; excretion happens in the field and handling for fuel or fertility amendment occurs when it is dry and methane emissions are negligible (Gonzalez-Avalos and Ruiz-Suarez, 2001). To some extent, emissions from manure might be curtailed by altering feeding practices (Külling et al., 2003; Hindrichsen et al., 2006; Kreuzer and Hindrichsen, 2006), or by composting the manure (Pattey et al., 2005; Amon et al., 2001), but if aeration is inadequate CH₄ emissions during composting can still be substantial (Xu et al., 2007). All of these practices require further study from the perspective of their impact on whole life-cycle GHG emissions.

Manures also release GHGs, notably N₂O, after application to cropland or deposition on grazing lands. Practices for reducing these emissions are considered in Subsection 8.4.1.1: Cropland management and Subsection 8.4.1.2: Grazing land management.