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.

8.4.1.7 Bioenergy

Increasingly, agricultural crops and residues are seen as sources of feedstocks for energy to displace fossil fuels. A wide range of materials have been proposed for use, including grain, crop residue, cellulosic crops (e.g., switchgrass, sugarcane), and various tree species (Edmonds, 2004; Cerri et al., 2004; Paustian et al., 2004; Sheehan et al., 2004; Dias de Oliveira et al., 2005; Eidman, 2005). These products can be burned directly, but can also be processed further to generate liquid fuels such as ethanol or diesel fuel (Richter, 2004). Such fuels release CO₂ when burned, but this CO₂ is of recent atmospheric origin (via photosynthetic carbon uptake) and displaces CO₂ which otherwise would have come from fossil carbon. The net benefit to atmospheric CO₂, however, depends on energy used in growing and processing the bioenergy feedstock (Spatari et al., 2005).

The competition for other land uses and the environmental impacts need to be considered when planning to use energy crops (e.g., European Environment Agency, 2006). The interactions of an expanding bioenergy sector with other land uses, and impacts on agro-ecosystem services such as food production, biodiversity, soil and nature conservation, and carbon sequestration have not yet been adequately studied, but bottom-up approaches (Smeets et al., 2007) and integrated assessment modelling (Hoogwijk et al., 2005; Hoogwijk, 2004) offer opportunities to improve understanding. Latin America, Sub-Saharan Africa, and Eastern Europe are promising regions for bio-energy, with additional long-term contributions from Oceania and East and Northeast Asia. The technical potential for biomass production may be developed at low production costs in the range of 2 US$/GJ (Hoogwijk, 2004; Rogner et al., 2000).

Major transitions are required to exploit the large potential for bioenergy. Improving agricultural efficiency in developing countries is a key factor. It is still uncertain to what extent, and how fast, such transitions could be realized in different regions. Under less favourable conditions, the regional bio-energy potential(s) could be quite low. Also, technological developments in converting biomass to energy, as well as long distance biomass supply chains (e.g., those involving intercontinental transport of biomass derived energy carriers) can dramatically improve competitiveness and efficiency of bio-energy (Faaij, 2006; Hamelinck et al., 2004).