Agricultural lands (lands used for agricultural production, consisting of cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops) occupy about 40- 50% of the Earth’s land surface. Agriculture accounted for an estimated emission of 5.1 to 6.1 GtCO2-eq/yr in 2005 (10-12% of total global anthropogenic emissions of greenhouse gases (GHGs)).
A variety of options exists for mitigation of GHG emissions in agriculture. The most prominent options are improved crop and grazing land management (e.g., improved agronomic practices, nutrient use, tillage, and residue management), restoration of organic soils that are drained for crop production and restoration of degraded lands.
Agriculture releases to the atmosphere significant amounts of CO2, CH4, and N2O and agricultural land occupied 5023 Mha in 2002 (IPCC 2007). Most of this area was under pasture (3488 Mha, or 69%) and cropland occupied 1405 Mha (28%). Animal manure releases 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, mechanicallyseparating solids from slurry, or by capturing the CH4 emitted. Economic growth and changing lifestyles in some developing countries are causing a growing demand for meat and dairy products, notably in China where current demands are low. Opportunities for mitigating GHGs in agriculture belong to three categories (IPCC 2007):
a. Reducing emissions (for instance by more efficient management of carbon and nitrogen flows in agricultural ecosystems, such as managing livestock for reduction of methane)
b. Enhancing removals, such as storage of amounts of vegetative carbon in agro-forestry systems or other perennial plantings on agricultural lands and others).
c. Displacing emissions: amounts of vegetative carbon can also be stored in agro-forestry systems or other perennial plantings on agricultural lands. The net benefit of these bio-energy sources to the atmosphere is equal to the fossil-derived emissions displaced, less any emissions from producing, transporting, and processing.
Livestock manures represent a valuable resource that, if used appropriately, can replace significant amounts of chemical fertilizers (van der Meer et al., 1987; Bremand and de Wit, 1987; van Boheemen, 1987). However, unless animal manure is managed carefully to minimize odour, nutrient losses and emissions, it becomes a source of pollution and a threat to aquifers and surface waters. The most known technology for manure management is the anaerobic digestion , which is a process in which organic matter from wet organic wastes (ie. liquid manure, food processing wastes, etc.) is converted into methane by bacteria in the absence of oxygen. The methane is then collected and may be used to generate electricity. In addition, the anaerobic digestions process creates potentially valuable by-products, such as the solids fraction - fiber, and liquid with available nutrients.
Another common technique is the aerobic digestion , which is useful in treating liquid manure for odour reduction, chemical oxygen demand (COD) and biochemical oxygen demand (BOD) reduction, and pathogen control. Aerobic treatment is usually a batch process or, semi-continuous (batch feed). In a batch process, all of the treated material is removed from the facility before refilling with untreated slurry. In a batch feed or semi continuous process, some of the treated material is displaced by the addition of untreated material to the digestor.
A third method widely applied in the agricultural sector worldwide is the composting , which is an aerobic digestion process used for solid wastes. Slurries or separated solids can be composted if mixed with a carbon source such as straw, peat or wood shavings. However, composting a slurry without separating the solids requires a great deal of additional material to retain the liquid. This would be very impractical due to the cost of the material and the energy required to turn or aerate the compost. Composted manure is a premium organic fertilizer and holds some potential as a marketable product in the gardening and landscaping market. For some markets, and even some on-farm application techniques, the compost would have to be pelleted so that the nutrient content could be upgraded to a specific blend with commercial fertilizers.
Based on the IPCC (2007), for most animals, worldwide thereis limited opportunity for manure management, treatment, or storage; excretion happens in the field and handling for fuelor 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 or bycomposting the manure, but if aeration is inadequate CH4 emissions during composting can still be substantial (Xu et al., 2007).
Cereal straw and other farm residues are already used in Denmark, Spain and Romania, for example, to produce significant heat and power production. In Brazil, Australia, South Africa and elsewhere, sugarcane bagasse is used for heat and power, both for use at the mill and for export to the grid. Vegetative grass crops such as Miscanthus and reed canary grass can be grown for combustion in commercial grate boilers for heat production. The co-firing of straw with coal is well demonstrated in fluidised-bed boilers. Small-scale (<500 kW) power generation plants based on the steam cycle have also been built, but they are relatively inefficient and hence have relatively high power production costs. Further RD&D in CHP would help reduce costs. In all cases, storing the biomass is important so that the bioenergy plant can be operated all year round, or at least for as long a season as possible, to spread the investment costs.
Some operational necessities are required for different types of manure management. For instance, in order for an anaerobic digester to operate properly a constant supply of “recoverable” manure is needed (Kubsch, 2003). Not all types of dairy manure are appropriate for anaerobic digestion purposes. Manure collected through a continuous scrape or other means from cows on cows kept on hard surfaces is better suited for use in an anaerobic digester. Manure which is left to dry in the pasture or drylot is not as useful for anaerobic digestion since drying reduces the methane producing properties.
The main barriers include fuel logistics, fuel quality fluctuations (due to variations in rainfall, for example) feedstock price fluctuations and delivery costs. Technical improvements in harvesting, storage, transport, fuel preparation and other measures are still possible for virtually all biomass feedstocks.
Technologies in manure management are developing rapidly and several countries in the developing world are implementing them to a certain extent. According to Brandjes et al (1996), manure management systems are highly diverse, among which the following can be distinguished:
-Grazing. Substantial losses through leaching may occur due to the uneven distribution of faeces and urine (urine patches may have a N load equal to 200-550 kg/ha). Volatilization of N may also be considerable (10-25%, see 3.3.5), but less important since part is deposited on nearby areas, though some of it on non-agricultural land.
-Kraals. These enclosures are often used as in-situ fertilization of arable land by moving the kraal regularly. Soil fertility of a larger area, used for grazing, is partially concentrated on the arable land, thus enabling crop production in resource-poor situations. Losses through leaching will be slightly higher than during grazing as equivalent N and K fertilization rates are increased.
-Dry lot storage. If urine is not collected and bedding is sparsely used, losses of N and K in particular will be high as most urine is lost. Depending on the storage facilities and storage time of the faeces part of the nutrients in faeces will also be lost through leaching and surface runoff, in the case of a precipitation surplus and uncovered manure heaps. Urine collection will minimize K losses but N losses will often remain high as volatilization will increase, though this is dependent on climatic conditions, storage time and storage method. Using bedding, with sufficient absorption capacity to capture urine, might reduce N losses with ca. 15% of the mineral N.
-Slurry storage. This system of manure storage, where faeces and urine are stored together, is the main system in intensive livestock systems in OECD countries, except for broilers. Volatilization losses are dependent on the level of ventilation, depth of storage tanks and storage time, but often range between 5 and 35% of the total N excreted.
-Lagoons. Lagoon systems are quite common at large livestock farms in Eastern European countries and, to a lesser extent, in Asia, while their importance is growing in the USA. Liquid manure, either before on after separating part of the solids, is treated in anaerobic lagoons. Organic material is decomposed, thereby mineralizing part of the nutrients. The liquid phase is either discharged into surface water or used for irrigation. The main problems are related to the discharge into surface water, leaching through the lagoon bottom, and odour. High NH3 emission will occur as a major part of the N in mineral form, while also high CH4 and N2O emissions are also common.
-Plastering material for house construction. This is particularly important in Africa, however the amount of manure involved on a global scale is considered to be too insignificant to be discussed here. In this system all nutrients are lost for agriculture.
-Fuel. In many developing countries, and particularly in India, manure is an appreciable fuel. If burnt directly, most of the C, and all the N and S will be lost; other nutrients may be recycled to arable land via the application of the ash. The production of biogas from manure is another method to valorize the energetic value of manure. The high water content of the slurry makes it more difficult to handle, and N losses via volatilization may be high, because most N in slurry is in mineral form. Though strongly promoted (e.g. in China) and applied to some extent in Asia, its present application is still limited mainly due to high investment costs (both for the digester and adjoining equipment) and technical problems (Henglian, 1994).
-Feed. Manure could be recycled by feeding it to animals, both livestock and fish (Müller, 1980), but this practice is limited. In addition to widespread reluctance to use manure as feed, probably originating from fear of health hazards, this can be explained by the low nutritive value of most types of manure, except for poultry manure as ruminant feed which is of a reasonable quality (ca. 55-60% TDN, 20-30% CP). Consequently, in intensive production systems where collectable manure is abundant, more economic feed is available, while in production systems where the use of low quality feeds is common, high collection costs and/or opportunity costs (manure as fertilizer or fuel) are prohibiting the use of manure as feed.
In addition, biogas produced from the anaerobic digestion of animal manure, green crops and other forms of organic waste can be used for heat and power generation as well as for transport fuels – after scrubbing to remove CO2 and H2S (IEA, 2008). Several research programs exist, which aim at diffusing information and best practices for manure management.
According to the IEA (2008), the use of residues and waste as biomass can reduce farmers’ costs and provide them with additional income. Based on IPCC (2007), an appropriate mix of rice cultivation with livestock, known as integrated annual crop-animal systems and traditionally found in West Africa, India and Indonesia and Vietnam, can enhance net income, improve cultivated agro-ecosystems, and enhancehuman well-being. Such combinations of livestock and cropping, especially for rice, can improve income generation, even in semi-arid and arid areas of the world. Furthermore, greater demand forfarmyard manure can create income for the animal husbandry sector where usually poorer population deals with. There are other benefits from managing manure properly. Properly managed feedlots, manure stacks, and manure spreading can minimize nutrients entering surface waters and impacts downstream. Nutrient rich waters promote excessive algae and aquatic plant growth which reduces wildlife habitat, and recreational activities, and may increase water treatment costs. In addition, bacteria and other pathogens may enter surface waters with run-off causing other human health concerns. With proper management, these adverse environmental impacts can be minimized .
In cooler climates, slurry in anaerobic digestors must be heated to between 25°C and 35°C to promote growth of methane-producing bacteria. The heat required to fuel the boiler can often be produced by using part of the biogas emitted from the slurry. The remainder of the biogas can be used to fuel an electrical generator or for space heating, reducing thus the need for importing fuels for such energy services.
The effects of manure management practices on the protection of the environment are rather diverse as they depend on the location and the method itself where it is implemented. As Brandjes et al (1991) demonstrate, for ammonia emissions for instance, when deposited on agricultural land it is a free of cost fertilizer. If, on the other hand ammonia is deposited on urban, recreational or nature reserve areas, negative effects become prominent. In many countries with abundant natural areas, the negative effect of ammonia deposition on some areas may be considered to be hardly problematic, though if rare species are at risk, high values may be given because of reduced biodiversity. In contrast, when ammonia is deposited few small areas intended for nature development, is viewed as highly problematic, partly because of loss of recreation space but, probably even more importantly because of emotional values attached to these small "natural" areas in a highly industrialized and small country.
The effect on the environment of the manure produced in a particular agricultural system should be assessed by considering its role in the total nutrient management of the system. If the import and export of nutrients in the system is in balance, and animal manure is to play a positive role, it implies that losses from animal manure must be minimal. It also implies that efficient use is made of the manure in crop production, i.e. a large fraction of the nutrients from the manure is taken up by the crop. Another example is with the aerobic treatment, which can control dangerous bacteria such as Cryptosporidium and Salmonella and they cannot exist in the presence of oxygen. On the down side, aerobic treatment can cause excessive loss of nitrogen as nitrogen gas, nitrous oxide or even ammonia if excessive aeration rates are used. This loss of nitrogen to the atmosphere can create con cerns of acid rain in some instances. Another concern is the potential loss of the economic value as nitrogen fertilizer.
Land application of raw or composted manure can be tailored to reduce the emission of GHGs and their impact on the environment. Application of more nitrogen than a crop needs via manure will result in excess nitrogen accumulation in soil and will increase the release of nitrogen as nitrous oxide. Application of manure at the wrong time of year, for example in the very early spring, will also increase the release of nitrous oxide, as will applying raw manure during wet conditions. Researchers believe that timing manure application correctly and ensuring proper application amounts will contribute to an overall reduction in GHG emissions from agricultural operations.
Recycling of agricultural by-products, such as crop residuesand animal manures, and production of energy crops provides opportunities for direct mitigation of GHG emissions fromfossil fuel offsets (IPCC, 2007). According to the IPCC (2010), considering all gases, the global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030 is estimated to be ~5500-6,000 MtCO2-eq/yr. The US EPA (2006) forecast that combined methane emissions from enteric fermentation and manure management will increase by 21% between 2005 and 2020, taking into account that the global livestock-related methane production is expected to increase by 60% up to 2030. The technical potential of emissions reductions from manure management in comparison to the other practices in the agricultural sector are presented below.
For calculation of these GHG emission reductions, it is recommended to apply the approved methodologies for GHG emissions reductions from manure management systems  (large scale), GHG mitigation from improved animal waste management systems in confined animal feeding operations  (large scale), methane recovery in agricultural and agroindustrial activities  (small scale), consolidated methodology for GHG emissions reductions from manure management system s (large scale), which have been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html .
The magnitude of manure treatment methods unveils a broad range of costs and financial requirements for their implementation, as these methods are dependent on concentration of the dispersion rates of harmful compounds, production of similar compounds by other sources and types of natural resources affected (for instance water). The costs thus are highly dependent on the specific location that the method takes place (Brandjes et al. 1991). The value of manure can be difficult to determine. From an agronomic perspective, manure has many beneficial characteristics. Aside from its nutrient benefits, manure also improves soil tilth, increases water holding capacity, and promotes beneficial organisms. However, a larger volume of manure must be applied to realize these benefits when using manure as a nutrient source. With larger quantities of manure being applied, greater application costs are incurred. Thus, the per unit nutrient value of manure may be less than for commercial fertilizer. Here, the beneficial attributes of manure plus its nutrient value are expected to be equal to the per unit nutrient value of commercial fertilizer (Rausch and Songen, 1999).
For water quality aspects the costs of removing polluting compounds can be assessed. These costs vary according to the technology used, local prices for energy, labour, etc. Exploitation costs for removing N and P were estimated at 20-37 ECU (i.e. ca. US$ 27-50) per kg nutrient emission from point source pollution (like manure directly discharged into surface water). These values refer to Eastern European countries; they are based on 8% discount rate and are dependent on the level of nutrient reduction aimed at (Haskoning, 1994).
Information on costs of liquid manure treatment is still scarce. The few available estimates on total operational costs of purification plants vary between 5 and 15 ECU (i.e. 6.75 and 19 US$) per m3 (Ten Have and Chiappini, 1993), while the required effluent quality (10-15 mg N/l and 1-2 mg P/l; EEC-Council Directive 91/271/EEC, 1991) is still not attained, even if the plants work at optimal level. The conclusion is that large-scale treatment of liquid manure as sewage is not economically justified, even if management and infrastructural problems are solved. Second, all losses of nutrients via emission, leaching, direct discharge to the environment, etc., can be valued according to current costs of inorganic fertilizer, by way of replacement costs or foregone benefits. World market price (Feb 2010) for urea is about US$ $270/st.
Indicatively, Rausch and Sohngen (1999) conducted an economic comparison of three manure handling systems: (1) earthen holding pond using drag-line direct injection; (2) earthen holding pond using a liquid tanker and; (3) stack pad using a conventional spreader. Some figures of these costs as a reference are presented below.
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of methane avoidance in manure and wastewater projects in the CDM pipeline mainly apply the following methodologies:
CDM projects based on methane avoidance represent 11.3% of all CDM projects in the pipeline. Methane emission avoidance CDM projects based on composting and wastewater are mainly based in Malaysia and Indonesia, while most of the projects based on manure are based in Brazil and Mexico.
Brandjes, P.J., de Wit, J. and Van der Meer, H.G., 1996. Environmental Impact of Animal Manure Management. Food and Agriculture Organization of the United Nations. Available at: http://www.fao.org/WAIRDOCS/LEAD/X6113E/x6113e00.htm#Contents 
Breman, H. and De Wit, C.T., 1983. Rangeland productivity and exploitation in the Sahel, Science 221, pp. 1341–1347.
Gonzalez-Avalos, E. and Ruiz-Suarez, L.G., 2001. Methane emission factors from cattle in Mexico. Bioresource Technology, 80, 63-71.
IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.
IPCC, 2007. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).
Kubsch, K., 2003. Cooperative Anaerobic Digestionas a Manure Management Alternativein Northeastern WisconsinA Research Study Summary. Focus on Energy report. Available at: http://www.focusonenergy.com/files/document_management_system/renewables/glacierlandresearchstudy_feasibilitystudy.pdf 
Rausch, J. and Sohngen, B., 1999. An economic comparison of three manure handling systems. Report Number AE-5-99. Ohio State University Extension, Agricultural Economics.
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