Synopsis

This review analyzes published data on manure management practices used to mitigate methane (CH4) and nitrous oxide (N2O) emissions from animal operations. Reducing excreted nitrogen (N) and degradable organic carbon (C) by diet manipulation to improve the balance of nutrient inputs with production is an effective practice to reduce CH4 and N2O emissions. Most CH4 is produced during manure storage; therefore, reducing storage time, lowering manure temperature by storing it outside during colder seasons, and capturing and combusting the CH4 produced during storage are effective practices to reduce CH4 emission. Anaerobic digestion with the combustion of the gas produced is effective in reducing CH4 emission and organic C content of manure; this increases readily available C and N for microbial processes creating little CH4 and increased N2O emissions following land application.

Commentary

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Analysis

Livestock Manure & Emissions

Animal manure is a nutrient resource containing most of the essential elements required for plant growth. It can be a significant source of N in both intensive and subsistence production systems. Application of manure to cropland increases soil OM, microbial biomass, and mineralization rate (Spiehs et al., 2010; Langmeier et al., 2002) and improves several soil properties, including soil tilth, water-holding capacity, oxygen content, and fertility. It also reduces soil erosion, restores eroded croplands, reduces nutrient leaching, and increases crop yields (Khaleel et al., 1981; Araji et al., 2001). Animal manure is an alternative to energy-intensive and high-cost synthetic fertilizer. It can be a very effective fertilizer source when the available nutrient content and mineralization rate are synchronized with crop nutrient uptake.

Unmanaged accumulation of organic wastes, however, presents environmental and health concerns for humans and animals. Concerns include leaching of nitrate (NO3–) and pathogens to groundwater, unbalanced algal growth and eutrophication of surface water and deterioration of sensitive ecosystems, degradation nutrients, salts, and metals, and emission of gases considered a health and environmental risk (Gerber et al., 2005; Steinfeld et al., 2006; USEPA, 2011). Manure management refers to all activities, decisions, and components used to handle, store, and dispose of feces and urine from livestock to preserve and recycle the nutrients in the livestock production system (Brandjes et al., 1996; IPCC, 2006a). This includes manure accumulation and collection in buildings, storage, processing, and application to cropland as well as deposition in pastures and rangelands in grazing systems.

Methane and Nitrous Oxide Emissions

In ruminant production systems, enteric CH4 production is the largest contributor to GHG emissions, followed by CH4 from manure and in beef feedlot systems, N2O from pen surface, and N2O emissions from soils. Emissions from nonruminant livestock systems are less than that of ruminants and are mostly CH4 and N2O from manure storage and land application (Hristov et al., 2013b). The contribution of manure management to global GHG emission was estimated by Steinfeld et al. (2006) to be 2.2 Gt of CO2 warming potential equivalents (CO2e) per year. Methane emissions from manure storage were estimated to be 470 Mt CO2e/yr in 2010 with an expected 11% increase by 2020 and N2O emissions from fertilizer use, manure application, and deposition by grazing livestock was estimated at 2,482 Mt CO2e/yr in 2010 with an expected 18% increase by 2020 (USEPA, 2006). Nitrous oxide emissions from soil application of manure are a significant contributor to total GHG emissions from agriculture (Davidson, 2009) with animal waste representing 30 to 50% of the global agricultural N2O emissions (Oenema et al., 2005).

Both CH4 and N2O are powerful GHG with global warming potentials (GWP) of 25 and 298 kg CO2e/kg, respectively (Solomon et al., 2007). Most of the CH4 emission from manure is produced under anaerobic conditions during storage with very little land application. Manure produces less CH4 when handled as a solid (e.g., in stacks or pits) or when deposited on pasture or rangelands (USEPA, 2005). Therefore, opportunities to reduce CH4 emission are centred on preventing anaerobic conditions during storage or capturing and transforming the CH4 that is produced, if anaerobic conditions are present. Data summarized by Chianese et al. (2009) indicate average CH4 emissions from the covered slurry, uncovered slurry, and stacked manure to be 6.5, 5.4, and 2.3 kg/m2 per yr. However, rates vary with temperature and time in storage. Direct emissions of N2O from manure storage are small when compared with CH4 emissions.

For N2O emissions to occur, manure must first be handled aerobically where ammonium (NH +) or N is converted to NO3– and nitrite (NO2–) during nitrification and then handled anaerobically where the NO3 – and NO2– are reduced to elemental N (N2), with intermediate production of N2O and nitric oxide (NO) through denitrification (USEPA, 2010). Most of the N2O resulting from manure is produced in manure-amended soils through microbial nitrification under aerobic conditions and partial denitrification under anaerobic conditions, with denitrification, generally producing the larger quantity of N2O (Tisdale et al., 1993; USEPA, 2010). Losses of N2O from the pen surface of open-lot dairy or beef feedlot facilities, however, can be significant. The fact that a large amount, up to 50%, of the excreted N by beef cattle is not recovered in manure has been well documented for various geographic locations (Loh et al., 2008; Cole and Todd, 2009).

Most of these losses are as ammonia (NH3), but N2O emissions are also significant (Leytem et al., 2011; Rahman et al., 2013) and depend on a variety of factors, including surface conditions (Aguilar et al., 2011). Manure management practices in beef feedlots vary, but usually, pens are cleaned when animals are marketed (i.e., several times a year) or once a year (Eghball and Power, 1994), which creates conditions for NH3 and GHG emissions off the pen surface. Recently, Rahman et al. (2013) reported CH4, CO2, and N2O emission rates from the pen surface of a North Dakota beef feedlot of 38 g, 17 kg, and 26 g/head per d, respectively. Somewhat lower N2O emissions were reported for an open-lot dairy in southern Idaho: 0.13, 0.49, 28.1, and 0.01 kg/cow per d (NH3, CH4, CO2, and N2O, respectively; Leytem et al., 2011). Considering the much greater GWP of N2O, compared with CH4 (Solomon et al., 2007), the emissions of these 2 GHG were comparable in that and other studies (Boadi et al., 2004).

Comparatively lower N2O emissions from a free-stall dairy in the Texas panhandle were reported by Borhan et al. (2011): 836, 5,573, and 3.4 g/head per d (CH4, CO2, and N2O, respectively). In the same study, emissions from a beef feedlot were reported as 3.8, 1,399, and 0.68 g/head per d, respectively. The pen surface was estimated to contribute about 84% of the aggregate N2O emission in this study.

Manure contains most elements necessary for stimulating soil nitrification and denitrification processes that form N2O. These processes are transient, depending on the amount and form of available N (NH4+ or NO3–), soil oxidation-reduction potential, degradable C sources, soil temperature, water content, and microbial population (Cavigelli and Parkin, 2012). Denitrifying organisms can further reduce N2O to N2 at rates dependent on soil conditions, with multiple factors controlling the ratio of N2O to N2 produced.

The fraction of N completely reduced to N2 also increases as soil water content approaches saturation. Nitrous oxide can also be produced indirectly when manure N is lost through volatilization as NH3, NO, and nitrogen dioxide (NO2) and is nitrified and denitrified in soil following redeposition (USEPA, 2010).

Being a result of microbial processes, the emission of N2O is highly variable as influenced by environmental and metabolic factors, which makes measurement of mitigation effects difficult. Nonetheless, the results of adopting mitigation practices can be estimated using the potential N2O emission reductions obtained when optimal conditions for nitrification and denitrification are assumed. This approach makes it possible to gauge the effectiveness of mitigation practices and their interactions within the livestock production system.

Due to the nature of the antagonistic processes resulting in CH4 and N2O emissions (CH4 is produced under anaerobic conditions whereas production of N2O requires sufficient levels of oxygen), some practices that result in the reduction of CH4 production increase N2O emissions. An example is the aeration of manure during storage to reduce CH4 emissions. This process may increase N2O emissions when the aeration rate is sufficient to create an aerobic environment. Opportunities to reduce N2O and CH4 emissions from livestock manure are illustrated in Fig. 1 and 2 according to the process of formation and emission of CH4 and N2O and the flow of organic C and N during the animal production cycle. The review of manure CH4 and N2O mitigation practices based on experimental and commercial-scale applications was summarized. Data related to the mitigation practices presented in this review may not account for losses, conversions or other differences caused by preceding processes in the system, study duration, and climatic impacts such as winter and summer differences.

There is still much to learn about the benefits of particular mitigation practices, the effect of combining mitigation practices, the response of environmental indicators such as nutrient conversion, volatilization, leaching, erosion, etc., and the impact on the environmental and financial performance of the production system (farm) as a whole. It is common for one environmental benefit of a technology to negatively impact processes in another area (Gerber et al., 2013).

Ammonia: An Important Component in Greenhouse Gas Mitigation.

Although not a GHG, NH3 (and its ionized form, NH +) is an essential component of the Ammonium in manure) is the first product of decomposition of urea through the action of the microbial enzyme urease after urine is deposited on barn floors and pastures. Urease is abundant in fecal matter and in soil, and thus urea excreted in urine is rapidly converted to NH + when the environmental conditions (temperature, pH) are favorable. Ammonium N can be converted under aerobic conditions to NO – and both forms of N are readily available to plants. In contrast, organic forms of manure N are generally not readily available (Beegle et al., 2008). Ammonium N is also the carrier of rapidly available N in the soil and a necessary precursor in the process that leads to N2O emissions from application of manure and fertilizers and urine deposition in pastures (Tisdale et al., 1993; de Klein et al., 2001). Ammonia is a volatile gas that escapes to the atmosphere reducing the amount of N transported to the soil, which may offset the benefits of manure storage and land application mitigation practices.

 Simultaneously, NH3 that escapes to the atmosphere can form particulate matter that may return to soil through a dry or wet deposition. This deposition can harm sensitive ecosystems and contribute to N runoff and groundwater pollution and be converted into N2O through denitrification (Galloway et al., 2004; USEPA, 2010; Hristov et al., 2013b). Ammonia volatilization is generally the largest pathway of loss for manure N (Harper et al., 2004; Lee et al., 2011a), with losses typically accounting for 30 to 70% of the NH4+ content of cattle manure (Thompson and Meisinger, 2002). Nitrogen emissions can also be in the form of N2 (Harper et al., 2004) but losses as N2 have not been well quantified. Thompson et al. (1987) estimated that 2 to 12% of applied manure in English grassland was lost to denitrification (N2 +N20) with a surface application and 7 to 21% was lost with the injection of manure. The portion of manure N lost as N2O is relatively low, generally below 2 to 3%, and only in a few reports has it reached 10% (de Klein et al., 2001).

Based on an N mass balance approach, Hristov et al. (2011b) estimated that more than 25% of the feed N input on a dairy farm was not accounted for in milk and manure after 24 h following excretion and this was mostly attributed to NH3 volatilization; losses from beef cattle feedlots with long term exposure were even greater, reaching 50% (Cole and Todd, 2009). The relationship between manure NH3 volatilization and N2O emission is also complex because 1) emissions of both may be reduced by diet manipulation or manure management and 2) if a mitigation technology reduces NH3 losses, the preserved NH4+ may later increase soil N2O emissions (Petersen and Sommer, 2011). On the other hand, gaseous losses of N will reduce the availability of N for nitrification and denitrification processes and, consequently, N2O formation (USEPA, 2010). Therefore, NH3 emission is considered an essential component of the analysis of N2O mitigation practices presented in this review.

Conclusion

Much public relations work must be done to counter the bad reputation that beef production has received in terms of its ecological damage. Steinfeld et al. (2006) of FAO published a comprehensive global lifecycle assessment of livestock agriculture’s environmental impact called “Livestock’s Long Shadow: Environmental Issues and Options” (LLS) concluding that global livestock agriculture contributes 18% of total anthropogenic greenhouse gasses. Livestock contributes more to climate change than the global transportation sector (Steinfeld et al., 2006). These conclusions have been widely quoted in the popular press and often used incorrectly in articles that make such comparisons as contrasting driving a different vehicle to not eating meat or comparing the greenhouse gas emissions from producing a pound of carrots to producing a pound of beef (Bittman, 2008; Rosenthal, 2008; and Place and Mitloehner, 2012).

These types of comparisons, though inappropriate, lead to misguided public policy decisions such as “Meatless Mondays,” which was adopted in San Francisco (Chang, 2009). A major inappropriate premise in LLS was that land used for cattle was converted from a carbon “sink” (a net sequester of CO₂ from the air into the soil, e.g., the Amazon rainforest) into a carbon source. It is inappropriate to blame beef cattle emissions for this difference since the comparison involves a confounded situation in that when the rainforest is clear cut to graze cattle, the loss of the trees dramatically reduces the CO₂ sequestered regardless of the CH₄ emissions from enteric fermentation by cattle (Place and Mitloehner, 2012). Deforestation alone represents 34% of the total CO₂ attributed to livestock production in LLS (Steinfeld et al., 2006). In the next E-letter, we will continue to bring you the advancements of this investigation. We want to proceed further, more in-depth on this controversial subject.

Therefore, please follow us on social media and join us on the (15^th) fifteenth of the month for Part (2!) two to learn more about the environmental impact of beef production. Thereafter, please join us on the (1^st) first and (15^th) fifteenth of each month for our fortnightly delivery of insightful, informative must-reads from some of the world’s scientific thinkers. Selected by our editors is a collection of current topics with a profound ability for beneficial improvements, guidelines, and process practices. Thank you for reading our publication entirely; please share it with others who also care. We look forward to your comments and having you with us again fortnightly; we will be thrilled in having you with us; thus, we will take your trust in us with great honor and appreciation.