Nitrous Oxide and Methane Emissions from Animal Wastes and Lagoons

Nitrous oxide is produced from a wide variety of biological sources in soil, water, and animal wastes. During the last two centuries, human activities have increased N2O concentration by 13% (EPA, 1998). The main activities producing N2O are fossil fuel combustion, agricultural soil management and industrial sources. Use of large amounts of N fertilizer creates secondary problems associated with N2O released in anaerobic conditions (Mosier et al., 1998a). Agricultural soil management activities such as fertilizer application and cropping practices were the largest source of N2O emission (56.5MMTCE), accounting for 43% of the US total (EPA, 1998). Manure management in feedlots (3.7 MMTCE) and agricultural residue burning (0.1 MMTCE) are small sources of N2O emissions.

Methane is second only to CO2 in contributing to GHG emissions. Landfills are the largest contributor to CH4 emissions in the USA, while the agricultural sector is responsible for 30% of US emissions. Of the total 176.7 MMTCE emitted in the USA in 1996 (EPA, 1998), agricultural emissions of CH4 were: ruminant livestock fermentation, 34.5 MMTCE; agricultural manure management, 16.6 MMTCE; rice cultivation, 2.5 MMTCE; and biomass burning (Mosier et al, 1998b).

Greenhouse gases are associated with storage and application of animal manure. Of these GHGs, the greatest attention has been given to CH4 emissions generated by animals. There has been very little attention given to CO2 production by manure storage systems. Among the agricultural sector's potential CH4 emission sources, manure appears to contribute approximately 5% of the total (Table 3.3). Within the manure portion of CH4 emissions, swine production constitutes the largest amount due to the type of manure handling and storage (Table 3.4). Nitrous oxide generation within manure is a result of the nitrification/denitrification process that occurs in manure storage and application. After field application, it would be difficult to separate the N2O from manure sources from that of commercial fertilizer sources in the soil. Methane has been the gas most often measured in various studies; however, data comparing different production practices are sparse.

Table 3.3. Agricultural sources of atmospheric methane emissions in the US (EPA, 1994a).

Methane emission

Table 3.3. Agricultural sources of atmospheric methane emissions in the US (EPA, 1994a).

Methane emission

Source

(Tg year-1)

(MMTCE)

Rice

65

372.3

Livestock

80

458.2

Manure

10

57.3

Biomass burning

30

171.8

Sum

185

1059.6

Table 3.4. Methane emissions from livestock manure in the USA and the World

Table 3.4. Methane emissions from livestock manure in the USA and the World

Methane

emissions

USA

World

USA

World

Species

(Tg year-1)

(Tg year-1)

(MMTCE)

(MMTCE)

Dairy

0.71

2.89

4.07

16.55

Beef

0.19

3.16

1.09

18.09

Swine

1.11

5.29

6.36

30.16

Sheep and goat

-

0.71

-

4.07

Poultry

0.23

1.28

1.32

7.33

Other

0.24

0.51

1.37

2.92

Sum

2.48

13.84

14.21

79.12

Production of GHG from manure storage systems has not been sufficiently measured over a large number of units and over a wide range of climatic conditions. Methane production within lagoons and earthen storage systems comes from the solid/liquid interface, with the CH4-producing bacteria present at this interface. Anaerobic digestion of manure leads to the production of CH4. Hill and Bolte (1989) described the anaerobic manure storage system as a complex set of interdependent biological systems. Methane production is part of the biological complex, and they proposed that loading rate, pH and temperature were factors causing shifts in the balance among the organisms. An illustration of these interactions is given by Burton (1992), who found that shifting the anaerobic manure storage to an aerobic storage reduced the potential NH3 loss to the atmosphere. Unfortunately, this shift can lead to production of N2O. However, he did not quantify the expected release of these gases. Safley et al. (1992) characterized the emission of CH4 from different livestock systems and concluded that anaerobic manure storage systems would convert non-lignin organic matter into CH4 under warm, moist, anaerobic conditions. Parsons and Williams (1987) developed a mathematical model for anaerobic storage systems based on these factors that could be adapted for prediction of GHG.

The annual release of CH4 from different manure storage systems associated with swine vary from 10 kg per animal for subconfinement pits within buildings to about 90 kg per animal in a lagoon system. This variation in CH4 production can be attributed to the amount of solids in the different manure systems and the bacterial populations present in the manure storage. Groenestein and Faassen (1996) found that deep-litter systems for swine reduced N2O emissions because of changes in the manure digestion systems within the manure. Changes in manure management have had a positive impact on emission rates. Prueger and Hatfield (unpublished data, 1997) positioned a trace gas analyser over a lagoon and found that there was variation in the CH4 fluxes throughout the day in response to diurnal changes in temperature. (Similar variation has been observed in rice fields; Sass et al., 1991b; Satpathy et al., 1997; Wang et al., 1997a.) Prueger and Hatfield also documented variation in the exchange coefficient between the lagoon surface and the atmosphere. However, these data were not collected for a sufficient length of time to quantify seasonal changes in CH4 production in response to a wide range of atmospheric conditions. Kinsman et al. (1995) measured CH4 and CO2 production from lactating dairy cows and found that stored manure contributed 5.8 and 6.1%, respectively, to CH4 and CO2 emissions under conditions of their experiment. Manure storage, particularly for ruminant animals, represents a small fraction of the total GHG load to the atmosphere.

Kuroda et al. (1996) measured the emissions of GHG emitted during composting of swine faeces under continuous aeration using laboratory-scale composting apparatus. Methane emission was observed within only 1 day from starting the composting, while N2O and NH3 repeatedly rose and fell after every turning. Of the total N loss during composting, the total amount of N2O emission was a small fraction of NH3 emissions. Lessard et al. (1996) measured N2O emissions from agricultural soils after application of dairy cattle manure to cultivated land planted to maize (Zea mays L.). The manure application rates were 0, 170 and 339 kg N ha-1, respectively. On the manured plots, 67% of the total N2O emitted during the growing season occurred during the first 7 weeks following manure application. High N2O fluxes coincided with periods when NO3-N levels and soil water contents were relatively high. Fluxes were highest the first day after manure application, but returned to near pre-application levels 7 days later. There were short-lived peaks of N2O flux, usually following rain. Only 1% of the manure N, which accumulated as N2O, was potentially mineralizable over the snow-free season. In a similar study, Wassman et al. (1996) evaluated the effect of fertilizers and manure on CH4 emission rates using an automated, closed-chamber system in Chinese rice (Oryza sativa) fields. The rate of increase in CH4 emission was dependent on the total amount of organic manure applied. A single application of organic manure increased the relative short-term CH4 emission rates by 2.7-4.1 times compared with fields without organic manure.

Reports on the literature indicate that there is a large amount of variation in the fluxes of GHG from animal manure storage and handling. These differences could be attributed to variations in species, diet, loading rates into the storage, type of storage and environmental conditions within the manure storage. Further studies giving greater attention to the physical and biological parameters affecting microbial production and emission of GHG are needed. These data will have to be coupled with dietary models for different species and a complete understanding of the chemical factors within manure storage systems in order to quantify the dynamics of GHG production and emission. This type of information will be essential in developing realistic mitigation scenarios.

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