Several processes associated with the production and land application of by-products can lead to greenhouse gas emissions. The primary gases of concern are CO2, CH4, and N2O. Carbon dioxide released from digestive or waste handling processes has no net effect on global warming potential because the C was taken from the atmosphere via photosynthesis. If C is released as CH4, however, there is an effect on global warming potential because CH4 is a more potent greenhouse gas than CO2. In addition, N2O is a greenhouse gas and plays a role in the destruction of stratospheric ozone and waste management practices can influence the amount released to the atmosphere.
Hao et al. (2001) measured C and N2O losses from passive (no turning) or active (turned six times) composting methods for cattle feedlot manure. Most C loss was as CO2; however, a substantial amount of C was lost by CH4 emission. Carbon losses as CO2 and CH4 were 73.8 and 6.3 kg C Mg–1 manure, respectively, for the passive method and 168.0 and 8.1 kg C Mg–1 manure, respectively, for the active method. Nitrogen loss through N2O emission was 0.11 and 0.19 kg N Mg–1 manure for the passive and active methods, respectively. Consumption of fuel to aerate and maintain the manure windrows added an additional C loss of 4.4 kg C Mg–1 manure for the active method. Total greenhouse gas emission, expressed as CO2–C equivalent, was 240 and 401 kg C Mg–1 manure for the passive and active composting methods, respectively. Aerating the manure increased gas emissions due to greater biological activity, increased N cycling, and increased gas diffusion. A smaller gas diffusion rate and incomplete decomposition reduced gas emissions in the passive manure treatment.
Most estimates of N2O emission have been based on laboratory tests or from short-term field studies. Lessard et al. (1996) found that about 1 kg N ha–1 was lost due to N2O emission during 185 d from soil that received two applications of cattle manure for 2 yr. The effect of long-term manure application on N2O emission has been largely ignored. Chang et al. (1998) studied the effect of long-term manure application (21 yr) on the annual emission of N2O and whether emission rate was related to various environmental factors. Emission rates ranged from 2 to 4% of total N applied manure. These rates are much greater than results from the Lessard et al. (1996) short-term study. Greater emission rates from the long-term study may be the cumulative effect of repeated manure applications over several years and/or the mineralization of organic N reserves. The relationship of different combinations of environmental factors only accounted for 30% or less of the variability in N2O flux. The rate of N2O emission was greatest in the spring, but flux rates were significant throughout the winter months.
Similar to NH3 emissions, application method can greatly influence N2O emissions. Flessa and Beese (2000) compared N2O flux from surface and injected liquid cattle manure and injection greatly increased flux (Table 3). The researchers did not measure NH3 emissions, but injection for the purpose of reducing NH3 emissions may increase N2O emissions.