| Global warming, as one of the most serious environmetal issues, is a great concern currently. As reported, the global warming potential of methane is a factor of 25 of carbon dioxide, which is the second important greenhouse gases following CO2. The fourth IPCC report indicated that methane emissions from dairy cattle are considered one of the most important sources in agricultural sector. Therefore, accurately characterzing daily and season emission patterns of methane at dairy operation scale and fully understanding the relation between methane emissions and respective management practices will certainly help establish suitable methane emission factors for China, screen potential mitigation practices and verify the validity of default emission factors by IPCC methodology as well.In this study, by using in-situ measurement with an inverse dispersion technique and analysis in lab, methane emissions from two dairy opeations in Northern China were quantified during spring, summer, fall and winter seasons in order to investigate daily and seasonal emission patterns of methane and thus calculate annual methane emission factors and methane conversion factor accordingly. The main results are as following:1. This is the first study in China which was in attempt to use open-path lasers in combination with an inverse dispersion technique to investigate diurnal and seasonal emission patterns of methane from dairy cattle. Near the dairy operations, methane background concentrations varied from 1.29 mg/m3 to 1.38 mg/m3, which is slightly greater than the value of 1.27 mg/m3 reported by the Third report by IPCC. During measurements, the range of methane concentrations over these two selected dairy operations were 1.43 mg/m3～7.14 mg/m3 and 1.46 mg/m3～5.24 mg/m3, which is much greater than the detection limit of used open-path lasers. It shows that the basic requirement of our measurement techniques was fully satisfied.2. Clear diurnal pattern of methane emissions were identified. At experimental site 1, methane emission peaks started from 05:00 am, 11:30 am and 04:00 pm, respectively and 05:00 am, 11:30 am and 04:30 pm, respectively at experimental site 2; each emission peak lasted about 2 hours and peak emission rates were about 2～3 times of the rates between these emission peaks. The occurance of these emission peaks basically corresponded to the digestive rule during a day.3. Significant seasonal emission patterns of methane were also identified. For example, at experimental site 1, the average emission rates during fall and winter seasons were 0.34 kg/head/d and 0.31 kg/head/d , indicating approximately 9.7% greater emission rate during fall season. At experimental site 2, the emission rates during spring, summer, fall and winter seasons were 0.28,0.32,0.33 and 0.30 kg/head/d respectively, where the emission rates during spring, summer and winter are about 14.3%, 17.9% and 7.1% higher than winter seasons, indicating a clear seasonal difference.4. In this study, annual methane emission rates for these two emxperimental sites were also calculated by using ensemble average of all measurements during different seasons for respective site. The annual emission rates for site 1 and 2 were 119 kg/head/ yr and 112 kg/head/yr, and the methane emission intensities on the milk production base were 0.028 kg CH_]4 (equivalent 35.90 L CH_]4) and 0.025 kg CH_]4 (equivalent 32.05 L CH_]4). Such methane emission intensities are much higher than the values in developed countries, indicating the low milk productivity in China.5. In addition, methane conversion factor Ym of feeds used on these two experimental dairy operations during measurements were also calculated using IPCC TierII methodology, where the Ym values were 7.3% and 6.7% for site 1 and 2 respectively. Through this study, the initial range of methane conversion factors was preliminarily established. |
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