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Swine production is a primary source of protein and fat to human society, in the meanwhile, it continuously emits undesirable pollutant gases, including ammonia (NH3), hydrogen sulfide (H2S), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), imposing adverse impacts on animal and human health as well as on the environment. Previous studies proved that reducing excessive dietary N was effective in mitigating NH3 emissions from pig production, but the reduction in NH3 emissions during different swine physiological stages were not reported yet as only a few studies were based on continuous monitoring in the field. Moreover, the effects of reduced dietary N on other gases were not clear based on the very limited number of studies. A comparison study with reduced dietary crude protein (CP) with supplemental amino acid (AA) was conducted with 720 pigs in a 12-room research building for 155 days that covered from weaned to finishing stages. The pigs were divided into three 4-room groups and fed with 2.1%–3.8% reduced CP (T1), 4.4–7.8% reduced CP (T2), and standard (control) diets, respectively. Pig production, including pig number and pig weight, was recorded. Manure volume was measured weekly. Manure was sampled periodically, and samples were analyzed for pH, nitrogen, and other parameters. Building environmental variables, including temperature, relative humidity, room static pressure, ventilation, and pig activities, were continuously measured. Gas concentrations at the exhaust fans were also continuously monitored. Gas emission rates were calculated using the measured ventilation rates and gas concentrations. Group cycle mean NH3 emissions from the control, T1, and T2 groups were 68.9 ± 40.0, 46.7 ± 19.5, and 29.8 ± 10.6 g d-1 AU-1 (AU = 500 kg live mass), respectively. Emissions from the T1 and T2 groups were reduced by 33.0% and 57.2%, respectively, compared with the control group. The peak of NH3 emissions appeared during the third nursery phase for the T1 and T2 groups but delayed to the first grower phase for the control group. Large variabilities in NH3 emissions were observed among the four rooms within the same group. To study these variabilities, the 12 pig rooms were divided into sixty-six 2-room pairs, and the variables measured in each pair of two rooms were differenced to create a dataset showing the variabilities of NH3 emission and other measured variables. By using multivariate linear regression, the pig diet, total pig weight, and the pit manure volume were identified as the major influencing factors for the variabilities. Using the Panel Data Analysis with Heterogeneous Time Trends, the primary time-relevant variables imposing significant effects on the NH3 emissions were pit temperature and total pig weight. Additionally, two other unidentified factors that imposed substantial influences on the NH3 emission variabilities were also detected and their dynamic trends were uncovered. Different from its effects on lowering NH3 emissions, reduced dietary CP and AA supplements increased hydrogen sulfide (H2S) emissions. The group cycle means H2S emission rates were 4.0 ± 2.9, 4.3 ± 3.2, and 5.4 ± 4.0 g d-1 AU-1, respectively, for the control, T1, and T2 groups. Emissions of H2S were enhanced by 10.0 and 36.7%, respectively, for the T1 and T2 groups (p < 0.001). The enhanced H2S emissions from the T1 and T2 groups were related to the reduced manure pH and were possibly affected by several pathways, which could involve volatile fatty acids and nitrogen concentrations, and microbial activities in the manure. The dietary treatment of CP-reduction also influenced the emission of greenhouse gases (GHG). The dietary CP reduction was related to higher CO2 emission (p < 0.05). The two CP reduced groups had lower N2O emissions (p < 0.01). The CH4 emission was highest from the group where the pH of the produced manure was closest to the optimal pH range of anaerobic digestion. The accumulated global warming potential (GWP) was 1300.2, 1436.6, and 1420.9 kg AU-1 CO2eq, for the control, T1, and T2, respectively, during the whole cycle.