Methane (CH4) and nitrous oxide (N2O) are two potent greenhouse gases which in sum contribute to more than one fourth of global warming caused by anthropogenic activities. In the meantime, CH4 and N2O play significant roles in ozone layer chemistry. Understanding and quantifying CH4 and N2O fluxes in terrestrial ecosystems at large spatial scales, therefore, become an urgent task for accurately predicting climate change. In this study, I enhanced and applied a process-based ecosystem model, in conjunction with a series of spatial dataset including climate variability, ozone (O3) pollution, nitrogen (N) deposition, land cover change, N fertilization, and elevated atmospheric carbon dioxide (CO2), to examine the terrestrial fluxes of CH4 and N2O over the continental North America during 1979-2008. Over the study period, approximately 14.69 ± 1.64 T g C a-1 (1T g = 1012 g) of CH4, with a 95% confidence interval of (5.95 T g C a -1, 23.10 T g C a-1), and 1.94 ± 0.16 T g N a-1 of N2O, with a 95% confidence interval of (0.75 T g N a-1, 3.38 T g N a-1), were released from terrestrial ecosystems in North America. Both the United States and Canada acted as CH4 sources to the atmosphere, yet Mexico mainly oxidized and consumed CH4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH4 source, while all other ecosystems acted as sinks for atmospheric CH4, of which forests accounted for 36.8%. Regarding N2O emission in North America, the United States, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to the continental N2O emission.
Our simulations indicate that over the past 30 years 440.75 ± 8.97 T g CH4-C was released from North Americas’ terrestrial ecosystems; baseline emission contributed 417.24 ± 6.83 T g CH 4-C and global change factors accumulatively contributed 23.51 ± 9.61 T g CH4-C. O3 pollution led to a reduced CH4 emission by 2.30 ± 0.49 T g CH4-C. All other factors including climate variability, N deposition, elevated atmospheric CO2, N fertilizer application, and land conversion enhanced terrestrial CH4 emissions by 19.80 ± 12.42, 0.09 ± 0.02, 6.80 ± 0.86, 0.01 ± 0.001, and 3.95 ± 0.38 T g CH4-C, respectively, and interaction between/among these global change factors led to a decline of CH4 emission by 4.84 ± 7.74 T g CH4-C. From 1979 to 2008, North America’s terrestrial ecosystems accumulatively emitted 58.17 ± 0.85 T g N2O-N, of which global change factors contributed 2.81 ± 0.98 T g N2O-N, and baseline emission contributed 55.35 ± 0.56 T g N2O-N. The elevated CO2 led to a decrease in terrestrial N2O emission at 0.51 ± 0.07 T g N2O-N. Climate variability, N deposition, O3 pollution, N fertilization, and land use conversion increased N2O emission by 0.56 ± 0.68, 0.50 ± 0.07, 0.10 ± 0.02, 0.92 ± 0.09, and 0.16 ± 0.01 T g N2O-N, respectively. The interactive effect among multiple factors enhanced N2O emission by 1.10 ± 0.37 T g N2O-N over the 30 years. The factorial attribution of terrestrial fluxes of CH4 and N2O varied across countries.
Local sensitivity and uncertainty analyses indicate that the uncertainties in terrestrial CH4 and N2O fluxes varied across the continental North America; the largest uncertainty in CH4 flux locates in wetland, while the largest uncertainty in N2O flux lies in tropical forest, followed by cropland. This study provides useful and valuable information to both scientific community and policy makers such as magnitude, spatiotemporal and underlying mechanisms of terrestrial CH4 and N2O fluxes over the continental North America.