Increased Soil Emissions of Potent Greenhouse Gases under Increased Atmospheric CO2

 Burning fossil fuels and frequently changing land use contribute to rapidly increasing atmospheric CO2 levels. An increase in CO2 can alter both abiotic and biotic conditions of soil and affect the levels of other important greenhouse gases (GHG) such as nitrous oxide (N2O) and methane (CH4). Several previous studies have shown that increased CO2 levels could slow climate change by increasing plant efficiency and soil carbon input and storage, however CO2 should not be examined alone because other gases also have high global warming potentials. For example, N2O  and CH4 have global warming potentials 298 times higher and 25 times higher respectively than that of CO2. In this study, Van Groenigen et al. (2011) examined the effects of increased atmospheric CO2 on N2O levels in upland soil and CH4 levels in rice paddies and natural wetlands and concluded that changes in these greenhouse gases can greatly affect how ecosystems influence climate change.—Taylor Jones
Van Groenigen, K. J., Osenberg, C. W., Hungate, B. A., 2011. Increased soil emissions of potent greenhouse gases under increased atmospheric CO2. Nature 475, 214–216.

          Kees Jan van Groenigen and colleagues completed a meta-analysis on 152 observations from 49 published studies to examine fluxes of CH4 and N2O in the presence of increased CO2. GHG emissions span a variety of ecosystems and the compiled meta-analysis, compared to an individual experiment, provides a more comprehensive study. The increased CO2 stimulated N2O  emissions in uplands by 18.8% and stimulated CH4 in wetlands by 13.2% and in rice paddies by 43.4%. The authors also examined the effect of increased CO2 on the possible causes for these changes in GHG emissions, soil water content and root biomass. Combining all three types of terrain, soil water content increased 6% and root biomass increased 18%.
          The authors also investigated the importance of the changes in GHG levels on fertilized (agricultural) and non-fertilized (natural) land. The model was tested on current CO2levels to confirm the accuracy of the scaling approach. NO2 stimulation in agricultural uplands indicated an increased 0.33 Pg CO2equivalents per year and an increased 0.24 Pg CO2 equivalents per year in natural areas. CH4 stimulation in agricultural rice paddies indicated an increased 0.25 Pg CO2 equivalents per yearand an increased 0.31 Pg CO2 equivalents per year in natural wetlands. In addition, carbon sink was larger for fertilized areas and GHG emissions could cancel the expected increase in carbon sink by 16.6%, based on the authors’ calculations.
          Van Groenigen and colleagues present three reasons that suggest this increase of carbon sink could be an underestimate because first, the majority of data collected during the growing season and some terrains have higher N2O emissions during the winter that could add up to 7% more N2O. Second, N2O emissions increased in studies that included additional nitrogen, so as nitrogen increases along with CO2, N2O levels may also increases. Last, the authors noticed a weak correlation between experiment duration and so the effect of CO2 is likely to increase over time.
          Increased CO2 levels stimulate denitrification, a major contributor to N2O levels in upland soil and in wetlands and rice paddies of various geographic regions, methanogenic archaea rely heavily on carbon levels as a source of organic substrates and with increased CO2 levels, more CH4 is produced. This study shows that not only do increased CO2 levels amplify climate change, but the increase of N2O in uplands and CH4 in wetlands and rice paddies could negate carbon sink percentages and further studies should consider the indirect effects of these other greenhouse gases on climate change.

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