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 CO₂ levels to confirm the accuracy of the scaling approach. NO₂ stimulation in agricultural uplands indicated an increased 0.33 Pg CO₂ equivalents 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 year and 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.