Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2007
Publication Date: 8/23/2007
Citation: Kandeler, E., Mosier, A., Morgan, J.A., Milchunas, D., King, J., Rudolph, S., Tscherko, D. 2007. Transient elevation of carbon dioxide modifies the microbial community composition in a semi-arid grassland. Soil Biology and Biochemistry 40:162-171. Interpretive Summary: It is widely accepted that the release of greenhouse gases into the atmosphere will have profound impacts on the earth’s climate, including global warming, altered precipitation patterns, and increased storm intensities. But greenhouse gases may have direct effects on plant ecology that are in addition to the more widely-publicized effects due to climate change. In particular, plants respond directly to the greenhouse gas carbon dioxide (CO2) since CO2 is a substrate for photosynthesis. Increases in this gas alone can enhance terrestrial photosynthesis, thereby increasing the amount of ecosystem C which eventually finds its way to belowground pools of C, which in turn can affect nutrient cycling, ecosystem performance, and eventually feed-back on soil available nutrients, thereby further modulating plant responses. Considerable research has been conducted understanding direct plant responses to CO2, but less is known about the consequences of these atmospheric changes on soil nutrient cycling, which ultimately may determine how plants respond to global change. This experiment investigated how doubling the CO2 concentration over a native Colorado prairie effects soil biology, in particular the effects on biological organisms which are key in the decomposition and recycling of soil nutrients essential for plant growth. The results indicate that increasing atmospheric CO2 may be leading to a soil community with increasingly larger amounts of fungal relative to bacterial organisms, a community shift that we believe is in response to the lower quality plant material that develops under elevated CO2. These results confirm our earlier findings which suggest that the long-term responses of such semi-arid grasslands to rising atmospheric CO2 will involve a host of biological reactions in the soil microbial community which may ultimately determine how these native grasslands adapt and change in response to altered CO2 and climate change.
Technical Abstract: Using open-top chambers (OTC) on the shortgrass steppe in northern Colorado, changes of microbial community composition were followed over the latter three years of a five year study of elevated atmospheric CO2 as well as during a period of 12 months after CO2 amendment ended. The experiment was comprised of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360 ' 20 ppm (ambient treatment), three chambered plots maintained at 720 ' 20 ppm CO2 (elevated treatment) and three unchambered plots. The abundance of fungal phospholipid fatty acids (PLFAs) shifted in the shortgrass steppe under the influence of elevation of CO2 over the period of three years. Whereas the content of the fungal signature molecule was similar in soils of the ambient and elevated treatments in 1999, during the third year of CO2 treatment, elevated CO2 increased the content of the fungal signature molecule by around 60 percent during the two subsequent years. The shift of microbial community composition towards a more fungal dominated community was likely due to slowly changing substrate quality; plant community forage quality declined under elevated CO2 because of a decline of N in all tested species as well as shift in species composition towards greater abundance of the low forage quality species (Stipa comata). In 2000 and 2001, elevated CO2 increased the relative proportion of Gram-negative bacteria mainly because enhanced rhizodeposition under elevated CO2 specifically favoured Gram-negative bacteria. After CO2 amendment ceased, lower abundance of fungal and bacterial PLFAs were detected in the post CO2 treatment in comparison to the ambient treatment. Therefore, quantity and quality of available substrates have not changed dramatically enough to shift the microbial community permanently to a fungi dominated community. We conclude from PLFA composition of soil microorganisms during the CO2 elevation experiment and during another year after cessation of CO2 that increased community metabolic efficiency due to higher relative abundance of fungi could be a primary mechanism leading to enhanced C storage in short grass steppe, when elevation of atmospheric CO2 will go on in the future.