Transient elevation of carbon dioxide modifies the microbial community composition in a semi-arid grassland

Abstract Using open-top chambers (OTC) on the shortgrass steppe in northern Colorado, changes of microbial community composition were followed over the latter 3 years of a 5-year study of elevated atmospheric CO 2 as well as during 12 months after CO 2 amendment ended. The experiment was composed of nine experimental plots: three chambered plots maintained at ambient CO 2 levels of 360±20 μmol mol −1 (ambient treatment), three chambered plots maintained at 720±20 μmol mol −1 CO 2 (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 CO 2 over the period of 3 years. Whereas the content of the fungal signature molecule (18:2 ω 6) was similar in soils of the ambient and elevated treatments in the third year of the experiment, CO 2 treatment increased the content of 18:2 ω 6 by around 60% 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 CO 2 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 the year after which CO 2 enrichment had ceased, abundances of fungal and bacterial PLFAs in the post-CO 2 treatment plots shifted slowly back towards the control plots. Therefore, quantity and quality of available substrates had not changed sufficiently to shift the microbial community permanently to a fungal dominated community. We conclude from PLFA composition of soil microorganisms during the CO 2 elevation experiment and during the subsequent year after cessation of CO 2 treatment that a shift towards a fungal dominated system under higher CO 2 concentrations may slow down C cycling in soils and therefore enhance C sequestration in the shortgrass steppe in future CO 2 -enriched atmospheres.