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

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 CO2 as well as during 12 months after CO2 amendment ended. The experiment was composed of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360±20 μmol mol−1 (ambient treatment), three chambered plots maintained at 720±20 μmol mol−1 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 3 years. Whereas the content of the fungal signature molecule (18:2ω6) was similar in soils of the ambient and elevated treatments in the third year of the experiment, CO2 treatment increased the content of 18:2ω6 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 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 the year after which CO2 enrichment had ceased, abundances of fungal and bacterial PLFAs in the post-CO2 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 CO2 elevation experiment and during the subsequent year after cessation of CO2 treatment that a shift towards a fungal dominated system under higher CO2 concentrations may slow down C cycling in soils and therefore enhance C sequestration in the shortgrass steppe in future CO2-enriched atmospheres.