Responses Of Soil Microbial Community To Elevated Atmospheric CO₂ In An Annual Grassland

Grassland is one of the most important ecosystems on earth. Microorganisms are the major engines that drive soil biogeochemical cycles. The impacts of climate changes, represented by elevated atmospheric CO2, on soil microbial communities will definitely affect these cycles, which affects the overall ecological functions of grassland ecosystem. There were a growing number of studies focusing on the responses of soil microbial communities to elevated CO2(eCO2), but only a few of them examined the long-term effect of eCO2, and these studies largely ignored functional gene compositions.Using 16 S rRNA amplicon sequencing and a microarray named GeoChip, together with soil and vegetation attributes, this study investigated changes of the taxonomic and functional compositions of soil microbial communities from a model ecosystem, a Californian annual grassland, after long-term(14 years) experimentally elevated CO2 exposure(~550 ppm), trying to link soil geochemistry and plant properties with microbial community composition. Microbial taxonomic and functional compositions were both altered in eCO2 treatment plots, while the functional composition was possibly more sensitive to eCO2, implying that the abundant soil microbial taxa didn’t exhibit a high degree of functional redundancy. Microbial taxa responded to eCO2 differently: the relative abundance of Bacteroidetes was significantly lower in eCO2 plots while that of Verrucomicrobia was higher; the relative abundance of ammonia-oxidizing bacteria(AOB) and nitrite-oxidizing bacteria(NOB) remain unchanged. Soil temperature and dissolved organic carbon had the largest impacts on microbial taxonomic composition. As for functional gene categories, the relative abundance of genes related to carbon cycling and sulfur cycling significantly increased in eCO2 plots whereas those of metal homeostasis genes decreased. Elevated CO2 significantly stimulated DC/HB and HP/HB carbon fixation pathways, suppressed rTCA cycles, increased the relative abundance of labile carbon degradation genes and potentially inhibited soil CH4 emission. Moreover, microbial metal resistance, microorganism-induced sulfur cycling and phosphorus cycling were weaken, accelerated and unchanged under the eCO2 condition, respectively. eCO2 alone had no significant effect on microbial N transformations, which was supported by the changes of AOB, NOB and soil(de) nitrifying enzyme activity; however, N cycling genes showed significant correlations with AOB /NOB, indicating a close linkage between soil microbial taxa and ecological functions. Soil N, as well as dissolved organic carbon and atmospheric CO2, was the key environmental factors shaping microbial functional structure, although soil N content was similar in aCO2 and eCO2 plots.Above all, this study discussed the responses of soil microbial taxonomic composition and function structure to elevated atmospheric CO2 from an annual grassland in detail, providing clues for the accurate feedback of natural ecosystems to climate changes.