Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

Abstract

The increase in atmospheric CO 2 content alters C 3 plant photosynthetic rate, leading to changes in rhizodeposition and other root activities. This may influence the activity, the biomass, and the structure of soil and rhizosphere microbial communities and therefore the nutrient cycling rates and the plant growth. The present paper focuses on bacterial numbers and on community structure. The rhizospheres of two grassland plants, Lolium perenne (ryegrass) and Trifolium repens (white clover), were divided into three fractions: the bulk soil, the rhizospheric soil, and the rhizoplaneendorhizosphere. The elevated atmospheric CO 2 content increased the most probable numbers of heterotrophic bacteria in the rhizosphere of L. perenne. However, this effect lasted only at the beginning of the vegetation period for T. repens. Community structure was assessed after isolation of DNA, PCR amplification, and construction of cloned 16S rDNA libraries. Amplified ribosomal DNA restriction analysis (ARDRA) and colony hybridization with an oligonucleotide probe designed to detect Pseudomonas spp. showed under elevated atmospheric CO 2 content an increased dominance of pseudomonads in the rhizosphere of L. perenne and a decreased dominance in the rhizosphere of T. repens. This work provides evidence for a CO 2 -induced alteration in the structure of the rhizosphere bacterial populations, suggesting a possible alteration of the plant-growthpromoting-rhizobacterial (PGPR) effect.

Nitrogen fertiliser rate affects the frequency of nitrate-dissimilating Pseudomonas spp. in the rhizosphere of Lolium perenne grown under elevated pCO2 (Swiss FACE)

“…Pseudomonas counts in both low and high-fertilised swards were higher in root-associated RE and RS fractions under elevated than under ambient pCO 2 (Fig. 1), confirming previous results obtained at this site using a 16S rDNA-based approach (Marilley et al, 1999). This was attributed to greater belowground C allocation associated with increased root biomass, turnover and exudation under elevated pCO 2 (van Kessel et al, 2000) favouring Pseudomonas populations that are known to be good heterotrophic competitors for C sources, irrespective of their denitrifying abilities (Tiedje, 1988).…”

Frequency and Diversity of Nitrate Reductase Genes among Nitrate-Dissimilating Pseudomonas in the Rhizosphere of Perennial Grasses Grown in Field Conditions

“…Moreover, Marilley and co-workers [33] showed that the proportion of Pseudomonas was enhanced in the rhizosphere of L. perenne grown under elevated pCO 2 . The nitrate-dissimilating Pseudomonas populations (size and composition) are therefore very likely to be altered in the rhizosphere of plants grown under elevated pCO 2 .…”

“…Two perennial grasses with different nitrogen requirements [55] were used as model plants: the nitrophilic Lolium perenne and the oligonitrophilic Molinia coerulea. Bacteria affiliated to the genus Pseudomonas were previously shown to account for a significant part of the microbiota associated to the rhizosphere of these two plants [33,52]. A large collection of nitrate-dissimilating Pseudomonas strains isolated from the root, rhizosphere, and surrounding soil of these plants was characterized regarding their frequency and their nitrate reductase genes.…”

Specific rhizosphere bacterial and fungal groups respond differently to elevated atmospheric CO2

“…Previous studies on the effects of elevated atmospheric CO 2 concentrations have either examined total microbial-community patterns (Griffiths et al, 1998;Kandeler et al, 1998;Marilley et al, 1999;Montealegre et al, 2002;Jossi et al, 2006;Lipson et al, 2006;Drigo et al, 2007), metabolic patterns (Grayston et al, 1998;Hodge and Millard, 1998; or have focused on the structure of specific microbial communities of interest, such as Rhizobium species (Schortemeyer et al, 1996;Montealegre et al, 2000), Pseudomonas species (Marilley et al, 1999;, /a-Proteobacteria or Acidobacteria (Lipson et al, 2006) and mycorrhizal fungi (Gamper et al, 2005). However, there is still a general lack of knowledge with respect to the relative responses of different specific microbial groups in response to elevated atmospheric CO 2 conditions.…”