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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.

The restriction enzyme profiles of 16s ribosomal DNAs (rDNAs) amplified by PCR from thermophilic heterotrophic bacterial strains isolated from composts were compared with those of reference strains. This allowed us to assign all but 1 of 16 strains to four different Bacillus species (namely, Bacillus stearothermophilus, Bacillus pallidus, Bacillus thermoglucosidasius, and "Bacillus thermodenitrijicans") . This study showed that PCR restriction analysis of 16s rDNA contributes to rapid and reliable identification of newly isolated strains belonging to recognized species.A few studies have reported the presence of thermophilic bacteria in hot compost (3,4,7,19,20). Strom (19,20) isolated more than 750 heterotrophic spore-forming strains from compost; very few of these strains grew at temperatures above 60"C, and growth at 65°C was restricted to Bacillus coagulans (type A) and Bacillus stearothennophilus. Until recently, only strains related to B. stearothennophilus were identified from the hottest compost samples screened (65 to 69°C) (7,19,20). The great diversity of thermophilic bacteria related to the genus Bacillus has frequently been emphasized (16,22), but it appears that only a few of the isolates have properly been identified to date. The morphology of sporulating cells, the shape of colonies, and growth abilities have proved to be insufficient for unequivocal identification of Bacillus strains (10, 11). The purpose of the present study was to identify heterotrophic, thermophilic, spore-forming strains isolated from hot composts by using a rapid molecular method based on the restriction profiles of 16s ribosomal DNA (rDNA) amplified by PCR.Serial dilutions of compost sample suspensions were carried out in five different media. B and DN media consisted simply of nutrient broth (Merck, Darmstadt, Germany); DN medium was supplemented with 2 g of KNO, per liter. GA, P, and PN media were synthetic media composed of a basal mineral medium (1) supplemented with various growth substrates at a concentration of 2 g liter-' [GA medium contained D-glucose and sodium acetate; P medium contained sodium pyruvate; PN medium contained sodium pyruvate and (NH4)2S04)]. The cultures were incubated under air at 65°C for 1 to 6 days, and pure colonies were isolated by repeated streaking on the same media solidified with agar. Colonies varying in appearance were picked deliberately to try to increase the number of different species isolated (the second and third letters of the compost strain designations in Fig. 1 refer to the isolation medium). Pure strains were then routinely cultivated at 60°C on B medium supplemented with 2 g yeast extract per liter and solidified with agar (NAY medium). The type and reference strains are listed in Metabolic tests were carried out at 55°C with API 20 NE strips (BioMkrieux, Marcy-l'Etoile, France) by using a few fresh colonies suspended in the basal mineral medium supplemented with 0

SummaryA great variety and high numbers of aerobic thermophilic heterotrophic and/or autotrophic bacteria growing at temperatures between 60-80°C have been isolated from thermogenic (temperature 60-80°C) composts in several composting facilities in Switzerland. They include strains related to the genus Thermus (T. thermophilus, T. aquaticus. and several other new strains). Bacillus schlegelii, Hydrogenohacter spp., and of course heterotrophic sporeforming Bacilli. This contrasts with the generally held beliefthat thermogenic composts (> 60°C) support only a very low diversity of heterotrophic thermophiles. This biodiversity suggests efficient decomposition of organic matter at temperatures above 60°C, and a good thermo-hygienization.During the terminal cooling or maturation phase of composts high numbers and a great metabolic diversity of mesophilic bacteria was observed, including nitrogen-fixers, sulfur-oxidizers, hydrogen-oxidizers, nitrifyiers, and producers of extracellular polysaccharides or bacterial humin. This microbial diversity plays an essential roJe for compost stabilization. It is suggested that mature compost application improves soil chemistry and microbiology, and can thus be regarded beneficial for agriculture. lntroductionAmong the various processes used to manage organic wastes (landfiiL incineration), only the biological process of composting can bring about a stabilization of * To whom correspondence should he addressed. M. de Bertoldi et al. (eds.), The Science of Composting