Rhizosphere microflora of winter wheat plants cultivated under elevated CO2

We studied an effect of elevated atmospheric CO₂ on rhizosphere microorganisms in a hydroponics system where young wheat plants provided the only source of C for microorganisms. Plants were cultivated in mineral solution in sterile silica sand and exposed to control (ambient) and elevated (double) CO₂ concentrations for periods of 13, 20, 25 and 34 days.Microbial biomass C (C content in fraction of size 0.3–2.7 µm) was not affected by the elevated CO₂ concentration during the first 25 days of plant growth and was increased after 34 days of plant growth. A content of poly-β-hydroxybutyrate (PHB) reserve compounds (measured as derivatized product of 3-hydroxy-butyric acid and N-tert-butyldimethylsilyl-N-methyltrifluoracetamide using GC–MS) was lowered significantly (p<0.001) in the elevated CO₂ after 25 and 34 days. It was accompanied with a shift of bacterial distribution towards the nutritional groups utilising more complex organic material (number of CFUs on media with different sources of C and N). A coincidence of several events connected with plant and microbial carbon economy (decrease of an assimilation rate and relative growth rate of plants, small increase of microbial biomass, PHB decrease and suppression within the bacterial nutritional group requiring the most readily available source of C and energy) was observed in the system under elevated CO₂ on the 25th day.A modification of the GC–MS method for the detection of low levels of PHB compounds in natural samples was developed. We excluded the lipids fractionation step and we used EI MS/MS detection of the main fragment ions of the derivatized compound. This guarantees that the ion profiles have high signal-to-noise ratio at correct retention time. The detection limit is then about 30 pg g⁻¹ of sand or soil.The rhizosphere microflora responded very sensitively to the short-term changes in C partitioning in plants caused by the elevated CO₂.