Microbial parameters of rhizosphere soil, such as bacterial numbers or microbial activities, depend on the distance that microbes have to the root surface. In this study we show that the number of ...bacteria found in rhizosphere soil from white clover is highly correlated with root length density of the rhizosphere soil. In contrast, bacterial numbers, microbial activity (measured as fluorescein diacetate hydrolytic activity), and the amount of extractable carbon (C) in the rhizosphere of perennial ryegrass were independent of the amount of soil recovered from the roots. The missing rhizosphere effect in perennial ryegrass soils can be explained by the high rooting density of ryegrass, whereas the low rooting density of white clover results in gradients of microbial numbers and activities in soils. Results of these studies indicate that it is important to express microbial parameters on root length and soil weight bases, especially for less densely rooted soils.
We studied an effect of elevated atmospheric CO^sub 2^ 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) CO^sub 2^ 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 CO^sub 2^ 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 CO^sub 2^ 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 CO^sub 2^ 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^sup -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 CO^sub 2^.PUBLICATION ABSTRACT
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 El 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.
