Browsing by Subject "Elevated CO2"

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Einfluss von erhöhtem atmosphärischen CO2 auf die N2-fixierende Symbiose von Trifolium repens L. und Rhizobium leguminosarum biovar trifolii
(2007) Stöber, Sara; von Wirén, Nicolaus
CO2 is one of the main greenhouse gases strongly influencing the climate and the terrestrial ecosystem. Up to know little is known about the impact of elevated atmospheric CO2 on symbiotic interactions in the rhizosphere, especially on the N2-fixing symbiosis between Trifolium repens and Rhizobium leguminosarum biovar trifolii. First results of a ten-year Free-Air CO2 enrichment experiment (Swiss FACE) showed that after three years of CO2 fumigation the genetic composition of the Rhizobium population in the root nodules of T. repens had changed. The first part of this thesis set out to clarify the question whether a genetic difference in the Rhizobium population of root nodules of white clover could still be detected after ten years of CO2 fumigation or if an adaptation of the nodule bacteria to elevated CO2 concentrations had occurred. Furthermore the thesis addressed the question whether elevated atmospheric CO2 leads to quantitative and qualitative changes in the root exudation of T. repens particularly with regard to exudation of signal substances during the nodulation process. In summer 2002 white clover plants were collected from plots fumigated with CO2 and control plots of the Swiss FACE. Rhizobium strains were isolated from the clover root nodules and used for rep-PCR DNA fingerprinting. Results clearly showed that after ten years of CO2 enrichment changes in the genetic composition of the R. l. bv. trifolii could no longer be observed. Thus, CO2-induced changes in the population structure of rhizobia seemed to be transient. This can be traced back to the possibility that over the experimental period a new C/N equilibrium in the grassland ecosystem has been established. At the beginning of the FACE experiment an increase in the C/N ratio of the soil was detected, which could be balanced in the course of time through enhanced symbiotic N2 fixation and consequently a higher N input into the ecosystem. The observed stabilisation of the grassland ecosystem most likely caused a reduction of the indirect CO2 impact on the microorganisms. This might explain why a change in the genetic composition of Rhizobium strains was not longer detected after ten years in the Swiss FACE. To investigate an influence of elevated atmospheric CO2 concentration on the release of signalling compounds clover plants were cultivated hydroponically in two independent climate chamber trials under axenic and non-axenic conditions at ambient and elevated CO2 concentrations (400 and 800 ppm) and different levels of N supply. Root exudates were collected over a period of seven hours and at three and four different plant ages, respectively. Phenolic compounds were extracted by solid-phase extraction and afterwards analysed with HPLC and LC-MS. Additionally, the isolated fractions were tested for their ability to induce the nodulation genes of R. l. bv. trifolii using a nod-gene induction test. The CO2 enrichment caused an increase in shoot and root growth in both experimental setups, but did not provoke a change in the C/N ratio of the roots. Besides the known signal compound 7,4?-dihydroxyflavone new phenolic substances could be detected, which have not yet been described in literature. The fractions were identified by their polarity, light absorption and molecular weight as aglyca and flavones. All of these had the ability for nod-gene induction except one fraction (fraction 2). CO2 influenced the exudation of signalling compounds quantitatively but not qualitatively. The enhanced exudation, especially of 7,4?-dihydroxyflavone, could be attributed to the higher root mass under elevated CO2 but also to a higher release rate on a root fresh weight basis. The CO2 reaction of the clover plants, for the biomass production as well as for the root exudation, was clearly dependent on the N supply and only significant under axenic conditions. In individual cases the N impact was more pronounced than the CO2 effect: with increasing N demand axenic clover plants enhanced the exudation of the nod-gene inducing fraction C. It is concluded that this fraction, identified as a hydroxyflavone, has therefore an important signal function under N limitation. Besides the CO2 concentration and N supply, root exudation by T. repens was considerably influenced by the plant age, which caused a reduction of the signal exudation in older plants and qualitative changes of the released phenols, especially under non-axenic conditions. The present study suggests that the genetic shift of R. l. bv. trifolii detected at the beginning of the Swiss FACE experiment was most likely a consequence of the enhanced exudation of phenolic signal compounds of T. repens under elevated atmospheric CO2 concentrations.
Function and composition of the soil microbial community in calcareous grassland exposed to elevated atmospheric carbon dioxide
(2003) Ebersberger, Diana; Kandeler, Ellen
Terrestrial ecosystems generally respond to rising atmospheric carbon dioxide (CO2) concentrations with increased net primary productivity and increased water use efficiency. This may change the amount and quality of organic substances entering the soil and fuelling microbial metabolism. Soil microorganisms and their activity might also be affected by increased soil moisture at elevated CO2. This thesis was designed to analyse the response of the soil microbial community in a species-rich calcareous grassland in the Swiss Jura Mountains, which had been exposed to ambient and elevated CO2 concentrations (365 and 600 ppm) for six growing seasons. In the first study, laboratory incubation experiments were conducted to explore the relationship between litter quality under elevated carbon dioxide and enzymes involved in carbon cycling. Naturally senescent, mixed litter from the long-term field experiment was incubated with soil material for 10, 30 and 60 days. Soil samples were then obtained close to the litter layer using a microtome cutting device. Litter and soil samples were analysed for invertase and xylanase activity. The lower litter quality produced under elevated CO2, i.e. wider C/N ratio, yielded lower invertase and xylanase activities of litter. Litter addition stimulated activities in adjacent soil. Invertase activities of adjacent soil were not affected by litter quality, while soil xylanase activity was higher in soil compartments adjacent to litter from elevated CO2 plots. The reduced enzyme activities of litter produced under elevated CO2 can slow decomposition, at least during the initial stages. Since the effects of litter quality on enzyme activities in adjacent soil were small, we conclude that CO2-induced belowground C-inputs (e.g. increased root mass) and altered moisture conditions are more important controls of enzyme activities than altered litter quality. In the second study, functional diversity of the soil microbial community was assessed by analysing N-mineralisation and activities of enzymes of the C-, N-, P- and S-cycle of soil samples taken in spring and summer 1999, in the 6th season of CO2 exposure. In spring, N-mineralisation increased significantly by 30% at elevated CO2, while there was no significant difference between treatments in summer. The response of soil enzymes to CO2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly, by 32% and 21%, respectively. In summer, activity differences between CO2 treatments were greatest in the case of urease and protease (+21% and +17% at elevated CO2). The significant stimulation of N-mineralisation and enzyme activities at elevated CO2 was probably caused by higher soil moisture and/or increased root biomass. In the third study, soil microbial community structure of soil samples taken in spring and summer 1999 was analysed by means of PLFA profiles and 16S rDNA fingerprints obtained by PCR-DGGE. PLFA profiles were not affected by elevated CO2. Ordination analysis of DNA fingerprints revealed a significant relation between CO2 enrichment and variation in DNA fingerprints. This variation must be attributed to low intensity bands because dominant bands did not differ between treatments. Diversity of the bacterial community (number of bands in DNA fingerprints and Shannon indices) was not affected. The observed minute, but significant changes in the structure of the soil bacterial community might be caused by changes in the quality of rhizodeposits at elevated CO2. These could either result from altered rhizodeposition of individual plants or from altered species composition of the calcareous grassland.The 4th part of the thesis compiles data on soil microorganisms, soil fauna, soil structure and nitrogen cycle of calcareous grassland after CO2 exposure for six growing seasons. Microbial biomass, soil basal respiration and the metabolic quotient were not altered significantly. PLFA analysis revealed no significant shift in the ratio of fungi to bacteria. Protozoans, bacterivorous and fungivorous nematodes, acarians, collembolans, and root-feeding nematodes were not affected by elevated CO2. Total nematode numbers averaged slightly lower (-16%) and nematode mass was significantly reduced (by 43%) due to fewer large-diameter nematodes classified as omnivorous and predacious. CO2 exposure resulted in a shift towards smaller aggregate sizes; this was caused by higher soil moisture. Reduced aggregate sizes result in reduced pore neck diameters. This can confine the locomotion of large-diameter nematodes and possibly accounts for their decrease. The CO2 enrichment also affected the nitrogen cycle. N stocks in living plants and surface litter increased, but N in soil organic matter and microorganisms remained unaltered. N mineralisation increased considerably, but microbial N did not differ between treatments, indicating that net N immobilization rates were unaltered.