Abstract
The exceptionally fast rise of atmospheric carbon dioxide partial pressure (pCO2), a key component of global environmental change, may have profound implications for the functioning of ecosystems in the future. Increased photosynthetic production and plant growth stimulated by the rise of pCO2 may lead to an accumulation of non-structural carbohydrates (C) and intensify the mineral nutrient demands of plants. As a consequence, the interaction between plants and their symbiotic soil fungal partners may be altered. This may particularly apply to the arbuscular mycorrhizal fungi (AMF, Glomeromycota), because AMF rely exclusively on C supplied by living plants and are strongly implicated in plant mineral nutrition. AMF growth rates, the structure of AMF populations and communities and the physiological functioning of mycorrhizal symbioses may change. Effects of seven to ten years free-air CO2 enrichment in permanent, agricultural monocultures of Trifolium repens (white clover), a legume, and Lolium perenne (perennial ryegrass), a grass, were studied. This included: (1) A field study, based on traditional microscopical techniques, (2) an inoculation experiment with T. repens and pure fungal cultures isolated from the experimental field site, and (3) a molecular ecological field study, using PCR assay and DNA sequencing. (1) In the first field study, root colonisation levels by AMF and spore densities were microscopically assessed in permanent monoculture plots of T. repens and L. perenne. Generally, abundance of AMF in roots and soils from plots exposed to elevated atmospheric pCO2 conditions for seven years was higher. In particular, AMF associated with roots of L. perenne a plant known to experience severe N-deficiency, produced more hyphae, arbuscules, vesicles, and spores. This suggests that AMF profited from an increased availability of C in the roots of plants grown under elevated atmospheric pCO2. Moreover, there were significant interactions among nitrogen (N) fertilisation, host plant species and levels of AMF root colonisation with hyphae and vesicles. N-fertilisation reduced the levels of AMF root colonisation in L. perenne, but did not alter them in T. repens, a finding best explained by differences in host plant N-metabolisms with the legume having access to additional N provided by nodule symbiosis. (2) In the pot experiment conducted under controlled conditions using nine T. repens genets and fourteen single spore isolates of AMF, we found that eight years of elevated atmospheric pCO2 had selected for more beneficial (symbiotically effective) AMF strains of Glomus claroideum and G. intraradices. The fungal strains originating from plots which had been exposed to elevated atmospheric pCO2 increased foliar N-concentration of T. repens host plants by simultaneously stimulating biological N2 fixation in the nodule symbiosis and increasing the fraction of N taken up from the potting substrate. This finding is contrary to initial expectations that assumed that fungal strains with a history of exposure to elevated atmospheric pCO2 should be more C-costly and thus depress host growth under non-limiting phosphorus conditions. Furthermore, host plants of T. repens inoculated with fungal strains isolated from T. repens monoculture plots showed lower root colonisation and increased biological N2 fixation, compared to plants inoculated with AMF strains originating from mixed culture plots planted with T. repens and L. perenne. Moreover, symbiotic functioning, measured by physiological indicators, was strongly influenced by the particular combinations of plant genet (nine) and fungal isolate (fourteen). (3) In the molecular ecological field survey on AMF we used newly designed morphospecies-discriminating PCR primers for fungal identification within roots of T. repens. The population genetic structure of an AMF species (G. mosseae) was affected by ten years of CO2 fumigation, but not the community composition of six AMF species, located within the same roots. This population-level change indicates that AMF may adapt to new conditions under elevated atmospheric pCO2, even if shifts in the community structure (species level) are lacking. Adaptations through changes in the relative frequency of AMF strains could also explain the difference in symbiotic functioning that was detected in regard to host N-nutrition. Additional findings were a surprisingly uniform, overall relative abundance of the six detected AMF species, although the communities in the six sampled plots were (marginally) significantly structured spatially. Overall, results of this PhD thesis contribute to a better understanding of the ecology of AMF under high input agricultural conditions and reveal possible responses of mycorrhizal fungi to rising atmospheric pCO2. Changes in the structure of AMF populations and altered symbiotic functioning of AMF strains seem likely under prolonged elevated atmospheric pCO2. Findings made here highlight the key role AMF will play for future ecosystems functioning under a scenario of global environmental change.