Submitted to: Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/28/2008
Publication Date: N/A
Citation: Interpretive Summary: Many plants form beneficial relationships with fungi known as arbuscular mycorrhizal fungi (AMF). Benefits to the plant include increases in nutrient and water uptake through the root system because AMF colonize plant roots. However, the range of benefits can vary from large to sometimes detrimental effects when AMF colonize plants. We hypothesized that this difference in functioning of AMF may be due to the existing nutrient levels in the soil and that soils that are deficient in phosphorus had the most beneficial AMF. This relationship is important because the actual make-up of plant communities may be influenced by how beneficial AMF are to the plant species present. We studied these relationships by growing different types of grass species in soils from three different sites with and without AMF. As expected, the fungi were most beneficial in the most phosphorus limited grassland soils. We also saw that grass species that were not as successful at taking up nitrogen from the soil benefited the most from AMF. Therefore, we did see an influence of AMF and other microbes in the soil on the functioning of the plant and determined that this could have some effect on the make-up of plant communities.
Technical Abstract: The degree to which plants benefit from mycorrhizal symbioses is influenced by soil fertility. Plants generally benefit the most from arbuscular mycorrhizal (AM) associations when they are growing in phosphorus (P) deficient soil; however soil nitrogen (N) availability can also influence mycorrhizal function. We hypothesize that mycorrhizae are most beneficial in P-limited soils and less beneficial when N is most limiting. Anthropogenic N deposition has the potential to influence mycorrhizal function, but this response may depend upon the ambient soil N:P ratio. Plant community changes in response to N deposition may be linked to changes in mycorrhizal function. We hypothesize that nitrophilic plant species benefit less from AM symbioses than plant species that do not respond positively to N enrichment; furthermore, plant community shifts with N enrichment may arise from either positive or negative feedbacks between plants and their associated AM fungi and other soil organisms (AMSO). We studied mycorrhizal function in long term N enrichment experiments at three different grassland systems with low, medium and high ambient soil N:P ratios. Soils, plants and AMSO were used for greenhouse experiments involving two plant species from each of the grasslands. As expected, AMSO were most beneficial in the most P-limited grassland (highest N:P ratio) and the least beneficial in the most N-limited site (lowest soil N:P ratio). Also, our results support the hypothesis that nitrophilic plant species benefit less from AM associations than N insensitive species. Our experiments provide evidence for negative but not positive feedbacks between communities of plants and AMSO; however positive feedbacks would only be observed if the nitrophilic grass species benefited from AMSO, and that was not the case in our experimental system.