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The arbuscular mycorrhizal (AM) fungi are ubiquitous biotrophic root endophytes that form a significant link in macronutrient cycles. The AM fungi occupy two niches simultaneously, the intraradical niche, from which they gain carbon, and the bulk soil. We predict that AM community structure will vary through time, depending upon season and plant identity. In this study, we compared the AM fungal community colonising eight species of Acer L. (maple) grown in an arboretum. DNA from root samples was analysed using terminal restriction fragment length polymorphism (TRFLP) of AM fungal sequences. Three time points were sampled during the growing season, and ordination analysis showed this was the most important factor discriminating the TRFLP profiles. Analysis of each month separately showed that the host species was a significant factor in all samplings, but that the response of each species through time was different. This study demonstrates the dynamic nature of AM communities, and the importance of sampling habitats throughout the season to generate a complete picture of the AM community. Key words: maple, TRFLP, arbuscular mycorrhizas, communities, seasonality. Les champignons mycorhiziens a arbuscules (MA) sont des endophytes biotrophes ubiquistes des racines qui forment un lien significatif dans les cycles des macronutriments. Les champignons MA occupent simultanement deux niches, la niche intra-radicalaire, a partir de laquelle ils acquierent le carbone, et le sol en vrac. Nous predisons que la structure de la communaute de MA variera en fonction du temps, dependamment de la saison et de l'identite de la plante. Dans cette etude, nous comparons la communaute de champignons MA colonisant huit especes d'erables (Acer L.) cultives en arboretum. L'ADN d'echantillons de racines a ete analyse par polymorphisme de longueur de fragments de restriction terminaux (TRFLP) des sequences de MA fongiques. Trois periodes de temps ont ete echantillonnees durant la saison de croissance, et une analyse d'ordination a montre que cela constituait le facteur le plus important permettant de discriminer les profils en TRFLP. Une analyse de chacun des mois a montre separement que l'espece d'hote constituait un facteur significatif dans tous les echantillonnages, mais que la reponse de chaque espece etait differente en fonction du temps. Cette etude demontre la nature dynamique des communautes de MA et l'importance des habitats d'echantillonnage au cours de la saison, afin de dresser une image complete de la communaute de MA. [Traduit par la Redaction] Mots-cles: erable, TRFLP, mycorhize a arbuscules, communautes, saisonnalite., Introduction The arbuscular mycorrhizal (AM) symbiosis is the most widespread and abundant of all terrestrial mutualistic interactions. The symbiosis is thought to be generalist, in that those fungal taxa that [...]

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2007.01239.x Byline: THORUNN HELGASON (*), JAMES W. MERRYWEATHER, J. PETER W. YOUNG, ALASTAIR H. FITTER Keywords: arbuscular mycorrhizal fungi; Ajuga reptans; host range; Glomeromycota; phosphorous; SSUrDNA Abstract: Summary The composition of communities of arbuscular mycorrhizal (AM) fungi can have a large effect on the performance of their plant hosts. The role of individual fungal species in shaping this response is as yet unresolved. We have used the fungicide benomyl to alter the community of AM fungi in undisturbed monoliths of soil in a natural community. Changes in the community were characterized by root colonization (%RLC), cloning, sequencing and tRFLP of a partial SSUrDNA fragment. Eleven plant species were sufficiently abundant in the monoliths to be examined. In the highly mycorrhiza-dependent perennial herb Ajuga reptans, phosphate concentration was significantly reduced after benomyl treatment over a full growing season. The other plant species showed low colonization and no significant difference in phosphate concentration after benomyl treatment. Although colonization in A. reptans was reduced, many mycorrhizal fungi survived in the roots. Some became more abundant following fungicide treatment, suggesting competitive release. Fungi that increased were generalists that have been identified in field samples from published studies colonizing a wide range of plant species. Those that declined were specialists with a narrow host range; five types had not been recorded previously in field samples. AM fungi in this study differed greatly in their response to perturbation, independent of the identity of the host plant. If such functional diversity is widespread, then elucidating the part played by AM fungal diversity in regulating plant community structure will be key to our understanding and management of ecosystems. Article History: Received 18 September 2006; revision accepted 23 February 2007 Handling Editor: Marcel van der Heijden Article note: (*) Correspondence: Thorunn Helgason (fax +44 0 1904 328505; e-mail th7@york.ac.uk).

Summary: Denitrification causes loss of available nitrogen from soil systems, thereby reducing crop productivity and increasing reliance on agrochemicals. The dynamics of denitrification and denitrifying communities are thought to be altered by land management practices, which affect the physicochemical properties of the soil. In this study, we look at the effects of long‐term tillage and fertilization regimes on arable soils following 16 years of treatment in a factorial field trial. By studying the bacterial community composition based on 16S rRNA amplicons, absolute bacterial abundance and diversity of denitrification functional genes (nirK, nirS and nosZ), under conditions of minimum/conventional tillage and organic/synthetic mineral fertilizer, we tested how specific land management histories affect the diversity and distribution of both bacteria and denitrification genes. Bacterial and denitrifier communities were largely unaffected by land management history and clustered predominantly by spatial location, indicating that the variability in bacterial community composition in these arable soils is governed by innate environmental differences and Euclidean distance rather than agricultural management intervention. [ABSTRACT FROM AUTHOR]