Fungal community composition and function after long‐term exposure of northern forests to elevated atmospheric CO 2 and tropospheric O 3

Abstract

The long‐term effects of rising atmospheric carbon dioxide (CO 2 ) and tropospheric O 3 concentrations on fungal communities in soil are not well understood. Here, we examine fungal community composition and the activities of cellobiohydrolase and N ‐acetylglucosaminidase (NAG) after 10 years of exposure to 1.5 times ambient levels of CO 2 and O 3 in aspen and aspen–birch forest ecosystems, and compare these results to earlier studies in the same long‐term experiment. The forest floor community was dominated by saprotrophic fungi, and differed slightly between plant community types, as did NAG activity. Elevated CO 2 and O 3 had small but significant effects on the distribution of fungal genotypes in this horizon, and elevated CO 2 also lead to an increase in the proportion of Sistotrema spp. within the community. Yet, although cellobiohydrolase activity was lower in the forest floor under elevated O 3 , it was not affected by elevated CO 2 . NAG was also unaffected. The soil community was dominated by ectomycorrhizal species. Both CO 2 and O 3 had a minor effect on the distribution of genotypes; however, phylogenetic analysis indicated that under elevated O 3 Cortinarius and Inocybe spp. increased in abundance and Laccaria and Tomentella spp. declined. Although cellobiohydrolase activity in soil was unaffected by either CO 2 or O 3 , NAG was higher (∼29%) under CO 2 in aspen–birch, but lower (∼18%) under aspen. Time series analysis indicated that CO 2 increased cellulolytic enzyme activity during the first 5 years of the experiment, but that the magnitude of this effect diminished over time. NAG activity also showed strong early stimulation by elevated CO 2 , but after 10 years this effect is no longer evident. Elevated O 3 appears to have variable stimulatory and repressive effects depending on the soil horizon and time point examined.