Relationships between succession and community structure and function of Alnus-associated ectomycorrhizal fungi in Alaskan boreal forests

Abstract

Rates of production and carbon cycling in northern ecosystems depend heavily on nitrogen (N) availability across the landscape. Since much of the available N enters these systems through biological N-fixation, Alnus, with its capacity to fix large amounts of N, plays a critical role in ecosystem response to environmental change. However, because of its high phosphorus (P) demands, the abundance, distribution, and N-fixing capacity of Alnus is tightly controlled by the availability of P and its ability to assimilate P by associating with ectomycorrhizal fungal (EMF) symbionts. We assessed the potential of A. tenuifolia-associated EMF to access organic P forms of varying complexities. More than half of the community on A. tenuifolia were individuals from the genera Alnicola and Tomentella, indicating that the community of EMF on Alnus is a relatively distinct group of host-specific ectomycorrhizal fungi. However, the aggregated acid phosphatase, phosphodiesterase and phytase activities of the Alnus-EMF community were not dramatically different from other boreal plant hosts on the root tip level. We detected variability in the activities of the two Alnus dominants to mobilize acid phosphatase and phosphodiesterase. However, it appears that contrary to the hypothesis that nitrogen-fixing species would associate with EMF types well suited to P acquisition, the potential acid phosphatase activity of Alnicola luteolofibrillosa was significantly below the community mean. Our finding that enzyme activities of Alnus-EMF are not substantially greater than those found on other plant hosts suggests that if host specific EMF on Alnus facilitates P mobilization and uptake, the steps between P hydrolysis and assimilation into plant tissue as well as other pathways of P acquisition may be of greater importance in determining P provisioning to Alnus by EMF.