Phosphorus Mobilizing Enzymes of Alnus-Associated Ectomycorrhizal Fungi in an Alaskan Boreal Floodplain

Roger W Ruess,Knut Kielland,D Lee Taylor,Jack W Mcfarland,Michaela M Swanson,Karl D Olson

doi:10.3390/f10070554

Abstract

Because of its high phosphorus (P) demands, it is likely that the abundance, distribution, and N-fixing capacity of Alnus in boreal forests are tightly coupled with P availability and the mobilization and uptake of soil P via ectomycorrhizal fungi (EMF). We examined whether Alnus shifts EMF communities in coordination with increasingly more complex organic P forms across a 200-year-old successional sequence along the Tanana River in interior Alaska. Root-tip activities of acid phosphatase, phosphodiesterase, and phytase of A. tenuifolia-associated EMF were positively intercorrelated but did not change in a predictable manner across the shrub, to hardwood to coniferous forest successional sequence. Approximately half of all Alnus roots were colonized by Alnicola and Tomentella taxa, and ordination analysis indicated that the EMF community on Alnus is a relatively distinct, host-specific group. Despite differences in the activities of the two Alnus dominants to mobilize acid phosphatase and phosphodiesterase, the root-tip activities of P-mobilizing enzymes of the Alnus-EMF community were not dramatically different from other co-occurring boreal plant hosts. This suggests that if Alnus has a greater influence on P cycling than other plant functional types, additional factors influencing P mobilization and uptake at the root and/or whole-plant level must be involved.

Highlights

Relative to nitrogen (N), comparatively little is known about terrestrial phosphorus (P) cycling in boreal forests [1,2] despite the tight stoichiometric coupling between N and P in both terrestrial and aquatic ecosystems [3]
Alder root tip acid phosphatase activity and phosphodiesterase activity showed strong responses to substrate concentration, with rates being approximately double at the higher substrate concentration for both acid phosphatase (6.06 ± 0.35 versus 2.59 ± 0.18 pmoles min−1 mm−2, respectively, p < 0.01)
(100 and 500 μM) were positively intercorrelated, while phytase activity was positively correlated only with acid phosphatase measured at 250 μM (p < 0.05) and phosphodiesterase measured at 100 μM (p < 0.01)

Summary

Introduction

Relative to nitrogen (N), comparatively little is known about terrestrial phosphorus (P) cycling in boreal forests [1,2] despite the tight stoichiometric coupling between N and P in both terrestrial and aquatic ecosystems [3]. During early phases of soil development, P is primarily found in highly reduced inorganic forms of insoluble calcium minerals, such as apatite, which are of limited availability to plants [1]. Labile inorganic P increases and, for a time, is more available to plants and microbes than in the earliest stages of succession. Mineral forms become progressively more occluded as Fe and Al oxides [4], and biological activity transforms mineral P into organic forms [5]. Organic P content increases with successional time [6], comprising between 30%–90%. Of total P in developed soils [7]. Dominant soil organic P compounds include phosphate monoesters (such as phosphorylated sugars and mononucleotides), complex phosphate diesters

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