# Phosphorus Mobilizing Enzymes of _Alnus_-Associated Etomycorrhizal Fungi in an Alaskan
## 1 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]. As soil development progresses, labile inorganic P increases and, for a time, is more available to plants and microbes than in the earliest stages of succession. However, 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 (predominantly DNA and phospholipids), and more recalcitrant inositol phosphates such as phytic acid [8]. In mid to late soil development, phytates and other large, positively-charged P molecules are tightly bound to soil particles and only available to plants through the release of specialized hydrolyzing enzymes, often with the aid of organic acids. N-fixing plants play a unique role in P biogeochemical cycling because of the high P demand associated with nitrogenase activity and production of root nodules, which explains the high rates of both N and P cycling in litter of N-fixing plants [9]. How these plants acquire sufficient P to maintain N fixation is somewhat of a puzzle, particularly early in primary succession when N-fixation rates are stimulated by low N and high light availabilities. In old, highly-weathered tropical forests where P availability is often chronically low, it has been suggested that N-fixing plants persist because of their ability to access low-solubility inorganic or recalcitrant organic P forms using extracellular enzymes [10]. In general, extracellular hydrolytic enzymes produced by mycorrhizal associations are a primary mechanism by which plants acquire organic P in soils [11]. Throughout interior Alaska, thin-leaf alder (_Alnus incana_ spp _tenuifolia_ (Nutt.) Breitung, hereafter, _A. tenuifolia_) forms dense riparian stands where it can contribute \(>\)100 kg N ha\({}^{-1}\) year\({}^{-1}\) to early successional floodplain soils through N fixation [9; 12; 13]. As stands develop, _A. tenuifolia_ remains an important understory species in both mid-successful hardwood forests dominated by baslam poplar (_Populus balsamifera_ [PERSON]), and in late-successful stands dominated by white spruce (_Picea glauc_ ([PERSON]). Field experiments have shown that ecosystem N inputs by _A. tenuifolia_ are dramatically increased by P fertilization via increases in both nodule biomass and nitrogenase activity [9; 13], suggesting that N fixation rates are strongly P-limited in these ecosystems. The fact that _A. tenuifolia_ persists and continues to maintain high nodule biomass and N fixation rates even into late succession [14] suggests this species might change strategies to optimize P access across this successional sequence. Red alder (_Alnus rubra_ [PERSON]) has been shown to increase inorganic soil P availability via phosphatase production [15]. Furthermore, enzyme activities measured on individual ectomycorrhizal root tips have shown that many species of ectomycorrhizal fungi (EMF) produce phosphorolyrolase enzymes when associated with both _Alnus_[16] and other plant hosts [17; 18]. In general, EMF community composition is affected by a number of factors, including soil chemistry [19], seasonality [20], and successional stage [21].