# Phosphorus Mobilizing Enzymes of _Alnus_-Associated Etomycorrhizal Fungi in an Alaskan
## 4 Discussion
### Patterns of EMF Enzyme Activities  
Our analysis of enzyme activities on _Almus_ root tips suggests pronounced physiological differences in the capacity of the two EMF dominants, _A. luteolofibrillosa_ 2 and _Tomentella_ sp. 6, to mobilize P from compounds across the successional gradient. _A. luteolofibrillosa_ 2 had significantly higher phosphodiesteraseerac activities (500 uM) than _Tomentella_ sp. 6, while _Tomentella_ sp. 6 had significantly higher acid phosphatase activity than _A. luteolofibrillosa_ 2 when measured at both high and low substrate concentrations (Figure 5). Differences in the enzyme activities between EMF dominants suggest that these mycorrhizal species differ in their capacity to access P from different substrates and will thus have differing benefits to _Almus_. There have been few other studies that have tested the functionality of _Almus_-EMF communities as they relate to P mobilization. However, [PERSON] et al. (2014) [16] found that the potential acid phosphatase activity of the EMF community on _Almus rubra_ was significantly higher than that of Douglas fir (_Pseudotsuga metziesi_ ([PERSON]) [PERSON]) at two sites in central Oregon. We found a similar tendency for higher acid phosphatase in alder relative to white spruce (Figure 2b). Interestingly, they also found that some of the most dominant fungi in the _Almus_-EMF community did not demonstrate superior enzymatic activity relative to the community average. Similarly, we found that the higher activities of _A. luteolofibrillosa_ 2 versus _Tomentella_ sp. 6 for phosphodiesteraseerase and acid phosphatase, respectively, were not significantly greater than the rest of the other EMF species when pooled (data not shown). Given mixed results across the few studies that have investigated the functionality of _Almus_-EMF communities, it remains unresolved how _Almus_ meets its P demands and achieves high foliar P concentrations. Our finding that the enzyme activities of _Almus_-EMF dominants and the _Almus_-EMF community as a whole are not substantially greater than those found on other plant hosts suggests that if the community of host specific EMF on _Almus_ are related to P mobilization other factors are involved. These could include differences in EMF foraging strategy, extent of hyphal proliferation in the soil, GPP partitioning to first-order root tips and EMF along a branch, EMF growth rate, or P uptake and transport [51], all of which would influence whole-plant P assimilation. Exploration type of the EMF assemblage is likely an important factor influencing P acquisition. There are significant differences in the mantles and exploration types of EMF fungi [55], and EMF taxa vary in the amount of mantle as a proportion of total structure [56]. A given EMF species may have lower enzyme efficiency or infection frequency across root tips, but a more extensive hyphal system that would facilitate access to P from greater distances could result in greater overall P uptake. Additionally, much of the P assimilated by the EMF is retained by the fungus [53; 57], despite a high potential for P hydrolysis at the root surface. Differences among EMF species in the proportion of P transferred to the host would not be detected by measuring root tip enzyme activities but may explain some of the specialization we detected in EMF. Future studies are needed to consider the steps between P hydrolysis and assimilation into plant tissue, and the time and scale at which these processes take place, to fully understand EMF-_Almus_ specificity. The similarity in activity rates of P-mobilizing enzymes measured in the laboratory for _Alnus_ growing across very different stand types, and in _Alnus_ relative to other dominant woody plant species may not necessarily translate to _in situ_ equivalences in the effects that these host species have on P biogeochemical cycling at the stand level.