Alder strategies for phosphorus assimilation across a boreal forest successional sequence
Ecosystem processes in northern systems depend heavily on inputs of biologically fixed nitrogen (N) from A. tenuifolia, which contributes the majority of N accumulated during boreal forest succession. However because of the high phosphorus (P) demands of this plant, we hypothesize that N-fixation inputs are controlled by the ability of alder to assimilate P through associations with ectomycorrhizal fungi (EMF), which produce enzymes that mobilize organic and recalcitrant P forms. Because the forms and availability of P are known to change throughout forest succession and between soil horizons, we expected to see parallel shifts in EMF communities and function at these same scales. Using fluorogenic substrates, we measured the activity of acid phosphatase, phosphodiesterase and phytase enzymes bound to the surfaces of individual mycorrhizally-infected alder root tips (n=420) collected from organic and mineral soils across early, mid and late successional stands along the Tanana river floodplain in interior Alaska. Activities of all enzymes were positively correlated across stages and horizons (all P< .001); however, the strongest relationship was between acid phosphatase and phosphodiesterase (P<.001, R2 = 0.32). Enzyme activities were nearly double in late compared to early and mid-succession (both P<.0001), which did not differ. Horizon effects were only seen for acid phosphatase (P<.001), which overall, was higher in organic vs. mineral soils, However, strong stage by horizon interactions were found for both acid phosphatase (P<.01), where in early succession activities were higher on tips from organic verses mineral horizons (P<.001), and phosphodiesterase (P<.001), where activities were higher in organic horizons of early (P<.05), and mid (P<.10) successional stands but lower in organic horizons of late succession (P<.01). Data suggest that the functional traits of individual EMF species vary between soil horizons and throughout succession. We are currently analyzing fungal ITS rDNA sequences isolated from individual alder root tips of known enzyme activity to determine whether communities of alder associated EMF are stratified in accordance with soil P availability and plant N:P balance. This work is part of a larger project at Bonanza Creek that is manipulating partner choice in alder/EMF and alder/Frankia mutualisms using N and P fertilization.