N₂ fixation and ectomycorrhizal root enzyme activity are coupled on Siberian alder across an Arctic-boreal gradient

Siberian alder is a tall shrub expanding across Alaska. Host to ectomycorrhizal (EcM) fungi and nitrogen (N)-fixing actinobacteria, alder not only facilitates the addition of new N to its ecosystems through fixation, it may also alter phosphorus (P) availability and transform soil organic matter (SOM) via the exudation of EcM degradative enzymes. To understand alder’s potential for overhauling nutrient dynamics in northern biomes as it expands, we identified drivers of EcM root tip enzyme activity and N fixation across a latitudinal gradient in Arctic and boreal Alaska. We paired measurements of EcM root enzyme activity and fixation, testing 1) for relationships between each symbiotic root process and environmental and alder properties and 2) if enzyme activity and fixation were linked. We found mean annual temperature was the largest driver of EcM root tip enzyme activity, correlating with 95% of the variation in enzyme profiles. Warmer, southern sites had high activities dominated by hydrolytic enzymes while colder, Arctic sites had low activities that were dominated by phenol oxidase. In contrast, fixation was best explained by depth of the soil organic layer and foliar δ¹³C rather than by temperature. The relationship between fixation and enzyme activity depended on nutrient limitation (indicated by foliar N:P value). When N was more limiting than P, high fixation rates were associated with lower phosphatase activities relative to phenol oxidase. When P was more limiting, high fixation rates were associated with higher phosphatase activities relative to phenol oxidase. Our results suggest that as alder expands with warming, its potential heightened influence on ecosystems will be mediated by local edaphic conditions. In particular, as warming stimulates nutrient releases, the resulting changes in N limitation relative to P will control whether inputs of fixed N are accompanied by EcM-induced increases in either soil P availability or the transformation of lignin-rich SOM.