Phosphatase activity is related to N availability but not P availability across hardwood forests in the northeastern United States

Allocation processes are central to understanding patterns of productivity and nutrient retention in forest ecosystems. Nutrient addition experiments support ideas of resource allocation theory in soil with, for example, the addition of nitrogen promoting the activity of phosphatase enzymes in many different ecosystems. However, empirical evidence is needed to understand whether similar mechanisms operate spatially or temporally to balance availability of nutrients over the long-term. Therefore we examined the general question of whether N availability can influence phosphorus availability through effects on phosphatase enzyme activity, across hardwood forests in the northeastern United States that differ in age and soil properties. Using resin-available P and net N mineralization as our indices of plant-available nutrients, we found that the availability of P increased in relation to that of N across sites, with no effect of forest age. Net N mineralization was a significant predictor of soil phosphatase activity, suggesting that allocation to enzyme activity is a mechanism that could contribute to coupling of N and P availability in successional as well as mature northern hardwoods. However, resin-available P was not correlated with phosphatase activity. Bicarbonate-extractable P, an index of potentially available P, was not related to other soil variables across the stands. While N may influence the activity of phosphatase enzymes, this interaction does not appear to drive availability of P across these hardwood forest ecosystems. Instead we suggest that it operates at smaller scales and over shorter time scales than processes mediating the broader-scale coupling of N and P availability.