Organic Nitrogen Cycling Along a Temperate Forest Fertility Gradient.

This project is centered around a model system in which systematic variation in glacial parent material, forest community composition, and soil resource availability are used to elucidate mechanisms controlling organic nitrogen (N) cycling in temperate forests.  This model system corresponds to a fertility/productivity gradient ranging from low mineral N availability, oak dominated forests to high mineral N availability, maple-basswood forests. Specific research objectives address the relative importance of organic vs. mineral N in internal cycling and plant nutrition, and controls over the production, composition and loss of dissolved organic N (DON).  To date, we have found that free amino acids make their greatest contribution to plant-available N pools, in both relative and absolute terms at low fertility. Several lines of evidence indicate that rapid consumption of free amino acids is responsible for extremely low pool sizes at high fertility sites.  We have also found that tree seedlings of species associated with low-fertility sites demonstrate relatively greater affinity for amino acids as an N source compared to species associated with high fertility sites.  Finally, we have found that DON leaching losses (100 cm depth) follow a hump-shaped pattern with greatest concentrations at intermediate fertility.  It appears that DON leaching is low at low fertility sites because little is being produced in surface horizons.  In contrast, DON concentrations are low at high fertility sites because DON is being removed from solution, presumably through sorption to the mineral fraction.  Ongoing research projects addressing in situ amino acid uptake by trees and  mechanisms of DOM sorption will also be presented.