Investigating the Use of Natural Abundance d15N to Predict Fertilizer Growth Response in Douglas Fir and Loblolly Pine.

Intensely managed forests regularly include the addition of nitrogen (N) to the system to maintain or increase productivity. These anthropogenic inputs coupled with atmospheric N deposition are cause for investigation into ecosystem-level N dynamics, particularly with respect to fertilizer growth response and the capacity of terrestrial ecosystems to retain this added N.  Patterns in the stable isotope ratios of N (N) can provide insight into ecosystem N dynamics.  This research focuses on the use of natural abundance 15N as a predictor of N fertilizer growth response with study sites located throughout the species ranges of both Douglas fir and loblolly pine.  Foliar 15N and the enrichment factor (EF = 15N foliage - 15N soil) of control trees were correlated to resulting fertilizer growth response metrics including basal area, height and volume growth.  Additional work includes the use of natural abundance 15N to predict ecosystem susceptibility to N loss and the use of hyperspectral remote sensing to quantify foliar 15N.  Natural abundance 15N may provide an efficient method to assess N dynamics and fertilizer growth response across spatial and temporal scales.