The response of tree ring δ15N to whole-watershed urea fertilization at the Fernow Experimental Forest, WV

Plant tissue δ¹⁵N is frequently used as a proxy for N availability and N cycle dynamics, and the δ¹⁵N signature of tree rings could potentially be used to reconstruct past changes in the N cycle due to forest disturbance or anthropogenic N deposition. However, there are substantial uncertainties regarding how effectively tree ring δ¹⁵N records N cycle dynamics. We used increment tree cores from a forested watershed that received a one-time application of urea, along with the long-term stream water chemistry record from that watershed and a nearby reference watershed, to determine the effectiveness of tree ring δ¹⁵N in recording a change in N availability, and whether its effectiveness differed by species or mycorrhizal type. Tree ring δ¹⁵N of three species increased rapidly (within ~1 to 3 years) following fertilization (Quercus rubra, Fagus grandifolia, and Prunus serotina), while that of Liriodendron tulipifera did not respond to fertilization but increased ~10 years later. Tree ring δ¹⁵N tended to remain elevated throughout the measured time period (1967–2000), well past the pulsed fertilization response in stream water. This extended δ¹⁵N response may be partially caused by chronic atmospheric N deposition in the region, which also contributed to greater losses of nitrate in stream water by ~1980. Additionally, local recycling of N compounds, and retranslocation of N within the trees, may account for the persistence of elevated tree ring δ¹⁵N levels beyond the direct fertilization effects. Collectively, these results confirm that tree ring δ¹⁵N from some species can document the onset of historical changes in the N cycle. We suggest that studies utilizing tree ring δ¹⁵N as a proxy for long-term N cycle dynamics should look for a consistent pattern of change among several species rather than relying on the record from a single species.