Assessing tree ring δ15N of four temperate deciduous species as an indicator of N availability using independent long-term records at the Fernow Experimental Forest, WV.

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood δ15N in 2014, and tested the strength of correlation between tree ring δ15N of four species and stream water NO3- loss from 1971 to 2000. Higher soil NO3- was weakly associated with higher wood δ15N across species, and higher soil net nitrification rates were associated with higher δ15N for Quercus rubra only. The δ15N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO3-. L. tulipifera tree ring δ15N had a stronger association with stream water NO3- than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring δ15N, with a strong correlation between the wood δ15N and NO3- leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring δ15N, and given the weak relationships between contemporary wood δ15N and soil N cycle measurements, caution is warranted when using wood δ15N to infer changes in the N cycle.