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 delta N-15 in 2014, and tested the strength of correlation between tree ring delta N-15 of four species and stream water NO3- loss from 1971 to 2000. Higher soil NO3- was weakly associated with higher wood delta N-15 across species, and higher soil net nitrification rates were associated with higher delta N-15 for Quercus rubra only. The delta N-15 of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO3-. L. tulipifera tree ring delta N-15 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 delta N-15, with a strong correlation between the wood delta N-15 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 delta N-15, and given the weak relationships between contemporary wood delta N-15 and soil N cycle measurements, caution is warranted when using wood delta N-15 to infer changes in the N cycle.