Certain tree species are capable of forming symbiotic relationships with nitrogen-fixing bacteria, and are therefore able to input readily available nitrogen into a forest ecosystem. A common assumption is that this nitrogen addition would facilitate the growth of plants surrounding a nitrogen-fixer. Although there is some evidence of facilitation, a number of recent studies have shown the opposite, that nitrogen-fixers inhibit their neighbors. A large unknown component of these recent studies is the amount of nitrogen-fixed by nitrogen-fixing trees. Many nitrogen-fixing trees down-regulate nitrogen-fixation as soil nitrogen supply increases, so they might not provide much new nitrogen to their neighbors. Traditionally, measures of nitrogen-fixation have been taken at a single point in time, which makes it difficult to assess how past nitrogen-fixation might have influenced the neighbors of nitrogen-fixing trees. To study past nitrogen-fixation rates, I retrieved cores from nitrogen-fixing black locusts (Robinia pseudoacacia) and their non-fixing neighboring trees at three sites in Black Rock Forest (Cornwall, NY). If the conditions are right, the natural abundance of nitrogen isotopes in these tree cores can be used to determine the percent of nitrogen each tree derived from fixation (%Ndfa) at times in the past.
Results/Conclusions
We studied nitrogen isotopes in tree cores of Robinia and neighboring non-fixing trees from three separate sites within Black Rock Forest. The δ15N of neighboring trees, and thus our ability to detect nitrogen-fixation in Robinia, varied between each site. At site 1, the mean reference tree δ15N was near 0‰, which does not allow for meaningful estimates of nitrogen-fixation. For site 2, the mean reference tree δ15N was in the 1 – 2‰ range, which is still not far away from the fixation signal to derive a meaningful %Ndfa estimate. In the third site, the mean reference tree δ15N was in the 3 - 4‰ range, with low enough variance to allow an estimate of nitrogen-fixation by Robinia at different times in the past. At this site, Robinia derived 54 - 23% of its nitrogen from fixation, and fixation decreased as the tree aged. These methods allow a more individualistic view of how nitrogen-fixation occurs in a forest ecosystem, and give us the opportunity to use tree rings widths to measure how nitrogen fixation in the past affects growth of neighboring trees.