Changes in forest nitrogen cycling across deposition gradient revealed by δ15N in tree rings

Tree rings provide valuable insight into past environmental changes. This study aimed to evaluate perturbations in tree ring width (TRW) and δ15N alongside soil acidity and nutrient availability gradients caused by the contrasting legacy of air pollution (nitrogen [N] and sulphur [S] deposition) and tree species (European beech, Silver fir and Norway spruce). We found consistent declines of tree ring δ15N, which were temporarily unrelated to the changes in the TRW. The rate of δ15N change in tree rings was related to the contemporary foliar carbon (C) to phosphorus (P) ratio. This observation suggested that the long-term accumulation of 15N depleted N in tree rings, likely mediated by retained N from deposition, was restricted primarily to stands with currently higher P availability. The shifts observed in tree-ring δ15N and TRW suggest that acidic air pollution rather than changes in stand productivity determined alteration of N and C cycles. Stable N isotopes in tree rings provided helpful information on the trajectory of the N cycle over the last century with direct consequences for a better understanding of future interactions among N, P and C cycles in terrestrial ecosystems. [Display omitted] •Measured 15N/14N isotope ratio in tree rings extending back to the 19th century.•Consistent declines in the tree-ring δ15N connected to the legacy of air pollution.•Forest productivity unrelated to the observed tree-ring δ15N changes.•Contemporary foliar C/P defined the trajectory of tree-ring δ15N.•Future pathways of nitrogen and carbon in forests are likely P availability dependent.