Changes in stable nitrogen and carbon isotope ratios of plants and soil across a boreal forest fire chronosequence

Background and Aim: Nitrogen (N) and carbon (C) isotopic signatures (δ¹⁵N and δ¹³C) serve as powerful tools for understanding temporal changes in ecosystem processes, but how these signatures change across boreal forest chronosequences is poorly understood. Methods: The δ¹⁵N, δ¹³C, and C/N ratio of foliage of eight dominant plant species, including trees, understory shrubs, and a moss, as well as humus, were examined across a 361 years fire-driven chronosequence in boreal forest in northern Sweden. Results: The δ¹³C and C/N ratio of plants and humus increased along the chronosequence, suggesting increasing plant stress through N limitation. Despite increasing biological N fixation by cyanobacteria associated with feather mosses, δ¹⁵N showed an overall decline, and δ¹⁵N of the feather moss and associated vascular plants diverged over time from that of atmospheric N₂. Conclusions: Across this chronosequence the N fixed by cyanobacteria is unlikely to be used by mosses and vascular plants without first undergoing mineralization and mycorrhizal transport, which would cause a change in δ¹⁵N signature due to isotopic fractionation. The decreasing trend of δ¹⁵N suggests that as the chronosequence proceeds, the plants may become more dependent on N transferred from mycorrhizal fungi or from N deposition.