Changes in stable nitrogen isotopes of plants, bulk soil and soil dissolved N during ecosystem retrogression in boreal forest

Stable nitrogen isotope ratios (δ15N) of plants and soil have been used to study changes in the N cycle during ecosystem succession and retrogression. However, little is known about how δ15N of soil mineral N and dissolved organic N (DON) change during retrogression, despite their potential to inform on processes contributing to N loss. Here, we examined the δ15N of NH4+ and DON together with δ15N of the dominant plant species and bulk soil across a 5,000‐year‐old retrogressive chronosequence of forested islands in northern Sweden. The δ15N of bulk soil N, NH4+ and DON did not change greatly during retrogression, suggesting that there are no major losses of N from the system. The δ15N of NH4+ and DON was significantly correlated with that of bulk soil N across islands, indicating that bulk soil N is an important determinant of the δ15N of dissolved soil N. The δ15N of DON was significantly higher than those of NH4+ and bulk soil N, probably because of the inclusion of microbial N to the DON fraction. Despite the lack of changes in δ15N of soil N as retrogression proceeded, the δ15N of most plant species increased. These results suggest that despite the relative importance of the three underlying mechanisms involved is unclear, the N resources of plants change in response to retrogression: they have an increasing reliance on DON, a decreasing dependence on N transferred from the mycorrhizal fungi and reduced reliance on N from surface soil layers as retrogression proceeds. Despite the lack of changes in δ15N of soil N ( the gray and pale gray bars indicate the ranges of δ15N of DON and NH4+ (mean ± SD) across all 30 islands, respectively) as retrogression proceeded, the δ15N of most plant species increased. These results suggest that the N resources of plants change in response to retrogression: they have an increasing reliance on DON, a decreasing dependence on N transferred from the mycorrhizal fungi and reduced reliance on N from surface soil layers (e.g., the litter layer) as retrogression proceeds.