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.