Contrasting isotopic responses of dryland and wetland plants to a century of global anthropogenic changes in nutrient cycling

Anthropogenic emissions of carbon dioxide and reactive nitrogen in various forms disrupt the functioning of ecosystems around the world. In Europe, many valuable habitats, particularly wetlands and semi-natural dry grasslands, are under threat from ongoing eutrophication. However, due to contrasting water regimes, the uptake of anthropogenic nitrogen by plants in these ecosystems is different and is also interrelated with an increase in trophic level in both habitats. In our study, we measured the δ ¹⁵ N and δ ¹³ C values, as well as the total nitrogen content (TN), of 99 pairs of foliar samples collected from 14 species of vascular plants in Poland: seven wetland species and seven dry grassland species. Each pair consisted of a historical sample collected from a herbarium voucher dating from before 1939 (i.e. before artificial fertilisers were widely used in agriculture) and a contemporary sample collected in 2024 from the same species in a similar location. We performed t  tests to determine whether there were significant differences in the means of δ ¹⁵ N, TN and δ ¹³ C between samples from the two different habitats. Next, we calculated the differences in δ ¹⁵ N, TN and δ ¹³ C between the contemporary and historical samples for each pair. Using the information on date of collection of each herbarium voucher we corrected δ ¹³ C values taking into consideration changes of atmospheric δ ¹³ C due to fossil-fuel combustion (so called Suess effect). We then tested whether the difference for each species and habitat type was significantly different from zero using 90 % confidence intervals. Using multiple linear regression, we analysed the relationships between differences in δ ¹⁵ N and TN over time and the following factors: habitat type, the proportion of farmland in the surrounding landscape, the consumption of synthetic nitrogen fertiliser, and NO _x deposition. δ ¹⁵ N and TN values were lower in dry grassland species than in wetland species, in both the contemporary and historical subsets. For dry grassland species, the mean δ ¹⁵ N value was lower in contemporary samples than in historical ones. For wetland species, the opposite was true. The difference in δ ¹⁵ N values between pairs of samples was positively related to the amount of farmland in the surrounding landscape. The mean TN was higher in contemporary than in historical wetland plants, but not in dry grassland plants. The mean δ ¹³ C value, corrected for the Suess effect, was lower in contemporary samples than in historical ones. The mean difference was −0.51 ‰ for dry grassland and −3.85 ‰ for wetland species. Our study revealed that the century of fossil fuel-derived carbon emissions, increased nitrogen input into the environment, and dominance of artificial fertilisers and combustion-derived nitrogen over biological nitrogen sources have not led to consistent responses across habitats and species. While the isotopic composition of nitrogen and carbon in plant leaves in Central Europe has undoubtedly changed, this change is highly context-dependent. Its magnitude and direction are impacted by the type of habitat and the identity and/or ecology of the species. As expected, man-made alterations appear to be more pronounced in wetland environments than in dryland habitats. Furthermore, the source of disruption may differ between the two habitat types. Specifically, wetlands are exposed to a multitude of anthropogenic nitrogen and carbon sources, whereas dry grasslands seem to be predominantly affected by changes in the composition of the atmosphere.