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

The release of carbon dioxide and reactive nitrogen in various forms by humans disrupts the functioning of ecosystems around the world. In Europe, many valuable habitats, particularly wetlands and dry grasslands, are under threat due to eutrophication. However, contrasting water regimes mean that the uptake of anthropogenic nitrogen by plants in these ecosystems differs, and this is also interrelated with an increase in trophic level in both habitats.

In our study, we measured the δ15N and δ13C values, as well as the total nitrogen content (TN), of 99 pairs of foliar samples collected from seven species of vascular plants in dry grasslands and wetlands in Poland. Each pair consisted of a historical sample, collected from a herbarium voucher dating from before 1939 (i.e. before the widespread use of artificial fertilisers 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 δ15N, TN, and δ13C between samples from the two habitats. Next, we calculated the differences in δ15N, TN, and δ13C between the contemporary and historical samples for each pair. We then tested whether the difference for each species and habitat type was significantly different from zero using 90% confidence intervals. We analysed the relationships between differences in δ15N and TN over time and the following factors using multiple linear regression: habitat type, the proportion of farmland in the landscape, the consumption of synthetic nitrogen fertiliser and NOx deposition. 

The δ15N and TN values were lower for dry grassland species than for wetland species in both the contemporary and historical subsets. For dry grassland species, the mean δ15N value was lower in contemporary samples than in historical ones. For wetland species, however, the opposite was true. The difference in δ15N values between pairs of samples was positively correlated with the proportion of farmland in the landscape. The mean TN value was higher in contemporary wetland samples than in historical ones, but not in dry grassland plants. The mean δ13C value, corrected for the Suess effect, was lower in contemporary samples than in historical ones. The mean difference was −0.51 ‰ for dry grassland species and −3.85 ‰ for wetland species.

Our study revealed that a century of carbon emissions, increased nitrogen input into the environment and the dominance of artificial fertilisers and combustion-derived nitrogen over biological nitrogen sources has not resulted in consistent responses across habitats and species. While the isotopic composition of nitrogen and carbon in plant tissues in Central Europe has undoubtedly changed, this change is context-dependent. Its magnitude and direction are impacted by the 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 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 atmospheric composition.