Nitrogen content of herbarium specimens from arable fields and mesic meadows reflect the intensifying agricultural management during the 20th century

Arable fields and mesic meadows have been affected by intensifying agricultural management and nutrient input during the 20th century, but direct evidence for the long-term impact of intensification on plant nutrient contents remains scarce. Non-destructive novel spectroscopic methods can produce such data from herbarium specimens, making it possible to investigate how contents of leaf nutrient traits, especially nitrogen and phosphorus, changed over the last century, and what role habitat type and management practices play.

We carried out a resurvey study of functional traits in arable field and mesic meadow communities. We used specimens from two German herbaria with a high coverage of their local floras: the Senckenberg Herbarium Görlitz and the Herbarium Haussknecht in Jena. Following specimen information, the same plant species were resampled in the field in 2022 at the same locations. We employed Near-Infrared Spectroscopy to predict leaf nitrogen, phosphorus, and carbon content of herbarium and field specimens. Nutrient content changes over time were compared with publicly available records of P and N fertilization.

Overall, 1270 specimens of 76 species from both herbarium and field were studied, the oldest from the 19th century. Leaf nitrogen increased significantly through time, with a corresponding increase in leaf nitrogen:phosphorus ratio in both habitats. Leaf phosphorus and carbon content decreased significantly over time, with the latter decreasing significantly stronger in mesic meadows compared to arable fields. The total amount of nitrogen or phosphorus fertilizer applied per year on a regional scale was found to be significantly correlated with the respective leaf nutrient content levels. Mesic meadow species showed a stronger response in leaf nitrogen and leaf phosphorus content over time.

Synthesis: Our study shows a long-term increase of leaf nitrogen in the studied habitats, running in parallel to increased chemical fertilizer application in Germany. Our data indicates a shift from predominantly N-limited towards more P-limited growth conditions. The stronger response of species from mesic meadows compared to species from arable fields could indicate a faster adjustment to environmental pressures. This study thus also serves to showcase the potential of the combination of herbarium collections and NIR spectroscopy. 

Methods
The dataset consists of metadata, functional trait data, and fertilization data for herbarium specimens and plant specimens sampled in 2022.The herbarium specimens are from the collections of the Herbarium Haussknecht in Jena, Germany (JE), and the Senckenberg Herbarium Görlitz in Görlitz, Germany (GLM). The field specimens were gathered from May to August of 2022, and were press-dried in the same way as the herbarium specimens. Near-Infrared Spectroscopy (NIRS) measurements were taken from both specimen types using an ASD FieldSpec 4 and a contact probe. A subset of the field specimens was selected to create a NIRS calibration dataset, which involved conventional wet lab analyses of the leaf nutrient contents of leaf nitrogen, leaf carbon and leaf phosphorus contents for these specimens. These wet lab measurements were used in conjunction with the NIRS measurements to build partial least squares regression (PLSR) models for each trait. The PLSR models were in turn used to predict the respective trait values from NIRS measurements alone, allowing for the non-destructive functional trait measurements of the other field specimens and the historical herbarium specimens. Because some of the wet lab measurements were log transformed to improve the calibration model accuracy, some of the predicted trait values are log transformed as well. Further details on the calibration workflow and the use of NIRS in herbaria can be found in Proß et al. 2023 (doi.org/10.1111/oik.10255) and Kühn et al. 2024 (doi.org/10.1186/s13007-024-01146-x).