Paul Kühn, University of Jena, discusses his article: Nitrogen content of herbarium specimens from arable fields and mesic meadows reflect the intensifying agricultural management during the 20th century
Biodiversity in agricultural areas
Agricultural habitats such as arable fields and meadows are shaped by human activity. Agricultural productivity is rising, but at the same time, many of the species that have been living in agricultural habitats for centuries are now in serious decline. Often this occurs because mowing and harvesting are more frequent, crops are bred to grow faster, and fertilization is significantly increased.
Plants can adjust their nutrient uptake and general growth strategy in response to changing environmental pressures. However, it is unclear how individual plants react to changes in nutrient supply, particularly on changes in the nitrogen and phosphorus content of leaves. Herbaria, i.e. archives of dried plant specimens, can be used to study changes in growth and nutrient uptake strategies on long time scales. It is highly desirable to keep specimens intact, and thanks to near-infrared spectroscopy (NIRS) analyses which do not destroy plant specimens, functional leaf traits can be measured on historical plant specimens that are decades old, sometimes even centuries. This allows ecologists to reconstruct a “snapshot” of how the plant was supplied with nutrients at the time of sampling.
The study
Our study was carried out at two herbaria in Central Germany: the Senckenberg Museum of Natural History in Görlitz and the Senckenberg Herbarium Haussknecht in Jena. Based on a list of 50 common central European arable field and meadow plant species, we selected specimens that had precise data on the date and place of sampling. We then went out to the field in the summer of 2022 and tried to find the same species at the same location as the herbarium specimen. We used wet-lab measurements of our field specimens to calibrate our NIR spectroscope, and then non-destructively measured leaf nitrogen content, leaf phosphorus content, and other traits. Fertilization data from statistical yearbooks was also analyzed as a rough reference for the inorganic fertilizer used in each given year of the studied time period.
We analyzed 325 paired samples, with one herbarium specimen and one precisely relocated field specimen each. The oldest herbarium specimen was a wild carrot (Daucus carota), sampled in 1878, and in 2022, we were able to find wild carrots still growing on the same hillside meadow.
Results and outlook
Leaf nitrogen content generally increased over the time period, indicating an increasing availability of this important nutrient, and a corresponding shift towards plants spending their resources faster to speed up growth. The high amounts of inorganic nitrogen fertilizer used over the last decades drove this trend. Leaf phosphorus content decreased over time, which was associated with phosphorus fertilization values that peaked in the 1970s in eastern Germany and have been declining ever since. This appears to form a shift from N-limited to P-limited growth. However, not all plants reacted in the same way to the changing environment: species like the field larkspur (Consolida regalis) had decreasing leaf nitrogen content, indicating that this charismatic arable wildflower might be outcompeted in the current highly fertilized agricultural habitats.
We hope that this study will serve as a blueprint for future research. The drivers of biodiversity change are varied, and with the NIRS-supported study of herbarium specimens, better understanding of the long-term impacts of climate change, urbanization, or invasive species, can be gained.
