The Investigation of the Soil Seedbank in a Long-term Mowing and Fertilization Experiment

Abstract

Seedbanks, the assemblage of dormant seeds found in soils, play a vital role in plant community dynamics and can aid in environmental restoration efforts. Studies have shown that fire suppression, anthropogenic nutrient addition, and human-altered hydrology can all have negative effects on aboveground vegetation, such as a decrease in biodiversity. However, little is known about the effects of these factors on seedbanks. Here I investigated how fertilization, drainage, and disturbance (mowing) have affected the seedbank community in a long-term experimental study of a coastal plain wetland. After 20 years of mowing, unmowed plots are dominated by trees with a limited understory, while the mowed plots are dominated by herbaceous perennial species. Long-term fertilization and drainage have also altered the species composition of the aboveground plant community, with these treatments causing a decrease in species richness. We collected soil samples from each plot at two depths and used the seedling emergence method to characterize the soil seedbank community in a growth room. Long-term fertilization could alter seedling emergence either through its effect on the aboveground community and the seeds that enter the seedbank or through effects on germination. To explore this, I conducted an additional growth room experiment comparing seedling emergence with or without the addition of nutrients to soils of each treatment plot. I then conducted multiple univariate and multivariate analyses to investigate the effects of both the long-term treatments and the nutrient addition experiment on diversity, density and composition of the seedbank and aboveground vegetation. A total of 2,509 seedlings emerged over the course of the experiment, including 20 families and 59 species. I found that fertilization, drainage, and mowing increased both the total number of seedlings and the number of species that emerged. The multivariate analysis revealed that species composition was also altered by the three treatments. The addition of nutrients increased both the number and diversity of the seedlings that emerged. I calculated a Sorenson similarity index to compare the belowground seedbank community to aboveground vegetation both at the start of the experiment and in the year that soil was collected. Greater similarity was found between the plots that were mowed in the year 2022. While the aboveground plant community demonstrated a decrease and loss of species because of the introduced treatments, the seedbank did not show this pattern. This could be due to the seeds not experiencing the competition the aboveground plant communities face and to a time lag in changes in the belowground environment. The seedbank had species from before the introduction of the treatments and also had species introduced from the new treatments. This time lag demonstrated between changes in the seedbank and aboveground vegetation in this experiment suggests that the seedbank can be valuable tool for conservation efforts. This data set is important because seedbanks are highly understudied, and they are vital to the future and potential restoration of plant communities.