The impact of nitrogen deposition on semi-natural ecosystems

Alharbi, Mahmoud and Stevens, Carly (2025) The impact of nitrogen deposition on semi-natural ecosystems. PhD thesis, Lancaster University.

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Abstract

In the context of increasing anthropogenic impacts, understanding the effects of nutrient addition, particularly N deposition, on plant communities and ecosystems has become increasingly important. This thesis explores the implications of N deposition and nutrient enrichment on different ecosystems, including grasslands and heathlands, through multiple long-term and short-term experimental approaches. The main questions addressed were: 1) how does N deposition affect plant traits such as biomass, flowering, and seed production in grassland species? 2) how does N deposition influence seed banks and species composition in heathland ecosystems? 3) how does nutrient addition alter soil characteristics, and interact with disturbance to impact species diversity and biomass in grassland habitats? The first part of thesis was focused on the study of the impact of N addition on traits associated with flowering in four plant species: Achillea millefolium L., Lotus corniculatus L., Plantago lanceolata L., and Prunella vulgaris L. Despite the well-documented effects of N deposition on plant growth, this study revealed no significant differences in the biomass, plant height and phenology across all four species but Lotus corniculatus L. and Achillea millefolium L. showed significant differences in the total number of seeds and flowers, respectively, indicating a potential reproductive response to N addition. Furthermore, carbon content and C:N ratios exhibited no change across all species, except Plantago lanceolata L., which exhibited significant changes in nitrogen content and C:N ratios. The second chapter, conducted in Ruabon and Budworth long-term N addition experiments, investigated seed banks. The study showed that Calluna vulgaris dominated the seed banks at both Ruabon and Budworth sites, correlating well with the vegetation present. While C. vulgaris exhibited strong seed bank presence, other species such as Deschampsia flexuosa and Rumex acetosella were less abundant, suggesting limited contributions to species diversity and potential ecosystem degradation in the long term. Reductions in some of these species were significantly impacted by N additions. The thesis further examined the impact of nutrient addition, disturbance, and their combined effects on species richness, diversity, light availability, as well as the biomass. Nutrient addition had limited effects on species richness and diversity in a nutrient-rich, species-poor grassland. Disturbance initially increased diversity by favoring less competitive species, but dominant species recovered quickly, reducing diversity in the following year. Grass biomass decreased in the second year, and forb benefits from disturbance declined, possibly due to seed bank depletion. The combination of disturbance and fertilization ultimately led to reduced diversity due to nutrient-fueled dominance by certain species. Lastly, a long-term study in Norway assessed the impact of recovery from N deposition on soil chemistry from 2015 to 2020. No significant changes were observed in soil chemistry due to N deposition, indicating recovery over time. Mowing had no significant impact on recovery. This finding is particularly relevant to the current understanding of ecosystem management, as it suggests that soil chemistry may recover from N deposition. However, literature suggests vegetation recovery might not follow as readily.