The impact of nitrogen deposition on semi-natural ecosystems
Creators: name: Alharbi, Mahmoud
Publisher: name: Lancaster University
Year: 2025

description: 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.