Understanding the links between soil, plants, and pollinators.

Global nitrogen (N) deposition has risen steeply since the mid-19th Century and is forecasted to rise further. This increased flux of N to ecosystems is increasingly considered as a worldwide driver of environmental change. Impacts to botanical communities have been shown across a range of ecosystems and regions, but evidence on consequential impacts to further trophic levels is currently limited. Our understanding of how N affects plant-pollinator interactions is relatively poor. Declining trends in pollinating insects have been widely reported during recent decades and a key factor in this is losses of floral resources.
The research chapters presented in this thesis seek to address the wide knowledge gap of how N affects pollinators by investigating how soil N affects 1) the composition of floral functional traits in a plant community, 2) intra-specific variation in key floral functional traits: phenology and nectar, and 3) the potential impacts to plant-pollinator interaction networks. This research was undertaken using the Park Grass Long-term Experiment (PGE) at Rothamsted Research, UK, a long-term nutrient-enrichment experiment. The study found extensive impacts of N on the provision of floral resources. Floral resources were lost with N-enrichment, with morphologically specialised floral units particularly threatened. Flowering phenology of plant species was influenced by N-enrichment with a notable contrast between early-season species, which had an extended flowering duration, and peak-season species that had a reduced flowering duration. Responses of nectar traits varied across species but were shown to respond to N applications. The observed pollinator visitation networks were impacted by the soil treatments; N additions typically led to Diptera-dominant communities and networks that were potentially less robust to stochastic events. The research project provided evidence that soil N can act as an underlying driver of plant-pollinator networks and should be considered as a factor in pollinator ecology and trends.