Nitrogen deposition is a recognised driver of global change with potential impacts on ecosystem function. It primarily affects plants through soil eutrophication and acidification. However, the effect of increased soil nitrogen on pollinators, and their interactions with plants, represents a gap in our understanding of pollinator ecology.
We set out to address this knowledge gap using the Park-Grass Long-Term Experiment at Rothamsted Research, Hertfordshire UK: the oldest ongoing ecological experiment. The Park Grass plots have had consistent soil treatments applied for over 160 years, leading to a patchwork of plant communities adapted to their local environments. The use of a liming treatment allows us to distinguish the effects of nitrogen amount, nitrogen form and pH. This presentation will focus on the impacts to flowering phenology.
We monitored all Park Grass plots, twice weekly for eight years, recording first-flowering date (FFD), last-flowering date (LFD), and flowering duration for various botanical species. Statistical analysis was performed with mixed-effects models.
Results/Conclusions
Nitrogen addition affects the FFD of many studied species, but the nitrogen form is also an important factor. The reduced form (NHx) delays flowering for many species, whereas the oxidised form (NOy) has a more varied effect. These two forms of nitrogen typically have different sources, NHx is associated with agriculture and NOy with fossil fuel combustion. Global deposition of these two forms varies spatially and temporally.
The flowering duration of the three earliest species was extended with higher nitrogen addition. This could be useful for early emerging pollinators, including some bumblebees. However, nitrogen addition curtailed LFD and reduced flowering duration for most later species. This could be due to increased competition from grasses, which typically begin to dominate nitrogen-rich plots from mid-spring.
We often found a significant interaction between nitrogen and pH. Therefore, soil pH has the potential to either exacerbate or lessen the phenological shifts and asynchrony caused by nitrogen addition. The NHx form strongly acidifies soils, so regions with poorly-buffered soils experiencing high NHx deposition may be more at risk of decoupled interactions caused by phenological asynchrony.