A tale of two grasslands: The interactions of nutrient limitation, nitrogen deposition and elevated CO2 on the biogeochemical cycling and biodiversity of two contrasting UK grasslands

Humans activities are dominant drivers of the biogeochemical cycling of carbon (C), nitrogen (N) and phosphorus (P) in terrestrial ecosystems worldwide. Atmospheric N deposition can reduce biodiversity and have long-lasting consequences for plants and soils alike. While the ecological consequences of elevated CO2 (eCO2) are less certain, the unprecedented rise in C availability may have important ramifications for ecosystems, particularly when combined with N. As C and N pollution continue to increase, understanding their effects on ecosystem functioning and biogeochemical cycling of one of the largest and most important ecosystems; grasslands, is of paramount importance. Most research focusses on N-limited grasslands, but due to considerable differences between N and P cycling, we require a separate understanding for grasslands whose productivity is limited by P availability, for which we know very little. This thesis combines empirical and computational methodology to investigate the consequences of N pollution, eCO2, and their combination, for P-limited grasslands. It does so by utilising a state-of-the-art CO2 fumigation experiment on two contrasting grasslands from a long-term nutrient manipulation experiment. Specifically, it explores their effects on soil C sequestration, P cycling and nutrient limitation, biodiversity, and plant nutrient stoichiometry. Responses of plants and soils consistently differed between grasslands, suggesting contrasts in soil conditions and plant communities may contribute to grassland responses to biogeochemical perturbations. Nitrogen deposition strongly influenced soil organic C storage and the cycling of P, where access to organic P forms by plants may play a key role in determining the response of P-limited grasslands to N deposition. Combinations of eCO2 and N deposition considerably affected grassland communities and foliar C, N and P content of a multitude of plant species. To conclude, this thesis contributes to the pressing need to understand the responses of P-limited ecosystems to a future of high C and N availability.