Impacts of climate change on carbon and nutrient cycling in oceanic-alpine ecosystems

Broyd, Rosanne and Britton, Andrea and Taylor, Andy and Ostle, Nick and Mills, Rob (2022) Impacts of climate change on carbon and nutrient cycling in oceanic-alpine ecosystems. PhD thesis, Lancaster University.

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Abstract

Mountains are global reservoirs of biodiversity, water, and soil carbon (C), but are warming at greater than average rates, with reducing snow cover and changing rainfall regimes. These changes may alter biogeochemical cycling, impacting the fate of soil C. For oceanic-alpine ecosystems, there is limited knowledge about basic attributes and functioning, particularly responses to environmental change. This thesis aimed to improve understanding of C cycling in these ecosystems and potential impacts of climate change. I conducted surveys and experiments across snow melt gradients, elevations, and contrasting habitats. Survey of vegetation and soils in the Cairngorm Mountains showed that snow cover duration, elevation, and topography (snow-collecting vs snow-shedding sites) drove vegetation community composition. However, differences in vegetation were not reflected by C pool size in the upper 15 cm organic topsoil. In laboratory experiments, I examined effects of drought and rewetting intensity on C and nitrogen (N) cycling. Gas fluxes differed between plant communities and drought generally reduced ecosystem respiration rates. Rewetting led to a pulse in ecosystem respiration rates in Nardus snowbeds, but the nature of rewetting did not determine the size of the gas flux. Total leachate losses of C and all forms of N were greater following high intensity rewetting (storm). Drought reduced total losses of C and some forms of N, but increased total nitrate losses. Nitrate losses were greatest when storm followed drought. Large losses of nitrate due to dry periods and heavy rain events could acidify soils and lead to eutrophication of surface waters, potentially impacting ecosystem functioning. I conclude that oceanic-alpine ecosystems hold significant carbon stocks and are resilient to drought events, but it will be important to evaluate the functions of soil microbial communities, including the source of C mineralised following rewetting events, as soil microorganisms determine the relative accumulation and release of soil C and N through soil-atmosphere exchange.