Seasonal Controls on Litter and Soil Carbon and Nutrient Cycling in Arctic Tundra Ecosystems and Potential Impacts of Climate Change

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Christiansen, Casper Tai
Climate Change , Winter , Ecosystem Ecology , Arctic
Climate change is leading to warmer temperatures and greater snowfall in Arctic regions. Microbial decomposition activities are strongly regulated by temperature, and therefore climate warming is projected to enhance decay of the vast tundra soil organic matter pool, releasing CO2 into the atmosphere and nutrients into the soil solution. By contrast, increased soil nutrient availability promotes plant growth and changes in vegetation, both of which may enhance plant uptake of CO2 from the atmosphere. Consequently, depending on the net balance between these increases in CO2 release and uptake, tundra ecosystems may end up contributing globally significant feedbacks to a changing climate, further exacerbating environmental change. In this thesis, I modelled the effects of changes in seasonal climate on litter decay rates and ecosystem carbon (C) and nutrient pools in a variety of tundra vegetation-types. Specifically, I investigated how short- (one year) and longer-term (up to 9 years) experimentally-deepened winter snow and summer warming impact microbial communities and biogeochemical dynamics using experimental plots located in distinct ecosystems across the Canadian, Greenlandic, and Norwegian Arctic. Summer warming reduced surface litter decomposition in both relatively dry and wet ecosystems, likely because of evaporation-induced desiccation. In contrast, deepened snow had negligible effects on litter decay rates. However, tall birch shrub vegetation significantly stimulated litter decomposition, presumably due to a positive feedback from their greater litter inputs, enhancing soil nutrient pools and thereby microbial decomposition activities. Regarding plant and soil dynamics, the longer-term impacts of deepened snow included enhanced evergreen shrub growth and dominance over deciduous shrubs, but this increase in ecosystem C storage was dwarfed by 60 times greater soil C loss from the subsoil mineral layer. Soil bacterial community structure was much more strongly influenced by depth variation in edaphic properties down the thawing soil profile than by seasonal change from winter to autumn. However, despite greater diversity with depth, the seasonal thaw significantly affected community structure and biomass similarly throughout the soil profile. Altogether, this research suggests that summer warming may slow surface litter decomposition, but that deepened snow may result in substantial soil C losses that greatly exceed plant C uptake.
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