Dissertation Abstract
Effects of climate change on carbon and nitrogen cycling in permafrost soils of Alaska
Treat, Claire C 2014
Natural Resources, Earth and Environmental Sciences, University of New Hampshire (United States), 164 pp.
Effects of climate change in northern latitudes include warmer temperatures, permafrost thaw, and altered hydrology. If thawed, permafrost soils store enough carbon to double atmospheric carbon reservoirs. However, controls on carbon losses from permafrost soils are not well understood. I used laboratory incubation experiments and data synthesis to investigate the effects of substrate quality, environmental conditions including temperature and moisture, and microbes on soil carbon and nitrogen losses following permafrost thaw.
Decomposition processes control soil carbon losses and depend on organic matter quality, environmental controls, and microbial controls. Permafrost organic matter is commonly more labile than overlying surface soils because frozen temperatures prevent decomposition. Permafrost formation history controls the lability of permafrost organic matter due to prior decomposition and results in organic matter quality variations across the permafrost region. I measured differences in aerobic and anaerobic carbon production among tundra and boreal biomes, landscape positions, and vegetation types, indicating differential responses to permafrost thaw across the landscape related to organic matter quality.
Environmental controls on decomposition include temperature and moisture. Soil warming resulted in higher decomposition rates and was the primary environmental control on decomposition. However, permafrost thaw can result in wetland formation, saturated soils, anaerobic conditions, and slower decomposition rates. While the importance of saturation increased at warmer temperatures, anaerobic carbon losses were a fraction of aerobic carbon losses. Production of methane, a more potent greenhouse gas than carbon dioxide, was much smaller than anaerobic carbon dioxide production.
Finally, microbial communities directly control carbon and nitrogen cycling in permafrost soils. Methanogen abundance and growth rate determined potential methane production, but organic matter quality ultimately controlled microbial activity. Competition between plants and microbes controlled nitrogen losses in permafrost soils through mineralization and assimilation. A warmer permafrost region will lead to larger soil carbon losses, but soil saturation and organic matter quality will decrease the magnitude of carbon losses. Changes in plant productivity due to soil nitrogen availability may partially offset soil carbon losses. Thawing permafrost will be a positive feedback to climate change but assessing the net effects of permafrost thaw requires a better understanding of plant-soil interactions.
Decomposition processes control soil carbon losses and depend on organic matter quality, environmental controls, and microbial controls. Permafrost organic matter is commonly more labile than overlying surface soils because frozen temperatures prevent decomposition. Permafrost formation history controls the lability of permafrost organic matter due to prior decomposition and results in organic matter quality variations across the permafrost region. I measured differences in aerobic and anaerobic carbon production among tundra and boreal biomes, landscape positions, and vegetation types, indicating differential responses to permafrost thaw across the landscape related to organic matter quality.
Environmental controls on decomposition include temperature and moisture. Soil warming resulted in higher decomposition rates and was the primary environmental control on decomposition. However, permafrost thaw can result in wetland formation, saturated soils, anaerobic conditions, and slower decomposition rates. While the importance of saturation increased at warmer temperatures, anaerobic carbon losses were a fraction of aerobic carbon losses. Production of methane, a more potent greenhouse gas than carbon dioxide, was much smaller than anaerobic carbon dioxide production.
Finally, microbial communities directly control carbon and nitrogen cycling in permafrost soils. Methanogen abundance and growth rate determined potential methane production, but organic matter quality ultimately controlled microbial activity. Competition between plants and microbes controlled nitrogen losses in permafrost soils through mineralization and assimilation. A warmer permafrost region will lead to larger soil carbon losses, but soil saturation and organic matter quality will decrease the magnitude of carbon losses. Changes in plant productivity due to soil nitrogen availability may partially offset soil carbon losses. Thawing permafrost will be a positive feedback to climate change but assessing the net effects of permafrost thaw requires a better understanding of plant-soil interactions.