Subsurface Controls on Carbon Dynamics in a Changing Arctic Ecosystem

With climate change in the Arctic, temperatures are expected to rise at twice the rate as in temperate latitudes. This rapid change has the potential to disrupt local ecosystems and feed back to the global climate as frozen soils thaw and warm. Large stocks of carbon have accumulated in Arctic soils, protected from decomposition by cold, wet, and frozen conditions. With warming and thawing due to climate change, decomposition of this carbon is expected to increase, releasing it to the atmosphere as the greenhouse gases CO2 and methane. While a number of modeling efforts have attempted to quantify this potential feedback, the future Arctic carbon balance remains unknown due to uncertain mechanisms stabilizing soil carbon and complex interactions between vegetation and soils. In studies based in Barrow, Alaska, I address three sources of this uncertainty: (1) the magnitude of methane emissions following soil thaw, (2) interactions between plants, soil carbon, and microbial decomposers, and (3) the sensitivity of soil carbon cycling changes in microclimate./nFirst, I ask how methane formation, consumption within the soil, and net emission to the atmosphere may change with soil thaw in the Arctic. Loss of permafrost (perennially frozen ground) can lead to large-scale landscape changes, redistributing water and soil. Such physical changes can strongly influence emissions of methane, a greenhouse gas roughly 25 times as potent as CO2, whose future emission rates are highly uncertain. Combining field measurements with statistical modeling, I assess soil methane emissions and microbial methane processes (production and consumption) across a gradient of permafrost thaw. In contrast with many previous studies, I find that more degraded sites have lower methane emissions, a different primary methanogenic pathway, and greater methane oxidation than intact permafrost sites. These differences are greater than soil moisture or temperature data can explain. Additional microtopographic controls accounting for these observations may include differences in water flow and vegetation between intact and degraded polygons./nSecond, I ask how changes in plant activity due to climate change may influence the rate of soil carbon decomposition (the priming effect), through interactions between plant roots, microbial decomposers, and soil carbon compounds. In a two-year field experiment, I simulate increased plant root activity and measure its influence on soil carbon decomposition, plant CO2 uptake, mineral nitrogen availability, and microbial communities. I find no measurable relationship between substrate additions and soil organic matter decomposition, nutrient supply, or microbial population size. Treatment-level differences in primary production, however, indicate possible longer-term interactions between vegetation and decomposition. The absence of a measurable priming effect contrasts with numerous published reports documenting a positive priming effect under tightly controlled conditions. This difference may be due to high background variability in ecosystem respiration, a property of this in situ experiment. This chapter is one of the first studies evaluating this plant-soil interaction in a field experimental context, with a representative degree of environmental variability./nThird, I ask how decomposition rates of fast-cycling and slow-cycling soil carbon may be influenced by microclimatic changes. The rate of soil carbon turnover and its sensitivity to environmental variables such as temperature and oxygen availability are both highly uncertain and highly influential for model predictions of the global carbon cycle. In two laboratory incubations, I use natural abundance radiocarbon measurements of CO2 and soil organic matter to determine how fast-cycling and slow-cycling carbon pools respond to temperature changes and transitions between anaerobic and aerobic conditions. Using a novel analytical approach, I find that fast- and slow-cycling carbon pools from these Barrow, Alaska soils have comparable temperature sensitivities, with decomposition from both pools increasing by ~40 % for a 5°C temperature increase. Similarly, decomposition rates were highly sensitive to aerobic vs. anaerobic conditions, with no significant difference in sensitivity between pools. Radiocarbon contents of CO2 and soil organic matter indicate that ancient, slow-cycling carbon is sensitive to decomposition under soil temperature increases and water table changes.

Advisor: Torn, Margaret S