| Northern terrestrial ecosystems located in the circumpolar boreal region between 50° and 70° north contain about one third of the world's soil organic carbon (C). The recent rise in air temperatures at northern latitudes may be stimulating decomposition of soil organic carbon in these ecosystems. A net release of C stored in boreal soils due to increasing temperatures could have a significant positive feedback on global warming by increasing atmospheric carbon dioxide (CO2) and methane (CH4). It is important, therefore, to understand the factors that control decomposition in northern terrestrial ecosystems. The main controls of decomposition in northern ecosystems include temperature, moisture, and substrate quality. In boreal forest ecosystems there are complex interactions between these variables, making it difficult to determine their relative importance as controls on decomposition. In an effort to improve our understanding of the controls on decomposition in boreal forest systems, I conducted a series of field and laboratory studies in black spruce (Picea mariana) forests in interior Alaska. By combining these approaches, I was able to examine how temperature, moisture, and organic matter chemistry affect decomposition rates under controlled conditions, and then use this knowledge to interpret decomposition dynamics under field conditions. Black spruce ecosystems inhabit a broad range of landscape types varying in drainage and permafrost regime, which creates natural gradients in soil temperature, soil moisture, and soil organic C accumulation within this major vegetation type of the boreal region. I conducted my research at three study sites that were characterized according to drainage status. My field studies focused on measuring CO2 and CH4 gas exchange, litter decomposition, and soil C stocks, while my laboratory studies focused on the influences of temperature, moisture, and organic matter chemistry on decomposition of soil and dissolved organic carbon (DOC) collected from the study sites. Decomposition is highly sensitive to changes in temperature, moisture, and is closely linked to substrate chemistry in the lab and in the field. Broad differences in soil C and DOC chemistry among the sites were evident, and were indicative of both potential decomposition and of the extent of prior decomposition in the field. |
