Boreal vegetation and cycling of carbon and nitrogen

The boreal region, located just below the Arctic Circle, covers large parts of Alaska, Canada, and Russia. Boreal soils store more carbon (C) than the soils of any other ecosystem around the globe, making it very important to the global carbon budget. Model projections of climate change indicate a number of potential impacts on the boreal region, including increases in soil temperature, reduced amounts of snow cover, and thawing of permafrost (see Hinzman, 2005). These changes would alter the vegetation that dominates the landscape, which in turn would affect the amounts of carbon and nitrogen (N) stored in the soil. Less carbon stored in the soil means more carbon released to the atmosphere.

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The boreal region is characterized by large forested areas, mostly comprised of coniferous trees such as spruce, pine, and larch. In Interior Alaska, these forests are dominated by black spruce trees (Figure 1). There also are ecosystems characterized by shrubs, grasses, sedges (grass-like plants that grow in wetter areas), as well as fens (a type of wetland that receives water from both groundwater and rainfall). Researchers from the US Geological Survey and University of Guelph measured the amount of carbon and nitrogen stored in soils of these five different ecosystems and age-dated materials within their soil profiles. With this information the scientists were able to determine carbon and nitrogen accumulation rates, over both decades and centuries (500-800 years). Their aim was to determine if, and how, these rates of varied by ecosystem type. With this information they could determine whether shifts in vegetation due to climate change would have an impact on the amount of carbon or nitrogen stored in soils.

The researchers found that, over the past 60 years, no significant differences in carbon accumulation rates occurred among the five ecosystem types. This result demonstrates that, although the amount of carbon that enters and is lost from the soil may vary, the balance between these inputs and losses on the short-term is similar for all five ecosystems. Nitrogen accumulation rates for black spruce systems were lower than the sedge and fen ecosystems.

Over the century timespan, only the fen differed from the other four vegetation types. Carbon accumulation rates were 2-4 times higher in fens than the other ecosystems; nitrogen accumulation rates were 2-12 times higher. These results suggest that long-term carbon and nitrogen cycling in fens is fundamentally different from the other ecosystems. This difference likely results from differences in wildfire regimes, because fire not only releases large amounts of carbon and nitrogen to the atmosphere (combustion), but also affects post-fire nutrient cycling due to changes in soil temperature and moisture. Their shallower water table makes fens less likely to burn, even in dry years. If they do burn, the fires are less severe than in the other ecosystem types. Therefore, even though many fires have occurred in this region over the last several millennia, these fires had a smaller impact on the fens, resulting in more carbon and nitrogen being stored in the soil.

The results of this study suggest that regional soil carbon and nitrogen accumulation rates will not change significantly in response to shifts in vegetation between non-fen ecosystems. However, if climate change is extreme enough to reduce the amount of fens on the landscape, there could be a large decrease in the amount of carbon and nitrogen being stored with this region. This dataset will provide a useful baseline for comparison with future accumulation rates.

The paper, “Decadal and long-term boreal soil carbon and nitrogen sequestration rates across a variety of ecosystems”, was published in Climatic Change.

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