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Active pools of Carbon and Nitrogen in Soils of the Alaskan Tundra

by Robert MacLean, Joshua Schimel and Allen Doyle, Department of Biology and Wildlife, Institute of Arctic Biology

From a global change perspective, the soils of areas like Alaska's North Slope tundra are important because they represent a significant pool of carbon. This carbon is mostly in the form of soil organic matter, that is, humus or peat. Carbon accumulates in tundra soils because cold temperatures and water logging keep decomposition rates low relative to the rate of input of organic material from surface vegetation.

Incubation studies with fertilized and unfertilized soil cores from different soil and vegetation types on the North Slope tell an interesting story about the way in which carbon and nitrogen are stored in these soils and how active or labile these pools of elements are. Tundra soils, it seems, do not behave in quite the same way as soils from other latitudes.

Computer models are being used to help predict changes in the terrestrial carbon pool in response to global climate change and to determine if soil organic carbon will be released into the atmosphere through enhanced microbial activity. These models have to simulate not only the decomposition rates of soil organic matter but the reuse of the products of this decomposition by plants, especially nitrogen, and the additional uptake of atmospheric carbon by these plants through photosynthesis. Nitrogen cycles are intimately linked to carbon cycles as both decomposition of soil organic carbon by microbes and the photosynthetic uptake of atmospheric carbon dioxide by plants is limited by nitrogen availability. One of the most important parameters these models need is some way of separating the organic C and N in the soil into rapidly cycling, easily decomposed pools and more recalcitrant, slowly cycling pools. The size and turnover rates of the "active pool" are important because it is this pool that will influence ecosystem processes on the scale of decades to centuries that we are worried about with global climate change. The objective of our research was to see if we could quantify an "active fraction" for carbon and nitrogen in some arctic tundra soils.

We sampled soils from four distinct vegetation types: dry heath soils on the crests of terraces and low ridges; acid tussock soils from amongst tussocks in waterlogged peaty areas; limestone tussock soils from areas with calcareous soil parent material; and shrub tundra soils from beneath alder and willow shrub lands that occur in snow sheltered depressions. These four vegetation types encompass much of the range of soil conditions occurring in the tundra around the Toolik Lake Long Term Ecological Research (LTER) area. Samples were taken from previously fertilized (nitrogen) and unfertilized soils at each site.

When soils are incubated under controlled water content and temperature, organic carbon and nitrogen are mineralized into nitrate, ammonium and carbon dioxide. Carbon mineralization rates are shown in Figure 1; nitrogen results are not shown. Unfertilized soils have lower carbon mineralization rates because microbial decomposition is more limited by nitrogen availability.

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Each data point represents the average for six replicate sample sites. Mineralization rates are expressed as micrograms carbon per day per gram organiz matter in soil sample.

Usually we expect to see the rates of mineralization to decline logarithmically for carbon and increase logarithmically for nitrogen before stabilizing. The amount of C and N released during this logarithmic phase can be interpreted as representing the "active" or rapidly cycling component. This logarithmic phase is normally seen within the first few months of incubation in soils from other ecosystems.

In tundra soils we are not seeing this logarithmic pattern. Mineralization of carbon and nitrogen seems to be a relatively linear process. This suggests that the concept of a "discrete" active pool of soil organic C and N may not fit well with tundra soils and may make parameterization of biogeochemical cycling models difficult for tundra soils. Our data also illustrate how soils from different vegetation types exhibit different mineralization patterns, particularly when comparing acid tussock tundra with other vegetation. These differences are as profound as those found between farmland and forest soils in more temperate areas. Thus the subtle landscape mosaic of the arctic tundra may be just as important as the more stark patchwork of human land uses at lower latitudes as a predictor of soil organic carbon and nitrogen dynamics.

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