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Physical Environment - Soil Microbial Processes


Microbial Processes and Plant Nutrient Availability in Arctic soils

Several characteristics of arctic soils influence microbial activity, nutrient mineralization, and nutrient availability to plants and will certainly figure prominently in changes in these processes in a warmer arctic climate. Arctic soils are generally overlain by a dense mat of organic matter and vegetation, wet for at least part of the year and permanently frozen at some depth. These factors combine to lower summer soil temperatures, impede the progression and decrease the depth of seasonal thawing, and maintain relatively high soil moisture content. Cold, wet soil environments and short summers slow organic matter decomposition and nutrient mineralization and severely restrict nutrient availability to plants.

The accumulation of organic matter in arctic soils is determined largely by the combined effects of temperature and moisture on decomposition and primary production. Because of climatic variation among arctic regions, the amounts of organic matter and nutrients in tundra soils vary across broad geographic scales. Organic matter often accumulates at depth in permanently frozen peats in relatively wet arctic regions such as the coastal plain on northern Alaska.

Organic carbon increases with moisture, from low amounts in well-drained beach-ridge ecosystems with cushion plant-lichen communities. Such an overall pattern of organic carbon increasing with moisture from well- to poorly drained ecosystems also occurs in Alaska's coastal and foothill tundra regions

Well-drained soils are less common in patterned ground regions with little relief, such as the Alaskan coastal plain, where more than 85% of soils are moist to poorly drained. Moist soils with dense organic mats(5-40cm thick), intermediate thaw depths, and divers plant communities dominated by tussock-forming sedges occupy gently sloping land in much of the Low Arctic.

Organic matter and moisture content are important determinants of soil temperature, thaw depth, cation exchange capacity, aeration, redox potential, and other properties affecting biological processes in soils. Decomposition rates and soil moisture balances will likely be affected by the warmer temperatures predicted for the Arctic. The resulting changes in soil organic matter, moisture and microbial processes in ecosystems will alter the amounts, seasonality, and forms of mineral nutrients available to plants. A warmer climate will likely have different overall effects on soil properties and on nutrient cycling in dry, moist, and wet arctic ecosystems.

Source: CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991. P. 281-283.


Microbial and soil Processes

Nutrient cycling and fertilization studies in arctic ecosystems show that plant growth is strongly limited by nutrient availability. Primary production is often nitrogen-limited, but phosphorus (especially in organic soils) or nitrogen and phosphorus together can also limit production.

Arctic ecosystems are generally conservative of nutrients accumulating large amounts in soil organic matter pools with very long turnover times. Because of these characteristically slow turnover rates and, in some ecosystems, the gradual burial of organic matter in permafrost, nutrients become available to plants at very low rates. Long turnover time result from slow decomposition, which can become a bottleneck in nutrient cycling rates. Differences among ecosystem types in soil microclimate and decomposition may explain the inverse relationships between soil nutrient stocks and nutrient cycling rates or primary productivity as reported, for example, on Alaska's northern coast. Slow decomposition leads to greater accumulation of organic matter in soil and can lower nutrient mineralization rates, thereby decreasing primary productivity.

CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991.

OECHEL, W.C., CALLAGHAN, T., GILMANOV, T., HOLTEN, J.I., MAXWELL, B.,MOLAU, U., SVEINBJORNSSON, B., Global Change and Arctic Terrestrial Ecosystems


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