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Physical Environment - Nutrient Cycles


In order to understand the ANWR ecosystem, it is also necessary to investigate the energy and nutrient cycles. The carbon balance of the ecosystem has been highly influenced by global climate changes and CO2 content changes. The arctic contains 11% of the world's organic matter pool, and within the arctic tundra ecosystems, there are both carbon sinks and carbon sources. Vegetation changes in the Alaskan tussock tundra over the past decade has brought about important feedbacks on the region's biogeochemical cycles, mostly through altered rates of carbon and energy exchange between biosphere and atmosphere. Modeling analysis suggests that the source/sink strength of tundra depends on changes in photosynthesis that result from the partitioning of nitrogen between vegetation and soils, and on changes in soil moisture, which affect soil respiration rates. All of these factors may be affected by machine and human activity in the region and disturbances in the permafrost.

Nutrient cycling and fertilization studies in arctic ecosystems show that plant growth is strongly limited by nutrient availability. Such activity depends highly on decomposition, nitrogen mineralization, phosphorous availability, and controls on carbon and nutrient cycles, which in turn depend on temperature, moisture, decomposability of litter inputs, depth of thaw, etc.


Current Net Ecosystem Carbon Storage and Flux

Carbon pools and rates of carbon accumulation vary, depending on vegetation type and environmental conditions.

More than 90% of the carbon in arctic ecosystems is located in soils, with even higher percentages (98%) in soils of northern peatlands. In upland boreal forest, in contrast, only about 55% of the ecosystem carbon is found in the soil. Not only is the proportion of soil carbon substantial, but so are the absolute amounts. Arctic tundra has 55 Pg (petagrams) of carbon stored as soil organic matter in the A horizon, compared with 87.5 Pg in non-peatland boreal forest and 122 Pg in forest peatlands and Gorham's estimates of 455 Pg C are considerably higher (1991).

Tussock and wet sedge tundra soils account for the bulk of circumpolar tundra carbon stores because they have large amounts of carbon per square meter and cover large areas. Although per unit area carbon storage in polar semi-deserts is only about one-half that in the wet sedge tundra, the greater extent of these semi-deserts results in carbon stores approaching those of wet sedge tundra.

For these large stores of soil organic matter to have accumulated in northern ecosystems, production must have exceeded decomposition at some time in the past. Recent estimates indicate that northern ecosystems still constitute a small net sink for atmospheric carbon; current accumulation rates, however, are difficult to assess (Post, 1990; Gorham, 1991). Because the rates vary with conditions and ecosystem type, soil carbon accumulation is positive in some areas and negative in others. The overall balance is still uncertain (Chapin 1991).


Carbon Balance of Arctic Plants and Ecosystems(and the relation with global warming)

The carbon cycle is strongly correlated with climate in the region, as well as the global climate dynamics. Because of the large amount of carbon present in northern soils and the presumed sensitivity of soil carbon accumulation or loss to climate change, northern ecosystems may be particularly important to global carbon balance in the future. Between 250 and 455 petagrams of carbon are present in the permafrost and seasonally thawed soil layers -- about one-third the total world pool of soil carbon. Warmer soils could deepen the active layer and lead to thermokarst and the eventual loss of permafrost over much of the Arctic and the boreal forest. These changes could in turn alter arctic hydrology, drying the upper soil layers and increasing decomposition rates. As a result, much of the carbon now stored in the active soil layer and permafrost could be released to the atmosphere, thereby increasing CO2 emissions and exacerbating CO2-induced warming. Alternatively, plant communities and vegetation can be changed because of the increase in atmospheric CO2 and nutrient availability. New communities might be taller and have higher rates of primary productivity than does extant vegetation. The net result could be higher primary productivity, increased carbon storage in plant biomass, and a negative feedback on global atmospheric CO2.

Arctic and boreal forest ecosystems are unique in their potentially positive and negative response to elevated CO2 and associated climate change. The arctic ecosystem is also unique in its capacity for massive continuing, long term carbon accumulation because of its permafrost, and they are also particularly sensitive to global warming. Thus it can be seen that it is important to discuss the major processes and controls on carbon cycling in arctic ecosystems, and the likely effects of elevated atmospheric CO2 and concomitant climate change on carbon storage" (Chapin 1991).


Global warming, vegetation changes, and the nutrient cycles

Under global warming, temperature and precipitation changes in arctic regions are occurring already. In much of Alaska, approximately 1ºC per decade of warming has been observed. Vegetation changes have been recorded in Alaskan tussock tundra over the past decade (Chapin et al. 1995), and these changes are expected to have important feedbacks on the region's biogeochemical cycles through altered rates of C exchange between biosphere and atmosphere, and changes in the region's energy balance.

The arctic tundra ecosystem consists of both C sinks and sources. Detailed modeling analysis of arctic biogeochemistry (McKane et al. 1997) suggest that the source/sink strength of tundra depends on changes in photosynthesis that result from the partitioning of nitrogen (NO) between vegetation and soils, and on changes in soil moisture, which affect soil respiration rates.

From Arctic Ecosystem in a Changing Climate: An Ecophysiological Perspective : "The limitations of photosynthesis by low temperatures and low solar radiation has been clearly demonstrated and simulated for arctic vascular plants(Miller et al. 1976; Limbach et al. 1982; Tenhunen et al. 1994) But, because of the saturating response of photosynthesis to light, day length is an additional important limiting factor to productivity." "While moisture stress has no impact on productivity at some of the mean climate conditions, photosynthesis is vulnerable to changes in soil water potential and hydraulic constraints on water transport." A reduction in soil water potential can cause stomatal closure, to balance transpiration against reduced soil water intake. Lower hydraulic conductance can lead to stomatal closure and reduce gross primary productivity as rates of water supply are constrained by the characteristics of the vascular system. Photosynthesis in arctic ecosystems is linked closely to the hydrological cycle. C loss from ecosystems is also linked to soil moisture (Oechel et al. 1993).


Works cited:

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

Williams, M.; Rastetter, E. Vegetation characteristics and primary productivity along an arctic transect: implication for scaling-up. Journal of Ecology 1999 87: 885-898.

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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