The ecosystem pools of mineral nutrients are divided between the above- and belowground biota, the dead organic matter, and an inorganic pool, which probably constitutes the major source of plant-available nutrients. The difference in distribution of nutrients between soil and vegetation is still more pronounced than that in the distribution of C because the soil organic matter generally is enriched in nutrients compared with the vegetation (Jonas-son and Miclielsen, 1996). For instance, tlie ratio of C to nitrogen (N) is typically about 20 for soils and 60 for arctic vegetation (McGuire et al., 1995). Hence, the soil N concentration is about three times higher than the concentration in the vegetation. Thus, given a soil-to-vegetation C ratio of 17 (Table 1), the soil-to-vegetation ratio of N in the arctic and alpine regions triples in comparison to the C ratio and reaches about 50.
In spite of the large soil nutrient stores and low requirements of nutrient absorption by the low plant biomass, arctic ecosystems are still sensitive to nutrient inputs. This is because the nutrient mineralization rate, i.e., the transformation of nutrients from organic, plant-unavailable form to inorganic, available form, is low and usually sets the limits to primary production (Nadelhoffer et al., 1992). Any changes in the ecosystems that trigger increased nutrient supply generally lead to increased primary production (Kielland and Chapin, 1992).
The major limiting elements are N, phosphorus (P), or both together, usually with N limitation in dry and mesic ecosystem types and P limitation in wet ecosystems (Shaver and Chapin, 1995).
Hence, while variation in hydrology seems to be the proximate source of large-scale variability in ecosystem structure and function across landscapes, variation in supply rate of inorganic N and P seems to regulate the more detailed structure and function within each ecosystem type.
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