Are there Unaccounted Plant Sources of Limiting Nutrients

The assumption hitherto has been that plants only, or almost exclusively, take up N in inorganic form as NH44 or NO," - This assumption has been questioned recently, and there are strong

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FIGURE 2 Biomass and nitrogen accumulation (means ± SE) in Erio-phontin vaginatiun grown in NO,, NH4, and amino acids (AA) during 24 days. Treatment responses with the same superscript letter above the bars are not significantly different at p £ 0.05. (Redrawn from Chapin et ill., 1993, with kind permission from Nature).

indications that tundra plants of different life-forms take up organic N either directly or through their mycorrhiza. It has long been known from laboratory experiments that plants having ecto-or ericoid mycorrhiza can access organic N through their fungal partner (Abuzinadah and Read, 1988; Read et al., 1989). Chapin et al. (1993) and Kielland (1994) showed more recently that some plants can take up amino acids more rapidly than inorganic nitrogen from hydroponic culture in the absence of mycorrhizae (Fig. 2). Many dry and mesic vegetation types in the Arctic are dominated either by ericaceous species with ericoid mycorrhiza, e.g., dry heathlike vegetation types, or by Salix and Betula shrubs that host ectotrophic mycorrhiza and dominate large areas of mesic tundra in the low Arctic. Hence, if organic N uptake also occurs in situ, the vegetation over large areas of tundra has the potential capability to access organic N directly, without being dependent on mineralization and possible competition with soil microbes. However, even if plants avoid the mineralization step by absorbing organic N, they are still dependent on soil microbes for protease activity and the solubilization of organic N. Organic N uptake has also been reported in a number of dominant species of graminoid-dominated tundra, ranging from wet Carex sedge meadows, through mesic Eriophorum vaginatum tussock tundra to Kobresia heaths (Chapin et al, 1993; Schimel and Chapin, 1996; Raab et al, 1996). These species generally lack ectomycorrhizae and may absorb this N primarily by root uptake. Plant organic N uptake may explain the discrepancy between annual net mineralization and annual plant uptake as pointed out above (Giblin etal, 1991; Schimel and Chapin, 1996).

Although the evidence for organic N uptake in situ is strong, it is not entirely conclusive. For instance, mycorrhizal organic N uptake has been inferred from different |SN levels in non- or VAM-mycorrhizal plants, compared with the natural |5N abundance in co-occurring ecto- and ericoid mycorrhizal species (Fig. 3; Michelsen et al, 1996, 1998). There are strong indications that a large part of this difference is because the potential soil sources of N to plants, i.e., NH4, NO,, and amino acids, have different isotopic composition, which is manifested in the plant tissue after the nutrient uptake (Michelsen et al, 1998). However, it has not yet been possible to analyze the isotopic composition of these sources separately. laN signature also differs with rooting depth, which could also contribute to species differences in plant 15N (Schulze etal, 1994; Nadelhoffer etal, 1996). Similarly, one of the most conclusive studies of in situ uptake of organic N in graminoids of mesic and wet tundra (Schimel and Chapin, 1996) could not entirely eliminate the possibility that the organic N taken up was first mineralized by microbes within or outside the rhizosphere. However, evidence of organic N uptake is also being reported from other ecosystems, so it appears that this alternative pathway for N uptake is progressively being confirmed (Nasholm etal, 1998).

The importance of organic N uptake for plant production in the Arctic is evident because most functional plant groups and species with potential N uptake capacity are those that dominate the vegetation. For instance, Empetrum hermaphroditum or E. nigrum having ericoid mycorrhiza often dominate southern dry arctic vegetation. In more mesic areas Cassiope tetragona with ericoid mycorrhiza, dwarf Salix with ectomycorrhiza or Eriophorum

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FIGURE 3 Mean SI5N (± SE) of the bulk soil organic matter and plant species without mycorrhiza (NON), with ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI) fungi at four different heath or forest tundra sites, n is number of species analyzed. (From Michelsen et al., t998, with kind permission from Springer-Verlag).

FIGURE 3 Mean SI5N (± SE) of the bulk soil organic matter and plant species without mycorrhiza (NON), with ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI) fungi at four different heath or forest tundra sites, n is number of species analyzed. (From Michelsen et al., t998, with kind permission from Springer-Verlag).

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Treatment vaginatum with ability to absorb organic N without mycorrhiza are prominent species across large areas. Similarly, wet tundra sites are dominated by other graminoids which, like E. vaginatum, have been shown to absorb amino acids. The effect on the N cycle of these species that are known to absorb organic N needs to be more closely evaluated and quantified.

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