Ecologists have long been intrigued by the ways co-occurring species divide limiting resources. Such resource partitioning, or niche differentiation, may promote species diversity by reducing competition. Although resource partitioning is an important determinant of species diversity and composition in animal communities, its importance in structuring plant communities has been difficult to resolve. This is due mainly to difficulties in studying how plants compete for below-ground resources. Here we provide evidence from a 15N-tracer field experiment showing that plant species in a nitrogen-limited, arctic tundra community were differentiated in timing, depth and chemical form of nitrogen uptake, and that species dominance was strongly correlated with uptake of the most available soil nitrogen forms. That is, the most productive species used the most abundant nitrogen forms, and less productive species used less abundant forms. To our knowledge, this is the first documentation that the composition of a plant community is related to partitioning of differentially available forms of a single limiting resource.

Contents Summary 31 Introduction 31 Availability 32 Uptake 34 Field studies of plant amino acid uptake 41 Conclusions and future perspectives 43 Acknowledgements 44 References 45 Summary Languishing for many years in the shadow of plant inorganic nitrogen (N) nutrition research, studies of organic N uptake have attracted increased attention during the last decade. The capacity of plants to acquire organic N, demonstrated in laboratory and field settings, has thereby been well established. Even so, the ecological significance of organic N uptake for plant N nutrition is still a matter of discussion. Several lines of evidence suggest that plants growing in various ecosystems may access organic N species. Many soils display amino acid concentrations similar to, or higher than, those of inorganic N, mainly as a result of rapid hydrolysis of soil proteins. Transporters mediating amino acid uptake have been identified both in mycorrhizal fungi and in plant roots. Studies of endogenous metabolism of absorbed amino acids suggest that L‐ but not D‐enantiomers are efficiently utilized. Dual labelled amino acids supplied to soil have provided strong evidence for plant uptake of organic N in the field but have failed to provide information on the quantitative importance of this process. Thus, direct evidence that organic N contributes significantly to plant N nutrition is still lacking. Recent progress in our understanding of the mechanisms underlying plant organic N uptake may open new avenues for the exploration of this subject.

Recent studies of nitrogen (N) cycling in arctic tundra have indicated that inorganic N supplied to plants by mineralization is not sufficient to meet the annual requirement of N by many tundra species. Whereas N mineralization is slow in tundra soils and concentrations of inorganic N are low, these soils have large stocks of both structural and soluble organic N. In light of these observations, kinetics of absorption of three amino acids (glycine, aspartic acid, and glutamic acid) were measured in dominant vascular plant species of the four major ecosystems types in arctic Alaska and compared with concentrations of free amino acids in soils. Absorption rates were measured on roots using 14C—labeled substrates. Concentrations of free amino acids in soil were measured on water—extracted samples by high pressure liquid chromatography. All species had higher capacity (Vmax) for ammonium uptake (measured using methylamine as an ammonium analogue) than for any amino acid. However, at concentrations observed in the field, uptake rates estimated for amino acids were similar to (glycine) or less than (aspartic and glutamic acids) that for ammonium. On the basis of these comparisons, uptake rates of the three amino acids together may account for between 10 and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than the more slowly growing evergreen shrubs, suggesting that new growth created a sink that strongly influenced capacity for amino acid uptake. In general, ectomycorrhizal species had higher amino acid uptake than did non—mycorrhizal species. In species that were sampled from more than one community, amino acid uptake rates were highest in the community where a given amino acid was most abundant in the soil. The results indicate that, in arctic tundra, plants short—circuit the mineralization step of decomposition by directly absorbing amino acids. This implies that in the organic soils of these tundra systems (1) inorganic nitrogen is an inadequate measure of plant—available soil nitrogen, (2) mineralization rates underestimate nitrogen supply rates to plants, (3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource, and (4) by short—circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.

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