Research on plant nitrogen (N) uptake and metabolism has more or less exclusively concerned inorganic N, particularly nitrate. Nevertheless, recent as well as older studies indicate that plants may have access to organic N sources. Laboratory studies have shown that ectomycorrhizal and ericoid mycorrhizal plants can degrade polymeric N and absorb the resulting products. Recent studies have also shown that some non-mycorrhizal plants are able to absorb amino acids. Moreover, amino acid transporters have been shown to be present in both plant roots and in mycorrhizal hyphae. Although both mycorrhizal and non-mycorrhizal plants appear to have a capacity for absorbing a range of organic N compounds, is this capacity realized in the field? Several lines of evidence show that plants are outcompeted by microorganisms for organic N sources. Such studies, however, have not addressed the issue of spatial and temporal separation between plants and microorganisms. Moreover, competition studies have not been able to separate uptake by symbiotic and non-symbiotic microorganisms. Qualitative assessment of organic N uptake by plants has been performed with dual-labelled glycine in several studies. These studies arrive at different conclusions: some indicate that plants do not absorb this organic N source when competing with other organisms in soil, while others conclude that significant fractions of amino acid N are absorbed as intact amino acid. These variable results may reflect species differences in the ability to absorb glycine as well as differences in experimental conditions and analytical techniques. Although theoretical calculations indicate that organic N might add significant amounts of N to plant N uptake, direct quantitative assessment of the fraction of plant N derived from uptake by organic N sources is a challenge for future research.

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A wide range of recent studies have indicated that organic nitrogen may be of great importance to plant nitrogen (N) nutrition. Most of these studies have, however, been conducted in laboratory settings, excluding important factors for actual plant uptake, such as competition, mycorrhizal associations and soil interactions. In order to accurately evaluate the importance of different N compounds to plant N nutrition, field studies are crucial. In this study, we investigated short- as well as long-term plant nitrogen uptake by Deschampsia flexuosa, Picea abies and Vaccinium myrtillus from 15NO3-, 15NH4+ and (U-13C, 15N) arginine, glycine or peptides. Root N uptake was analysed after 6 h and 64 days following injections. Our results show that all three species, irrespective of their type of associated mycorrhiza (arbuscular, ecto- or ericoid, respectively) rapidly acquired similar amounts of N from the entire range of added N sources. After 64 days, P. abies and V. myrtillus had acquired similar amounts of N from all N sources, while for D. flexuosa, the uptake from all N sources except ammonium was significantly lower than that from nitrate. Furthermore, soil analyses indicate that glycine was rapidly decarboxylated after injections, while other organic compounds exhibited slower turnover. In all, these results suggest that a wide range of N compounds may be of importance for the N nutrition of these boreal forest plants, and that the type of mycorrhiza may be of great importance for N scavenging, but less important to the N uptake capacity of plants.