Introduction

Recent studies on the comparative ecology of vascular plants have shown that interspecific variation in functional traits can help us to understand ecosystem functioning and to predict ecosystem responses to environmental change (Walker et al. 1999; Grime et al. 2000; Lavorel and Garnier 2002; Westoby and Wright 2006). This contrasts sharply with our poor knowledge of the comparative ecology of non-vascular cryptogams, especially when it comes to their traits related to biogeochemistry (Cornelissen et al. 2007). Mosses are particularly abundant, and often dominant, in cold and cool biomes, especially in peatlands, boreal forest and high-latitude tundra (Gorham 1991; Woodin and Marquiss 1997; Forsum et al. 2006). In such ecosystems they control biogeochemical processes (Clymo and Hayward 1982; Bates and Farmer 1992; Cornelissen et al. 2007), either directly or indirectly via their impact on soil temperature and moisture regimes (Longton 1997; Woodin and Marquiss 1997; Van der Wal and Brooker 2004). The often cold and wet soils in these ecosystems have low nutrient mineralization rates and, consequently, generally low availability of inorganic nutrients. Nitrogen is usually the principal nutrient that limits plant assimilation and growth here, in some cases together with phosphorus limitation (Shaver and Chapin 1991; Chapin et al. 1995). However, in arctic tundra soil concentrations of organically bound nitrogen in the form of free amino acids may be an order of magnitude higher than those of inorganic nitrogen (Kielland 1995). Several predominant higher-latitude vascular plant species seem to have adapted to this naturally high amino acid availability in that they have a high capacity for the uptake of amino acids, whether or not with the aid of mycorrhizal fungi (Kielland 1994; Näsholm et al. 1998; Schimel and Chapin 1996; Näsholm and Lipson 2001). By this mechanism such plants short-circuit the N cycle by by-passing the phase of N mineralisation by soil microorganisms (Chapin 1995). Can arctic cryptogams do this as well? Indirect evidence from seven sub-arctic/arctic moss species and two lichens, based on natural abundance signatures of the isotope 15N in their tissues, raised the suspicion that these cryptogams might derive significant amounts of N from organic matter relative to inorganic sources (Michelsen et al. 1998). Labeling studies have shown that several lichen species (Dahlman et al. 2004) and two bryophyte species have the ability to take up amino acids directly from their environment. Based on a laboratory assay, the peat moss Sphagnum rubellum can probably take up the small amino acid glycine directly from the soil solution (Kielland 1997). Forsum et al. (2006) used dual labelling with 13C and 15N to show that the moss Hylocomium splendens took up glycine under real field conditions. If the ability to take up and process both smaller and larger amino acids is shared among many of the predominant higher-latitude mosses, it could be an important factor contributing to their strong position there. Our study is, to our knowledge, the first to test this experimentally.

Specifically, we tested the following hypotheses: (1) the amino acids aspartic acid and glycine can be taken up by a range of different moss species and types, both in the lab and in the field; (2) there are differences in the relative uptake of the amino acids between the moss species, as related to their growth forms or their adaptations to habitat moisture and/or N availability. To answer our hypotheses we tested 11 sub-arctic moss species (four peat mosses Sphagnum sp., seven other mosses) for their potential uptake of dual 13C and 15N labelled glycine and aspartic acid, two amino acids known to be abundant in high-latitude organic soils (Kielland 1994, 1995; Weintraub and Schimel 2005). Thereto we injected these amino acids into tundra soil both in a standardized lab experiment and in situ.

Methods

Lab experiment: sampling

We selected 11 moss species of abundant and widespread, often circum-arctic, occurrence in tundra. Several of these species also occur in cool-temperate peatlands. We collected fresh turfs of these species within a 2 km radius around the Abisko Research Station, Abisko, Northern Sweden (68°21′ N, 18°49′ E, alt. c. 340–370 m), in mid July 2006. These species were tentatively classified into four groups based on (a) widely recognised overall differences in morphology and functioning between the phylogenetically distant Sphagnum versus non-Sphagnum mosses (Chapin et al. 1996); and (b) differences in typical habitat wetness and turf density within each of these higher taxa. Thus, we distinguished between: (1) ‘wet’ Sphagnum species (S. riparium Aongstr., S. lindbergii Schp.) which have a rather loose turf structure and grow in or just above standing water in mires; (2)’drier’ Sphagnum species (S. fuscum (Schimp.) Klingg., S. cf. warnstorfii Russ.), which grow in denser turfs above the water table in bogs and mires; (3) non-Sphagnum mosses that grow in rather dense turfs and are in relatively close contact with the water table in mires like the species in group 2 (Aulacomnium palustre (Hedw.) Schwaeg., Paludella squarrosa (Hedw.) Brid., Tomentypnum nitens (Hedw.) Loeske, Drepanocladus revolvens (Swartz) Warnst. s.l.); and (4) non-Sphagnum mosses that are more loosely packed on relatively dry substrata (Hylocomium splendens (Hedw.) Schimp., Polytrichum commune Hedw., Pleurozium schreberi (Brid.) Mitt.). Within group (4), P. commune stood out by the fact that it is endohydric (with water conducting tissues), which may help it to take up inorganic nutrients efficiently from the soil.

For each species 15 samples were collected from five blocks with mutual distances of at least 3 m. A PVC tube (15 cm deep, 5 cm diameter) was drilled into a moss turf in nearly complete monoculture, including both its green and brown parts. The tubes with the mosses were taken back to a greenhouse in day light (see below), where each moss sample was gently rinsed from its soil solution for 5–10 min in a 0.05 M KCl solution. Then they were placed back into the tube in the same density, put onto water-saturated foam blocks for 2 h in order to acclimate.

Lab experiment: amino acid uptake

The standardized tubes filled with the mosses were subjected to three treatments of five replicates each (blocks). In the control treatment, 5 ml demineralised water was injected. In the amino acid treatments 5 ml of 1 mM double labelled glycine (15N, 13C-2 glycine; one C atom labelled) or 5 ml of 1 mM double labelled aspartic acid (15N,13C4 aspartic acid; all four C atoms labelled) was injected at five points halfway down the tube, around the transition of the green and brown moss parts. We considered this the relevant point in the profile with respect to amino acids in natural soil solutions, which are predominantly break-down products of organic matter (Kielland 1995), although we can not exclude the possibility that sub-arctic mosses might presently also intercept very small amounts of amino acid from anthropogenic N deposition in precipitation (cf. Forsum et al. 2006 and Discussion). We assumed that virtually all 15N that was accompanied by corresponding amounts of 13C (1:1 ratio for glycine, 1:4 ratio for aspartic acid) retrieved from a moss could be attributed to the uptake of whole, non-mineralised amino acids. Any refixation of respired 13CO2 was probably negligible (see Discussion). After injection of the solutions the mosses were allowed to take up the amino acids for 24 h in the greenhouse (temperature close to 15oC, slightly lower than under ambient light regime). After 24 h the pH of the remaining water in the tubes was measured. Then the mosses were harvested and washed in 0.05 KCl solution for 10 minutes to make sure no labelled amino acids were left on the surface of the moss shoots. Subsequently the samples were dried at 70oC, weighed and ground.

Field experiment

There had been sufficient precipitation in the weeks and days before the start of the field experiment, so that we could assume none of the moss species to be desiccated during the experiment (authors’ observations). The field experiment was carried out following the same methods and the same blocked design as for the lab experiment, with the difference that the tubes with the mosses were placed back into their own holes in the moss cushions in the field directly. The solutions were injected with demineralised water, glycine or aspartic acid in the same way as in the lab experiment and uptake was allowed again for 24 h. After harvesting the tubes were taken back to the lab, rinsed, dried, weighed and ground as above. Subsequent analysis was the same for both lab and field experiment.

Rinsing control experiment

To make sure only the amino acids taken up by the plant and not the amino acids on the surface of the moss were measured, an additional experiment was carried out to check whether 10 min of washing was sufficient. In that case, additional rinsing would not result in different (lower) concentrations of labelled isotopes. The methods used in this experiment were the same as in the lab experiment. For this experiment two species of contrasting morphology and habitat were used: Hylocomium splendens and Sphagnum fuscum. Each of these species was injected with either glycine or aspartic acid, and, upon harvesting after 24 h, rinsed either once for 10 min or three times for 10 min at a time. Replication was 3. Subsequent analysis was the same as for all the samples in the lab and field experiments (see below).

Stable isotope analysis

Nitrogen and carbon concentrations and atom percentages of 15N and 13C of the dried, ground samples were determined in separate runs for N and C using an elemental analyzer (NC2500, ThermoQuest Italia, Rodana, Italia), coupled with a continuous-flow isotope ratio mass spectrometer (Delta Plus, ThermoQuest Finnigan, Bremen, Germany). The IRMS also measured the total mass percentage of C and N in the samples, therefore together with the atom percentages of 15N and 13C, the fraction of amino acids taken up could be calculated using the formula:

$${{\text{Fraction uptake}} = {\left( {C_{{{\text{sample}}}} } \right)}*{\left( {{\text{F}}^{{{\text{13}}}} C_{{{\text{sample}}}} - {\text{F}}^{{{\text{13}}}} C_{{{\text{nat}}}} } \right)}} \mathord{\left/ {\vphantom {{{\text{Fraction uptake}} = {\left( {C_{{{\text{sample}}}} } \right)}*{\left( {{\text{F}}^{{{\text{13}}}} C_{{{\text{sample}}}} - {\text{F}}^{{{\text{13}}}} C_{{{\text{nat}}}} } \right)}} {^{{{\text{13}}}} C_{{{\text{added}}}} }}} \right. \kern-\nulldelimiterspace} {^{{{\text{13}}}} C_{{{\text{added}}}} }$$

where C sample is the total amount of C in the sample (g); F13 C sample is the fraction (13 C/total C) found after amino acid treatment; F13 C nat, is the natural abundance of 13C in the species, here represented by the average fraction of 13C in the control treatment; and 13 C added is the amount (g) of 13C added. Since all moss species were placed in tubes with a surface area of 19.63 cm2 at their field density, the total amino acid uptake represents their uptake per unit surface area. We did not carry out a correction for plant mass, because at unlimited supply a given moss species will presumably take up an amount of amino acid proportional to its own mass. The mass of the plants is already represented by C sample in the calculation of the fraction uptake.

Statistics

Data analysis was performed using SPSS 10.1. We carried out a three-way ANOVA on atom percentages of 13C, with species, amino acid type and rinsing treatment (rinsing once versus three times) as fixed factors. We used untransformed 13C data, since their variances did not differ significantly (Levene’s test, P = 0.17).

Differences between the amino acid addition treatments (control versus glycine and control versus aspartic acid) were tested using two-way Analysis of Variance (ANOVA) on untransformed atom percentages of 13C, with species and amino acid treatment as independent variables. Although in one of the groups the frequency distributions were not normally distributed, we proceeded with the analyses. Analyses of variance are known to be robust to deviations from normality as long as the sample sizes are nearly equal (Zar 1999). To test whether there were species-specific differences in the relative rate of amino acid uptake, a one-way ANOVA with a post hoc Tukey’s test was carried out on the fraction of injected 13C taken up. This test was carried out on untransformed data. We used linear regression to compare interspecific rankings for 15N versus 13C.

Results

Rinsing controls

There was no significant effect of rinsing the mosses once versus three times on 13C % in the 3-way ANOVA (rinsing treatment, F = 0.35, P = 0.56), as contrasting with a significant difference between the two species (F = 45.6, P < 0.001) and between glycine versus aspartic acid added (F = 24.4, P < 0.001). There were no significant first or second order interactions. See Supplementary figure 1 for further details. These results indicate that any 13C adhering to the outer surfaces of the mosses had been removed already with the first rinsing treatment. We therefore assume that 13C enrichment in the lab and field experiments (see below) would be due to uptake.

Amino acid uptake

All species tested had the capacity to take up amino acids. In all species and all treatments the atom percentages of 13C in the plants were higher than in the controls after addition of either of the amino acids, both in the lab and the field experiment (see Supplementary figure 2), The only exception was Polytrichum commune, which showed clear 13C uptake in the lab experiment but not in the field. The ANOVA on percent 13C confirmed that amino acid uptake was highly significant across the species set both for glycine and aspartic acid, both in the lab and in the field (Table 1). Also, in each of the treatments, both in the lab and in the field, there was significant overall variation in the uptake of amino acids among species, as indicated by the interaction term. While both the lab and the field experiments showed an enrichment of 13C and thus amino acid uptake in the treated samples, the fraction of the added amino acid that was taken up by the plants was in most cases lower in the field experiments than in the lab experiments (Fig. 1). A general pattern in all treatments was that 13C was taken up in lower amounts than 15N, and that the relative difference was greater for aspartic acid (mean 13 C/15 N = 0.41) than for glycine (mean 13 C/15 N = 0.86); see Figs. 1 and 2.

Fig. 1
figure 1

The fraction of added amino acid retrieved in the plant after uptake for 24 h for a glycine in the lab; b aspartic acid in the lab; c glycine in the field; d aspartic acid in the field. White bars show the uptake of 13C, dark bars that of 15N. Standard errors are shown one-sided (n = 5). The species are grouped as follows, from left to right: (1) wetter Sphagnum spp.; (2) drier Sphagnum spp.; (3) denser, wetter non-Sphagnum mosses; (4) loose, drier non-Sphagnum mosses. For 13C, bars that have no letters in common are significantly different in Tukey’s post hoc tests. The fraction of field uptake of 13C in aspartic acid by P. commune was actually −0.020 ± 0.024

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Fig. 2
figure 2

Regression of uptake of 13C versus 15N for a glycine in the lab experiment (dotted line, slope 0.87, R 2 = 0.73), b glycine in the field (irregularly dashed line, slope 0.96, R 2 = 0.98), c aspartic acid in the lab (dashed line, slope 0.47, R 2 = 0.79), d aspartic acid in the field (solid line, slope 0.41, R 2 = 0.94). Each data point represents a species mean value. All 11 species are included

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Table 1 Results of the 2-way ANOVAs on percent 13C data with the independent variables amino acid addition treatment (2 levels) and species (11 levels), for glycine and aspartic acid in laboratory and field experiments, respectively (n = 5)
Full size table

Whilst the numbers of species for each a priori functional group were not sufficient for formal statistical analysis, some trends can be derived from Fig. 1, as indicated partly by the Tukey’s post hoc test results for the component species involved. The looser, ‘wetter’ Sphagnum species (group 1) tended to take up more 13C than the denser, ‘drier’ Sphagnum species (group 2), especially in the field. Within the non-Sphagnum mosses, the denser, ‘moister’ species (group 3) appeared to generally take up more than the looser, ‘drier’ ones (group 4), particularly so for glycine in the lab, but for the field uptake of both amino acids this tendency was explained mostly by the lack of uptake by Polytrichum commune. Groups 2 and 3 showed broadly similar uptake patterns, with substantial interspecific differences within each group.

We also tested for each of the amino acids in lab or field whether uptake, as indicated by fraction of 13C taken up, was negatively related to total percent tissue N across the 11 species. Although all regression slopes showed a negative trend, none was significantly different from zero (0.084 ≤ R 2 ≤ 0.18). Since we thought that underlying relationship might be obscured by overall differences between Sphagnum and non-Sphagnum mosses, we also tested the same relationships for the subset of seven non-Sphagnum species. We found significantly negative slopes for both glycine (R 2 = 0.72, P < 0.05) and aspartic acid (R 2 = 0.62, P < 0.05) in the field, but not in the lab experiment (glycine, R 2 = 0.022; aspartic acid, R 2 = 0.12).

We also report consistent positive interspecific relationships between the natural abundance of 13C in control plants and the fraction 13C uptake in the amino acid addition experiments. The accompanying linear regression slopes were significantly greater than zero for glycine uptake in the lab as a function of natural 13C abundance (R 2 = 0.64, df = 9, P < 0.01) and for glycine uptake in the field (R 2 = 0.46, df = 9, P < 0.05). However, for aspartic acid uptake such positive regression trends were not significant in the lab (R 2 = 0.18, df = 9) or in the field (R 2 = 0.34, df = 9). This would imply that amino acid carbon taken up from tundra soil has a higher 13C percentage than plant carbon after fractionation of atmospheric carbon through photosynthesis processes. However, since we could only speculate about the underlying mechanisms at this stage, we shall not discuss these results below.

Discussion

In support of our first hypothesis, we have demonstrated that a wide range of sub-arctic moss species possess a high capacity for uptake of amino acids from their direct environment. This also confirms our suspicion based on earlier investigations on 15N natural abundance signatures of seven sub-arctic/arctic mosses (three of which in common with our study), which implied possible long-term incorporation of N derived from organic rather than inorganic sources in soil or precipitation (Michelsen et al. 1998). Thus, many of the abundant higher-latitude mosses have the ability to short-cut the nitrogen cycle by being relatively independent of ammonium or nitrate for their N nutrition, which would give them a great ecological advantage in tundra soils with low availability of inorganic N forms. However, in order to interpret and refine our findings, several methodological and ecological factors need to be addressed first.

Methodological issues

Rinsing moss samples three times versus once as in the main experiments, had no effect on the fraction of 13C taken up. Although ideally we should have tested this for all 11 species, the lack of a ‘rinsing’ effect in two contrasting species in terms of phylogeny, morphology and habitat choice, may justify our assumption that the elevated 13C in the plants resulted predominantly from real uptake of the amino acids by the mosses, rather than adherence to their surface.

For the calculation of the fraction of amino acids taken up, we had to subtract the natural ‘background’ 13C content of the mosses by averaging the 13C content of control samples, since cutting off parts of moss samples to be injected with labelled amino acid would have damaged the mosses and interfered with uptake. However, the background 13C may differ between samples of the same species as well, making such subtractions doubtful for individual injected samples. To tackle this problem, the two-way ANOVAs, testing the control versus the amino acid treatment, were carried out on raw, uncorrected percentage 13C data. We assumed the species by treatment interactions from these ANOVAs to best represent any interspecific differences in uptake.

Amino acid uptake: fact or artefact?

This study has shown that all 11 mosses tested here could take up two representative amino acids of contrasting molecular size, viz. glycine (small) and aspartic acid (larger). With the notable exception of Polytrichum commune (20 and 10% uptake for glycine and aspartic acid respectively in the lab, but negligible or no uptake in the field), this uptake was generally substantial and occurred both in a standardized albeit rather artificial lab environment and in a realistic but heterogeneous field setting. The fractions of amino acid taken up by the other 10 mosses ranged from 12–65% (average 36 ± 5%) and 5–61% (average 24 ± 5%) for glycine in lab and field, respectively, and from 5–34 (average 18 ± 2%) and 0–26% (average 10 ± 2%) for aspartic acid in lab and field. Thus, uptake was on average lower in the field than in the lab. This may be explained by a range of factors, including exchange of the added amino acid solution with water in the surrounding environment (via the open tube bottoms), differences in hydrology or microbial activity between the field and lab experiment, pH differences, and/or competition with vascular plants or other mosses for amino acids or other sources of nitrogen in the field (Näsholm et al. 2000).

Our main analyses were carried out on the 13C data. For the 13C measured in the plant after the injection of the amino acid, we assume the whole amino acid, or at least the part of the amino acid that is still in an organic form, to have been taken up. A general pattern in all treatments was that 15N was taken up in greater amounts than 13C, especially for aspartic acid, which could indicate the breakdown of the amino acids before uptake by for instance the microorganisms in the soil environment (Jones 1999; Lipson and Monson 1998, 1999). Theoretically shoot 13C enrichment could also result from re-fixation of respired 13CO2 upon such amino acid mineralization. However, this is unlikely to have caused any significant 13C enrichment of moss shoots in our experiment since, similar to Näsholm et al. (2000), we did not measure any 13C enrichment of adjacent moss shoots in our control treatment.

The isotopes 13C and 15N were in general taken up in more equal amounts in the glycine addition treatment than in the aspartic acid treatment. This seems to agree with a study by Lipson et al. (1999), who showed that microorganisms have low affinity for glycine compared to other, larger amino acids like for instance glutamate, which is more similar to aspartic acid.

Some 13C may also have been lost initially inside the mosses, because inside plants both glycine and aspartic acid can be converted into glycolysis- or TCA-cycle intermediates and thus some of the 13C could be respired relatively easily. Aspartic acid can be converted into oxalo-acetate (Mazelis 1980) while glycine can be transformed into pyruvate via serine respiring the carboxylic-C in the process (Oliver 1994). To avoid losing the 13C via respiration after it entered the plant we used 15N, 13C-2 glycine, which is labelled on the non-carboxylic C atom. The aspartic acid (15N, 13C4 aspartic acid) was labelled on all C atoms, including a carboxylic part, so here loss via internal respiration could be a factor determining the low 13C/15N ratio after uptake. However, the loss of the carboxylic C atom could also be due to breakdown by microorganisms, as discussed above. Differences in respiration rates between mosses may not only be species dependent but may depend also on abiotic factors such as water availability (Dilks and Proctor 1979). However, any such differences in respiration rate are unlikely to explain the variation in 13C and 15N content among mosses. The breakdown of the amino acids in the soil by microorganisms is probably the main contributor to the differences in 13C/15N ratio found (Jones 1999).

Interspecific differences in amino acid uptake

For comparing species belonging to different functional groups for amino acid potential (hypothesis 2), the lab experiments are probably the most revealing, since the water availability was standardised across all species here. Owing to small numbers of species within functional groups as defined here a priori, we could not test differences in amino acid uptake among groups statistically. However, some trends can be observed. Both within Sphagnum and within the non-Sphagnum mosses, the species from relatively wet habitats appeared to take up greater amounts of amino acids, although more clearly so for glycine than for aspartic acid. These patterns were also somewhat apparent in the field experiment. We might tentatively explain these observations from the growth circumstances of the species. The species adapted to wetter environments might hold more water in their cushions in order to maintain, and compete in, their wet environment. This way, they may have had more intense contact with the injected amino acid solution, which could have explained their higher amino acid uptake. For species from drier environments the water availability in the lab experiment may have been much higher than in the field, even though the weather before the experiment had not been dry. This might explain why they tended to show much greater uptake in the lab than in the field in relatively terms, compared to the small relative differences in uptake for the wetter Sphagnum species, for instance. However, we need data for the actual traits underpinning such apparent differences among species groups. One trait that showed some interesting pattern was total tissue N concentration, which among the non-Sphagnum mosses in the field experiments was negatively correlated with amino acid uptake capacity. We speculate that species more dependent on amino acids would be found in the more N-stressed environments, as tentatively indicated by low tissue N concentrations. In such environments a conservative N economy as supported by amino acid uptake would be beneficial. The low amino acid uptake of Polytrichum commune in the lab, and the virtual absence of uptake by this moss in our field experiment, is of particular interest, since this is the only endohydric species among the 11 studied here. Since P. commune has internal conducting tissues which may also transport inorganic nutrients from the soil relatively efficiently (Ligrone et al. 2000), it may rely mostly on inorganic N sources in the soil rather than on amino acids. However, injecting amino acids broadly at the boundary of brown and green parts means that potential uptake through rhizoids could have been partly missed in our experiment. Several other morphological features on the leaf surface could play a positive or role in amino acid uptake too, for instance protective waxes in certain species (e.g. P. commune) from less wet habitats might be expected to interfere with the permeability of the cell wall (Claytongreene et al. 1985). If lower photosynthetic rates are also associated with conservative N economy and amino acid uptake in mosses, this would be somewhat supported by the reported maximum photosynthetic rates (P max) of some of the mosses. Skre and Oechel (1981) found that P. commune (low amino acid uptake) had high P max compared to intermediate H. splendens and P. schreberi (rather low amino acid uptake), while two Sphagnum species (although different spp. from the ones in our study, which had relatively high amino acid uptake) had low P max. It is clear that, in order to develop a framework for predicting amino acid uptake among wide-ranging moss species, we need to not only test for the uptake potential of more species, but also screen them for the morphological, chemical and/or physiological traits determining and predicting it.

Conclusions

In support of our first hypothesis, at least 10 out of 11 abundant and widespread subarctic moss species studied here show a substantial ability to take up nitrogen from free amino acids in the soil, represented here by glycine and aspartic acid. The next step in this research could be to study the relative preference for different N sources in different moss species, by supplying inorganic and organic N forms in different combinations in the same experiment (cf. Schimel and Chapin 1996, Forsum et al. 2006). Anthropogenic nitrogen deposition plays a relatively small but perhaps not negligible role in high-latitude regions (Woodin and Marquiss 1997) and a relative increase in ammonium compared to amino acids in tundra soils could potentially reduce the advantage of amino acid uptake as reported here. There is recent evidence that anthropogenic N deposition in lower-latitude regions also contains an organic fraction, including amino acids (Cornell et al. 2003, Forsum et al. 2006), and it would be interesting to investigate whether this could be of any consequence to the nitrogen nutrition of higher-latitude bryophytes.

There was a significant difference in the uptake of the two amino acids among the species tested. Thus, it would be useful to know whether the species that had low amino acid uptake, have a preference for \({\text{NH}}_{\text{4}}^{\text{ + }} \) or NO3 – and vice versa (Schimel and Chapin 1996; Forsum et al. 2006), and whether that would relate to their site preference. We also need to do more empirical work to be able to link the amino acid capacities of different species to the morphological, chemical and/or physiological traits determining them. Even so, by linking our results to the high concentrations of amino acids and the low concentrations of inorganic N in widespread higher latitude soils (Shaver and Chapin 1991; Kielland 1995), it is likely that the uptake of free amino acids in N-stressed arctic ecosystems generally provides a significant proportion of the nitrogen demand of a wide range of mosses. This might be a key factor explaining how they can maintain their strong position in extensive N-stressed areas in cool and cold northern biomes.