Figure 1.
Dose response curves of (a) ammonium uptake rates (filled circles; µmol NH4+ gDW−1 h−1) and (b) nitrate uptake rates (open circles; µmol NO3− gDW−1 h−1) of Sphagnum magellanicum (n = 25) from the pristine site.
The rates represent the average uptake rates after 0.5(n = 5). Note that y-axes differ by one order of magnitude. Saturation curves (Eq. 1) were fitted to rates: Vmax of ammonium (28 µmol NH4+ gDW−1 h−1) was higher than Vmax of nitrate 2.5 µmol NO3− gDW−1 h−1), whereas Km-values were similar (6.5 µM for ammonium and 3.5 µM for nitrate, respectively). The linear component k was higher for ammonium (0.022 l g−1DW h−1) than for nitrate (0.004 l g−1DW h−1). Dashed lines show the fit to uptake measurements using only two parameters (Vmax and Km).
Figure 2.
Average uptake rates (µmol N gDW−1 h−1) after 0.5 (dark grey), 2 (grey) and 72 (light grey) hours, respectively in Sphagnum magellanicum (n = 5) from the pristine site (Argentina).
The upper panel (a) shows ammonium (NH4+) uptake, the lower panel (b) nitrate (NO3−) uptake.
Figure 3.
Decrease in ammonium uptake at the pristine site with increasing exposure time on.
Eq. 2 fitted well (r2 = 0.99) for the uptake data for both 10 µM (black circles) and 100 µM (grey triangles). The half-time value ‘b’ was 2.1 hours for both concentrations. The constant (C) was lower in the 10 µM treatment (0.22 µmol N gDW−1 h−1) compared to the 100 µM treatment (0.88 µmol N gDW−1 h−1). Dashed line indicates critical N-uptake to maintain biomass production (0.8 µmol N gDW−1 h−1). Note that uptake rates at 24 hours and later were based on the depletion of ammonium in the experimental solution, whereas other rates presented in this paper are average rates calculated from 15N uptake.
Figure 4.
Differences in uptake rates (µmol N gDW−1 h−1) were dependent on the origin of the mosses.
N-Uptake rates are shown at 1 µM (dark grey), 10 µM (grey) and 100 µM (light grey), respectively, of the pristine site (Argentina, left) and the N-polluted site (The Netherlands, right). Upper panel (a) shows ammonium (NH4+) uptake. The lower panel (b) shows shows nitrate (NO3−) uptake. Experiments lasted 72 hours.
Table 1.
Minimal adequate models1 used for data presented in Fig. 2.
Table 2.
Accession effect: linear models of data presented in Fig. 4.
Figure 5.
Differences between average uptake rates (µmol N gDW−1 h−1) after 0.5 hours in different fractions of Sphagnum magellanicum from the pristine site.
We analysed capitula and stems separately, while whole plant values represent the DW weighted mean of both fractions.
Table 3.
Differences between uptake rates in capitulum and stem tissue, as shown Fig. 5.
Figure 6.
Exposure time of mosses to nitrogen increases non-linearly at increasing nitrogen concentrations.
We used a conceptualized relationship (log-log) between nitrogen concentration in rain and exposure time for a living Sphagnum layer that would need to retain some 90% of the nitrogen load. The area below the horizontal dots (average residence time of rain) indicates an increasing potential of nitrogen leaching through the living layer of mosses. Maximum uptake rates Vmax differ between scenarios: black “ammonium pulse” 28 µmol N gDW−1 h−1, green “ammonium long” 2.8 µmol N gDW−1 h−1, red “nitrate pulse” 2.5 µmol N gDW−1 h−1, purple “nitrate long” 0.8 µmol N gDW−1 h−1 and grey dashed line “nitrate low affinity” 0.8 µmol N gDW−1 h−1, respectively. The model was parameterised with a Km of 6.5 µM for ammonium and a Km of 3.5 µM for nitrate. The “nitrate low affinity” was calculated with a Km of 35 µM. An increasing potential of leaching is expected above the dotted black line that indicates the upper limit of residence time of rain. Average nitrogen concentrations in rain (µmol N l−1) relate to yearly wet deposition of nitrogen (kg ha−1) by a factor 0.1 (yearly rainfall 750 mm).