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soils (Schmidt et al., 1999). The high N accumulation could be due rather to a particular
adaptation of spruce to the cold environment, which results in a lower growth with a greater
photosynthesis and a higher N incorporation in the needles (Oleksyn, 1998). Moreover, the N
accumulation in the soil followed by a decrease of C could be attributed to a selective
degradation of only easily decomposable O-containing compounds (Saiz-Jimenez, 1996).
This would affect the degradation of N-containing compounds and the production of
dissolved inorganic and organic forms.
The inorganic N content in both sites was significantly higher in the OF horizon than in
the OH horizon, evidencing how the upper organic horizons have a high N absorption
capability. Low quantities of N were found into the OH horizon, where the major part of roots
developed.
Ammonium in the late fall was present in very lower amounts in the dwarf forest and
prevailed to the nitrate in all the OF and OH horizons, probably in relation to a stronger action
of
Sphagnum
that can assimilate even short NO
3
-
pulses through a re-adsorption from
senescing lower stem parts (Jauhiainen et al., 1998).
During the winter and early spring season, NH
4
+
was reduced while nitrate increased in
the buried bags in both sites but the changes were always more expressed in the reference
site. The NH
4
+
consumed in the buried bags was likely used by microbes and recycled, ruling
out a possible uptake by plants, since no differences between the incubated and the
undisturbed soil were found. The increase of nitrate concentration during winter and early
spring was in agreement with several studies (for instance Williams et al., 1996). In more
temperate sites the increasing nitrification in early spring is related to a higher activity of soil
microorganisms when air temperature is still low and plants remained dormant. The opposite
occurs in the permafrost zone: a protraction of negative soil temperatures also when the air
temperature started to increase and reached values of +15°C in April (Delaloye et al., 2003;
Korner and Hoch, 2006) could cause an unbalance between the nutrient availability in the
rizosphere and the spring plant request. In the dwarf forest the production of nitrate might be
not exploited by spruce since soil temperatures until the end of May were still below that
threshold temperature of +3.2 °C in soil indicated for the beginning of the growing season
(Korner and Hoch, 2006). By contrast in the reference site the soil temperature was above this
threshold. The major amount of NO
3
-
inside the bag compared to outside suggested a large
nutrient consumption during the early spring. It can be inferred that while in the stratum II the
higher amount of NO
3
-
produced during winter and early spring was likely consumed through
plant uptake and leaching, in the dwarf forest the lower amount of NO
3
-
produced was lost
only trough leaching and/or translocation into depth, highly favoured by voids between
boulders.
This suggests that the major part of nitrate was no more available when roots started later
their activity. Therefore there is as asynchronies between the availability of critical resources
and their utilization in the dwarf forest. To confirm this hypothesis further studies with
different incubation techniques are currently being investigated.
Although net N mineralization followed the same trend in the two sites, the major soil N
immobilization in the microorganisms of the site I compared to the other site suggested that
the critical conditions under the dwarf trees could have selected a different microbial
community particularly tolerating cold temperatures and then more active and more resistant
to the winter season (Brooks et al., 1996) and to moderate freeze/thaw events (Lipson and
Monson, 1998; Freppaz et al., 2007). Such tolerance has been observed in Antarctic peat
