Agriculture Reference
In-Depth Information
has been collected from a whole winter (November 2001-May 2002). In the same period
the N and C dynamics in the organic OH horizon were measured by the buried bag
technique and in the undisturbed soils.
Tree growth was significantly lower in the permafrost affected sites, where the
winter soil temperature was significantly lower than the reference forest.
During winter an increase of nitrate concentration was recorded in all sites, providing
an inorganic N pool ready available for plant growth under the reference forest, but not
under the dwarf trees, due to the lower soil temperature which inhibit plant nutrient
absorption. Moreover, under the dwarf forest the microbial N immobilization, with a
corresponding DON and NH
4
+
decrease, was more evident than in the reference site. The
critical conditions under the dwarf trees could have selected a microbial community
particularly tolerating cold temperatures, and then more resistant to moderate freeze/thaw
events.
Thus, an inorganic N pool, constituted mainly by the leachable NO
3
-
, is available in
the early growing season in the cold site and in the reference site, but the lower soil
temperature under the dwarf trees may inhibit soil nutrient adsorption by plants.
Therefore in the cold site there is as asynchronies between the availability of nutrients
and their utilization, which may affect the plant growth.
I
NTRODUCTION
Permafrost distribution ranges from continuous coverage in the northern continuous
permafrost zone to discontinuous ice lenses in the localized permafrost zone. Discontinuous
alpine permafrost exists roughly above 2400 m ASL.
Below the timberline, permafrost is assumed to exist only at scree slopes located at foot
of high cliffs, at very shaded sites as reported by Kneisel et al. (2000). Slopes composed of
large blocky materials are reported to determine a mean annual ground temperature colder
than in fine materials (Harris and Pedersen, 1998). Unexpectedly low ground temperatures
and ground ice during the summer were reported in talus at low altitudes (200-1000 m ASL)
throughout the Daisetsu Mountains (Hokkaido Island) (Sawada et al., 2003) and in the
mountains of Central Asia (Altai, Northern and Inner Tien Shan) (Gorbunov et al., 2004).
The difference in air temperature between the open space of the block slope and the
atmosphere is thought to create an air circulation phenomenon which is supposed to be the
predominant factor for the preservation of permafrost on mountain belts (Wakonigg, 1996;
Tanaka et al., 2000). In winter, the warmer air in the blocks tends to rise and escape in the
upper part of the slope, displaced by cold air, entering through holes in the snow cover. In the
absence of snow cover in summer, the cold air trapped between the blocks sinks downslope
through the blocks and escapes into the air at the bottom of the deposit, permitting warm air
to replace it. These processes are assumed to operate in conjunction with other factors, such
as slope and aspect which modify the potential radiation at a given site (Harris and Pedersen,
1998).
Due to the occurrence of permafrost conditions in the mountain belt these sites are
vegetated with plants typical of subalpine and alpine areas and some species display signs of
severely limited growth (Wegmann, 1995; Kneisel et al., 2000; Gorbunov et al., 2004). The
ground may be covered by a thick layer of mosses (e.g.
Sphagnum
), due to the high soil
humidity linked to heavy precipitation and air condensation. The vegetation composition and
