Agriculture Reference
In-Depth Information
microbes (Wynn-Williams, 1982) and in pure cultures of cold-adapted bacteria (Panoff et al.,
1995). The microbial activity seems to continue down to -5°C, although around theses values
the free water availability starts to be a limiting factor (Coxson and Parkinson, 1987). The
microbial C increase is linked to a significant reduction of DOC concentration, both outside
and outside the bags, revealing how the microbial access to labile C was equal.
The greater immobilisation of N in the microbial biomass and the comparable behaviour
inside and outside the bags revealed a higher competition of microorganisms with plants for
nutrients under the dwarf forest than in the reference site. However, the microbial
immobilisation was not so competitive to determine a net negative N mineralization, as found
by Bauhus and Barthel (1995) and Raubuch and Joergensn (2002). The winter N
mineralization created a significant pool of N still available for plant. In essence, spruce could
survive on the exceeding inorganic N discarded by microbes (Foster et al., 1989; Lipson and
Monson, 1998) if other phenomena such as leaching would not cause a large depletion of the
few nutrients produced under the dwarf forest.
C
ONCLUSION
Tree growth was limited in the permafrost affected site, where the winter and early spring
soil temperature was significantly lower than in the reference forest. A smaller amount of
ammonium was available in the dwarf forest and was consumed during winter while nitrate
seemed not a limiting factor and was produced during winter and early spring. However, the
low soil temperatures did not allow plant uptake and the nutrient was lost trough leaching
and/or translocation into depth before roots started their activity. Moreover under the dwarf
forest the greater microbial N immobilization, with a corresponding DON and NH
4
+
decrease
revealed a significantly higher competition of microorganisms with plants for nutrients. The
critical conditions under the dwarf trees could have selected a microbial community
particularly tolerating cold temperatures, and then more active and more resistant to the
winter season and to moderate freeze/thaw events.
In conclusion, an inorganic N pool, constituted mainly by the leachable NO
3
-
, was
available in the early growing season in both sites, but the lower soil temperature under the
dwarf trees may inhibit soil nutrient absorption by plants. Therefore in the cold site there is
asynchrony in the early spring season between the availability of critical resources and their
utilization by plants, with potential loss of nutrients.
A
CKNOWLEDGEMENTS
We are grateful to the Swiss Federal Institute for Snow and Avalanche Research for the
logistic and technical support.
