Agriculture Reference
In-Depth Information
To compare soil characteristics and tree growth (
Picea abies
) in areas with different
evidence of permafrost occurrence the site was divided into two strata, corresponding to the
prevailing tree growth form: I) stunted forest and II) reference forest (Figure 1).
Tree Growth, Age and Size
Ring width is the most commonly studied parameter. Large, thin-walled cells are laid
down early in the growing season when conditions are favourable for growth. These are
followed by smaller, thick-walled and denser cells late in the growing season which form due
to the onset of cooler temperatures, lack of soil moisture, and shorter days. The production of
latewood cells terminates abruptly, followed by larger cells the following growing season.
This abrupt termination at the end of one year and the beginning of the next year marks a ring
boundary. Ring width is measured between two successive ring boundaries.
Around each soil profile in every stratum, three trees exhibiting representative sizes were
selected. The height of the trees was estimated and the tree stems were cored perpendicular to
their axes at heights of 30 cm above the ground. In the laboratory, the wood cores were
processed with the usual dendrochronological methods (Schweingruber 1988).
Soil Chemical and Physical Characterization
The soil under the dwarf trees was classified as a Histic Organosol Insaturé and under the
reference trees as an Organosol Insaturé (Gobat et al, 2004).
The soil samples were collected from the different soil horizons, at depth between 5 and
75 cm. The electrical conductivity was measured by a conductivity cell, measuring the
resistance of a 1:20 soil/water suspension. The bulk density was determined weighing an
intact soil core (500 mL). The pH was determined in a 1:20 soil-water suspension. Total soil
carbon and nitrogen were measured using a THERMOQUEST NC 2005 combustion
analyzer. CaCO
3
content was determined by a Scheibler's calcimeter. The total P was
determined colorimetrically after solfo-perchloric digestion (Martin et al. 1999).
To determine ammonium and nitrate concentrations, 10 g of soil was added to 50 mL of
0.5M K
2
SO
4
and shaken for 1 h, centrifuged and filtered. NH
4
+
in 0.5M K
2
SO
4
extracts was
diffused into 0.01M H
2
SO
4
after treatment with magnesium oxide (Bremner 1965) and the
trapped NH
4
+
was determined colorimetrically (Crooke and Simpson 1971). NO
3
-
in the same
extracts was determined colorimetrically as NH
4
+
after the reduction with Devarda Alloy. The
organic nitrogen (N
org
) was determined as the difference between total N and the inorganic N
species.
For dissolved organic compounds, sub-samples (10 g fresh weight) were shaken with 100
mL 0.5M K
2
SO
4
for 1 h and the suspension filtered at 0.45 μm under suction. Total dissolved
N (TDN) in the extracts was measured as NH
4
+
after oxidation of aliquots of extracts with
alkaline persulphate and subsequent reduction with Devarda Alloy (Williams et al., 1995).
Dissolved organic N was calculated as DON = TDN - (NH
4
+
+ NO
3
-
).
Dissolved inorganic phosphorus (DIP) in the extracts was measured using the malachite
green method (Ohno and Zibilske, 1991). Total dissolved P (TDP) concentrations in the
