Abstract
The purpose of this experiment was to understand and evaluate the effects of
season and vegetation on annual nitrogen cycling in a high latitude birch forest
ecosystems. A solution of NH4Cl was added to the soil, plants and microorganisms in
early October and then measured at intervals throughout the winter, spring and summer
of the following year. Direct influences of vegetation on ecosystem nitrogen cycling were
investigated. Results indicated that intact plants had little effect on seasonal patterns of
microorganism growth or nitrogen accumulation in the soil.
Evergreen and deciduous species differed in nitrogen uptake capacity and
contributed to the ecosystems productivity differently. Vascular plants uptake of nitrogen
and allocation in roots was at the onset of winter however came to an end once snow
covered the ground. Plant nitrogen uptake was restricted to the snow free season. There
was a large release of nitrogen, recently acquired from microorganisms, during the spring
when the snow cover melted. During the summer months plant uptake and distribution of
nitrogen was to above ground tissues. Findings indicated that seasons exert primary
control over annual nitrogen cycling in sub artic birch forests.
Introduction
In high latitude ecosystems plant growth is limited by nitrogen availability.
Seasons exert strong control over element cycling in the sub artic ecosystems.
Considerable biological activity takes place beneath the snow cover in winter.
Microorganisms decompose plant litter under the snow cover and nitrogen accumulates
in the soil. During the spring thaw period, nitrogen flushes in these high latitude
ecosystems resulting in nitrogen loss. Winter time nitrogen accumulation and plant
species nutrient uptake capacity play an important role on annual patterns of nitrogen
cycling in sub artic birch forests.
Hypothesis:
(I)
Seasons affect annual nitrogen cycling and nitrogen availability to plants for
growth and production
(II)
The presence of plants influence patterns of annual microorganism nitrogen
cycling in sub artic birch forests
(III)
Differences between evergreen and deciduous species influence nitrogen
uptake capacity during fall and spring in sub artic high latitude ecosystems
The experiment was carried out in Northern Sweden where the snow cover season
begins in early October to late May. Average range in temperatures between summer
and winter months was 10 C to –9 C. The experimental design was six areas of
similar vegetation all within two metres of a birth tree. Two plots were selected
within each area (control plot and treatment plot).
Hypothesis I
Do seasons affect annual nitrogen cycling and nitrogen availability to plants for
growth and production? Two areas were used to test this hypothesis, plots with plants
species and plant with plants removed. A solution of NH4Cl was added to the centre of
each treatment plot in early October and then measured at intervals until July of the
subsequent year. The reason to use NH4Cl was to determine any period with rapid
nitrogen turnover within the microorganism pool.
Test results indicated at the beginning of winter there was substantial nitrogen
uptake in vascular plant roots. During the winter the snow cover provides a strong
insulating effect on the ground and plant nitrogen uptake ceases. Microbial biomass
nitrogen (MBN) pools were not significantly affected by seasons suggesting a switch
from mostly fungal soil microorganisms in winter to mostly bacterial soil
microorganisms in summer. During winter nitrogen mineralizes, changing form from
organic to inorganic by microorganisms under the snow. This resulted in the soils
ammonium (NH4 ) concentration to increase throughout the winter. During late spring
snow melts and micro organism’s release nitrogen into the soil, a rapid rise in plant
nitrogen uptake during the spring snowmelt suggests plants can take up released nitrogen.
Late spring and early winter have strong controls on annual nitrogen cycling in high
latitude ecosystems, affecting the availability of nitrogen for plant growth.
Dissolved total nitrogen (DTN) was greatly influenced by seasons as well as plant
removal in treatment plots. DTN increased gradually in the control plots through the
winter until May, where it reduced dramatically until July. Plant removal treatment plots
had increasing DTN for all months except May. When the snow melted and nitrogen was
released into the soil, nitrogen in treatment plot (plant removal plot) was not taken in by
plants and dissipated by leaching. By understanding the controls of the seasonal nitrogen
cycle and nitrogen availability to plants for growth and production help predict
ecosystems response to disturbances.
Hypothesis II
Test whether vegetation influence patterns of annual microorganism nitrogen
cycling in sub artic birch forests. Similar procedure was followed for testing effects of
vegetation on nitrogen availability to plants for growth and productivity as was used in
Hypothesis I. Control plot plants were present and treatment plot plants removed
(including roots). A solution of NH4Cl was added to the centre of the treatment plot in
early October and then measured at intervals until July of the subsequent year. Results
indicate annual patterns of microorganism nitrogen cycling are independent of plant
influences. There were no significant changes in microbial biomass nitrogen (MBN)
pools in control and treatment plots or between sampling months.
Plots with no plants had slightly warmer soils during the summer, however during
the winter plant removal plots had no effect on soil temperature. The organic layer of soil
had a wide range of moisture recordings for both the control and treatment plots. Soil
became increasingly wet through the winter, adding to the moisture from melting snow
during spring promoted downward leaching of nitrogen. Fast drying after spring thaw
with minimal moisture levels in mid-July. Presence of plants had little impact on soil
microbial growth, nitrogen accumulation or microbial nitrogen turnover, therefore likely
determined by seasonal environmental factors not plant association.
Hypothesis III
Do differences between evergreen and deciduous trees influence nitrogen uptake
in spring and fall in sub artic high latitude ecosystems? Two areas were used to test this
hypothesis, plots with dominate evergreen vegetation and plots with dominate deciduous
vegetation. The same sampling procedure was used from the previous two hypothesis. A
solution of NH4Cl was added to the centre of each treatment plot in early October and
then measured at intervals until July of the subsequent year.