326
Isabel Serrasolses and V. Ramon Vallejo
clearcut during the first year after the fire, but the differences were reduced
with time and disappeared in autumn 1989. The mineral nitrogen exported
from the forest floor layers was higher in the burned plot (35 kg N ha- I year-I)
than in the clearcut and control plots (16 kg N ha- I year-I). Under the 5-cm
mineral soil the differences between plots were not significant and the losses
were around 18 kg N ha- I year-I (Fig. 22.5).
From the data obtained in zero-tension lysimeters in the studied plots
(Poch 1992; Gazquez 1994), the burned plot lost more nitrate than the control and clearcut plots. The increase in nitrogen availability in the soil solution of the burned plot exceeded the plant uptake, leading to leaching from
the soil.
22.7 Overall Effects of Clear-Cutting and Slash-Burning
on Soil Fertility
Slash-laying-down on the forest floor contributed to an increase in the content of nutrients in the forest floor, especially in the L layer during the first
years after clear-cutting. This treatment increased N availability in the year
and a half following the disturbance, producing a net loss of nitrate from the
forest floor.
The effects of fire were dramatically expressed in the direct volatilization
of a major proportion of the labile N pool in the ecosystem. In the post-fire
period, the residual forest floor layers suffered a reorganization by means of
the progressive incorporation of ash into the H layer and upper mineral soil,
increasing the spatial heterogeneity in the burned plot. The fire led to high
nutrient availability during the first post-fire year. This availability was in excess to plant uptake, and resulted in leaching losses for most of the basic
cations and nitrate. The heavy rainfalls 1 year after burning led to detectable
losses of soluble P and mineral N. Later on, the burned plot showed lower
contents of these nutrients in the remaining H layer. However, the amount of
nutrient loss by erosion and leaching was not relevant with respect to the
ecosystem pools.
The disturbances studied, clear-cutting and slash-burning, increased soil
fertility in the short term. In the medium term, the nutrient content returned
to the undisturbed levels by means of different mechanisms (immobilization,
leaching). Clear-cutting and slash-laying-down presented lower leaching and
erosion losses than were found with slash-burning.
Isabel Serrasolses and V. Ramon Vallejo
clearcut during the first year after the fire, but the differences were reduced
with time and disappeared in autumn 1989. The mineral nitrogen exported
from the forest floor layers was higher in the burned plot (35 kg N ha- I year-I)
than in the clearcut and control plots (16 kg N ha- I year-I). Under the 5-cm
mineral soil the differences between plots were not significant and the losses
were around 18 kg N ha- I year-I (Fig. 22.5).
From the data obtained in zero-tension lysimeters in the studied plots
(Poch 1992; Gazquez 1994), the burned plot lost more nitrate than the control and clearcut plots. The increase in nitrogen availability in the soil solution of the burned plot exceeded the plant uptake, leading to leaching from
the soil.
22.7 Overall Effects of Clear-Cutting and Slash-Burning
on Soil Fertility
Slash-laying-down on the forest floor contributed to an increase in the content of nutrients in the forest floor, especially in the L layer during the first
years after clear-cutting. This treatment increased N availability in the year
and a half following the disturbance, producing a net loss of nitrate from the
forest floor.
The effects of fire were dramatically expressed in the direct volatilization
of a major proportion of the labile N pool in the ecosystem. In the post-fire
period, the residual forest floor layers suffered a reorganization by means of
the progressive incorporation of ash into the H layer and upper mineral soil,
increasing the spatial heterogeneity in the burned plot. The fire led to high
nutrient availability during the first post-fire year. This availability was in excess to plant uptake, and resulted in leaching losses for most of the basic
cations and nitrate. The heavy rainfalls 1 year after burning led to detectable
losses of soluble P and mineral N. Later on, the burned plot showed lower
contents of these nutrients in the remaining H layer. However, the amount of
nutrient loss by erosion and leaching was not relevant with respect to the
ecosystem pools.
The disturbances studied, clear-cutting and slash-burning, increased soil
fertility in the short term. In the medium term, the nutrient content returned
to the undisturbed levels by means of different mechanisms (immobilization,
leaching). Clear-cutting and slash-laying-down presented lower leaching and
erosion losses than were found with slash-burning.