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30 cm soil frost
DOC (mg l -1 )
No soil frost
DOC (mg l -1 )
0
50
100
150
200
0
50
100
150
200
-10
-10
-30
-30
April 1
April 1
-50
-50
May 1
May 1
June 1
-70
-70
June 1
July 1
July 1
-90
-90
Figure 3.14 Concentration profiles of DOC in the soil during winter, spring and early
summer (April-July) at five soil depths in plots without and with deep soil frost.
The quality and quantity of DOC in the soil frost experiment during the
winter seemed to be controlled by lysis of cell structures and by limiting the soil
microbial activity. While freeze-out processes are believed mainly to control the
concentration of DOC in soil solution below the expanding ice during freezing,
lysis of cells may release highly bioavailable organic compounds of low molecular
weight and low C/N ratio (Stepanauskas et al . 2000). Temperatures below
freezing are unfavourable for heterotrophic microbial activity. This may result in
undecomposed organic material of high substrate quality that subsequently will
be decomposed during unfrozen conditions. As a consequence, the soil frost
experiment has shown that the thermal conditions in the soil ecosystem influence
the soil organic matter decomposition rate and CO 2 production (Ă–quist &
Laudon 2008).
The effect of summer droughts on soil solution DOC - the Clocaenog
experiment (the United Kingdom)
In many areas of Europe, climate change is projected to lead to increased
frequency and severity of summer droughts. At Clocaenog, a heathland site in
North Wales, the United Kingdom, repeated summer droughts have been
experimentally induced each year since 1999, initially as part of the CLIMOOR
(Climate Driven Changes in the Functioning of Heath and Moorland Ecosystems)
and VULCAN (Vulnerability assessment of shrubland ecosystems in Europe
under climatic changes) projects. A retractable transparent roof system was used
to reduce summer rainfall by around 60%, for a set of replicated 20 m 2 plots
(Beier et al . 2004).
Droughts were found to reduce rates of soil respiration, as biological activity
became moisture limited. Measurements of soil solution DOC (Fig. 3.15) suggest
that DOC production is similarly affected; in the control plots, DOC concentrations
consistently increase in summer, but in the drought plots concentrations can fall
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