Geoscience Reference
In-Depth Information
Land surface subsidence is undoubtedly the next greatest problem (after drainage)
when peat is drained for agriculture. As a result of drainage, the continuous and
dynamic process of peat subsidence is triggered. Draining the peat initially reduces
its buoyancy and leads to compaction of the organic column under its weight. This
then increases the pressure exerted by the upper peat layers (whose specific gravity has
been increased by drainage) on the peat layers beneath. As a result, all the peat layers
subside and it is not only the surface (drained) layers that are affected. Subsidence is
greatest in the uppermost peat layers because they are the most aerated and exposed
to the effects of land use (shrinkage, materialization and the mechanical pressure of
machines).
Peat subsidence is a physical process that depends on factors such as peat depth,
peat type, degree of decomposition, mineral matter content, porosity and moisture
content, and especially on drainage depth and density. Thus subsidence is a combina-
tion of shrinkage, compaction, biochemical oxidation and burning of peat materials.
Waterlogged and anaerobic peat becomes aerobic upon drainage. This leads to changes
in hydropedological parameters such as hydraulic conductivity, bulk density, pore vol-
ume and moisture content. It needs to be realized that a change in ground surface level
of a few centimetres, where the natural variation in ground level is less than one metre,
can have dramatic hydrological effects.
In the case of tropical peat, subsidence of 0.60m has been recorded for a drained
deep peat (water level of 0.75-1.0m) (Melling and Hatano, 2003). Subsidence of
1.0-1.5m can be expected within the first three years of an agricultural development
(Melling, 2000). This is especially so in the Alan forest areas because of the widespread
existence of a vacant layer (Melling, 2000) and the high porosity of the peat. The
heterogeneous nature of peat also causes differential rates of subsidence upon drainage
making the peat surface quite hummocky. The micro-relief variation can be as much as
0.5m. This also complicates the estimation of peat subsidence rate. The height loss of
the peat resulting from agricultural development will lead to a complete transformation
of the peat land landscape.
Peat subsidence has several serious consequences. Drainage needs to be regularly
adapted to new levels and conditions, otherwise inundation and flooding will recur.
The rooting systems, particularly of perennial species, become exposed, and top-heavy
crops such as oil palms start to lean over and are partly uprooted. Roads and other
structures become unstable. Annual subsidence rates of cultivated peat are a function of
the degree of aeration and temperature (McAfee, 1989). Subsidence rates are greatest
in areas nearest the main drains and decrease with distance from the drain.
Oxidation refers to microbial decomposition of the soil material, ultimately con-
verting it to carbon dioxide and water. The entry of air into the soil causes organic
matter to be oxidized at a more or less constant rate, through biochemical processes
under the influence of microorganisms. A microorganism that oxidizes the peat and
cause the most serious form of subsidence requires oxygen from the soil air. However,
the soil profile below the water table contains essentially no air or dissolved oxygen,
so the microorganisms are unable to function, and the soil is protected from this type
of subsidence.
Peat subsidence due to consolidation and oxidation leads to an increase in peat bulk
density. Consolidation does not involve a loss of soil material compared to oxidation.
The bulk density is closely associated with the hydrological properties of the peat
Search WWH ::




Custom Search