Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

forest, and this indicates that they are affected by oligotrophic waters. The difference in pH and
nutrient status between glades and forest decreases with decreasing altitude. The water originating
from high altitude is naturally oligotrophic and it progressively charges with cations downstream.
Alongside this simple model, we must consider in this case other mechanisms resulting in water
oligotrophy, such as podzolization and drainage from acidic badrocks.
In circumstance where SOM turnover rates are reduced, such as anaerobic condition due to
water logging, low temperature due to high altitude, extremely acidic parent materials or production
of low quality OM (such as litter from coniferous trees, Erica spp., etc.), the net accumulation of
organic acids will lead to podzolization even in the tropics (FAO 2001). Hydromorphic podzols are
caused by waterlogging and have predominant lateral spread, with groundwater enriched in OM
travelling long distances (FAO 2001) and eventually emerges at lower altitudes or in depressions
in the form of black rivers (such as the Rio Negro in S. America) and lakes. pH levels are a good
indication of a cause for podzolization, favoured at pH5
(Shoji et al. 1993).
In the toposequence studied, looking at pH values, we could predict that podzolization would
occur above S-1850 and ferralitization downstream. Evidence of ferralitization is found in the high
chroma of the soil in the landscape surrounding Delo-Mena (South of the studied toposequence),
indicating enrichment of sesquioxides in the topsoil. In accordance to what described by Chauvel
et al. (1996) for the Rio Negro basin, if podzolization occurs at higher altitudes, it could generate a
deep flow of organic acids and Al that would extend into the ferralitic zone downstream; this flow
will stop in the soil at saturation or will emerge in depressions (black lakes) or streams (black rivers)
at lower altitudes (Fig. 3).
Figure 3. Sketch diagram (not in scale) summarizing the elements supporting the theory of a deep
“black water” flow, and integrating them in a simplified model of the escarpment.
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 140