Geoscience Reference
In-Depth Information
through their mound-building activities (
Fig. 53
). The scale of these effects can be
illustrated by estimating the depth to which the ground surface would be covered by
the soil in the ant-hills if the mounds were evenly spread. This depth reaches a max-
imum of 20 millimetres on the oldest undisturbed grassland, but this underestimates
the overall effect since it ignores reworked soil below the present ground surface, and
also the past product of abandoned and decayed mounds. However, it shows the scale
of physical modification achieved by a small creature when present in large numbers.
To summarize the picture at Porton, the present substantial variation in chemistry
within soils of a single class is a direct consequence of ecological succession and bio-
logical activity acting over varying lengths of time. It is not the result of distinct par-
ent materials, and hence different soil classes, within a site.
In the United States, the entire A horizon of some virgin Brown Podzolic Soils
consists of material brought to the surface from the B horizon by ants. This materi-
al reaches thicknesses of 30-40 centimetres which have accumulated at an estimated
rate of one centimetre per century. It is thus not only the larger earthworms that can
stratify soils and develop clear profile characteristics. Of course, if we were not con-
centrating on British habitats, then the effects of termites are even more conspicuous.
They can build mounds up to 2-3 metres high and 6-10 metres across in long-estab-
lished locations.
Our consideration of soils and the role of organisms in semi-natural soils began
with a look at the detail of humus profile formation in a woodland situation. Similarly,
for grasslands, earlier work by B. Barratt produced a 'classification of humus forms'
for grassland soils from a wide range of English and New Zealand sites. She used thin
sections to identify and illustrate the relative frequency of fungal hyphae and resting
bodies, faunal droppings and partially eaten leaf fragments and roots within the differ-
ent humus forms. In most grassland soils with mull humus and high base status (pH
above 5.5), there is no conspicuous and discrete humus layer. Instead, a thin, rapidly
disappearing litter layer passes into a mineral horizon intensively worked by earth-
worms. On somewhat more acid Brown Earths (pH 4.5-5.0), an H layer is present
with enchytraeid worms more prominent in the soil fauna. Moder humus profiles in
grasslands that are intermediate in profile character between Brown Earths and Brown
Podzoloic Soils typically have distinct L, F and H layers as in the Dutch woodland
soils. The well humified H horizon is due to microbiological activity and the action
of mites and enchytraeids, rather than earthworms. At the acidic end of this spectrum
of humus types on grassy moorland, the surface horizons are mor humus, with mites
and springtails conspicuous in the soil fauna.
