Conservation and Sustainable Use of the Biosphere - WBGU

120 E Diversity of landscapes and ecosystems
the extent of the reaction of the cations and the
anions is known. In the formation and depletion of
organic matter, the generation of protons means
acidification, their consumption deacidification or
alkanization of soils of water bodies.
The balance of many opposing processes can be
recorded at the level of the annual biogeochemical
cycle. The fluxes into the ecosystem or ecosystem
compartments (input) and out of the system are measured
or calculated. Depending on the knowledge of
the processes and resolution with regard to the fluxes
measured and the compartments, information can be
gained about whether certain processes are in
dynamic equilibrium (input = output) or whether
they lead to linear (same annual difference between
input and output) or non-linear changes (growing
annual difference between input and output) of
material stocks in the system. In terrestrial ecosystems,
input-output differences usually reflect
changes in quality, eg in biomass, in the soil’s humus
content and in the composition of the organism communities.
If, however, the new production of biomass
and its destruction are balanced in the long term, this
is usually associated with a characteristic, site-related
species composition expressed in the plant community
and the humus formation (as an expression of
the decomposer community).
E 3.2.2
Biogeochemical fluxes in the soil
In the stationary, that is balanced, state the net
turnover of protons is also zero: the outputs of material
correspond to the inputs; the state of soil does not
change. However, it is not possible to achieve
absolute equality of input and output because a small
amount of the assimilated CO 2
is always washed out
with the leached water as hydrogen carbonate
together with Ca 2+ ions, which causes alkalinity in the
water body. This process is necessarily linked to the
dealkinization or acidification of the soils, which is
therefore a natural, very slow process. The dealkinization
can be compensated for to a certain degree
by the weathering of silicates or by inputs from the
atmosphere. However, this presupposes that silicates
are still present in the soils. In very old soils, eg in the
tropics, this is frequently no longer the case.
In this context, the soil plays a decisive role
because it is the reaction medium in which the two
major processes in terrestrial ecosystems, biomass
production and biomass decomposition (mineralization)
meet. The soil acts as a nutrient pool (weathering)
and nutrient reservoir (mineralization,
exchange) and allows a connection between nutrient
absorption and mineralization to take place under
changing environmental conditions. Biomass production
also always means an accumulation of substances
that are temporarily being removed from the
environment. But the material ‘depletions’ of the
environment take place on a very small scale, eg in
the area of the rhizosphere, and they are compensated
for by the decomposition of biomass. The biogeochemical
gradients that originate in this process
are weakened again by burrowing animals, with the
result that on average the conditions for root growth
do not change. Only thanks to this property is it possible
for ecosystems, even in climates characterized
by excess precipitation and marked seasons, not to
deplete nutrients quickly and to ensure luxuriant
plant growth.
In natural terrestrial ecosystems, which are more
or less in a state of dynamic equilibrium, the internal
nutrient cycle is largely closed. Nutrients that are
absorbed by plants return to the soil by the process of
organic decomposition. The principle of the biological
cycle presupposes that a balance is maintained
between nutrient output and input and, consequently,
also the size of the internal pool of matter.
Nutrient losses through soil erosion, leaching, evaporation
or the removal of living or dead organic matter
are considered to be insignificant under these
conditions. Above and beyond this, it is assumed that
these losses are compensated for by nutrient influxes
with precipitation, mineral weathering, biotic fixing
or nitrogen from the air and inputs of mineral or
organic matter from other systems (Fig. E 3.2-1).
Because of their different structures, terrestrial and
aquatic habitats are fundamentally different from
each other with regard to the biogeochemical cycles
that take place within them.
E 3.2.3
Biogeochemical fluxes in water bodies
Under constant conditions there may be a quasi-stable
dynamic equilibrium in inland aquatic ecosystems,
just as in terrestrial ecosystems. The main reason
for this is that the vast majority of the conversions
take place in the open water, which has a vertical
temperature stratification during the growing
season, and the biogeochemical conversions are
therefore distributed asymmetrically along the gradients.
Only in water layers near the surface is there
sufficient light for the photosynthetic production of
living organic matter (primary production). The
thickness of the productive (euphotic) layer in inland
waters fluctuates between a few centimetres and
20m; in the ocean it reaches a maximum thickness of
80–100m (Wetzel, 1983; Lalli and Parsons, 1997).As a
result of the use and subsequent remineralization of