Conservation and Sustainable Use of the Biosphere - WBGU

Sustainable land use E 3.3
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vation generated as a result of nutrient concentration
was a consequence of far reaching degradation phenomena
among grazing and forest land.
E 3.3.4.10
Multifunctional land use
With a current population of 6 thousand million and
10–12 thousand million in the foreseeable future, the
need for biomass for food is enormous. For countless
years to come, every year thousand millions of tonnes
of plant and animal matter will have to be made
available. These masses are provided by just a few
plant and animal species, requiring large areas of cultivable
land (Sections D 3.4 and E 3.3.4.1).To survive,
people are forced to use state-of-the-art technology
in order to develop and maintain highly productive,
but also sustainable, land use. In this respect, the
degradation of soil has to be prevented and the competitor
organisms that are constantly developing
through evolution, such as weeds, pests or pathogens,
have to be kept under control.
In the past this was achieved by the selection and
breeding of high yielding and resistant varieties as
well as the use of plant protection agents and fertilizers.
But in future, ‘green’ genetic engineering will
have to be examined without any prejudice as a possible
way of solving the growing problems. The risks
that may be associated with it should be minimized
by appropriate testing procedures and controls
(WBGU, 1998a). New methods of soil tilling will
have to be found to reduce physical and chemical soil
degradation. In this process, above all the use of the
turning plough, which has been used for millennia to
prepare for sowing and combat weeds, will have to be
replaced by other implements that make the soil less
susceptible to erosion.
As shown in the previous sections, the potential
for increased production of biological resources lies
largely in increasing area productivity or in substituting
terrestrially-generated raw materials and foods.
One means of assessing the yield potential of various
sites and regions at global level is offered by the
net primary production of the natural vegetation. Net
primary production (NPP) refers to the biomass per
unit area formed in a year by the primary producers.
This is a net factor because the losses that occur
through the plant’s gas exchange are deducted from
gross production. NPP is a good indicator of the site’s
yield potential because the natural plant communities
of the ecosystems concerned have adapted to the
site conditions over very long periods and have optimized
biomass production in the process.
The NPP of natural ecosystems is not spread
evenly over the land surface; it has zones of varying
productivity. The inner tropical area, with the exception
of the Horn of Africa, is a zone of high to maximum
productivity of the terrestrial biosphere. In the
subtropical and temperate zones there are also large
regions with high and very high primary production,
but the picture here is more subtly differentiated. Fig.
E 3.3-8 shows the global distribution of NPP. These
are modelled data that represent an average of 17
global vegetation models. This procedure minimizes
the differences in the geographic and temporal allocation
of net primary production that occur with the
different types of vegetation models. In addition to
solar radiation, temperature, precipitation levels and
distribution, NPP is heavily dependent on the fertility
of the soils. Carbon is closely linked to soil’s
organic matter content (humus) and can be seen as
an indicator of the organic providers of soil fertility.
It can be seen that the fertility of the soils in the tropics
is largely based on organic matter, whereas in the
temperate zones great importance is attached both to
the mineral and the organic components. This has
consequences for the productivity of agricultural and
forestry cultures because the organic matter in soils
reacts more quickly and more intensely to human
interventions than does mineral matter. The reasons
for the marked fall in NPP after man has started cultivation
(grazing, crop growing, plantations) include
the following:
• The cultivation of monocultures or very simplified
crop rotations leads to the decoupling of biogeochemical
cycles and, thus, to nutrient depletion or
acidification.
• The removal of biomass and its limited return
accentuates the above processes.
• Soil tilling leads to humus depletion and, thus, to
the depletion of water and nutrient stores as well
as soil stabilizers.
• Infestation with pests increases with monocultures.
• Increasing homogeneity reduces beneficial organisms.
• Intensive grazing reduces diversity and the density
of plant stocks.
• The adaptation of crops to site conditions is usually
not as good as that of native plants.
• Symbioses are destroyed.
• Adapted crop plants are increasingly being
replaced by higher yielding, but less well adapted
cultivars.
• Periods without plant coverage and the thinning
out of plant stocks increase soil degradation.
• Burning vegetation litter leads to nutrient losses
and the reduction of biodiversity.
• Machine use compacts the soil and destroys its
regulatory function for the hydrological and biogeochemical
regimes.