Important soil ecosystem services and
functions underlying ISFM
Carbon transformations:
The decomposition of organic materials
into simpler molecules is one of the most
important ecosystem services performed by
soil organisms. Decomposition is also defined
as the mineralization of carbon; 90% is carried
out by microorganisms such as bacteria and
fungi greatly facilitated by soil meso and
macrofauna that fragment residues and
disperse microbial propagules.
Nutrient cycling:
The cycling of nutrients is a critical
ecosystem function that has positive direct
impacts (through plant-microbial symbiotic
relationships) on crop yield due to increases
in plant available nutrients, especially
nitrogen (N) through biological nitrogen
fixation (BNF) by soil bacteria (e.g. Rhizobium)
and phosphorus (P) through arbuscular
mycorrhizal fungi (AMF). As said above, it
literally means that these microorganisms
release nutrients to the roots in exchange for
carbon to feed on, either from root exudates,
or from plant/root organic material. Increasing
populations of bacteria and fungi provides
more food for protozoa (feeding on bacteria)
and nematodes (feeding on bacteria, fungi,
protozoa, other nematodes and roots) and
their waste (manure) is directly available to
plants as nutrients.
On a global scale, BNF accounts for around
65% of the nitrogen used by crops and
pastures. There is scope for considerable
increase. The supply of nitrogen is
inexhaustible, as nitrogen comprises almost
80% of the earth’s atmosphere. While
estimates of symbiotic BNF can be as high as
400 kg N ha −1 yr −1 , average BNF is about 10-fold
lower. Growing legume rotational and cover
crops adds biologically fixed N.
Most soils in South Africa contain low amounts
of soluble phosphorus due to the parent
material and/or P being fixed in acid soils.
However, if levels of AMF colonisation are high,
there will be no need to add large quantities of
inorganic P, or in some cases, none.
The additional plant diversity and growth
period obtained with cover crops promotes
root proliferation and activity, stimulates a
greater variety of soil microorganisms and
enhances carbon and nutrient cycling. The
soil surface is covered for a longer period
during the year, so nutrient losses from runoff
and erosion are reduced. This longer period
of plant growth substantially increases the
amount of plant biomass produced, which
in turn increases organic matter additions to
the soil. It also traps excess soluble nutrients
not used by the previous crop, prevents them
from leaching, and stores (recycles) them for
release during the next growing season.
Photo 4: Permanent organic soil cover pro-
vides a favourable habitat for many organ-
isms
AgriKultuur |AgriCulture
Nutrients provided by CA and enhanced
ecosystem services
Table 1 illustrates the amount of nutrients
potentially available to the next crop through
CA and various soil ecosystem processes –
only N, P and K are included. As an example,
an average Dry Matter (crop residue) quantity
of 12 t/ha (typically produced by a mixed
summer cover crop) was used, a soil depth of
10cm, a SOM of 2.5% and soil bulk density of
1.3 g/cm 3 .
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