Download Organic matter and biological activity

Organic matter and
biological activity
Soil organisms
use the plant residues and organic matter
as food
any excess nutrients (nitrogen,
phosphorus and sulphur) are released into
the soil in forms that plants can use
their waste products are called soil
organic matter
Soil cover
Organic materials above and on the
surface of the soil can provide physical
‘buffering’ against raindrop impact and
direct insolation
Aggregate stability
Sticky substances on the skin of
earthworms and those produced by fungi
and bacteria help bind particles together
Earthworm casts are also more strongly
aggregated (bound together) than the
surrounding soil from the mixing of
organic matter and soil mineral material,
as well as the intestinal mucus of the
worm.
Guidance of rainwater
decomposition of dead roots provides
downward-penetrating channels, through
which rainwater can quickly reach lower
levels of the root-zone.
meso-organisms such as worms and
termites create burrows with the same
result.
Decomposition of organic
matter is a biological process
It depends on:
• soil organisms,
• the physical environment (oxygen,
moisture and temperature) and
• the quality of the organic matter.
Soil food web
The organisms and the interactions
among organisms
The energy needed for all food webs is
generated by primary producers: the
plants, lichens, moss, photosynthetic
bacteria and algae
Most other organisms depend on the
primary producers for their energy and
nutrients: they are called consumers
The living part of soil
organic matter includes:
microorganisms such as bacteria, fungi,
protozoa, nematodes, viruses and algae.
macroorganisms include vertebrates, such as
moles, and invertebrates. The latter are
those organisms that lack a backbone and
rely on an external covering. This group of
organisms includes arthropods, ranging
from mites to larger beetles, millipedes and
termites, and earthworms, slugs and snails.
Decomposition
the physical breakdown and biochemical
transformation of complex organic
molecules of dead material into simpler
organic and inorganic molecules
These carbon chains, with varying amounts
of attached oxygen, hydrogen, nitrogen,
phosphorus, and sulphur, are the basis for
both simple sugars, amino acids and plant
nutrients.
Humus
Successive decomposition of dead material
and modified organic matter results in the
formation of humus.
Humus affects the soil properties, as it
colours the soil darker; increases soil
aggregation and aggregate stability;
increases the cation exchange capacity; and
contributes nitrogen, phosphorus, and other
nutrients as it slowly decomposes.
Humus vs soil structure
Without humus, soils with high lime or clay
content would compact easily when
worked.
Polysaccharides are the actual substances
that glue the soil particles together.
The more resistant soil organic matter
(humic
humic acids) hold together the
microaggregates while fulvic acids bond the
microaggregates into macroaggregates
Earthworms
The burrowing activity provides channels
for air entrance and passage of water, which
has an important effect on oxygen diffusion
in the root zone and drainage.
Shallow-dwelling earthworms create
numerous channels throughout the topsoil.
The large vertical channels greatly increase
water infiltration under intense rainfall or
waterlogged conditions.
Humus consists of
different humic
substances:
Fulvic acids
Humic acids
Humins
Humic and fulvic substances enhance plant
growth directly through physiological and
nutritional effects. Some of these substances
function as natural plant hormones (auxines
and gibberillins) and are capable of
improving seed germination, root initiation,
uptake of plant nutrients and can serve as
sources of nitrogen, phosphorus and
sulphur.
Non-humic, organic
molecules
This active fraction of soil organic matter is
the main supply of food for various
organisms living in the soil. The active
fraction is strongly influenced by weather
conditions, moisture status of the soil,
growth stage of the vegetation, addition of
organic residues, and cultural practices, like
tillage.
Effect of CA
The greater production of foliage in a
system with cover crops and reduced or zero
tillage compared to monocrop cultures with
conventional tillage, leaves a protective
blanket of leaves, stems and stalks from the
previous crops on the surface. In this way
organic matter can be built up on the soil
surface, which creates favourable conditions
for the activity and the population
development of the microorganisms.
OM increase
Organic matter content (%)
Soil depth (cm)
0
0.5
1
1.5
2
0-3
3-13
13-26
Direct seeding
26-52
Minimum tillage
Conv entional tillage
Microbial biomass under
conventional tillage (CT)
and direct seeding (DS)
600
Conventional tillage
500
-1
Microbial biomass (ug C-CO2 g soil)
Direct seeding
400
300
200
100
0
Fallow
Hairy v etch
Lupin
Lathy rus
Wheat
Ry e
Italian
ry egrass
Oats
Oil radish
Population size Bradyrhizobium (# cells *100)
Root nodule bacteria
60
50
40
30
20
10
0
S/W/M
S/W
M/W
Conventional tillage
S/W/M
S/W
M/W
Conservation agriculture
Average root colonization (% )
Infestation of crop roots
with Mycorrhizal fungi
70
Maize
60
Soy a
50
40
30
20
10
0
Natural vegetation
Conv entional
Conservation
Conservation
tillage (1 y ear)
agriculture (10
agriculture (20
y ears)
y ears)
Number of earthworm
burrows
Number of earthw orm burrow s per m 3
1200
Conventional tillage
Conservation agriculture
1000
800
600
400
200
0
10
20
30
Soil depth (cm)
40
50