Science Forums 2

[Michaelangelica]

The activity of soil organisms can be divided into four functions:
1. Regulation of OM turnover & nutrient cycling,
2. Biological degradation
3. Maintenance of soil structure, and
4. Interaction with plants.
1a. Organic Matter (OM) Turnover:
• Carbon is a core element of OM and a vital energy source for soil biota.
• By decomposing OM the soil biota gain access to this carbon.
• Microbial biomass, the population of micro-organisms, acts as the engine for
OM turnover and nutrient release.
• Soils with high levels of OM support a greater number and a more diverse
range of biota.
• Where OM energy is plentiful, crop residue decomposition and OM
accumulation will occur.
• Specific organisms breakdown different types of OM. e.g. cellulolytic micro-
organisms only decompose cellulose and not lignin.
• The rate of OM breakdown relates to the soil environment, the number and
type of organisms present and the chemical structure of the plant residues.
Breakdown may occur in months or several thousand years.
1b. Transformation of Nutrients:
• The conversion of OM, by soil organisms, to available nutrients is called
mineralisation. This process is a key element of soil fertility.
• Whilst decomposing OM to obtain carbon, other nutrients are released.
These may be: soluble and leached (e.g. nitrate [NO3]), volatile and lost to
the atmosphere (e.g. nitrogen as N2 & N2O, sulphur as H2S) or readily
available to the plant (e.g. nitrates, phosphates and sulphates).
• In order to increase the up-take of a specific nutrient, many plants form
mutual relationships (symbioses) with soil micro-organisms. Examples of
symbiotic relationships include: legumes with the bacteria Rhizobium
species to fix atmospheric nitrogen gas, and most crops with mycorrhizal
fungi to absorb phosphorus and other nutrients from the soil environment.
• Mycorrhiza have been found to improve plant uptake of phosphorus. This is
thought to be due to the vast ‘collection structure’ provided by the hyphal
network of fungi.
. . .

[url=http://72.14.253.104/search?q=cache:AYNOHBwvwTYJ:www.csiro.au/files/files/pcz9.pdf+CSIRO+no+till+farming+soil+microorganisms &hl=en&ct=clnk&cd=4&gl=au&client=firefox-a]The

The relationship between agricultureand soil organisms

Tillage:
Cultivation alters the physical, chemical and biological components of the soil
system. No-till, direct-drill systems result in significant differences in soil
organism activity compared to conventional deeper tillage.
No tillage:
• OM levels are high and micro-organisms become concentrated at the soil
surface.
• Residue decomposition and nutrient mineralisation is slower.
• Fungal hyphae are more prolific in the top 5cm of soil. This is beneficial in
terms of desirable fungi such as mycorrhizae but negative in relation to
pathogenic fungi such as Rhizoctonia.
• Fungal feeding nematodes, protozoa and macro fauna increase.
• 10 - 100 times more fungal feeding protozoa were counted under no-till and
stubble retention treatments. These may provide controls for pathogenic
fungi.
• Narrow points on cultivators used at seeding result in soil disturbance below
the seed. This in combination with a three week chemical fallow, prior to
seeding, reduces the severity of the pathogenic fungus Rhizoctonia .
• Deep burrowing earthworms in direct drilled plots improve soil structure
assisting root growth and increasing the yield of annual crops.

Conventional Tillage:
• This favours organisms with short generation times, rapid dispersal and high
metabolic rates. Bacteria and bactivorous fauna are dominant in cultivated
soils.
• Fungal hyphae are broken by cultivation and therefore reduced.
• Organisms are distributed more deeply into the ploughed layer .
• Residue decomposition and nutrient mineralisation is more rapid due to
better soil-stubble contact.
• The rapid activity results in a higher level of breakdown and a lower level of
OM accumulation.
• Earthworm populations significantly decrease.
• Increased cultivation has been shown to reduce the number of root lesion
nematodes, but no effect on cereal yield has been recorded.
influence soil biota activity?
Stubble incorporation favours bacteria and bactivorous fauna whereas stubble left on soil surface supports more fungi and fungivorous fauna.
. . .

http://72.14.253.104/search?q=cache:AYNOHBwvwTYJ:www.csiro.au/files/files/pcz9.pdf+CSIRO+no+till+farming+soil+microorganisms &hl=en&ct=clnk&cd=4&gl=au&client=firefox-a