growers@sgcotton.com.au Roger Tomkins - Greenmount Press

Nutrition
feature…
control (no stubble) treatments were included. The incorporated
treatments and a control were tilled weekly prior to all pots being
irrigated to near field capacity.
Results
Field study (Experiment 1)
SOC increased in the 0–30 and 30–60 cm depths of soil, but
no significant change was apparent below 60 cm (Figure 1).
Between 0 and 60 cm depth, SOC increased by about 1.24 tonnes
C per hectare per year (or 4.4 tonnes of CO equivalent (eCO ) per
2 2
hectare per year). The increase in SOC was much greater in the
30–60 cm depth in the soil profile.
Controlled environment study (Experiment 2)
SOC was slightly lower where the soil was tilled compared
with no tillage (Figure 2) – but the three treatments were not
statistically different. Had the tillage operations been more
vigorous, deeper or more often, or the experiment continued for
a longer period, the loss of SOC may have been greater.
At the end of the experiment, only 22 per cent of the
stubble-C applied to the soil surface was retained by the soil,
compared with incorporating the stubble, where 54 per cent of
the stubble-C was retained.
Also, of high importance is the effect of stubble addition on
soil micro-organisms which use the carbon from the stubble as
their energy source. The soil microbes that can fix nitrogen from
the atmosphere (outside of the legume/rhizobia association) do
so using the energy released as they decompose crop stubble.
Incorporated stubble supported a greater amount of freeliving
N fixation compared with the surface stubble treatments.
About 20 per cent of the stubble-N was lost where the stubble
was applied to the soil surface stubble treatments, but total soil
N increased by more than 50 per cent where the stubble was
incorporated.
Over three drying cycles, water loss was 10 per cent lower in
the surface-applied stubble treatment, compared with the stubble
incorporation treatments, as the soil was disturbed. Water loss
averaged 2.5 and 2.7 mm/day for the respective treatments.
Discussion
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Significant increases in SOC can occur where management
practices encourage retention of the stubble-C added to the soil,
FIGuRE 1: SOC at two profile depths
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32 — The Australian Cottongrower August–September 2012