Inorganic nutrients mediate the

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• There is much current interest in the
  sequestration of soil C
• - improve soil fertility, C-farming, C-tax,
  mitigate climate change
• However, there are many examples that show that
  retaining
• crop residues doesnt necessarily build
  soil-C
• - Rumpel (2008) Soon (1998) Campbell et al.
  (1991)

Inorganic nutrients mediate the sequestering of
soil C as humus Clive Kirkby

John Kirkegaard Alan Richardson Graeme
Batten Chris Blanchard
Len Wade
CSIRO Plant Industry Charles Sturt University EH
Graham Centre for Agricultural Innovation
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Humus (passive SOM) has constant CNPS ratio
C N P S
humus 10000 918 180 141
Details in Kirkby et al. 2011
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Crop residues are nutrient poor cf with humus
C N P S
Humus 10000 918 180 141

Wheat straw 10000 155 15 14

C that can be humified by stubble nutrients C that can be humified by stubble nutrients 1860 750 979
maximum (possible) humification efficiency 17 8 10
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Hypothesis
The conversion of crops residues to humus may be
limited by available nutrients in soil (nutrient
availability could alter the humification
efficiency, HE)

• Tested with two laboratory incubation studies
• (and a field trial not discussed today)
• lab incubations
• soil stubble with/without nutrients
• 13C-stubble to assess the dynamics of new and
  old C

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1st expt measuring the net humification
efficiency Wheat stubble incubated with
without supplementary nutrients (7 cycles)
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Microbial Biomass
Improving microbe food quality (not amount) has
led to an increase in microbe numbers (and
passive SOM?)
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1st expt measuring the net changes in humus-C,
-N, -P and -S wheat stubble incubated with
without supplementary nutrients (7 cycles)
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Where is this soil C
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When fresh crop residues added
It doesnt stop them eating this
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Total soil C lost or retained
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Total soil C lost or retained
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Total soil C lost or retained
Nutrient addition always increased the new C
retained Nutrient addition always increased the
net C level Nutrient addition had a varied
effect on the old C lost
Just as important to understand why we still lose
old humus as it is how to make new humus
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Main message Given that the N, P and S must
remain sequestered for as long as the C is
sequestered the potential value of these
nutrients must be taken into account
The hidden cost of building soil carbon
Every 1 tonne of C-sequestered requires
nutrient Amount (kg) Approx price/kg nutrient Approx Cost ()
N 92 1.50 138
P 18 3.50 63
S 14 1.00 14
215
Australian government currently values CO2 at
23 / tonne ! equates to 84 per tonne
of C