1. Inorganic nutrients mediate the sequestering of soil C as humus Clive Kirkby CSIRO Plant Industry Charles Sturt University EH Graham Centre for Agricultural Innovation John Kirkegaard Alan Richardson Graeme Batten Chris Blanchard Len Wade  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 doesn’t necessarily build soil-C - Rumpel (2008); Soon (1998); Campbell et al. (1991)

2. Humus (passive SOM) has constant C:N:P:S ratio CNPS humus 10000 918180141 Details in Kirkby et al. 2011

3. CNPS Humus10000918180141 Wheat straw100001551514 C that can be ‘humified’ by stubble nutrients 1860750979 Crop residues are nutrient poor cf with humus maximum (possible) humification efficiency ~17%~8%~10%

4. 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  13 C-stubble to assess the dynamics of ‘new’ and ‘old’ C Hypothesis

5. 1 st expt – measuring the net humification efficiency 1 st expt – measuring the net humification efficiency Wheat stubble incubated with & without supplementary nutrients (7 cycles) Straw addition = 10t/ha/cycle

6. Microbial Biomass Straw addition = 10t/a Improving microbe food quality (not amount) has led to an increase in microbe numbers (and passive SOM?)

7. 1 st expt – measuring the net changes in humus-C, -N, -P and -S 1 st expt – measuring the net changes in humus-C, -N, -P and -S wheat stubble incubated with & without supplementary nutrients (7 cycles) Straw addition = 10t/ha/cycle

8. Where is this soil C

9. When fresh crop residues added It doesn’t stop them eating this

10. Total soil C lost or retained

12. 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

13. nutrientAmount (kg) Approx price/kg nutrientApprox Cost ($) N921.50138 P183.5063 S141.00 14 $215 The ‘hidden’ cost of building soil carbon Every 1 tonne of C-sequestered requires Australian government currently values CO2 at ~$23 / tonne ! = equates to $84 per tonne of C = equates to $84 per tonne of C Main message potential 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