One of the biggest puzzles about soil carbon has been solved. In 2008, 5 scientists published a short paper called "The Hidden Cost of Carbon Sequestration"*. Many people thought it shot a big hole in any prospect of Soil Carbon trading. Effectively, it made the claim that a farmer could not afford to increase carbon levels in their soils because humus ties up nitrogen and other nutrients needed by plants to grow. The farmer would have to buy extra fertiliser to replace that stolen by the humus and it would cost more to do that than soil carbon trading would pay.
The lead author told me that, based on his paper's argument, the increases in soil C achieved by leading carbon farmers were doubtful. "
I am aware of Colin Seis's remarkable achievements, and I have wondered
how he has succeeded in increasing soil organic matter in the topsoil by
2%. If that increase is largely humus, then it is likely to contain, in
organically bound form, about 2 tonne/ha of N, 400 kg/ha of P and 300
kg/ha of S. I puzzle about where such large amounts could have come
Well, now science has solved the puzzle. Free-living nitrogen-fixing bacteria are supplying 75% of the N a 2t/ha cereal wheat paddock in the Mallee uses, according to the Victorian DPI. A 12-year trial found bacteria are delivering 35kg/ha each year. In an intensive cropping regime the organic carbon level rose from 0.80% to 1% between 1997 and 2011. Cropping is usually a carbon-exporting activity. The CSIRO's Dr Margaret Roper has published a review of literature that estimates that the theoretical potential of the contribution of these bacteria is up to 150kgN/ha.
The DPI's Ron Sonogan reported the Mallee trials: "Assuming a 0.2% increase in OC each year, this may well have added another 120kg/ha of nitrogen to the system over 14 years." The widespread shift to no-till and stubble-retention over the last 20 years has increased the carbon inputs which are a key driver for bacterial N2 fixation. Estimates of fixation were set more than 20 years ago and are therefore in need of up-dating, say the scientists. Australian Farm Journal reported the findings earlier this year, proving that the nutrients incorporated in humus don't have to come out of a bag.
*GRDC Groundcover Magazine Issue 76, p.19 (2008)