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
 from.

 Regards, 

John Passioura".  

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.