I was amazed to read in this month's Scientific American that converting 2% of the world's agricultural land to 'no till' cultivation would halt the rise in CO2 levels.
Furthermore, converting all of it would reduce CO2 to pre-industrial levels!
Corn, wheat, maize etc. are forms of grass and are perenniels but mankind decided several thousand years ago that replanting annually leads to improved yields. The 'no till' movement is currently underway and has numerous advantages: higher soil organic carbon, better water retention, lower fertiliser use, less energy intensive and, most importantly, lower soil erosion but offset by lower yields.
The claim is made by Prof. Douglas Kell of the Bioanalytical Sciences Group of Manchester University. He is also CE of the BBSRC - Biotechnical and Biological Science Research Council. He has produced a calculator which can be found here.
This claim may not hold up in the UK because of climate factors. See Chapter 7 of the Fourth Carbon Budget from the Committee on Climate Change which can be found here.
I have not read all this literature yet but any comments, particularly by farmers, would be most welcome. BTW, the ploughing up of grasslands has been one of the major contributors to increased CO2 levels, Wiltshire please note.