No SOC loss.

The effect of no-till conversion of land that had been in smooth bromegrass for 13 years to no-till corn production on soil organic carbon in eastern Nebraska was observed for 6 years by USDA scientists.

The bromegrass was killed with herbicides in late fall of 1998 and corn was planted directly into the killed sod in the spring of 1999. No plowing or cultivation was conducted during the entire period of the study. Weeds were controlled with herbicides. Soil samples were collected at three different depths at the beginning and several times during the study and were analyzed for soil organic carbon. Carbon isotope ratio analyses made it possible to determine the amount of soil organic carbon that originated from bromegrass or corn. . .

During the 6 years of the study, the origin of the soil carbon in the two upper soil layers (0- to 5-, and 5- to 10-cm depths) changed with the soil carbon from bromegrass gradually replaced by that from corn. Total soil organic carbon, however, did not change significantly at any depth during the 6 years of the study. There was no loss of sequestered soil carbon during 6 years of continuous no-till corn production.

If grasslands must be converted back to croplands then this is a better way to do it. But there have been many studies that found continuing increase in SOC in grasslands. That depends on management. A well managed conversion of grasslands to crop land can avoid loss of SOC, but well managed grasslands increase SOC. I think that increased SOC is of more value than the grain, especially if it merely feeds the ethanol industry.

It may be that management of crop lands can be further improved by the use of biochar as a durable carbon soil amendment. It doesn't need to be worked into the soil, so the benefits of no till can be preserved. In this way the soil can continue to improve even when cropped.