In collaboration with the Organic Grains Research group, Mary Parr, graduate student in the Soil Science department at North Carolina State University, has been investigating nitrogen fixation and release in a system involving roll-killed winter legume cover crops followed by no-till organic corn.

Parr has quantified the total amount of nitrogen in the biomass of hairy vetch and crimson clover at the time of roll-kill, and is in the process of calculating the portion of this nitrogen coming from the atmosphere versus the soil.

Beginning with corn planting in May 2009, she monitored the decomposition and release of fixed nitrogen from hairy vetch and crimson clover mulches that had been roll-killed. One of the novel ways in which she quantified nitrogen released from the decomposing mulch was by using “resin probes” inserted into the soil that collected the nitrogen released by mulch over a 2-week period.

She found that the hairy vetch plants contained up to 195 pounds of nitrogen per acre by mid-May, and crimson clover contained between 120 and 140 pounds of nitrogen per acre.

As a rolled surface mulch, nitrogen from the hairy vetch cover crop was made available more quickly to the corn crop — with peak availability between 2 and 6 weeks, but the crimson clover nitrogen did not become available until later in the season — between 6 and 10 weeks after planting.

This experiment was conducted at the Tidewater station in Plymouth, N.C., and the Piedmont station in Salisbury, N.C. It will be repeated on both sites in the coming year.