"Robin Hood" Fungus Presents Agricultural Dilemma

A fungus that transfers nutrients from the soil to plants’ roots seems like a blessing from Mother Nature. But a type of fungus known as mycorrhizae will feed any needy plant, including sicklepod and other weeds, according to scientists at the Agricultural Research Service.

Most soybeans form nodules, or small knobs, on their roots. Beneficial bacteria that live on the nodules draw nitrogen from the atmosphere. But a genetic mutation can prevent some soybean varieties from forming the nodules. These non-nodulating plants have now supplied new clues for scientists at ARS, the U.S. Department of Agriculture's chief scientific agency.

Since the 1970s, researchers have noticed that a row of nodulating soybeans will share nitrogen with a neighboring row of non-nodulating plants. The closer the two rows are, the taller and healthier are the non-nodulating soybeans. Scientists have also suspected that mycorrhizae can play an important role in nutrient uptake by plant roots.

ARS agronomist Joe Burton in Raleigh, N.C., tied these two facts together by chemically tracing the path of nitrogen between crop rows. The nodulating plants themselves are not being “charitable.” Instead, Burton found, mycorrhizae were busily transporting nitrogen away from the nodule-producing “rich” plants to the non-nodulating “poor” ones.

What does this mean to farmers? They don’t grow non-nodulating soybeans; the nitrogen-fixing nodules make for high-protein soybeans. The question: where else might the mycorrhizae take that extra nitrogen? The answer: sometimes to weeds--including sicklepod, Palmer amaranth and morning-glory, the scientists have learned.

Because mycorrhizae are probably beneficial for acquisition of all soil nutrients, getting rid of the helpful microbes is not desirable, even if it were possible.

Now that agricultural researchers have demonstrated the mycorrhizae’s indiscriminate generosity, they will be exploring ways to make it shun sharing nitrogen with weeds.

Scientific contact: Joseph W. Burton, ARS Soybean and Nitrogen Fixation Laboratory, Raleigh, N.C., phone (919) 515-2734, fax (919) 515-7959, joe_burton@ncsu.edu