Microbes Do It; Now So Do Plants

A stage in the nitrogen cycle previously known to occur only in microorganisms has now been shown to occur in some plants, according to a new study.

The nitrogen cycle is mediated by interactions among plants, animals, and microorganisms. Some plants are key because they harbor bacteria that can reduce atmospheric nitrogen to ammonia, the stage called nitrogen fixation.

In the stage called nitrification, organic nitrogen is converted to nitrite and nitrate. This process is mediated by microorganisms, primarily bacteria. Researchers at the University of Wales, Swansea, now find that nitrification also occurs in some plants—in particular, the horseshoe vetch (Hippocrepis comosa), a nitrogen-fixing member of the legume family [Nature, 430, 98 (2004)].

This plant accumulates 3-nitropropionic acid (3-NPA) in its leaves. This highly toxic compound shields the plant from herbivores, says Charles R. Hipkin, one of the researchers. "When its leaves are dying, the plant breaks down 3-NPA to avoid killing itself." The metabolism yields 3-oxopropanoic acid, as well as nitrite and nitrate. When the leaves fall, these ions return to the soil.

The work indicates that nitrate can be deposited in the soil even in the absence of microbial nitrifiers, comments Willy Verstraete, a professor of microbial ecology and technology at Ghent University, in Belgium.

The extent of nitrification due to this route is several orders of magnitude less than that due to microbial action, Hipkin says. But that this route even exists is of significance, he adds.

Furthermore, bacterial nitrification is suppressed under certain conditions, such as in highly acidic soils or in waterlogged soils. "Under those conditions, if you find these legumes growing, the nitrification they are undergoing may exceed bacterial nitrification," Hipkin says.