Long-term nitrogen fertilizer use disrupts plant-microbe mutualisms

http://www.eurekalert.org/pub_releases/2015-02/uoia-lnf022315.php

Public Release: 23-Feb-2015
University of Illinois at Urbana-Champaign

When exposed to nitrogen fertilizer over a period of years, nitrogen-fixing bacteria called rhizobia evolve to become less beneficial to legumes - the plants they normally serve, researchers report in a new study.

These findings, reported in the journal Evolution, may be of little interest to farmers, who generally grow only one type of plant and can always add more fertilizer to boost plant growth. But in natural areas adjacent to farmland, where fertilizer runoff occurs, or in areas where nitrogen oxides from the burning of fossil fuels settle, a change in the quality of soil rhizobia could have "far-reaching ecological and environmental consequences," the researchers wrote.

"The nitrogen that we apply to agricultural fields doesn't stay on those fields, and atmospheric nitrogen deposition doesn't stay by the power plant that generates it," said University of Illinois plant biology professor Katy Heath , who led the study with Jennifer Lau , of Michigan State University. "So this work is not just about a fertilized soybean field. Worldwide, the nitrogen cycle is off. We've changed it fundamentally."

Not that long ago, before the advent of industrial fertilizers and the widespread use of fossil fuels, soil nitrogen was a scarce commodity. Some plants, the legumes, found a way to procure the precious nitrogen they needed - from rhizobia.

"The rhizobia fix nitrogen - from atmospheric nitrogen that we're breathing in and out all the time - to plant-available forms," Heath said. "Plants can't just take it up from the atmosphere; they have to get it in the form of nitrate or ammonium."

In return, legumes shelter the rhizobia in their roots and supply them with carbon. This partnership benefits the bacteria and gives legumes an advantage in nitrogen-poor soils. Previous studies have shown that nitrogen fertilizers can affect the diversity of species that grow in natural areas, Heath said. In areas polluted with fertilizer runoff, for example, legumes decline while other plants become more common.

•••••

A genetic analysis of the microbes revealed that the composition of the bacterial populations was similar between fertilized and unfertilized plots: The same families of rhizobia were present in each. But rhizobia from the fertilized plots had evolved in a way that made them less useful to the legumes, Heath said.

•••••