When exposed to nitrogen fertiliser over a period of years, nitrogen-fixing bacteria called rhizobia evolve to become less beneficial to legumes, with which they normally have a symbiotic relationship.

These are the findings of new research by experts at the University of Illinois. They said their findings may be of less interest to farmers, “Who generally grow only one type of plant and can always add more fertiliser to boost plant growth,” but that run-off could be affecting soil life in areas near to farmland.

In natural areas adjacent to farmland, where fertiliser 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 a colleague at Michigan State University. "So this work is not just about a fertilised soybean field. Worldwide, the nitrogen cycle is off. We've changed it fundamentally."

Legumes have found a way to procure the precious nitrogen they need to grow - from soil-dwelling bacteria called 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 fertilisers can affect the diversity of species that grow in natural areas, Heath said. In areas polluted with fertiliser runoff, for example, legumes decline while other plants become more common.

In the new analysis, Heath and her colleagues looked at six long-term ecological research fields at Michigan State University's Kellogg Biological Station. Two experimental plots were located in each of six different fields. One plot in each field had been fertilised with nitrogen for more than two decades; the other, a control plot, had never been fertilised.

The researchers isolated rhizobia from the nodules of legumes in fertilised and unfertilised plots. In a greenhouse experiment, they tested how these bacteria influenced legume growth and health. The researchers found that the plants grown with the nitrogen-exposed rhizobia were smaller and less healthy (they produced significantly less chlorophyll) than plants grown with rhizobia from the unfertilised plots.

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

"This study tells us something about mutualisms and how they evolved," she said. "Mutualisms depend on this balance of trade between the partners, this special nitrogen-carbon economy in the soil, for example. And when the economy changes - say when nitrogen is no longer scarce - these mutualisms might go away.”