Plant-Nitrogen Mutualistic Relationship Doesn't Always Last

While plants and nitrogen normally benefit from one another in a mutualistic relationship, a new study shows that too much of a good thing doesn't always last, as nitrogen fertilizer use over a long period of time eventually disrupts this beneficial rapport.

Before industrial fertilizers were invented and fossil fuels became wildly popular, soil nitrogen was scarce. To compensate, some plants and legumes found a way to make this precious nitrogen using bacteria called rhizobia.

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

So what do rhizobia get in return? The legumes shelter the bacteria in their roots and supply them with carbon. This partnership not only benefits the bacteria but also gives legumes an advantage in nitrogen-poor soils.

It would seem that everyone is happy. However, previous research has found that in areas adjacent to farmland where fertilizer runoff occurs, or in areas where nitrogen oxides from the burning of fossil fuels settle, legumes decline while other plants thrive. That's because long-term nitrogen fertilizer use reportedly changes the quality of soil rhizobia, which could have "far-reaching ecological and environmental consequences," the researchers wrote in their study, published in the journal Evolution.

"This study tells us something about mutualisms and how they evolved," Heath 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."

To get to the bottom of how these mutualisms work, the researchers studied six long-term ecological research fields. In each field, one plot of land was fertilized with nitrogen for more than 20 years, while the control plot was never fertilized. After isolating rhizobia from the legumes, the team found that plants grown with the nitrogen-exposed rhizobia produced 17 to 30 percent less biomass and significantly less chlorophyll compared to plants grown with rhizobia from the unfertilized plots.

Essentially, as time passes nitrogen-fixing rhizobia evolve to become less beneficial to legumes - the plants they normally serve, breaking up what was once a happy, healthy relationship.

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