When exposed to nitrogen fertilizer over a period of years, nitrogen-fixing bacteria called rhizobia evolve to become less beneficial to legumes according to US researchers at the University of Illinois.
“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.”
The researchers isolated rhizobia from the nodules of legumes in fertilized and unfertilized 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 produced 17 to 30 percent less biomass and significantly less chlorophyll than plants grown with rhizobia from the unfertilized plots.
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.