Human-Caused Increase In Nitrogen Is Reducing Diversity In Nitrogen-Fixing Plants, Study Finds

An increase in nitrogen levels in the atmosphere due to human activities, including fertiliser use and pollution is reducing diversity among nitrogen-fixing plants, posing a threat to both biodiversity and ecosystem stability, a study has found.

Nitrogen-fixing plants convert nitrogen gas from the air into products, like ammonia, which is then used by the plant to grow. These plants help maintain healthy ecosystems by adding nitrogen to the soil.

Researchers, including those from Mississippi State University, US, found that excessive nitrogen from agriculture and industry makes these plants less competitive, leading to simplified plant communities with fewer species.

"While others predicted climate change might benefit nitrogen fixers, our research shows otherwise. Humans are changing Earth in multiple ways that affect nitrogen fixers, and nitrogen deposition is overwhelming as a harmful effect," Ryan A Folk, an assistant professor at Mississippi State University and co-author of the study published in the journal Science Advances, said.

Folk said that nitrogen, the first number listed on a bag of fertiliser, is often the most important plant macronutrient in natural and agricultural systems, so the loss of these plants threatens both biodiversity and ecosystem stability.

The researchers selected 971 pairs of plots from 53 sites, that included at least one nitrogen-fixing plant in either a baseline survey conducted between 1940 and 1999, or a last resurvey conducted between 1995 and 2019.

Data for analysis was taken from the forestREplot database, which is maintained by Ghent University, US, and helps assess how forest plant communities around the world are changing.

"From the baseline survey to the last resurvey, most plots lost N-fixers (65%), losing, on average, 72% of the baseline nitrogen-fixer richness. Not only the number but also the proportion of nitrogen-fixer species in the community declined, on average, by two per cent between the baseline and the last resurvey," the authors wrote.

The authors said that assessments of changes in long-term diversity among nitrogen-fixing plant communities are rare.

Human-caused increase in nitrogen levels in the air reduces diversity in nitrogen-fixing plants in ways that could strongly affect natural nitrogen fixation, they said.