Many trees can generate their own fertilizer from nitrogen in the air. But it remains a mystery as to why they grow where they do – for example, few grow in the nitrogen-poor soils of temperate forests but many thrive in the nitrogen-poor soils of tropical forests.
Now researchers suggest that these plants act in smart, strategic ways, not only benefiting themselves but influencing the composition and productivity of their surroundings – and profoundly shaping ecosystems on land. The work also points to the potentially dire consequences of massive amounts of human-generated nitrogen pollution.
“The idea that plants are ‘smart’ enough to actively adjust what they are doing as conditions change opens the door to a new way of understanding how they may respond to change in the local and global environment,” says study senior author Lars Hedin, an ecosystem biologist at Princeton University. Hedin, along with study lead author Efrat Sheffer at the Hebrew University of Jerusalem and their colleagues, detailed their findings online November 23 in the journal Nature Plants.
Nitrogen is an essential nutrient for all life on Earth, needed to create proteins, chlorophyll and DNA. Although nitrogen makes up most of Earth’s atmosphere, this nitrogen is largely unavailable to life because the nitrogen atoms are bound up by powerful chemical bonds. The vital process known as nitrogen fixation breaks these bonds and incorporates the freed nitrogen atoms into biologically useful molecules.
Nitrogen-fixing plants host soil bacteria known as rhizobia in their root cells, feeding the microbes sugar. These bacteria harbor the nitrogen-fixing enzyme nitrogenase. Although nitrogen fixation is a costly, resource-intensive activity, it can reap major dividends for plants.
Curiously, nitrogen-fixing trees struggle in seemingly ideal conditions but flourish in less friendly environments. In the nitrogen-rich soils of tropical forests, where nitrogen fixers would not seem to have a competitive edge, there are hundreds of species of nitrogen-fixing trees that handily compete with other trees. In contrast, in the nitrogen-poor soils of boreal or temperate forests, the ability to produce fertilizer would seemingly grant an advantage. However, nitrogen-fixing trees are not nearly as prevalent or long-lived in these higher-latitude forests as they are in the tropics, and they literally become overshadowed by other trees over time.
To solve this mystery, Hedin, Sheffer and their colleagues developed an evolutionary game theory model to analyze how fixers compete with non-fixers following disturbances such as fires, mudslides and hurricanes endured by nitrogen-fixing trees in tropical and non-tropical forests. After such events, plants often need nitrogen to make the most out of vacancies that disturbances open up in forests.
A major feature of this new model is that tropical nitrogen-fixers have evolved to switch nitrogen fixing on and off. In contrast, higher-latitude nitrogen-fixers usually cannot stop fixing nitrogen.
The researchers discovered that as tropical forests recovered from disturbances, nitrogen-fixing trees generated fertilizer to boost early growth. However, they switched off nitrogen-fixing when soil nitrogen reached high levels, diverting that energy to better compete for sunshine.
In contrast, as higher-latitude forests recovered from disturbances, nitrogen-fixing trees kept on generating fertilizer, a costly activity that both hampered their growth and helped their neighbors. As a consequence, these trees did not succeed past the early stages of the recovery of those forests.
“Plants can be highly strategic in when and how they deploy important functions such as nitrogen fixation,” Hedin said. “We ascribe agency to plants the same way that agency traditionally has been ascribed to animals.”
These findings suggest that widespread human-induced nitrogen pollution, largely as a result of agriculture, might have strong effects on nitrogen-fixing trees, says biogeochemist Benjamin Houlton at the University of California-Davis, who did not take part in this research and who wrote a commentary about it for Nature Plants.
The work implies, says Houlton, that excessive amounts of nitrogen could disrupt nitrogen-fixing trees, particularly in temperate forests, potentially leading to local extinctions. “If true, this disruption could impact the resiliency of ecosystems in a major way,” Houlton says via email. “For example, nitrogen-fixing shrubs and trees naturally fertilize ecosystems following fires and other kinds of disturbances. What happens if these kinds of plants are no longer around?”