Researchers suggest how animal herbivores hinder nitrogen fixation in tropical forests
Woodhouse, UK: The results of recent research into the restrictions on nitrogen-fixation">nitrogen fixation among plants may help reforestation initiatives to be more successful.
Some trees, like those in the Fabaceae family of legumes, have a symbiotic connection with bacteria that enables them to absorb nitrogen from the atmosphere. Nitrogen is a nutrient that most plants obtain from the soil, but some soils--especially those in newly replanted or disturbed tropical forests--can be deficient in nitrogen, which inhibits tree growth.
Nitrogen is a key nutrient needed for photosynthesis.
Using a process known as nitrogen-fixation">nitrogen fixation, some plants have adapted to take-up nitrogen from the air using the services of friendly bacteria. This enables trees to grow in habitats where nitrogen levels are low. And there are other benefits.
Higher levels of nitrogen can result in higher levels of photosynthesis and removal of carbon dioxide, a potent greenhouse gas, from the atmosphere, helping to offset some of the carbon emissions from human activity.
Nitrogen fixation also raises the nitrogen levels in the soil, encouraging non-fixing species to survive. But this apparent win-win situation has drawbacks.
An international team of scientists, led by Will Barker, a doctoral researcher from the School of Geography at the University of Leeds in the UK, found that trees with the ability to fix nitrogen attracted insects and other animals that would eat their leaves, a process known as herbivory.
In a scientific paper published in the journal Nature, the researchers report that in a tropical forest in Panama, the nitrogen-fixing trees experienced 26 per cent more herbivory than non-fixing trees.
They found that by being consumed, the nitrogen-fixing trees experienced lower growth and lower rates of survival when compared to non-fixing trees. This would have an impact on the amount of new nitrogen getting into the forest soils and would constrain the role that trees were able to play in taking carbon dioxide out of the environment.
Barker said: "These findings give us new insights into the function of different types of trees in tropical forests and could help inform efforts to reforest tropical regions that have been degraded for logging and agriculture.
"For example, people reforesting sites may consider including a diverse mix of nitrogen-fixing and non-fixing trees so that they have enough to ensure nitrogen fixers can bring in new nitrogen, even when there will be constraints by herbivory.
"They also would not want to plant all nitrogen-fixing trees, however, because then those regenerating forests may disproportionately attract animal pests that could wipe out the nitrogen-fixing trees."
Dr Sarah Batterman, Associate Professor in the School of Geography at Leeds, who supervised the research, said: "These findings are significant because for decades people have been interested in how abiotic factors like the availability of soil nitrogen or phosphorus or temperature constrain symbiotic nitrogen-fixation">nitrogen fixation.
"We've found that interactions with animals through herbivory on leaf tissue may be critical for determining how fertile tropical forest soils are in terms of nitrogen.
"We expect the patterns that we found in Panama will hold across a wide variety of ecosystems. We look forward to exploring that further in the future."
