image: Tropical forests in Panama are threatened due to agriculture, mining, development, and climate change. Reforestation is an important strategy to protect the tropical carbon sink. Nitrogen-fixing trees are known to be especially effective at helping regenerating forests grow quicker and store more carbon.
A new study, published this week in Proceedings of the National Academy of Sciences, reveals that nitrogen-fixing trees play an underrecognized role in recovering tropical forests by enriching nutrient-poor soils with scarce elements such as phosphorus and molybdenum.
Coauthor Sarah Batterman, a tropical forest ecologist at Cary Institute of Ecosystem Studies, explains, "We've long known that nitrogen-fixing trees work with soil microbes to make atmospheric nitrogen available to plants. Our study found that nitrogen fixers also play a vital role in unlocking other nutrients by weathering rocks beneath their roots. These nutrients include things like phosphorus, which is very limited in tropical forest soils and important to plant growth."
In this study, the team compared the soils of nitrogen-fixing and non-fixing trees across young recovering tropical forest plots that are part of the Agua Salud Secondary Forest Dynamics Network in the Panama Canal Area. They measured soil pH and mineral weathering - a process that releases nutrients trapped in rocks. They also assessed the composition of the soil microbial community.
To measure weathering, they buried 504 mesh bags of rocks (crushed dunite) under trees, and measured nutrient weathering from the rocks over eight months. Dunite was chosen because it weathers at a rate comparable to minerals in local soils. Trees sampled included five species of nitrogen fixers (51 trees), five species of non-fixers situated in a legume-rich area (39 trees), and five species of non-fixers situated far from any legumes (36 trees).
Batterman explains, "Mineral weathering under nitrogen fixers happened twice as fast as weathering under non-fixers. The soil under nitrogen fixers was far more acidic than the soil under non-fixers. This increased acidity promotes weathering and the release of chemical elements locked in minerals as they break down."
The team also discovered that soils beneath nitrogen fixers were teeming with a bacterial genus new to science (Candidatus Acidoferrum) that thrives in the acidic conditions created by nitrogen-fixing trees. These bacteria are highly enriched under nitrogen fixers, where they enable phosphorus release by breaking down iron-bearing minerals.
Two important products of weathering include available phosphorus and molybdenum. Phosphorus boosts plant growth, and molybdenum helps plants assimilate nitrogen. Before weathering, these scarce but critical elements are trapped in the aluminum and iron found in Panamanian soils. The acidic conditions found beneath the roots of nitrogen fixers catalyze the weathering needed to release phosphorus and molybdenum in bioavailable forms that plants can use.
Co-author Kristin Saltonstall of Smithsonian Tropical Research Institute explains, "Together, acidic soil conditions and a distinct community of bacteria enhance nitrogen fixers' ability to weather rocks and access nutrients. Benefits aren't limited to fixers themselves. We also found that nitrogen fixers help fertilize other plants throughout the forest, with broad effects on nutrient cycling in the forest community."
Lead author Dimitar Epihov of the University of Sheffield points out, "Tree diversity has long been recognized as vital for successful forest recovery. Current views dictate that nitrogen-fixing trees supply nitrogen to help speed up the recovery process. Our work expands these views by revealing that these trees also modulate the microbiome of soils belowground, enabling the release of other non-nitrogen nutrients from rocks. Benefits can be passed on to neighboring trees, and aid in the productivity of regrowing forests in the tropics."
Batterman notes, "Nitrogen-fixing trees have presented ecologists with a longstanding paradox. They are abundant in tropical forests, but so is nitrogen in the soil. Why are they so common, if access to nitrogen does not typically limit growth? Our findings offer an answer to this puzzle. Regardless of whether they are actively fixing, nitrogen fixers also have a unique ability to unlock phosphorus, which is very limited."
Tropical forests are responsible for 70% of the global forest carbon sink, yet they are under threat due to mining, logging, agriculture, development, and climate change. Understanding how different tree species influence tropical forests is essential to informing science-based reforestation strategies.
Batterman concludes, "Tropical soils tend to be nutrient-poor, which can slow forest recovery. Including nitrogen-fixing species in reforestation projects could helping neighboring trees obtain more nutrients, grow quicker, and ultimately help protect the tropical carbon sink."