Dhe tropical rainforests are shrinking rapidly. In doing so, chainsaws and fires not only destroy hotspots of biological diversity, they also heat up the global climate. But how big are the chances that former forest areas will regenerate and become a sink for the greenhouse gas carbon dioxide again? That depends, among other things, on whether there are any forest areas in the area that can provide enough seeds for new generations of trees. If microorganisms are also involved, the young trees will grow quickly even on poor soil. This is what an international group of biologists found out during field research not far from the Panama Canal.
All over the world, an abundance of legume tree species grows in tropical forests. These plants, also known as legumes, are characterized by the fact that they are supplied with nitrogen from the air: At their roots they form nodules that harbor special bacteria. There the microbes find a suitable atmosphere to bind nitrogen from the air and thus produce ammonium. Farmers appreciate this symbiosis. For example, when you grow clover or peas, nitrogen accumulates in the soil.
Telltale DNA in the ground
This type of fertilization works just as well with trees from the legume family. The black locust, for example, originally at home in North America, can therefore also thrive on poor sandy soils. However, tropical soils often lack not only nitrogen, but also other mineral nutrients. Phosphorus, for example, is often firmly bound to iron or aluminum. Likewise the trace element molybdenum, part of an enzyme with which the nodule bacteria produce ammonium. But how do legumes manage to obtain the necessary minerals anyway? To find out, the scientists led by Dimitar Z. Epihov from the University of Sheffield and Sarah A. Batterman from the University of Leeds studied the biological activities underground in detail.
In areas where the rainforest is regenerating, the biologists have buried more than five hundred fine-meshed nets, filled with crushed silicate rock from the Oliving group, representing the silicates in the local soils. As it turned out eight months later, the rock in the root space of legumes weathers much faster than that of other trees. Unless these trees grow in the immediate vicinity of legumes.
In search of microorganisms that release mineral nutrients, Epihov and his colleagues analyzed the DNA isolated from soil samples. As the researchers in the Proceedings of the American National Academy of Sciences report that microbes that are able to free phosphorus from its binding to oxidized iron populate the root space of legumes in remarkable numbers. Using DNA databases, the two most common ones could be identified as Acidobacterium capsulatum and identify a previously unknown species from the group of acidobacteria. These microbes, as their name suggests, thrive best in an acidic environment, which makes it easier for them to attack oxidized iron.
Perfect nutrient cycle
Legumes apparently create a suitable ambience because their roots excrete acid. The older the trees get, the more and more spaciously they influence the degree of acidity. In this way, they favor bacteria that can still release mineral nutrients even where the soil is completely depleted by frequent downpours. This type of fertilization also benefits neighboring trees, which also benefit from the nitrogen that legumes use to enrich the soil. With the help of nodule bacteria in their roots and acidobacteria outside, tree-shaped legumes therefore play an important role when the deforested rainforest is gradually regenerating. This probably not only applies to rainforests in Panama.
Since young forests are constantly producing growth, they need more mineral nutrients than they can recover from dead plant parts. In contrast, the nutrient cycle works almost perfectly in old, largely undisturbed forests. Because organic matter is recycled very quickly in the warm, humid tropical climate, the forest floor, unlike in temperate latitudes, is not a large storage facility for mineral nutrients. Most of them are found in the living inventory of tropical rainforests. The fact that much of it is lost when such a forest is cut down makes regeneration difficult. All the more important are legumes, which, with their symbiotic bacteria, manage to tap into otherwise inaccessible sources of nutrients.