Tropical Rain Forest: Soils and Nutrients Recycling
The ecosystem of the tropical rain forest is very efficient by making maximum use of its limited mineral nutrients, thus being one of the most productive habitats on the planet. The cycling of nutrients in the tropical forest is fast and efficient, occurring within an almost "leak-proof" ecosystem. Despite this quality, the ecosystem does have limitations. The tropical soils are nutrient deficientas a result of the wet climate and as many plant communities store a lot of these nutrients in their vegetation.
soil of the tropical rain forest is a laterite soil
that has a characteristic reddish color. It is composed of
sheets of crust made up of iron and aluminum oxides. This
gives the soil an acidic quality. Due to its age and long
time weathering, silica, sodium, magnesium, potassium,
calcium, phosphorus, and sulfur are all available in very
low levels. Other reasons for their low concentrations
are the fact that the plants have absorbed the nutrients
before they reach the soil, and as they are leached out with
the continual rainfall.
The multilayered structure of the forest filters the nutrients out of the rain water as it falls through. The nutrients themselves come from winds which may carry in N, S, P or from those more likely leached out the vegetation above. Each level of the canopy traps a certain amount of the water and the nutrients that come along with it before it reaches the soil. The leaves of the trees are long lived, tough, and resistant to herbivores. Some form "drip tips" that help channel rain water drain from the leaf faster, thereby reducing the time water is on the leaves - thus reducing lose of nutrients to leaching as well as discouraging epiphyte growth.
Diagram from Univ. of Michigans Climate Change course.
The root systems of the trees of the tropical rain forest have developed a symbiotic relationship with the fungi and bacteria that live in the soil. The tree species and the fungi are codependent. The microorganisms of the soil help the tree roots absorb phosphorus, zinc, copper, molybdenum, and other minerals from the organic layer of the soil. Aside from helping the plants to absorb nutrients, microorganisms also provide them with resistance to root pathogens. The microorganisms receive their energy from the plant host in the form of fixed carbon ( sugars plus).
The tropical rain forest has a variety of mechanisms to speed up the absorption of nutrients. Most of the minerals are absorbed before reaching the soil. This causes a rapid recycling of the nutrients. There are many nutrient conserving mechanisms in action in the rain forest, that allow this region to be highly productive despite the lack of nutrients in the soil.