5) Nutrient cycling

Most tropical forests live “on the edge.” The input of nutrients into a tropical rainforest is generally lower than the demand, so the plants must recycle a high percentage of their nutrients in order to survive. This occurs through the decomposition of dead leaves, plants, and animals by soil microbes and the uptake by plants of chemicals released during decomposition. Some additional nutrients may be derived from leaves leached by rainfall; others may drift in from smoke or aerosols. But the rainforest is to a great extent a closed system. It modifies its own supply of resources and thus growing conditions through its influences on the soil and on nutrient cycling. The cycling of nutrients in rainforests is a very important conservation mechanism, as, since nutrients are rapidly taken up from the soil into plants, nutrient leaching by high rainfall is minimized. High biodiversity is beneficial, because different species of plants have varied chemical compositions and nutrient-cycling patterns, and so are able to exploit many opportunities in the environment.

Of course the forest must have ample supplies of oxygen, hydrogen and carbon (supplied from the atmosphere or water or decomposing organisms; normally these are not limiting because of the recycling of these elements). Carbon and nitrogen enter the soil when plants die or shed leaves, and these elements augment the fertility and water-holding capacity of the soil. Calcium, phosphorus, potassium, magnesium and selenium are also essential minerals which are scarce in certain soils, and may become limiting elements (i.e., compounds or elements which are essential to plants, and, when scarce, restrict growth) for rainforest growth and productivity. Micronutrients such as iron, boron, manganese, copper, zinc, molybdenum and chlorine are necessary in tiny quantities, and may also act as limiting elements. In some soils of the tropics (certain oxysols and ultisols), minerals such as calcium, potassium and magnesium, which, since they are derived from the weathering of rock, have been exhausted by leaching during centuries of heavy rainfall, and may be limiting factors. In other tropical soils, either phosphorus or nitrogen may be an important limiting factor. Phosphorus is often a limiting factor in lowland forests, and nitrogen in montane forests with shallow soils. Forests on nitrogen-impoverished ultisols in lowland Amazonia are often filled with leguminous trees, which have nitrogen-fixing microbes associated with their roots. Plant growth is also regulated by a complex of interactions among nutrients. One element may limit the cycling or accumulation of other elements, as, for example, nitrogen accumulation may be limited by the low availability of water or other nutrients. Little is known of these interactions.

Even forests on poor soils may grow very well. The soils on which Brazilian forests grow are generally relatively infertile, but the leaves of the trees accumulate high concentrations of nutrients. Nevertheless, Brazilian forest litterfalls are just as nutrient-rich as those in forests at La Selva, Costa Rica, where soils have much higher nutrient levels (Proctor, 1995).

The roots of most forest plants lie fairly shallow, within a foot of the surface. Because of this, any disruption to the soil surface will have serious consequences for root structure. In rainforests on deeper soils, as in river valleys, many nutrients seep into the soil and nutrient cycles are mainly closed systems, with few nutrients entering from outside. In forests which lie on shallow soils, such as hillsides, there may be some nutrient input from the decomposition and weathering of rock.