Muddy Highways

The highway is the Amazon--a true turnpike in every sense of the word! It's the second largest river, at 6514 km (4050 m), with the largest volume of water (95,000 cu m (3.3 million cu ft) per second. How carefully did you observe the rivers you traversed as you traveled to your destination? Did you note the change of color, as the boat moved from the Itaya River into the Amazon?

Rainforest soils generally fall into the categories of ultisols, oxisols, or alfisols. Utisols are well-weathered and filled with materials leached from other parts of the soil. Oxisols are old and often acidic. They may be reddish due to oxidation. The most common form of soil, however, is the more neutral alfisol, characterized by gray to brown surface horizons of clay.

Remember the soil horizons we reviewed in Week 1? The B horizon, just below the roots of the rainforest plants, is clay. (Remember that sand is a particle size but clay is both a particle size (tiny and slimy) and also a mineral. The high clay composition of the B horizon makes it unstable. (Watch out when you leave the boat! It's easy to slip.) Clay acts as a sealant. Because the particles are so small, they remain suspended in water for a long time. So the spring outwash from the Andes may still be evident in the Amazon as you cross it. Each year, the Amazon transports about 1240 metric tons of Andean clay and 3200 metric tons of silt from the lowlands it traverses.  You may also note globs of what seems like "suds" on the surface of the water. That's not pollution; it's a combination of clay and oils draining from wetlands.

In the rainforest, no nutrients can be stored beneath the A horizon--the roots and decomposing litter. The clay is a sealant, almost like visqueen. So preservation of the topsoil is absolutely vital. Without it, the rainforest cannot survive.





Nutrient Recycling

Feeling moldy yet? It's inevitable, you'll get something damp...and it may just stay damp for a long time. One of the most important groups

 of organisms in the rainforest are fungi, and they rule! (If you've walked the canopy walkway at night, you'll know what I mean!)

In the forest, mycelial meshes cover parts of the forest floor and aid in keeping the precious nutrients from leaching from the thin surface layer of soil during the constant rains. Fungi, along with insects, bacteria, archaeans, and other organisms, help return over 80% of the matter in fallen leaves back to the soil in a form that other organisms can use.

Mycorrhizae are symbiotic relationships between fungi and the roots of plants. They assist the roots in working fast--catching the nutrients before they wash away. They are vital partners for rainforest trees.

In the world's croplands, legumes are the primary source of nitrogen fixation--the chemical process that takes N2 from the air and changes it to an NOx radical form that can be used to make proteins. In the forest, this process is assisted by the bacteria in the guts of termites. Termites have a tremendous value, too, to the human residents. The insects are fed to chickens, and the nests boiled to make an insect repellent. They are able to break down the wood that falls to the forest floor, releasing its nutrients for future generations of plants and animals, because of their symbiotic bacterial partners.


(Image source: USDA)