Chemical Composition:
As we would expect for extremely weathered soils, the pH of the tropical soils are acidic, as most basic compounds have been washed away by leaching. The average pH for the Amazon Basin is between 4.17 – 4.94 (Negreiros, G. H. de; Nepstad, D. C. 1994, Mapping deeply rooting forests of Brazilian Amazonia with GIS, Proceedings of ISPRS Commission VII Symposium - Resource and Environmental Monitoring, Rio de Janeiro. 7(a):334-338.). While the pH remains relatively constant over time, we can see that it has effects on the biological and chemical characteristics of the soil. Most microbiological life in the Amazon soils is fungi rather than bacteria, which prefer basic conditions. Chemically, soils below a pH of 6.0 are more likely to be deficient in certain nutrients optimal for plant growth, including Ca, Mg, K, and phosphate ions. Acidic soils are also more susceptible to Al3+ toxicity, as aluminum ions become more soluble as pH decreases (Tan, Kim H. Environmental Soil Science, 2000, Marcel & Dekker Inc.). Of course, native rainforest species are adapted to these conditions, thus an acidic pH, and its effects, only become an issue when we attempt to use the land for other purposes.
Another important factor to consider in characterizing soil chemical composition is Effective Cation Exchangeability (ECE). This is a measure of a soil’s ability to exchange positively charged ions with plant roots. In soils with high ECE, plants can easily exchange H+ for important nutrients, such as Ca, Mg, and K ions. As we have already mentioned, in tropical soils, these ion concentrations are already quite low in tropical soils, therefore, so is their exchangeability. For example, in data collected from the Cerado region of Brazil, K+ exchangeability was found to be 0.14 milliequivalents per 100g in the A Horizon (Mendonca, Eduardo S. & Rowell, David L. (1996), "Mineral and Organic Fractions of Two Oxisols and Their Influence on Effective Cation-Exchange Capacity" Soil Science Society of America Journal, 60(6)). This is compared to the 0.2 milliequivalents per 100g of K+ exchangeability that is considered to be a critically low level by agricultural standards (Jones, Benton J., Laboratory Guide for Conducting Soil Tests and Plant Analysis, 2001, CRC Press, Appendix E). ECE is generally considered to be determined by the soil’s physical qualities and organic matter content, thus it is also essential to look at organic matter levels in the Amazon Basin.
Organic C and N are the principle organic compounds in tropical soils. As expected, their percents compositions are low, being 0.95% for C and 0.10% for N (Mendonca, Eduardo S. & Rowell, David L. (1996), "Mineral and Organic Fractions of Two Oxisols and Their Influence on Effective Cation-Exchange Capacity" Soil Science Society of America Journal, 60(6)). This does not mean that there little organic matter in the system itself. Unlike other forests, most of the tropical rainforest’s biomass is stored in the plants themselves, while rapid bacterial decay ensures nutrients from decomposition are rapidly available for reabsorption (Encarta 2002, “Rain Forest”, 2002, Microsoft Co.). When land is cleared for agricultural purposes, therefore, plant species that do not have this ability to store large amounts nutrients are introduced to the soil. As a result, N and C are left in the poorly covered soil to be leached away by water.
Average composition data from the Amazon Basin
Negreiros, G. H. de; Nepstad, D. C. 1994. Mapping deeply rooting forests of Brazilian Amazonia with GIS. Proceedings of ISPRS Commission VII Symposium - Resource and Environmental Monitoring. Rio de Janeiro. 7(a):334-338.
Negreiros, G. H. de; Nepstad, D. C.& Davidson, E. (In Press ). Profundidade Mínima de Enraizamento das Florestas na Amazônia Brasileira. Book of the Workshop between The Woods Hole Research Center and Smithsonian Institute in Manaus in 1994.
Negreiros, G. H. de; Nepstad, D. C.; Potter, C &.Davidson, E.(In Preparation)- Mapping Potential Rooting Depth in Brazilian Amazon Forests.
Important Cycles:
To maintain a chemical equilibrium in any soils, there must be cycles where compounds pass in and out of the soil. It is important to understand the nature of these processes if we want to assess the health of the current forest, as well as feasibility of human use of this land. The most significant of these cycles when considered the soil’s importance to the surrounding flora are cycles involving organic material, most notably the carbon and nitrogen cycles.
The carbon cycle is the means by which organic wastes can be recycled into useable forms (Tan, Kim H., Environmental Soil Science, 2000, Marcel Dekker Inc.). Most carbon enters the system as CO2 from the atmosphere, which is then absorbed by plants and converted into carbohydrates through photosynthesis. These carbohydrates are incorporated into the plant matter and released into the soil through decomposition with leaf-fall or once the plants die. Microorganisms in the soil are responsible for the decomposition of this organic material, producing humic matter and eventually gaseous CO2, which is released into the air, where it can again be absorbed by plants.
The nitrogen cycle is very similar to the carbon cycle, though slightly more complicated. Like the carbon cycle, the system’s initial source of nitrogen is N2 gas from the atmosphere. When it reaches the soil, this gas must be converted into NO3- before it can be used by plants. This process, called nitrogen fixation, is carried out by microorganisms in the soil that are either symbiotic or nonsymbiotic. The symbiotic organisms are directly involved in symbiosis with another plant species, receiving glucose in exchange for useable nitrogen, while the nonsymbiotic are independent organisms living in the soil. Measurements have shown that nitrogen is more effectively fixed by symbiotic organisms, converting up six times as much N per year as nonsymbiotic organisms (Tan, Kim H. Environmental Soil Science, 2000, Marcel & Dekker Inc.). In an environment such as the rainforest, where the soil is unable to hold many nutrients, symbiotic nitrogen fixation is prevalent as an adaptation to nitrogen deficiencies. However, it is important to note that if there is an excess of inorganic nitrogen available for absorption, nitrogen fixing is reduced. Thus nitrogen fertilizers actually inhibit this aspect of the nitrogen cycle (Committee on Tropical Soils, Soils of the Humid Tropics, 1972, National Science Academy). Once the nitrates are absorbed by the plants, they are held until the plants die, releasing nitrogenous compounds back into the soil. These compounds are converted into ammonia, which can either be absorbed directly by plants or converted into nitrates by microorganism, which are then absorbed. Any remaining nitrogenous material is converted back into N2 and released into the atmosphere by bacteria. Unfortunately, in soils such as those of the tropical rainforest with little nutrient storage capacity, much nitrogen can be lost through NO3- leaching with exposure to water, which not only depletes the soil of nitrogen but also degrades the water quality (Tan, Kim H. Environmental Soil Science, 2000, Marcel & Dekker Inc.). Therefore, deforestation in the tropics has a pronounced disruption of the nitrogen cycle.
Impacts of Deforestation on Cycles (illustrated by diagram):
These diagrams have been constructed for the purpose of illustrating the relationship between different states of nitrogen in the global environment. Human impact factors relevant to the Amazon region are illustrated in red, with blue arrows indicating their either decreasing or increasing effects on various stages of the nitrogen cycle. The Global Biogeochemical Cycle of Nitrogen diagram illustrates this dynamic process for the globe. The Inorganic Nutrient Cycle diagram shows the path of nitrogen and associated cations on a more detailed and rainforest-specific level. From these figures, the data obtained through experimental procedures can be more easily visualized and explained.
Disruptions to the Amazonian nitrogen cycle can be studied on both global and local scales. Considered globally, we can see human contributions to increased nitrous oxide levels (greenhouse gases) in the atmosphere due to slash-and-burn techniques for clearing the land. This method is combustion of organic matter, causing a release of this biological nitrogen in the form of N20 and NOx at a estimated global rate of 40 Tg (1012) per year .
Observed on a local scale of the Amazon basin itself, increases in fertilizer use have certain consequences for the native vegetation. The most measurable effect is that of water contamination due to fertilizer run-off. When excess nitrogen is applied to a region in the form of fertilizers, it is not retained by the soil as added fertility. Rather, this nitrogen (in the form of NO3-) is leached from the soil into the surrounding water system. This leads to eutrophication of surrounding bodies of water . In the soil itself, this process depletes the reservoir of positive ions (Ca2+, K+, etc.), which are transported with the negative nitrate ions. The presence of this available nitrogen also disrupts the natural nitrogen fixation sequence, as microorganisms are no longer required to provide a nitrogen source for the surrounding vegetation, and thus no longer participate in fixation . This presumably adds to the complexity for any kind of land rehabilitation, as the natural nitrogen fixation has been temporarily disabled. Not only is the use of the land disruptive to the nitrogen cycle, but also its conversation to such uses. When land is initially stripped of its natural vegetation, the decaying biomass infuses the soil with nitrogen compounds. If left fallow, this nitrogen is leached away, again with associated cations, as there are no roots to absorb it . Also, without vegetative cover, decomposer, such as worms and termites, and microorganism populations, important for nutrient cycling, are diminished . If cultivated, the demands of the crops often exceed the holding capacity of the soil and fertilization is required, often carried out to excess, with the effects previously outlined.
Important Microorganisms:
In the soils of the Amazon, microorganisms play an important role in not only the carbon, nitrogen, and nutrient cycles, but also in aiding plant ion absorption. One of the most striking microorganisms is the Mycorrhizae fungi, which are involved in a symbiotic relationship with many species of plants, particularly tropical trees. They invade the primary cortex of the root system, but leave the main roots and secondary cortex intact. This effective increases the active surface area of the plant roots by as much as a factor of ten (Tan, Kim H. Environmental Soil Science, 2000, Marcel & Dekker Inc.). These fungi supply the plants with P, N, and K in a usable form, as well as limit pathogen entry through the roots. This results in increased water regulation, allowing for a more rapid recovery from droughts and abiotic stresses. In exchange, these plants provide the fungi with sugars produced through photosynthesis (Harley, J.L., Smith, S.E, Mycorrhizae Symbiosis (1983), Academic Press, London).
Recently, a study done in Venezuela suggests that mycorrhizae innoculation could be used to aid in rehabilitation of deforested soils. The experimenters attempted two methods of treatments (as well as controls). One involved phosphorus fertilizers and mycorrhizal inoculation (I+P) while the other was only innoclutation with mycorrhizal fungi (I). The (I+P) treatment caused a 60% increase in above ground biomass after a five month regrowth period as compared to a control, and twenty times that of the (I) treatment. The chemical analysis of these soils showed that while no exchangeable P was detected in the controls, there was about 2.17 mg/g in inoculated and fertilized soils. The researchers believe that this is because in general, plants in mature tropical ecosystems depend on presence of mycorrhizae for their development. Therefore, when disturbance, such as deforestation, causes a loss of mycorrhizae, “recovery of the degraded areas is only possible if these propagules are reintroduced by natural processes or human intervention” (Cuenca, G., De Andrade, Z., Escalante, G, 1998, Arbuscular mycorrhizae in the rehabilitation of fragile degraded tropical lands. Biol Fertil Soils, 26).
THE NITROGEN CYCLE
Background: Plants require nitrogen for healthy growth. Most of the nitrogen in organic matter is in a form that plants cannot use. Bacteria found in the siol converts these organic forms of nitrogen into forms that the plant can use. When plants die, they decompose and become part of the organic matter of the soil and the cycle is repeated again.
The nitrogen cycle is one of the most important nutrient cycles found in terrestrial ecosystems. Nitrogen is the building block of many complex molecules formed by plants and animals; some examples are amino acids, proteins, and nucleic acids used in DNA.
Most of the world's nitrogen is located in the atmosphere as N2 gas. Still, atmospheric nitrogen is unuseful to the plants. This is because plants can only consume nitrogen in two solid forms: the ammonium anion (NH4+) and the nitrate anion (NO3-). However, the former is more preferred than the latter because large concentrations of ammonium is extremely toxic. (1)
In most ecosystems, nitrogen is primarily sotred in living and dead organic matter. This nitrogen is converted into inorganic forms (forms usable by plants) when it re- enters the biogeochemical cycle (Cycling of chemicals through the biosphere, lithosphere, hydrosphere, and atmosphere) via decomposition. (2) Decompositoin is achieved by various decomposers found in the upper soil layer. They alter the nitrogen found in organic matter into such forms as ammonium salts. This process, known as mineralization, is carried out by a variety of bacteria, actinomycetes, and fungi. (3)
Animals also require a certain amount of nitrogen in their system for survival. Animals, however, secure their nitrogen fixation through plants or other animals that have fed on plants.
Four processes participate in the cycling of nitrogen through the biosphere: Nitrogen Fixation Decay Nitrification Denitrification (4)
Nitrogen Fixation:The nitrogen molecule (N2) is inert. It is also stable because of the strengh of the tripple bond that it possesses. Thus, breaking it apart will require a substancial amount of energy. Most industrial fixation is achieved under great pressure, at temperatures of 600 degrees celcius, and with the use of a catalyst. Under these conditions, atmospheric nitrogen and hydrogen can be combined to form ammonia. This can be used directly as a fertilizer, however, it is further processed into more useful forms such as urea and ammonium nitrate.
The ability to fix nitrogen is found only in certain bacteria. The first step in this process produces ammonia. However it is quickly incorporated into protein and other organic nitrogen compounds.
Decay:When plants die, they decay and leave behind organic forms of nitrogen. This is changed into more useful inorganic forms by certain bacteria.
Nitrification:Ammonia can be taken up directly by plants- usually through their roots. However, most of the ammonia produced by decay is convertd into nitrates. This is accomplished in two steps: *Bacteria of the genus Nitrosomonas oxidize ammonia to nitrites. *Bacteria of the genus Nitrobacter oxidize nitrites to nitrates. These two groups of bacteria are called nitrifying bacteria. (5) Through their activities, nitrogen is made available to the roots of plants.
Denitrification:The previous processes remove nitrogen from the atmosphere and pass it through the ecosystems. Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere. Denitrification is achieved by bacteria that live deep in the soil and in aquatic sediments. Because the conditions are anaerobic, they use the nitrates as an alternatie to oxigen for the final electron accceptor in their respiration. (5)
Another process, called volatilization turns urea fertilizers and manures on the soil surface into gases that also join the atmosphere. (5) This completes the nitrogen cycle.
Carbon is stored in the following five major sinks (1) As organic molecules found in the biosphere (2) As the gas carbon dioxide in the atmosphere (3) As organic matter in soils (4) In the Lithosphere as fossil fuels and sedimentary rock (5) In the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.
The carbon cycle is the circulation of carbon in the form of the simple element and its compounds through nature. The source of carbon in living things is carbon dioxide (CO2), from air or dissolved in water. Algae and green plants (producers) use CO2 in photosynthesis to make carbohydrates, used in the processes of metabolism tomake all other compounds in their tissues and those of animals that consume them (herbivores). The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and (at night) plants return the CO2 to the atmosphere as a byproduct of respiration. The carbon in animal wastes and the bodies of organisms is released as CO2, in a series of steps, by decay organisms (decomposers), chiefly bacteria and fungi. Some organic carbon (the remains of organisms) has accumulated in the earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere Here is an illustration of the carbon cycle
Carbon dioxide levels in the atmosphere have been rising drastically in the past 50 years and based on the trend are continuing to do so. As seen in the graph of atmospheric carbon dioxide, levels have risen drastically in the pacific over the past fifty years. The small spikes seen to be caused by seasonal changes from changing foliage levels.
CO2 effects on the rainforest and discovered the drastic influence the rainforest plays as a carbon dioxide sink (due to photosynthesis reaction [ + H2O => C6H12O6 + O2]).
All rainforests (tropical, subtropical and temperate) are under threat from human activity at the present time. They are being destroyed at an alarming rate, that could potentially lead to many different types of environmental catastrophe, not only in the local forest zone, but globally. The greatest threats comes from deforestation (tree removal by various means and for various purposes) and mining.
Deforestation may be done to create farmland, to build hydro-electric plants, to sell the lumber, or through careless or accidental burning. Rainforest microbes are extremely efficient at breaking down and recycling waste organic matter - the leaf litter and layers of detritus on the ground. As a result, almost no nutrients reach the forest soil and it is consequently poor. Removal of the trees allows the soil to dry out and the little exists to deteriorate. This causes the rainforest microbes to die and the soil becomes largely inert, biologically. The degraded soil is also prone to erosion by wind and when land floods, it can be washed away.
Landsat Image © ESA Eurimage, 1992
The rainforest soil is often of a reddish color, rather than brown. Nutrient levels in the soil are very low. This can be attributed to:
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The chemical composition of the soil. The clays in the rainforest soil are less chemically active and therefore less efficient at retaining nutrients.
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At depths of just a few centimeters below the soil surface, there is practically no organic matter at all.
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The amount of litter, or detritus, on the surface of the soil is low. At first sight this might seem somewhat paradoxical, since the rate of production of biomass in the tropical forest may be at least four to six times that in a temperate forest. However, under the hot and humid conditions found in these regions, the rate of decomposition of the litter by saprophytes (, and certain types of and ) is accelerated. In other words, the bacteria, protists and fungi in the rainforest are very active and effective.
The root systems of the trees are confined to the topmost layers of the soil. This is not unexpected, since the deeper layers of the soil do not contain appreciable quantities of nutrients.
Because of the resulting poor soil conditions, sustained farming after clearance is difficult and people regularly move on, looking for more fertile soil. The types of crops grown take the few remaining nutrients that are present from the ground, without putting anything back. Typically farmers move on after just three years and each time they move, they "slash and burn" virgin forest, destroying everything that grows.
If total deforestation of an area of rainforest occurs and the topmost layers of nutrient-deficient soil are removed by erosion, the virgin forest can once again be regenerated by a new primary succession. However, this is not possible if the underlying rock (such as weathered sandstone and alluvial sands) is low in nutrients. In this case only a much simpler ecosystem develops, such as a heath-population or a sparse savannah. Once destroyed, the rainforest is gone for ever.
Typically in an area like the Amazon basin, which is home to the world's largest single rainforest, only around 4% of land is suitable for agriculture of any type. 75% of the land is so poor that cultivators are unlikely to get more than a single crop from the soil, before it is completely exhausted.
Settlers tend to cut the largest trees, thinking that the soil is the most fertile there, but the rainforest natives know that the places where the trees have thin trunks often have the best soil. The soil infertility has caused the large trees to develop a highly efficient system of nutrient extraction. Tree roots can extend up to 100 meters along the ground from the tree trunk and form a root mat 30 centimeters (a foot) or more thick. This mat can capture over 99% of the nutrients that fall on it.
On the one hand rainforest soil is permanently damaged by water erosion. Deforested areas are exposed to rain and flooding, both of which washes away top-soils. Tree roots tend to hold soils together, act to provide physical reservoirs for water by making barriers and slow the rate of flow of moving water, thus reducing its eroding effects. In Nepal and India, for example, deforestation in the foothills of the Himalayas has led to catastrophic flooding of the river Ganges. Bangladesh has been in receipt of the results, with well known effects. Forests act as giant sponges, absorbing enormous quantities of water in the rainy or monsoon seasons. Normally this water is released in a controlled way over the following year, providing those downstream with a steady and sustained flow. Remove the rainforest sponge and the waters simply rush downstream and flood, often taking soils, humus, and the micro-fauna with them.
Mycorrhizal fungi © IMPACT Project
Conversely, soil can be baked by the sun. One consequence is that the important are destroyed by dehydration. The live in a symbiotic relationship with trees, and every rainforest tree species may have its own, very specialized, fungal species associated with it. These unique fungi enable the tree to absorb more minerals from the soil than it would otherwise be able to, in exchange for energy. These fungi are similar to those associated with mushrooms, that are commonly found in temperate forests. In both instances, most of the fungus is in tiny filaments that surround the tree's roots. When the are not present, the trees cannot grow. In deforested areas, fungi will not grow in the warmer and drier soil that results when the forest canopy is removed. The degraded soil is taken over by coarse grasses and other hardy species.
Mining activity not only destroys trees with clearings and roads for the mines, it also pollutes rivers and water tables with heavy metal toxins that are almost impossible to remove. Rainforest areas downstream of mines can be affected for hundreds of miles. These toxins - often mercury-based compounds - not only kill animals and plants, they can affect the microorganisms as well.
The microorganisms are at the bottom of the food chain. Their survival is essential for the survival of all the other species. Sad as it may be to lose something as beautiful as one of the jungle cats to extinction, through human abuse of the rainforest, the forest itself and most fauna would survive nevertheless. Loss of rainforest microbes, however, would have a severe effect throughout the entire rainforest ecology. The rainforest is dependent upon the actions of microbes, to sustain the base-level of food supply. Microbes dispose of dead matter by decaying and rotting it. They provide nutrients both from their by-products and from themselves, for other usually larger and more complex life-forms - they are food.
Many species of animal and plant could consequently suffer extinction, if microbial populations were destroyed or degraded. Although microbes are numerous and there are many species, they can be sensitive to the smallest of environmental change - changes in water acidity, levels of sunlight, toxins, etc. They can be quite fragile. However, they and their diversity are key to the survival of the forest.
The microbial world of the rainforest, though lacking the appeal of the more exotic larger creatures, should be regarded as just as important as any other rainforest population. Therefore efforts should be directed towards understanding and preserving their habitat and the way they interact with the forest environment.
The removal of forests from the land. Deforestation occurs when humans cut down trees to use as fuel or for building materials or to obtain land for agricultural, developmental, or other purposes. Deforestation results in the loss of an important sink for carbon dioxide during photosynthesis and an important element of the local ecosystem -- forests provide a home for plants and animals, stablize soils, and help regulate local climate conditions. Most deforestation results from burning to clear ground for agricultural production, leading to the release of more carbon dioxide and other greenhouse gases.
