Chemical Characterization

1. Chemical Composition

2. Important Cycles (and the related impacts of deforestation)

3. Important Microorganisms

 

 

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

Horizon	pH	C% composition	N% composition
A (humus, nutrients)	4.95	2.17	0.16
A1	4.98	2.24	0.15
A12	5.00	2.02	0.13
A2	4.92	0.49	0.05
A3	4.67	1.04	0.10
B (clay)	4.95	0.45	0.05
C (bedrock)	5.11	0.32	0.03

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).