The large-scale deforestation of tropical rainforest has had a great impact towards the environment, from the loss of habitats, to driving climate change. However what one often neglects is destruction of soil as a result of deforestation. Recently while watching a documentary about the deforestation in the Amazon I’ve noticed that the deforested areas were like deserts, which strongly contrasted with the lush lives among the rainforest floors. This is seen in the image below taken by John Michael Fay of National Geographic.
Figure 1: Deforested Amazon Rainforest (Fay, J.M., n.d. )
This led me to my question, what is the main factor(s) which determine the health of the soil or the extent of soil degradation?
Rainforest soil have little or no nutrients as they are rapidly used up by the flourishing plant life (Sayre, 1994). However a tropical rainforest has its own nutrient cycle, where it recycles the nutrients released by the decomposition of the organic matter on the forest floor. The Nitrogen cycle and Water cycles within the Tropical Rainforest, are the 2 major cycles which allows the rich existence of life among the forest (Witherick, M.E., 2010).
As learned from GCSE Biology, Nitrogen is a essential element for all living things. This is because the enzymes in and living organism which facilitates all kinds of reaction and processes is a form of protein. And from IB Chemistry we know that a protein molecule is identified by the functional group of an amine.
Figure 2: A Protein Molecule (University of New Mexico, n.d.)
Although the air is composed of 79% Nitrogen gas, but it is in a chemical form which is inaccessible to the majority of living organisms. (Kiera, S., 2009). This form of Nitrogen can be converted to the usable form of nitrates and nitrites through Nitrogen Fixation, a process limited to some microorganisms. (Nitrogen Cycle, n.d.). The nitrogen fixation mechanism is undergone exclusively with prokaryotes using the complex enzyme of nitrogenase and can be seen from the following equation (Deacon, J., n.d.):
N2 + 8H+ + 8e– + 16 ATP ↔ 2NH3 + H2 + 16ADP + 16 Pi
As seen with the visible layer of humus and rotting leaves of the rainforest floor,the hot and humid climate of the rainforest, this creates an ideal condition for decomposition. The decaying leaves, animal and insect droppings are decomposed by fungi and bacteria, which releases Ammonium (NH4+). Then through the process of Nitrification by bacteria, nitrate and nitrite compounds can be formed and assimilated into plants. This cycle is represented in the following diagram by Sierra Kiera, 2009:
Figure 3: The Nitrogen Cycle (Sierra Kiera, 2009).
Water is also an essential substance for all living organism as it is a component of aerobic respiration. The water cycle consist of the processes of: evaporation, precipitation, transpiration, through flow, surface runoff. In the diagram below by BBC Bitesize, it shows the uniqueness of the water cycle and the features that help the rainforest retain and store the precious water.
Figure 4: The Rainforest Water Cycle (BBC, n.d.)
These cycles are examples of close systems. As we have just learned in Chemistry, the closed system allows the rainforest to sustain a natural occurring equilibrium of its nutrient usage and creation and water storage and transfer. By removing the trees from the cycle, we are in fact “opening” the system, creating an imbalance in nature, interfering with the equilibrium. Since the trees are a large store of water in the cycle and prevent direct rainfall, the removal of the trees will cause the rain to wash away any nutrients and physically erode the soil. Since there are no more decaying roots and leaves from trees, there will be a lack of supply to nitrogen, which inhibits the growth of other organism. Also the lack of decomposition of these organic substances, will cause problems with water retention. Water plays a vital role not just in aerobic respiration but assimilating important dissolved minerals such as calcium and magnesium into plants. Due to the copious amount of rain in a tropical climate, the remaining nutrients in the soil is quickly washed away, leaving a sheet of bare land with no capability of plant growth.
In conclusion, the ratio of organic matter in soil is a major factor in determining the fertility of the soil. The decomposers (fungi and bacteria), the amount of nutrients, the decaying matter, and the water retention rate are all necessities in soil for the growth of plants. (Lewandowski, A., 2002). With the lack of either component, the soil can be easily susceptible to erosion, degrading the soil, in the long-run causing desertification. This poses a threat to the available arable land available for crops and other agricultural use, which directly affects our supply of food.
Sayre, April Pulley. Exploring Earth’s Biomes: Tropical Rainforest. New York: Henry Holt and Company, Inc, 1994, pp.1-56
Nitrogen Cycle. (n.d.). Microbiology The Beginning. Retrieved September 5, 2013, from http://www.theguardians.com/Microbiology/gm_mbr14.htm
Kiera, S. (2009, October 13). Nitrogen Cycle in the Rainforest. Biology of Tropical Rainforest. Retrieved September 5, 2013, from https://sites.google.com/site/biologyoftropicalrainforests/classroom-news/reminderthatihttps://sites.google.com/site/biologyoftropicalrainforests/classroom-news/reminderthatitsashortweekthisweektsas
ACEER. (n.d.). Water Cycle. West Chester University. Retrieved September 5, 2013, from http://www.wcupa.edu/aceer/amigos/cd/water_cycle.htm
Lewandowski, A. (2002). Organic Matter Management – Soil Scientist. University of Minnesota Extension. Retrieved September 5, 2013, from http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
BBC. (n.d.). BBC – GCSE Bitesize: Rainforest water and nutrient cycles. BBC – Homepage. Retrieved September 5, 2013, from http://www.bbc.co.uk/schools/gcsebitesize/geography/ecosystems/tropical_rainforests_rev2.shtml
Deacon, Jim. “The Microbial World: The Nitrogen cycle and Nitrogen fixation.” Biology. The University of Edinburgh, n.d. Web. 5 Sept. 2013. <archive.bio.ed.ac.uk/jdeacon/microbes/nitrogen.htm>.
Witherick, M. E., & Milner, S. (2010). Edexcel IGCSE geography. Harlow: Edexcel.
“Biological Macromolecules.” UNM Biology Department Home Page. N.p., n.d. Web. 5 Sept. 2013. <http://biology.unm.edu/ccouncil/Biology_124/Summaries/Macromol.html>.
Fay, J.M., n.d. Retrieved from: http://environment.nationalgeographic.com/environment/photos/rainforest-deforestation/#/madagascar-slash-burn_278_600x450.jpg