module_02 Carbon Transformations and Organic Matter
lesson_01 What happens to organic materials once they are added to soil?

Be able to discuss the formation of soil organic matter

Material vs matter Plant and animal residues which are not yet added to soil are organic material. Examples of organic material added to rice paddies include rice straw and animal manure. Once organic material is in soil, it is referred to as organic matter.

Did you know? About organic matter...

• Soils rich in organic matter are characteristically dark in color.

• Soil organic matter contains 3-4 times as much carbon as is found in all the world’s living vegetation.

• The mean level of organic carbon in the top soils of lowland rice areas across tropical Asia is between 1-2%

Decomposition Organic material added to soil is food for decomposing organisms. The quality of this food depends on several factors like particle size, the types of carbon compounds in its composition, and the amount of nitrogen it contains. Organic materials with fewer complex carbon compounds will decompose faster. Materials with more nitrogen also decompose faster because nitrogen is needed by decomposing organisms.

Soil factors like pH, temperature, moisture, and aeration also affect decomposition rate. Decomposition proceeds more quickly when pH is neutral, temperature is warm, and both oxygen and moisture are available. anaerobic soil The lack of oxygen that occurs in submerged soils results in slower decomposition. Relatively few types of decomposing organisms are adapted to an anaerobic environment - only facultative and obligate anaerobes (refer to Module 2 Lesson 3). Also, less energy is produced during anaerobic respiration compared to aerobic respiration. For this reason, partially decomposed organic materials often accumulate in an anaerobic environment.

Next lesson

The next lesson explains how the products of decomposition differ in an anaerobic rice paddy compared to aerobic soil.


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