4. Soil Organic Matter

A soil’s parent material determines

• the development of soil horizons

• the organic matter content of the soil
• the mineral composition of the soil
• the depth of the soil profile

A soil's ability to retain water and nutrients is largely a function of its 

• surface area to volume ratio
• carbon to nitrogen ratio
• biotic community
• mineralogy
  

The highest soil organic matter content would be expected

• In cold environments, due to slow decomposition
• In temperate environments, due to the balance of intermediate decomposition and plant productivity
• In hot environments, due to high plant productivity
• Soil organic matter content is ultimately not a function of climate

Within any particular climate, the rate of organic matter decomposition is primarily dependent upon

• the chemical composition of the organic matter
• which decomposer microbes are present
  
• the species diversity of the decomposer community
  
• the amount of sun- and ultraviolet light reaching the organic matter
  

Why is soil organic matter important?

• It promotes soil structural stability

• It helps prevent soil erosion

• It promotes water retention within the soil matrix

• All of these answers are correct

Which of these situations will favor the highest soil decomposer activities?

• When soil pores contain both air and water

• When soil pores are mostly dry and filled with air

• When soil pores are mostly filled with water

• All of these conditions favor high soil decomposer activities

Most organic matter entering the soil ultimately gets converted to

• carbon dioxide
• microbial biomass

• soil aggregates
• plant biomass

Which of these is likely to be respired into CO2 by decomposition the fastest?

• Soluble carbohydrates leached from freshly senesced plant litter

• Cellulose in the cell walls of freshly senesced plant litter

• Lignin in the cell walls of freshly senesced plant litter

• Hemicellulose in the cell walls of freshly senesced plant litter

A soil's ability to retain organic matter is largely a function of its:

• Age

• Carbon to nitrogen ratio

• Biotic community

• Climate

Gross primary production is best defined as:

• All carbon captured by plant photosynthesis

• Carbon captured by plant photosynthesis minus Carbon lost from autotrophic respiration

• Carbon captured by plant photosynthesis minus Carbon lost from both autotrophic and heterotrophic respiration

• All carbon lost from both autotrophic and heterotrophic respiration

• All carbon lost from autotrophic respiration