ESA ESA Soil Science & Biogeochemistry 1 — Questions and Answers
Question 1: Which soil horizon is characterized by the accumulation of leached materials such as clay, iron oxides, and organic matter?
- O horizon
- A horizon
- B horizon (Correct answer)
- C horizon
Correct answer: B horizon
The B horizon (subsoil) is the zone of illuviation where materials translocated from upper horizons accumulate.
Question 2: The primary driver of nitrogen fixation in terrestrial ecosystems is:
- Abiotic lightning discharge
- Biological N2 fixation by diazotrophic microorganisms (Correct answer)
- Atmospheric deposition of NOx
- Mineralization of soil organic matter
Correct answer: Biological N2 fixation by diazotrophic microorganisms
Biological nitrogen fixation by diazotrophs (e.g., Rhizobium, free-living cyanobacteria) is the dominant pathway converting atmospheric N2 to bioavailable forms.
Question 3: Cation exchange capacity (CEC) of a soil is primarily determined by:
- Soil bulk density
- Clay content, clay mineralogy, and organic matter content (Correct answer)
- Soil pH alone
- Parent material particle size
Correct answer: Clay content, clay mineralogy, and organic matter content
CEC reflects the abundance of negatively charged sites on clay particles and organic matter surfaces that retain exchangeable cations.
Question 4: Which process converts ammonium (NH4+) to nitrate (NO3−) in soils?
- Denitrification
- Mineralization
- Nitrification (Correct answer)
- Assimilation
Correct answer: Nitrification
Nitrification is a two-step aerobic microbial process: ammonia-oxidizing bacteria convert NH4+ to NO2−, which nitrite-oxidizing bacteria then convert to NO3−.
Question 5: Soil organic matter decomposition is primarily regulated by which set of factors?
- Parent material and slope aspect
- Temperature, moisture, and substrate quality (C:N ratio) (Correct answer)
- Vegetation type and canopy cover alone
- Soil texture and bulk density alone
Correct answer: Temperature, moisture, and substrate quality (C:N ratio)
Decomposition rates are controlled by temperature and moisture availability, which govern microbial activity, and by litter chemistry (C:N ratio, lignin content).
Question 6: Which ectomycorrhizal adaptation allows host trees to access phosphorus in organic soil layers?
- Production of phosphatase enzymes by fungal hyphae (Correct answer)
- Direct root elongation into organic matter
- Nitrogen fixation by fungal symbionts
- Secretion of organic acids to dissolve parent rock
Correct answer: Production of phosphatase enzymes by fungal hyphae
Ectomycorrhizal fungi produce phosphatase enzymes that mineralize organic phosphorus, making it available for plant uptake.
ESA ESA Soil Science & Biogeochemistry 1
Which soil horizon is characterized by the accumulation of leached materials such as clay, iron oxides, and organic matter?