B.F. Bachelor of Forestry Forest Soils 2 — Questions and Answers
Question 1: Which macronutrient is most commonly limiting to tree growth in boreal and many temperate forest soils?
- Phosphorus
- Potassium
- Nitrogen (Correct answer)
- Calcium
Correct answer: Nitrogen
Nitrogen is typically the most limiting macronutrient in boreal and temperate forests because slow decomposition rates limit nitrogen mineralization from organic matter.
Question 2: What type of symbiotic association do most conifers and many hardwoods form with soil fungi to enhance nutrient uptake?
- Rhizobium nodules
- Mycorrhizae (Correct answer)
- Actinomycete associations
- Lichen partnerships
Correct answer: Mycorrhizae
Mycorrhizae are mutualistic root-fungus associations that greatly enhance the absorption of phosphorus, nitrogen, and water in exchange for plant photosynthates.
Question 3: What is humus?
- The partially decomposed layer of fresh leaf litter at the soil surface
- Stable, well-decomposed organic matter that improves soil structure and fertility (Correct answer)
- The living microbial biomass within the soil matrix
- A type of clay mineral common in temperate forest soils
Correct answer: Stable, well-decomposed organic matter that improves soil structure and fertility
Humus is the dark, stable end product of organic matter decomposition that improves soil structure, water retention, CEC, and nutrient availability.
Question 4: Which biological process converts atmospheric nitrogen (N₂) into plant-available ammonium in forest soils?
- Denitrification
- Nitrification
- Nitrogen fixation (Correct answer)
- Ammonification
Correct answer: Nitrogen fixation
Nitrogen fixation is performed by specialized bacteria and cyanobacteria that enzymatically reduce atmospheric N₂ to ammonia, the first step toward plant uptake.
Question 5: How does soil compaction from heavy logging equipment primarily affect forest regeneration?
- It increases soil temperature and promotes seed germination
- It reduces soil porosity, limiting root penetration and water infiltration (Correct answer)
- It increases nutrient availability by breaking up organic matter
- It improves drainage and reduces waterlogging risk
Correct answer: It reduces soil porosity, limiting root penetration and water infiltration
Compaction reduces pore space, restricts oxygen availability, impedes root penetration, and decreases water infiltration, all of which hinder seedling establishment and early growth.
Question 6: What is the primary role of the soil food web in nutrient cycling within forest ecosystems?
- It mineralizes nutrients from inorganic compounds through chemical weathering
- It decomposes organic matter and releases nutrients into plant-available forms (Correct answer)
- It fixes atmospheric gases and incorporates them into the soil
- It transports nutrients from deep soil layers upward to the surface
Correct answer: It decomposes organic matter and releases nutrients into plant-available forms
The soil food web — including bacteria, fungi, nematodes, and invertebrates — decomposes organic matter and mineralizes bound nutrients, releasing them in forms plants can absorb.
Question 7: What is the process called by which organic nitrogen is converted to ammonium (NH₄⁺) in forest soils?
- Nitrification
- Denitrification
- Ammonification (Correct answer)
- Volatilization
Correct answer: Ammonification
Ammonification (mineralization) is the microbial breakdown of organic nitrogen compounds into ammonium, making nitrogen plant-available from decomposing organic matter.
B.F. Bachelor of Forestry Forest Soils 2
Which macronutrient is most commonly limiting to tree growth in boreal and many temperate forest soils?