ESA ESA Soil Science & Biogeochemistry 2

Question 1

In the global carbon cycle, the largest active reservoir of carbon is found in:

Accepted Answer

Soils (organic carbon)

Answer

Soils contain approximately 1,500–2,000 Pg C in organic form, making them the largest active terrestrial carbon reservoir—roughly twice the atmospheric pool.

Question 2

Denitrification is favored under which soil conditions?

Accepted Answer

Anaerobic, carbon-rich soils

Answer

Denitrifying bacteria use NO3− as an electron acceptor when O2 is limiting, so anaerobic, carbon-rich (electron donor-rich) conditions maximize denitrification.

Question 3

Which soil texture class has the highest water-holding capacity per unit volume?

Accepted Answer

Silt loam

Answer

Silt loam soils have a fine pore structure that retains more plant-available water than coarser-textured soils while still allowing drainage.

Question 4

The Redfield ratio describes the stoichiometric relationship in marine phytoplankton as approximately:

Accepted Answer

C:N:P = 106:16:1

Answer

The Redfield ratio (C:N:P ≈ 106:16:1) reflects the consistent elemental composition of marine phytoplankton and guides understanding of nutrient limitation.

Question 5

Phosphorus availability in soils is typically lowest at:

Accepted Answer

Acidic pH below 5.5 and alkaline pH above 7.5

Answer

At low pH, phosphorus is fixed by iron and aluminum; at high pH, it precipitates with calcium—so availability peaks near neutral pH.

ESA - Ecological Society of America Certified Ecologist Practice Test

ESA - Ecological Society of America Certified Ecologist Practice Test

ESA ESA Soil Science & Biogeochemistry 2