ESA Soil Science & Biogeochemistry Flashcards

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

Answer: Soils (organic carbon)

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.

Denitrification is favored under which soil conditions?

Answer: Anaerobic, carbon-rich soils

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

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

Answer: Silt loam

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

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

Answer: C:N:P = 106:16:1

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

Phosphorus availability in soils is typically lowest at:

Answer: Acidic pH below 5.5 and alkaline pH above 7.5

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

Which process is responsible for the conversion of organic nitrogen in decomposing litter to inorganic ammonium?

Answer: Mineralization (ammonification)

Mineralization (ammonification) is the microbial decomposition of organic nitrogen-containing compounds that releases NH4+ into the soil solution.