Soil Biology and Soil Ecology
Domain scope
This domain examines soil organisms, organic matter decomposition, nutrient cycling, bioavailability, biodegradation kinetics, and ecotoxicology. Questions often connect microbial processes to contaminant attenuation and remediation design.
Core rules and concepts
- The soil food web includes bacteria, fungi, protozoa, nematodes, and earthworms. Mycorrhizal fungi extend plant nutrient uptake and can affect metal tolerance.
- Decomposition is controlled by C:N ratio, temperature, moisture, oxygen, and substrate quality. A C:N near 25–30 supports active microbial growth without severe nitrogen immobilization.
- Bioavailability, not total concentration, often governs toxicity and biodegradation. Aging and sorption reduce bioavailability.
- First-order biodegradation follows Ct = C0 e^(−kt); half-life t1/2 = ln2/k.
- Biostimulation adds nutrients or electron acceptors; bioaugmentation adds specific organisms.
Worked example
A first-order rate constant k = 0.05/day. Half-life = 0.693/0.05 = 13.9 days. After 60 days, fraction remaining = e^(−0.05×60) = e^(−3) = 0.050, so about 5% remains. The 90% removal time is ln(10)/0.05 = 46 days.
Traps that cost marks
Assuming total contaminant concentration predicts toxicity; ignoring sorption and aging when estimating bioavailability; and treating soil respiration as equivalent to microbial biomass.