A & C only
A & C only are correct about biogeochemical cycling.
Statement Analysis
A. TRUE – Carbon cycling between CO₂ (inorganic) ↔ organic forms occurs via aerobic (photosynthesis/respiration) and anaerobic (methanogenesis, fermentation) microbial pathways. Methanogens reduce CO₂ to CH₄ anaerobically.
B. FALSE – Biogeochemical cycles are primarily energized by solar energy driving photosynthesis (C, N, S fixation), primary production, and trophic transfers. Only phosphorus cycle lacks direct solar input.
C. TRUE – Phosphorus cycle involves only PO₄³⁻ (orthophosphate) forms—no gaseous phase exists. Weathering → solution → uptake → sedimentation → uplift cycle operates entirely in solid/liquid phases.
D. FALSE – Methane (CH₄) has high global warming potential (GWP ≈ 28-34× CO₂ over 100 years, 84-87× over 20 years), making it major climate concern despite shorter atmospheric residence.
E. FALSE – Nitrogen (N₂, NOₓ), sulfur (SO₂, H₂S) have gaseous components; phosphorus cycle lacks gaseous phase. E incorrectly claims all three gaseous only.
Option Breakdown
| Option |
Evaluation |
Reason |
| A & C only |
Correct |
Validates microbial C-cycling + sedimentary P-cycle facts. |
| A, B, C only |
Incorrect |
B contradicts solar energy dependency. |
| A, B, D only |
Incorrect |
B false; D reverses methane GWP reality. |
| A, C, E only |
Incorrect |
E ignores phosphorus sedimentary nature. |
Biogeochemical cycling inorganic organic carbon transformations by microbes and phosphorus phosphate no gaseous form define key ecological processes tested in NEET/GATE environmental science.
Carbon Cycle Microbial Pathways (Statement A)
Aerobic: CO₂ + H₂O → [CH₂O] + O₂ (photosynthesis); C₆H₁₂O₆ + 6O₂ → 6CO₂ (respiration).
Anaerobic: CO₂ → CH₄ (methanogenesis); fermentation yields organic acids → methanogenesis. Methanotrophs oxidize CH₄ aerobically.
Phosphorus Sedimentary Cycle (Statement C)
No gaseous phase: Apatite (PO₄) → weathering → HPO₄²⁻/H₂PO₄⁻ → plant uptake → detritus → sedimentation → geological uplift (10⁵-10⁸ yr). Slowest biogeochemical cycle.
Common Exam Distractors
Solar energy (B): Drives autotrophic fixation powering all food webs.
Methane GWP (D): 28× CO₂ (100-yr), 84× (20-yr)—potent short-term warmer.
N/P/S gaseous (E): N₂ (78% atm), SO₂ volcanic; P uniquely sedimentary.
Cycle Classification
| Cycle |
Gaseous Reservoir |
Primary Energy |
| Carbon |
CO₂, CH₄ |
Solar |
| Nitrogen |
N₂ (78%) |
Solar/bacterial |
| Sulfur |
SO₂, H₂S |
Solar/volcanic |
| Phosphorus |
None |
Geological |
NEET Application
A & C only tests microbial metabolism knowledge + sedimentary vs. gaseous cycle distinction—recurring pattern in ecosystem dynamics questions.
