Questions: Biogeochemical Cycles: Carbon, Nitrogen, and Phosphorus

Question 1

Which biogeochemical cycle lacks a significant atmospheric phase and is therefore the slowest to replenish on ecological timescales?

• A. Carbon cycle
• B. Nitrogen cycle
• C. Phosphorus cycle
• D. Sulfur cycle

Answer: (C)

Phosphorus has no significant atmospheric reservoir. Carbon cycles through the atmosphere as CO₂; nitrogen cycles through the atmosphere as N₂ (the largest reservoir). Phosphorus is locked in rock and sediment and enters ecosystems almost entirely through slow geological weathering. This makes phosphorus the classic 'limiting nutrient' in many terrestrial and aquatic ecosystems — it cannot be replenished quickly when depleted.

Question 2

Nitrogen fixation — the conversion of atmospheric N₂ to biologically usable forms — can be performed by most soil bacteria when environmental nitrogen is scarce.

• T. True
• F. False

Answer: False

Nitrogen fixation is performed by only a small number of specialized prokaryotes (e.g., Rhizobium, cyanobacteria, Azotobacter) because it requires the enzyme nitrogenase and is extremely energetically costly. The triple bond in N₂ is one of the strongest in chemistry. Most bacteria cannot fix nitrogen at all, even when nitrogen is limiting. This is why legume-Rhizobium symbioses are so ecologically important.

Question 3

Explain why the nitrogen cycle is described as the most 'biologically mediated' of the three major biogeochemical cycles.

Model answer: Unlike the carbon cycle (which has major abiotic drivers like volcanic emissions and chemical weathering) and the phosphorus cycle (driven largely by geological weathering), nearly every key transformation in the nitrogen cycle depends on specialized microbial activity. Nitrogen fixation (N₂ → NH₄⁺) requires nitrogen-fixing prokaryotes. Nitrification (NH₄⁺ → NO₃⁻) requires nitrifying bacteria. Denitrification (NO₃⁻ → N₂) requires denitrifying bacteria. Without these microbes, nitrogen would remain locked in the atmosphere as inert N₂, unavailable to plants and animals.

The key insight is that each step in the nitrogen cycle requires specific enzymatic machinery found only in certain prokaryotes. No abiotic process under normal conditions can efficiently fix nitrogen from the atmosphere or perform nitrification. This biological bottleneck means that disrupting microbial communities (through pesticides, waterlogging, or tillage) directly impacts nitrogen availability for the entire ecosystem.