Questions: Nutrient Cycling: Phosphorus and Sulfur Cycles

Question 1

A freshwater lake near intensive farmland develops severe algal blooms after years of agricultural runoff. The limiting nutrient responsible is most likely:

• A. Nitrogen — because nitrogen fixation makes it highly mobile and easily transported from agricultural soils to lakes
• B. Phosphorus — because it is the limiting nutrient in most freshwater ecosystems and has no atmospheric reservoir, so agricultural runoff delivers phosphate that was previously scarce, triggering explosive algal growth
• C. Carbon — because agricultural CO₂ emissions dissolve in the lake and fertilize photosynthesis
• D. Sulfur — because acid rain from SO₂ emissions destroys competitors of algae and releases sulfur nutrients

Answer: (B)

Phosphorus is the limiting nutrient in most freshwater ecosystems precisely because it has no atmospheric reservoir — only slow rock weathering supplies it naturally. When agricultural fertilizers (rich in phosphate) wash into lakes, they relieve this natural limitation and trigger eutrophication: explosive algal growth, oxygen depletion, and ecosystem disruption. Nitrogen can be a co-limiting nutrient in some marine systems, but the freshwater case is classically phosphorus-driven. The absence of a biological 'fixation' shortcut for phosphorus is what makes agricultural runoff so ecologically potent.

Question 2

Why can't terrestrial ecosystems replenish phosphorus the same way they replenish nitrogen after phosphorus is exported to deep ocean sediments?

• A. Phosphorus is chemically inert and cannot be processed by soil bacteria the way nitrogen can
• B. There is no atmospheric reservoir for phosphorus and no biological process equivalent to nitrogen fixation — phosphorus locked in deep sediment can only return via tectonic uplift over millions of years
• C. Phosphorus bonds irreversibly with calcium in ocean water and can never be recycled under any conditions
• D. Decomposers cannot break down phosphorus compounds, so phosphorus is lost at every trophic level

Answer: (B)

The key contrast is nitrogen fixation: nitrogen-fixing bacteria (Rhizobium, cyanobacteria) can pull N₂ directly from the atmosphere, replenishing fixed nitrogen on timescales of years to decades. Phosphorus has no atmospheric gas phase and no biological mechanism to extract it from air or water when it's locked in deep sediment. The geological timescale of phosphorus recycling (tectonic uplift over millions of years) is why phosphorus is a chronic limiting nutrient — what weathering releases is essentially all that ecosystems get on a human timescale.

Question 3

Unlike the nitrogen cycle, the phosphorus cycle has no significant atmospheric phase, making phosphorus unavailable through atmospheric deposition in most ecosystems.

• T. True
• F. False

Answer: True

This is the defining structural feature of the phosphorus cycle as a sedimentary cycle. Nitrogen cycles through N₂ gas (78% of Earth's atmosphere); carbon cycles through CO₂ and CH₄. Phosphorus has no stable volatile compound under normal biospheric conditions — no atmospheric gas phase, no atmospheric reservoir, no biological pathway equivalent to nitrogen fixation to replenish what is lost. Phosphorus moves: rock → weathering → soil/water → organisms → decomposition → soil/sediment → (eventually, over millions of years) back to rock. Every step is in the solid or dissolved phase.

Question 4

The sulfur cycle, like the phosphorus cycle, is primarily a sedimentary cycle with no significant atmospheric component.

• T. True
• F. False

Answer: False

False — the sulfur cycle has a substantial atmospheric component that phosphorus entirely lacks. Volcanoes and hot springs release SO₂ and H₂S into the atmosphere; bacterial decomposition releases H₂S from anaerobic sediments; industrial combustion of fossil fuels releases SO₂. These atmospheric sulfur compounds form sulfate aerosols (affecting climate by reflecting sunlight) and produce acid rain when they dissolve in water. This atmospheric dimension gives sulfur cycling outsized importance for climate and air quality — a stark contrast to phosphorus, which stays almost entirely in the solid and dissolved phases.

Question 5

Why does the absence of an atmospheric phase make phosphorus uniquely vulnerable to permanent loss from terrestrial ecosystems, compared to nitrogen?

Model answer: Nitrogen lost from an ecosystem — through denitrification to N₂, or leaching — can be replenished because nitrogen-fixing bacteria and cyanobacteria pull N₂ directly from the atmosphere, which contains an essentially inexhaustible supply. Phosphorus has no atmospheric reservoir and no biological fixation mechanism. Once phosphorus washes from soil to rivers to the deep ocean and settles into sediment, it is effectively unavailable to terrestrial ecosystems until tectonic uplift re-exposes it — a process requiring millions of years. This one-way drainage makes phosphorus a non-renewable resource on ecological timescales.

The contrast illuminates why the phosphorus cycle's lack of an atmospheric phase is so consequential. Atmospheric gases can be drawn down by biological processes (nitrogen fixation, photosynthesis for carbon), creating renewable sources. Phosphorus has no such renewal pathway. The practical consequence is eutrophication: when humans add phosphorus faster than ecosystems can immobilize it, the excess flows to aquatic systems and disrupts them. There is no natural atmospheric buffer to absorb the excess.