Matter cycles

Explain the difference between a gaseous and a sedimentary biogeochemical cycle, and classify the carbon, nitrogen, and phosphorus cycles into one of the two types, justifying your choice.

Define reservoir and flow in a biogeochemical cycle, and identify a specific reservoir and a specific flow in the carbon cycle, explaining which process maintains that flow.

Write the overall chemical equations for photosynthesis and aerobic cellular respiration, and explain the role both processes play, in opposite directions, within the carbon cycle.

Petrol combustion and cellular respiration both release $\text{CO}_2$. Explain what the two processes have in common chemically, how they differ, and why only one of them has been part of the natural carbon cycle for millions of years.

The diagram represents the main reservoirs and fluxes of the carbon cycle. Name the processes marked with the letters a, b, and c, indicate which process corresponds to the arrow marked with '?', and explain how this flux alters the balance of the cycle.

Describe the role of nitrogen-fixing bacteria and nitrifying bacteria, indicating which chemical transformation each group performs and why they are essential for plant nutrition.

Name the main chemical forms nitrogen adopts throughout its cycle (atmospheric $\text{N}_2$, ammonium, nitrite, nitrate and organic matter) and explain why most plants cannot directly use the $\text{N}_2$ in the air.

Order the following stages of the nitrogen cycle as they occur in an ecosystem and briefly describe what each consists of: nitrification, fixation, denitrification, and ammonification.

Phosphorus is often the limiting nutrient in terrestrial ecosystems. Explain what it means for a nutrient to be 'limiting', why the phosphorus cycle is so slow, and what consequence its lack of a gaseous phase has.

Unlike carbon and nitrogen, the phosphorus cycle is predominantly sedimentary. In an agricultural region, a paradox is observed: soil analyses show a high accumulation of total phosphorus after decades of phosphate fertilization, but crops show symptoms of chlorosis and phosphorus deficiency unless mineral fertilizers continue to be added, while adjacent surface waters suffer from eutrophication.

1. a) Justify why the lack of a significant gaseous phase constrains the natural recycling rate of phosphorus back to the continents, and explain which biological and geological processes allow its return to dry land on a macroscopic scale.
2. b) Explain the process of phosphorus fixation or insolubilization in soil: explain how pH (highly acidic or strongly calcareous soils) influences the formation of insoluble salts with iron, aluminium, or calcium, making phosphorus unavailable for root uptake.
3. c) Given the anticipated depletion of apatite mines ("peak phosphorus"), design two circular economy measures for an agricultural-livestock farm aimed at mobilizing the phosphorus already retained in its soils and recovering it from wastewater effluents without relying on synthetic fertilizers.

The roots of legumes host nodules containing bacteria of the genus Rhizobium. Explain what the plant and the bacterium exchange in this symbiosis, which process of the nitrogen cycle the bacterium performs, and what agricultural advantage there is in including legumes in crop rotation.

An environmental monitoring team studies a coastal lagoon after an agricultural runoff discharge and records the following data:

• Day 5: Surface reactive phosphorus concentration of $0,8\text{ mg/L}$ and chlorophyll $a$ of $65\ \mu\text{g/L}$; dissolved $\text{O}_2$ saturation of $145\,\%$ at noon in the top $40\text{ cm}$ of depth.

• Day 20: Surface $\text{O}_2$ saturation of $35\,\%$ at dawn; extreme turbidity ($40\text{ NTU}$); dissolved $\text{O}_2$ at the bottom ($3\text{ m}$) at $0,3\text{ mg/L}$, with the release of hydrogen sulfide ($\text{H}_2\text{S}$) and benthic fauna die-off.

A municipal technician states: "It is impossible for fish to die of asphyxiation if algae produce plenty of oxygen through photosynthesis; the cause must be direct chemical toxicity from nitrate."

1. a) Refute the technician's claim using biogeochemical arguments: justify why a massive autotrophic bloom inevitably leads to hypolimnetic and nocturnal anoxia.
2. b) Explain which trophic group proliferates from day 15 onwards and which anaerobic metabolic pathways account for the presence of $\text{H}_2\text{S}$ at the bottom.
3. c) Predict what immediate ecological consequences treating the lagoon with a chemical algaecide to eradicate phytoplankton would have on the matter cycle.

A maize farm requires an input of $180\text{ kg N/ha}$. The agronomist is hesitating between two management options:

• Option 1: Apply synthetic mineral fertiliser (urea/ammonium nitrate) produced industrially via the Haber-Bosch process.

• Option 2: Sow a cover crop of common vetch (Vicia sativa, a legume) in autumn-winter, chop it as green manure in spring, and supplement with composted pig slurry.

1. a) Compare both options in terms of energy consumption and carbon footprint: justify how the $\text{N}\equiv\text{N}$ bond is chemically broken in industry compared to how symbiotic nitrogenase does it in root nodules.
2. b) Following a spring storm, nitrate concentrations of $95\text{ mg/L}$ (the safe threshold is $50\text{ mg/L}$) and emission peaks of nitrous oxide ($\text{N}_2\text{O}$) into the atmosphere are recorded in wells near plots treated exclusively with Option 1. Explain which microbial transformations of the nitrogen cycle give rise to both problems from the surplus fertiliser.
3. c) Design a combined strategy that takes advantage of the immediate availability of mineral fertiliser and the sustainability of biological fertiliser, minimising leaching and gaseous losses.