Exercises: 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.

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.

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.

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.

Explain what the decomposition of organic matter is, which groups of organisms carry it out, and why the cycles of matter would come to a halt without them even if producers were still present.

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.

Explain the role phytoplankton plays in the oceanic carbon cycle and why the oceans are said to act as a sponge for atmospheric $\text{CO}_2$.

Gasoline 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.

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.

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 possible journey of a carbon atom from being part of atmospheric $\text{CO}_2$ to becoming incorporated into a muscle in your body and then returning to the atmosphere, naming the biological process behind each step (photosynthesis, feeding, respiration...).

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.

A mature forest fixes $10\ \text{t}$ of $\text{CO}_2$ per hectare per year, but its trees and decomposers release $9.5\ \text{t}$ through respiration over the same period. Calculate the forest's net annual carbon sequestration and explain why a young, growing forest sequesters much more than a mature one even though it fixes less $\text{CO}_2$ in total.

Explain how the deforestation of a tropical basin disrupts both the carbon cycle and the water cycle at the same time: what happens to the carbon stored in the biomass, to evapotranspiration, to the region's rainfall and to runoff.

Cattle emit methane ($\text{CH}_4$) during digestion. Explain which anaerobic process produces it, which cycle of matter it belongs to, and why its emission is a concern even though it is much smaller in quantity than that of $\text{CO}_2$ (compare their greenhouse warming potentials).

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.

Two high-mountain lakes, located at equal distances from an industrial park with coal-fired power plants, receive continuous precipitation with an average pH of $4.1$. Biological and chemical monitoring reveals very contrasting data:

• Lake A (situated on a granitic basin): Has lost its salmonid and amphibian populations; a dissolved $\text{Al}^{3+}$ concentration of $0.35\text{ mg/L}$ is detected in its waters.

• Lake B (situated on a limestone basin): Keeps its fish richness intact and shows no detectable dissolved aluminum.

1. a) Justify on a lithological and chemical scale why the same flux of acid rain causes a biological collapse in lake A but proves harmless in lake B.
2. b) Explain the biogeochemical mechanism by which acidification of the surrounding soil mobilises toxic ions such as $\text{Al}^{3+}$ and leaches essential nutrients ($\text{Ca}^{2+}$, $\text{Mg}^{2+}$), and predict the impact of this phenomenon on the forest in basin A.
3. c) To restore lake A, an association proposes periodically adding large volumes of slaked lime or calcium carbonate directly to the water. Argue the limitations and potential harmful effects of this measure compared to reducing $\text{SO}_2$ emissions at the source.

A science sceptic posts the following message: "The atmosphere contains about $850\text{ Gt}$ of carbon and the oceans more than $38\,000\text{ Gt}$. Our annual emissions from burning fossil fuels are barely around $10\text{ Gt}$. Since the planet's total carbon is constant and anthropogenic emissions are less than $0.03\,\%$ of the marine reservoir, it is absurd to claim that we can alter the global carbon cycle."

1. a) Refute their argument by distinguishing between the dynamics of the biological (fast) cycle and the geological (slow) cycle: use the concepts of flux, residence time, and dynamic equilibrium to justify why $10\text{ Gt/year}$ destabilises the global climate.
2. b) If natural sinks (biosphere and ocean surface) absorb on average $45\,\%$ of the anthropogenic carbon emitted each year, calculate how many ppm of $\text{CO}_2$ accumulate annually in the atmosphere, knowing that $1\text{ ppm} \approx 2.13\text{ Gt}$ of atmospheric carbon.
3. c) Predict what will happen to the ocean's capacity to continue acting as a chemical sink for $\text{CO}_2$ as surface water temperature increases and its pH decreases, justifying through the chemical equilibrium of carbonic acid and carbonates.

To combat global warming, a geoengineering project has been proposed consisting of "fertilising" high-nutrient, low-chlorophyll (HNLC) oceanic zones by adding soluble iron salts to enhance the biological carbon pump.

1. a) Justify what nutritional limitation iron relieves in these areas and describe the biogeochemical pathway that the fixed carbon must follow so that sequestration is effective on a geological timescale (centuries or millennia) rather than a mere rapid biological recycling.
2. b) Analyse the benthic ecological risks: predict what will happen in the deep water column if a massive rain of organic marine snow descends to aphotic zones with a low rate of water turnover.
3. c) Current global warming intensifies the thermal stratification of the surface ocean (creating a sharper thermocline). Predict and argue how this stratification will affect the natural functioning of the biological pump and what feedback it will cause on atmospheric $\text{CO}_2$.

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.

An environmental impact assessment report compares two newly built hydroelectric dams:

• Dam A: Floods a rocky, mountainous valley in a temperate climate, where all tree biomass was previously felled and cleared. Average bottom water temperature: $9\ ^\circ\text{C}$.

• Dam B: Floods a tropical rainforest plain without clearing the vegetation or the thick soil humus layer. Average bottom water temperature: $26\ ^\circ\text{C}$.

During its first decade of operation, Dam B emits more greenhouse gases into the atmosphere per MWh produced than a natural gas thermal power plant.

1. a) Justify biogeochemically why dissolved $\text{O}_2$ is depleted at the bottom of Dam B and which microbial consortia act so that the carbon stored in plant biomass is released as methane ($\text{CH}_4$) instead of carbon dioxide ($\text{CO}_2$).
2. b) Knowing that the 20-year global warming potential of $\text{CH}_4$ is about 80 times greater than that of $\text{CO}_2$, critically discuss the common claim that "hydroelectric power is always a clean, carbon-neutral energy source."
3. c) Design two pre-filling measures and one technical management measure at the turbine water intake (intake elevation) to prevent anoxic waters rich in dissolved gases from being discharged downstream and suddenly degassing.

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, aluminum, 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.

An agricultural consortium discharges its runoff water loaded with nitrates ($80\text{ mg/L}$) and soluble phosphorus through a constructed wetland before reaching a protected coastal lagoon. At the outlet of the wetland, analyses indicate:

• Nitrate concentration reduced to $9\text{ mg/L}$.

• Continuous release of diatomic nitrogen ($\text{N}_2$) into the atmosphere with near-zero $\text{N}_2\text{O}$ emissions.

• Net carbon accumulation in the sediment at a rate of $400\text{ g C/m}^2\times\text{year}$.

1. a) Analyze the biogeochemical microzonation of the wetland: justify how the oxidized rhizosphere of aquatic macrophytes and the underlying anoxic mud interact spatially to successfully complete sequential nitrification and denitrification.
2. b) Explain the mechanism by which wetlands retain and accumulate carbon over millennia at rates higher than those of many mature forests, relating water saturation to the low activity of bacterial and fungal hydrolytic enzymes.
3. c) Predict and discuss the biogeochemical consequences on the nitrogen and carbon cycles if this wetland were artificially drained to convert it into developable land.