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 global 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 city 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.