Matter cycles
The carbon in your breath today may once, millions of years ago, have been part of a dinosaur. Unlike energy, which is dissipated as heat and does not come back, the atoms of matter are not destroyed: they pass from living things to the atmosphere, the water or the rocks, and return again and again. These journeys are called biogeochemical cycles.
Reservoirs, fluxes and types of cycle
A mature forest in the Pacific Northwest stores tons of carbon in its trunks, but when the wood burns or decomposes, that mass disappears from view even though the atoms have not been destroyed. Where is the planet's matter stored, and by what mechanisms does it travel from one place to another?
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,…
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…
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.
Decomposers
A fallen leaf in a Texas Hill Country oak woodland disappears in a few months. Where does it go?
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…
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.
The carbon cycle
Write the overall chemical equations for photosynthesis and aerobic cellular respiration, and explain the role both processes play, in opposite directions, within the…
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.
Oceans, forests and methane
The oceans are a huge carbon reservoir. $\text{CO}_2$ dissolves in surface water, and phytoplankton use it for photosynthesis. When they die, some carbon sinks to the deep ocean and sediments, so the ocean acts like a sponge for atmospheric $\text{CO}_2$.
Disrupting the carbon cycle has consequences for whole ecosystems: extra atmospheric $\text{CO}_2$ warms the planet and dissolves in the oceans, making seawater more acidic, which stresses organisms such as corals and shellfish. When a tropical forest is cleared, the carbon in its biomass is released as $\text{CO}_2$ by burning and decay. Fewer trees means less evapotranspiration, so less moisture returns to the air, rainfall can decrease, and bare soil increases runoff and erosion.
Methane ($\text{CH}_4$) is produced by archaea in oxygen-free places, such as the stomachs of cattle and wetlands, by anaerobic digestion (methanogenesis). It is part of the carbon cycle. Per molecule, methane traps much more heat than $\text{CO}_2$ (a much higher global warming potential), so small amounts matter.
Explain the role phytoplankton plays in the oceanic carbon cycle and why the oceans are said to act as a sponge for atmospheric $\text{CO}2$.
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…
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.
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 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…
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...).
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…
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,…
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…
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).
The nitrogen cycle
Although the air you breathe contains $78\%$ nitrogen and plants are immersed in it, many corn fields turn yellow and stop growing if fertilizer is not added to the soil. Why does an element that is so abundant in the atmosphere turn out to be unavailable to plants, and how do they manage to take it in?
Describe the role of nitrogen-fixing bacteria and nitrifying bacteria, indicating which chemical transformation each group performs and why they are essential for plant…
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…
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…
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.
The phosphorus cycle and other sedimentary cycles
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…
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.
Eutrophication: when the cycles become unbalanced
After the neighboring fields have been heavily fertilized, a pond of clear water becomes covered within a few days by a dense, opaque green layer, and soon afterwards dead fish appear floating at the edge. How can a massive input of nutrients, which stimulates the primary producers, end up wiping out the aquatic fauna?
Oxygen in a bloom changes through the day. In daylight, algae photosynthesize and surface water can become supersaturated with dissolved $\text{O}_2$ (for example, 145% saturation at noon). At night, algae and bacteria only respire, so surface oxygen falls (for example, 35% at dawn). When the algae die, decomposition near the bottom can push oxygen close to zero (hypoxia), and anaerobic microbes may release hydrogen sulfide ($\text{H}_2\text{S}$), killing bottom-dwelling animals.
The Gulf of Mexico dead zone forms this way: nutrient runoff from farms in the Mississippi River basin feeds algal blooms, and decomposition uses up the oxygen near the bottom.
An environmental monitoring team studies a coastal lagoon after an agricultural runoff discharge and records the following data: Day 5: Surface reactive phosphorus…
An environmental monitoring team studies a coastal lagoon after an agricultural runoff discharge and records the following data:
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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.
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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."
- a) Refute the technician's claim using biogeochemical arguments: justify why a massive autotrophic bloom inevitably leads to hypolimnetic and nocturnal anoxia.
- 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.
- c) Predict what immediate ecological consequences treating the lagoon with a chemical algaecide to eradicate phytoplankton would have on the matter cycle.
The sulfur cycle
Sulfur is part of amino acids such as cysteine and methionine. Plants take up sulfate from the soil and the sulfur passes to consumers through feeding; decomposition returns it to the environment. Microorganisms transform reduced and oxidized compounds: in anoxic environments sulfide can be produced, and oxidizing bacteria convert it into sulfate.
Back to the beginning: The carbon atom you breathe out today could have belonged to a dinosaur because, after the reptile died and decomposed, that carbon passed into the atmosphere as $\text{CO}_2$ or was trapped in sedimentary rocks; millions of years later, a plant fixed it again by photosynthesis and it reached you through the food chain.