Biogeochemical Cycles - Comprehensive Notes

Biogeochemical Cycles

Overview

In biogeochemical cycles, substances are temporarily stored in "nutrient reservoirs" such as organisms, soil, air, and water.
Rapid cycling: substances move between nutrient reservoirs quickly (e.g., producer to consumer).
Slow cycling: substances move between nutrient reservoirs slowly (i.e., fossils).

Components

Biotic component, available as nutrients: living organisms.
Biotic component, unavailable as nutrients: living or recently living organisms (coal, oil, peat) undergoing fossilization.
Abiotic component, available as nutrients: atmosphere, soil, water undergoing weathering and erosion.
Abiotic component, unavailable as nutrients: minerals in rocks undergoing the formation of sediments.
Processes connecting these components include respiration, decomposition, erosion, burning of fossil fuels, excretion, and photosynthesis.

Carbon and Oxygen Cycles

Plants take in more carbon in the form of $CO_2$ every year than plants and animals make.
Most of the carbon released back into the atmosphere is from forest fires and the breakdown of organic material by decomposers.
Plants, animals, and decomposers play a vital role in the rapid cycling of carbon.

Carbon Cycle Details

Burning fuels (wood and fossil fuels) releases carbon into the atmosphere.
Oxygen in the atmosphere $(O2)$ and carbon in the atmosphere $(CO2)$ are key components.
Photosynthesis converts atmospheric carbon dioxide into plant matter, releasing oxygen.
Cellular respiration by higher-level consumers, decomposers, and plants returns carbon dioxide to the atmosphere.
Decomposers break down detritus (dead organic matter).

Rapid Cycling of Carbon

Used by plants in photosynthesis to make oxygen and energy.
Given off by plants and animals during cellular respiration.

Slow Cycling of Carbon

Much of the carbon in the environment is held in living organisms.
Trees are known as “carbon sinks” as they store large amounts of carbon.
Carbon is only released through decomposition or forest fires.

Fossilization

Some organic matter is not decomposed once the organism dies.
It will settle and become fossilized.
Fossils hold onto carbon for thousands or millions of years until they are burned as fossil fuels.

Carbon Sinks

The ocean is the largest carbon sink.
Weathering of certain rocks such as limestone also releases small amounts of carbon into soil, air, and water.
Limestone is formed from calcium carbonate in the shells of aquatic organisms $(CaCO_3)$ .

Carbon Cycle Diagram

Rapid cycling involves processes like combustion, evaporation, respiration, and photosynthesis.
Atmosphere: $\text{CO}_2$ dissolves in water.
Biotic environment: interaction between living organisms.
Abiotic processes contribute to the cycle.
Slow cycling involves petroleum deposits (fossil fuels) in the Earth's crust and deep ocean.

Human Impact on the Carbon Cycle

Human activities disrupt the natural cycling of carbon.
This disruption can have significant effects on the environment.

Greenhouse Effect

Solar energy from the sun passes through the atmosphere.
The Earth's surface is heated by the sun and radiates the heat back out towards space.
Some energy is reflected back out to space.
Greenhouse gases in the atmosphere trap some of the heat.

Carbon Dioxide Variations

The industrial revolution has caused a dramatic rise in $\text{CO}_2$ .
Ice Age cycles show variations in carbon dioxide concentration over thousands of years.

Oxygen Cycle

The oxygen cycle is closely linked to the carbon cycle.
Oxygen is also cycled by means of cellular respiration and photosynthesis.

Sulfur Cycle

All organisms require sulfur as it is an important part of proteins and vitamins.
Plants and algae use sulfur in the form of sulfate $(SO_4^{2-})$ .
Decomposers break down plants and animals and return sulfur to the soil or atmosphere as hydrogen sulfide $(H_2S)$ .
Hydrogen sulfide is the rotten egg smell in muddy ponds and wetlands.

Sulfur Cycle - Bacteria

Bacteria are an essential part of the sulfur cycle.
They use sulfur-containing compounds in photosynthesis or cellular respiration.
Sulfate reducers convert sulfate to sulfide.
Sulfur oxidizers convert sulfide to elemental sulfur and sulfate.

Acid Deposition

Some sulfur is taken out of rapid cycling as it becomes part of rocks.
Fossil fuels (oil, coal, and natural gas) contain sulfur.
Sulfur is also found in rocks and is released by weathering.

Acid rain

Acid rain: sulfur dioxide reacts with oxygen and water vapor to form sulfurous acid $(H2SO3)$ and sulfuric acid $(H2SO4)$ .
These acids return to earth as rain, snow, or sleet.
It can damage vegetation, acidify lakes, leach nutrients from the soil, and corrode buildings.

Sulfur Cycle (Diagram)

Sulfur dioxide $(SO_2)$ is emitted from volcanoes and through geologic uplifting, upwelling in groundwater, and mining and fossil fuel burning.
This sulfur dioxide becomes sulfate in the atmosphere $(SO_4^{2-})$ . Volcanoes also produce sulfate.
Acid precipitation, uptake by plants, decomposition, and sedimentation are key processes.
Bacterial interface plays a crucial role (e.g., sulfate to inorganic sulfur $(S2)$ , and hydrogen sulfide $(H2S)$ formation).
Inorganic sulfur present in rocks, coal, oil, ores, and seafloor vents.

Nitrogen Cycle

Nitrogen gas makes up 78.1% of the Earth’s atmosphere.
Nitrogen is an essential part of proteins and the structure of DNA.
Nitrogen is not easily accessible to organisms as most cannot use atmospheric nitrogen.
To be useful to plants, nitrogen must be in the $NO3^-$ (nitrate ion) form or $NH4^+$ (ammonium).

Atmospheric Composition

Nitrogen: 78%
Oxygen: 21%
Argon: ~1%
Traces of Water Vapor, Carbon Dioxide, Helium, Neon, Methane, Nitrous Oxide

Nitrogen Fixation

Nitrogen fixation: the changing of atmospheric nitrogen into $NO3$ and $NH4^+$ .
This can be done in two ways:
- Lightning: causes nitrogen to react with oxygen to make $NO_3$ .
- Some bacteria found on the roots of legumes (e.g., peas, clover) can convert $N2(g)$ into ammonium $(NH4^+)$ .

Ammonification

Ammonification: when decomposers break down organic matter and make ammonium $(NH_4^+)$ .

Nitrification

Nitrification: bacteria will convert ammonium $(NH4^+)$ into nitrite $(NO2^-)$ or nitrate $(NO_3^-)$ .

Denitrification

Denitrification: bacteria will convert nitrite $(NO2^-)$ or nitrate $(NO3^-)$ back into nitrogen gas $(N_2)$ and give it off into the atmosphere.
Denitrification occurs in environments with very little oxygen.

Nitrogen Cycle Details (Diagram)

Nitrogen-fixing bacteria in nodules and soil perform $N_2$ fixation.
Human activities, nitrification by nitrifying bacteria.
Denitrification of $NO_3$ by denitrifying bacteria occurs.
Cyanobacteria in biotic communities also fix nitrogen.
Decomposers break down dead organisms and animal waste to produce $NH_4^+$ .

Phosphorus Cycle

Phosphorus is an essential nutrient for DNA and ATP and a major component of bones and teeth.
It is available in very limited quantities.
Phosphorus does not cycle through the atmosphere.
It is found in soil, water, and rocks.

Phosphorus Uptake

Animals obtain phosphorus through eating grain, meat, and drinking milk.
Plants can only use phosphorus in its $PO_4^{3-}$ (phosphate) form as it dissolves in water.
Algal bloom: overgrowth of algae caused by excess phosphorus in lakes.
Algal blooms are detrimental to aquatic environments as the decomposers breaking down the organic matter use up all the oxygen.

Phosphorus Cycle (Diagram)

Geological uplifting and weathering of phosphate from rocks, run-off.
Phosphate in solution, chemical precipitation.
Detritus settling to the bottom, sedimentation forms new rock.
Animals consume plants, decomposers break down organic matter, phosphate in soil, leaching.