2.5 Biogeochemical Cycles, Biomes & Human Impact

2.5 Biogeochemical Cycles, Biomes & Human Impact

GED Exam Focus: Environmental science items test your knowledge of nutrient cycle steps (especially carbon and nitrogen), biome climate characteristics, succession sequences, and the chemical/ecological mechanisms behind human impacts like eutrophication, global climate change, and ocean acidification.

Biogeochemical Cycles: Matter Recycling

While energy flows one-way through ecosystems and eventually dissipates as heat, matter (chemical elements) is conserved and continuously recycled between biotic communities and the abiotic environment via biogeochemical cycles.

Biogeochemical Cycle Principle:
[Abiotic Reservoir: Soil / Air / Water]
       |                          ^
 (Assimilation / Uptake)   (Decomposition / Respiration)
       v                          |
[Biotic Community: Producers -> Consumers -> Decomposers]

1. The Carbon Cycle

Carbon is the backbone of organic biological molecules (carbohydrates, lipids, proteins, nucleic acids).

• Carbon Fixation (Photosynthesis): Autotrophs remove $CO_2$ gas from the atmosphere to synthesize glucose ($C_6H_{12}O_6$). $6CO_2 + 6H_2O + \text{light} \longrightarrow C_6H_{12}O_6 + 6O_2$
• Respiration: Plants, animals, and decomposers break down glucose, releasing $CO_2$ back into the atmosphere. $C_6H_{12}O_6 + 6O_2 \longrightarrow 6CO_2 + 6H_2O + \text{ATP}$
• Combustion: Burning fossil fuels (coal, oil, natural gas) and wood releases stored fossil carbon into the atmosphere as $CO_2$.
• Ocean Absorption: Oceans act as a major carbon sink, dissolving atmospheric $CO_2$.

2. The Nitrogen Cycle

Nitrogen is required to build amino acids (proteins) and nucleotides (DNA/RNA). Although nitrogen gas ($N_2$) makes up $78%$ of Earth's atmosphere, plants cannot absorb $N_2$ directly due to its triple covalent bond.

1. Nitrogen Fixation: Specialized nitrogen-fixing bacteria (e.g., Rhizobium residing in legume root nodules, or free-living soil cyanobacteria) convert inert $N_2$ gas into ammonia ($NH_3$) and ammonium ($NH_4^+$).
2. Nitrification: Soil nitrifying bacteria convert ammonium into nitrites ($NO_2^-$) and then nitrates ($NO_3^-$).
3. Assimilation: Plants absorb nitrates from soil to build plant proteins; herbivores eat plants to acquire organic nitrogen.
4. Ammonification: Decomposers break down nitrogenous waste and dead organisms, returning nitrogen to the soil as ammonium.
5. Denitrification: Denitrifying bacteria in anaerobic soils convert nitrates back into $N_2$ gas, completing the cycle.

3. The Water & Phosphorus Cycles

• Water Cycle: Driven by solar energy. Involves evaporation (water to vapor), transpiration (water evaporation from plant stomata), condensation (cloud formation), precipitation (rain/snow), and surface runoff/infiltration into aquifers.
• Phosphorus Cycle: Unique because it has no atmospheric gas component. Phosphorus cycles locally through rock weathering, soil absorption by plants, consumption, and marine sediment deposition.

Global Terrestrial Biomes

Biomes are large geographic regions characterized by distinct climate patterns (temperature and precipitation) and specialized ecological communities.

Ecological Succession

Ecological succession is the predictable, gradual process of community development and species replacement in an area over time following a disturbance.

Primary vs. Secondary Succession

Primary Succession Sequence:
Bare Rock  --->  Lichens/Mosses  --->  Grasses/Shrubs  --->  Softwood Trees  ---> Climax Community
              (Pioneer Species)    (Soil Formation)       (Intermediate)        (Hardwood Forest)

• Primary Succession: Begins in virtually lifeless environments devoid of topsoil (e.g., exposed bare rock from retreating glaciers, cooled volcanic lava flows).
  • Pioneer Species: The first organisms to colonize bare rock (typically lichens and mosses). They break down rock chemically to form initial organic topsoil.
• Secondary Succession: Occurs following a disturbance that clears an existing community but leaves topsoil intact (e.g., forest fires, abandoned agricultural fields, logging).
  • Because fertile soil is already present, secondary succession proceeds much faster than primary succession.
• Climax Community: The stable, mature, long-term ecological community achieved at the final stage of succession.

Human Impacts on Ecosystems & Biodiversity

Human industrial and agricultural activities significantly alter global ecosystems. Key environmental issues tested on the GED include:

1. Agricultural Eutrophication & Aquatic Dead Zones

Eutrophication occurs when excess synthetic agricultural fertilizers (containing high concentrations of nitrogen and phosphorus) run off into lakes, rivers, and coastal estuaries during heavy rain.

Step-by-Step Eutrophication Sequence:
1. Fertilizer Runoff (N & P) enters water body
        |
        v
2. Rapid Algal Bloom blocks sunlight to underwater plants
        |
        v
3. Algae die and sink to the bottom
        |
        v
4. Population of Aerobic Bacteria surges to decompose dead algae
        |
        v
5. Bacterial Respiration depletes Dissolved Oxygen (Hypoxia)
        |
        v
6. Fish and aquatic organisms suffocate -> "Dead Zone"

2. Global Climate Change & Ocean Acidification

• Greenhouse Effect: Combustion of fossil fuels increases atmospheric concentrations of greenhouse gases ($CO_2$, $CH_4$, $N_2O$). These gases trap infrared heat radiation reflected from Earth's surface, causing global average temperature increases.
• Ocean Acidification: As atmospheric $CO_2$ increases, oceans absorb excess gas. $CO_2$ reacts with seawater to form carbonic acid ($H_2CO_3$), releasing hydrogen ions ($H^+$) that lower ocean pH. This depletes carbonate ions, preventing marine shellfish and coral reefs from building calcium carbonate ($CaCO_3$) protective structures.

3. Habitat Loss & Invasive Species

• Habitat Fragmentation: Clearing land for agriculture or urban development breaks large habitats into small isolated patches, threatening species requiring wide ranges.
• Invasive Species: Non-native species introduced to a new environment (e.g., Kudzu, Zebra Mussels, Cane Toads). Lacking natural predators, invasive species outcompete native species for resources, destabilizing native food webs.