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## Nutrient Cycles in Ecosystems

Nutrients cycle within and among ecosystems, ensuring the continuous supply of essential elements.

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### Hydrologic (Water) Cycle

Definition: Collects, purifies, and distributes the Earth’s water supply.

- Precipitation: Falls on terrestrial ecosystems.

- Surface Runoff: Movement of water over soil after precipitation.

- Glaciers: Precipitation converted to ice and stored.

- Groundwater: Water that seeps deeper into the soil.

- Aquifers: Underground layers of sand and water-bearing rock.

#### Human Impacts on the Hydrologic Cycle

- Over-withdrawal of Freshwater: Using water faster than it can be replaced.

- Deforestation: Reduces water infiltration into soil.

- Wetland Destruction: Affects natural flood control.

- Air Pollutants (Aerosols): May weaken rainfall patterns.

- Climate Change:

- Increases glacial melting and evaporation.

- Alters global water distribution.

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### Carbon Cycle

Definition: Carbon cycles through the biosphere via photosynthesis and aerobic respiration.

- Carbon dioxide (CO₂):

- Makes up 0.040% of the atmosphere.

- Stored in deeply buried deposits of dead plant matter and algae, which are converted into fossil fuels (coal, oil, and natural gas).

- Decomposers: Release stored carbon from insoluble carbonate minerals and rocks in sediment.

#### Human Impacts on the Carbon Cycle

- Burning Fossil Fuels: Adds CO₂ to the atmosphere faster than natural cycles can absorb.

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### Nitrogen Cycle

Definition: Nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems.

- Essential for: Proteins, vitamins, and DNA.

- Five Steps of the Nitrogen Cycle:

1. Nitrogen Fixation: Conversion of atmospheric nitrogen into a usable form by nitrogen-fixing bacteria, volcanic activity, lightning, and industrial processes.

2. Nitrification: Soil bacteria convert ammonia to nitrate, providing energy for nitrifying bacteria.

3. Assimilation:

- Plants absorb ammonia or nitrate to create plant compounds (e.g., proteins).

- Animals assimilate nitrogen by consuming plants or other animals.

4. Ammonification:

- Bacteria decompose nitrogen-containing waste (e.g., urine), producing ammonia.

5. Denitrification:

- Nitrate is converted back to nitrogen gas by denitrifying bacteria in oxygen-poor environments (e.g., deep soil near water tables).

#### Important Nitrogen-Processing Organisms

- Rhizobium: Nitrogen-fixing bacteria found in root nodules of legumes (e.g., beans, peas).

- Cyanobacteria: Photosynthetic bacteria that fix nitrogen in moist environments.

#### Key Nitrogen Compounds

- Ammonia (NH₃)

- Ammonium ions (NH₄⁺)

- Nitrate ions (NO₃⁻)

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### Phosphorus Cycle

Definition: Movement of phosphorus through water, the Earth’s crust, and living organisms.

- Phosphorus is essential for:

- DNA and cell membranes.

- Bone and teeth formation.

- Phosphate Ions (PO₄³⁻):

- Main form of phosphorus available to plants.

- Phosphorus cycles slower than water, carbon, and nitrogen because it does not have a gaseous phase.

#### Human Impacts on the Phosphorus Cycle

- Fertilizer Use: Increases phosphorus runoff, leading to aquatic ecosystem disruptions.

- Deforestation: Reduces soil phosphate levels due to erosion.

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### Human Activities & Biogeochemical Cycles

Human actions significantly alter these natural cycles:

- Burning fossil fuels → Increases CO₂ emissions.

- Deforestation → Reduces water and carbon absorption.

- Fertilizer use → Alters nitrogen and phosphorus balance in ecosystems.

- Aerosols & Climate Change: Affect global water cycles and temperature balance.

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### Freshwater Availability

- Only 0.024% of Earth’s freshwater supply is accessible to humans and other species.

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