Phosphorus and Sulfur Cycles Study Notes

Phosphorus and Sulfur Cycles

Overview

• Topic: Phosphorus and Sulfur Cycles

• Course: AP Environmental Science

• Focus on understanding interactions in ecosystems through phosphorus and sulfur cycles.

Learning Objectives

• Enduring Understanding (ERT-1): Ecosystems result from interactions between biotic (living organisms) and abiotic (non-living elements) factors.

  • Learning Objective (ERT-1.F): Explain the steps and reservoir interactions in the phosphorus cycle.

Phosphorus Cycle

Essential Knowledge

1. Definition (ERT-1.F.1): The phosphorus cycle involves the movement of atoms and molecules containing phosphorus between sources and sinks, with sources being where phosphorus originates and sinks being where it accumulates.

2. Major Reservoirs (ERT-1.F.2): The primary reservoirs of phosphorus are rock and sediments enriched with phosphorus-bearing minerals, critical for the cycling of this element in ecosystems.

3. Atmospheric Component (ERT-1.F.3):

   • Absence of Atmospheric Component: Unlike other biogeochemical cycles, the phosphorus cycle does not include a significant atmospheric element.

   • Consequences: This absence results in the limited return of phosphorus from oceanic sources back to land, leading to its scarcity in both aquatic and many terrestrial ecosystems.

   • Consequences in Ecosystems: In undisturbed ecosystems, phosphorus often acts as a limiting nutrient, meaning its availability restricts biological productivity.

Vocabulary

• Phosphate: Refers to the phosphorus that can exist in various forms:

  • Forms: phosphate rock, soil phosphate, dissolved phosphates.

• Chemical Symbol: $PO_4^{3-}$

• Bioavailable Phosphorus: Phosphorus in a form that can be absorbed and utilized by living organisms, including plants, algae, and bacteria.

• Biological Importance: Integral in the formation of DNA, RNA, ATP (adenosine triphosphate), and phospholipids.

• Physical role in Organisms: Essential component of bones and teeth in vertebrates.

• Eutrophication: Nutrient enrichment process that leads to excessive plant growth and decay, causing oxygen depletion in the water, resulting from high phosphate levels.

Phosphorus Cycle Process

Step-by-Step Breakdown

• Phosphate Mining: Extraction of phosphate for agricultural fertilizers.

• Fertilizer Application: To crops, contributing to increased phosphorus in soils.

• Biological Consumption: Incorporation of phosphates into living organisms (plants consume phosphates, which are then passed to herbivores and carnivores).

• Erosion: Natural weathering of rocks contributes dissolved phosphates into soils and water systems.

• Animal Interactions:

  • Animal Waste: Both excretion and decomposition return phosphates back into the soil environment.

• Marine Influence: Phosphates can dissolve in marine environments and contribute to oceanic life.

Human Impacts on the Phosphorus Cycle

• Sources of Eutrophication:

  • Fertilizer runoff, including mine-tailings from phosphate mining.

• Environmental Consequences: Increased nutrient loads in aquatic systems lead to cultural eutrophication, a phenomenon that drastically affects water quality and marine ecosystems.

Sulfur Cycle

Overview

• Components: Involves several forms of sulfur in the ecosystem, primarily as sulfur dioxide ($SO<em>2$), hydrogen sulfide ($H</em>2S$), and various sulfate ions.

Key Processes

1. Volcanic Eruptions: Natural source of sulfur emissions into the atmosphere as $H_2S$.

2. Atmospheric Sulfur: Sulfur dioxide ($SO_2$) can undergo precipitation as acid rain, impacting both terrestrial and aquatic systems.

3. Decomposition: As organic matter decomposes, $H_2S$ is released into the soil.

4. Soil and Marine Contributions: Sulfates (e.g., $SO_4^{2-}$) are produced, entering both soil and marine environments.

Human Impacts on the Sulfur Cycle

• Fossil Fuels:

  • Combustion of coal and diesel fuels releases sulfur in the forms $H<em>2S$ and $SO</em>2$ into the atmosphere.

  • Sulfur dioxide can convert to sulfuric acid, contributing significantly to acid rain issues.

• Rate of Release: Current rates of sulfur release into the environment may be unprecedented compared to historical natural cycles.

  • Impact on Waterways: Elemental sulfur ($S$) or sulfates ($SO_4^{2-}$) can be released into waterways, further impacting aquatic ecosystems.

  • Major Contributors: Pyrite ($FeS_2$), commonly known as fool's gold, serves as a substantial natural source of sulfur emissions in natural geological processes.

Mercury Cycle

• Importance of Mercury: Not essential for biological functions but critically relevant for pollution concerns.

  • Extensive discussion on the mercury cycle is reserved for later study in the pollution unit.