Environmental Cycles: Sulfur, Phosphorus, and Nitrogen – Lecture Notes

Sulfur Cycle: Overview and Key Concepts

• The sulfur cycle is described in the lecture in parallel to the carbon cycle, highlighting the movement of sulfur through rocks, oceans, biomass, and the atmosphere, driven by geological and biological processes.
• Major reservoirs and forms:
  • Rocks containing sulfur (e.g., sulfide minerals such as iron sulfide, FeS$_2$).
  • Ocean water: dissolved sulfur species present in seawater.
  • Biomass: sulfur incorporated in organic molecules and amino acids.
  • Atmosphere: sulfur gases and aerosols (e.g., sulfur dioxide, SO$2$; hydrogen sulfide, H$2$S).
• Geological and volcanic sources:
  • Volcanic activity releases sulfur compounds (e.g., SO$2$) along with carbon compounds (CO$2$).
  • Iron sulfide (FeS$_2$) in rocks is a key sulfur carrier; heating or weathering can separate iron from sulfur.
  • Volcanic emissions also contribute to atmospheric sulfur species that can form sulfuric acid upon oxidation.
• The sulfur cycle involves the following transformations:
  • Sulfur in minerals (e.g., FeS$2$) is exposed by weathering and can be released as sulfur gases (SO$2$, H$_2$S).
  • In the atmosphere, sulfur gases oxidize to sulfuric acid-forming species; sulfuric acid (H$2$SO$4$) can precipitate as acid rain or dry deposition.
  • Acid rain deposits sulfur back to soils and water, where it can be incorporated into soils or minerals and re-enter the cycle via weathering and biological uptake.
  • The cycle can repeat as sulfur moves between rocks, soils, oceans, biomass, and the atmosphere.
• Biological significance and amino acids:
  • Sulfur is essential in biochemistry; two of the 20 amino acids contain sulfur in their side chains: cysteine and methionine.
  • Sulfur availability influences protein synthesis and metabolic pathways.
• Forms of sulfur and relevant chemistry:
  • Sulfides: S$^{2-}$; sulfides occur in minerals like FeS$_2$.
  • Sulfates: SO$_4^{2-}$; common in soils and oceans.
  • Sulfites: SO$_3^{2-}$; transient intermediate in some sulfur reactions.
  • Sulfate versus sulfide distinction: the lecture emphasizes that the practical distinction often matters for environmental processes (e.g., acid rain formation from sulfates).
• Atmospheric component and cycle speed:
  • The sulfur cycle has a clear atmospheric component, making it a relatively fast cycle compared with carbon and phosphorus cycles.
  • The phosphorus cycle is highlighted as the one cycle without a significant atmospheric component (in other words, it cycles on a much longer timescale).
• Human relevance and emissions:
  • Sulfur enters the atmosphere primarily through the burning of sulfur-containing fuels and mining/smelting activities, increasing SO$_2$ emissions and contributing to acid rain.
  • Ultra-low sulfur fuels (e.g., ULSD with ~15 ppm sulfur) are discussed as a way to reduce atmospheric sulfur emissions from transportation.
• Nutritional and biological notes:
  • Sulfur-containing amino acids are critical for protein function and metabolism.
  • Some biological observations mentioned include the idea that trace sulfur can affect oxygen transport in hemoglobin, potentially influencing athletic performance when sulfur compounds are present in small amounts (note: this is anecdotal in the lecture).
• Key takeaway about sulfur in environmental context:
  • Sulfur originates from rocks, oceans, biomass, and atmosphere and cycles through these compartments via geological and biological processes.
  • Human activities (fossil fuel combustion, mining, and smelting) intensify the cycle by increasing atmospheric SO$_2$, leading to acid rain and ecosystem impacts.

Sulfur Cycle: Sulfides, Sulfates, and Acid Rain

• Sulfides (e.g., FeS$2$) and sulfates (SO$4^{2-}$) are sulfur-containing species in rocks and soils; oxidation or weathering releases sulfur into the environment.
• Volcanic processes emit sulfur-containing gases, notably sulfur dioxide (SO$2$) and sometimes hydrogen sulfide (H$2$S).
• In the atmosphere, SO$2$ is oxidized to sulfate aerosols, which can convert to sulfuric acid (H$2$SO$_4$) and fall as acid rain (wet deposition) or dry deposition.
• Acid rain chemistry (illustrative):
  • SO$2$ + H$2$O + O$2$ → H$2$SO$_4$ (approximate path in the atmosphere leading to rain acidity).
• Acid rain impacts:
  • Lowers soil and water pH, increases mobility of toxic metals, and affects plant and aquatic life.
  • About 70% of acid rain is estimated to be deposited on oceans, influencing ocean chemistry and nutrient cycles.
• Biological and ecological significance:
  • Sulfur in ambient science influences enzyme function and respiration in some organisms; trace amounts of sulfur can affect oxygen transport in the bloodstream under certain conditions (as discussed in the lecture).
• Practical sulfur chemistry concepts from the lecture:
  • Sulfates (SO$_4^{2-}$) and sulfides (S$^{2-}$) are both important sulfur forms; the emphasis is on sulfur starting with sulfur-containing compounds (S-containing rock minerals, sulfates) rather than the exact structural distinction.
  • The sulfur cycle involves rocks, oceans, biomass, atmosphere, and volcanic sources as key reservoirs and fluxes.

Human Impacts on the Sulfur Cycle

• Fossil fuel combustion: refining and burning coal and diesel increases sulfur emissions; sulfur dioxide (SO$_2$) is released into the atmosphere.
• Vehicle emissions: ultra-low sulfur diesel (~15 ppm sulfur) reduces SO$_2$ emissions from traffic, but broader fossil fuel use still contributes to atmospheric sulfur.
• Mining and smelting: higher rates of mining and smelting increase SO$_2$ emissions, amplifying acid rain formation.
• Environmental consequences:
  • Acid rain alters soil chemistry, lowers pH in freshwater systems, and contributes to metal mobility and ecosystem stress.
  • Acid rain and sulfur deposition are linked to ecological and health concerns (asthma and respiratory issues cited in the lecture).
• Connections to other cycles:
  • The sulfur cycle interacts with the carbon cycle via volcanic emissions and atmospheric chemistry; sulfur compounds can influence biological processes and energy metabolism in organisms.

Plant Nutrients: Calcium, Magnesium, and Potassium (K), and Their Cycles

• Calcium (Ca), Magnesium (Mg), and Potassium (K) are essential macronutrients for plants.
• Practical example: a head of broccoli can contain more calcium than a glass of milk, illustrating nutrient density differences.
• Biological relevance:
  • Calcium is involved in nerve impulses and signaling; the sodium-potassium pump is a fundamental cellular mechanism mentioned in the lecture.
• Cycles:
  • These elements cycle similarly to phosphorus in terms of source and sink dynamics: rocks weather to release ions, which are taken up by plants and return to soils/biomass; some fraction remains stored in minerals, some is leached away, and some is returned via decomposition.
• Note on naming:
  • Potassium starts with the letter P in the mnemonic, but its chemical symbol is K (potash is commonly a potassium-containing fertilizer). The lecture hints at this distinction and the practical use of these nutrients in agriculture.

Phosphorus and Nitrogen Cycles: Fertilizers, Detergents, and Ecological Impacts

• Fertilizers and major players:
  • The two big fertilizer ingredients discussed are phosphorus (P) and nitrogen (N).
  • Detergents historically contained phosphates to enable cleaning; modern formulations increasingly advertise phosphate-free options to reduce eutrophication.
  • Phosphates were used in dishwashing detergents to remove stains; this practice contributed to nutrient loading in water bodies when wash water entered waterways.
• Detergent phosphate issue and environmental impact:
  • Phosphates in wastewater promote algal growth in surface waters, reducing sunlight penetration and harming coral reefs and overall aquatic health.
  • Reformulations to phosphate-free detergents aimed to reduce eutrophication and protect aquatic ecosystems.
• Phosphorus cycle: sources and sinks
  • Major sources: weathered rocks, mineral deposits, and biomass turnover.
  • Sinks: incorporation into soils and sediments; uptake by plants and microorganisms; some phosphate remains in soluble forms and can be leached into waterways.
• Nitrogen cycle (briefly referenced):
  • Not discussed in depth in the transcript, but nitrogen is also a key fertilizer component and participates in similar source-sink dynamics (soil, water, biomass).
• Eutrophication and algal blooms:
  • Phosphorus (and nitrogen) enrichment leads to algal blooms that can block sunlight, disrupt coral reefs, and create dead zones (hypoxic, anoxic waters) in aquatic systems.
• Agricultural and urbanization connections:
  • Excess nutrients enter waterways through runoff from agriculture and impervious surfaces (roads, roofs) that reduce infiltration and increase surface runoff.
  • This nutrient loading can fuel excessive plant and algal growth downstream, impacting water quality and ecosystem health.

Water Cycle, Land Use, and Ecosystem Impacts

• Urbanization and hydrology:
  • Impervious surfaces (roads, buildings) reduce infiltration, increase runoff, and alter the natural water cycle.
  • Reduced groundwater recharge can necessitate more irrigation and groundwater pumping in downwind agricultural areas.
• Deforestation effects:
  • Deforestation reduces canopy interception and evapotranspiration, impacting local and regional rainfall patterns.
  • Increased runoff can lead to soil erosion and nutrient loss from soils, affecting downstream water quality.
• Biodiversity and nitrogen:
  • Increased nitrogen availability can alter plant community composition and reduce biodiversity (example: wetland grasses in South Carolina).
  • Invasive species such as Phragmites australis (Australian origin) can outcompete native Spartina alterniflora under higher nitrogen scenarios, changing marsh ecology and food webs.
  • Spartina is native and tolerant of low-nitrogen environments; Phragmites can thrive with added nitrogen, displacing native species and altering trophic relationships.
• Food web consequences in marshes:
  • Changes in vegetation affect herbivores and higher trophic levels (e.g., marsh periwinkle snails feeding on Spartina and blue crabs depending on the native community).
  • Shifts in species composition propagate through the food web, with economic implications for fisheries and ecosystem services.
• Nutrient enrichment and ecosystem health:
  • Phosphorus enrichment can cause eutrophication and dead zones, leading to hypoxic or anoxic conditions that are detrimental to aquatic life.
• Sulfur, acidity, and heavy metals:
  • Burning coal and other sulfur-containing fuels contributes to acid rain, which increases the solubility of toxic heavy metals and affects ecosystem health and water quality.
• Synthesis: why this matters for environmental science
  • Understanding how fertilizers disturb sulfur, phosphorus, and nitrogen cycles helps explain real-world environmental problems (e.g., acid rain, eutrophication, coral reef degradation, biodiversity loss).
  • The lecture emphasizes tying substance properties, human activities, and ecological outcomes together to explain environmental consequences.

Integrated Takeaways and Connections

• Humans disturb natural cycles primarily through fossil fuel combustion, mining, and agricultural practices, with nutrient runoff and acid deposition being central concerns.
• The sulfur cycle is notably accelerated by atmospheric processes (SO$2$, H$2$S, H$2$SO$4$ formation) compared to the relatively slower carbon and phosphorus cycles.
• Phosphorus, unlike carbon and sulfur, has no robust atmospheric pathway in the cycle, making it more dependent on geological and hydrological transport and thus relatively slow.
• Fertilizers (N and P) are major drivers of eutrophication, influencing both freshwater and marine ecosystems and affecting coral reefs and downwind agricultural areas.
• The interaction between land use (deforestation, urbanization, mining) and nutrient cycling has broad implications for water quality, biodiversity, climate relevance, and ecosystem services.

Note: Some minor factual points in the transcript (e.g., specific claims about sulfur effects on hemoglobin-mediated oxygen transport and potash/phosphate mining distinctions) are presented here as described in the lecture. Where applicable, I’ve mirrored the instructor’s statements and indicated potential ambiguities or common real-world notes (e.g., potash vs. phosphate sources) to support exam-style recall and critical understanding.