9 Sulfur Cycle and Interconnection 1

Sulfur: Fundamental Properties and Historical Context

• Periodic Table Placement: Sulfur ($S$) is located in Column 16, grouped alongside Oxygen ($O$) and Selenium ($Se$). Its behavior is logically similar to oxygen, particularly in its ability to form a diverse array of chemical compounds.

• Cosmic and Terrestrial Abundance:

  • It is the tenth most abundant element in the universe by mass.

  • It is the fifth most abundant element on Earth.

• Biological Importance: Sulfur is essential for all life. Its primary biological role involves its association with two specific amino acid units (cysteine and methionine) which are critical for the manufacture and structure of proteins.

• Historical Significance:

  • It has been isolated and utilized throughout history.

  • The Ancient Greeks famously used sulfur as a fumigant, likely utilizing its elemental form sourced from volcanic eruptions.

• Industrial Applications: Sulfur is used extensively in modern industry, specifically in:

  • The manufacture of sulfuric acid ($H_2SO_4$), which is a precursor for fertilizers and essential salts.

  • Pesticides.

  • The pharmaceutical industry.

  • Battery manufacturing.

  • The food industry.

• Winogradsky and Microbiology: The scientist Winogradsky is noted for his work with the Winogradsky column, a glass jar ecosystem. He discovered Beggiatoa, a bacterium associated with sulfur-rich environments that can both oxidize forms of sulfur and reduce them to generate hydrogen sulfide ($H_2S$).

• Nomenclature Note: The transcript acknowledges two spellings: "Sulfur" (Americanized) and "Sulphur" (traditional UK), noting that global standards may eventually converge.

The Global Sulfur Cycle: Pathways and Processes

• Atmospheric Inputs:

  • Industrial Source: The burning and smelting of fossil fuels generate sulfates ($SO_4$) that enter the atmosphere.

  • Volcanic Source: Volcanic eruptions release sulfur directly into the air.

• Deposition Mechanisms:

  • Dry Deposition: Sulfur falls directly from the sky onto vegetation, grasslands, forests, and other natural or artificial environments.

  • Wet Deposition: Sulfur dissolves in precipitation and falls as rain, snow, or cloud moisture. This was notably associated with "acid rain" in the 1970s and 80s, where dissolved sulfur caused a significant drop in environmental $pH$.

• Biotic Transformation: Sulfur is assimilated by plants and biomass inorganically and then exuded or transformed into organic forms. This leads to "organic deposition."

• Soil and Sediment Dynamics:

  • Both organic and inorganic forms of sulfur accumulate in soils and sediments.

  • Decomposition of living biomass returns organic sulfur to the environment.

  • Microorganisms in terrestrial environments assimilate sulfates (oxidized mineral sulfur) and transform them into organic sulfur.

• Marine Cycles:

  • Deposition into the marine environment creates sulfate pools in the water.

  • At depth, sulfates are deposited into sediments.

  • In anoxic (oxygen-depleted) conditions, sulfates are reduced to sulfides ($S^{2-}$).

• Geological Sequestration: Over geological timescales, sulfur becomes associated with sediments and fossil fuels, forming iron sulfides ($FeS_2$), coal/oil deposits, or ores. These are eventually reactivated via volcanic activity or human extraction (mining and smelting).

Terrestrial Sulfur Cycling: Analysis of Inputs and Losses

• Inputs (Blue Squares):

  • Atmospheric Deposition: Wet or dry, predominantly inorganic.

  • Animal Manures and Biosolids: Inorganic sulfur source.

  • Plant Residues: Primarily organic sulfur.

  • Mineral Fertilizers: Predominantly human-influenced inorganic inputs.

• System Components and Transformations (Green Circles):

  • Atmospheric Pool: Sulfur from volcanic or anthropogenic pollution sources.

  • Plant/Solid Environment: Direct elemental or inorganic deposition onto plants.

  • Mineralization: The transformation of organic sulfur (from residues/manures) into inorganic sulfur, such as sulfate ($SO_4^{2-}$).

  • Immobilization/Assimilation: Microbes take up inorganic sulfate and convert it back into organic forms.

  • Adsorption: Inorganic sulfur can be adsorbed onto particle exchange sites (specifically the anion exchange) or synthesized into minerals.

• Losses from the System (Red Symbols):

  • Leaching: The primary loss of sulfur occurs through the leaching of the highly mobile sulfate pool.

  • Runoff and Erosion: Particulate-associated sulfur is lost during precipitation events.

  • Volatilization: Reduced sulfur (sulfide) can become gaseous and escape into the atmosphere.

  • Crop Harvesting: The removal of biomass constitutes a direct removal of sulfur from the soil system.

Redox Potential and the Microbial Electron Tower

• Energy Hierarchy: Bacteria utilize terminal electron acceptors based on the highest potential energy yield. They follow a specific sequence (the "Tower") as resources are depleted.

• The Electron Tower Hierarchy:

  1. Oxygen ($O_2$): High energy yield, aerobic conditions.

  2. Peroxide: Aerobic conditions.

  3. Nitrate ($NO_3^-$): Once oxygen is exhausted, microbes switch to nitrate.

  4. Sulfate ($SO_4^{2-}$): Becomes the primary acceptor once nitrate is depleted (Anoxic environment).

  5. Iron ($Fe^{3+}$): Anaerobic conditions.

  6. Carbon Dioxide ($CO_2$): Anaerobic conditions.

  7. Hydrogen ($H^+$): Deep anaerobic conditions.

Global Sulfur Pools and Fluxes: Quantitative Data

• Unit Definition: $1 \text{ Teragram (Tg)} = 10^{12} \text{ grams} = 10^6 \text{ tonnes} (1 \text{ megaton})$.

• Major Global Pools:

  • Atmosphere: $0.8$ Tg (includes dust, $SO_2$, and $SO_4$; approximately $10\%$ circulates annually).

  • Soil: $3 \times 10^5$ Tg (Estimated turnover: 1,000 years).

  • Rivers and Lakes: $300$ Tg.

  • Surface Ocean (Water): $1.3 \times 10^9$ Tg.

  • Surface Ocean (Biomass - Plankton/Fish): $30$ Tg.

  • Rocks (Lithosphere): $2.4 \times 10^{10}$ Tg (A massive, largely inaccessible pool).

• Fluxes (Per Annum):

  • River/Lake to Surface Ocean: Pre-industrial was $104$ Tg/year; currently doubled to $213$ Tg/year due to human activity.

  • Ocean Sedimentation: $135$ Tg/year.

  • Fossil Fuel Burning: $70$ to $100$ Tg/year.

  • Mining: Approximately $160$ Tg/year.

  • Atmospheric Deposition to Ocean: Increased from $159$ Tg/year to $231$ Tg/year.

  • Surface to Slats/Atmosphere Assimilation: $140$ Tg/year.

  • Volcanic Processes: Fixed at approximately $10$ Tg/year.

Comparative Analysis: The Global Carbon Cycle

• General Properties: Carbon ($C$) is the 15th most abundant element in the Earth's crust and 4th in the universe. It forms many allotropes (organic, inorganic, organometal) and has a high propensity for polymerization.

• Unit Definition: Quantities expressed in Gigatons of Carbon (GtC).

• Carbon Pools:

  • Rocks and Sediments: $77,000,000$ Gt.

  • Lithosphere (Inaccessible): $50,000,000$ Gt.

  • Deep Oceans: $37,000$ to $38,000$ Gt.

  • Soils and Permafrost: $3,600$ to $3,650$ Gt.

  • Fossil Resources: $7,500$ Gt.

  • Surface Oceans: $900$ to $1,000$ Gt.

  • Atmosphere: $829$ to $875$ Gt.

  • Vegetation: $550$ to $670$ Gt.

  • Marine Biota: $3$ Gt.

• Carbon Fluxes:

  • Photosynthesis: Assimilates $120$ Gt/year (one slide) to $133$ Gt/year (another slide).

  • Respiration and Wildfires: Releases $60$ Gt/year.

  • Vegetation Death/Respiration: $60$ and $59$ Gt/year respectively.

  • Industrial Emissions: $10$ Gt/year.

  • Land Use Change: $1$ to $1.1$ Gt/year.

  • Riverine Migration to Oceans: $1$ Gt/year.

  • Marine Sedimentation: $0.2$ Gt/year (Total balance of $10$ Gt into deep ocean).

Comparative Analysis: The Global Nitrogen Cycle

• General Properties: Nitrogen ($N$) makes up $78\%$ of the atmosphere and is the 7th most abundant element in the solar system. It is essential for all living matter, though most organisms require "pre-fixed" forms.

• Anthropogenic Impact: The Haber Bosch process allows for synthetic manufacture of ionic nitrogen.

• Nitrogen Pools (Teragrams N):

  • Atmosphere (Nitrous gases): $1.4 \times 10^5$ Tg.

  • Atmosphere (Nitrogen Oxides - $NO_x$): $1.3$ to $1.4$ Tg.

  • Soil (Total N): $9.5 \times 10^4$ Tg.

  • Oceanic Dissolved Organic N: $2.1 \times 10^5$ Tg.

  • Oceanic Sediment: $4$ to $5 \times 10^8$ Tg.

  • Biomass (Plankton/Fish): $470$ Tg.

• Nitrogen Fluxes (Per Annum):

  • Industrial Fixation: $80$ Tg/year (Outweighs many natural processes).

  • Natural Fixation/Deposition: $90$ to $140$ Tg/year.

  • Crop Fixation (Legumes): $40$ Tg/year.

  • Natural Denitrification: $12.2$ Tg/year.

  • Fertilizer Denitrification: $6.9$ Tg/year (approx. $8\%$ of industrial fixation lost).

  • Oceanic Biological Fixation: $100$ to $200$ Tg/year.

  • Marine Sedimentation: $14$ Tg/year.

Comparative Analysis: The Global Phosphorus Cycle

• General Properties: Phosphorus ($P$) is highly reactive, present at $0.1\%$ in the crust. It exists in highly insoluble minerals. Essential for cell walls, DNA, and RNA. It lacks a significant atmospheric gas phase (unlike N, C, and S).

• Phosphorus Pools (Teragrams P):

  • Rock Phosphate: $4 \times 10^8$ Tg.

  • Oceanic Sediment: $4 \times 10^8$ Tg (Turnover: 2 million years).

  • Deep Ocean: $8.7 \times 10^4$ Tg.

  • Soils: $2 \times 10^5$ Tg (Turnover: 2,000 years).

  • Surface Ocean Water: $2,700$ Tg.

  • Mineable Resource: Only $1 \times 10^4$ Tg.

  • Plant Biomass: $3,000$ Tg.

  • Atmospheric Dust: $0.028$ Tg.

• Phosphorus Fluxes (Per Annum):

  • Soil to Plants: Increasing between $10.5$ and $15.5$ Tg/year.

  • Soil to Rivers/Oceans: $7$ to $9$ Tg/year.

  • Rock Phosphate to Animal Feed: $2$ Tg/year.

  • Animal Assimilation: $15$ Tg/year.

  • Plankton/Fish Intake: $140$ Tg/year.