Lecture 13: Phosphorus, Sulfur and Mercury Cycling

Why is sulfur biogeochemistry important?

• Anaerobic OM mineralization

• Toxicity: H₂S, Regulates heavy metal storage / toxicity

• Important control on wetland pH

• Wetland Management: Smells

• Nutrient – Not limiting

What are the sources of wetland S?

Weathering from soil minerals

• Internal

• With Si, Gi and Ti

Wet deposition of Sulfate (H₂SO₄)

• Volcanoes

• Acid rain

How is wetland S lost?

Gaseous losses: H2S, DMS

Dissolved in water: SO42-

What is sulfate reduction?

An anaerobic process:

OM + SO₄^2- + H⁺ ← → CO₂ + H₂O + H₂S

OM = oxidized, SO₄^2- = reduced

• Yields little energy

• Increase in pH

What is sulfate oxidation?

An aerobic process:

H₂S + 2O₂ ← → H₂SO₄

O₂= oxidized, H₂S = reduced

• Yields little energy

• Decrease in pH

How does pH and DO vary in a salt marsh?

→ Tides bring in O₂, SO42-

• pH of ocean water is ~8.2

• Flooded marsh sediments use up O₂ then start sulfate reduction

• Outgoing tides thus have lower O₂ and lower pH

What does sulfide often bind with in a wetland?

Will bind and precipitate with reduced iron to form iron sulfide

Often FeS is the main store of S in mineral wetlands

FeS can dissolve again if H2S [ ] drop under oxic conditions

Why does s cause ‘dirty’ coal?

Formation of pyrite in wetlands is the same mechanism which caused coal to be such a dirty fossil fuel: high S in coal

T/F acidiication can persist in wetlands?

True

Wetlands that have built up large stores of sulfide due to acid rain, can continue to cause acidification of downstream lakes for decades after acid rain has stopped if there are droughts

T/F Prairie wetlands have acid pulses?

False

• Bedrock is composed of calcium carbonate which buffers acidification

What heavy metals will sulfide bind with? what does this mean for heavy metal storage?

Fe, Cu, Zn, Hg, Pb, Cd etc …

Act as sinks for heavy metals: sulfide metals are insoluble and not bioavailable

But when the wetland dries out the heavy metals are released and become bioavailable

What is the characteristics of acid mine drainage?

Low pH

High in SO42-, FeOH and heavy metals

How can wetlands help restore acid mine drainage?

Improve water quality

Act as a sink for acidity, Fe and Heavy metals due to sulfide reduction and sulfide precipitation

What are the sources of Hg or M to wetlands?

Weathering

Atmospheric inputs

• Anthropogenic

• Volcanoes

T/F Hg and Mg are non toxic and do not bioaccumulate

False

They are toxic but do not bioaccumulate; insoluble in soils

What are the characteristics of methylmercury?

• Microbially produced

• Mobile (soluble)

• Bioaccumulates, highly toxic

What increases the conversion of mercury to methyl mercury?

A source of sulfate

Anaerobic conditions

Labile organic matter

A storage of ionic mercury

Which peatland has higher MeHg?

Thermokarst fens

• Groundwater connection

• Thaw and collapse becoming anoxic

What is the importance of P?

Limiting nutrient

Eutrophication

why is P different than N?

• No gaseous phase

• P is sedimentary cycle

• Not directly involved in redox reactions

What are the forms of orthophosphorus in wetlands?

4 forms (low pH → high pH):

1. H₃PO₄

2. H₂PO₄^-

3. HPO₄^2-

4. PO₄^3-

what is orthophosphorus also called? what does it measure?

Also called Soluble Reactive Phosphorus (SRP)

Used to measure total phosphorus

What are impacts the availability of Phosphorus?

Can be tied up in organic compounds:

• Released through OM break down; depends on redox, temperature, and OM quality

• Some microbes have enzymes that target P

Strongly adsorbed to minerals, mainly inorganic Ca, Fe and Al

• Very low solubility, but depends on pH and redox conditions

What does P availability look like under Aerobic conditions?

• Tied to Fe, Al under low pH

• Tied to Ca under high pH

• High phosphate solubility / availability at a pH of 6.5

• P keeps cycling

What are the sources // losses of P to and from wetlands?

Mineral particles (Al, Fe, Ca -P) :

• To wetlands as dry deposition

• With water (Si, Ti)

Dissolved phosphates:

• With water (Si, Ti, Gi)

Organic P (Particulate / Dissolved):

• With Water (Si, Ti, Gi)