Biogeochemical Cycles, Feedbacks, Water & Phosphorus

Why is a holistic study of global biogeochemical cycles important?

• It helps understand the interconnectivity between Earth's spheres and predict impacts of environmental change.

• It informs climate modelling, pollution control, and sustainable resource use.

What are key applications of studying biogeochemical cycles?

• Climate change modelling

• Nutrient management

• Environmental remediation

• Tracking anthropogenic influence

• Understanding palaeoclimate.

What are feedbacks in the Earth system?

Processes that can amplify (positive) or dampen (negative) changes in Earth systems

Give an example of a positive feedback in historical climate events.

During the PETM, warming released methane hydrates, increasing greenhouse gases and enhancing warming (methane feedback loop).

How does a negative feedback regulate climate?

Higher CO₂ boosts plant growth (photosynthesis), which removes CO₂ from the atmosphere, acting as a stabilising force.

How do feedbacks affect the CO₂ and CH₄ budgets?

• Warming increases permafrost thaw (releasing CH₄) → enhances ocean stratification (reducing CO₂ drawdown) and affects vegetation cover → alters CO₂ sequestration.

What forms does phosphorus take in the environment?

Phosphate (PO₄³⁻), organic phosphorus compounds (e.g. phospholipids), and mineral-bound forms (apatite, vivianite).

What controls phosphorus mobility?

pH, redox conditions, mineral associations (e.g. Fe or Al), microbial activity, and organic matter content.

What are key reservoirs in the natural phosphorus cycle?

• Rocks (apatite)

• Soils

• Sediments

• Biomass (plants/animals)

• Rivers

• Oceans

What are major fluxes in the phosphorus cycle?

• Weathering

• Uptake by organisms

• Mineralisation

• Decomposition

• Sedimentation

• Burial

What is the difference between reactive and refractory phosphorus?

Reactive P is bioavailable (e.g. orthophosphate), while refractory P is bound in minerals or complex organics and not easily accessible.

What is eutrophication and how is it linked to phosphorus?

Excess phosphorus leads to algal blooms, oxygen depletion (hypoxia), and ecosystem collapse. Often results from fertiliser runoff and sewage discharge.

How do humans use phosphorus and how does it affect the cycle?

• Fertilisers, detergents, food additives.

• Mining and usage concentrate P in aquatic systems, accelerates eutrophication and alters the natural P cycle.

What are mitigation strategies for phosphorus pollution?

• Wastewater treatment (e.g., Enhanced Biological P Removal)

• Chemical precipitation

• Buffer zones

• Wetland restoration

• Regulation of phosphate-based products.

What makes water a unique molecule for the Earth system?

• High heat capacity

• Universal solvent

• Expands upon freezing

• Strong surface tension

• High latent heat of vaporisation and fusion

What are key processes in the water cycle?

• Evaporation

• Transpiration

• Condensation

• Precipitation

• Infiltration

• Percolation

• Surface runoff

• Groundwater flow

• Storage.

How does the water cycle affect the transport of other elements?

Moves nutrients (N, P, C), dissolved gases, pollutants, and sediments across ecosystems, shaping chemical and biological cycles.

How does the water cycle influence climate?

Water vapour contributes to the greenhouse effect; clouds influence albedo; latent heat transport affects energy redistribution globally.

What are direct impacts of human activity on the water cycle?

• Urbanisation (impermeable surfaces)

• Damming

• Irrigation

• Land use change

• Groundwater extraction

• Subsurface water movement.

What are indirect impacts of human activity on the water cycle?

Climate change alters evaporation, snowmelt timing, glacial retreat, and precipitation patterns; affects ecosystem function and water availability.

What is residence time in a reservoir and how is it calculated?

The average time a substance stays in a reservoir. Calculated as:

Residence Time(τ) = Reservior Size/Outflow Rate

What is the role of wetlands in nutrient and water cycles?

Wetlands store water, slow runoff, promote denitrification, trap sediments, and remove nutrients (P, N), supporting water purification and flood control.

How does the phosphorus cycle differ from nitrogen and carbon cycles?

Phosphorus lacks a significant gaseous phase and is primarily sedimentary, while N and C have major atmospheric components.

What is internal loading of phosphorus and where is it important?

Release of phosphorus from sediments under anoxic conditions; common in lakes and reservoirs experiencing eutrophication.

What is evapotranspiration and why is it important?

Combined water loss from evaporation and transpiration. Critical for returning water to the atmosphere and influencing energy and moisture fluxes.

What are the major reservoirs in the carbon cycle?

• Atmosphere

• Terrestrial biosphere

• Oceans (surface and deep)

• Fossil fuels

• Sediments.

What are the major fluxes in the carbon cycle?

• Photosynthesis

• Respiration

• Decomposition

• Ocean-atmosphere exchange

• Fossil fuel combustion

• Land-use change.

What processes sequester carbon over long timescales?

• Organic matter burial in sediments

• Carbonate precipitation and burial

• Silicate weathering.

How does ocean stratification affect the carbon cycle?

• Reduces mixing between surface and deep layers

• Limits CO₂ uptake by deep ocean

• Decreases carbon sequestration efficiency.

What are major sources of anthropogenic CO₂?

• Fossil fuel burning

• Cement production

• Deforestation

• Biomass burning.

What is the isotopic signature of CO₂ from fossil fuels?

• Depleted in ¹³C → leads to a measurable decrease in atmospheric δ¹³C values (Suess effect).

What are the key reservoirs in the nitrogen cycle?

• Atmosphere (as N₂)

• Soils

• Biomass

• Ocean (nitrate and ammonium)

• Sediments.

What are the key microbial processes in the nitrogen cycle?

• Nitrogen fixation

• Nitrification

• Denitrification

• Ammonification

• Assimilation.

What is nitrogen fixation and which organisms perform it?

Conversion of N₂ gas to ammonia (NH₃) by diazotrophs (e.g. cyanobacteria, rhizobia).

What is denitrification?

Microbial conversion of nitrate (NO₃⁻) to N₂ gas under anoxic conditions, removing bioavailable nitrogen.

How do human activities alter the nitrogen cycle?

Synthetic fertiliser production (Haber-Bosch process), fossil fuel combustion (NOₓ emissions), and land-use change increase reactive nitrogen in ecosystems.

What are environmental consequences of excess nitrogen?

• Eutrophication

• Acid rain

• Greenhouse gas emissions (N₂O)

• Biodiversity loss

• Water contamination.

How does the nitrogen cycle interact with other cycles?

• Affects carbon sequestration (e.g. fertilisation boosts photosynthesis)

• Drives oxygen demand in water (eutrophication)

• Links to sulphur and phosphorus cycles.

What is the role of isotopes in the nitrogen cycle?

• δ¹⁵N is used to trace nitrogen sources and transformations.

• Enrichment occurs during denitrification and assimilation.

What are δ¹³C, δ¹⁵N, δ³⁴S?

• These are isotope ratios — they describe the relative abundance of heavy vs light isotopes in a sample compared to a standard.

• They are expressed as delta (δ) values in per mil (‰), which means "parts per thousand"

What do δ values mean?

• Positive δ (e.g., δ¹³C = +5‰) → Sample is enriched in the heavy isotope compared to standard.

• Negative δ (e.g., δ¹³C = –25‰) → Sample is depleted in the heavy isotope compared to standard

• Biological and chemical processes often prefer lighter isotopes → called isotopic fractionation.

What do δ¹³C, δ¹⁵N, δ³⁴S tell us?

Isotope	Tells us about...	Example Usage

δ¹³C

Carbon sources (C3 plants, C4 plants, methane, marine algae)

Identify primary producers, methane release

δ¹⁵N

Nitrogen cycling (fertiliser use, denitrification, nitrogen source)

Trace eutrophication, soil N transformations

δ³⁴S

Sulphur sources (volcanic SO₂, biogenic DMS, sediment sulphides)

Track sulphur cycle, photic zone euxinia

Why are δ values useful?

• Record environmental processes

• Link sources to their origins

• Reveal ancient climates and biogeochemical changes

• Diagnose pollution sources

Examples of δ¹³C values

• C3 plants (e.g., trees): δ¹³C ≈ –26 to –30‰

• C4 plants (e.g., maize): δ¹³C ≈ –10 to –14‰

• Methane: very depleted, δ¹³C ≈ –50‰ to –70‰

• Marine algae: δ¹³C ≈ –20‰ to –22‰

→ So if you find δ¹³C = –27‰ in sediments, it's likely from terrestrial C3 plants

Examples of δ¹⁵N values

• Fertilisers: δ¹⁵N ≈ –2‰ to +2‰

• Sewage/manure: δ¹⁵N ≈ +10‰ to +20‰

• Denitrified nitrate: enriched δ¹⁵N (+10‰ and higher)

→ So δ¹⁵N = +12‰ in a river suggests sewage pollution and/or strong denitrification.

Examples of δ³⁴S values

• Marine sulphate: δ³⁴S ≈ +21‰

• Volcanic SO₂: δ³⁴S ≈ 0‰ to +5‰

• Biogenic DMS (dimethyl sulphide): δ³⁴S ≈ –2‰

→ So δ³⁴S = +2‰ in a coastal aerosol suggests volcanic influence