Lecture 4 - Nutrient Cycling: Nitrogen and Phosphorus

Why are nitrogen and phosphorus essential for all living organisms?

• N is important component of amino acids and nucleic acids

• P is the structural link in DNA/RNA and is critical for formation of ATP for respiration

How is nitrogen found?

• unreactive N₂

• organic N - from living or dead tissues

• inorganic N - ammonia, ammonium, nitrite, nitrate, etc.

How is phosphorous found?

• Organic and inorganic - within these groups can be soluble or insoluble

  • PO₄³⁺ most common in water, reacts with N compoinds

  • Not found in a gaseous form

What are the sources of nitrogen and phosphorus?

• External loading

  • N: Rainfall, aerial deposition, dust particles, fertiliser application  (as NO₃); burning of fossil fuels, planting N-fixing crops, wastewater disposal (as NH₄)

  • P: From weathering of rocks, human activities e.g. pollution from detergents, fertilisers

• Internal loading - sediments and nutrient cycling

Describe the nitrogen fixation stage

• Nitrogen fixing organisms have specialised features called heterocysts capable of fixing nitrogen gas when N is in limited supply

  • e.g. diazotrophs (e.g. bacteria, azotobacter, rhizobia and cyanobacteria)

• Takes place in anaerobic conditions

  • N₂ + 3H₂ (+ nitrogenase) → 2NH₃

Describe the ammonification stage

• Ammonium ions (NH₄⁺ ) can be converted to nitrate (NO₃) directly in the presence of decomposers

  OR

• Converted to NO₂ and then NO₃ through nitrification

Describe the nitrification stage

• NH₄⁺→ NO₂^- → NO₃^-

  • Nitrosomonas sp. convert NH₄⁺→ NO₂^-

  • Nitrobacter sp. convert NO₂^- → NO₃^-

• Can be used by organisms in lakes and streams (e.g. algae and plants) by diffusion

Describe the denitrification stage

•  Nitrate in soils is reduced back to N₂ (g) and released in the atmosphere in the presence of denitrifying bacteria (e.g. pseuedomonas)

  • Anaerobic conditions

  • Wastewater treatment: the anoxic conditions in wetlands means the waterlogged marsh soils encourage denitrification

Why is phosphorus a growth limiting nutrient?

• P is bound to soils and rocks

  • Soluble PO₄ is rapidly adsorbed onto particles or precipitated with other compounds

  • e.g. apatite (Ca₃(PO₄)₂)

• No gaseous phase so no atmospheric pool

• No specialised microbial processes like N

• Plants only take up in organic forms

• Through decomposition, organic P dissociates into inorganic forms

• Fungi and bacteria take up to 2 and 9 times more P than plants respectively

Challenges of overuse of phosphorous

• Species shift due to eutrophication

  • Harmful algae grow rapidly → changes the aesthetics/toxins/taste/odour → loss/reduced aquatic life

• Increased productivity of aquatic plants

  • Increased organic matter → reduces water clarity and increases plant respiration → decreases oxygen levels → loss/reduced aquatic life