Nutrient Cycles and Decomposition Study Notes

Nutrient Cycles Overview

• Carbon, Oxygen, nitrogen, and water are essential nutrients for life.

• Decomposers (bacteria and fungi) break down dead organisms to return nutrients to the physical environment for reuse by green plants.

Carbon Cycle

• Processes removing atmospheric $CO_2$:

  • Photosynthesis: Green plants absorb $CO_2$ to synthesize carbohydrates, proteins, and fats.

  • Feeding: Animals consume plants, incorporating carbon compounds into their bodies.

• Processes releasing atmospheric $CO_2$:

  • Respiration: Plants and animals release $CO_2$ into the atmosphere.

  • Decay: Decomposers (bacteria and fungi) break down dead organic matter.

  • Combustion: Burning fossil fuels (coal, natural gas, oil) releases $CO_2$.

• Importance of the carbon cycle:

  • Ensures continuous supply of $CO_2$ for photosynthesis.

  • Enables a linear flow of energy by trapping solar energy in carbon compounds.

  • Maintains constant atmospheric $CO_2$ concentration.

Nitrogen Cycle

• Processes adding nitrates to soil:

  • Nitrification: Aerobic nitrifying bacteria in well-aerated soils convert ammonium ions to nitrites (Nitrosomonas) and then to nitrates (Nitrobacter).

  • Nitrogen fixation:

  • Free nitrogen-fixing bacteria (Azotobacter) in well-aerated soil convert $N_2$ gas into ammonium ions.

  • Nitrogen-fixing bacteria (Rhizobium) in root nodules of leguminous plants (peas, beans, clover) convert atmospheric nitrogen to ammonium ions.

  • Lightning and thunderstorms: Provides energy for nitrogen and oxygen to combine into nitrogen dioxide, which dissolves in rain as nitric acid to yield nitrate ions.

  • Decomposition (Ammonification): Putrefying bacteria and fungi decompose dead bodies, faeces, and excretory products (urea) into ammonium ions.

• Processes removing nitrates from soil:

  • Absorption by roots: Plants absorb nitrates to produce amino acids and proteins.

  • Leaching: Rainwater dissolves and carries away soluble nitrates.

  • Denitrification: Anaerobic denitrifying bacteria (Pseudomonas denitrificans) in waterlogged soil break down nitrates into $N_2$ gas.

Role of Microorganisms in Decomposition

• Saprophytic fungi and bacteria break down dead organisms and waste via external digestion using enzymes:

  • Proteases break down proteins into amino acids.

  • Amylase breaks down starch to maltose, and maltase breaks down maltose to glucose.

• Small amounts of nutrients are absorbed for respiration and growth; remaining nutrients are released into the surrounding air and soil.

• Inorganic products released include $CO_2$, ammonium compounds, hydrogen sulfide, water vapour, sulfates, phosphates, and potassium ions.

• Microorganisms recycle ecosystem nutrients, prevent waste accumulation, and curb disease spread.