Nutrient Recycling

Decomposers are organisms that break down the remains of dead animals and plants. They are vital for returning nutrients to living organisms in the ecosystem.

Key decomposers:

• Bacteria - microscopic single-celled organisms that secrete enzymes directly onto organic matter to digest it externally.
• Fungi - including yeasts, moulds, and mushrooms - target tougher materials like lignin and cellulose found in plant cell walls.

Detritivores are organisms like earthworms, woodlice, and insects that physically break down organic matter into smaller pieces, increasing the surface area for bacterial decomposition.

Factors affecting decomposition rate:

• Temperature - higher temperature increases enzyme activity (to a point)
• Moisture - decomposers need water for metabolic reactions
• Oxygen - aerobic decomposition is faster than anaerobic
• Chemical nature of material - soft tissue decomposes faster than lignin/cellulose

Carbon is found in all organic molecules - carbohydrates, lipids, and proteins. It cycles continuously between the biotic and abiotic parts of the ecosystem.

Key processes in the carbon cycle:

• Photosynthesis - removes CO₂ from atmosphere; plants convert it to glucose. This brings carbon from the abiotic to the biotic part of the ecosystem.
• Respiration - all organisms release CO₂ back to the atmosphere as they break down glucose for energy.
• Decomposition - bacteria and fungi break down dead organisms, releasing CO₂ (aerobic) or methane CH₄ (anaerobic).
• Combustion - burning fossil fuels releases stored carbon as CO₂.
• Fossil fuel formation - over millions of years, dead organisms that didn't decompose were compressed into coal, oil, and gas.

The Greenhouse Effect: CO₂ and methane trap heat in Earth's atmosphere. This is a natural process, but human activities (burning fossil fuels, deforestation) have intensified it, causing global warming.

Carbon sinks are reservoirs that absorb more carbon than they release. Examples: forests, oceans, soil, peat bogs. Carbon sequestration is the process of capturing and storing atmospheric carbon.

Nitrogen makes up 78% of the atmosphere but atmospheric N₂ is unreactive - plants cannot use it directly. It must be converted to nitrates (NO₃⁻) before plants can absorb it.

Key stages of the nitrogen cycle:

• Nitrogen Fixation - converting N₂ gas into ammonium (NH₄⁺) or nitrates. Done by:
  • Lightning - reacts N₂ and O₂ to form nitrates that dissolve in rain
  • Nitrogen-fixing bacteria (e.g. Rhizobium) in root nodules of legumes (peas, clover, beans)
• Nitrification - nitrifying bacteria convert ammonium → nitrites → nitrates in the soil.
• Assimilation - plants absorb nitrates through their roots and use them to make amino acids and proteins.
• Ammonification - decomposers break down nitrogen compounds in dead organisms, releasing ammonia (NH₃).
• Denitrification - denitrifying bacteria convert nitrates back into N₂ gas, returning it to the atmosphere. These bacteria thrive in waterlogged, anaerobic soils.

Human impact: Burning fossil fuels and using nitrogen-based fertilisers have doubled global nitrogen fixation since the 1940s. Excess nitrogen leaches into waterways causing eutrophication. The Haber-Bosch process industrially fixes nitrogen for fertiliser production.

Not all dead material decomposes at the same rate. Ecologists identify five key factors:

• Temperature - decomposer enzymes work faster in warm conditions, but very high temperatures denature them. Decomposition slows to almost nothing in frozen soils.
• Water - decomposers need moisture for their metabolic reactions, but waterlogged conditions push out oxygen and slow things down.
• Oxygen - aerobic decomposition is fast and releases CO₂. Anaerobic decomposition is slower and can release methane (CH₄), a potent greenhouse gas.
• Detritivores - earthworms, woodlice and insects shred dead matter into smaller pieces, exposing more surface area for microbes to attack.
• Type of material - soft tissue decomposes within days; woody plant material (rich in lignin) and animal bone/tendon can take years.

Peat bogs are a classic example of very slow decomposition. Waterlogged, acidic, low-oxygen conditions almost stop microbial activity, so dead moss builds up as peat instead of returning to CO₂. This is why bogs are enormous carbon stores and draining them releases greenhouse gases that were locked away for thousands of years.

Whale falls show the opposite extreme. When a whale dies and sinks to the deep sea floor, cold water, high pressure and near-total darkness slow decomposition dramatically. Specialist bone-eating worms (Osedax) and chemosynthetic bacteria form a temporary ecosystem that can persist for decades on a single carcass.