Circulation of nutrients is essential for the continuation of life as elements are cycled between living organisms and the environment through the absorption of nutrients and release of wastes. Soil consists of inorganic and organic components and contains microorganisms, oxygen, and moisture that decompose dead and decaying materials, recycling their nutrients. Decomposers like fungi and bacteria break down organic matter chemically while detritivores like earthworms fragment materials physically, increasing the surface area for decomposers and improving soil structure and fertility.

1. Circulation of Nutrients Despite an inexhaustible influx of energy in the form of sunlight, continuation of life depends on recycling of essential chemical elements. These elements are continually cycled between the environment and living organisms as nutrients are absorbed and wastes released. The cycling of nutrients from the decomposition of dead or decaying matter , provides essential elements required for metabolic processes, such as photosynthesis, and constructing fundamental organic molecules, such as amino acids and nucleic acids.

2. The role of soil in decomposition Soil consists of 2 main components… INORGANIC ORGANIC Oxygen and moisture , trapped in the spaces (pores) between the soil particles, are required by micro-organisms to decompose materials. Dead/Decaying organisms, parts of organisms, faeces and urine derived from weathering of rocks. Type determined by relative proportions of SAND , SILT and CLAY particles very important agriculturally.

3. The structure of soil The structure of soil is vital to nutrient recycling in terrestrial ecosystems Organic litter : plant/animal debris Topsoil containing humus : roots, invertebrates, micro-organisms Leaching of nutrients from the soil Subsoil : rich in minerals & organic material, some roots Weathered rock : sand, gravel, clay Impermeable Bedrock

4. Soil fauna Wide variety including … FUNGI BACTERIA - [Insert Soil micro-organisms table] INVERTEBRATES - worms, woodlice, spiders, nematodes, larvae Essential to soil productivity - directly affect quality of soil

5. Rhizosphere The area where plant roots and soil come into contact. There are large numbers of micro-organisms here that differ in species composition. The major micro-organisms found are the bacteria whose growth is stimulated by various nutrients released by the plant roots. In exchange, the by-products of microbial metabolism that are released into the soil stimulate plant growth.

6. Decomposers and Detritivores These organisms feed on waste or dead organic matter such as dead leaves, dead bodies, or waste products, decomposing it by producing enzymes to break it down. Action of detritivores increases the activity of decomposers :- DETRITIVORES : Detritus eating soil invertebrates e.g. Earthworms, woodlice, spiders and nematodes . Physically reduce detritus particle sizes to produce humus [ FRAGMENTATION = Larger surface area for decomposers to work on!] Enhance fertility of the soil by incorporating leaf surface litter into the soil PHYSICAL DECOMPOSITION DECOMPOSERS : Fungi & Bacteria Use waste materials as energy, carbon & nutrient sources Carry out respiration to release CO 2 Chemically breakdown [using ENZYMES ] detritus (decaying matter) to release inorganic ions ( mineralisation ) CHEMICAL DECOMPOSITION

7. Detritus Food Chain A food chain based on dead organic material A food chain containing a primary producer which is a soil invertebrate e.g. Earthworm. A Detritus food chain is essential to the energy flow of an ecosystem.

8. Decomposition The breakdown of dead organic matter(involving both physical and biological processes) with the release of inorganic nutrients into the soil. These nutrients are then available for uptake by plants and other primary producers Undecomposed material = LITTER [DETRITUS] Completely decomposed matter = HUMUS

9. Humus Completely decomposed material Dark brown in colour Composition varies depending on which organic molecules are present Improves aeration, water and nutrient retention; and so soil structure  Humus content =  soil fertility

10. Mineralisation Occurs at the same time as humus is being formed Process which changes essential minerals e.g. N , P and S from Organic  Inorganic compounds e.g. ammonium , orthophosphate and sulphate Results in nutrients and minerals being released from dead organisms into a food chain

11. Rate Of Decomposition This varies within different biomes and is dependent on several factors: Type of organic matter Number and types of decomposers and detritivores Environmental conditions i.e. temp, moisture etc

12. The Carbon Cycle Carbon is removed from the atmosphere by fixation during photosynthesis and returned by respiration, decomposition & burning fossil fuels

13. The Nitrogen Cycle 1) FIXATION : the reduction of atmospheric nitrogen to ammonia by CYANOBACTERIA . Rhizobium fix nitrogen in the root nodules of legumes. Catalysed by enzyme complex NITROGENASE . LEGHAEMOGLOBIN is a molecule made by both the plant and the bacteria which limits the amount of O 2 reaching the bacteria. This is important as nitrogen-fixing is an anaerobic process 2) NITRIFICATION : the conversion of ammonium to nitrite to nitrates by NITROSOMONAS and NITROBACTER. Nitrates and ammonium are assimilated by plants into proteins and amino acids. They are lost by leaching and denitrifying bacteria. Aerobic process. 3) DENITRIFICATION : returns nitrogen to the atmosphere 4) AMMONIFICATION : the decomposition of organic nitrogen to ammonia Water saturation of the soil affects the cycling of nitrogen. i.e.  H 2 0 =  O 2 anaerobic/aerobic affect different stages of cycle

14. The Nitrogen Cycle

15. The Phosphorus Cycle Phosphorus is a major element of ATP, Nucleic Acids, Phospholipids 1) PHOSPHATE added to the soil by the weathering of rocks 2) Taken up by primary producers incorparated into molecules 3) PHOSPHORUS is taken up by consumers 4) PHOSPHORUS returned to the soil by decomposition (faeces/detritus) Phosphate is a limiting factor in the productivity of aquatic ecosystems as it has LOW SOLUBILITY (needed by ALGAE!) Phosphate enrichment can lead to eutrophication (as can excess Nitrogen ) 1 2 3 4