3.5.4 nutrient cycles

nutrients are recycled

play a vital role in recycling chemical elements such as phosphorus and nitrogen

• DNA/RNA • ATP • phospholipid bilayer

• phosphorus isn’t found as a gas, so isn’t in the atmosphere • it is mainly found as a phosphate ion in mineral form in sedimentary rock

fungal associations between plant roots and beneficial fungi

• the fungi increase the surface area for water and mineral absorption • the mycorrhizae act like a sponge so they hold water and minerals around the roots • this makes the plant more drought resistant and able to take up more inorganic ions (mutualistic relationship as plant also provides carbohydrates to fungus)

• phospate ions are found dissolved in water eg. rivers, ponds, water in soil • plants absorb the phosphate ions by active transport and incorporate them into DNA, phospholipids etc • animals eat and digest the plants, so the phosphate ions are incorporated into their biological molecules • excretion from the animals contains phosphate ions and some goes directly back into the water where the ions where originally found • when animals die and are decomposed, phosphate ions are found in their remains eg. bones and shells, as well as excretion like guano • the remains of animals like bones and shells erode over time, releasing the phosphate ions back into the water • over time deposition occurs, so some phosphate ions from the excretion and decomposition of animals are deposited into rocks • these rocks are then eroded, so the phosphate ions return to the oceans and soil • sediments from oceans and rivers also builds up and creates rocks over time

• plants and animals can’t obtain nitrogen through gas exchange, partly because it contains a triple bond • microorganisms are therefore needed to convert nitrogen gas into nitrogen containing substances which can be absorbed

• proteins • ATP • nucleic acids

• nitrogen gas from the atmosphere is converted into nitrogen-containing compounds • this may be done in the root nodules of leguminous plants by mutualistic bacteria which convert nitrogen gas into ammonium or nitrates (bacteria get carbohydrates from plants and plants get amino acids from bacteria) • in plants which aren’t leguminous it is done by naturally occuring bacteria in the soil

• ammonium ions are formed from ammonia • some ammonia is produced by the bacteria in nitrogen fixation • some comes from saprobionts which decompose the proteins, ATP and DNA from dead plant and animal cells using enzymes, as well as the urea in animal waste

• nitrifying bacteria in the soil oxidise ammonium ions to form nitrite ions • more nitrifying bacteria oxidise the nitrite ions into nitrate ions

• nitrate ions are absorbed into plants by active transport • they are then assimilated into cells and used to create ATP, amino acids, DNA etc

• denitrifying bacteria in the soil convert nitrate ions into atmospheric nitrogen • these bacteria are anaerobic so this only occurs when there is a lack of oxygen in the soil

lightning: 1. it breaks nitrogen molecules into nitrogen atoms 2. when it rains, the nitrogen atoms combine with oxygen to form nitrates 3. the nitrates are carried to the Earth and used for nitrogen fixation by plants

to replace the nitrates and phosphates lost from soil by harvesting plants and removing livestock, and increase productivity and yield

manure, compost, sewage

chemicals

• they are cheap, or free if a farmer owns animals • the organic matter within them can improve soil structure or water retention • nutrients are released slowly over time, so are less likely to be washed away after rain • they are less water soluble so there is less leaching

• it is difficult to control the exact nutrients present and their proportions • the nutrients are less concentrated so more fertiliser is needed

• they can be created to contain exact proportions of nutrients (as too much of one ion may be toxic) • they are highly soluble, making it easier for plants to absorb the nitrates and phosphates • the nutrients are concentrated and easy to apply

• they don’t improve soil structure • they are more water soluble so the ions within them will dissolve in the water surrounding the soil and may be leached when it rains, which negatively impacts the environment

when water-soluble compounds from fertilisers are washed away, often into rivers or ponds

when nitrogen fertilisers are leached into waterways from fertilised fields and stimulate algae growth