Nutrient Cycles

• Mychorrhizae are associations between plant roots and fungi, they extend the root system, increasing surface area for absorption
• Mychorrhizae hold water and nutrients close to the roots and, in return, the fungi receive sugars from the plant
• The nitrogen cycle is the cycling of nitrogen from gaseous to solid forms
• Nitrogen begins as a gas in the atmosphere, which is fixated by nitrogen fixing bacteria in the root nodules
• Nitrogen is fixed by nitrogen fixing bacteria as ammonium, this is also made by saprobionts decomposing dead organisms
• Ammonium is oxidised into nitrites / NO2- and then nitrates / NO3- by nitrifying bacteria in the soil
• Nitrates are absorbed from the soil by plants which assimilate them into amino acids, when the plants die, the nitrates are denitrified back to ammonium
• Nitrates in the soil can also be denitrified by denitrifying bacteria in anaerobic conditions such as waterlogged and compacted soil, nitrates are released as nitrogen gas
• Animals that eat plants also assimilate their nitrates and, when they decompose, saprobionts also release their nitrates as ammonium
• Nitrogen can be assimilated by all organisms to make amino acids, proteins, DNA and ATP
• Nitrogen gas is inert so cannot be assimilated into most organisms from the air
• Nitrogen fixing bacteria can be free living in the soil or water as well as in mutualistic relationships in root nodules of plants
• The phosphorus cycle is the process of phosphates moving through an environment entirely as solids
• Phosphates are essential in making ATP, DNA, and the phospholipid bilayer
• Phosphate begins as rock, where it is weathered by rain into rivers, from here, it dissolves in the water
• Phosphates dissolved in rivers can be compacted into sedimentary rocks or fixed by algae then assimilated into animals
• When animals containing phosphates die - land and water animals - their phosphates are released into the soil by saprobionts and reassimilated into plants
• Phosphates used by farmers can also leach into the soil and rivers, as well as phosphates from animal waste
• Plants assimilate phosphates in the soil from dead plants and animals and from leaching
• Fertilisers are necessary as nutrients from the soil are assimilated into crops which are then removed from the environment in harvest, stripping the soil of nutrients
• Inorganic fertilisers increase crop yield the most due to their high ion concentration, however, after a certain concentration, root damage occurs and water diffuses from the roots by osmosis due to high water potential gradients
• NPK fertilisers (nitrates, phosphate ions and potassium) favour certain species such as grasses which outcompete other species, reducing food sources and species diversity
• Eutrophication occurs when fertilisers such as nitrates and phosphate ions leach into water, causing algal bloom
• Algal bloom blocks out light, preventing photosynthesis of plants, this leads to excessive plant death
• Dead plant matter as well as rapidly dying algal matter, is swiftly decomposed by saprobionts, this increases rates of aerobic respiration, reducing dissolved O2 in the water
• A decrease of O2 dissolved in the water leads to death of fish and animals, wiping out life in the water
• Anaerobic bacteria are also active in cases of eutrophication, increasing concentrations of toxins in the water