5.4 Nutrient Cycles
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Name the general stages in the phosphorus cycle
1.weathering 2.runoff 3.assimilation 4.decomposition 5.uplift
Why is the phosphorus cycle a slow process?
Phosphorus has no gas phase, so there is no atmospheric cycle. Most phosphorus is stored as PO4^3- in rocks
What happens during weathering and runoff?
Phosphate compounds from sedimentary rocks leach into surface water and soil
Explain the significance of phosphorus to living organisms
Plants convert inorganic phosphate into biological molecules e.g. DNA,ATP,NADP Phosphorus is passed to consumers via feeding
What happens during uplift
Sedimentary layers from oceans (formed by the bodies of aquatic organisms) are brought up to land over many years
How does mining affect the phosphorus cycle?
Speeds up uplift
Name the 4 main stages of the nitrogen cycle
1.Nitrogen fixation 2.Ammonification 3.Nitrification 4.Denitrification
Why can’t organisms use nitrogen directly from the atmosphere
N2 is very stable due to strong covalent triple bond
What happens during atmospheric fixation of nitrogen?
• High energy of lighting breaks N2 into N • N reacts with oxygen to form NO2- • NO2- dissolves in water to form NO3-
Outline the role of bacteria in nitrogen fixation
Mutualistic nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil Use the enzyme nitrogenase to reduce gaseous nitrogen into ammonia
Outline the role of bacteria in ammonification
• Saprobionts feed on and decompose organic waste containing nitrogen • NH3 released • NH3 dissolves in water in soil to form NH4+
Outline the role of bacteria in nitrification
2-step process carried out by saprobionts in aerobic conditions: 2NH4+ + 3O2 —> 2NO2- + 2H2O + 4H+ 2NO2- + O2 —> 2NO3-
Outline the role of bacteria in denitrification
Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen
Explain the significance of nitrogen to living organisms
Plant roots uptake nitrates via active transport & use them to make biological compounds e.g: -amino acids -NAD/NADP -nucleic acids
Outline the role of mycorrhizae
Mutualistic relationship between plant and fungus increases surface area of root system = increases uptake of water and mineral Ions
Give 3 benefits of planting a different crop on the same field each year
-Nitrogen-fixing crops e.g. legumes make soil more fertile by increasing soil nitrate content -Different crops have different pathogens -Different crops use different proportions of certain ions
Name the 2 categories of fertiliser and state the purpose of using fertiliser
Organic:decaying organic matter & animal waste Inorganic: minerals from rocks, usually containing nitrogen, phosphorus,potassium. To increase gross productivity for higher yields
At a certain point, using more fertiliser no longer increases crop yield. Why?
A factor unrelated to the concentration of mineral ions limits the rate of photosynthesis, so rate of growth cannot increase any further
Outline 2 main environmental issues caused by the use of fertilisers
Leaching:nitrates dissolve in rainwater and ‘runoff’ into water sources Eutrophication: water source becomes putrid as a result of algal bloom
What happens during eutrophication?
• Aquatic plants grow exponentially since nitrate level is no longer a limiting factor • Algal bloom on water surface prevents light form reaching the bottom and plants die. 3.oxygen levels decrease as population of aerobic saprobionts increases to decay dead matter, so fish die. • Anaerobic organisms reproduce exponentially and produce toxic waste which makes water putrid.
How can the risk of eutrophication be reduced?
Sewage system marshes on farm Pumping nutrient-enriched sediment out of water Using phosphate-free detergent