5.4 Nutrient Cycles Flashcards
Name the 5 stages of the phosphorus cycle.
Weathering
Runoff
Assimilation
Decomposition
Uplift
Why is the phosphorus cycle slow?
Stored as PO₄³⁻
Phosphorus has no gas phase so no atmospheric cycle
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.
Converted from inorganic phosphate into biological molecules e.g. DNA, ATP, NADP
What happens during uplift?
Sedimentary layers from oceans formed by 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.
Nitrogen fixation
Ammonification
Nitrification
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 lightning 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.
Nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil
Use enzyme nitrogenase to reduce gaseous nitrogen into ammonia
Outline the role of bacteria in ammonification.
Saprobionts feed on and decompose organic waste containing nitrogen (e.g. urea, proteins, nucleic acids etc)
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⁻ + H₂O + 4H⁺
2NO₂⁻ + O₂ -> 2NO₃⁻
What are saprobionts?
Decomposers that digest their material externally by secreting enzymes onto dead organisms and waste material
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 for uptake of water and mineral ions
Give 3 benefits of planting a different crop on the same field each year.
Nitrogen-fixing crops (legumes) make soil more fertile by increasing soil nitrate content
Different crops have different pathogens
DIfferent crops use different proportions of certain ions
Name 2 categories of fertiliser.
Organic
Inorganic
State the purpose of fertiliser.
To increase gross productivity for higher yield
What is organic fertiliser?
Decaying organic matter & animal waste
What is inorganic fertiliser?
Minerals from rocks, usually nitrogen, phosphorus, potassium
Why doesn’t more fertiliser increase crop yield further at a certain point?
It is no longer the limiting factor
Another factor is limiting the rate of photosynthesis
Rate of growth cannot increase any further
Outline 2 environmental issues caused by fertiliser use.
Leaching - nitrates dissolve in rainwater and run off into water sources
Eutrophication - algal bloom due to excessive nutrients
What happens during eutrophication?
Aquatic plants grow exponentially since nitrate level no longer limiting factor
Algal bloom on surface kills other plants by blocking light
Aerobic saprobionts increases to decay dead matter and use oxygen so fish die
Anaerobic organisms reproduce and produce toxic waste
How can the risk of eutrophication be reduced?
Sewage treatment on farm
Pumping nutrient-enriched sediment out of water
Using phosphate-free detergent
