Nutrient Cycle Flashcards
How do saprobionts digest material externally?
- secrete extra cellular enzymes onto their food
Why do living organisms need nitrogen?
- to be able to make amino acids and proteins needed for growth and repair (most plants only take up nitrogen in the form of ammonia or nitrate as it is unreactive)
Nitrogen fixing bacteria
- fix nitrogen gas from the air into ammonia which dissolves in water to form ammonium ions which can be oxidised to form nitrate ions
Saprobionts
- decomposes protein/DNA in dead remains and waste products, releasing ammonium ions
Nitrifying bacteria
- convert ammonium ions to nitrites, then nitrates
Denitrifiying bacteria
- convert nitrates back into nitrogen gas
Where do nitrogen fixing bacteria live?
- in soil or in the root nodules of leguminous plants
What does a plant gain from having the nitrogen fixing bacteria and what would the bacteria get from the plant?
- ammonium ions that form after bacteria fix nitrogen can be used by the plant to make amino acids and in return, the plant provides bacteria with a constant source of organic compounds like glucose for respiration
What does lightning provide?
- energy for nitrogen to react in the air to form oxides which are carried (fixed) into soils by rainwater forming nitric acid which gets converted to nitrates
What happens when soil becomes waterlogged or too acidic?
- oxygen levels drop causing denitrifying bacteria to thrive as they work in anaerobic conditions and nitrates get converted to nitrogen gas which plants can’t use so the cycle is interrupted and soil fertility is reduced
What does digging drainage ditches and ploughing do?
- both add oxygen to the soil which reduces denitrification rates and encourages more nitrate to form in the soil by nitrifying bacteria
Why is nitrate easier to absorb than ammonium ions?
- nitrate ions are more water soluble
Describe how the action of microorganisms in the soil produces a source of nitrates for crop plants?
- protein is formed into ammonium compounds by saprobionts
- ammonium into nitrite and into nitrate by nitrifying bacteria
- nitrogen into ammonium by nitrogen fixing bacteria in the soil
Why does applying high concentrations of fertiliser to the soil reduce plant growth (refer to water potential)?
- fertiliser salts dissolve and lowers water potential of the soil so roots have higher water potential
- water moves by osmosis out of the roots into the soil so the plant does not have enough water
What are mycorrhizal fungi?
- fungi that has mutualistic association with roots forming an extension designed to increase surface area for absorption of water by osmosis
- acts like a sponge to hold water to avoid drought and makes it easier to take up ions like phosphates
State the differences between the phosphorus and nitrogen cycle
- phosphorus cycle has no atmospheric involvement whereas nitrogen cycle has nitrogen gas in the atmosphere
- phosphate in phosphorus cycle has low solubility in water whereas nitrate in nitrogen cycle is highly soluble in water
- phosphorus cycle is slower due to the above two points whereas nitrogen cycle is faster due to the above points
- in phosphorus cycle phosphorus is very reactive and never exists naturally in its elemental form whereas in nitrogen cycle nitrogen gas is unreactive and exits naturally in its elemental form
First step of eutrophication
- extra nutrient added from leaching causes algae and other surface plants to grow very rapidly (algal bloom)
Second step of eutrophication
- this blocks out light making it limiting factor and reduces photosynthesis so rooted plants die providing more nutrients
Third step of eutrophication
- saprobiontic bacteria multiply as they feed on dead algae
Forth step of eutrophication
- as bacteria multiply they respire aerobically a lot more so take up oxygen decreasing oxygen content and a biological oxygen demand
Fifth step of eutrophication
- less oxygen for fish to respire so migrate or die and body produces more nutrient for bacteria so process continues
What is leaching?
- some fertiliser washed from the soil by rain into the nearest pond, lake or river
Purpose of fertilisers
- increase efficiency of energy transfer along food chains, improving productivity so crops grow better and are cheaper
Why do we need fertilisers?
- replenishes minerals removed from soil when crops are harvested or animals eat them to prevent them becoming a limiting factor of plant growth and to help sustain ecosystem
What do natural fertilisers consist of?
- dead and decaying remains of plants and animals
- animal waste (manure, slurry, bone meal and urea)
Why may artificially made fertilisers not be used?
- expensive
- contaminate soil, water, air
- lead to pH imbalance of soil
- increases pests and affect health
- contributes to climate change
Why would overuse of nitrogen fertiliser lead to reduced species diversity?
- nitrogen rich soils favour rapidly growing species like grasses and nettles which outcompete others which would die reducing species diversity, reducing number of habitats and number of different food sources for other organisms
Causes of eutrophication
- leaching of nitrate and phosphates from fertiliser
- raw sewage
- manure washed out of farmlands
What is the overall effect of eutrophication?
- decreased oxygen levels in water leading to death of aquatic organisms and increase in pathogen bacteria
Suggest a practical for measuring algal growth
- using colorimeter
- measure the absorbency of light
- zero the colorimeter
Why use logarithmic scale?
- values have large range
Evaluate why farmers should not use fertilisers to increase yields when growing plants
- Should not use: prevents development of mycorrhizae, mycorrhizae help plants defend themselves, mycorrhizae help plants take up nitrates and phosphates (increasing crop yield)
- should use: increases gross primary production so increases yield, most soil is poor in phosphate so without fertiliser, plant might not get enough phosphate
