The nitrogen cycle Flashcards
Describe the nitrogen cycle (10)
-nitrogen gas in the air
- nitrogen fixing bacteria in root nodules convert it into nitrates in soil
- also converted by lightning
- plants absorb nitrates
- animals eat plants/animals and absorb nitrates
- animals die/excrete proteins and urea
- decomposers (e.g. soil bacteria) break them down into ammonium compounds
-ammonium compounds converted by nitrifying bacteria into nitrates in the soil
- some nitrates broken down by soil bacteria and nitrogen is returned to the air
- other nitrates taken up by plants again
What do nitrogen fixing bacteria do?
- in root nodules of legume plants
- also in the soil naturally
-convert nitrogen gas from the air into nitrates in the soil
Why does planting legumes in a field replenish nitrates faster than leaving it fallow if soil already contains nitrogen fixing bacteria?
it happens faster as there are more bacteria contained within the root nodules
What is nitrogen used for in animals and plants?
- makes amino acids, which form proteins
- DNA contains nitrogen
- muscle, skin, antibodies, cell membranes
nitrates and lightning
- lightning provides energy needed for the nitrogen in the air to react with the oxygen in the air
- this forms nitrates, which fall to the soil in rain
Why do plants need nitrates in the soil?
they can’t absorb nitrogen from the air, so absorb it as nitrogen compounds dissolved in soil water
What is natural fertiliser?
manure
- contains nitrogen, which decomposers in the soil break down into nitrates to be used by plants
- also contains urea (toxic to plants) and other larger molecules which are broken down over time
advantages of natural fertiliser
- cheaper
- high levels of nitrogen which are broken down into nitrates
advantages of artificial fertiliser
- don’t need to be broken down, so crops can be planted straight away (unlike manure)
- allow farmers to control the amount and type of nutrients put into the soil
disadvantages of fertilisers
can cause eutrophication
crop rotation
- different crops are grown in a field each year
- growing plants takes nitrates out of the soil, decreasing the quality and making the next plants grow slower
- a fallow (empty field) lets the bacteria in the soil slowly replenish the nitrates in the soil
- legumes have nitrogen fixing bacteria in their roots, so add nitrogen back to the soil quicker than leaving it empty
advantages of crop rotation
- decomposers in the soil have time to replenish the nitrates that have been removed by plants, which are needed for growth
- legumes add nitrates back to the soil (by nitrogen-fixing bacteria in their roots) without using fertiliser and risking eutrophication
- legumes replenish nitrates quicker than leaving the field fallow
- allows the farmer to still grow produce
Name types of legume
potatoes, clover, beans, peas
Why is the relationship between legumes and nitrogen fixing bacteria mutualistic?
- bacteria provide nitrates needed for growth
- plant provides nutrients and a habitat
-both benefit
Plants grow well in fertile soil. Explain how bacteria help to keep soil fertile. [4]
- plants need nitrogen for making proteins
- can only absorb it as nitrogen compounds (nitrates) from soil
- soil bacteria act as decomposers
- release ammonia from proteins in dead bodies and from urea
- bacteria in the soil make nitrates (from ammonia)
- nitrogen fixing bacteria in the soil and roots of some plants convert nitrogen gas from the air into nitrogen compounds the plant can use
