The nitrogen cycle

Question 1

Describe the nitrogen cycle (10)

Answer 1

-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

Question 2

What do nitrogen fixing bacteria do?

Answer 2

• in root nodules of legume plants
• also in the soil naturally

-convert nitrogen gas from the air into nitrates in the soil

Question 3

Why does planting legumes in a field replenish nitrates faster than leaving it fallow if soil already contains nitrogen fixing bacteria?

Answer 3

it happens faster as there are more bacteria contained within the root nodules

Question 4

What is nitrogen used for in animals and plants?

Answer 4

• makes amino acids, which form proteins
• DNA contains nitrogen
• muscle, skin, antibodies, cell membranes

Question 5

nitrates and lightning

Answer 5

• 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

Question 6

Why do plants need nitrates in the soil?

Answer 6

they can’t absorb nitrogen from the air, so absorb it as nitrogen compounds dissolved in soil water

Question 7

What is natural fertiliser?

Answer 7

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

Question 8

advantages of natural fertiliser

Answer 8

• cheaper

- high levels of nitrogen which are broken down into nitrates

Question 9

advantages of artificial fertiliser

Answer 9

• 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

Question 10

disadvantages of fertilisers

Answer 10

can cause eutrophication

Question 11

crop rotation

Answer 11

• 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

Question 12

advantages of crop rotation

Answer 12

• 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

Question 13

Name types of legume

Answer 13

potatoes, clover, beans, peas

Question 14

Why is the relationship between legumes and nitrogen fixing bacteria mutualistic?

Answer 14

• bacteria provide nitrates needed for growth
• plant provides nutrients and a habitat

-both benefit

Question 15

Plants grow well in fertile soil. Explain how bacteria help to keep soil fertile. [4]

Answer 15

• 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

Question 16

role of ammonia and soil bacteria in making nitrates

Answer 16

• decomposers/soil bacteria form ammonia from proteins/urea in dead bodies/plants
• other soil bacteria use this to form nitrates

Question 17

Explain why a farmer might decide one year to grow clover plants in a field instead of a crop. [2]

Answer 17

clover roots contain nitrogen fixing bacteria which means that fewer nitrates are removed from soil

adds nutrients to the soil when clover plants ploughed in

Question 18

Farmers plough crop stubble back into the soil after harvesting. Explain how this helps to improve soil fertility. [4]

Answer 18

the stubble contains proteins

decomposer break these down and release the nitrogen compounds as ammonia into the soil

makes the ammonia available