Nutrient Cycles

Question 1

general stages of phosphorus cycle

Answer 1

1. weathering and absorption
2. feeding and excretion
3. decomposition
4. erosion

Question 2

why is phosphorus cycle a slow process

Answer 2

phosphorus has no gas phase so there is no atmospheric cycle
most phosphorus as PO4 3- in rocks.

Question 3

weathering and absorption

Answer 3

weathering and erosion of rocks helps phosphate ions to become dissolved and so available for absorption by plants which incorporate them into their biomass.

Question 4

feeding end excretion

Answer 4

the phosphate ions pass into animals which feed on the plants. Excess phosphate ions are excreted by animals and may accumulate in waste materials such as guano (excretory products of some sea birds)

Question 5

decomposition

Answer 5

on death of plants and animals, decomposers break down releasing phosphate ions into the water or soil

Question 6

erosion

Answer 6

phosphate ions in excreta , released by decomposition and dissolved out of rocks, are transported by streams and rivers into lakes and oceans where they form sedimentary rocks- completing the cycle.

Question 7

significance of phosphorus in living organisms

Answer 7

plants convert inorganic phosphate into biological molecules e.g. DNA, ATP, NADP
phosphorus is passed to consumers via feeding.

Question 8

4 main stages of nitrogen cycle

Answer 8

1. nitrogen fixation
2. ammonification
3. nitrification
4. denitrification

Question 9

why can’t organisms use nitrogen directly from the atmosphere?

Answer 9

N2 is very stable due to strong covalent triple bond

Question 10

atmospheric fixation

Answer 10

high energy of lightning breaks N2 into N.
N reacts with oxygen to form NO2-
NO2- dissolves in water to form NO3-

Question 11

industrial fixation- Haber Process

Answer 11

artificially uses a catalyst, under high temperature and pressure to make masses of ammonia that is then used as fertilisers.

Question 12

biological fixation- bacteria

Answer 12

includes mutualistic bacteria (Rhizobium) in root nodules of legumes and free-living bacteria such as Azotobacter.
Rhizobium contains enzyme nitrogenase to reduce gaseous nitrogen into ammonia.

Question 13

use of ammonia in legumes + how is bacteria mutualistic to legumes

Answer 13

used to make amino acids which the legume can use to make proteins in return for supplying the bacteria with carbohydrates

Question 14

role of bacteria in ammonification

Answer 14

organic nitrogen compounds return to enivironment through excretions of animals.
saprobiants feed on and decompose organic waste containing nitrogen.
releases ammonia
ammonia dissolves in water in soil to form NH4+ (ammonium cation).

Question 15

role of bacteria in nitrification

Answer 15

2 step process carried out by saprobiants in aerobic conditions:
-Bacteria of genus Nitrosomonas oxidise ammonia (NH3) to nitrates (NO2-)
-Bacteria of the genus Nitrobacter oxidise the nitrates (NO3-)

Question 16

role of bacteria in denitrification

Answer 16

Anaerobic denitrifying bacteria such as Pseudomonas convert soil nitrates back into gaseous nitrogen thus replenishing the atmosphere, closing nitrogen cycle.
-takes place in wet and waterlogged soil.

Question 17

significance of nitrogen in living organisms

Answer 17

plant roots uptake nitrates via active transport and use them to make biological compounds e.g. amino acids, NADP, nucleic acids.

Question 18

role of mycorrhizae

Answer 18

mutualistic relationship between plant and fungus increases surface area of root system=increases uptake of water and mineral ions in return of sugars from plant

Question 19

benefits of planting a different crop on same field each year

Answer 19

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.

Question 20

purpose of using fertilisers and 2 categories of fertiliser

Answer 20

-to increase gross productivity for higher yield- to replace nitrates and phosphates lost by harvesting plants and removing livestock.
Natural- organic, decaying organic matter and animal waste e.g. manure
Artificial- inorganic, made using industrial processes e.g NPK fertilisers often delivered as soluble powders or pellets

Question 21

2 main environmental issues caused by use of fertilisers

Answer 21

leaching- nitrates dissolve in rainwater and ‘runoff’ into water sources
eutrification- water sources become putrid as a result of algal bloom.

Question 22

what happens during eutrification

Answer 22

1. aquatic plants grow exponentially since nitrate level is no longer a limiting factor.
2. algal bloom on water surface prevent light from reaching bottom and plants die.
3. oxygen levels decrease as population of aerobic saprobiants increases to decay dead matter, so fish die.
4. anaerobic organisms reproduce exponentially and produce toxic waste which makes water putrid.

Question 23

how can risk of eutrification be reduced

Answer 23

sewage treatment marshes on farms
pumping nutrient-enriched sediment out of water
using phosphate-free detergent.