Nitrogen cycle

Assimilation (2)

plants taking in ammonium/nitrate, making them into proteins, nucleic acids

Why is nitrogen inert? (2)

nitrogen has a triple covalent bond, so it is unreactive

What is the nitrogen cycle?

flow of nitrogen atoms between organic and inorganic nitrogen compounds and atmospheric nitrogen gas in an ecosystem

Why is nitrogen important?

nitrogen is required to make amino acids, proteins and nucleic acids

How are plants involved in the nitrogen cycle?

Plants absorb nitrates into their roots

How are organic nitrogen compounds produced by plants transferred through the food chain?

primary consumers eat plants

What processes release minerals back into the soil? (2)

decomposition of plants and animals after death, decomposition of egested and excreted products of animals

What key organisms are involved in these processes?

bacteria

Describe the biological process of ammonification/putrefaction as a part of the nitrogen cycle (2)

decomposers secrete enzymes that decay dead organisms and animal products, from ammonia to nitrites to nitrates

Describe the biological process of nitrification as a part of the nitrogen cycle (3)

nitrification adds nitrites or nitrates to the soil, ammonium ions formed in putrefaction/ammonification are converted to nitrites by Nitrosomonas, nitrites are converted to nitrates by Nitrobacter

What is the word equation for the first part of nitrification?

ammonium ions + Nitrosomonas → nitrites

What is the word equation for the second part of nitrification?

nitrites + Nitrobacter → nitrate

Why do the reactions of nitrification require aerobic conditions? (2)

They are oxidation reactions, the nitrogen atom in the ammonium progressively loses hydrogen atoms and gains oxygen atoms

Explain the process of denitrification as a part of the nitrogen cycle (2)

loss of nitrate from the soil, anaerobic bacteria Pseudomonas convert nitrate ions to nitrogen

What is the word equation for the process of denitrification?

nitrate + Pseudomonas → nitrogen

Why is the denitrification process favoured by anaerobic soil conditions? (3)

it’s a reduction reaction, oxygen is lost, it is a problem in waterlogged soils

Explain the process of nitrogen fixation as a part of the nitrogen cycle

the reduction of nitrogen atoms in nitrogen molecules to ammonium ions, nitrogen-fixing organisms Azotobacter in the soil convert nitrogen molecules to ammonium ions, symbiotic nitrogen-fixing bacteria Rhizobium in root nodules of plants

How do the Rhizobium bacteria and plant have a mutualistic relationship? (2)

the bacteria are given a habitat and source of glucose, the plant receives ammonium ions from the nitrogen-fixing bacteria

Explain stage 1 of nitrogen-fixation occurring in root nodules

nitrogen gas diffuses into the legume root nodule

Explain stage 2 of nitrogen-fixation occurring in root nodules

nitrogen are catalyses its reduction to ammonium ions using energy from ATP

Explain stage 3 of nitrogen-fixation occurring in root nodules (3)

root nodules contain a type of haemoglobin called leg-haemoglobin, that binds molecular oxygen in the nodules and protects the reactions from oxidation, makes nodules pink

Explain stage 4 of nitrogen-fixation occurring in root nodules

ammonium ions converted to organic acids and then into amino acids for incorporation into bacterial proteins

What is the word equation for nitrogen fixation most of the time?

nitrogen atoms + Azotobacter → ammonium ions

What is the word equation for nitrogen fixation in root nodules?

nitrogen atoms + Rhizobium → ammonium ions

Explain stage 5 of nitrogen-fixation occurring in root nodules (2)

some amino acids and ammonium ions are diverted into the vascular strand connecting the root nodule to the plant and in this way, the plant obtains nitrogen for its own metabolism

Explain stage 6 of nitrogen-fixation occurring in root nodules (2)

radicles of newly germinated legumes do not contain Rhizobium, in the soil the newly emerged radicle and Rhizobium both secrete chemo-attractants

Explain stage 7 of nitrogen-fixation occurring in root nodules

the radicle grows towards Rhizobium cells and the Rhizobium cells use their flagella to move towards the radicle

Explain stage 8 of nitrogen-fixation occurring in root nodules

Rhizobium cells invade the cortex of the radicle and the large number produced when they replicate makes the swelling which is the root nodule

Explain stage 9 of nitrogen-fixation occurring in root nodules

the nodules and the bacteria they contain enable leguminous plants to grow successfully even when soil nitrates are scarce

Explain stage 10 of nitrogen-fixation occurring in root nodules (5)

on the death of the plant, both the plant and the Rhizobium that the nodules contain are decomposed into ammonium compounds, which are released into the soil, the ammonium ions are nitrified to nitrites and nitrates, improving the soil quality further

Why do farmers want to maximise nitrate and ammonium concentrations in the soil? (4)

nitrate and ammonium are the nitrogen compounds that plants can absorb, the more absorbed then the higher the levels of protein and nucleotides leading to a larger plant, the more absorbed the more chlorophyll and so a higher rate of photosynthesis produces more glucose leading to a larger plant, a larger plant means the farmer will have a larger yield and so a larger profit

How can farmers maximise nitrate and ammonium concentrations in the soil? (4)

fertilisers provide a source of ammonium and nitrate, these can be artificial or natural, drainage and ploughing provides aerobic conditions promoting nitrification and nitrogen fixing and reducing denitrification, planting legumes through crop rotation leads to more Rhizobium bacteria and so more nitrogen fixing into ammonium ions

What are some disadvantages of these methods to increase soil nitrate and ammonium concentrations? (3)

reduces biodiversity as some plants prefer lower levels of nitrogen compounds, digging of ditches removed habitats decreasing biodiversity, fertilisers run off fields into water ways causing eutrophication

Explain stage 1 of eutrophication

excessive nutrients containing nitrogen from fertilisers run off from the land into water

Explain stage 2 of eutrophication

aquatic plants flourish and grow rapidly

Explain stage 3 of eutrophication (2)

algae also grows rapidly forming an algal bloom, water becomes green and light is unable to penetrate to any depth

Explain stage 4 of eutrophication (3)

algal bloom prevents sunlight from reaching aquatic plants, so water oxygen levels fall, plants in lower regions can’t photosynthesise and they die

Explain stage 5 of eutrophication (5)

decomposition rate increases, aerobic respiration of decomposers reduces dissolved oxygen further, decrease in animal species diversity as they relied on plants for food and shelter, short-lived algae soon die and are decomposed by saprobiontic fungi, causing a biochemical oxygen demand

Explain stage 6 of eutrophication (2)

all the water except the upper layers exposed to air becomes deoxygenated, death of organisms requiring dissolved oxygen in water

Explain stage 7 of eutrophication (2)

anaerobic bacteria in the water reduce nitrate to nitrite, they flourish and some species release gases with hydrogen sulphide

What are some non-biological processes that have an impact on the nitrogen cycle? (3)

application of agricultural fertilisers adds nitrogen to the soil, lightning adds a small amount of nitrogen to the soil, leaching of minerals removes nitrogen from the soil

How does increasing human population impact the nitrogen cycle? (3)

increases the amount of food needed, plant breeding and genetic modification and pesticides help, treating soil and maintaining its structure

How does ploughing fields to improve soil aeration impact the carbon cycle? (3)

helps aerobic organisms like free-living nitrogen fixers enhance the formation of ammonium ions in the soil, enhances the conversion of ammonium into nitrites and nitrates by nitrifying bacteria, plant roots respire aerobically and generate ATP fuelling their active uptake of minerals

How does draining land impact the nitrogen cycle? (4)

allows air to enter the soil, reducing the anaerobic conditions, which favour denitrifying bacteria, loss of nitrates is reduced

How does artificial nitrogen fixation impact the nitrogen cycle? (4)

the Haber process converts nitrogen to fertilisers, essential to produce high volumes of good quality food, fertilisers are mainly compounds containing ammonium and/or nitrate ions, like ammonium nitrate augmenting ammonium ions and nitrates produced by nitrogen-fixing and nitrifying bacteria

How do large amounts of animal waste impact the nitrogen cycle? (4)

improves soil structure so soil holds more nutrients and water and is more fertile, encourages microbial activity promoting the soil’s mineral supply improving plant nutrition, transported by road in large containers from areas where animals are farmed, manure releases nitrogen compounds to the soil gradually over a prolonged period

What is slurry and its function? (3)

a liquid made from manure and water produced by more intensive livestock rearing systems, normally injected into soil because of the smell, herbivore manure has milder smell due to less protein

What are biosolids? (2)

treated sewage sludge, sustainable alternative to inorganic fertilisers

What is significant about planting fields of legumes? (4)

enhances nitrogen fixation, when the crop dies, it is ploughed back into the soil as green manure, it has a high nitrogen content

What are the effects of nitrogen fertilisers on habitats?

eutrophication

Eutrophication (2)

the artificial enrichment of aquatic habitats by excess of nutrients, often caused by run off of fertilisers

oilgotrophic

upland streams have very few minerals dissolved in them

How is eutrophic water produced?

as water flows over the rocks minerals dissolve in the water

What is dystrophic water and its problem?

when mineral content of water is too high, it causes organisms to die

What are problems of excess nitrates in soils? (2)

reduces species diversity on grassland, increase the growth of grasses and plants such as nettles that shade out smaller plants

What are problems of nitrates leaching into rivers?

additional fertilisers and sewage result in eutrophication of lakes and rivers

What strict legislations are put in place for farmers to prevent eutrophication? (4)

restrict amount of fertiliser applied to soil, only apply fertiliser at times crops are actively growing so it’s readily used, leave a strip at least 10 metres wide near watercourses so nitrates can’t directly enter water, dig drainage ditches to protect watercourses

Explain what occurs in drainage ditches (3)

water flows slowly in the ditches and they accumulate landfill, lead to a local drop in invertebrate biodiversity and the water’s high nitrate concentration reduces species diversity in adjacent grassland, but rare species are reported to grow in drainage ditches in response to unusual conditions