Cycles Part 3

Question 1

What is the largest reservoir for nitrogen?

Answer 1

The largest reservoir of nitrogen is the atmosphere (3.8 × 1018 kg N). Atmospheric nitrogen is predominantly in the form of the diatomic molecule N2. Although N2 is commonly referred to as nitrogen or nitrogen gas, it should be more correctly referred to as dinitrogen, a term indicating its molecular composition.

Question 2

How many atoms is a nitrogen made up of and how many bonds does it have?

Answer 2

The two nitrogen atoms in the N2 molecule are joined by a very strong triple bond.

Question 3

If dinitrogen dominates the atmosphere, why might the availability of nitrogen often be a limiting factor in plant growth?

Answer 3

The strong triple bond is a major factor in the low reactivity of the N2 molecule. Plants require nitrogen in a form that can be used for the synthesis of proteins.

Question 4

What are the two naturally occurring ways for nitrogen fixation?

Answer 4

Nitrogen fixation can be achieved in one of two naturally occurring ways:
via biological nitrogen fixation, where dinitrogen combines with hydrogen from water, and the ultimate product is the ammonium cation, NH4+
during lightning strikes, where dinitrogen combines with atmospheric oxygen, O2, and the ultimate product is the nitrate anion, NO3−.

Question 5

What is nitrogen fixation?

Answer 5

For dinitrogen to be used by plants, it has to be converted into a usable form, or ‘fixed’. For this to occur, the triple bond has to be broken during nitrogen fixation reactions, which involve either hydrogen or oxygen.
The common term for biologically active nitrogen is fixed nitrogen.

Question 6

What is biologically active nitrogen?

Answer 6

Chemical species such as NH4+ and NO3− can collectively be termed biologically active nitrogen because they can readily be incorporated by all plants, unlike dinitrogen, which can be considered to be biologically inactiv

Question 7

What common farming practice suggests that the abundant pool of atmospheric dinitrogen cannot be used directly by most plants?

Answer 7

The use of nitrogen-containing fertilizers in the production of food crops and animal fodder.

Question 8

What term describes the cycling of nitrogen, a significant amount of nitrate, NO3−, is converted back to dinitrogen (and minor amounts of two oxides of nitrogen: nitrous oxide, N2O, and nitric oxide, NO) by a bacterial process

Answer 8

denitrification

Question 9

If denitrification occurred in isolation, how would it affect the levels of fixed nitrogen in an ecosystem?

Answer 9

Denitrification would cause levels of fixed nitrogen to fall.

Question 10

If denitrification were the only major process operating in the biogeochemical nitrogen cycle, then the levels of fixed nitrogen would decline so rapidly that the biosphere would cease to function in less than 200 years. Common experience tells us that this is not the case, and the losses due to denitrification are balanced by a process that replenishes fixed nitrogen. suggest a mechanism for the replenishment of fixed nitrogen.

Answer 10

The biological fixation of atmospheric N2; this is partly what makes it such a vital process.

Question 11

Nitrogen is essential to life, and is found in which of the following?

Fats
Carbohydrates
Proteins
Vitamins

Answer 11

Nitrogen is an important component of amino acids, which are the building blocks of protein as well as RNA and DNA. Nitrogen is also necessary for making chlorophyll. Proteins are necessary for the growth of any organism while RNA and DNA comprise the genetic material of cells. Without chlorophyll the majority of plant life would be unable to harness the Sun’s energy through photosynthesis.

Question 12

What process do living organisms rely on to obtain nitrogen for building amino acids?

Assimilation
Fixation
Excretion
Digestion

Answer 12

Living organisms rely on nitrogen fixation, as the nitrogen in the atmosphere (N2) is not biologically active (i.e. in a form that organisms can use to build amino acids and proteins).

Question 13

Which of the following organisms fixes nitrogen?

Trees
Fungi
Viruses
Bacteria

Answer 13

Nitrogen fixing bacteria, often found in root nodules but also free-living in the soil, fix nitrogen into a form that can be used by plant life. Note that these root nodules are different to mycorrhiza (which are fungi and are largely involved in phosphorous uptake).

Question 14

Which chemical substance is produced through nitrogen fixation?

Oxygen (O2)
Carbon dioxide (CO2)
Ammonium ions (NH4+)
Nitrate ions (NO3−)

Answer 14

Ammonium ions (NH4+) are produced by nitrogen fixation, when nitrogen gas is converted into these ions. The ions then bind with the soil particles – largely the clay and humus colloids. It should be noted that ammonium ions can be stored in the clay–humus complex for long periods of time. If the nitrogen fixers live in root nodules then this symbiotic relationship provides ammonium ions for the plant and carbohydrates for the bacterium.

Question 15

What do the nitrifying bacteria convert?

Soil-bound ammonia to nitrite, and then nitrate
Dead plant materials to ammonia
Nitrogen to ammonia
Nitrates into nitrogen gas

Answer 15

Check answer
Your answer is correct. Nitrifying bacteria release soil-bound ammonia and convert it to nitrites (NO2−) and then nitrates (NO3−). Both forms of nitrogen are highly soluble. Note that nitrite seldom accumulates in the soil, as the conversion from NO2− to NO3− is generally much faster than the conversion from ammonium (NH4+) to NO2−.

Question 16

What is the process that decomposers use to break down complex organic materials in simpler compounds, including ammonia?

Respiration
Excretion
Assimilation
Mineralisation/ammonification

Answer 16

Decomposers, particularly the fungi, break down complex organic materials through the process of mineralisation. The specific production of ammonia from complex organic materials is known as ammonification.

Question 17

What is the name of the process that describes living organisms taking up nitrogen?

Digestion
Assimilation
Ingestion
Respiration

Answer 17

Your answer is correct. Assimilation is the process by which living organisms take up nitrogen so that it can be converted into amino acids and other important molecules.

Question 18

What is the name of the process by which nitrogen is returned to the atmosphere, completing the cycle?

Nitrification
Ammonification
Denitrification
Respiration

Answer 18

Nitrate from the overlying aerated soil is washed into the waterlogged zone, and nitrate is a strong oxidising agent. Under these conditions, denitrifying bacteria, such as Pseudomonas, can continue to oxidise the organic matter using nitrate instead of oxygen. The end-products of this reaction are N2, and the two oxides N2O and NO, which can all be returned to the atmosphere.

This happens in aquatic systems too, where nitrate in the upper levels will diffuse to lower oxygen-free levels and will be used by denitrifying bacteria. The same process occurs, but on a vast scale in both lakes and the ocean.

Question 19

What would happen if the nitrogen cycle in soil and lakes were closed?

Answer 19

Nitrogen fixation and denitrification will be occurring in both the lake and soil environments, and if each environment were a totally closed system, then nitrogen fixation would exactly balance the losses due to denitrification

Question 20

How are large amounts of NO3- and NH3 be lost from soil processes?

Answer 20

Large amounts of NO3− and NH3 can be lost from soils by processes known as nitrate leaching and ammonia volatilisation respectively, and some of this fixed nitrogen could be washed into the lake

Question 21

What term defines The loss of nitrogen as nitrate ions dissolved in water?

Answer 21

nitrate leaching

Question 22

What term defines The loss of NH3 from terrestrial ecosystems to the atmosphere?

Answer 22

Ammonia volatilisation

Question 23

What happens when levels of fixed nitrogen fall below optimum value in the soil?

Answer 23

these nitrogen-fixing organisms are capable of detecting when the levels of fixed nitrogen fall below the optimum value, and when this occurs, nitrogen-fixing genes (nif genes) are activated in the organisms. The activation of these genes initiates biochemical and physiological changes that convert the organisms into mini nitrogen-fixing factories, allowing them to thrive in an otherwise sterile environment.

Question 24

hat are the consequences of the activity of nitrogen-fixing organisms?

Answer 24

The levels of fixed nitrogen in soils will be brought back up towards their optimum value. When this occurs, non-fixing organisms will recolonise the environment, because they are also opportunists, and simply benefit from the hard work of the fixers. However, the nitrogen-fixing organisms would be at a distinct ecological disadvantage once nitrogen levels in the soil began to rise. Remember that the triple bond of the dinitrogen molecule is very strong, and the breaking of this bond requires a lot of energy. Therefore, during nitrogen fixation, energy that could have been used for plant growth, for example, is diverted towards the breaking of the triple bond of dinitrogen

Question 25

What happens when nitrogen fixing organisms stop using atmospheric nitrogen when optimum soil levels are low?

Answer 25

Non-fixing organisms simply assimilate fixed nitrogen, in which the N≡N bonds have already been broken for them, and hence more of their energy can be used directly for plant growth. In order to remain competitive when the levels of fixed nitrogen have been brought back to the optimum value, the nitrogen-fixing organisms will cease the fixation of atmospheric dinitrogen and simply assimilate fixed nitrogen, like all of the other organisms.

Question 26

What is optimum value?

Answer 26

The Earth’s biosphere contains a constant level of fixed nitrogen that can be termed the ‘optimum value’.

Question 27

What is considered as biologically active nitrogen that helps plants grow?

Answer 27

Biologically active nitrogen species, e.g. NH4+ and NO3−, can be used by plants for growth.

Question 28

What are the 4 mechanisms of nitrogen fixation?

Answer 28

lightning strikes
natural biological nitrogen fixation
agricultural nitrogen fixation
industrical nitrogen fixation.

Question 29

What percentage of total annual nitrogen fixation do lightening strikes account for?

Answer 29

it appears that lightning strikes account for about 10% of the total annual amount of natural nitrogen fixation.

Question 30

Although the amount of nitrogen fixed by lightning strikes is insufficient for the needs of the biosphere, can you think of another ecological reason why the nitrogen fixed by this process may not be vital for modern ecosystems?

Answer 30

Thunderstorms are entirely random processes that will deposit fixed nitrogen on ecosystems irrespective of whether it will be used or not. For example, a thunderstorm could deposit fixed nitrogen on a lake that was already heavily polluted with nitrates, which would simply exacerbate an existing ecological problem.

Question 31

What are the only known free living organisms that can naturally fix nitrogen?

Answer 31

The only confirmed free-living organisms that are capable of fixing nitrogen are certain bacteria, including free-living soil bacteria and cyanobacteria. Cyanobacteria are important nitrogen fixers in the oceans, although they are not found solely within the marine environment.

Question 32

What the nitrogen fixing organisms that aren’t free living?

Answer 32

There are also nitrogen fixing organisms that aren’t free-living, but exist in association with other organisms. Perhaps the most famous of these are bacteria of the genus Rhizobium, which enter into a symbiotic relationship with legumes (peas, beans, clover, etc.). Such relationships exist where there is a close association between two distinct species where both organisms benefit from this situation.

Although these specialised organisms can fix atmospheric dinitrogen, their nitrogen-fixing capabilities may lie dormant if there is abundant fixed nitrogen in their immediate environment. For example, if a soil is rich in NH4+, then peas will thrive without requiring Rhizobium to fix atmospheric dinitrogen.

Question 33

Can you think of any environment on land where you would find only nitrogen-fixing organisms?

Answer 33

Freshly exposed nitrogen-free rocks, such as lava flows or rocks that have recently been freed from glacial ice. The initial colonisation of such rocks can be achieved only by nitrogen-fixing organisms.

Question 34

Why is ammonia so difficult to make?

Answer 34

It is because nitrogen is relatively unreactive (the molecules are held together by strong triple bonds). When trying to create ammonia using the catalyst (heated iron wool) the equation goes both ways – the reaction is reversible – ammonia can easily return to its constituent elements of hydrogen and nitrogen.

Question 35

What is the most striking difference between industrial nitrogen fixation and biological nitrogen fixation?

Answer 35

Whereas bacteria can fix nitrogen at ambient temperatures and pressures using sunlight as the energy source, industrial nitrogen fixation is energy intensive, requiring both elevated temperatures and elevated pressures.

Question 36

What percentage of nitrogen is lost when using it as fertilizer?

Answer 36

Furthermore, in the symbiotic relationships in legumes the biologically fixed nitrogen is incorporated directly into plant protein, whereas fertilizers are applied to the soils in the hope that the nitrogen will be assimilated by plants. However, fertilizers are actively cycled within the soil before they can be assimilated by plants. This results in the formation of soluble inorganic and gaseous forms of nitrogen that can be lost from the soils, and can lead to problems of pollution. Indeed, as much as 50% of fertilizer nitrogen can be lost from soils.

Question 37

Name a present-day environment that contains ‘fossil’ organic matter that is not being recycled.

Answer 37

A peat bog is a good example. This is a highly reducing environment (i.e. one in which hydrogen is added or oxygen removed), and organic matter can remain largely intact.

Question 38

What molecule is the most important for decomposition of organic nitrogen into an inorganic species.

Answer 38

However, despite this complexity, the end-point of the decomposition of organic nitrogen is simple inorganic species. Of these, NH4+ is the most important;

Question 39

What term defines the breakdown of complex organic molecules, often involving the release of energy.?

Answer 39

Catabolism

Question 40

What term defines Bonds between amino acids in a protein?

Answer 40

peptide bonds

Question 41

What is the dominent species of nitrogen in living organisms?

Answer 41

NH4+ is the dominant species in living organisms

Question 42

Can you think of a common experience that reveals the loss of soil nitrogen from animal wastes?

Answer 42

Walk close to a pig farm in summer and breathe deeply through your nose. Among those healthy smells, you will detect NH3.

Question 43

How long does NH3 last in the environment?

Answer 43

However, although much nitrogen may be lost by volatilisation, NH3 has only a very short residence time in the atmosphere (six days). Much of the gas will therefore be redeposited rapidly in the fields, either directly or dissolved in rainwater (although unavoidable losses of NH3 will occur). As previously suggested, NH3 may be blown away from the fields into oceans, lakes, forests and other ecosystems, but in addition, much of the volatilised NH3 may be consumed by certain atmospheric reactions.

Question 44

How does nitrogen and sulfur affect the atmosphere?

Answer 44

Human activity contributes significantly to the amount of nitrogen oxides, NOx, and – more importantly – sulfur dioxide, SO2, in the atmosphere. Here, these gases react to form nitric acid, HNO3, and sulfuric acid, H2SO4

Question 45

What can neutralise sulfuric and nitric acids and how?

Answer 45

The only major basic chemical compound in the atmosphere that can neutralise these acids is the ammonia molecule, NH3.

Ammonia reacts with these acids to form fine particles of ammonium nitrate, NH4NO3, and ammonium sulfate, (NH4)2SO4. There have been some suggestions that aerosols containing these molecules may have some localised effect on climate. Their high solubility in cloud and rainwater means that they will be deposited rapidly, and their contribution to global climate change is probably negligible. However, what this effect does highlight is that human interferences in biogeochemical cycles can cancel each other out, at least partially. Increasing emissions of agricultural NH3 therefore help to neutralise increasing emissions of industrial SO2.

Question 46

How much more effective is nitrous oxide at retaining heat that CO2?

Answer 46

Nitrous oxide is a greenhouse gas that is over 300 times more effective in retaining heat than carbon dioxide.

Question 47

can you think of one way that human activity may be leading to an increase in atmospheric nitrous oxide?

Answer 47

Modern agriculture requires the use of synthetic fertilizers that are fixed via the Haber–Bosch process. Some of this fixed nitrogen is returned to the atmosphere by denitrification, which will increase net levels of atmospheric nitrous oxide.

Question 48

How does organic nitrogen convert to ammonium ion?

Answer 48

how organic nitrogen is converted to the ammonium ion, NH4+, during mineralisation.

Question 49

What can repeat planting and harvesting of soils remove?

Answer 49

Repeated planting and harvesting of a non-fixing plant such as wheat will eventually remove so much fixed nitrogen from the soils that they will lose their fertility. In order to avoid this, farmers must apply fertilizers to the fields. These fertilizers are commonly ammonium salts, ammonia solutions, nitrates, urea and animal wastes. For economic reasons, they are often applied in a single large dose, whereas natural biological fixation is a continuous process.

Question 50

What is the most mobile form of nitrogen in soil?

Answer 50

NO3− is the most mobile form of nitrogen in most soils

Question 51

What are the potential threat so of nitrates in drinking water?

Answer 51

Two potential threats to human health have been identified for the consumption of nitrates in water, although opinions differ widely as to the level of risk involved. The first of these is a condition known as methaemoglobinaemia, or ‘blue baby’ syndrome. Here, the supply of oxygen to the bloodstream is restricted because nitrite ions, NO2−, formed from ingested nitrate ions, compete with oxygen for uptake by haemoglobin. The symptoms are similar to those of carbon monoxide poisoning.

The second hazard associated with consuming nitrates in water is that the ions can react with amines in the gut to form carcinogenic (potentially cancer-causing) compounds.

Question 52

What levels does WHO recommend NO3- and NO2 should be in drinking water?

Answer 52

Because of the uncertainties surrounding the extent of the risks, the World Health Organization (WHO) recommends that levels of NO3− and NO2− in drinking water should be less than 50 mg l−1 and less than 3 mg l−1 respectively.

Question 53

How can excessive nitrogen affect aquatic environemnts?

Answer 53

Similarly, surface algal blooms in a restricted environment, such as a small lake, can seriously disrupt the ecology, simply because most aquatic organisms will be deprived of sunlight. This can lead to the death of many organisms, and their decomposition will consume oxygen, resulting in stagnation.

Question 54

What problems can occur with ploughing of the soil?

Answer 54

In addition to the enhanced production of nitrates, the continued ploughing of agricultural land has also resulted in many other long-term problems such as soil erosion, a general decrease in the amount of organic matter present in the soil (enhanced mineralisation), and loss of soil moisture.

Question 55

What is zero tillage seeding?

Answer 55

By definition, zero-tillage seeding is a one-step operation that places seed and fertilizer into an undisturbed seedbed, and retains sufficient stubble from the previous year’s crop to prevent soil erosion and retain soil moisture. Specialised machines cut a thin slit in the soil where fertilizer and seeds are deposited in precisely the right quantities, at precisely the right depth. Although not suited to all climates, the results have justified the effort and, with proper management, yields may be as good as or better than those obtained with traditional techniques.

Question 56

Ammonia volatilisation is the next most important fixed nitrogen loss from terrestrial sources (122 × 109 kg y−1), yet most of this ammonia is eventually redeposited. Under what conditions is ammonia volatilised, and why is most of it redeposited?

Answer 56

Ammonia is volatilised from dry alkaline soils, but most importantly from animal wastes and urea-based fertilizers. It is rapidly redeposited. Ammonia is very soluble, so readily dissolves in cloud and rainwater. Its atmospheric residence time of only six days means that most of it is redeposited.

Question 57

Rivers and runoff constitute a major loss of nitrogen from the land to the oceans. In what form is this nitrogen?

Answer 57

Rivers may contain several different nitrogen-containing components, such as the ions NH4+ and NO3−, particulate organic matter and plant and animal debris.

Question 58

What is the most important store for nitrogen in terrestrial systems?

Answer 58

Organic matter is the most important store of nitrogen in terrestrial ecosystems but much of this is not readily recycled.

Question 59

What processes is nitrogen in organic matter recycled?

Answer 59

Nitrogen in organic matter is recycled through a variety of processes:
mineralisation, where organically bound nitrogen is released into the environment as NH4+ (also urea and uric acid)
ammonia volatilisation, which mainly involves the breakdown of urea and the loss of NH3
nitrification, where nitrifying bacteria oxidise NH4+ to NO2− and NO3− in oxidising environments

Question 60

Describe the denitrification process of bacteria?

Answer 60

Denitrification is a process were denitrifying bacteria use NO3− instead of O2, during the oxidation of organic matter in reducing environments

Question 61

What is the assimilation process of nitrogen to plants?

Answer 61

Assimilation is a process where soil NH4+ and NO3− are incorporated into plants

Question 62

Which of the following is a good example of where we can obtain sulfur to keep us healthy?

Water
Air
Diet
Our skin makes it

Answer 62

The primary source of sulfur comes from meat and fish, but other good sources are eggs and certain sulfur-rich vegetables.

Question 63

Which is the most important natural source of sulfur dioxide gas?

Volcanoes
Phytoplankton
Sedimentary rocks
Fertilisers

Answer 63

The correct answer is ‘phytoplankton’. Such organisms (e.g. Emiliana huxleyi) are the most important source of sulfur dioxide, which is released into the ocean and then bubbles up into the atmosphere when the plant dies. It is then deposited across the globe as acid rain.

Question 64

Under which conditions can bacteria produce H2S gas?

Aerobic conditions
During digestion
Anaerobic condition
During lightning storms

Answer 64

The correct answer is that anaerobic bacteria can produce H2S under anaerobic conditions (without oxygen). These bacteria thrive in conditions of no oxygen molecules per se – but obtain oxygen by reducing calcium sulfate (which is very common in seawater).

Question 65

Which of the following habitats might most likely provide anaerobic conditions?

Oak woodland
Desert
Vegetated shingle
Saltmarsh

Answer 65

Anaerobic bacteria are common in any anaerobic conditions, such as saltmarsh/estuarine mud or mangrove swamps, but also (and most importantly) in the mud at the bottom of the oceans. Here the mud is turned to shale over long periods of time, and the sulfur is transformed into iron pyrite. So begins the long journey of the rock (and its sulfur) back to the surface via geological events (such as volcanoes and uplift) and the rock cycle.

Question 66

Which fuel, in particular, is adding to the natural acidity of rain water?

Diesel
Coal
Nuclear
Hydroelectric

Answer 66

The burning of coal, in particular, adds sulfur dioxide to the atmosphere (especially low-grade coal). Sulfur dioxide is highly soluble, as so is easily transformed into sulfuric acid.

Question 67

Which of the following is least likely to be affected by SO2 pollution?

Limestone buildings
Human health
Chalk downland plant-life
Upland lakes and trees

Answer 67

Chalk downland plant-life is least likely to be affected, as it is growing on shallow basic soils that are close to the calcium carbonate bedrock that would neutralise acid rain.

Question 68

Apart from the expense, what is the main problem with neutralising SO2 emissions (or acid lakes for that matter) with lime?

NO2 is emitted
CO2 is emitted
H2 is emitted
CH4 is emitted
Check answer

Answer 68

The correct answer is that CO2 is emitted when any acid acts on calcium carbonate. This adds to the greenhouse effect (and ironically ocean acidification). It is therefore ideal to use less energy, in particular coal.

Question 69

Do you think the sulfur cycle has more similarities to the carbon or nitrogen cycle? Explain your answer.

Answer 69

Both carbon and sulfur cycles are dominated by large sedimentary reservoirs. The nitrogen cycle, in contrast, is dominated by a large atmospheric reservoir.

Question 70

A minor, but important, natural source of sulfur in the atmosphere is the biological production of H2S through sulfate reduction. Identify where the major sulfur flux

Answer 70

The major source of biogenic H2S is from anaerobic saltwater wetland soils and sediments, i.e. the biogenic gases shown in Figure 5.3.10. Since wetland soils and sediments are included as terrestrial soils, these fluxes are shown as a terrestrial source in the global cycle.

Question 71

What accounts for 99.99% of sulfur on the Earth?

Answer 71

rocks and seawater account for over 99.99% of the Earth’s sulfur.

Question 72

determine how else sulfur is transported to the ocean by natural processes.

Answer 72

Some sulfur enters the ocean as atmospheric fallout, having originated from volcanic emissions.

Question 73

suggest how human activities may affect fluxes of sulfur from the crust.

Answer 73

Human extraction and mining have accelerated the movement of sulfur from the crust.

Question 74

Which natural atmospheric process do you think has been exacerbated by the anthropogenically increased flux of sulfur from the crust to the atmosphere?

Answer 74

The production of acid rain by the reaction of atmospheric water with the sulfur-containing gases generated by volcanoes and marine organisms has been increased by the burning of fossil fuels and the release of crustal sulfur. Acid rain causes lakes and rivers to become acidic, killing off fish and other organisms. It also damages trees and buildings.`

Question 75

What two sources can sulfur be split in to?

Answer 75

The sources of sulfur can be split into two types – natural and anthropogenic sources.

Question 76

Where does sulfur occurr naturally?

Answer 76

Sulfur occurs naturally in all parts of the environment, including the atmosphere, oceans, soils, rocks and living organisms.

Question 77

How can sulfur be assimilated by plants directly?

Answer 77

Sulfate produced by the weathering of pyrite can be assimilated directly by plants.

Question 78

What natural flux of sulfur is produced by marine organisms?

Answer 78

Dimethyl sulfide, (CH3)2S is the largest natural flux of sulfur gas, which is produced by marine organisms.

Question 79

What are the three main sources of natural sulfur?

Answer 79

There are three main natural sources of atmospheric sulfur:

the oceans
the decay of organic matter
volcanic emissions.

Question 80

In what form is sulfur present in the sea?

Answer 80

The element is primarily present within oceans in the form of dissolved salts, principally sulfates.

Question 81

How is sulfur released in to the atmosphere from the sea?

Answer 81

. The mechanism by which sulfate is released into the atmosphere is via sea spray. This evaporates, leaving fine particles containing sulfur in the atmosphere. The gaseous forms of sulfur enter the atmosphere through gaseous exchange at the ocean’s surface.

Question 82

What two principal anthropogenic sources are sulfur emission created?

Answer 82

Sulfur emissions are also caused by human activity. These emissions originate from two principal sources:

Most importantly, from the burning of fossil fuels for heat and electricity generation. The relevant fossil fuels are coal and oil.
Also through industrial processes such as the smelting of iron and other metals whose ores contain significant amounts of sulfur, and through the manufacture of sulfuric acid.

Question 83

All fossil fuels contain sulfur from at least one common source. What is it?

Answer 83

Sulfur is one of the macronutrients necessary for all life. It is incorporated into the amino acids of all living things, including the vascular plants that eventually form coal and the algae and other plankton that form oil.

Question 84

What is released in to the atmosphere when fossil fuels are burned?

Answer 84

When fossil fuels are burned, the carbon and sulfur from these compounds are released into the atmosphere as CO2 and SO2.

Question 85

What is the percentage of sulfur in coal?

Answer 85

The total sulfur content of coal can be as high as 6–8%.

Question 86

In which ecosystems on Earth would you be most likely to find significant biological production of H2S? Explain your answer.

Answer 86

Saltmarshes, mangrove swamps and estuaries where anaerobic soils are exposed to high concentrations of sulfate ions are the most likely ecosystems for biological production of H2S. In these ecosystems, the combination of anaerobic conditions, large amounts of available carbon (sediments rich in organic matter) and a large supply of SO42− from seawater provide the ideal conditions for sulfate reduction to occur.

Question 87

Global dimming means that sunlight across the globe has reduced in strength over the last few decades. (True or false?)

Answer 87

True. Research has shown that sunlight levels have reduced worldwide with, for example, a 22% reduction in Israel and an 18% reduction in Britain.

Question 88

Global dimming is caused by CO2 emissions. (True or false?)

Answer 88

False. Global dimming is caused by particulates otherwise known as aerosols and these provide platforms for water to condense and create clouds. Note that aerosols can be primary (such as natural dust from deserts, sea spray and pollutant soot) or secondary (largely based on transformed pollutant gases, such as SO2 from power stations which transforms to various forms of sulfate in the air). NO2, for example from exhaust fumes, also creates important secondary aerosols.

Question 89

Global dimming causes a warming of the globe. (True or false?)

Answer 89

False. Global dimming causes cloud formation, which results in the heat of the sun being reflected away from the planet.

Question 90

Global dimming is helping to dampen the effects of global warming. (True or false?)

Answer 90

True. It is suggested that if it wasn’t for the effects of pollutant aerosols in the atmosphere then anthropogenic-induced global warming would be far greater.

Question 91

Governments should encourage companies to the pump more aerosol pollutants into the atmosphere to increase global dimming and decrease the effects of global warming. (True or false?)

Answer 91

False. Apart from the obvious direct health problems and detrimental environmental effects associated with aerosol pollutants (such as acid rain), global dimming appears to have been responsible for droughts in the Sahel in the 1970s and 1980s. It is suggested that global dimming could also threaten the path of the monsoon in Asia, which could affect food production and water supplies for billions of the world’s population.

Question 92

Governments should tackle both global warming and global dimming at the same time. (True or false?)

Answer 92

True. It is important that both greenhouse gas emissions and aerosols should be tackled if we are to safeguard the Earth, its environment and resources for future generations.

Question 93

What contains the majority of phosphorus?

Answer 93

the Earth’s crust contains the majority of phosphorous

Question 94

What reservoir does not contain phosphorus?

Answer 94

the phosphorus cycle does not contain a gaseous reservoir

Question 95

Where is the majority of the phosphorus that is used for fertilizer derived from, and what issue(s) might this raise for humans?

Answer 95

Most of the phosphorus is derived from rock, and this is arguably a finite resource when considered on an anthropogenic timescale. There are also social and environmental issues associated with the heavy industry required to extract the element from the rock.

Question 96

What other reasons are there for a need for humans to recycle phosphorus, aside from concerns about its availability?

Answer 96

We need to recycle phosphorus (i.e. prevent it getting into oceans and rivers) because not only would we stem destructive mining for the resource, we might also, more importantly, stem the pollution (in the form of eutrophication and algal blooms) that phosphorus pollution from fertilizers and wastewater can cause.

Question 97

What makes the phosphorus cycle different from the other cycles?

Answer 97

Phosphorus has no gaseous reservoir.

Question 98

What natural gas is formed from phosphorus?

Answer 98

Phosphorus does, however, naturally form a gas called phosphine (PH3), but it is restricted to very specialised local conditions.

Question 99

By considering the chemical formula of phosphine, what sort of environment would it be present in?

Answer 99

Phosphine is a reduced gas and hence it is restricted to oxygen-poor reducing environments such as saltmarshes.

Question 100

What form of phosphorus can be assimilated by organisms?

Answer 100

Phosphate is a form of phosphorus that can be assimilated by organisms. However, most phosphate reacts with soil minerals such as iron and aluminium oxides and becomes unassimilable

Question 101

What type of system is the phosphorus cycle?

Answer 101

In the terrestrial ecosystem, the flux of phosphorus between the soil and biota is almost a closed system and 60 × 1012 g P y−1 is rapidly recycled in this way (Figure 5.3.12). Most of the phosphate only breaks free of the terrestrial phosphorus cycle when soil erosion allows solid particles to be transported by running water.

Question 102

What do regions of upwelling phosphorus tend to be associated with?

Answer 102

In some parts of the world, the phosphorus-rich waters of the deep ocean can upwell and reintroduce phosphorus to surface waters. This action stimulates the growth of phytoplankton, so regions of upwelling tend to be associated with high levels of primary production.

Question 103

As there are no significant gaseous losses of phosphorus from the oceans, outputs of phosphorus are restricted to the 2.0 × 1012 g P y−1 buried in marine sediments. Examine Figure 5.3.12 and suggest how this output is replenished.

Answer 103

The outputs of phosphorus to marine sediments are balanced by the inputs of dissolved phosphorus from rivers.

Question 104

What is phosphorus reliant on to facilitate recycling?

Answer 104

the phosphorus cycle is reliant on tectonic activity to facilitate recycling.

Question 105

What problems are associated with anthropogenic phosphorus harvesting?

Answer 105

In common with many other biogeochemical cycles, human activities have disturbed the natural phosphorus cycle. Around 12 × 1012 g P y−1 is released from the geosphere by mining, most of which goes to the production of phosphate fertilizers. The phosphorus incorporated into agricultural crops from fertilizers is removed through harvesting and is eventually released as human or animal sewage. Sewage treatment removes around 40% of this phosphorus but the remainder finds its way into rivers, lakes and the sea.

Question 106

What sort of activities might cause aquatic pollution associated with phosphorus?

Answer 106

Over fertilisation on neighbouring fields and gardens and run off from household drains.

Question 107

Over fertilisation on neighbouring fields and gardens and run off from household drains.

Answer 107

a. Phosphate that is applied to soil in the form of fertilizer becomes bound to soil particles and then is not leached through the soil into the groundwater. This property can allow phosphorus to build up to very high and even toxic levels in the soil.
b. Runoff from agricultural land, which carries sediment into watercourses, introduces extra phosphorus into rivers and lakes, where it may become released into solution. As the productivity of many aquatic ecosystems is limited by phosphorus availability, an influx of phosphorus often results in a large increase in net primary production, which in turn depletes the water body of its oxygen as the organic matter begins to decay. This oxygen depletion can result in the death of fish and other aquatic animal