Nutrient Cycles (A Level only) (AQA A Level Biology)

The diagram in Figure 1 shows a stream that runs past a farm. A group of students took water samples at various locations along the stream and measured the nitrate concentration in parts per million (ppm) in those water samples. The students have marked these nitrate concentrations on the diagram in Figure 1.

Figure 1

Explain how the farm may be causing the increase in nitrate concentration in the water of the stream, after it has flowed past the farm.

The students have been told that nitrate concentration above 200ppm encourage eutrophication in the stream.

Describe the processes that may be occurring in the stream in Figure1 just after it has passed the farm

Explain why there is increased algal growth in the section of the stream in Figure 1 just after it has flowed past the farm.

The students investigating the stream in Figure 1 compared the oxygen levels of the water in the stream before it had passed the farm (upstream) and the water of the stream after it had passed the farm (downstream). They found that the water downstream of the farm had a significantly lower oxygen concentration. The students hypothesised that the low oxygen concentration might change the community of organisms in this section of the stream.

What effect(s) might these low oxygen levels have on the community in the water downstream of the farm.

Scientists have been monitoring the phosphate concentration in the water of Loch Lomond in Scotland since 1990. The results are shown in the graph in Figure 1.

Figure 1

Considering Figure 1, give:

i) One possible source of the phosphate in the
Loch.

ii) The percentage increase in phosphate between 2000 and 2020.

In 1990 the phosphate levels in the water of Loch Lomond (see Figure 1) were considered optimal for the growth of aquatic plants.

Explain why phosphates are essential for plants to grow.

Herbivores living in and on Loch Lomond, such as fish and wading birds, also require phosphorus.

Explain how herbivores obtain phosphorus from their food.

Phosphorus (in the form of phosphates) is absorbed and used by the animals.

Describe how the phosphorus gets recycled back into the lake ecosystem of Loch Lomond.

The diagram in Figure 1 shows part of the nitrogen cycle occurring on a farmed field where crops are being grown.

Figure 1

Name the processes labelled X, Y and Z in the diagram in Figure 1.

Describe how the saprobionts in Figure 1 obtain their nutrients from dead organisms and animal waste products.

As seen in Figure 1, a key step in the nitrogen cycle that is essential for successful growth of crops is the take up of nitrogen compounds from the soil by the plants. Saprobionts release nitrogen into the soil in the form of ammonia/ammonium ions but another key process must occur before the crops can use this nitrogen.

i) Name this process.

ii) Describe this process.

Adding inorganic fertilisers to a field, as in Figure 1, is a farming practice that can increase the amount of nitrogen-containing compounds in the soil. Describe and explain one other farming practice that can increase the amount of nitrogen-containing compounds in the soil.

Two fields, C and D, were used to grow corn. Each field was divided into six plots. Different masses of fertiliser containing ammonium nitrate were applied to each plot. After one month, samples of corn plants were collected from each plot and their mass calculated. The results are shown in the table in Figure 1.

Figure 1

Describe the results of the study shown by the table in Figure 1.

Explain why the mass of corn produced from Field C in Figure 1 increased whenthe mass of fertiliser added was increased from 0 to 40 kg ha^–1 but no longerincreased when more than 60 kg ha^–1 of fertiliser was added.

In the year before the investigation in Figure 1 was conducted, Field C had been used for grazing sheep. Field D, however, had been used to grow the same crop as the current year in which this investigation took place. When the mass of fertiliser added was 0 kg ha^–1, the mass of corn from Field C was higher than from Field D.

Assuming all other conditions were the same during the investigation, give a possible explanation for this finding.

Using the results table in Figure 1, determine which field (Field C or Field D) showed the greater percentage increase in mass of corn. State the difference in percentage increase between the two fields?

The graph in Figure 1 below shows the effect of adding different amounts of two types of nitrogen-containing fertiliser (fertiliser X and fertiliser Y) on the yield of wheat crops.

Figure 1

Considering the graph in Figure 1:

i) The curves for both fertilisers do not start at the origin of the graph.

Explain why.

ii) Suggest the mass of nitrogen-only fertiliser (fertiliser Y) a farmer should use per square metre for growing wheat.

Looking at the curve between 0 and 15 g m^-2 for fertiliser Y in Figure 1, it appears that the amount of nitrogen added in the fertiliser is limiting the yield of wheat. Past this point, it appears that something else is limiting the yield.

i) Using the information in Figure 1, suggest what limits yield after this point.

ii) The wheat plants receiving fertiliser X can form more ATP and showed a greater yield as a result. Describe the structure of ATP.

Other than their importance in the production of ATP, state and explain two reasons why phosphate ions are important for the growth of plants.

Using artificial nitrogen-containing fertilisers, such as in Figure 1, is one way to increase crop yield. There are alternative methods for increasing crop yield, such as including leguminous plants in a crop rotation. Explain how this technique can also improve crop yield.

A suitable soil for agriculture ensures nutrient cycling for optimum plant growth for agricultural production. A farm’s ability to produce good crop yields is often limited by available soil nutrients, especially nitrogen. Nitrogen is not present at high levels in soil material despite the fact that nitrogen constitutes about 78% of all the atmospheric gases. 

Outline four ways in which nitrogen and its compounds are introduced into soil as a mineral nutrient for plants.

Nitrifying bacteria in soil convert ammonia into nitrites and nitrates. Researchers examined two soils with different pH for the effect on the communities of nitrifying bacteria found in these soils. They sampled soil from two sites, X and Y.

The pH of the soil samples were:

• Site X - pH 6.9

• Site Y - pH 4.3

Ammonia concentrations were measured in the soil samples for the subsequent 20 days. Each sample contained the same concentration of ammonia at the beginning of the experiment and had the same mass. They recorded the ammonia concentrations in:

• Soil X alone

• Soil Y alone

• A mixture of equal masses of soils X and Y with its pH adjusted to 6.9

Their results are shown in Figure 1.

Figure 1

Suggest why the researchers chose the units of ammonia concentration as shown in Figure 1. 

Calculate the difference in the mean rate of breakdown of ammonia per day in the first three days in soil X and soil Y.

Show your working and the units for your answer.

The researchers concluded that the mixing of soils demonstrated that there were different communities of bacteria in soils X and Y.

Use Figure 1 to provide evidence to support their conclusions. Give reasons for your suggestions.

Farmers regularly plough their arable fields. Use your knowledge of the nitrogen cycle to explain the benefits of this practice.

Describe the nature of mycorrhizae and their role in facilitating the uptake of water and inorganic ions by plants. 

In the phosphorus cycle, outline three ways in which phosphorus is introduced into soil and into water in oceans, lakes and rivers

Figure 1 shows the measured phosphate levels in river water upstream and downstream of a large US city during the period 1980 - 1995. The city is surrounded by productive arable land in all directions.  

Figure 1

In 1990, the city authorities introduced restrictions on the use of phosphorus detergents in the city. 

Explain the effect of these restrictions and suggest an additional measure the authorities can take to further reduce phosphate levels in river water.  

The bison is a large, herbivorous mammal that lives on the prairies and plains of North America. Their diet is varied, consisting largely of a mixture of grasses and sedges. Rainfall is high in April to May and also in November. A dry season runs from July to October, during which time rainfall is very low.

Figure 1 shows the pattern of the protein content of all the plants that could form part of the bisons’ diet. Also shown is the protein content of the food the bison actually eat.

Figure 1

In periods of zero or low rainfall, the plants’ protein content reduces. Suggest one way in which zero or low rainfall could account for this change.

At all times of the year, the mean protein content of the food actually eaten by the bison differs from that of the plants which could be eaten. 

Suggest one explanation for this difference.

Referring to Figure 1, deduce the months in which the difference between protein available and protein eaten is at its minimum and maximum levels. Account for these differences.   

At times when bison eat food containing less than 5% protein, the protein content of their body tissues has been observed to decrease. 

Suggest and explain how a lack of one named protein makes the bison less able to avoid predation by, for example, wolves.

Lentil plants are leguminous and have nodules on their roots which contain bacteria that are able to fix nitrogen. Figure 1 shows some of the processes involved in nitrogen fixation by these bacteria.

Figure 1

Name substances P and Q.

The bacteria in the root nodules respire anaerobically. This produces hydrogen and ATP used in nitrogen fixation. The hydrogen comes from reduced NAD. 

Explain how the regeneration of NAD allows ATP production to continue.

Other than planting leguminous crops and spreading artificial fertilisers, describe andexplain how one farming practice results in the addition of nitrogen-containing compounds to a field.

Describe and explain how two different farming practices result in the removal of nitrogen-containing compounds from a field.

Certain species of nitrogen-fixing bacteria (e.g. Azotobacter and Cyanobacter species) are free-living soil organisms that are not associated with plants mutualistically. As such,they need to find a different source of energy. 

Suggest two means by which these free-living bacteria could generate the energy required for nitrogen fixation.

One method of sewage treatment is called activated sludge, in which organic matter is fed, as untreated sewage, to bacteria in the treatment tank. These bacteria include decomposers and nitrifying bacteria. A food chain exists within the tank, in which the bacteria are eaten by ciliated protoctists, which are, in turn, eaten by carnivorous protoctists.

Explain the roles of the decomposers and the nitrifying bacteria in converting nitrogen from organic compounds in the sewage into a soluble, inorganic form.

Describe the importance to ecosystems of the role played by nitrogen-fixing bacteria in the nitrogen cycle.

Leaching of nitrate in groundwater, often found in run-offs from arable farms, can lead to environmental problems such as eutrophication of ponds, waterways and lakes. Farmers wishing to alleviate this problem have sought to capture their agricultural run-off and treat it as a form of sewage. The treatment employs another group of bacteria to take advantage of a process which occurs naturally in the nitrogen cycle.  Suggest which process this is and provide reasons for your suggestion.