I) What Roles Do Earthworms Play in Maintaining Soil Stability? (2 Points
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(a) Explain the consequences of the observations seen in the studies described and comment on the long-term sustainability of each agricultural practice.
(i) What roles do earthworms play in maintaining soil stability? (2 points)
They are essential for mixing soil and for shredding detritus. This breaks detritus down into smaller pieces that can be chemically degraded and release nutrients and minerals into the soil.
The burrowing earthworms also help with the aeration and water infiltration of the soil. Without earthworms the soil community would become less stable and degrade more quickly.
(ii) How does the presence of organic matter benefit the soil? (2 points)
Depending on the soil texture (the amount of sand, silt, and clay), organic matter or humus can improve water and aeration properties, workability, and as the organic matter is broken down can release mineral nutrients and other material into the soil that benefit plants.
Having a good supply of organic matter in the soil, located in the O horizon, enhances the stability of the soil and has a positive effect on sustainability.
(iii) Suggest why there is less nitrate contamination of groundwater from the permanent pastures. (2 points)
When animal feed-grain is continually grown on agricultural land, inorganic fertilizers have to be applied to the fields in order to attain good crop yields. These inorganic fertilizers contain nitrates, phosphates, and potassium compounds which are obtained by mining rocks that are rich in the mineral resources.
This can have a negative impact on the rock cycle and the environment, and is clearly a non- sustainable practice. The nitrates from the application of inorganic fertilizer can dissolve in rainwater, infiltrate the soil, and build up in the groundwater below.
When animals are allowed to graze in pastureland, it is unnecessary to apply large amounts of inorganic fertilizers, and therefore fewer nitrates will be found in the groundwater. Fertilizer in the pastureland may be in the form of animal dung which organic. (iv) In what ways could the stream quality have improved? (2 points)
The stream quality improved as a result of less fertilizer runoff reducing the problem of eutrophication. When tested, the stream water will probably have lower amounts of nitrates and phosphates than would be found from land continually used for grain production.
As less soil erosion takes place from pastureland, the sediment accumulation in the stream will be lower and not cover the streambed, allowing larvae to build up on the rocks and provide habitat for small aquatic organisms. With lower amounts of sediments in the stream, light penetration will enable benthic plants to survive.
Grain production requires the application of pesticides and herbicides and a switch to pasture grazing will result in lower amounts of these chemical pollutants in the runoff into the stream. All of these examples will result in improved stream quality.
(b) Discuss one negative consequence of grazing cattle on pastureland instead of feeding on grain. (1 point)
A farmer who feeds his beef cattle a diet of grain will generally be able to raise a high number of animals in a relatively small area. This could be in a building designed to house cattle in stalls. They often can be brought to market sooner due to the diets.
A farmer may not want to switch to permanent pasture grazing as the number of cattle that can be raised will drop.
Economically, the farmer may be against such a move as his profit margin will decrease, although the fewer cattle may drive the price of beef up causing less beef to be sold.
(c) Describe one potential negative effect on rivers and streams of grazing animals on pastureland. (1 point)
There are several possibilities here. Animal manure contains nutrients that would diminish water quality of the stream.
That might mean eutrophication or increase in pathogens such as E. coli which would make the water unfit for uses, such as drinking downstream.
Another possibility is that the animals may eat vegetation around the stream making soil erosion of the bank and flooding more likely.