7.1 Human influences and industry

Human influences on the nitrogen cycle

Humans have contributed significantly to the nitrogen cycle in a number of ways.

• Atmospheric pollution is another problem. The main culprits are nitrous oxide (N ₂ O), nitric oxide (NO) and nitrogen dioxide (NO ₂ ). Most of these gases result either from emissions from agricultural soils (and particularly artificial fertilisers), or from the combustion of fossil fuels in industry or motor vehicles. The combustion (burning) of nitrogen-bearing fuels such as coal and oil releases this nitrogen as NO ₂ or NO gases. Both NO ₂ and NO can combine with water droplets in the atmosphere to form acid rain . Furthermore, both NO and NO ₂ contribute to the depletion of the ozone layer and some are greenhouse gases . In high concentrations, these gases can contribute towards global warming .
• Both artificial fertilisation and the planting of nitrogen fixing crops , increase the amount of nitrogen in the soil. In some ways this has positive effects because it increases the fertility of the soil and means that agricultural productivity is high. On the other hand, however, if there is too much nitrogen in the soil, it can run off into nearby water courses such as rivers or can become part of the groundwater supply as we mentioned earlier. Increased nitrogen in rivers and dams can lead to a problem called eutrophication . Eutrophication is the contamination of a water system with excess nurtrients, which stimulates excessive algae growth at the expense of other parts of the ecosystem. This occurs as eutrophication reduces oxygen levels in the water. Sometimes this can cause certain plant species to be favoured over the others and one species may 'take over' the ecosystem, resulting in a decrease in plant diversity. This is called a 'bloom'. Eutrophication also affects water quality. When the plants die and decompose, large amounts of oxygen are used up and this can cause other animals in the water to die.

Case study : fertiliser use in south africa

Refer to the data table below, which shows the average fertiliser use (in kilograms per hectare or kg/ha) over a number of years for South Africa and the world. Then answer the questions that follow:

1. On the same set of axes, draw two line graphs to show how fertiliser use has changed in SA and the world between 1965 and 2002.
2. Describe the trend you see for...
   1. the world
   2. South Africa
3. Suggest a reason why the world's fertiliser use has changed in this way over time.
4. Do you see the same pattern for South Africa?
5. Try to suggest a reason for the differences you see in the fertiliser use data for South Africa.
6. One of the problems with increased fertiliser use is that there is a greater chance of nutrient runoff into rivers and dams and therefore a greater danger of eutrophication. In groups of 5-6, discuss the following questions:
   1. What could farmers do to try to reduce the risk of nutrient runoff from fields into water systems? Try to think of at least 3 different strategies that they could use.
   2. Imagine you are going to give a presentation on eutrophication to a group of farmers who know nothing about it. How will you educate them about the dangers? How will you convince them that it is in their interests to change their farming practices? Present your ideas to the class.