Question

human activity and nitrogen cycling
nitrogen is often a limiting nutrient for plant and algal growth (as is phosphorus). a limiting nutrient is the one that is in shortest supply and therefore limits growth.
application of nitrogen fertilizers at rates that exceed the rate at which plants can use it leads to accumulation of nitrogen in the soil. nitrates can then leach (move down through the soil) into groundwater or run - off into surface waters.
this extra nitrogen load is one of the causes of increased enrichment (eutrophication) of lakes and coastal waters so that algal blooms are often seen at the surface (right). an increase in algal growth can also increase decomposer activity, depleting oxygen and leading to the death of fish and other aquatic organisms (far right). anoxic bottom waters also cause the release of phosphorus from sediments, making enrichment worse.
the rate at which nitrates are added has increased faster than the rate at which nitrates are returned to the atmosphere. this has led to the widespread accumulation of nitrogen in the biosphere.
global changes in nitrogen inputs and outputs between 1860 and 1995 in million tonne
n₂ = unreactive nitrogen
n₂ = unreactive nitrogen
n₂o, noₓ
nh₃
n₂o, noₓ
nh₃
nitrates = reactive nitrogen
nitrates = reactive nitrogen
1860
1995

1. (a) how does excess nitrogen enter waterways?

(b) describe one important effect of nitrogen pollution:

1. using the quantitative models of nitrogen fluxes (1860 and 1995) above, calculate the increase in nitrogen deposition in the oceans from 1860 to 1995 and compare this to the increase in release of nitrogen from the oceans:
2. work in groups of 3 - 5 to develop a strategy to reduce the impact of human activity on the nitrogen cycle. support your group’s strategy as a viable solution by providing an evidence - based argument that shows your understanding of the processes involved in nitrogen cycling. write your strategy and its defense on a separate sheet and attach it to this page.

Explanation:

Step1: Answer question 10(a)

When nitrogen - based fertilizers are applied at rates that exceed the needs of plants, the excess nitrogen can move down through the soil into groundwater or run - off into surface waters. Also, the release of nitrogen from sediments due to increased decomposer activity can contribute to excess nitrogen in waterways.

Step2: Answer question 10(b)

One important effect of nitrogen pollution is eutrophication. Excess nitrogen in water bodies causes algal blooms. The rapid growth and subsequent decay of these algae deplete oxygen in the water, leading to the death of fish and other aquatic organisms.

Step3: Answer question 11

From the models:
In 1860, nitrogen deposition in the oceans is 27 million tonne and in 1995 it is 60 million tonne. The increase in nitrogen deposition in the oceans is \(60 - 27=33\) million tonne.
In 1860, nitrogen release from the oceans is 90 million tonne and in 1995 it is 121 million tonne. The increase in nitrogen release from the oceans is \(121 - 90 = 31\) million tonne.
The increase in nitrogen deposition in the oceans (\(33\) million tonne) is greater than the increase in nitrogen release from the oceans (\(31\) million tonne).

Answer:

10(a) Excess nitrogen enters waterways when nitrogen - based fertilizers are over - applied and move through soil into groundwater or runoff, and from the release of nitrogen from sediments due to increased decomposer activity.
10(b) Eutrophication: Algal blooms caused by excess nitrogen deplete oxygen in water, killing fish and other aquatic organisms.
11 The increase in nitrogen deposition in the oceans from 1860 to 1995 is 33 million tonne. The increase in nitrogen release from the oceans is 31 million tonne. The increase in deposition is greater than the increase in release.