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The question was

Which of the following statements is supported by these data?

A. Phosphorus was the first limiting nutrient followed by nitrogen.

B. Nitrogen was the first limiting nutrient followed by phosphorus.

C. Phosphorus was limiting but nitrogen was not limiting.

D. Nitrogen was limiting but phosphorus was not limiting.

E. Neither nitrogen nor phosphorus was limiting.

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The answer was B: Nitrogen is the first limiting nutrient followed by phosphorus, but I do not know why. I thought it would be only phosphorus as the Nitrogen values are always greater than the control values.

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up vote 2 down vote accepted

The control graph shows the baseline against which we can compare the other graphs. It is the graph without any changes, and hence is the graph in which one of the nutrient is limiting.

In the graph for +P, there is no rise in biomass and the graph resembles the control. This shows that Phosphorous is not the first(initial) limiting nutrient, as its addition did not affect the biomass production.

The +N graph shows a significant rise in biomass, indicating that N was indeed the initial limiting nutrient. On its addition, the biomass production increased.

The +N and +P graph shows a higher biomass than +N and the control. This indicates, that while N was established to be the first(initial) limiting reagent, addition of Phosphorous also significantly improves the biomass yield, and hence P must also have become a limiting nutrient once N was added and its limiting effect removed. That is the reason why +N+P graph is higher than just +N graph. If phosphorous would not have been limiting even on addition of N, then the +N+P graph would have resembled +N graph.

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