Question

Soils undergo weathering over large time-scales (millions of years). Over time, concentration of phosphorus (P) declines monotonically (solid line), while nitrogen (N) shows a unimodal pattern (dotted line) as shown. 

Based on this pattern, which one of the following diagrams represents the likely trend in N:P ratio? 

• A. (i)
• B. (ii)
• C. (iii)
• D. (iv)

Hint:

In ecosystems, nutrient ratios like N:P reflect the availability and cycling of these nutrients. A unimodal pattern for N suggests a peak in availability before declining.

Answer 1

The Correct Option is A

As soils weather over time, phosphorus (P) concentration declines steadily (monotonically), while nitrogen (N) follows a unimodal pattern. The key observation here is that nitrogen increases, reaches a peak, and then decreases, while phosphorus declines continuously. This suggests that the N:P ratio will initially rise as nitrogen increases relative to phosphorus in the younger soils, then reach a peak, and finally decrease as both nitrogen and phosphorus concentrations decline.
[6pt] Graph (i) shows a similar trend where the N:P ratio increases initially (as N rises relative to P) and then decreases over time, which aligns with the observed patterns of nitrogen and phosphorus in the soils.
[6pt] - Graph (ii) shows a constant N:P ratio, which does not fit the expected trend.
- Graph (iii) and (iv) show linear increases and decreases, which do not align with the unimodal pattern of nitrogen.
Thus, the correct diagram is (i), showing the expected changes in N:P ratio over the soil's age.