Final answer:
The soil of tropical rainforests is poor in nutrients because excessive rainfall d) leaches them away, leaving the soil less fertile than that of temperate forests.
Step-by-step explanation:
The soil of tropical rainforests is poor because most of the nutrients are d) leached away by excessive rainfall.
While tropical rainforests have high net primary productivity due to ideal temperature and precipitation, this productivity leads to a rich layer of decaying plant matter on the forest floor rather than in the soil itself.
In tropical climates, there's no winter dormancy that allows the buildup of nutrients in the soil like in temperate zones. Instead, the constant rainfall continuously leaches nutrients, preventing their accumulation within the soil profile.
In contrast, temperate forests have a thick layer of leaf litter that decomposes into humus, enriching the soil with organic matter.
The biodiversity and complex structure of tropical rainforests, including the forest floor and canopy layers, rely on the nutrients from the surface layer rather than the soil which is nutrient-poor due to leaching.