Which of the following is an effect of forest fires on soil?

When considering the effects of forest fires on soil, one significant impact is that they can cause some soils to repel water. This phenomenon occurs due to the formation of a hydrophobic layer, which can happen when organic materials, such as plant residues, are heated and burned. As a result of intense heat, properties of the soil, including the presence of certain compounds, can change to create this water-repellent layer.

While other options might seem plausible, they do not hold true as universally as the correct option. For instance, forest fires can reduce soil moisture rather than increase it, as the intense heat drives moisture away and can create conditions that prevent water from properly infiltrating the soil. Although forest fires can temporarily release some nutrients into the soil, the overall effect often includes the loss of organic matter, which is essential for healthy nutrient levels. Finally, while fires may encourage new vegetation to grow after an initial recovery period, this is not a direct effect on the soil itself, but rather a consequence of ecological succession following the disturbance caused by the fire.

Thus, the formation of a water-repellent layer in some soils as a response to fire is a clear and direct effect that stands out in the context of soil science.