What is the primary concern regarding phosphorus movement during heavy rainfall?

The primary concern regarding phosphorus movement during heavy rainfall is the loss of phosphorus through runoff. When heavy rainfall occurs, the excess water can lead to surface runoff, which is a process where water flows over the soil surface instead of soaking into the ground. This runoff can carry away not only water but also nutrients, including phosphorus, which can accumulate in the top layers of soil.

Phosphorus is critical for plant growth, but its movement away from the intended area can lead to several environmental issues. For instance, when phosphorus enters waterways from runoff, it can cause eutrophication, a situation where nutrient overload leads to excessive algae growth. This can deplete oxygen levels in the water, harming aquatic life.

The other options, while related to soil and nutrient management, do not address the primary concern of phosphorus loss during heavy rainfall. For example, increased soil structure stability and enhanced soil fertility are generally beneficial aspects, and oxygen depletion in soil can be a consequence of other factors, but they do not directly correspond to the immediate concern of phosphorus runoff during heavy rain events.