What contributes to subsurface compaction?

Subsurface compaction is primarily influenced by the weight and pressure exerted on the soil, which is most directly related to axle load. When heavy machinery or vehicles pass over the soil, the weight transferred through the axles compresses the soil particles together. This compaction can lead to reduced porosity and permeability of the soil layers below the surface, affecting root growth and water movement.

The other options, while they may impact soil structure and health, do not contribute to subsurface compaction in the same direct manner. Pest activity might disrupt the soil in certain ways, but it does not apply the same type of pressure that causes compaction. Surface vegetation can help protect against compaction by providing cover and reducing the impact of rainfall on the soil surface. Organic matter content is crucial for soil health and structure, promoting aeration and water retention, but it does not inherently cause compaction. Thus, axle load is the primary factor in the context of this question.