Topsoil Compaction Risk Based on the Different Responses of Soil Structure to Compaction Stress

Compaction leads to reduced crop yields, as the soil structure is destroyed. As soil structures respond differently to different degrees of compaction stress, early warnings for the risk of soil compaction caused by agricultural machinery need to be provided based on changes to the soil structure. In this study, we quantified the changes in the aeration porosity, aggregate mean weight diameter, structure coefficient, and cone index of different soil layers in response to compaction stress under different tyre axle weights (7.0 kN, 11.5 kN, 15.8 kN, and 20.4 kN) to analyse the risk of soil compaction in the topsoil layer (0–25 cm). The results showed that the compaction stresses that led to significant changes in soil structure in the 0–5 cm, 5–10 cm, 10–15 cm, and 15–20 cm soil layers were 130 kPa, 156 kPa, 111 kPa, and 103 kPa, respectively, and were significantly greater than the precompression stress of the soil in each layer. This finding proves that the changes in soil volume and structure caused by compaction are significant but not exactly equivalent; moreover, a threshold past which the stress caused by compaction results in soil structure failure still exists. Under 180 kPa of surface contact stress, the soil cone index, aeration porosity, aggregate mean weight diameters, and structure coefficient of the 0–5 cm and 5–10 cm soil layers gradually moved closer to the soil parameter levels of the subsoil layer before compaction. We suggest that the response of the soil structure to compaction stress proceeds along three stages, elastic deformation, plastic deformation without structure failure, and soil structure failure, within which soil structure failure stress and precompression stress are the two key threshold stresses. This study provides a more reliable theoretical basis upon which field managers can warn of soil compaction risk.