Field-scale Assessments of Subsoil Compaction and Soil Structural Functions in Lower Saxony, Germany

Subsoil compaction a one of the major threats for crop production and soil ecological functioning in Europe. To quantify its field-scale extent, field investigations were conducted on 26 agricultural fields across Lower Saxony state in northern Germany, covering four dominant soil textures (clay loam, sandy loam, silt, and silty loam) and two field management zones (machinery turning area and main field area). In-situ measurements of penetration resistance and shear strength were conducted on field at 10 cm (topsoil) and 40 cm (subsoil), and bulk density (ρ_b), saturated hydraulic conductivity (Ks), air permeability (Ka) were measured on undisturbed soil samples at the same depth. The results showed that most machinery turning zones showed clear signs of subsoil compaction. About 80% fields showed 5~60% higher penetration resistance and shear resistance in machinery turning area. For sandy loam soil in the east and north part of Lower Saxony, penetration resistance frequently exceeded 3 MPa, reaching >5 MPa at 40 cm, which is above root limiting thresholds. Compared to main field zone, bulk density in machinery turning area increased by 2-5%, with silty loam field exhibiting the largest increase of bulk density. Additionally, soil pore functions of Ka and Ks in machinery turning area exhibited obvious decline compared to field area especially in the subsoil of silty loam and sandy loam fields. For both machinery turning area and field area, a significant correlation was found between hydraulic conductivity and soil bulk density (R²=0.21, p=0.023) in topsoil with ρ_b from 1.25~1.60 g cm^-3, while in subsoil with ρ_b from 1.45~1.65 g cm^-3 no such correlation was found, indicating the pore functions in subsoil mainly depends on connective pores during the structure formation process. These results demonstrate that traffic-induced subsoil compaction is widely altering soil physical structure and pore functions in Lower Saxony, and the texture-dependent responses highlight the need for specific compaction mitigation strategies during the future field managements.