Legacy effects of subsoil compaction and earthworm biopores on soil hydrophysical properties and root density in a temperate silt loam soil

The short- and medium-term effects of agricultural soil compaction are well documented, but its long-term impacts and the role of biopores in subsoil recovery remain poorly understood. This study evaluated legacy effects of subsoil compaction on soil hydrophysical properties and root density and assessed the contribution of earthworm biopores to soil recovery. The compaction experiment was established in 1995 on a silt loam Haplic Luvisol by applying six passes of a wheel loader with a maximum wheel load of 5 Mg. Soil measurements were conducted immediately after compaction (1995) and repeated in 2019 and 2023. Measurements in 1995 included soil penetration resistance (PR), bulk density (BD), air-filled porosity ( ɛ ₐ), and relative gas diffusivity (D s /D 0 ). Winter wheat root density was assessed in 2019. In 2023, PR, BD, ɛₐ, D s /D 0 , saturated hydraulic conductivity ( K s ), earthworm abundance, and subsoil structural quality (Ssq) were measured. Intact soil cores (30–35 cm) collected in 2023 were grouped according to the presence or absence of visible earthworm biopores. Compaction effects persisted after 28 years. Compared with the control, compacted soil had higher BD (+7.4%), PR (+78% at 20–40 cm), Ssq (+133%), and lower ɛ ₐ (−31.7%) and D s /D 0 (−33.1%). Root density, K s , and earthworm abundance were statistically identical between treatments. These hydrophysical differences persisted strongly in samples without biopores, but largely disappeared in biopore-containing samples. Comparison with 1995 data indicated partial recovery of aeration and gas transport, especially in biopore-rich samples. These results show that subsoil compaction can persist for decades, but recovery of pore connectivity is strongly mediated by earthworm biopores.