Impact of long-term sub-soiling tillage on soil porosity and soil physical properties in the soil profile

Long-term conventional tillage leads to soil compaction and formation of a plough layer, which affects soil physical properties, transport of water, and growth of crop roots toward deeper soil, resulting in soil degradation and a decline in crop productivity. Long-term sub-soiling tillage has proven to be an effective measure for remediating the plough layer and improving the structure and physical properties of the soil. In this study, we experimentally investigated the effects of long-term sub-soiling tillage in an arid region of Henan Province, China, along a deep soil (0–100 cm) profile over 8 years of sub-soiling tillage at a depth of 30 cm and compared the results against conventional tillage at a depth of 15 cm. We measured soil pore distributions of macropores (> 1 mm in diameter), mesopores (0.16–1.0 mm), and total pores (> 0.16 mm) measured by X-ray computed tomography (CT), soil total porosity (ϕ) and > 0.16 mm equivalent porosity measured by conventional methods, soil bulk density (ρ_s), soil organic carbon content (SOC), the proportion of macroaggregates (> 0.25 mm) (PMA), soil field moisture capacity (fc), available moisture content, and saturated hydraulic conductivity (K_sat). The results indicate that long-term sub-soiling tillage increased soil pore numbers at 0–35 cm depth (macropores, mesopores, and total pores), improved pore shape, and significantly increased porosity at 0–20 cm depth (macropores, mesopores, and total pores; p < 0.05) compared to conventional tillage. In addition, ϕ and > 0.16 mm equivalent porosity were increased by 10.4 and 87.1% at depths of 0–60 cm under sub-soiling tillage, respectively. SOC (0–55 cm depth), fc (0–45 cm depth), available moisture content (0–40 cm depth), K_sat (0–40 cm depth), and PMA (0–50 cm depth) were increased by 16.7, 14.3, 23.8, 471.5, and 98.3%, respectively, and ρ_s (0–60 cm depth) was reduced by 8.6%. Observed correlations between SOC, soil pore parameters, and soil physical properties suggest that soil pore parameters and soil physical properties can be improved by increasing SOC. These correlations are stronger under sub-soiling than that under conventional tillage. Therefore, sub-soiling is an effective technique for improving soil pore characteristics and physical properties while preventing soil degradation.