Preferential flow in a long-term no-tillage experiment on a silt loam soil in Mediterranean conditions

Water flow throughout the soil allows and regulates life on the Earth's surface. Knowing where this flow mainly takes place (preferential flow) is critical i) to measure it appropriately, ii) to take advantage of it for a more efficient use of water. Soil management has great impact on soil hydrological properties and can have an effect at catchment scale, while knowing within plot variability can improve flow estimations at plot level. On a 22-year-old experiment comparing intensive (IT) and no-tillage (NT), soil hydrological properties were determined within (W-row) and between (B-row) crop rows several times along two cropping years (2018-19 and 2019-20) on undisturbed soil cores. Tillage significantly influenced soil water retention being higher under IT than NT in the wet range above -10 cm soil matric potential. The cause was a larger volume of mesopores (1000 to 300 µm in diameter) in IT. Despite that, hydraulic conductivity was significantly higher in NT in this range, especially because mesopores in NT revealed greater pore continuity than in IT. No differences in soil hydraulic conductivity were found at lower soil matric potentials. These results suggest that, although IT increases soil porosity creating new pores regularly, these pores are less interconnected than the long-standing pores created in NT by the roots and fauna activity. The lower hydraulic conductivity in IT can reduce infiltration and increase runoff losses resulting in less water available for crops.  The position with respect to the crop row (W-row or B-row), did not have an impact on soil water retention but on soil hydraulic conductivity, that was significantly higher under W-row than B-row above -10 cm H₂O soil water potential. Although the volume of pores of different size classes did not differ between both row positions, continuity of macropores (>1000 µm) was significantly higher under W-row than B-row and tended to be higher W-row also for the other pore classes. The effect of the sowing slot, the growth of the plant roots, and the protective effect of the plant cover itself can explain the preferential flow pathway found W-row. The differences between flow regimes under different tillage systems found at the small scale highlight the importance of considering the site-specific management impacts on soil structure and pore geometry, as these will affect hydrological flow processes at the catchment scale. Differences between positions with respect to the plant row need to be considered to properly characterize hydrological flow phenomena in soils, even under the same management practices.