Factors governing soil water repellency under tillage management: the role of pore structure and hydrophobic substances

Soil water repellency (SWR) has significant consequences for crop yield, carbon sequestration, aggregate stability, soil erosion, and water movement. It is known to be linked to hydrophobic substances and pore structure. Conservation agriculture could affect SWR through both aspects. However, most of the studies have only focused on hydrophobic substances due to the complexity of soil pore structure measurement and quantification. In this study, X-ray computed tomography at a resolution of 27.27 μm was used to calculate the shape, porosity, and connectivity of the pore network and reveal the impact of hydrophobic substances and pore structure on SWR. All samples were collected from two long-term experimental fields. The treatments were conventional tillage with residue removal (CT), reduced tillage with residue incorporated (RT), and no-tillage with residue mulch (NT) in both of the fields. The water repellency index was determined using the intrinsic sorptivity method by measuring the water and ethanol sorptivity. The results showed that RT and NT treatment increased the porosity of pores of 55-165 μm in diameter that had a positive relationship with ethanol sorptivity and water repellency index, respectively. However, the total porosity and the porosity of >165 μm in diameter had no significant link with SWR properties. RT and NT treatments could enhance ethanol sorptivity by increasing pore connectivity. However, pore connectivity had no effect on water sorptivity because of the hydrophobic substances. NT treatment also reduced water sorptivity by increasing pore surface area and hydrophobic substances. Soil organic carbon and microbial biomass carbon, both of them as hydrophobic substances, were higher under RT and NT treatment than CT. Microbial biomass carbon was more positively correlated to SWR than soil organic carbon, which indicates that microbial biomass carbon is a better indicator explaining tillage effects on SWR. Overall, RT and NT treatment could increase the water repellency index, which was a result of the interactions between pore structure and hydrophobic substances. In order to unravel the mechanisms underlying conservation tillage impacts on SWR more accurately, it is essential to determine both pore structure and hydrophobic substances at the same time.