Spatio-temporal variation of anisotropy of saturated hydraulic conductivity in a tilled sandy loam soil

Knowledge on anisotropy of saturated hydraulic conductivity can improve the understanding of transport phenomena in soil. We hypothesized that saturated hydraulic conductivity (K_s) in the upper part of the root zone of an agricultural sandy loam soil was anisotropic at different soil depths and times after tillage. K_s was measured on undisturbed 100 cm³ core samples taken in the horizontal and vertical directions in up to four soil layers (Surf: surface layer (0–5 cm); Top: topsoil (10–15 cm); Trans: transition layer between topsoil and subsoil; Sub: subsoil (40–60 cm)) 1, 8 and 32 months, respectively, after mouldboard ploughing and drilling. The ratio between estimated geometric mean values for K_s in the vertical and the horizontal directions (K_ms,v/K_ms,h) was used to test the hypotheses. A total of 669 soil samples were analysed.K_ms,v/K_ms,h varied with time after tillage and between soil layers. One month after ploughing, K_ms,v/K_ms,h was <0.23 (P = 0.975) in the Trans layer with an average value of 0.084, i.e. K_ms,h was 12 times larger than K_ms,v. Anisotropy could not be documented in this layer 8 or 32 months after ploughing, i.e. K_ms,v/K_ms,h was not significantly different from 1.0. For the Surf and Top layers 32 months after ploughing, K_ms,v/K_ms,h was in the intervals 1.4–50 and 3.1–77, respectively, (P = 0.95) with average values of 8.4 and 15, respectively. Thus, K_ms,v was 8.4 respectively 15 times larger than K_ms,h in the two layers. Anisotropy was not found in these layers 1 or 8 months after tillage. Strong anisotropy was found in the Sub layer with K_ms,v/K_ms,h averaging to 14 and 32, respectively, 8 and 32 months after tillage. K_ms,v and K_ms,h generally decreased with time in the Surf, Top and Trans layers, except in the vertical direction in the Top layer between 8 and 32 months after ploughing, and in the Trans layer between 1 and 8 months after ploughing. Overall, the geometric means of K_s varied between 10^−4.0 and 10^−7.1 m s⁻¹.The results may reflect systematic effects of soil settlement and drying/wetting phenomena coupled with biological activity and the existence of stable, vertically oriented biopores in the subsoil. It appears to be necessary to consider anisotropy of K_s and its variation in the analysis and modelling of water flow and chemical transport in agricultural soils, particularly to explain heterogeneous flow phenomena at the plot and field scales.