Anisotropy of unsaturated soil hydraulic properties of eroded Luvisol after conversion to hayfield comparing alfalfa and grass plots

The ploughing-induced compaction of the interface between topsoil and subsoil negatively affects the connectivity and continuity of the complex pore system through plough pans as artificial boundary resulting in water-logged conditions. The conversion of arable land into hayfield is an opportunity for breaking up plough pans and recovering pore networks in the long-term. The basic idea of the current study was to investigate the potential pore structure recovery effect by growing either deep-rooting alfalfa or shallow-rooting grass on former conventionally-tilled cropland. The alfalfa and grass plots located in moraine region in Northeast Germany on erosion-affected truncated Albic Luvisol, were sampled nine years after the conversion to hayfield. Undisturbed soil cores (300 cm3) were extracted in vertical (0°) and horizontal directions (90°) from the boundary between Ap and Al-Bt- horizons (about 25−35 cm soil depth). The soil water retention and the hydraulic conductivity under variable-saturated conditions were determined by using a combination of methods including suction plate, pressure chamber, double membrane through-flow, and evaporation experiments. The anisotropy ratio, AR, (horizontal versus vertical) of the unsaturated hydraulic conductivity function, K(Se), was less pronounced for the soil at the alfalfa as compared to that of the grass plot, especially for wide coarse pores (wCP: >−60 hPa). The AR-ratio was differently depending on the pore size for soil from grass and alfalfa plot, and ranged between AR-values of 1 and 7. The pore size class-related matric flow potential, ϕ, derived from K(Se) function was used for comparing the volume fraction of pore size classes with a pore size-related fraction of the capillary potential. The volume fraction of the wide coarse pores was negatively related with ϕ (r2: 0.91). The smaller AR-values for the soil of the alfalfa plot suggest a tendency for better changing the platy structure into a prismatic-platy structure through the more intense rooting in the vertical direction than in the soil of the grass plot.