1 Department of Agroecology - Soil Physics and Hydropedology, Department of Agroecology, Science and Technology, Aarhus University2 Agroscope Research Station ART, Department of Natural Resources & Agriculture3 University of Helsinki, Department of Food and Environmental Sciences4 University of Helsinki, Department of Agricultural Sciences5 Swedish University of Agricultural Sciences, Department of Crop Production Ecology6 Department of Agroecology - Soil Physics and Hydropedology, Department of Agroecology, Science and Technology, Aarhus University
Anisotropy and long-term effects of compaction
Arrangements of elementary soil particles during soil deposition and subsequent biological and physical processes in long-term pedogenesis are expected to lead to anisotropy of the non-tilled subsoil pore system. Soil compaction by agricultural machinery is known to affect soil pore characteristics, but few studies have addressed the effect on soil pore anisotropy. This study was conducted within two long-term field experiments on soil compaction, established in 1981 on a clay soil in Finland (60º49N, 23º23'E) and in 1995 on a sandy clay loam in Sweden (55º49'N, 13º11'E). In 2009/2010, soil cores were sampled in vertical and horizontal directions from 0.3, 0.5, 0.7 and 0.9 m depth (the two lower depths only in Sweden). In the laboratory, water retention, air permeability (ka) and gas diffusivity (Ds/D0) were determined. For the sandy clay loam, morphological characteristics of pores (effective pore diameter, dB; tortuosity, τ; the number of effective pores per unit area, nB) were calculated using a tortuous tube model at -100 hPa matric potential and blocked air-filled porosity (εb) and pore continuity index (N) were estimated from the relationship between ka and air-filled porosity (εa) for a range of matric potentials. Factor of anisotropy (FA) was determined as the ratio of a given property measured in the horizontal direction to that in the vertical direction. For both soils, ka was anisotropic at some depths (FA<1), while Ds/D0 displayed anisotropy only for the clay soil (FA<1). In the sandy clay loam soil, dB and nB displayed significant anisotropy (FA<1) except at 0.9 m. We interpreted this as a minor effect of biological activities and physical processes in the C-horizon (0.9 m depth), which is likely to exhibit intrinsic isotropic pore characteristics because of its origin (glacial till soil). Compaction generally reduced ka, Ds/D0 and pore morphology indices for both sampling directions. Compaction had an effect on anisotropy of ka and dB only in the sandy clay loam at 0.3 m depth, where it reduced the anisotropy of the pore system due to lower reduction in these parameters in the horizontal than in the vertical direction. Compaction reduced anisotropy for the N parameter, indicating soil pore continuity at macropore scale, while it increased the anisotropy for εb. Our data thus indicate that soil compaction affects anisotropy, especially that of macropores.