Gas transport and subsoil pore characteristics: Anisotropy and long-term effects of compaction

Abstract

Anisotropy of soil pore functions significantly affects the transport of gas and water in soil. This paper quantifies anisotropy of subsoil pores and investigates the long-term impact of soil compaction by agricultural machinery. Two long-term field experiments on soil compaction formed the basis for the investigation, one established in 1981 on a clay soil in Finland (60°49′N, 23°23′E) and another in 1995 on a sandy clay loam in Sweden (55°49′N, 13°11′E). In 2009/2010, soil cores were sampled in vertical and horizontal directions from 0.3, 0.5, 0.7 and 0.9m depth (the two lower depths only in Sweden). In the laboratory, water content, air-filled porosity (εa), air permeability (ka) and gas diffusivity (Ds/D0) were determined at selected matric potentials. 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 −100hPa matric potential. Blocked air-filled porosity (εb) and a pore continuity index (N) were estimated from the relationship between ka and εa for a range of matric potentials. A factor of anisotropy (FA) was determined as the ratio of a given property measured in the horizontal direction to that in the vertical direction. ka showed anisotropic behaviour (FA<1) for the clay soil and for the 0.3m depth of the non-compacted sandy clay loam soil, while Ds/D0 displayed anisotropy for the clay soil (FA<1). In the sandy clay loam soil, dB and nB displayed significant anisotropy (FA<1) except at 0.9m. We interpreted this as effects of biological activities and physical processes in the B-horizon not being active in the C-horizon (0.9m depth). Compaction generally reduced ka, Ds/D0, dB, nB and increased τ for both sampling directions. Compaction had an effect on anisotropy for soil drained to −100hPa, but only for ka and dB in the sandy clay loam at 0.3m depth. Compaction reduced anisotropy for the N parameter, i.e. effects on soil pore continuity at the macropore scale, while it increased the anisotropy for εb. Our data thus indicate that compaction had persistent effect on soil physical properties and also affects anisotropy, especially that of macropores.