Soil compaction: effects on soil hydraulic properties and preferential water flow

Sammanfattning: Soil compaction caused by passage of agricultural machinery over the surface is an issue in many agricultural soils with a high clay content. Compaction is known to modify soil pore structure and soil hydraulic properties, but can also affect the occurrence of preferential flow. Water flow through preferential flow pathways can facilitate transport of fertilisers, pesticides and contaminants to groundwater, creating harmful environmental problems. In two complementary studies, this thesis examined soil hydraulic properties and preferential water flow on soil cores sampled in the field and quantified flow patterns in situ at two sites approximately 300 m apart in central Sweden. One year after compaction of field plots, X-ray computed tomography (CT) imaging was used to visualise and quantify soil pore structures in soil columns taken from the subsoil (30-50 cm depth). Degree of preferential water flow and transport were derived from non-reactive tracer breakthrough curves. Dye tracing experiments were performed in the field and different soil mechanical and hydraulic properties were measured to help explain the dye patterns. Contrasting effects of wheel traffic were observed at the two neighbouring sites. Thus, even quite small differences in initial soil and site conditions can significantly influence the extent to which applied compaction stresses affect the connectivity of structural pore space in soil and, consequently, water flow patterns. These results contradict previous findings of an increase in preferential flow following soil compaction. This discrepancy in results was analysed with the help of a conceptual model, which suggested that preferential flow is greatest at some intermediate level of compaction at which macropore continuity is still maintained, despite reductions in macroporosity. The model also illustrated how compaction, and subsequent recovery from compaction, might affect susceptibility to preferential flow and surface runoff. To prevent subsoil compaction, it is important to consider soil conditions at the time of trafficking, which can significantly influence the effect of compaction on soil pore connectivity and associated water flow.

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