Effect of Subsoil Compaction On Hydraulic Parameters.

Soil compaction is a major threat to sustainable soil quality and is increasing since agricultural machinery is becoming heavier and is used more intensively. Compaction not only reduces pore volume, but also modifies the pore connectivity. The inter-Nordic research project POSEIDON (Persistent effects of subsoil compaction on soil ecological services and functions) put forward the hypothesis that due to a decrease in the hydraulic conductivity in the soil matrix, compaction increases the frequency of preferential flow events in macropores and therefore increases the leaching of otherwise relatively immobile agrochemicals. In a morainic clay soil, undisturbed soil cores (6280 cm³) were sampled at 20-40 and 60-80 cm depth in the spring 2009 fourteen years after operation with a heavy sugar beet harvester. Soil cores were sampled both from uncompacted reference blocks and from compacted blocks. In the field the near-saturated hydraulic conductivity was measured with a tension infiltrometer in the same treatments at a depth of 30 cm. In the laboratory saturated and near-saturated hydraulic conductivity and the bulk density were measured as well. Also, macropores in the large soil cores were made visible by CT scans performed at a water content at field capacity. Prior to the soil sampling campaign, measurements of mechanical strength in terms of cone penetration resistance gave a clear indication of lasting compaction effects below the plough layer. Measurements on the soil columns showed that for the upper soil depth, a significant increase in bulk density was measured for the compacted treatment. For the lower depth differences were less pronounced. For the saturated hydraulic conductivity, the results indicated a decrease of the hydraulic conductivity for the compacted treatment for the upper depth whereas the near-saturated hydraulic conductivity did not show any significant differences between treatments at the two depths due to a high variability between measurements. Results from the measurements with the tension infiltrometer were in accordance with the hydraulic conductivity measured in the laboratory showing no difference in the hydraulic conductivity at low matric potentials, but with a tendency for larger values of the conductivity in the reference blocks at matric potentials close to saturation. Results from the CT scans were in accordance with the measurements of bulk density showing a significant trend of reduced macroporosity for the compacted upper depth. We conclude that the measured changes in the analyzed transport parameters support our hypothesis that the colloid-facilitated transport of agrochemicals in spatially connected macropores leads to a higher risk of contamination of water bodies under compacted agricultural soils.