Using X-ray computed tomography to assess changes in soil structure and pore space of Retisol upper horizons during compaction by logging equipment

This study investigates the alterations in pore space morphology resulting from modern harvesting activities. The explored plots are located in the middle taiga zone, Komi Republic, near the city of Syktyvkar. The pre-harvest ecosystem was characterized by a coniferous-deciduous forest and the original soils were identified as Albic Retisols. Logging was conducted in December when the top 10 cm of soil were frozen. Sampling was carried out in an undisturbed felling area and in the skidding trails subjected to three and ten forwarder passes. Additionally, a plot that underwent ten passes followed by rut levelling was examined. Soil micromonoliths from upper mineral horizons were collected from every treatment and analysed using X-ray computed tomography. The grayscale images obtained from the scans were segmented, and the resulting data were quantitatively analysed. The findings reveal sensitivity of total and especially air-filled porosity to the number of forwarder passes. Three passes resulted in the compression of air-filled pores without expressed destruction of the soil profile structure. Ten passes led to an unsignificant compaction comparing to the three-pass treatment, but caused mixing of the upper mineral horizons and the forest litter. Therefore, the pore space geometry underwent substantial modification. The soil structure at this treatment can be described as massive, characterized by numerous newly formed small closed pores replacing larger horizontally oriented ones. Levelling of skidding trails facilitated soil loosening which was evidenced by the analysis of greyscale and pore size distribution images and calculated data.