Soil aeration and soil water tension in skidding trails during three years after trafficking

Abstract

Parameters of soil aeration and of soil water tension were measured for three years in skid trails of a 34 year old beech forest from natural regeneration which received its first thinning. The investigation took place in the Solling (Germany, Lower Saxony) at 400m a.s.l. where cambisols have developed from silty Pleistocene deposits on Triassic sandstone. During thinning skid trails were laid out with a harvester, followed by a forwarder. The following measurements were made (i) in the undisturbed soil, (ii) in the wheel track, (iii) in the middle line between the wheel tracks: Continuous monitoring of water tension in 6–10cm soil depth and soil air CO2-concentration in 6 cm soil depth. Iron rods in the soil (down to 27cm) were taken as indicators for soil aeration (redox indication) and were exposed for four weeks in late summer every year.Research questions were: How does the soil air CO2-concentration and soil water tension change in time? How is soil air CO2-concentration related to soil water tension and to soil temperature? What is the course of CO2-concentration and soil water tension in the skid trail middle lane compared to undisturbed soil and wheel-track soil? Can iron rods reflect the soil aeration difference between trafficked and undisturbed soil?CO2 measurement, monitoring of soil water tension, and redox indication with iron rods showed that driving with harvester and forwarder not only affected the wheel tracks but also the unpassed middle lane of the skidding trails. Decrease of CO2-concentration in the soil air indicated an initial regeneration of air diffusivity in the first 6cm of the impacted soil within the first three years after trafficking. Iron rods had significantly different frequencies of reducing conditions in the order wheel tracks > middle lane between wheel tracks > undisturbed soil. Iron rods indicated no recovery of soil aeration in depth 12–24cm during the three years of observation. The soil water tension reflected the transpirational water extraction by trees in the undisturbed soil in the course of the vegetation period (from spring to summer/autumn). In the skid trail the water tension indicated a more water filled porosity than in the undisturbed soil. The generally weaker water tension in the skid trails indicated, that also the middle lane between the wheel tracks was separated from the transpirational flow of soil water to the trees. CO2-concentration in soil air at 6cm depth in summer was more related to soil water tension than to soil temperature.