Terrestrial microcosms (TMC) are frequently used for testing the durability of wood and wood-based materials as well as the protective effectiveness of wood preservatives. In contrary to experiments in soil ecology sciences, the experimental set-up is usually rather simple. However, for service life prediction of wood exposed in ground, it is of immanent interest to better understand the different parameters defining the boundary conditions in TMC. This study focussed therefore on the soil-wood-moisture interactions and their potential effect on durability testing in TMC. TMC were prepared from the same compost substrate with varying water holding capacity (WHC) and soil moisture content (MCsoil). Wood specimens were made from English oak, Beech, Douglas fir, and Scots pine sapwood and exposed to in total 48 different TMC and wood moisture content (MCwood) was studied as well as its distribution within the specimens. For this purpose the compost substrate was mixed with sand and turf and its WHC was determined using two methods in comparison, i.e. the ‘Droplet counting method’ and the ‘Cylinder sand bath method’ in which the latter turned out advantageous over the other. MCwood increased generally with rising MCsoil, but WHC was often negatively correlated with MCwood. Instead, the degree of water saturation Ssoil could serve as a more predictive measure for MCwood in soil exposure scenarios. With increasing Ssoil the MCwood increased, but followed wood species-specific curves with differently steep increase and a plateau at Ssoil = 0 %. In addition, Ssoil from which MCwood increased most intensively was found to be wood-species specific and might therefore require further consideration in soil-bed durability testing and service life modelling of wooden components in soil contact.

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