**Christian Brischke \* and Friedrich L. Wegener**

Wood Biology and Wood Products, University of Goettingen, Buesgenweg 4, D-37077 Goettingen, Germany; f.wegener@stud.uni-goettingen.de

**\*** Correspondence: christian.brischke@uni-goettingen.de; Tel.: +49-551-3929514

Received: 20 May 2019; Accepted: 30 May 2019; Published: 4 June 2019

**Abstract:** Terrestrial microcosms (TMCs) 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 setup is usually rather simple. However, for service life prediction of wood exposed in ground, it is of imminent interest to better understand the different parameters defining the boundary conditions in TMCs. This study focused, therefore, on soil–wood–moisture interactions. Terrestrial microcosms were prepared from the same compost substrate with varying water holding capacities (WHCs) and soil moisture contents (MCsoil). Wood specimens were exposed to 48 TMCs with varying WHCs and MCsoil. The 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 peat 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. The MCwood increased generally with rising MCsoil, but WHC was often negatively correlated with MCwood. The distance to water saturation 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 wood in soil contact.

**Keywords:** decay; ENV 807; soft rot test; soil moisture content; use class 4 (UC4)

## **1. Introduction**

For determining the durability of wood or the protective effectiveness of wood preservatives against soft rot fungi and other soil-inhabiting micro-organisms, terrestrial microcosms (TMCs) can be used. For this purpose, natural top soil or a fertile loam-based horticultural soil should be used and various requirements need to be fulfilled with respect to the soil substrate.

It is well known that many parameters affect the decay activity of soils [1–5]. Therefore, it is recommended to consider more than one in-ground field test site for durability testing of wood and more than one soil substrate for laboratory studies using TMCs [2,6–9].

Consequently, for standardized test protocols several parameters are more or less strictly defined. For instance, according to the European standard CEN/TS 15083-2 [10], the following soil-related boundary conditions need to be assured:


Finally, a moisture content of the soil (MCsoil) equivalent to 95% of its WHC is required and the TMC should be stored at 27 ◦C ± 2 ◦C and 70% ± 5% relative humidity (RH) during the whole period of exposure in a dark room.

A previous study by Wälchli [11] showed that MCwood decreased with both decreasing MCsoil and WHC as determined for two different soils and five different MCsoil. However, mass loss (ML) by decay of untreated and differently copper–chromium–boron (CCB)-treated Scots pine sapwood was neither correlated with MCwood nor with MCsoil. Similarly, Mieß [12] found an increase in MCwood with rising MCsoil in three different soil types and for different untreated and modified timbers. Furthermore, she found a gradient of MCwood in untreated wood from the highest MCwood in the bottom part and lowest MCsoil in the top part of the buried test stakes. In contrast, a remarkable 20% of the Scots pine sapwood specimens showed the highest MCwood in the top or central part of the specimens. Mieß [12] suggested that the MCwood gradients were the consequence of vertical gradients of MCsoil, which were differently severe due to the different soil wetting and re-drying regimes. It is further likely that the gradients were the consequence of ML gradients along the stake-shaped specimens, because the MCwood of the different specimen segments had been determined not before the end of the test after 17 weeks of incubation when significant ML had already occurred.

Gray [13] performed durability tests in TMCs using different soils at different MCsoil and found that the highest ML occurred at an MCsoil between 108% and 148% of the WHC of the respective soil. The highest MCwood after harvesting was found at an MCsoil between 120% and 218% of its WHC referring to an MCsoil at approximately 40% in all soil types used. Thus, ML increased with increasing MCsoil, but found an optimum, which was, however, far beyond the recommended 95% WHC. Again, MCwood data are needed to obtain a set perspective, since they refer to the different severely decayed specimens after harvesting.

In summary, it becomes evident that both WHC and MCsoil influence MCwood and ML through fungal decay, and do seemingly interact. Clear relationships between the three moisture-related parameters have not yet been established.

Others [6,12,14,15] previously demonstrated that all three rot types, i.e., brown, white, and soft rot, occur in TMCs complemented by tunneling, erosion, and cavity bacteria. However, neither MCwood nor MCsoil seemed to limit their occurrence. Solely, soft rot apparently copes better with very high moisture contents, which are not favorable for brown and white rot fungi. Nevertheless, soft rot fungi can degrade wood in a rather large moisture range. They are early colonizers, so-called "ruderal organisms" [16], which, in contrast to basidiomycetes ('combative organisms'), are rarely able to take over a substrate [17].

The WHC of soil substrates can vary remarkably, and therefore, it needs to be determined before each test. In both standards, CEN/TS 15083–2 [10] as well as ENV 807 [18], a suitable method for determining the WHC of soil is described: the so-called "droplet counting method". The method is based on determining the ability of a sample of a test substrate to retain water against the pull of a vacuum pump, as a measure of its WHC. However, the method is rather laborious and time consuming. Furthermore, the standards lack a definition of the vacuum that needs to be applied, wherefore one might question the reproducibility of the test results.

Within this study, we conducted comparative WHC measurements on a series of different mixtures of compost and silica sand using the "droplet counting method" and an alternative method according to ISO 11268-2 [19], where wet soil samples are allowed to drain on a sand bath. Based on this comparison of methods, TMCs should be prepared representing soil substrates of varying WHCs and MCsoil. The overall objective of this study was to establish relationships between WHC, MCsoil, and the resulting MCwood of different wood species after exposure in the TMC.

#### **2. Materials and Methods**

#### *2.1. Soil Substrates*

Three soil substrates were used to prepare TMCs of defined water holding capacities (WHCs). The basis substrate was a compost produced by the University of Goettingen from horticultural waste (i.e., leaf litter, grass, cut softwoods, and hardwoods, sand). To lower its WHC, silica sand (grain size > 0.2 mm) was added; to increase its WHC, peat (moderately-to-severely decomposed high-moor peat (H3–H8), total nitrogen 0.35%, magnesium 0.15%, organic substance 30%) was added. Both peat and compost were passed through a sieve of a nominal aperture size 8.5 mm. The soil moisture content (MCsoil) and the WHC were determined according to the "droplet counting method" and the "cylinder sand bath method".
