*3.2. Impact of WHC and MCsoil on the Moisture Content of Wood (MCwood) Exposed in TMCs*

After three weeks of exposure in different TMCs, the average MCwood was highest in Scots pine sapwood (88%), followed by English oak (75%), Beech (67%), and Douglas fir (48%). In general, MCwood increased with increasing MCsoil, but was strongly dependent on the WHC of the soil. The lower the WHC, the higher was the MCwood at a given MCsoil, which coincided with previous findings [12]. Lower WHC in this study corresponded with a higher percentages of silica sand, which can only physically absorb water in contrast to organic soil substrates such as compost soil and peat, which also form chemical bonds with water [20]. The capacity to bind water is therefore higher in organic substrates which restricts the amount of available water which potentially wets the wood specimens.

This effect became especially prominent when considering MCsoil as a percentage of the WHC of the soil as illustrated in Figure 4. The lower MCsoil (%WHC), the lower the MCwood was at a given WHC of the substrate in the TMCs. However, it also became apparent that the effect of MCsoil became more pronounced at higher WHCs, i.e., the range of MCwood between 30% and 120% MCsoil expressed as percentage of its WHC was higher by up 2 factors in substrates of high WHC (120%) compared to those with very low WHC (30%).

**Figure 4.** The interrelationship between wood moisture content (MCwood) and WHC for different MCsoil expressed as a percentage of the WHC of the TMC (regression functions are shown in Table 5).


**Table 5.** Regression functions for fitting curves shown in Figure 5 (*y* = wood moisture content MCwood; *x* = water holding capacity WHC).

The difference between MCwood results achieved after three weeks of in-soil exposure was surprisingly small between Beech and English oak heartwood, because the latter is known to take up water more slowly due to the formation of tyloses in the vessels. In contrast, Beech wood—apart from false heartwood which was excluded in this study— usually takes up liquid water very easy, although its vessel diameters are much smaller compared to the early wood vessels of English oak. Similarly, the maximum MCwood of Douglas fir heartwood was in the same range of that of Scots pine sapwood when exposed in soil at an MCsoil of 120% WHC. Solely, at a lower MCsoil, the more permeable Scots pine sapwood showed higher MCwood compared to the refractory heartwood of Douglas fir. In summary, it became evident that already after a short exposure period of three weeks in wet soil, wood anatomy-induced differences in moisture uptake diminished confirming previous findings [12].

To further illustrate the interdependency between WHC and MCsoil and their effect on MCwood, the distance to water saturation of the soil substrate (*Ssoil*) was determined according to Equation (7) and correlated with MCwood (Figure 5).

$$S\_{\rm soll} = \text{MC}\_{\rm soll} - \text{WHC} \left( \%-\text{points} \right) \tag{7}$$

where *Ssoil* is the distance to water saturation of the soil substrate, in %-points; MCsoil is the soil moisture content, in %; *WHC* is the water holding capacity, in %.

Generally, with increasing *Ssoil*, the MCwood increased as well, but followed wood species-specific curves with differently steep increases and a plateau at *Ssoil* = 0%, i.e., soil water saturation. For English oak, Beech, and Douglas fir, the MCwood stayed approximately constant at *Ssoil* > 0%. Solely, for Scots pine sapwood, MCwood dropped significantly after exceeding the saturation point, although unlimited uptake of liquid water was expected to be provided above this threshold.

The distance to water saturation from which MCwood remarkably rose differed also between wood species and was approximately at −55%-points for English oak, −25%-points for Beech and Douglas fir, and −20%-points for Scots pine sapwood. Scots pine sapwood showed also by far the highest increase in MCwood with increasing *Ssoil*, i.e., between 32% and up to 180% MCwood between −20 and 0% *Ssoil*.

**Figure 5.** The interrelationship between the distance to water saturation of the soil substrates (Ssoil) and the wood moisture content (MCwood).

#### *3.3. Moisture Content Gradients in Buried Wood Specimens*

The MCwood in specimens buried to 4/5 of their length in TMCs showed partly drastic gradients from high moisture content in the bottom to less in the upper part, which was not buried (Figures 6–9). Solely, Scots pine sapwood specimens exposed at high MCsoil (95%WHC) showed barely significant gradients, but very high MCwood in all parts of the specimen. Similarly, deviating MCwood gradients were reported by Mieß [12] for Scots pine sapwood specimens. As expected, generally, the highest difference in MCwood was found between the upper segments and upper next segments.

The MCwood in the upper segments of English oak and Douglas fir specimens was in the range of their equilibrium moisture content (EMC) at fiber saturation. In contrast, the upper segments of Scots pine sapwood and Beech specimens showed MCwood up to 180 and 80% respectively, indicating strong capillary water transport along the specimen axis.

The MCwood gradient in the specimens was positively correlated with MCsoil (%WHC). However, two types of MC gradients became apparent: (1) increasing MCwood from the upper to the next segment, but rather constant MCwood below (e.g., English oak at WHC = 30%), and (2) a steady increase of MCwood from the upper to the bottom segment (e.g., Beech and Scots pine sapwood).

**Figure 6.** Distribution of MCwood in English oak specimens buried to 4/5 of their length (20–100 mm) in different TMCs. Different letters indicating significant differences between groups at *p* < 5% according to a Student *t*-test for non-paired samples.

**Figure 7.** Distribution of MCwood in Beech specimens buried to 4/5 of their length (20–100 mm) in different TMCs. Different letters indicating significant differences between groups at *p* < 5% according to a Student *t*-test for non-paired samples.

**Figure 8.** Distribution of MCwood in Douglas fir specimens buried to 4/5 of their length (20–100 mm) in different TMCs. Different letters indicating significant differences between groups at *p* < 5% according to a Student *t*-test for non-paired samples.

**Figure 9.** Distribution of MCwood in Scots pine sapwood specimens buried to 4/5 of their length (20–100 mm) in different TMCs. Different letters indicating significant differences between groups at *p* < 5% according to a Student *t*-test for non-paired samples.

#### **4. Conclusions**

The findings from this laboratory study on the soil–wood–moisture interactions in terrestrial microcosms led us to the following conclusions:

• The more advantageous "Cylinder sand bath method" should consequently be seen as an adequate alternative for the "Droplet counting method", which turned out disadvantageous regarding practical applicability, reproducibility, and reliability.


Based on the findings from this study, further experiments have been initiated to examine the effect of soil–wood–moisture interactions on fungal decay, in particular soft rot decay. In addition, the outstanding role of soil and wood temperature on decay in constantly wet wood will be further investigated.

**Author Contributions:** F.W. together with C.B. were mainly responsible for the conceptualization, methodology used, data evaluation, data validation, and formal analysis. Investigations and data curation were conducted by F.W. The original draft of this article was prepared by C.B. who was also responsible for the review and editing process of this article. C.B. and F.W. oversaw the visualization.

**Funding:** Parts of the research were funded in the frame of the research project 'CEMWOGEO—Cement coating of wood for geotechnical applications'—Funding Program for Renewable Resources—by the German Federal Ministry of Food and Agriculture. Fachagentur Nachwachsende Rohstoffe e.V. (FNR)—Project reference number: 22007617.

**Acknowledgments:** The authors gratefully acknowledge support with the TMC experiments from Brendan Marais.

**Conflicts of Interest:** The authors declare no conflict of interest.
