*2.2. Determination of the soil Moisture Content (MCsoil)*

Soil samples of 7–64 g (depending on the soil density) were taken for determining the soil moisture content (MCsoil). Three replicate samples were taken, weighed to the nearest 0.01 g, oven-dried at 103 ◦C, and weighed again. MCsoil was calculated as follows:

$$\text{MC}\_{\text{soil}} = \frac{\mathbf{m}\_{\text{soil, wet}} - \mathbf{m}\_{\text{soil,0}}}{\mathbf{m}\_{\text{soil,0}}} \times 100 \,\left[ \, \% \right] \tag{1}$$

where MCsoil is the soil moisture content, in %; msoil,wet is the wet soil mass, in g; msoil,0 is the oven-dry soil mass, in g.

#### *2.3. Determination of the Water Holding Capacity (WHC) of Soil*

#### 2.3.1. "Droplet Counting Method"

A small quantity of water was added to soil samples of 200 g, the substrate was mixed well, and the operation was repeated until the soil particles stuck to another (crumb structure). Further, 25 mL water were added, mixed well, and allowed to stand for 2 h. A coarse filter paper was placed in the bottom of a Buchner funnel (100 mm diameter) and moistened to seal the filter paper to the funnel. The prepared test sample was transferred into the funnel and spread evenly. The bottom of the Buchner funnel was covered by soil substrate to a height of at least 10 mm. Suction was applied using a vacuum pump until no more than five drops of water per minute were being withdrawn from the sample, increasing the suction slowly to avoid perforation of the filter paper. The sample was transferred to an aluminum container of known mass and weighed. The container was oven-dried at 103 ◦C ± 2 ◦C and weighed again. The *WHC* of *n* = 5 peat samples and *n* = 3 sand and compost samples was determined and calculated as follows:

$$\text{WHC} = \frac{\text{m}\_{\text{soil, saturated}} - \text{m}\_{\text{soil,0}}}{\text{m}\_{\text{soil,0}}} \times 100 \text{ [\%]} \tag{2}$$

where *WHC* is the water holding capacity, in %; msoil,saturated is the soil mass at saturation, in g; msoil,0 is the oven-dry soil mass, in g.

#### 2.3.2. "Cylinder Sand Bath Method"

Soil was inserted into polyethylene cylinders with 4 cm diameters. The bottoms of the cylinders were covered with a fine polymer grid and filter paper (MN 640 W 70 mm). All cylinders were placed in a vat for 3 h, which was filled with water to a height 1 cm above the soil filling height of 7 cm. After soaking the soil in water, the cylinders were placed on a water-saturated sand bath for 2 h to

allow the unbound water to drain. The soil samples were then weighed wet, oven-dried at 103 ◦C ± 2 ◦C, and the WHC of the soil was calculated according to Equation (2) analogously to the "droplet counting method".

## *2.4. Preparation of Mixed Soil Substrates*

For a comparison of the "Droplet counting method" and the "cylinder sand bath method" and for establishing a regression between mixing ratios of the different soil substrates and their resulting WHC, a total of 22 soil substrate mixtures were prepared as summarized in Table 1.

**Table 1.** Mixing ratios of soil substrates for water holding capacity (WHC) tests. Percentage is based on the oven-dried mass.


For preparing mixed soil substrates, the following equation was used:

$$m\_{\text{soil x,turget, net}} = m\_{\text{turget, total, }0} \times (\frac{a\_{\text{turget, 0}}}{100}) \times (\frac{100}{100 - MC\_{\text{soil x}}}) \text{ (g)}\tag{3}$$

where *msoil x,target, wet* is the target mass of the wet substrate x, in g; *mtarget, total, 0* is the target oven-dry mass of the total mix, in g; *atarget,0* is the target percentage of substrate x based on the oven-dry mass, in %. *MCsoil x*, is the soil moisture content of substrate x, in %.

#### *2.5. Terrestrial Microcosms (TMCs)*

Miniature terrestrial microcosms were prepared in polypropylene containers of 110 (height) × 110 <sup>×</sup> 80 mm3 and a volume of 500 mL. In total, 48 different substrates were filled in the containers each to a height of 100 mm. The combinations of the parameters WHC and MCsoil are summarized in Table 2, where the latter is expressed as (%WHC). The containers were weighed to the nearest 0.01 g, closed with a lid, and their total mass maintained over a period of three weeks.

The following regression functions were used (see also Section 3.1):

$$\text{WHC}\_{\text{mix}; \text{compost}-\text{samd}} = -0.586 \times a\_{\text{target}, \text{sand}, 0} + 80.81 \text{ (\%)}\tag{4}$$

$$\text{WHC}\_{\text{mix}; \text{compost}-\text{turf}} = 2.499 \times a\_{\text{target}, \text{turf}, \text{0}} + 87.15 \text{ (\%)}\tag{5}$$

where *WHCmix:compost-sand* is the water holding capacity of compost mixed with silica sand, in %; *WHCmix:compost-peat* is the water holding capacity of compost mixed with peat, in %; *atarget, sand, 0* is the target percentage of sand based on the oven-dry mass, in %; *atarget, peat, 0* is the target percentage of peat based on the oven-dry mass, in %.

**Table 2.** Soil moisture content MCsoil (%) for combinations of target WHC <sup>1</sup> and target MCsoil expressed as (%WHC).


<sup>1</sup> WHC determined according to the "cylinder sand bath method" according to ISO 11268-2 [19]. <sup>2</sup> WHC of pure silica sand was 21.8% and consequently the lowest WHC achieved.

The WHC of the substrates used for TMCs were determined exclusively according to ISO 11268-2 [19]. The basic substrate was compost. Silica sand and peat were added according to the regression obtained by comparative WHC measurements as described in Section 2.4 (Figure 1).

**Figure 1.** Interrelationship between the percentage of added sand and peat and the WHC of the substrate mixtures: (**a**) WHC determined according to the "droplet counting method" [10]. (**b**) WHC determined according to the "cylinder sand bath method" [19].

The soil mixtures used for the TMCs are summarized in Table 3.


**Table 3.** WHC of different mixtures of compost with sand and peat.

<sup>1</sup> WHC determined according to the "cylinder sand bath method" according to ISO 11268-2 [19]. <sup>2</sup> WHC of pure silica sand was 21.8% and consequently the lowest WHC achieved.

#### *2.6. Preparation and Exposure of Wood Specimens*

Specimens of 5 <sup>×</sup> <sup>10</sup> <sup>×</sup> 100 (ax.) mm<sup>3</sup> were prepared from Scots pine sapwood (*Pinus sylvestris* L.), Douglas fir heartwood (*Pseudotsuga menziesii* Franco), English oak heartwood (*Quercus robur* L.), and European beech (*Fagus sylvatica* L.). All specimens were free from defects such as cracks, decay, and discoloration. For each of the 48 combinations of WHC and MCsoil, *n* = 5 replicate specimens of each species were prepared, which corresponded to a total of 960 specimens.

In total, 48 soil substrates, i.e., combinations of WHC and MCsoil, were prepared and each was used to fill two containers (miniature TMCs). Wood specimens were conditioned at 20 ◦C/65% RH until constant mass before soil exposure. Afterwards, ten wood specimens were buried to 4/5 of their length in each container and exposed for three weeks. The MCsoil was maintained by adding water about every third day if needed.

## *2.7. Determination of the Wood Moisture Content (MCwood)*

Specimens from selected TMCs were used to determine the MCwood distribution within the specimens. Therefore, after harvest, the specimens were cleaned from adhering soil particles, cut into five segments of 20 mm length using ratchet scissors, weighed, dried, and weighed again in each step separately. Segment-wise MCwood was determined on specimens after exposure in TMC with substrates of 30, 60, and 90% WHC, and a MCsoil of 30, 75, and 95% of its WHC.

$$\text{MC}\_{\text{wood}} = \frac{\text{m}\_{\text{wood, wet}} - \text{m}\_{\text{wood, 0}}}{\text{m}\_{\text{wood, 0}}} \times 100 \ (\%) \tag{6}$$

where MCwood is the wood moisture content, in %; mwood,wet is the wet mass of the wood specimen, in g; mwood,0 is the oven-dry wood mass, in g.

#### *2.8. Statistical Analysis*

Regression functions between different variables were established using the method of least squares to achieve the best fit. Statistical differences between collectives were considered significant at a probability of error less than 5% according to a modified Student *t*-test (Welch test).

#### **3. Results and Discussion**

#### *3.1. Water Holding Capacity (WHC) of Soil Mixtures*

The WHC of the three initial substrates was highest for peat, followed by compost and sand according to both methods applied (Table 4).

**Table 4.** WHC (%) of the initial soil substrates determined according to the "droplet counting method" and the "cylinder sand bath method". Standard deviation in parentheses.


<sup>1</sup> Number of replicate samples was *n* = 3 and *n* = 5 for peat tested according to the droplet counting method.

According to both methods, the WHC of the different soil mixtures was linearly correlated with the percentage of added sand or peat, respectively. With increasing percentage of sand and decreasing percentage of peat, the WHC decreased (Figure 1).

It became evident that: (1) single WHC values scattered more and (2) the regression between substrate ratios and WHC was less pronounced when using the "droplet counting method". Furthermore, the following advantages of the "cylinder sand bath method" over the "droplet counting method" became apparent:


funnel and affected the WHC, as shown for peat in Figure 2. With increasing sample size (= sample mass) the WHC increased.

**Figure 2.** Interrelationship between the WHC of peat according to the 'droplet counting method' (CEN/TS 15083-2, 2005) and the mass of the peat sample.

Generally, it was observed that substrates with very high WHC, such as peat, require longer wetting periods than specified by the standards, i.e., 1–2 h according to CEN/TS 15083-2 [10] and 3 h according to ISO 11268-2 [19]. After 3 h of submersion, the peat was still not fully water saturated when oven-dried before, which consequently led to an underestimation of its WHC.

The WHC determined according to both methods were highly correlated, especially for WHC below 200%, as shown in Figure 3. Therefore, and regarding its numerous advantages, in the following, all WHC measurements were conducted using the "cylinder sand bath method".

**Figure 3.** Interrelationship between the WHC of different substrate mixtures according to the "droplet counting method" [10] and the "cylinder sand bath method" [19].
