Studies into Fungal Decay of Wood In Ground Contact—Part 1: The Inﬂuence of Water-Holding Capacity, Moisture Content, and Temperature of Soil Substrates on Fungal Decay of Selected Timbers

: This article presents the results from two separate studies investigating the decay of wood in ground contact using adapted versions of laboratory-based terrestrial microcosm (TMC) tests according to CEN / TS 15083-2:2005. The ﬁrst study (A) sought to isolate the e ﬀ ect of soil water-holding capacity (WHC soil [%]) and soil moisture content (MC soil [%WHC soil ]) on the decay of ﬁve commercially important wood species; European beech ( Fagus sylvatica ), English oak heartwood ( Quercus robur ), Norway spruce ( Picea abies ), Douglas-ﬁr heartwood ( Pseudotsuga menziesii ), and Scots pine sapwood ( Pinus sylvestris ), while keeping soil temperature (T soil ) constant. Combinations of soil mixtures with WHC soil of 30%, 60%, and 90%, and MC soil of 30%, 70%, and 95%WHC soil were utilized. A general trend showed higher wood decay, measured in oven-dry mass loss (ML wood [%]), for specimens of all species incubated in soils with WHC soil of 60% and 90% compared to 30%. Furthermore, drier soils (MC soil of 30 and 70%WHC soil ) showed higher ML wood compared to wetter soils (95%WHC soil ). The second study (B) built on the ﬁrst’s ﬁndings, and sought to isolate the e ﬀ ect of T soil and MC soil on the decay of European beech wood, while keeping WHC soil constant. The study used constant incubation temperature intervals (T soil ), 5–40 ◦ C, and alternating intervals of 10 / 20, 10 / 30, and 20 / 30 ◦ C. A general trend showed drier MC soil (60%WHC soil ), and T soil of 20–40 ◦ C, delivered high wood decay (ML wood > 20%). Higher MC soil (90%WHC soil ) and T soil of 5–10 ◦ C, delivered low wood decay (ML wood < 5%). Alternating T soil generally delivered less ML wood compared to their mean constant T soil counterparts (15, 20, 25 ◦ C). The results suggest that di ﬀ erences in wood species and inoculum potential (WHC soil ) between sites, as well as changes in MC soil and T soil attributed to daily and seasonal weather patterns can inﬂuence in-ground wood decay rate.


Introduction
Wood is one of the oldest raw materials used worldwide. As a construction material, it is used in a variety of manners, both indoors and outdoors. Due to its renewable and biodegradable properties, wood is becoming increasingly important when considering more environmentally friendly and sustainable construction materials. However, being renewable and biodegradable also poses considerable challenges to its utilization, requiring design measures to prevent degradation and extend service-life [1].
Finér et al. [25], found soil temperature, and to a lesser extent, soil phosphorous concentration and available nitrogen, to be the best explanatory variables for the differences in wood decay between sites.
To investigate the effect of the abiotic, soil-level variables on the durability of wood used in ground contact, TMC experiments according to CEN/TS 15083-2:2005 [19] were undertaken in two separate studies: Study (A) examined the effect of (1) WHC soil and (2) MC soil on decay progress while keeping (3) T soil constant. Study (B) kept examined the effect of (2) MC soil and (3) T soil while keeping (1) WHC soil constant.
Software packages such as TimberLife [26] have developed a means towards modeling in-ground wood decay progression, and by extension durability and service-life. Durability and service-life show a clear link through the overlapping of data requirements used in these study fields [27]. The service-life of a product or component incorporates the concept of durability, but with additional information relating to usable lifespan. Part 2 of this research article series seeks to use a 'dosimeter' approach in modeling of in-ground wood decay [28]. Originally developed for aboveground wood, a dose-response approach will be applied to the obtained data sets.

Standard Test Requirements
Due to the similarity in methodology of study (A) and study (B), the following section regarding experimental methods is presented as relevant to both. Where necessary, differences in experimental methods are distinguished using capital lettering, study (A), or study (B). If relevance to (A) or (B) is not stipulated, the methodology applies to both.
Terrestrial microcosms (TMCs) in accordance with CEN/TS 15083-2:2005 [19] were utilized in semifield experiments. The standard stipulates that a natural topsoil or a fertile loam-based horticultural soil substrate is used, with pH 6-8 and no additives. The soil should have a WHC soil of 20-60%, MC soil equal to 95%WHC soil , and the test should be conducted in a dark, climate-controlled room set to a temperature of 27 • C and relative humidity of 65%.

Soil Substrates
The basis of the substrate was a horticultural compost produced at the forest botanical garden at the University of Göttingen's North Campus. The compost comprised fallen leaves and cuttings from grass and trees. Soil was passed through a sieve with nominal aperture size of 8.5 mm. WHC soil was then determined according to the 'cylinder sand bath method' according to ISO 11268-2 [29]. To lower the WHC soil of the base compost substrate, silica sand (0-0.2 mm grain size) was added. Study A used substrates with WHC soil of 30%, 60%, and 90%, while study B only used substrates with WHC soil of 60%.

Determination of the Soil Moisture Content (MC soil )
Soil samples of 50-90 g (depending on the soil density) were taken for determining the soil moisture content (MC soil ). Three replicate samples were taken, weighed to the nearest 0.01 g, oven-dried at 103 • C for 24 h, and weighed again. MC soil was calculated according to Equation (1) below.
where MC soil is the soil moisture content [%]; m w is the wet soil mass [g]; m 0 is the oven-dry soil mass [g].

Determination of the Soil Water-Holding Capacity (WHC soil )
Soil was inserted into polyethylene cylinders 10 cm long with 4 cm diameter. The bottoms of the cylinders were covered with a fine polymer grid and filter paper (MN 640 W 70 mm). All cylinders were filled with soil to a height of 5-7 cm and saturated in an 8 cm high water bath for 3 h. After the saturation period, the cylinders were placed on a water saturated sand bath for 2 h to allow unbound water within the soil-filled cylinders to drain to reach the equivalent of field capacity. The soil samples were then weighed wet, as well as after oven-drying at 103 ± 2 • C for 24 h. WHC soil [%] was calculated according to Equation (2) below.
where WHC soil is the soil water-holding capacity [%]; m s is the saturated soil mass [g]; m 0 is the oven-dry soil mass [g].

Preparation of Mixed Soil Substrates
To mix the different soil substrates of compost and sand to the predetermined WHC soil of 30%, 60%, and 90%, the WHC soil of soils mixed in incremental ratios based on oven-dry mass was first determined. where WHC soil is the target water-holding capacity of the soil mixture [%]; x is the fraction of sand substrate in the total soil mixture based on oven-dry mass [%]; R 2 is the coefficient of determination between actual and predicted values. Table 1 below shows the incremental soil mixtures used to establish a WHC soil regression equation for the substrates sand and compost. To prepare mixed soil substrates for testing WHC soil , Equation (3) below was used.
where m x, wet is the mass of the wet substrate x [g]; m total, dry is the oven-dry mass of the total soil mixture [g]; x is the fraction of the substrate (sand or compost) in the total soil mixture m total, dry based on oven-dry mass [%]; MC x is the moisture content of the soil substrate x [%]. A regression between the incremental mixing ratios of the two substrates sand and compost and their resulting WHC soil was determined. Equation (4) below shows the regression relationship for WHC soil of the two substrates used to define the mixture percentages to attain mixed soil substrates with WHC soil of 30%, 60%, and 90% for study (A), while Equation (5) below shows the regression relationship for WHC soil for study (B) to attain mixed soil substrates with WHC soil of 60%. where WHC soil is the target water-holding capacity of the soil mixture [%]; x is the fraction of sand substrate in the total soil mixture based on oven-dry mass [%]; R 2 is the coefficient of determination between actual and predicted values. Table 1 below shows the output from computations using Equations (4) and (5).
where WHC soil is the target water-holding capacity of the soil mixture [%]; x is the fraction of sand substrate in the total soil mixture based on oven-dry mass [%]; R 2 is the coefficient of determination between actual and predicted values. Once the soil mixtures with target WHC soil of 30%, 60%, and 90% were attained, three MC soil were decided on for study (A); equal to 30, 70 and 95%WHC soil . Two MC soil were decided on for study (B); equal to 60 and 90%WHC soil . Distilled water was added to the soil mixtures to reach MC soil equal to 30, 70 and 90%WHC soil , shown here as MC soil , target [%]. Therefore, distilled water was added to reach MC soil,target as shown below in Table 2. Equation (6) below was used to calculate the mass [g] in distilled water required to add to the soil mixture to reach MC soil,target . If the soil was too moist to start with, the soils were placed in drying ovens set to 30 • C to dry out until the MC soil,target was reached. To account for losses in MC soil resulting from fungal activity and evaporation, rewetting to MC soil,target occurred once per week throughout the 16-week incubation period.
where m water is the mass of distilled water to add to the soil mixture [g]; MC soil,target is the target soil moisture content [%]; MC soil,current is the current moisture content of the soil mixture before adding any additional water [%]; m total, dry is the oven-dry mass of the total soil mixture [g].

Soil Temperature (T soil ) Control
For study (A), TMC boxes were stored in a temperature-controlled room set to a temperature of 20 ± 2 • C, while for study (B), climate chambers were used to incubate the TMCs and simulate differences in T soil . In total, eight constant T soil and three alternating T soil were investigated. Constant T soil rose in 5 • C intervals from 5 to 40 • C, while alternating T soil cycled to 10/20, 10/30, and 20/30 • C. Temperature was changed once every 7 days for TMCs of alternating T soil , meaning that a full cycle of alternation lasted 14 days, or 2 weeks. Two TMCs per T soil were prepared. In total, 22 TMCs were prepared; one TMC per T soil (eight fixed, three alternating), per MC soil (60 and 90%WHC soil ). Relative humidity of 65% was maintained in all climate chambers.

Preparation and Exposure of Wood Specimens
In study (A) European beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), Scots pine sapwood (Pinus sylvestris L.), English oak heartwood (Quercus robur L.), and Douglas-fir heartwood (Pseudotsuga mensziesii Franco.) were used. Specimens of 5 × 10 × 100 (ax.) mm 3 were prepared, in accordance with CEN/TS 15083-2:2005 [19]. Kiln-dried boards (>60 • C) were conditioned to wood moisture content (MC wood ) of 12 ± 2%. Specimens were then prepared from planed strips of the boards, with a cross-section of 10 ± 0.1 mm × 5 ± 0.1 mm. Annual rings were orientated 90 ± 15 • to the broad face of the specimen (i.e., 10 mm face). Transverse cuts of the cross-section delivered sharp edges and a fine-sawn finish to the end-grain surface, with a final specimen length of 100 ± 1 mm. All specimens were free from defects such as cracks, decay, and discolouration.
After specimen preparation, all specimens were oven-dried at 103 • C for 24 h and weighed for oven-dry mass to the nearest 0.001 g. Prior to soil exposure, all specimens were again conditioned to MC wood of 12 ± 2% (confirmed by Equation (7) below) and buried 4/5 of their length into the soil substrate with 120 specimens per TMC box. In total, 1080 test specimens were used; eight replicate specimens for each of the five wood species, three specimen removal intervals (8, 12, 16 weeks), and nine different soil conditions. After soil exposure, specimens were removed, cleaned of remaining soil, and again oven-dried at 103 • C for 24 h. Specimens were then weighed again to the nearest 0.001 g with oven-dry wood mass loss (ML wood ) calculated according to Equation (8) below. Mean ML wood and standard deviation of mean ML wood was calculated according to Equations (9) and (10) below.
where MC wood is the wood moisture content [%]; m 3 is the wood specimen's mass after TMC exposure [g]; m 2 is the wood specimen's oven-dry mass after TMC exposure [g].
Oven-dry mass loss (ML wood ) and wood was calculated according to Equation (8) below.
where ML wood is the wood specimen's oven-dry mass loss [%]; m 1 is the wood specimen's oven-dry mass before TMC exposure [g]; m 2 is the wood specimen's oven-dry mass after TMC exposure [g]. Mean ML wood was calculated according to Equation (9) below.
where mean ML wood is the arithmetic mean of the oven-dry mass loss of the sample population [%]; x i is the oven-dry mass loss (ML wood ) of each individual wood specimen in the sample population [%]; n is the total number of wood specimens in the sample population. Standard deviation of mean ML wood was calculated according to Equation (10) below.
where s is the standard deviation of the sample population; x i is the oven-dry mass loss (ML wood ) of each individual wood specimen in the sample population [%]; x the mean oven-dry wood mass loss (mean ML wood ) of the sample population [%]; n is the total number of wood specimens in the sample population.
In study (B), European beech wood specimens of 5 × 10 × 100 (ax.) mm 3 were prepared in the same manner as study (A), i.e., in accordance with CEN/TS 15083-2:2005 [19]. Specimens were also buried 4/5 of their length into the soil substrate, but rather with 80 specimens per TMC. A subset of 10 replicate specimens was removed every 2 weeks. In total, eight exposure intervals, 11 T soil (eight fixed, three alternating), and two MC soil (60 and 90%WHC soil ), delivered a total of 1760 beech wood specimens used in the study.

Impact of WHC soil and MC soil on Fungal Decay (Study A)
The mean ML wood of all five wood species used in study (A) during 16 weeks of incubation is shown in Figure 1. Oak seemed the most resilient to high ML wood of all tested wood species with the maximum mean ML wood not exceeding 20%. When considering mean ML wood across all soil conditions measured at the 16 week incubation interval, wood species with low durability (in ground contact) such as beech and Scots pine sapwood [30] showed the highest mean ML wood , i.e., 18% and 17%, respectively (Table 3). All species generally showed higher mean ML wood in the soil mixtures with WHC soil of 60% and 90% compared to 30%. This was attributable to the high percentage of silica sand constituting almost 100% of the soil mixtures with WHC soil of 30%, which showed a lower presence of microorganisms compared to the soil mixtures with WHC soil of 60 and 90%. Although only limited ML wood occurred in soils with WHC soil of 30%, the result remains important nonetheless, to gain an understanding of wood decay across a broad range of WHC soil . Soil particle size distribution is an underlying determinant of WHC soil [31]. However, other sandy soil types with comparable particle size distribution and WHC soil can possess a different (and potentially higher) wood decay potential than the sandy soils used in this study. The sand used in this study was purchased from a supplier and packaged in 25 kg bags. This means the sand was subject to industrial processes such as sieving for consistent particle size distribution (0-0.2 mm grain size) and drying to ensure consistent packaging quantities. This would naturally result in a lower inoculum potential compared to undisturbed sandy soils.
Soils with MC soil of 95%WHC soil showed consistently lower ML wood than drier soils with MC soil of 30 and 70%WHC soil . Increased moisture also stimulates microbial activity; however, decay was impaired once an optimal level of MC soil and MC wood was exceeded. Full mean ML wood data with accompanying standard deviation can be found in Appendix A. Tables A1-A5 in Appendix A are presented in order of wood species. Besides a source of fungal inoculum itself (mycelium or spores), Zabel and Morrell [32], list four critical requirements for fungal growth in wood, namely, a source of free or unbound water, favorable temperatures (approximately 0-42 • C), atmospheric oxygen, and a digestible carbon substrate. In addition to these requirements comes the added complexity of understanding which specific fungi types are active throughout various ranges of these critical requirements, and if any of these requirements (reagents) were only available in limited quantity. Extensive research has already been conducted to illustrate that different soils in different locations can deliver different dominating decay rates and types [8,[33][34][35][36][37][38].
While this study used one soil characteristic, WHC soil , as a metric to describe the soil's capillarity or moisture retention ability, the broader concept of pedogenesis (soil formation or development) begins to show relevancy for outdoor, in-field wood decay testing. Soil genesis describes how a soil, in all of its layers, came to be in its present state. Environmental factors in parent material (underlying geology), climate, biota (organisms), topography, and time, operate through soil processes of additions, losses, translocations, and transformations to form soils [39]. This means that soils from different locations, and soil layers (horizons) within a single soil profile can show varying characteristics in their physical, biological, and chemical composition. All of these can influence microorganism decay activity. However, within this study's controlled environment, WHC soil , MC soil , and wood species still showed promise as prediction variables to ML wood , due simply to the role of moisture in fungal wood decay, that is without moisture, wood decay ceases. Besides a source of fungal inoculum itself (mycelium or spores), Zabel and Morrell [32], list four critical requirements for fungal growth in wood, namely, a source of free or unbound water, favorable Oxygen availability in soil should also be kept in mind when considering moisture availability. Under conditions of high MC soil , oxygen availability decreases, since the soil pore spaces become filled with liquid, which in-turn slows wood decay. Examinations of wooden foundation piles have confirmed a protective effect of high soil moisture levels. In waterlogged, anaerobic soils, wood decay is found to progress slowly through wood-decaying bacteria while aggressive wood-decaying fungi are suppressed [40,41].
3.2. Impact of T soil and MC soil on Fungal Decay (Study B)

Constant T soil
The mean ML wood of 10 beech wood specimens for every T soil and every 2-week specimen removal interval over the 16-week incubation period is shown in Figure 2. At constant T soil , TMCs with lower MC soil (60%WHC soil , Figure 2a,c below) delivered higher ML wood than those with higher MC soil (90%WHC soil , Figure 2b,d below). Lower MC soil in combination with T soil of 15-40 • C were favorable decay conditions (ML wood > 20%), with optimum ML wood occurring at 35 • C. However, 35 • C also showed the highest standard deviation of 13.9% (Appendix A: Table A6).
Higher MC soil in combination with low T soil of 5-10 • C showed unfavorable decay conditions (ML wood < 5%), where temperature alone could not be held accountable for the inhibited decay, with increased MC soil also contributing [42]. For higher MC soil , the optimum T soil was 25 • C with ML wood at 16.0% after 16 weeks of incubation. This value corresponded to ML wood at 10 • C for specimens exposed to lower MC soil . Interestingly, 40 • C seemed to show consistently higher ML wood compared to 25 • C throughout the early and middle incubation periods, however 25 • C ultimately showed higher ML wood after 16 weeks.
For both MC soil conditions, ML wood also increased with incubation time. Generally, for both MC soil conditions tested, ML wood increased with T soil . However, some T soil intervals showed exceptions to this trend, such as 15 and 35 • C for MC soil of 90%WHC soil . It was assumed that problems related to ventilation within the climate chamber led to inconsistent results here.
Previous studies investigating in-ground wood decay have confirmed an interactive relationship between MC soil and T soil on wood decay [42,43]. Soil moisture has the potential to alter the response of fungal growth to warming, meaning that wood decay can be inhibited if soil is too wet or too dry [44]. Elevated temperature (3 • C above ambient of 15 • C) did not significantly increase wood decomposition rate alone or in combination with increases in MC wood and MC soil [42]. However, drying (of both soil and wood), results in a decreased wood decay rate [42], coincidently illustrated for 15 • C with MC soil of 90%WHC soil , where excessive ventilation (drying) could be to blame. Figure 2c,d plotted ML wood against T soil as a function of incubation period (2-16 weeks), for both lower and higher MC soil . From this, the effect that changes in T soil had on ML wood could be deduced. Decay rate can increase more drastically with an increase in T soil later in the incubation period compared to earlier in the incubation period. This can be seen from the steeper gradient of ML wood with increasing incubation period, and is illustrated clearly in Figure 2c, where clear differences in ML wood are discernible.
For MC soil of 60%WHC (Figure 2c above), the effect that an increase in the T soil from 5 to 35 • C had on ML wood could be seen by comparing ML wood at the exposure intervals of 4 and 16 weeks. After 4 weeks, increasing T soil by 1 • C, ML wood increased by 0.6%; towards the end of the study period, after 16 weeks, ML wood changed by 1.0%. It can be assumed that the increased rate of decay is related to the development of wood-decaying microorganisms. This assumption is consistent with the observation of Eaton and Hale [45], that at the beginning of exposure there was a comparatively low mass of microorganisms in the test soil as well as in the wooden test specimens. After an initial phase of fungal colonization, decay of the wooden substrate began. Changes in temperature therefore have a greater effect the further decay progresses [46]. For both MCsoil conditions, MLwood also increased with incubation time. Generally, for both MCsoil conditions tested, MLwood increased with Tsoil. However, some Tsoil intervals showed exceptions to this trend, such as 15 and 35 °C for MCsoil of 90%WHCsoil. It was assumed that problems related to ventilation within the climate chamber led to inconsistent results here.
Previous studies investigating in-ground wood decay have confirmed an interactive relationship between MCsoil and Tsoil on wood decay [42,43]. Soil moisture has the potential to alter the response of fungal growth to warming, meaning that wood decay can be inhibited if soil is too wet or too dry [44]. Elevated temperature (3 °C above ambient of 15 °C) did not significantly increase wood decomposition rate alone or in combination with increases in MCwood and MCsoil [42]. However, drying (of both soil and wood), results in a decreased wood decay rate [42], coincidently illustrated for 15 °C with MCsoil of 90%WHCsoil, where excessive ventilation (drying) could be to blame. Mean oven-dry mass loss (ML wood ) of European beech wood specimens incubated for 16 weeks at constant soil temperature (T soil ) with soil water-holding capacity (WHC soil ) of 60% and soil moisture content (MC soil ) of 60%WHC soil (a), and 90%WHC soil (b). Mean oven-dry mass loss (ML wood ) of beech wood specimens plotting against increasing soil temperature (T soil ) for a specified incubation period (# wks), incubated in soil with water-holding capacity (WHC soil ) of 60% and soil moisture content (MC soil ) of 60%WHC soil (c), and MC soil of 90%WHC soil (d).
For a given fungi species (and isolate), above the lower T soil decay activity limit, the "reaction speed-temperature (RST) rule" begins to take effect, which states that in a certain temperature range, increasing the temperature by about 10 • C, enzyme activity (and therefore decay rate) runs faster by a factor of 2-4. Frequently, the optimum lies, depending on the species (and isolate) between 20 and 40 • C [4,47].
For lower MC soil (Figure 2c above), increasing T soil from 5 to 15 • C increased ML wood by a factor of 1.97 (10 weeks incubation) to 5.42 (2 weeks incubation) and thus a factor of 2.0 for almost all incubation intervals of this T soil range was exceeded. When T soil was increased from 10 to 20 • C, the factor increases only exceeded 2.0 for the first three incubation intervals (2, 4, and 6 weeks). The RST rule was thus only confirmed for lower T soil and especially at the beginning of the incubation period.
A review of the RST rule for MC soil of 90%WHC soil showed an overall more heterogeneous picture due to the peaks at 25 and 40 • C and low ML wood values at 15 • C (Figure 2d above). Consequently, T soil increase from 15 to 25 • C, resulted in ML wood increasing by a range of factors, starting at 3.55 (4 weeks incubation) to 7.92 (6 weeks incubation). With T soil increased from 30 to 40 • C, the factor only exceeded 2.0 for the incubation intervals of 4-8 weeks. The low ML wood of T soil at 15 • C, did indeed confirm the RST rule for T soil interval from 15 to 25 • C, but was only of limited significance due to low ML wood at 15 • C causing an overrated ML wood at higher T soil . Overall, it was found that the RST rule could only be confirmed for individual T soil intervals and therefore could not be confirmed generally for both MC soil ranges. It should also be mentioned that ML wood was used to test the validity of the RST rule because it was assumed that ML wood was proportional to fungal enzyme activity and therefore wood decay rate. However, fungal enzyme activity and wood decay rate should not be used synonymously since many factors determine wood decay rate as measured by oven-dry mass loss, such as wood species, fungal community composition, fungal community succession, and MC soil , to mention but a few. More information regarding various factors influencing wood decay and service-life can be found in Marais et al. [48]. Full mean ML wood data with accompanying standard deviation for constant T soil can be found in Appendix A: Tables A6 and A8.

Alternating T soil
The test specimens in TMC with alternating T soil showed a similar trend to those with constant T soil conditions. ML wood of test specimens exposed to MC soil of 60%WHC soil showed an influence from T soil at the start of the test period, which became clearer throughout the course of the test (Figure 3a,c,e below). Here too, ML wood after 16 weeks was greater than 20%, except for T soil at 10/20 • C with ML wood of 19.4%. As with constant T soil , ML wood for MC soil of 90%WHC soil was considerably lower than for MC soil of 60%WHC soil . However, contrary to constant T soil , no increase in ML wood with increasing T soil was detected. The alternating T soil pair of 10/20 • C showed the highest ML wood , but this was still low after 16 weeks (ML wood < 10%). Problems related to the maintenance of temperature, humidity, and air movement within the climate chamber may be to blame for this (i.e., high evaporation between rewetting intervals). Full mean ML wood data with accompanying standard deviation for alternating T soil can be found in Appendix A; Tables A7 and A9.

Comparison: Constant vs. Alternating T soil
Alternating T soil (10/20, 10/30, 20/30 • C), delivered lower ML wood compared to their mean constant T soil counterparts (15,20,25 • C) regardless of MC soil (Figure 3), but with the exception of 10/20 • C at higher MC soil (Figure 3b below), which delivered higher ML wood than its mean constant T soil counterpart of 15 • C. These results correspond with previous findings concerning pure fungal cultures where alternating incubation temperatures rarely led to a higher fungal growth rate than a mean constant temperature counterpart [49]. Furthermore, indiscriminate temperature fluctuation across a defined temperature range was more indicative of a natural environment than alternating temperature aligned to the minimum and maximum of the defined range [50].
Fungal dormancy may also explain the decreased ML wood when compared to its mean constant T soil counterparts. Dormancy refers to a physiological state of fungal activity defined by strongly reduced respiration reflective of a form of resting, which does not contribute to turnover processes (i.e., wood decay and the organic matter in the soil). Once dormant, the requirements for reactivation are limited to rewetting and/or the addition of new organic material to the substrate [51]. However, since soil moisture was maintained at constant levels throughout all the TMC setups and organic material was in abundance, the decreased ML wood was most likely the result of fungal respiration reacting to altered T soil , which includes a lag period. Furthermore, the removal of specimens every 2 weeks made it difficult to understand the influence of a single T soil interval in the alternating cycle, since T soil was altered every week. Since a fungal characterization study also did not form part of this article, knowing the exact temperature and moisture boundaries (or curve) and subsequently stating which fungi groups were active, partially active, dormant, or dead, remains speculative.  : (b,d,f)).

Conclusions
The studies presented in this article show a clear influence of WHCsoil, MCsoil, and Tsoil on the decay of wood in soil contact. Fungal activity was either inhibited or promoted depending on the combination of these abiotic soil-level conditions. For all five wood species tested in study A, Figure 3. Comparisons of mean oven-dry mass loss (ML wood ) of beech wood specimens incubated at constant and alternating soil temperature (T soil ) with soil water-holding capacity (WHC soil ) of 60% and soil moisture content (MC soil ) of 60%WHC soil (left panel: (a,c,e)), and 90%WHC soil (right panel: (b,d,f)).

Conclusions
The studies presented in this article show a clear influence of WHC soil , MC soil , and T soil on the decay of wood in soil contact. Fungal activity was either inhibited or promoted depending on the combination of these abiotic soil-level conditions. For all five wood species tested in study A, European beech, Douglas-fir heartwood, English oak heartwood, Scots pine sapwood, and Norway spruce, decreased ML wood occurred under conditions of high MC soil (i.e., MC soil = 95%WHC soil ).
In study B, both T soil and MC soil showed an influence on the decay activity of soil-inhabiting microorganisms. Both abiotic factors MC soil and T soil influenced each other in such a way that wood decay increased or came to a standstill. Already at the beginning of the test period a reciprocal effect was noticed, where higher T soil in combination with lower MC soil resulted in an increased decay rate, while at low T soil , especially in the wetter soil environment (90%WHC soil ), only slight decay took place. This trend continued over the entire trial period of 16 weeks.
Alternating T soil conditions decreased wood decay activity of soil microorganisms compared to their corresponding constant T soil counterparts. It was assumed that weekly T soil changes required soil microorganisms to adapt to the altered environmental conditions, which impaired wood decay rate.
A graphical comparison of ML wood after different exposure intervals showed that when using ML wood as indicator, the reaction-speed temperature (RST) rule, which establishes a connection between T soil and wood decay rate, could not be generally confirmed. When considering MC soil of 60%WHC soil , which was responsible for more favorable decay conditions, an increase in T soil at the end of the trial period showed a stronger influence on increases of ML wood compared to an earlier stage of the trial period. This indicates that an initial fungal settlement phase was required before T soil increases could be linked to increases in wood decay rate. It would be of interest here to obtain a differentiated picture of the antagonistic and synergetic interactions between the microorganisms by demonstrating the organisms involved in wood decay. Inferences regarding decay severity can be made once the conditions surrounding microbial invasion of the wood substrate are understood (i.e., T soil , WHC soil , and MC soil ). Irrespective of the RST rule, this data is valuable nonetheless due to the frequent specimen removal interval throughout the 16-week incubation period and the range in T soil , which can allow for further use in modeling the service-life of wood in ground contact. Responses of ML wood to changes in T soil at different stages of decay progress were therefore quantifiable in percentage of oven-dry mass loss (ML wood [%]).
Advancements in wood service-life prediction have incorporated a distinction between biological factors causing degradation and structural effects on timber, and the structural response of timber to both loads and loss of resistance to loads due to decay [27,28,52]. Software packages such as TimberLife [26] have modeled decay progression of wood in ground contact by using indirect macroclimate data, such as that from the Scheffer Climate Index [53], coupled to 35 years of field trial data to develop regional wood decay rates, presented as a risk map for the entire continent of Australia. These are species-specific, which include mostly Australasian, and some prominent European and North American wood species. Other methodologies of modeling decay progression, such as dose-response, rather use wood microclimate data in MC wood and T wood (dose) to calculate the number of ideal expose days required for decay onset until ultimate failure of the wooden component to occur (response). The method uses a '0-4 limit-state' evaluation system to describe stages of decay progress as a function of decay depth and spatial distribution. In this case, the wood-and-soil microclimate is sought to be defined through the addition of soil-level variables as investigated in this article. Further differentiation between these two methodologies and the advantages and disadvantages of each can be expected in Part 2 of the article series.
The complexity of the relationship that the parameters T soil , MC soil , and WHC soil have to one another and the wood decaying microorganisms that are active at various dose loads (i.e., combinations of WHC soil , MC soil , and T soil ) was evident. Additionally, Part 2 will present deeper statistical analyses into significant differences in ML wood between these various dose loads.

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