1 Introduction

Drylands and deserts currently cover about 41% of the global land area and they are located especially in arid and semi-arid regions being home to three billion people (United Nations Organization (ONU) 2020). The biggest part of this area is covered by sandy soil, characterized by a lack of structure, poor water retention, high water permeability, and low soil plant-available water (Valentin 2005). Therefore, management is needed to improve this soil’s properties for agricultural activities. One of the alternatives to enhance the soil’s physical and hydraulic qualities is the application of organic amendments such as organic compost and/or biochar.

Biochar is a solid porous material, rich in carbon content, with a large surface area and cation exchange capacity (CEC), produced from the pyrolysis process in an oxygen-limited environment (Hussain et al. 2017, Mansoor et al. 2021). This material is a promisor for improving soil physical and hydraulic qualities such as soil structure, water retention, water available content, porosity, infiltration rate, bulk density, and hydraulic conductivity (de Jesus Duarte et al. 2019; de Jesus Duarte et al. 2020; Laird et al. 2010, Herath et al. 2013 and Abrol et al. 2016. Recent studies had proved that the incorporation of biochar with other organic amendments can maximize the potential benefits of soil properties (Izilan et al. 2022). Compost is one of the best choices to be combined with biochar because of its positive effects on soil chemical, physical-hydric, and biological contributions (Agegnehu et al. 2016; Jaafar et al. 2014; Pandit et al. 2020; de Jesus Duarte et al. 2019).

Many findings had demonstrated that biochar-compost application had contributed to reducing costs of inorganic fertilizer because biochar is rich in carbon content but does not provide sufficient nutrients for crops. In addition, this combination contributes to improving the root growth by modifying the soil’s physical properties, such as the aggregate stability, reduction of soil bulk density, increase of soil porosity, soil water retention and water availability (Sharma et al. 2021; Lu and  Sun 2014).

The biochar-compost application is a winning strategy because the high quantity of pores and exchangeable cations in the organic compounds contributes to water and nutrient retention promoting plant development and the increase of the bacterial community in soil (Reeves 1997; Tisdall and Oades 1982). In addition, its high carbon content (60–80%), high specific surface area (3000m2g-1)2, and high volume of pores (70%–90%), where the majority are mesopores and micropores (90%) promote the high capacity to retain water (Major et al. 2009).

The challenge in the use of biochar and/or compost is that both of them normally are hydrophobic. If on one hand we can select biochar with low hydrophobic by selecting the correct raw material produced at high pyrolysis temperature (Kinney et al. 2012), on the other hand, the hydrophobicity in compost is due to many chemical compounds with hydrophobic effects (Doerr and Shakesby 2000). The amendment, when added to the soil, normally changes the soil wettability or hydrophobicity (Doerr and Shakesby 2000). Because of this effect, the study of sandy soil treated with compost plus biochar or compost alone is necessary.

The effects of a hydrophobic amendment certainly can eliminate the mentioned benefits promoted by the biochar and/or compost addition (Dekker et al. 2001; Briggs et al. 2012; Doerr and Shakesby 2000). This is why it necessary to know the hydrophobicity of soil treated with an organic amendment in small scale such as this experiment, before applying it on the field. However, methods to determine the hydrophobicity in biochar and/or compost are still unclear.

The existing hydrophobicity tests such as Water Droplet Penetration Time Test (WDPT), Wilhelmy Plate Method (WPM), and Capillary Rise Method (CRM) are applied respectively to determine the wettability by liquids on soil particles by the knowledge of surface energy in soil samples, measurement of contact angles and surface energies on soil samples and to analyze the powders hydrophobicity used in physics, chemistry, and pharmacy industries, but have not been used on soil samples (Bachmann et al. 2003) that need to be tested in order to recommend the best test to be applied in sandy soils treated with compost plus biochar or compost alone.

As we saw, the interaction of biochar and compost can be used as a soil amendment for improving the soil’s physical and hydraulic properties. However, there is a lack of information about the composition proportion of biochar and/or compost to be added to sand soil (Blanco-Canqui 2017), and it is necessary to establish a method to evaluate the biochar-compost hydrophobicity in sand soil. These necessities are necessary to be accomplished in order to better understand the interaction between biochar and compost and its effects on soil hydrophobicity, soil physical and hydraulic properties and to recommend the best proportion of biochar and/or compost to be added in sand soil (Glaser et al. 2002a, b; Lehmann et al. 2003; Moberly and Meyer 1978).

We hypothesize that an increase in compost and biochar amounts will improve the water storage capacity and the plant-available water holding capacity and the higher the amounts of biochar and compost are, the higher the contact angles will be, resulting in long infiltration times. In addition, because the measuring principle of the hydrophobicity methods differs, the methods lead to different results. Therefore, this study aims to investigate sand soil treated with biochar and/or compost application: the adequate method to evaluate its hydrophobicity, the best proportion composition of biochar and/or compost, and its influence on the sandy soil water balance.

2 Material and methods

2.1 Experimental setup

The samples were prepared with mixtures of one to three components (sand, sand + compost, or sand + compost + biochar) in different proportions by weight of dry matter with particle size for each fraction < 2 mm. A total of 10 treatments with a volume of 100 cm3 each, and six replicates per treatment, were analyzed (Table 1).

Table 1 Composition of the 10 treatments with relative amounts (weight%) of biochar (B) and compost (C)
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The sandy soil used in the mixtures came from the Ecological Botanical Garden of the University of Bayreuth, Germany. It could be classified as pure sand because the silt and clay were smaller than 10% and the sand fraction was dominated by medium sand (61%). The biochar was mainly made from beech wood (Fagus sylvatica L.) pyrolyzed at 500 °C and the compost was made from organic waste (50% sewage sludge, 25% green cuttings, and 25% pieces of knots). After 3 months of the trial period, six-cylinder samples of each treatment were used for the preparation of the water retention curves and to determine the WDPT, WPM, and CRM. The chemical characterization of the beech wood biochar, pyrolyzed between 500 to 550 °C was performed by Karki (2018) and a miscroscopy wood characterization of the charcoal was made by Balzano et al. (2020).

2.2 Water holding capacity, field capacity and porosity

To assess the plant-available water holding capacity and its ratio to the soil pore size distribution (Hartge and Horn 1999), we determined the soil water balance and water movement using soil water retention curves. For this purpose, samples were placed in a cylinder (5 cm in diameter and 5 cm in height), which was protected against sample loss at the bottom with a water-permeable membrane. Complete water saturation (θs) was achieved and samples were placed on the tension table, then the following tensions were applied: 0.003, 0.009, 0.02, 0.03, 0.06, 0.1, 0.3 bar equivalent to the respective water potential (pF 0.5, 1, 1.25, 1.5, 1.8, 2, 2.5, 3, 4.2), pF is the logarithm transformation of soil tension in cm of water.

Using water-saturated ceramic plates, the pressure 1 bar (pF 3) and 15 bars (pF 4.2) were applied in a pressure pot (Eijkelkamp Agrisearch Equipment). After the samples stabilized at each pressure potential, they were weighted and dried at 105 °C and weighted again for the calculation of θr, and the saturation water contents θs were later determined by substance density determinations on the basis of the pore volume determined from calculated porosity. The data adjustment for the water retention curves were performed via HYPROP Data Evaluation Software V.1.0, UMS GmbH, 2011 (Pertassek, Peters, and Durner), using the classic van Genuchten model (van Genuchten 1980).

Macroporosity and microporosity were calculated by soil water retention curve using theoretical values for macroporosity superior to 50 μm, obtained under pressure − 0.06 bar and microporosity inferior to 50 μm obtained under pressure between − 0.06 and 15 Pa (Richards 1965; Unidos 1972).

2.3 Substance density

In order to determine the porosity and to obtain information about the pore volume at saturation conditions (pF 0) the substance density ρ g cm− 3 was determined. To calculate the substance density, the soil samples were dried at 105 °C, finely grounded in a mortar, and about 10 g of soil was weighed into a pycnometer and deionized water was added until half of the amount in the recipient. The pycnometers were put together in the desiccator without a lid with filled cups of pure deionized water. The desiccator became airtight sealed and coupled with a vacuum pump, which worked for one hour to degas water. After the evacuation, the temperature of the water was measured, the pycnometer was completely filled and the water rose up by capillary. After weight, the pycnometer was emptied and cleaned, the water temperature was measured for the density calculation, and then the pycnometer with degassed deionized water was weighed. The substance density ρs was calculated according to eq. (1).

$${\uprho}_S=\frac{\left({w}_{as-}{w}_a\right)}{\left(\frac{w_{w-}{w}_a}{\uprho_{w_i}}\right)-\left(\frac{w_{sw-}{w}_{as}}{\uprho_{s_2}}\right)}$$
(1)

Wa [g] is the weight of the empty pycnometer, Was [g] is the weight of the pycnometer with approximately 10 g dry soil sample, Wsw [g] denotes the weight of the pycnometer with a soil sample and deionized water and Ww [g] is the weight of the pycnometer with deionized water, ρwi [g cm− 1] is the temperature-dependent density of the water when weighing Wsw and ρs2 is the density of water when weighing Ww. All weightings were carried out with a special pycnometer lid, which was provided by a thin capillary traversed and was used to calibrate the vessel to 100 ml (DIN ISO 11508).

2.4 Texture

To determine the soil texture, the organic material, carbonates, and iron hydroxides were eliminated (Durner and Nieder 2003), and then the pipette method (Köhn 1928) and laser-optical particle analyzes (Grashey Jansen 2007) were used. The sand fractions in the sieves were quantitatively transferred into vessels and dried at 105 °C and then weighed (Durner and Nieder 2003). The silt and clay fractions were determined by the pipette method.

2.5 Water droplet penetration time (WDPT)

The WDPT test allows indirect measurement of the soil particles surface energy. If the soil surface energy is known, it can be used to determine its wettability by liquids. Probably, the surface of the soil changes physically and energetically through the application of the water, affecting the soil water infiltration behavior (Bachmann et al. 2003).

The WDPT was performed on the same 60 cylinders samples that were used for the determination of the water retention curves during the different moisture levels (pF levels). The first WDPT measurement took place at pF 0.5 with a hanging water column. This was followed by a further eight steps up to pF 4.2, and the last two pF steps being set used the overpressure method (pressure pot). The exact pF levels were pF 0.5, 1, 1.25, 1.5, 1.8, 2, 2.5, 3, 4.2 at which the WDPT was measured. With a 50 μl pipette, three drops were placed in random places, and dripped onto the sample surface in the cylinder. With a stopwatch the time was measured from the moment when the drop came to rest on the sample until it completed infiltration or when the sample surface at this point no longer shone.

The shortest measurable period was considered to be 1 s. Everything below that was with 0 s and the median was obtained from the three repetitions of measurements per sample. Since for pF 0.5–1.8 values between 0 and 1 s were measured, and for pF 2 values of 0–2 s were measured. The infiltration time was measured on a random basis, for these six pF levels gave each variant a mean WDPT of 0 s.

2.6 Wilhelmy plate method (WPM)

The WPM serves to determine the progressive and retrogressive contact angle of a sample. For this purpose, a metal plate with defined dimensions, those with a thin layer of dried and sieved (< 2 mm) soil material coated was immersed in a test liquid at a constant speed, and subsequently pulled out (Bachmann et al. 2003).

The hydrophobicity was determined with contact angle measurements in all treatments. Previously the sand-biochar-compost mixtures were dried and sieved (< 2 mm) at 45 °C, and 40 small steel plates (31 mm wide and 15 mm high) on both sides were covered with double-sided adhesive tape, resulting in 4 repetitions each. Next, the protective strip on the adhesive tape was removed and the panels in the respective soil samples were turned over until they were filled with soil material.

The Wilhelmy panels covered with soil material were inserted one after the other into the bracket provided for the dynamic contact angle measuring device DCat 11 (Manufacturer: Data Physics Instrument GmbH, Filderstadt) clamped. The DCat 11 measures fully automatically and processes the data using the SCat software (version 2.8.1.77).

The contact angles were determined in pure deionized water and it was changed before each new measurement. For this purpose, a glass bowl was filled with deionized water and placed on a stand of the DCat 11, and then the immersion speed was entered as 0.2 mm s-1. The depth was set at 10 mm and the frequency with which the data was acquired was at 30 Hz. The water surface tension σw [mN m-1], was read before each measurement and it was carried out using the SCat software. The four weight measurements were used to determine the contact angle of the variant 0 (PK); 6.1 (K), and for this determination eq. (2) was used.

$$\cos \left({\upvarphi}_w\right)=\frac{F_{Ges}}{l_w{\upsigma}_{lv}}$$
(2)

FGes is the total force (N), lw is the wetted sample length [m], and σlv is the surface energy between the liquid phase and gas phase [mJ m− 2]. Theoretically, the WPM can be used to determine contact angles over the entire range from 0° to 180° (Bachmann et al. 2003).

2.7 Capillary rise method (CRM)

The CRM has not become yet the standard method for contact angle measurements on soil samples, but it is used to analyze the physical and chemical properties of the powders of pharmacy (Bachmann et al. 2003). The disadvantage of this method is that reliable values can only be obtained for soils with contact angles < 85° (Bachmann et al. 2003). Even so, this method will be applied in this investigation to make comparisons between WPM, as well as WDPT test.

The soil samples were sifted to 2 mm and dried at 45 °C and were placed in glass tubes of 6 cm high and 9 mm of diameter (type GT11 from Data Physics instruments GmbH). It was applied three replicates for each analysis with water as the test fluid and three for n-hexane.

To get comparable results, in the tubes a uniform storage density of the samples was adopted. To do this, each filled tube was carefully placed on a vertically movable table with speed op to 0.2 mm s− 1 and frequency of 30 Hz. The contact angle measurements were made on the DCat 11, using the SCat software. The measurement process happened via software. The change in weight over time was squared and recorded by the software on the computer graphically mapped. The three repeated measurements with deionized water followed three measurements with n-hexane in the same way, Fig. 1 shows the immersion and wetting process of treatment 0 (C); 6.1 (C) with the test liquids water (curves 1–3) and n-hexane (curves 4–6).

Fig. 1
figure 1

Microporosity and macroporosity of sandy soil with relative amounts of biochar (B) and compost (C). only sand (0 (B); 0 (C)) compared with 1,6% of compost addition and 0,2 and 0,4 biochar plus 1,4 and 1,2 % of compost addition respectively. B: only sand, compared with 3,2% of compost addition and 0,3 and 2,8 biochar plus 0,8 and 2,3% of compost addition respectively. C: only sand, compared with 6,1% of compost addition and 0.6 and 1.5% biochar plus 5.4 and 4.5% of compost addition, respectively

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2.8 Statistical analysis

Normal distribution was checked by Q-Q plots, as well as using the Kolmogorov-Smirnov test. The homogeneity of the variances was determined by Levene test and it was applied the ANOVA according to Kruskal-Wallis with K-samples, as well as the Bonferroni post-hoc test. To group significantly different values with a 0.05 (confidence interval was 95.0%) level of significance the Scheffé procedure was applied and to analyze the method accuracy. We applied the correlation test between the methods tested based on standard deviations.

3 Results

3.1 Porosity and water balance

Low quantities of compost (1.6% and 3.2%) and biochar (0.2% plus 1.4% of compost) had no significant effect on soil porosity (Fig. 1). Increasing amounts of compost increased microporosity and decreased microporosity (Fig. 1). A similar trend occurred for increasing biochar amounts although biochar amounts were generally lower and this trend was not consistent, especially for medium biochar amounts (Fig. 1).

The increase of the microporosity is reflected in the soil water holding capacity and field capacity (Figs. 2, 3, and 4).

Fig. 2
figure 2

Modeled water retention curves of the treatments with relative amounts of biochar (B) and compost (C) A: only sand (0 (B); 0 (C)) compared with 1,6% of compost addition and 0,2 and 0,4 biochar plus 1,4 and 1,2 % of compost addition respectively. B: only sand, compared with 3,2% of compost addition and 0,3 and 2,8 biochar plus 0,8 and 2,3% of compost addition, respectively. C: only sand, compared with 6,1% of compost addition and 0.6 and 1.5% biochar plus 5.4 and 4.5% of compost addition, respectively

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Fig. 3
figure 3

Usable field capacities [% by volume] with relative amounts of biochar (B) and compost (C), calculated from the volumetric water content. Error bars indicate the standard deviation. Same bar color shows equal SOM proportion (0% by weight; 1.6% by weight; 3.2% by weight; 6.1% by weight)

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Fig. 4
figure 4

Water drop penetration time (WDPT) differentiated by pF levels for treatments with relative amounts of biochar (B) and compost (C). Error bars show standard deviations. Different letters (a and b) show significant differences between the treatments within a pF level (Bonferroni post hoc test with a significance level of 0.05)

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All treatments showed typical water retention curves of sandy soil, the addition of organic matter such as biochar and/or compost increased the soil water retention. Even the low quantity of compost (1.6%) and the low addition of biochar (0.2% and 0.4% plus compost (1.4% and 1.2%), respectively, increased the soil water holding capacity (Fig. 2A), and this effect was significant (p ≤ 0.05) even at the low potential (pF 0.5; 1.0 and 3.0) with the addition of 3.2% of compost and 0.6 and 0.8% of biochar to plus 2.% 8 and 2.3% of compost, respectively (Fig. 2B). Overall, the three treatments with a proportion of 6.1% of compost and 0.6 and 1.5% of biochar plus 5.4% and 4.5% of compost, respectively, showed over the whole drainage period higher water content than pure sand (Fig. 2C).

Field capacity increased with increasing amounts of biochar or compost (Fig. 3). However, especially for compost, only higher amounts significantly increased field capacity, while for biochar lower amounts achieved the same effect (Fig. 3). The lowest field capacity, 6.0% by volume, was found for the control (pure sand), and the highest, 9.6%, for 0.6% biochar plus 5.4% compost (Fig. 3). This means an increase of 60% in the field capacity. Field capacity increased with the increasing amount of compost or biochar. Striking is a comparison of the three treatments with the same SOM showing that the FC was always higher for the variants with higher biochar and SOM (Fig. 3).

3.2 Wettability methods

3.2.1 Water droplet penetration time test (WDPT test)

In general, for all pF levels and treatments, short water infiltration times were measured (Fig. 4). Thus, all treatments were classified as wettable (Table 2). Pure sand had the most rapid water infiltration while increasing compost amounts increased water infiltration times (Fig. 4). The infiltration retardation of compost could be lowered by adding biochar (Fig. 4). Surprisingly, water infiltration was generally better in moist soils, i.e., as the proportion of compost increased, the infiltration decreased in sand soil. With the same proportion of SOM, the pure compost treatments showed up to 100% longer infiltration times compared to the treatments with biochar. Significant reduction in wetting class from wetting class 1 (wettable) to wetting class 2 (slightly water-repellent) was only shown at pF 4.2 and one pure compost application of 6.1% by weight. The standard deviations were sometimes very high (up to 10 s, Fig. 4).

Table 2 Classification of infiltration times [s] (WDPT test) of the treatments with relative amounts of biochar (B) and compost (C)
Full size table

3.2.2 Wilhelmy plate method (WPM)

No significant difference between the treatments with WPM could be found (Table 4). This meant that all measured contact angles were 0° and the wettability was the same, which had an infiltration time of 0 seconds. According to the classification in Table 3, all treatments were classified as fully wettable (Table 4).

Table 3 Contact angle classification according to Karoly Müller (unpublished)
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Table 4 Classification of contact angles [°] (CRM and WPM) of the treatments with relative amounts of biochar (B) and compost (C)
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3.2.3 Capillary rise method (CRM)

The measured contact angles φw for all treatments were in the range of 0°- 83°, which were in the range of fully wettable, moderately wettable, or not sufficiently wettable (Table 4). Pure sand and all pure compost treatments exhibited contact angles between 50º and 60° with no significant difference (Fig. 5) corresponding to moderately wettable (Table 3). Contact angles of biochar treatments significantly increased with the increase of biochar amounts from 0º to 80° (Fig. 5), thus covering a range from completely wettable to not sufficiently wettable (Table 3). Therefore, low amounts of biochar addition to compost has a beneficial effect on wettability, medium amounts of biochar addition to compost has no effect and high amounts of biochar addition to compost has a negative effect (Fig. 5).

Fig. 5
figure 5

Contact angle φw [°] of the treatments with relative amounts of biochar (B) and compost (C) determined according to CRM. Black points show outlier values and black crossbar medians. Different letters (a - d) show significant differences between the treatments (Bonferroni post-hoc test with a significance level of 0.05)

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A clear effect in terms of increasing the hydrophobicity of the sand with an increasing proportion of biochar could not be determined, but the wettability of the two variants with the highest biochar applications was significantly lower (ẋ (φ) = 69.8° and 78.3°; moderate - not sufficiently wettable) than pure sand. A significant improvement in the wettability of the sandy soil was achieved with the lowest biochar application of 0.2% by weight (ẋ (φ) = 14.0°; fully wettable). The standard deviations were up to 18.2°, sometimes very high.

3.2.4 Comparison between wettability methods

Water infiltration times at pF 2.5, pF 3.0, and pF 4.2 were significantly (p < 0.001) correlated with volumetric water content at pF 2.5, pF 3.0, and pF 4.2 (Table 5). All combinations of pF values were also correlated significantly (p < 0.001, Table 5). The same was true for the CRM but based on p < 0.05 (Table 5). However, CRM was not significantly correlated with WDPT (Table 5, Figs. 6, 7, and 8).

Table 5 Correlation matrix of the contact angle φw [°] (CRM). the infiltration times [s] (WDPT test; pF 2.5 / 3.0 / 4.2) and the water content θv [% by volume] (water tension curves of pF 2.5; 3 and 4.2) according to Spearman-Rho (SPSS)
Full size table
Fig. 6
figure 6

Correlation of the infiltration times at pF 4.2 of treatments with relative amounts of biochar (B) and compost (C) with the volumetric water content θv at pF 4.2. R² is the coefficient of determination of the black regression line (SPSS)

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Fig. 7
figure 7

Correlation of the contact angle φw of treatments with relative amounts of biochar (B) and compost (C) with the volumetric water content θv at pF 4.2. R² is the coefficient of determination of the black regression line (SPSS)

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Fig. 8
figure 8

Correlation of the contact angle φw [°] (CRM) and the logarithmic infiltration times [s] at pF 4.2 for treatments with relative amounts of biochar (B) and compost (C) (A); at pF 2.5. (B); at pF 3 (C) R² is the coefficient of determination of the black regression line (SPSS)

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4 Discussion

4.1 Porosity and water balance

Biochar addition normally changes the soil porosity by contributing with new pores (Major et al. 2009). We showed that even low quantities of biochar are enough to change the soil porosity, we verified a significant increase in microporosity after the addition of 0.8% of biochar, 2.3% of carbon and 96.89% of sand and 1.4% of biochar, 4.5% of compost and 93.96% of sand. The high contribution of the increase of microporosity was associated with the high biochar rate compared to the other treatments, which resulted in high addition of a product with high quantity of micropores as the biochar, approximately 90% of the pores in biochar were micropores. When this product is added to soil, it normally changes the pore size distribution which increased microporosity (Major et al. 2009) (Fig. 3). Similar results were found by de Jesus Duarte et al. (2019) when biochar plus sugar cane filter cake was applied to sandy soil under field conditions.

The contribution of biochar plus compost in microporosity to sandy soil was mainly related to the high quantity of micropores in the organic material and the biochar that was a porous material with 70%–90% of pore volume, and 95% of these pores were mesopores and micropores. The contribution of microporosity in this soil certainly affected the soil water holding capacity and the water movement (Conte et al. 2014). Certainly, soil water retention depends on the biochar physicochemical properties, which, in turn, are most affected by feedstock type, pyrolysis conditions, and duration of charring (Huang et al. 2021; Wiedner et al. 2013). However, a recent study has proved that the biochar pore size distribution was more important than chemical composition in affecting water movement in charred systems (Conte et al. 2014). The water retained in the charred materials is subjected to a slow-motion regime defunding quickly within the channels connecting pores between one another and different surface pores or char surface with the interior part of the same material, and the water move from the largest pores to the smallest pores and the micropores may adsorb water with high capillary forces so that it is not available for most plants (Conte et al. 2014). But, the creation of new pores in the soil, especially micropores could contribute to increasing the soil water retention and the plant-available water holding capacity.

The assumption that an increase in compost and biochar amount will improve the water storage capacity was confirmed, whereby biochar had a higher effect than compost. In detail, 6.1% compost had the same effect as 2.4% or 4.5% compost when mixed with only 0.8% or 1.5% biochar, respectively. It is possible to save money and use the compost without biochar to obtain similar results such as an increase in microporosity and the soil water holding capacity. As some studies had proven, an increase in the water retention of 20% and 36% when 5% and 10% of biochar from wood pyrolyzed at 300 °C was added (Huang et al. 2021). However, it is necessary to take into consideration that the decomposition of the compost is really fast compared to biochar which required about 4000 years under optimal field conditions (Kuzyakov et al. 2014). This is why the use of compost alone needs to be continued to maintain these effects, affecting the cost of production, while biochar can be added only once or less often to achieve similar effects.

The treatments with the highest amount of compost (6.1%) alone or in combination with biochar (0.6% biochar and 5.4% compost) exhibited the highest plant-available water-holding capacity of about 10% compared to about 6% for pure sand (Fig. 3). Abel et al. (2013) reported an increase in field capacity with a pure biochar application, around 10% and 5% by weight and 16.3% by volume, and Novak et al. (2009) described an increase of 15.9% by volume and 2% by weight of biochar in loamy sand soil. Liu et al. (2012a, 2012b) reported an increase in plant-available soil water retention compared to pure compost of about 100% when 20 Mg ha-1 biochar was applied together with 30 Mg ha-1 compost to sandy soil in NE Germany under realistic agricultural practice. In a global meta-analysis with 158 observations, Omondi et al. (2016) reported an increase in plant-available water holding capacity of about 30% when biochar was added to coarse-textured soils. Therefore, compost and biochar additions to dry sandy soils lead to about 50% longer plant survival after rainfall followed by longer drought periods.

In the temperate latitudes, this is a crucial advantage because arid or semi-arid areas only have 3 weeks of water supply per year, so the dry phases are common for several months. In these areas, water storage capacity could be improved by promoting the reduction of water in the irrigation, reducing costs, and increasing the irrigation efficiency, in areas with no irrigation system biochar can contribute to maintain the water on the soil for a longer period of time. A positive balance in terms of increasing water storage capacity and FC of the soil through biochar-compost applications also attracted research attention (Liu et al. 2012a, 2012b; de Jesus Duarte et al. 2019; Blanco-Canqui 2017). Downie et al. (2011) mentioned that biochar applications could also be an option for climate change adaptation when drought periods increased or evaporation increased due to increased temperature.

The improvement on soil physical and hydraulic properties will be favorable to root development and plant production. When a plant is added in a soil with good physical and hydraulic qualities such as with high porosity, low bulk density and high-water availability. The root system does not have resistance to grow, and the presence of a plant normally contributes to improve the aggregate stability, increase the soil microporosity and reduce the soil bulk density. In addition a good soil quality promotes a good root system which is normally associated with a healthy plant with high production. Igalavithana et al. (2016) verified that the addition of 25 Mg ha− 1 of biochar Fagus salvativa increased 71.05% of production of the Dactylis glomerata. In our study, the same biochar was used plus compost, increasing the potential of contributing to plant production.

4.2 Wettability methods

4.2.1 Water droplet penetration time (WDPT test)

As expected, we observed a decrease in the wettability by increasing the time of infiltration by the WDPT test at higher pF levels (Figs. 4 and 5). Pure sand did not show this behavior (WDPT always 0 s) and in contrast to the results of Liu et al. (2012a, 2012b), which indicated that pure sand was slightly water-repellent at pF ≤ 1.8 and extremely water-repellent at pF=2.5, contrary to our expectations. In addition, there was no increase in the duration of the infiltration time when the proportion of biochar was increased. On the contrary, the biochar was able to reduce significantly the hydrophobic effect of the compost. Verheijen et al. (2010) mentioned the possibility of washing out hydrophobic-acting organic acids in this regard.

The capacity of the biochar to reduce compost hydrophobicity is new in the literature. Studies have found that fresh biochar is highly water-repellent (WDPT 92 h) (Briggs et al. 2012). Glaser et al. (2002) explained that the biochar hydrophobicity is related to its hydrophobic polycondensed aromatic structure. However, we found that biochar-rich samples, which were taken directly under the litter layer were only slightly water-repellent (WDPT < 10 s). It can happen because the use of distilled water can reduce the biochar hydrophobicity (Briggs et al. 2012) and its hydrophobicity is eliminated after 1 year of interaction with soil particles (Ojeda et al. 2015).

The observed increase in the infiltration time with an increasing proportion of compost or the hydrophobic properties of the compost could be due to microbial activity in compost, according to which enriched hydrophobic excretion products of microorganisms and fungi increased the hydrophobicity of the soil (Cosentino et al. 2010). Coatings of organic compounds and non-polar groups on inorganic soil particles reduce their surface energy and thus reduce their wettability (Bachmann et al. 2003). An investigation into whether the microbial activity during the increased drainage period could show whether hydrophobicity developed in the samples over time. Abel et al. (2013) verified that biochar applications up to 5% in sand soil, neither in the field nor in the laboratory recognize water-repellent behavior (WDPTs < 5 s). Considering the same amount of SOM, the samples with more biochar content showed shorter infiltration times, possibly due to their high porosity and its water sorption properties.

4.2.2 Wilhelmy plate method (WPM)

The contact angle of 0° determined for all treatments suggested that the method was not sensitive compared to WDPT and CRM methods. It can be assumed that the proportion of SOM applied was so low that the Wilhelmy plates during sample preparation with only a little bit of biochar and compost particles had been stocked. During the test, it was observed that the coarse grain size of the sand caused a rough relief which does not occur in soils under natural conditions.

In the case of clayey or silty samples, the method is more sensitive, since the adhesion of the much finer grains on the plates is closer to natural storage in the field. The natural pore structure is lost using the WPM. A homogeneous planar surface is essential when measuring the contact angle, otherwise, roughness and pore structure will affect the result (Cosentino et al. 2010), which contributes to the underestimation of the contact angle of wettable samples (Bachmann and McHale 2009) and increases errors in the graphic evaluation.

4.2.3 Capillary rise method (CRM)

Although CRM contact angles significantly increased with increasing biochar amounts, CRM contact angles were not higher compared to pure sand or pure sand plus compost (Fig. 5). Only the treatments with both the highest compost and biochar amounts had significantly higher CRM contact angles (Fig. 5).

4.3 Comparison between wettability methods

The contact angles from WPM and CRM are not correlated. The methods are based on the same measuring principle but differ fundamentally in theory and structure and therefore the comparison is limited (Goebel et al. 2008). The WPM is two-dimensional, and the CRM is a three-dimensional method (Ramírez-Flores and Bachmann 2012). The reproducibility of the repeated measurements of the CRM is presumably due to their three-dimensionality, less than WPM (Ramírez-Flores and Bachmann 2012). However, it can probably be more sensitive to subtle differences in the sample properties, it was confirmed by the results of the CRM, since showed subtle differences in the wettability of the samples.

According to the common classification, all samples were classified as wettable. According to Table 4 one treatment was classified as fully wettable and one as not sufficiently wettable, while the rest were classified as moderately wettable. The results of the WPM showed no differences and were all to be classified as fully wettable.

The assumption that large contact angles result in long infiltration times and small contact angles in short infiltration times could not be confirmed (Figs. 7 and 8). The methods used attributed to different soil properties, different sample properties, and moisture (Cosentino et al. 2010).

The WDPT test was not performed on completely dried samples, unlike CRM and WPM. Dekker et al. (2001) noted that air-dried or oven-dried samples only reduce the potential water-repellent properties of the soil, but the actual from the field cannot dry soil. This makes it possible to determine the actual hydrophobicity of an estimated soil (Dekker et al. 2001). The drying temperature is also crucial, whereby drying at 105 °C gives the best results (Dekker et al. 2001). Dekker et al. (2001) recommend the methods to be carried out on samples naturally dried in the field.

The different drying processes (oven drying (40 °C) for CRM and WPM versus dewatering and oven drying (105 °C) for WDPT test) could have affected compost and biochar properties differently. The results of the CRM suggested that the biochar in the dried condition shows a hydrophobic behavior, but in wet conditions behaved rather sportively (WDPT test). When dried, compost showed no hydrophobic behavior (CRM), but probably caused increased microbial and organismic activity, which involved the accumulation of excretory products hydrophobicity generated (WDPT test). Each of the methods was more or less operator-dependent, WPM and CRM, the evaluation of the graphs and WDPT test in time measurement and interpretation from the start to end of the infiltration process, as well as during the application of drops (Bachmann et al. 2003).

Examining the different classification systems for evaluating wettability is necessary to continue to search for a uniform nomenclature for future investigations, because the previously used nomenclatures for the characterization of the infiltration times and contact angles were not satisfactory and compatible. It is urgent to need new nomenclatures as in the case of Table 3. However, before doing so, it is necessary to check whether the methods are comparable in all delivery results.

5 Conclusions

The addition to 0.6% of biochar, 5.4% of compost in 93.99% of sand is recommended to improve water balance and wettability sand soils. This composition contributed to an increase of 38% of water volume in the field capacity compared to the control (only sand soil). An increase in the plant-available water holding capacity of 38% in sand soil is a positive result for agriculture in arid regions or temperate regions with increased drought periods as currently observed in the whole Europe. The soil using this composition was also wettable according to WDPT test. This test differed from the other tests and it seems to be the most suitable to analyze sand soil treated with biochar-compost since: 1) WDPT appeared to allow for the best estimates of wetting behaviors of the different mixtures; 2) the column samples were the least disturbed, and 3) the test was additionally feasible at different water content levels, and 4) it is easy to conduct, even in the field or without access to a laboratory. This method can be used both in laboratory and field condition and is feasible on undisturbed soil of varying moisture. However, this method is strongly dependent on the conductor of the experiment and the exact measurement of short infiltration times (0 s – 2 s) proved challenging.