1 Introduction

Biochar application to agricultural soils is becoming more widespread as a way to regulate soil moisture content, particularly in light-textured soils (Buchkina et al. 2017; Igaz et al. 2018; Razzaghi et al. 2020), reduce greenhouse gas emissions into the atmosphere, mainly N2O and in some cases CH4 and CO2 (Buchkina et al. 2019; He et al. 2017), and control the mobility of various environmental pollutants (García et al. 2020; Wang et al. 2022). In addition, it is suggested that biochar application to soils can increase their fertility and crop productivity by reducing leaching or even supplying plants with nutrients (Hagemann et al. 2017; Yao et al. 2012). Biochar is produced from organic materials such as wood scraps, stems, haulms and other forestry and agricultural wastes through pyrolysis by exposure to heat without oxygen. Biochar made at low pyrolysis temperatures (< 500 °C) is hydrophobic due to pore clogging by tars, which make the biochar exhibit aliphatic groups at its surface. Being produced at high pyrolysis temperatures (> 500 °C), biochar has more affinity to water (Gray et al. 2014) as only a small quantity of aliphatic groups remains on its surfaces with most of the hydrophobic material burnt out. However, biochar production at high temperature requires energy and cost (Ahmed et al. 2016), and also leaves less carbon in the product when the temperatures are above 500–550 °C (Yang et al. 2020), therefore it is more common to produce biochar at relatively low pyrolysis temperatures.

Water absorbing capacity of biochar is a crucial property that enables biochar, being applied to soils, to improve water balance of soils, accumulate nutrients, and stimulate the growth of microorganisms involved in important biogeochemical processes (Ajayi et al. 2016; Batista et al. 2018; Burrell et al. 2016; Gholami et al. 2019; Gray et al. 2014; Hagemann et al. 2017; Ibrahini et al. 2022; Kinney et al. 2012; Li et al. 2021; Novak et al. 2014; Santos et al. 2022; Villagra-Mendoza and Horn 2019). Studies have shown that in addition to high porosity and large surface area of biochar (Batista et. al. 2018; Gray et al. 2014), one of the important properties determining the high water holding capacity of biochars is high water wettability (Batista et al. 2018; Dieguez-Alonso et al. 2018; Schimmelpfenning et al. 2011; Ojeda 2015; Paetsch et al. 2018; Usevičiūtė and Baltrėnaitė, 2020). Initially, biochar is a rather hydrophobic material that repels water, therefore finding approaches to improve its wettability by selecting feedstock and preparation methods to produce more hydrophilic biochar is an important task (Adhikari et al. 2022; Batista et al. 2018; Dieguez-Alonso et al 2018; Usevičiūtė and Baltrėnaitė 2020). Reducing the hydrophobicity of biochar is considered as one of the ways to reduce the negative impact of drought on soils, increase soil water content and, as a result, reduce water stress in plants (Adhikari et al. 2022; Batista et al. 2018).

Usevičiūtė and Baltrėnaitė (2020) investigated the wettability of biochar derived from bark and wood of pine and birch, and hemp under low and high temperature pyrolysis from 450 to 700 °C. The authors showed that the wettability of biochar surface with a water was increasing (the contact angles were decreasing) with the pyrolysis temperature and holding times increase. The wettability of biochar also depended on the nature of biomass from which it was produced. Regardless the pyrolysis conditions, hemp biochar exhibited moderate hydrophobicity with the contact angles in the range from 80° to 64° that was supported by the water penetration test. The similar trend was obtained by Dieguez-Alonso et al. (2018) in their study of the water absorbing capacity and wettability of biochars obtained from pine wood biomass and corn digestant at temperatures of 400–600 °C with different inert gas blowing rates. It was found that the wetting contact angles of all the studied samples decrease from 90 to 74° with the inert flow gas rate or temperature of pyrolysis increase. The authors concluded that, despite the fact that the wetting angles were less than 90°, the samples were still quite hydrophobic. To make the biochar sample more hydrophilic the higher inert flow gas rates during the pyrolysis have to be used to prevent the condensation of volatiles on the biochar surface. Despite similar wettability, water absorption capacity of the corn digestant biochar at high pyrolysis temperature was higher compared to that of the pine wood biochar. The authors suggested that probable reason was higher porosity and reduction in the number of surface functional groups.

Batista et al. (2018) studied the influence of properties responsible for high water retention using biochar obtained from a variety of biomass under low temperature pyrolysis (350 °C). They revealed that in addition to porosity, zeta-potential and the cation exchange capacity, good water retention was provided by the high value of the free energy of the biochar surface particularly its higher polar component. Previously, Schimmelpfenning et al. (2011) noted that the presence of polar groups on the surface of biochar promoted the water absorption and accumulation of moisture due to the interaction of the groups with water molecules. This, in turn, allowed efficient wetting of the biochar surface with water. The specific surface area of biochar increases (Eibisch et al. 2015) when the biochar production conditions change from low temperature hydrothermal carbonization (200–250 °C) of biomass to high temperature pyrolysis (700 °C) and result in a significant reduction of the contact angle and an improvement in biochar water holding capacity.

Reduction in biochar hydrophobicity can also be achieved by artificial aging of biochar through the post-pyrolysis physical and chemical processing. Such treatments lead to the oxidation of the biochar surface and change microbiological and chemoadsorption activity of biochar (Adhikari et al. 2022; Ghaffar et al. 2015; He et al. 2021; Huang et al. 2018; Lawrinenko et al. 2016; Mukherjee et al. 2014). For example, the study of Mukherjee et al. (2014), where biochar samples were mixed with a soil and stored under laboratory conditions, has shown an increase in the amount of adsorbed phenols and quinones and the coating of the biochar surfaces with organic substances and microbial colonies during their interaction with the soil for 15 months. This, according to the authors, can contribute to the process of humification and improve soil fertility. In the study of Huang et al. (2018) the aging process of biochar has been achieved by biochar incubation under constant conditions in the dark for 300 days without any contact with soil. It was found that in the process of aging there was a significant oxidation and, as a result, a change in the elemental composition of the biochar surface. As the authors noted, such changes could significantly reduce the biochar stability and have important consequences for the migration and transformation of pollutants in the soil. Aller et al. (2017) have shown that biochar aging in laboratory conditions under chemical exposure for a year led to the hydrophilization of the biochar surface and increased its water holding capacity compared to the fresh biochar.

Biochar aging in the field, in natural soil conditions, had improved wetting properties and consequently resulted in increased capacity of the soils to retain water and nutrients (Ojeda et al. 2015; Paetsch et al. 2018). Ojeda et al. (2015) showed that surface wettability of biochars obtained from different biomass sources and through different pyrolysis processes (slow, fast or gasification) substantially increased after a year in a sandy-loam soil. However, the wettability of the internal surface of the biochar did not change significantly. Paetsch et al. (2018) revealed significant increase in wettability of biochar after a three-year field experiment and concluded that the aging of biochar significantly increased the plant available water in drought-affected soils.

Despite the active research on biochar optimization by physical and chemical methods in laboratory conditions and a number of short-term field experiments studying the increase in biochar water holding capacity after its application to soil, very little is known about the long-term effects of soil processes on the wettability, physicochemical properties and structure of biochar surface. Natural biochar aging in the soil takes long time and requires long-term experiments, which are not always easy to conduct. At the same time, understanding such changes in biochar on a long time scale is vital in terms of soil productivity and water-holding capacity prediction.

In this study the effect of biochar aging in an agricultural soil for 6 years on the wetting properties and water absorbing capacity of the biochar was studied using a small-scale field experiment with a crop rotation. The effect of mineral fertilizer application on the surface properties of the biochar and its water absorbing capacity was evaluated. Changes in the soil water retention capacity were also studied.

2 Materials and methods

2.1 Experimental site and soil

A small-scale field experiment was established at Menkovo Experimental Station of the Agrophysical Research Institute in the St. Petersburg region of Russia (59°340′ N, 30° 080′ E) in 2015. Average 10-year annual rainfall in the area was 1109 mm, with up to 60% falling during the growing season (May–September). Average annual air temperature was + 4.5 °C, and average air temperature of the growing season  was + 13.6 °C. The soil of the experiment was the most typical soil in the area: a sandy loam Spodosol containing 7–8% clay, 7–15 mg of mineral N kg−1 soil, 239–438 mg of available P kg−1 soil, 67–145 mg of available K kg−1 soil, 14–15 g of SOC kg−1 soil, with bulk density ranging from 1.1 to 1.5 g cm−3 soil, and pHKCl—from 5.0 to 5.9. The biochar was commercially produced from birch wood by slow pyrolysis at temperature of 400–500 ℃ with holding time between 1 and 2 h. Biochar pieces of 0.5–1.5 cm in size were applied to the soil. Three treatments were studied in the field: experiment on the plots 4 m × 2 m in four replicates: (1) control soil, (2) application of biochar (5 t ha−1), and (3) application of biochar (5 t ha−1) with complex mineral fertilizer (90 kg N ha−1, 90 kg P ha−1, and 90 kg K ha−1). Biochar was applied only once, in spring of 2015. From 2015 to 2021 the following crops were grown in the experiment: 2015—potato (Solanum tuberosum L.), 2016—winter rye (Secale cereale L.), 2017—mixture of oats (Avena sativa L.) and common vetch (Vicia sativa L.) with undersown perennial grasses, and 2018–2021—perennial grasses: clover (Trifolium pretense L.) and timothy (Phleum pretense L.). The complex mineral fertilizer (NPK, Azophoska) was applied into the soil in spring of 2015, 2016, 2017 and 2018 shortly before the crops were sown. Conventional plowing to the depth of 22–24 cm was used to till the soil in spring of 2015, 2016 and 2017 while for the period of 2018–2021, when the soil was under the perennial grasses, it was left untilled.

2.2 Materials

Three groups of the biochar samples were studied for wetting and water absorbing characteristics: (1) C—the control group of biochar samples: the same biochar that was applied to the soil in 2015 but was just stored in a tightly closed plastic box at room temperature for 6 years after the production, (2) S—the group of biochar samples collected from the experimental plots without the mineral fertilizer application, and (3) S + N—the group of biochar samples collected from the experimental plots amended with the mineral fertilizer. The biochar particles of about the same size (1.0–1.5 cm) were collected from the experimental soil in the spring of 2021—the sixth year of the field experiment. During the study, the samples were stored in plastic containers under laboratory conditions at the temperature of 20–24 °C and the air humidity of no more than 30%. To study the wettability and the water absorbance capacity of the biochar samples, distilled water and aqueous solution of nonionic surfactant Triton X-100 (Sigma Aldrich, Russia), with critical micelle concentration (CMC) of 0.24 g l−1  , and the minimum surface tension of the aqueous solution \({\gamma }_{cmc}=\) 31 mN m−1  at \(T=\) 298 K were used. The molecule of this surfactant consists of both hydrophilic and hydrophobic parts that enable the molecule to be adsorbed on solid surfaces of any nature through appropriate interaction mechanisms. Modification of biochar with the adsorbed surfactant can influence its specific surface area, affect the surface energy and absorption capacity (Gęca et al. 2023). Recently (Wiśniewska et al. 2021) the effect of such a surfactant was used to modify biochar for efficient adsorption of water contaminations. In the present study the treatment of biochar with Triton X-100 aqueous solution was used to study changes in the wetting properties and the water holding capacity of both the biochar aged in the soil and the control samples.

The soil samples for the soil water retention measurements were taken with cutting rings in four replicates for every treatment (only the control soil and the soil with biochar application were studied) a year after the experiment was established. Soil moisture pressure plate apparatus (Soilmoisture, USA) was used to measure the soil water retention in the water potential range from 0 to − 1500 kPa (− 5 kPa, − 10 kPa, − 35 kPa, − 50 kPa, − 100 kPa, − 330 kPa, − 500 kPa, and − 1500 kPa). Average values and mean errors were calculated and one-way analysis of variance (ANOVA) was applied to evaluate the significance of differences (p ≤ 0.05) between the means of the normally distributed data. Based on the experimental results, water retention curves, characterizing different forms of soil water, were plotted in log scale.

2.3 Free surface energy measurements

The free energy of the lateral surface of the biochar samples was evaluated using the geometric mean method by Owens and Wendt (1969), which takes into account the polar interactions between the solid and the liquid materials:

$$\left(1+\mathrm{cos}\theta \right)\cdot{\gamma }_{l}=2\cdot\left(\sqrt{{\gamma }_{s}^{p}{\gamma }_{l}^{p}}+\sqrt{{\gamma }_{s}^{d}{\gamma }_{l}^{d}}\right)$$
(1)

where \({\gamma }_{l}\) is the surface tension of the test liquid, the subscripts \(s\) and \(l\) denote the sample and liquid surfaces, and superscripts \(p\) and \(d\) denote polar and dispersive (non-polar) components of the surface energy, respectively. Diiodomethane was chosen as a non-polar liquid (\({\gamma }_{l}={\gamma }_{l}^{d}=\) 50.8 mN m−1) and water—as a liquid with dominated polar component of the surface energy (\({\gamma }_{l}^{d}=\) 46 mN m−1 and \({\gamma }_{l}^{s}=\) 26 mN m−1). Static contact angles, \(\theta\), for both liquids on the lateral surface of the biochar samples were measured using the contact angle measuring and contour analysis system (OCA 25, DataPhysics Instruments, Germany). The data were collected in a second after a 3-μl droplet of the test liquid was deposited on the studied surface. The measurements were performed in three replicates for each type of biochar. Then the obtained values of surface energy were averaged and the absolute errors were calculated using the Kornfeld method with a confidence level of 0.75. The table presents the average values of the surface energy and its components.

2.4 Wetting and penetration kinetics measurements

The wettability of the biochar samples was studied by measuring the contact angle evolution of water and surfactant solution droplets on the sample surfaces. The contact angle of the liquid on the sample surface was measured immediately after the deposition of a liquid drop (3 μl) using the automated goniometer (OCA 25, DataPhysics Instruments, Germany). The contact angle was measured both on the lateral surface of the sample, i.e., along the fibers, and on the radial surface, i.e., across the fibers. The changes of the contact angle—\(\theta (t)\), the base diameter—\(D(t)\), and the volume—\(V(t)\) with time were measured for (1) the droplets of distilled water on the studied surfaces of the biochar without any preliminary sample treatment; (2) the droplets of aqueous solution of Triton X-100 at concentration of 1 CMC on the studied surfaces of the biochar without any preliminary sample treatment; (3) the droplets of distilled water on the studied surfaces of the biochar, but the biochar samples were preliminary soaked in aqueous solution of Triton X-100 at 1 CMC for 24 h, then dried in an oven at 120 ℃ for 7 h, and stored at room conditions for 24 h before the measurements.

Measurements were carried out at least in 10 replicates to check the reproducibility of the results. Then the values of each parameter corresponding to each moment of time were averaged, and standard deviations and absolute errors were calculated using Student's distribution with a confidence level of 0.95. The average values were used in the discussion.

The wetted area of the biochar surface—\({S}_{w}\) was calculated using the method of Ivanova et al. (2016) of the automatic summing up of pixels of a given intensity on the image sequence, taking into account the scale factor. The area of the droplet base at its maximum diameter—\({D}_{max}\) on the surface of biochar samples was estimated according to the formula \({S}_{d max}=(\pi {D}_{max}^{2})/4\) with an assumption that the perimeter of the droplet base had the shape of a circle.

2.5 Water absorption capacity measurements

Water absorption capacity indicates the ability of a material to accumulate water when immersed in water. To evaluate the water absorption capacity of the biochar samples, several pieces of approximately the same size and a total weight, \({m}_{0}\), ranging between 1.5 and 3 g were placed in beakers filled with either distilled water or aqueous Triton X-100 solution at 1 CMC, and left for around 20 h. Then the samples were extracted from the liquids and placed on a sieve to remove any excess liquids from their surfaces. After that, the water-saturated samples were weighed, \({m}_{w}\). The mass water absorption capacity was calculated according to the formula (2):

$${W}_{m}=\frac{{m}_{w}-{m}_{0}}{{m}_{0}}\times 100\mathrm{\%}$$
(2)

The measurements were conducted in five replicates for each type of biochar and both liquids. The obtained data were averaged, and standard deviations and absolute errors were calculated with the Student's distribution at a confidence level of 0.95. Statistical significance of difference between the mean values of \({W}_{a}\) in the three groups of biochar for pure water and separately for the surfactant solution was tested using one-way analysis of variance for p values < 0.05.

2.6 Scanning electron microscopy study

The microtextural characteristics of the lateral (along the fibers) and the radial (across the fibers) surfaces of the biochar samples were studied by scanning electron microscopy (SEM, TESCAN Mira 3 LMU, Czech Republic) with SE detector (20 kV) to visualize the samples. In addition, the internal structure of the biochars within the soil (S and S+ N samples) in the area of the artificial experimental fracture of the lateral surface was studied. The elemental composition and the relative concentrations of the elements on the sample surfaces were determined by the energy dispersive X-ray spectroscopy (EDS) using the Oxford Instruments Ultim Max accessory, allowing to obtain characteristic X-ray spectra of the sample surface. The elemental composition was determined in three replicates on each type of biochar. Then the data for each chemical element obtained for each type of biochar were averaged and the absolute errors were calculated using the Kornfeld method with a confidence level of 0.75. Error bars on the histogram were added for chemical elements with a content of more than 1%. The presence of statistically significant differences between the average values of the major elements content in the three groups of biochar was determined using one-way analysis of variance with a significance level p < 0.05.

3 Results and discussion

3.1 Microstructure and elemental analysis of surfaces

Figure 1 shows SEM surface images of the C, S + N and S biochar samples in lateral and radial positions. Figure 2 presents the composition of elements on the lateral surfaces of the biochar samples. The surface of the C biochar in the lateral position constituted of a system of hollow destroyed vessels of prismatic shape without foreign inclusions (Fig. 1a). The walls of some individual vessels were perforated and the pore size varied from 20 to 30 nm. The radial surface of the C sample was porous and represented by the walls of parenchyma cells. The pore size ranged from 30 to 80 µm, which corresponded to macro- and mesopores. The vessels were mostly empty, only some of those were filled with small inclusions, which were fragments of the same biochar (Fig. 1d). Energy dispersive X-ray spectroscopy of the C sample expectedly showed a small range of chemical elements on its surface (Fig. 2). There were six elements in the spectrum composition, among which carbon was dominating and oxygen was making up a significant portion, while silicon was completely missing. The presence of oxygen is obviously associated with some degradation of chemical properties of the C sample surface due to its long storage in the air (the oxidation process) as was shown in other studies (Huang et al. 2018).

Fig. 1
figure 1

SEM images of the lateral surfaces of biochar samples: a C—the control biochar, b S + N—the biochar collected from the experimental plots amended with the mineral fertilizer, and c S—the biochar collected from the experimental plots without the mineral fertilizer application and SEM images of the radial surfaces of the biochar samples: d—C, e—S + N, and f—S

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Fig. 2
figure 2

Elemental composition by EDS analysis of biochar sample surfaces: C—the control biochar, S—the biochar collected from the experimental plots without the mineral fertilizer application, S + N—the biochar collected from the experimental plots amended with the mineral fertilizer. Histogram shows the elements with concertation > 0.1%. The error bars (only for chemical elements with the content of more than 1%) indicate the standard error of the mean (n = 3)

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The biochar samples, which were collected from the soil, had remarkable differences from the C sample: the lateral surfaces were covered with fine mineral soil particles. The same effect of biochar aging in soils was reported earlier (Wang et al. 2021). At the same time, there were obvious differences between S and S + N as well. Thus, the S + N sample was coated with a continuous layer made of fine particles, and the soil particles were glued together so tightly that the pattern of the vessels network underneath was clearly visible on the crust surface (Fig. 1b). The lateral surface of the S samples also had mineral coating, but it was scattered and friable (Fig. 1c), mainly because it consisted of larger soil particles, such as fine sand and silt, whereas the mineral crust of the S + N sample  largely contained clay and silt. Regarding the radial surfaces, it is clear from the pictures that there were no visual differences between the S + N and S samples: in both cases, the pore space of parenchyma was filled with loose fine-dispersed mixture of sand, silt and clay particles (Fig. 1e, 1f).

To study the internal structure of the S and S + N biochar samples, the pieces were broken along the fibers. It was found that the nature of the internal structure of these two samples did not differ from that of the C biochar, i.e., the pore space of the samples did not contain any foreign inclusions inside (Fig. 3). Soil particles were present in a trace amount only in the edge areas of the samples and therefore, soil particles and microorganisms penetrated only into the near-surface layer of the biochars to a depth of several vessel diameters.

Fig. 3
figure 3

SEM images of internal structure of biochar samples: a S + N sample—the biochar collected from the experimental plots amended with the mineral fertilizer; b S sample—the biochar collected from the experimental plots without the mineral fertilizer application

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The elemental composition of the S + N and S samples was richer than that of the C sample: the latter did not contain any of Si, Fe, Ti or Na found in the samples affected by the soil (Fig. 2). In addition to increased content of ash elements and decreased relative content of carbon, the most important difference was the appearance of silicon. The chemical composition of the S + N and S sample surfaces is more "soil-like", which is explained by the presence of the mineral crust on the biochar surface. For example, the content of Si fraction on the S sample surface reached 22%, that corresponded  to about half of the average concentration of this element in most of the soils. Iron and aluminum were also present in the biochars composition being the most quantitatively representative soil chemical elements after silicon.

The obtained results indicate that the control biochar sample (C) was more hydrophobic than the samples under S and S + N treatments. The higher affinity to water of the latter two samples, presumably, was related to the interaction of biochar with soil water, root exudates and microorganisms during the time when the biochar was in the soil. This interaction might have resulted in an increased density of oxygen containing functional groups (carboxylic, hydroxylic, and  phenolic) associated with structural fragments of different organic compounds on the biochar surfaces as was shown earlier for the biochar samples collected from the similar experiment but with much shorter exposition of biochar to soil (Balashov et al. 2022). Water film on the biochar surface attracts microorganisms and finer negatively charged soil particles that form hydrophilic coating on the biochar surface with high content of Si and O.

3.2 Surface free energy of biochar samples

Free surface energy is a fundamental property that determines the wetting and adhesive properties of surfaces and allows predicting the nature of the liquid-surface interaction and the ability of the surface to absorb and retain moisture. The low values of the free surface energy indicate weak attraction forces, mainly due to dispersive interaction, between water and the solid material with a consequence of poor wettability. The large value of the polar component of the surface energy reflects the strong attraction of polar liquids, such as water, to the material. It serves as an indicator of the high wettability of the material by water, which is the most polar liquid. Table 1 shows the values of the free surface energy of the lateral surfaces of the studied biochar samples calculated using OWKR model (Eq. (1)). According to the obtained results, the free surface energy of the C sample was lower than that of the two biochar samples. The presence of small polar component (\({\gamma }_{s}^{p}=\) 0.8  mNm−1) may be caused, according to Huang et al. (2018), by the adsorption of the oxygen-containing groups on the C sample surface, Fig. 2, and that may be the reason for slight hydrophilisation of the biochar surface. The highest total free surface energy and its polar component (\({\gamma }_{s}^{p}=\) 26.4  mNm−1) were found on the S sample (Table 1) that may be determined by the adhesion of the naturally hydrophilic sand particles. This result in turn   suggested better wetting and water absorption capacity of the S sample. The relatively small polar component on the lateral surface of the S + N samples can probably be explained by the presence of organic or bacterial material on the biochar surface (Zhu et al. 2017) that results in lower affinity to water compared to S sample. According to one-way analysis of variance the difference between the mean values of the free surface energy (\({\gamma }_{s}\)) and its polar component (\({\gamma }_{s}^{p}\)) for the samples of biochar exposed to the soil with (S) or without fertilizer (S + N) and the control samples of biochar (C) were significantly different with p < 0.05. At the same time, for the mean values of the dispersive component (\({\gamma }_{s}^{d}\)) no statistical difference between the samples was found (p = 0.45). This fact suggests that the biochar aging process does not affect the dispersive component of the free surface energy, but affects its polar component, which is generally consistent with the physics of the process.

Table 1 Contact angles and the free surface energy \(({\gamma }_{s}\)) of biochar samples and its dispersive \(\left({\gamma }_{s}^{d}\right)\) and polar \({(\gamma }_{s}^{p})\) components
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3.3 Wetting and penetration of liquids on biochar samples

3.3.1 Water droplets

Figure 4 shows the time evolution of the normalized base diameter (\(\overline{D}={D}_{i}/{D}_{0}\)), contact angle (\(\overline{\theta }={\theta }_{i}/{\theta }_{0}\)) and volume (\(\overline{V}={V}_{i}/{V}_{0}\)) of a distilled water droplet on the lateral surfaces of the studied biochar samples. Values \({\theta }_{0}\), \({D}_{0}\) and \({V}_{0}\) were taken immediately after the droplet deposition onto the biochar surface. On the lateral surfaces of C and S + N samples, the partial wetting took place with the initial contact angles \({\theta }_{0}\) equal to 74° and 80°, respectively. The process of wetting and evaporation of the droplet for C and S + N biochar samples can be divided into 3 stages (Fig. 4a): (I) the spreading over the surface (10–30 s) in the stepwise mode, (II) the evaporation with the constant diameter (\(D\left(t\right)\) ≈ const), i.e. with the pinning of the contact line, for another 60 s, and (III) a mixed mode of evaporation (Shaikeea et al. 2017) with the receding contact line (dewetting). The steps in \(D\) and \(\theta\) values indicate that wetting of the surface by the water droplet occurs due to an abrupt change in the liquid/substrate interfacial energy at the contact line, that is caused by textural and chemical heterogeneity of the surface due to the presence of different materials such as soil particles of different size and other adsorbents. This inhomogeneity initiates the stepwise transition to a more stable wetting state, which is characterized by the minimum energy of the system. Interestingly, the step size, \(\Delta D\), is 60–200 µm for the C samples, which corresponds to the pore width of the destroyed vessels in the biochar. Thus, it can be assumed that the droplet spreading occurs in the pinning-depinning mode of the contact line.

Fig. 4
figure 4

Wetting properties of the biochar samples. The time dependence of normalized contact angle, base diameter and volume of water droplets on the lateral surfaces of the samples: a C—control biochar, S + N—biochar from soil with mineral fertilizer application and b S—biochar from the soil without mineral fertilizer application. Stages: I—the spreading of the water droplet on the surface (the wetting of the surface); II—evaporation of the water droplet from the surface of C and S + N samples with the constant diameter; penetration of the water droplet into S sample with the constant diameter; III—the dewetting stage with evaporation in the mixed mode (C and S + N samples); the dewetting stage caused by penetration of the droplet into the sample (S sample). Subscripts C, S + N and S denote samples

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An important result is that the wetting properties of samples C and S + N are quite similar, which means the addition of the nitrogen fertilizer to the soil with biochar did not practically change its properties compared to the C sample, aged during storage. In other words, the effect of the mineral fertilizer on the biochar samples in the soil is comparable to the effect of oxidation of the C samples in the air. In addition, as can be seen in Fig. 4a, the water droplet do not penetrate into the lateral surface of either sample for a time sufficient for its complete evaporation. This is due to the formation of the crust coating on the lateral surface (Sect. 3.1),  which prevents the water penetration into the layer as illustrated schematically in Fig. 5a.

Fig. 5
figure 5

Illustrations of the coating layer on the lateral surface of biochar samples and the wetting behavior of water droplets: a S + N sample—the biochar collected from the experimental plots amended with the mineral fertilizer. The ample coated with the crust layer, the case of the partial wetting, a water droplet evaporates without penetration into the sample; b S sample—the biochar collected from the experimental plots without the mineral fertilizer application. The sample coated with the loose porous layer of soil particles on surface, the case of complete wetting and penetration of the water droplet

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The nature of the interaction between water and the S sample differs from that for the C and S + N samples, Fig. 4b. The initial contact angle on the lateral surface of the S sample is about \({\theta }_{0}\)≈ 60°. When comparing this value with \({\theta }_{0}\) for the C sample, it can be assumed that biochar exposure to the agricultural soil for 6 years increases the wettability of biochar only by 18%, whereas Ojeda et al. (2015) showed a 69.5% improvement in the wetting properties of biochar after just a year in the soil. However, for a correct assessment of the biochar surface properties, it is important to analyze the entire processes of wetting and absorption. In our case, 3 stages of wetting (spreading) and penetration of the water droplet into the S sample can be distinguished: (I) a short-term spreading at \(V(t)\approx\) const (about 250 ms), (II) penetration with the constant diameter (\(D(t)\approx\) const) and pinning of the contact line (about 2 s), and (III) penetration accompanied by receding of the droplet contact line—the dewetting process. At the stage (II) a slight receding of the droplet contact line was also observed, which was caused by the penetration of water into the surface, but it was negligible compared to the stage (III). According to Krainer and Hirn (2021), when a droplet is deposited onto a porous or rough surface, a certain decrease in its volume can occur due to the filling of cavities under its base, if the characteristic scale of the cavity is larger than the droplet size. In our case, the initial droplet diameter was from 3 to 3.5 mm, and the scale of the cavity estimated from the SEM images was in the order of magnitude 0.1 mm. Thus, the decrease in the droplet volume due to the drawdown effect was negligible.

Figure 6a shows the frame sequence of the process of wetting and penetration of a water droplet into the lateral surface of S sample. As can be seen, the penetration into the sample occurs predominantly along the sub-surface layer, which is the destroyed pores filled with various soil and clay particles. Figure 5b schematically illustrates the droplet spreading and penetration into this porous layer. Figure 6b shows the time dependence of the relative area, (\({S}_{w}(t)/{S}_{d max}\)), of the wetted surface of the S sample with a water droplet. The results have shown that the wetting properties and the water absorbing capacity of the lateral surface of the biochar aged in the agricultural soil for 6 years have improved dramatically—the sample exhibits hydrophilic properties, not only due to the formation of organo-mineral complexes on the surface (Paetsch et al. 2018), but also due to a formation of a specific surface layer consisting of biochar and soil particles.

Fig. 6
figure 6

The frame sequence of spreading and penetration of water in the sub-surface layer of the S sample. Scale bar, 5 mm (a). Time changes in the relative area (Sw(t)/Sd max) of the S sample lateral surface wetted by a water droplet (b), where Sw is the area of the wetted surface, Sd max is the area of the droplet base at its maximum spread. The error bars show the standard error of the mean (n = 10)

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The mechanism of the wetting and the lateral water penetration into thin porous materials for both partial and complete wetting cases were studied in details in works of Arjmandi-Tash et al. (2017), Kumar et al. (2019), Starov et al. (2003), and Starov (2004). According to Starov (2004) the droplet volume penetrated into a substrate can be written as \({V}^{*}={V}_{0}-\phi \delta {S}_{\infty }\), where \({V}_{0}\) is the initial volume of the droplet, \(\phi\) is the porosity, \(\delta\) is the thickness of the porous layer, and \({S}_{\infty }\) is the maximal area of the wetted porous layer. For the case of complete wetting, \({V}^{*}=\) 0, the thickness of actively absorbing porous layer of the biochar sample can be estimated as

$$\delta ={V}_{0}/\phi {S}_{\infty }$$
(3)

As noted above, the top lateral layers of the S and S + N biochars are formed of destroyed vessels, which are filled with soil particles. Thus, the porosity of this layer can conditionally be considered similar to the average value for soddy-podzolic sandy loamy soil (\(\phi\) = 0.4). Using the known values of \({V}_{0}\) and \({S}_{\infty }\) one can obtain the thickness of the layer \(\delta \cong\) 150 µm, which has comparable order of magnitude to the depth of the grooves estimated from the SEM images. Thus, we can conclude that water spreads out on the lateral side in a thin surface porous layer by analogy with absorption and imbibition in a thin membrane, see (Fig. 5b).

As can be seen in Fig. 7, the wettability of the radial surface of the studied samples differs significantly from that of the lateral surface. The radial surface of the C sample exhibits superhydrophobicity with the initial contact angle of the water droplet reaching \({\theta }_{0}\)≅ 140°. In this case, the droplet sits on a solid surface consisting of a bundle of vessel ends of different diameter (see, for instance, Fig. 1d), which resembles a hydrophobic patterned surface with air pockets in accordance to the Cassie-Baxter wetting for hydrophobic surfaces. The C-B model equation is as follows:

Fig. 7
figure 7

Wetting properties of the radial surfaces of the biochar samples. The time dependencies of normalized contact angles, base diameters and volumes of water droplets on the surfaces: a C—control biochar, S + N—biochar from soil with mineral fertilizer application and b S—biochar from the soil without mineral fertilizer application. Stages: I—the spreading and penetration of the water droplet on surface (S and S + N samples); II—the dewetting and penetration of the water droplet on surfaces (S and S + N samples). Subscripts S, C and S + N denote samples

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$$\mathrm{cos}\,{\theta }_{CB}={f }_{sv}\mathrm\,{cos}\,{\theta }_{Y}-{f }_{lv}$$
(4)

It is possible to predict the Young contact angle, \({\theta }_{Y}\), which reflects the inherent wettability of that material without influence of surface structural effects. Here, \({\theta }_{CB}\) is the apparent measured contact angle (which is \({\theta }_{0}\)≅ 140°) \({f }_{lv}\) and \({f }_{sv}\) is the ratio of the liquid–vapor area, \({A}_{lv}\), and the solid–liquid area, \({A}_{sl}\), to the droplet base area, \({A}_{d}\). Applying method of Ivanova et al. (2016) for the analysis of SEM images of the radial surface of the C sample, the following values have been obtained: \({f }_{lv}\approx\) 0.55 and \({f }_{sv}=1-{f }_{lv}\approx\) 0.45. Equation (4) yields for the Young contact angle value \({\theta }_{Y}\approx\) 115° that testifies the originally high hydrophobicity of this carbon-rich material. This result is close to the contact angle (\({\theta }_{0}\approx\) 101°) obtained by Usevičiūtė and Baltrėnaitė (2020) for birch biochar powder produced at low pyrolysis temperature (450°C and holding time of 1 h). At the same time, this value is higher than that obtained on the lateral surface (\({\theta }_{0}\)≅ 74°). That can be explained by some hydrophilicity of the lateral surface due to its accessibility to oxidation (Adhikari et al. 2022; Huang et al. 2018). High hydrophobicity of the radial surface of C biochar sample resulted in a long-term evaporation of the droplet, when both \(\theta (t)\) and \(D(t)\) continuously decreased and there was no pinning of the receding contact line (Fig. 7a).

After a water droplet was placed onto the radial surfaces of S + N and S samples, the initial contact angle turned out to be very large: \({\theta }_{0}\cong\) 110° and \({\theta }_{0}\cong\) 130°, respectively. However, the hydrophobic Cassie-Baxter state lasted for about 0.1 s, and then transition to the Wenzel wetting state, which is more energetically favorable, took place. This transition is known as a spontaneous Cassie-Baxter hydrophobicity breakdown (Sbragaglia et al. 2007) and can be initiated by spontaneous disturbances of the contact angle due to imperfections on surfaces. As a result, the wetting and penetration of the liquid into the sample in the complete wetting mode occurs within a short period. The transient spreading is followed by the receding of the contact line due to volume loss caused by penetration (Fig. 7a,b). The total process of interaction of water with the substrate ends with water droplets absorption within 10 and 20 s for the S + N and S samples, respectively. It is important that penetration of water droplets occurs mainly into the depth of the samples through vessels of various diameter accessible for penetration. Considering the porosity of biochar, the penetration depth can be roughly estimated using Eq. (3). The porosity (p) of biochar produced from various tree species by the low-temperature pyrolysis is ranged between 0.55 and 0.86 (Baltrėnas et al. 2015; Gray et al. 2014; Lu and Zhong 2018). The porosity of biochar produced from birch wood at 450 °C pyrolysis equals to 0.792 (Baltrėnas et al. 2015). Using the known average values of \(V\) and \({S}_{\infty }\) of droplets on both samples yields, the penetration depth into both the S+N and S sample was obtained (δ ≈ 1–2 mm).

Thus, the long-term interaction of the biochar with the soil significantly improves the biochar surface layer wetting and, therefore, water absorption, in comparison with the control biochar samples, aged in the air. Interestingly, both the lateral and radial surfaces of the S biochar sample were involved in the process of water retention. Adding mineral fertilizer to the soil, on the contrary, reduces the wettability of the lateral surface of the samples and the ability of this surfaces to absorb water due to the surface crust formation. Meanwhile, the radial surfaces of the S + N showed show a greater wetting and absorption rate compared to the S samples. Extra information on water-holding capacity of the studied biochar samples was obtained by studying their absorption capacity when the samples were exposed to the long-term contact with water (Fig. 8). It can be seen that all the samples absorb water being immersed in it for a long time. Since the biochar density is obviously lower than the density of water and the raw material for the biochar was wood, the mass water absorption capacity calculated according to Eq. (2) will be higher than 100%. The water absorption capacity of the S samples is higher, as can be expected from the results above. The presence of the thin soil crust on the S + N samples resulted in a slight decrease in the water absorption capacity, Fig. 8, because this layer was not saturated with water, unlike in the S sample. Data analysis showed that the effect of the biochar aging process (in air, in soil and in soil with fertilizer applied) on water absorption capacity of samples was statistically significant with p < 0.05.

Fig. 8
figure 8

The water absorbing capacity of biochar samples measured in pure water and in water with added surfactant (Triton X-100) at concentration of 1 CMC. C—the control biochar, S—the biochar collected from the experimental plots without the mineral fertilizer application, S + N—the biochar collected from the experimental plots amended with the mineral fertilizer. The error bars show the standard error of the mean (n = 5)

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3.3.2 Droplets of aqueous surfactant solution

One of the ways to modify the wettability of biochar, making it more hydrophilic and more surface active, is the treatment of the surface with various substances, including polymethylsiloxanes, surfactants and others (Adhikari et al. 2022; Gęca et al. 2023). In this study a non-ionic surfactant was used as a surface modifier due to the amphiphilic nature of its molecules. It aimed to evaluate the possibility of enhancing the water absorbing capacity and wettability of the biochar already hydrophilized either by the soil process or by air for 6 years. Figure 9a shows the time evolution of \({D}_{i}/{D}_{0}\) and \({V}_{i}/{V}_{0}\) of the droplet of aqueous solution of Triton X-100 at concentration of 1 CMC on the lateral and radial surfaces of the C and S + N samples. Adding surfactant to water significantly changes the nature of the interaction of the liquid with the C and S + N samples. On the lateral surfaces, the spreading and penetration of the liquid into the samples occurs in the complete wetting mode, contrary to pure water (Fig. 4a). The wetting (spreading) stage was by 2 orders of magnitude faster compared to pure water. The effect of the surfactant drastically improved the wetting of the radial surface of the C samples and ensured the fast (~ 10 s) absorption of water into the pore space (Fig. 9a). The absorption rate of the surfactant solution into the lateral surface of the S samples increased several times compared to pure water. The droplet almost instantly penetrated into the sub-surface layer filled with soil particles, with the lateral spreading along the surface, as it was described above for the experiment with pure water.

Fig. 9
figure 9

The time dependencies of \({D}_{i}/{D}_{0}\) and \({V}_{i}/{V}_{0}\) of droplets on the lateral and the radial surfaces biochar samples. a The droplets of aqueous solution of Triton X-100 at 1 CMC on C and S + N samples. b The pure water droplets on the biochar samples treated with nonionic surfactant Triton X-100 at 1 CMC. Subscripts L and R denote the lateral and the radial surfaces of the samples

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Generally, the addition of surfactant to water improved the wetting/absorption characteristics of all the sample types. The initial contact angles of the radial surfaces of the samples decreased to \({\theta }_{0}<\) 90°, and were: 80° for the C samples, 87° for the S samples, and 58° for the S + N samples. Despite this fact, the water absorption capacity of the samples measured after their prolonged contact with the surfactant solution turned out to be ambiguous (Fig. 8). On one hand, the amount of water absorbed by the C samples increased expectedly compared to the samples exposed to pure water. It can be assumed that adsorption of surfactant molecules onto the hydrophobic surface of the C sample led to hydrophilisation of the skeleton of the sample and made the macro- and micropores of the sample available for water penetration. On the other hand, the water absorption capacity of the S and S + N samples did not change significantly compared to the same samples exposed to pure water (Fig. 8). The explanation could be that the surfaces of the S and S + N samples, as was shown above, were covered with hydrophilic soil particles, which may have high affinity to water. In this case, the conditions are not exactly suitable for the surfactant molecules adsorption, but nonetheless the adsorption can occur through attachment of hydrophilic heads of molecules to the hydrophilic material on the biochar surfaces, eventually making the surface layer of the samples more hydrophobic than before the experiment. This way, a certain barrier layer is created, impeding the penetration of water into the sample thickness. Such an effect is comparable to the autophobic effect described by Bera et al. (2016). Moreover, washing soil particles off the biochar sample surfaces when they are in the solution or when they are being withdrawn from the solution, may be an additional reason leading to some errors and underestimation of the wet samples mass. However, the results of the data statistical analysis testify that the effect of surfactant on the water absorption capacity of the three types of biochar samples was statistically significant at p < 0.05.

3.3.3 Pure water droplets on modified samples

Treatment of the samples with the surfactant solution (1 CMC, Triton X-100) showed that the initial contact angle \({\theta }_{0}\) of the pure water droplet was reduced on the surfaces of all the samples. On the lateral surfaces of the C and S + N samples \({\theta }_{0}\) of the water droplet decreased 2.0 and 1.6 times, respectively, compared to the untreated samples. The contact angle on the S sample has changed insignificantly. Overall, the simultaneous wetting and penetration of water droplets into the lateral surface of all the studied samples occurred within 1 s. (Fig. 9b). Modification of the properties of the radial surfaces was significant—the samples S and S + N become less hydrophobic with a decrease in \({\theta }_{0}\) from 130° and 110° to 68° and 60°, respectively. In this case, the penetration of the water droplet into the samples occurs in less than 1 s (Fig. 9b). Changes in the radial surface properties of the C sample were barely noticeable, \({\theta }_{0}\) decreased from 140° to 120° and, in general, the evaporation of the droplet in the non-wetting mode occurred the similar way to the biochar sample without any treatment (Fig. 4a). Such a small decrease in the wettability of the C sample can be due to the small area of the solid part of the radial surface that was in contact with the droplet that might have not been enough to effective adsorption of the surfactant molecules and improving the wetting properties of the biochar.

3.4 Soil water retention

Water retention properties of the studied sandy loam Spodosol without and with biochar applied are characterized by the two water-retention curves given on Fig. 10. The results of the study have shown that the water retention capacity of the soil has changed only in the range of moisture potentials from − 5 to − 100 kPa, with the plant-available water corresponding to the range of water potentials from − 5 to − 300 kPa (Datta et al. 2017). In the range of water potentials from − 5 to − 100 kPa water retention of the biochar untreated soil decreased from 32.7 to 15.8% with the soil holding 16.8% of water, while for the soil treated with biochar it decreased from 34.5 to 16.3%, with the soil holding 18.2% of water. The 1.4% increase in the water-holding capacity of the soil is very valuable as all this water is available for plants. The changes in the soil water retention 1 year after biochar application were statistically significant (p < 0.05) only at a moisture potential of − 5 kPa and − 10 kPa. As the moisture potential decreased to − 300 kPa (the lowest border of the plant-available water, Datta et al. 2017), and to − 1000 kPa, the differences between the biochar treated and untreated soil were becoming insignificant or non-existent. The results are similar with our earlier findings for the soil from the same area (Rizhiya et al. 2015, Abramova and Buchkina 2021)—biochar can change soil pore distribution mainly due to its porous structure and high surface area, increasing the water-holding capacity of the soil in the range of the plant-available water. It was also shown by Razzaghi et al. (2020), as a result of a statistical meta-analysis of literature published between 2010 and 2019, that biochar significantly increased plant available water content in the coarse-textured soils (by 45%) compared to the medium- and fine-textured soils (by 21% and 14%, respectively).

Fig. 10
figure 10

Water retention curves (in log scale) for loamy sand Spodosol one year after biochar application. P—water potentials, W—soil water content. The error bars show the standard error of the mean (n = 4)

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4 Conclusions

Biochar aging in the agricultural sandy loam Spodosol with crop rotation for 6 years made the biochar surfaces hydrophilic (with an exception of the lateral surface of the biochar from the treatment with the mineral fertilizer) compared to the control biochar oxidized in air. However, the increase in the biochar water absorbing capacity relative to the control biochar sample was statistically significant but not very high (28% and 10% for the treatments without and with the mineral fertilizer, respectively). The main reason for that was, presumably, in the unchanged internal pore space of the biochar in the soil, as the SEM images have shown, water absorption requires the internal space to be involved into the process.

The analysis of the surface structure and the free surface energy of the biochar revealed significant changes caused by the action of biochar aging in the soil. The most significant changes were found in the lateral surface of biochar exposed to the soil with no mineral fertilizer. Biochar was covered with soil particles and exhibited properties of a thin hydrophilic porous membrane with a large polar component of the free surface energy. This thin layer of soil particles contributed to an increase in the biochar water holding capacity compared to the other samples. The application of the mineral fertilizer to the soil resulted in accumulation of finer soil particles on the biochar lateral surfaces in a form of a crust. Such crusts were preventing water penetration to the biochar surfaces and as a result the biochar samples were exhibiting lower water absorbing capacity compared to the biochar samples collected from the soil with no fertilizer.

The treatment of biochar with the non-ionic surfactant to increase its water absorbing capacity produced the significant effect on the control biochar samples only. The lateral surface of the biochar became hydrophilic and, despite the persistent large contact angle (> 90°) on the radial biochar surface, which was generally related to specific structural features of the biochar and the small contact area between the sessile droplet and the radial biochar surface, the relative increase in water absorbing capacity reached 66% compared to the control biochar. To predict for the duration of this positive effect in the soil, additional investigations are necessary. However, these results can help to improve the biochar water retention. For both biochar samples, aged in the soil, surfactant treatment accelerated the wetting of surfaces, but slightly lowered the biochar water capacity, which allowed us to raise the question on the nature of relationships between biochar surface wettability and its water capacity in the presence of surfactant molecules.

The findings allow the following general conclusion: the hydrophilisation of biochar caused the contact with the agricultural sandy loam Spodosol and modification of the biochar surface by amphiphilic molecules before embedding the biochar into the soil, supported the better water holding capacity of the soil in the range of plant-available water and can have an effect on plant growth and nutrient migration within the soil.