1 Introduction

Biochar production and application to soil can serve as an environment-friendly strategy for the disposal of increasingly abundant organic waste (Zhang et al. 2018). The ability to use biochar for waste disposal and the benefits of using biochar to improve soil quality and rehabilitate degraded soils have attracted considerable attention (Berek 2014). The main mechanisms of soil improvement include the ability of biochar to increase cation exchange capacity (CEC), pH, and water holding capacity and to directly add mineral nutrients (Wang et al. 2014; Chathurika et al. 2016; Hilioti et al. 2017; Cornelissen et al. 2018). The high pH and CEC of biochar result in increased pH and CEC of soils (Zhang et al. 2018). The ability of biochar to increase soil pH and CEC mainly results from its composition of alkaline substances, including ash and carbonates of Ca2+, K+, and Mg2+ (Yuan et al. 2011; Jien and Wang, 2013); its surface properties; and the ability of biochar to reduce exchangeable acidic cations (Al3+ and H+) (Masud et al. 2014).

Much of the work on biochar has focused on specific soil conditions, primarily acidic to slightly acidic, low in CEC, and degraded, poor soils. For example, work done by Martinsen et al. (2015) reported an increment in pH, CEC, and exchangeable bases after application of three biochars produced from cacao (Theobroma cacao) shell, oil palm (Elaeis guineensis) shell, and rice (Oryza sativa) husk on 31 soils. However, all 31 soils used for the study were acidic (pH (H2O) ranging from 3.5 to 5.6) and had CEC ranging from low (16.1) to medium (153.9 mmolc kg−1), and the majority of the soils were very acidic with low in CEC. Increment in soil exchangeable base cations (Ca2+, K+, and Mg2+) has been also reported following the application of biochar in acidic soil (pH (H2O), 3.8) (Xu et al. 2013). Similarly, biochar significantly increased soil pH and exchangeable cations in two acidic soils with pH (H2O) of 4.12 and 4.75 (Wang et al. 2014). In another recent study, biochar increased CEC, pH, and exchangeable K+, Mg2+, and Ca2+ in soil that was moderately acidic (pH (CaCl2) 4.5; pH (H2O) 5.1) with low CEC (60.5 mmolc kg−1) (Pandit et al. 2018). The increment in pH and CEC of soils after application of biochar to three acidic to slightly acidic soils (pH (CaCl2) 3.95, 5.12, and 5.85) with low CEC (18.3, 27.6, 35.4 mmolc kg−1) has also been reported (Martinsen et al. 2014). Hence, additional studies are needed to investigate the effects of biochar additions on soil pH, CEC, and exchangeable base cation contents across soils with a wider range of physicochemical properties under the same environmental conditions. Based on this gap among previous studies, our work attempts to address the following objectives:

  1. 1)

    To elucidate the soil factors, which predominantly governs the effect of high-temperature pyrolyzed biochar (700 °C) application on soil pH, CEC, and exchangeable base cations (Ca2+, K+, and Mg2+)

  2. 2)

    To identify level of soil pH, CEC, and exchangeable base cations (Ca2+, K+, and Mg2+) content for the application of high-temperature pyrolyzed biochar (700 °C) with particular characteristics for most effective alteration of the soil properties

2 Materials and methods

2.1 Soil sampling and biochar production

Soils were selected based on their individual properties (pH, CEC, nutrient content, and textural classes) from 10 different agricultural sites in the Czech Republic, namely Malin, Suchdol, Kbely, Poděbrady, Hněvčeves, Červený Újezd, Lhota, Humpolec, Žamberk, and Lukavec, with conventional crop rotation in all cases. Each soil was collected randomly from the top layer of 0–20 cm, air-dried, and passed through a 2-mm sieve prior to use. The biochar was produced from coniferous wood chips by fast pyrolysis in a fluid reactor at 700 °C and ground to 2 mm. The specified high temperature was selected to produce quite stable biochar with the highest production temperature and exploitation of the highest possible energy from the biomass. The location and specific properties of soils and biochar are presented in Table 1.

Table 1 Selected physicochemical properties of soils and biochar
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2.2 Experimental design

A 12-week incubation experiment was set up using 500-ml plastic pots to which 200 g of dry soil was added. Pots were kept in a greenhouse with room temperature (25 ± 2 °C) controlled and the moisture content in pots was kept at 60% of a given soil’s water holding capacity. The experiment was designed with 5 treatments for each soil: control (soil + no biochar), soil + 0.5% biochar, soil + 2% biochar, soil + 4% biochar, and soil + 8% biochar (w/w ratio). The selected biochar application rate is in regard of considering field applicable rate 0.5 and 2% biochar rate and slightly higher biochar rate 4 and 8% to observe the highest possible biochar effect at higher rate in laboratory scale. Each soil–treatment combination was replicated three times. All pots were filled with the soil–biochar mixture and thoroughly mixed individually. Every pot was irrigated every other day in order to reach 60% water holding capacity. For the determination of soil 60% water holding capacity, first soil maximum water holding capacity was determined. It was done by filling Mitscherlich columns with air-dried soils of known weight and moisture content. Subsequently, the column was soaked in water for 2 h to fully saturate the soil in the column and after full saturation for 2 h, the water in the soil was drained for 12 h. After draining, the maximum water holding capacity was determined gravimetrically as the amount of water retained by the known amount of soil (dry weight) in the Mitscherlich columns. Samples of incubated soils and soil–biochar mixtures (50 g) were taken randomly on the 7th day (week 1) to assess the immediate response of soils on the biochar application. After taking these initial samples from each pot, the soil remaining was left to incubate until the end of our incubation period. The second samples of 50 g were taken at day 84 (week 12) of incubation after thoroughly mixing. The duration of the experiment approximated the vegetation period of many crops.

2.3 Chemical analysis

The biochar and soil pH were determined after extraction of samples with 0.01 M CaCl2 (w/v = 1/5) according to ISO 10390 (2005) using an Argus pH meter (Sentron) with a transistor CupFET probe. For the determination of CEC, 2.5 g sample was added in 50-ml Nalgene tube agitated for 1 h and the supernatant collected after centrifugation. The saturation was done three times filling the supernatant to 100-ml flask for the determination of exchangeable cations (Ca2+, K+, and Mg2+). After the three-step saturation of the soil sample with 0.1 mol L−1 BaCl2 solution, the exchanged Ba2+ was released by agitating the centrifuged pellet with 0.02 mol L−1 MgSO4 solution for 2 h. After 2-h agitation, the solution was centrifuged and the remaining Mg2+ in the supernatant was determined for estimation of CEC (Gillman 1979).

2.4 Statistical analyses

One-way analysis of variance (ANOVA) was used to analyze the effect of biochar under Tukey’s significance difference test. Multivariate analysis of variance was employed to investigate the general effects of biochar application rate, incubation period, soil type, and interactions with soil properties.

Relative changes (%) of CEC and exchangeable Ca2+, K+, and Mg2+ in biochar-amended soils were calculated using Eq. (1).

$$ X\ \left(\%\right)=\left(\frac{C_n-C}{C}\ \right)\times 100 $$
(1)

where X denotes the changes in CEC or exchangeable Ca2+, K+, and Mg2+ (%); C is the concentration in the control soils mmol kg−1; and Cn is the concentration in the biochar-amended treatments mmol kg−1.

Pearson correlation tests were performed among values of biochar-induced changes in CEC, pH, and exchangeable Ca2+, K+, and Mg2+ of soils using the Pearson two-tailed test with α = 0.05.

In this paper, mean value differences at α = 0.05 were considered statistically significant and all statistical analyses were performed using SPSS 17.0 software.

3 Results

3.1 Effect of biochar on soil pH

The application of 0.5% biochar to soil did not induce significant changes to soil pH at either incubation period (1 and 12 weeks), with the exception of a few cases after 1 week (Table 2). Significant pH increment began with a 2% biochar application rate in seven soils, which had a pH ≤ 6.2 (Poděbrady, Hněvčeves, Červený Újezd, Lhota, Humpolec, Žamberk, and Lukavec). A pH rise of up to 1.17 units was observed in these seven soils. For the remaining three soils with pH > 6.2 (Malín pH = 7.1, Suchdol pH = 6.9, and Kbely pH = 7.01), 8% biochar induced a significant increase at both incubation periods, and a 4% biochar addition also increased pH in these soils in some cases. In these three neutral soils, the addition of the highest amounts of biochar (8%) increased soil pH only to a maximum of 0.4 units. We have also calculated the theoretical expected increment of pH by considering the percentage of biochar added to the incubated soils and comparing with the effect of tested biochar in our incubation experiment. The results revealed that the calculated pH of the soil–biochar mixture was in most cases lower than the actual pH, with the variation reaching up to 0.69 units.

Table 2 Effect of biochar on soil pH
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3.2 Effect of biochar on soil CEC

Both a significant increase and a decrease in the CEC of incubated soils were observed following the addition of biochar to soils. The “break point” of the change in the direction of biochar effect on soil CEC was identified (Fig. 1). Biochar addition increased the CEC of seven soils (Poděbrady, Hněvčeves, Červený Újezd, Lhota, Humpolec, Žamberk, and Lukavec), which all had a lower original content of exchangeable Ca2+ compared with biochar. In these seven soils, the relative increment of CEC compared with the control treatment ranged between 0.2 and 35.4%. This group of soils was characterized by lower CEC, lower exchangeable Ca2+, and lower pH relative to the remaining three soils. The addition of 0.5 and 2% biochar induced a considerable change only in a few cases, and the higher biochar rates (4 and 8%) induced a significant increase in all seven soils.

Fig. 1
figure 1

Effect of elevated biochar rates on soil CEC (mmol(+) kg−1) at the 12th week of incubation and interaction with the original content of soil exchangeable Ca2+ (mmol kg−1). CEC of all soils after biochar addition with significance difference is stated at Table S1 (ESM); the vertical line indicates the exchangeable Ca2+ content of added biochar (mmol kg−1); indicates “break point,” i.e., the soil original exchangeable Ca2+ content, where biochar-induced increment in soil CEC is altered toward decline for this particular biochar

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A decline in CEC was observed in soils that had a higher original exchangeable Ca2+ content than the biochar, namely the Malín, Suchdol, and Kbely soils. The relative decline of soil CEC compared with the control treatment in these three soils varied from 0.2 to 7.9%. Application of 0.5% biochar did not induce a significant CEC decrease in all soils (Table S1, Electronic Supplementary Material—ESM). The biochar addition rate of 2% induced significant CEC decreases in Malín and Suchdol soils at 12 weeks of incubation, whereas 4% caused a substantial decrease in Malín and Kbely soils at both incubation periods, and in Suchdol at week 12. However, the addition of 8% biochar significantly decreased soil CEC in all three soils at both incubation periods.

We also calculated the change in CEC according to the amount (percentage) of biochar added and the properties of soil mixture incubated in order to compare with the biochar effect in soil environments. This figure was higher than the actual CEC recorded from our incubation in all soils, with a difference ranging up to 25%.

3.3 Effect of biochar on soil exchangeable cations (Ca2+, K+, and Mg2+)

The biochar addition showed the break point in the magnitude of biochar-induced change in the exchangeable Ca2+ content of soils (Fig. 2). An increment in exchangeable Ca2+ was observed in seven soils (Poděbrady, Hněvčeves, Červený Újezd, Lhota, Humpolec, Žamberk, and Lukavec) at 4 and 8% biochar application rates. These seven soils had a lower original exchangeable Ca2+ content relative to the biochar. A decrease in exchangeable Ca2+ was found in three soils (Malín, Suchdol, and Kbely) that had a higher original exchangeable Ca2+ content than the biochar.

Fig. 2
figure 2

Effect of elevated biochar rates on the content of soil exchangeable Ca2+ (mmol kg−1) at the 12th week of incubation and interaction with the original content of soil exchangeable Ca2+ (mmol kg−1). Exchangeable Ca2+ content of all soils after biochar addition with significance difference is stated in Table S1 (ESM); the vertical line indicates the exchangeable Ca2+ content of added biochar (mmol kg−1); indicates “break point,” i.e., the soil original exchangeable Ca2+ content, where biochar-induced increment in soil CEC is altered toward decline for this particular biochar

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The decline in soil exchangeable Ca2+ content was up to 11.4% relative to the control at a biochar application rate of 8%. This decline was not significant in most cases for 0.5 and 2% biochar additions, whereas 4 and 8% biochar addition rates induced a considerable decline in all three soils (Table S2—ESM). On the other hand, a significant increment was observed in four soils (Poděbrady, Humpolec, Žamberk, and Lukavec) ranging up to 38.6% relative to the control treatment at the 8% biochar rate. In these four soils, the exchangeable Ca2+ increment was at 4% biochar rate in all soils except for soil Humpolec, whereas the increment was observed only in some cases for 0.5 and 2% biochar addition rate. The exchangeable Ca2+ contents of the remaining three soils (Hněvčeves, Červený Újezd, and Lhota) were unaffected by biochar additions. These three soils had similar contents of exchangeable Ca2+ as that of the added biochar.

The addition of biochar was able to increase exchangeable K+ content in all soils except some in the 0.5% biochar addition rate treatment. At 0.5% biochar addition, the effect was significant in all soils at least at one of the incubation periods, except for the Malín and Humpolec soils (Table 3). The biochar addition rate of 2% induced significant K+ content increases in all soils except for Humpolec after only 1 week of incubation. Biochar additions of 4 and 8% substantially increased the exchangeable K+ of all soils at both incubation periods. The proportion of biochar-induced increment relative to the control was up to 242% at a biochar addition rate of 8% in soil with an acidic pH (pH = 4.8) and with very low exchangeable K+ content (1.6 mmol kg−1) as compared with other soils. The increment in exchangeable K+ was as low as 0.7%, and even declined in two cases of the 0.5% biochar application rate in Suchdol and Malin soils with neutral pH (pH = 6.9 and 7.0, respectively). Biochar-induced decreases in exchangeable K+ occurred in only two cases at the 5% biochar application rate: in Malin soil, a decrease of up to 5.4% after 1 week of incubation, and in Suchdol soil, a decrease of up to 1.6% after 12 weeks of incubation. The effect of biochar application on soil exchangeable Mg2+ content was inconsistent and insignificant in most incubated soils (Table S3—ESM). A significant increment was only observed in two soils (Žamberk and Lukavec), both of which were characterized by very low original exchangeable Mg2+ contents, were acidic, and had low CEC. This increment ranged up to a maximum of 63% compared with the control treatment at the 8% biochar addition rate.

Table 3 Effect of biochar addition on exchangeable K+ content of soil
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3.4 The interrelationships of the determined soil characteristics

Based on our results from the multivariate analysis of variance (Table 4), all factors evaluated (biochar rate, period of incubation, and soil type), and in many cases their interactions, significantly affected soil pH and exchangeable Ca2+ (p < 0.05). With respect to CEC, exchangeable K+, and exchangeable Mg2+, all the evaluated factors and their interactions were significant (p < 0.05).

Table 4 Multivariate analysis of variance in soil pH, CEC, and exchangeable Ca2+, K+, and Mg2+ with interaction and single effect of factors
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Pearson correlations among original soil properties with the amount of change in soil pH, percentage of change in CEC, and exchangeable Ca2+, K+, and Mg2+ relative to the control after 12 weeks of incubation were calculated (Table 5). The amount of biochar-induced change in soil pH relative to control soil was significantly and negatively correlated with the soil’s original pH, CEC, exchangeable Ca2+ content, and percentage of soil clay fraction. The amount of biochar added had a positive significant correlation with the percentage of soil sand fraction.

Table 5 Pearson’s correlation coefficients among percentage of 8% biochar rate induced changes (CEC, exchangeable cations, and pH values) relative to control at the 12th week of incubation and soil original properties
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The biochar-induced decline (%) in soil CEC and exchangeable Ca2+ at 12 weeks of incubation behaved in the same way and both parameters were negatively correlated with the original soil pH, CEC, exchangeable Ca2+ and Mg2+ content, and the percentage of soil clay fraction. In the case of K+, the biochar rate was negatively correlated with soil exchangeable K+ and Mg2+ content, and with the percentage of soil clay fraction. Exchangeable Mg2+ was negatively and significantly correlated only with the initial soil exchangeable Mg2+ content and the percentage of soil clay fraction.

The regression analysis for pH including all ten soils was used to model equations integrating the biochar addition rate and other soil parameters, which have a significant regression coefficient (Table 6). All original soil properties included were significant (p < 0.01). The whole model of soil pH was significant (p < 0.001) and could explain 97% of the variability in soil pH after the addition of biochar.

Table 6 Regression model for pH including soil parameters and biochar addition rate at 12th week of incubation, regression coefficients are displayed
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Another model for CEC and exchangeable Ca2+ was constructed after grouping our soils into two groups. One group contained soils with a lower exchangeable Ca2+ content than added biochar (Table 7), and the other contained soils with a higher exchangeable Ca2+ than the added biochar (Table 8). The entire model was significant (p < 0.001) and explained more than 99% of the variability in both CEC and exchangeable Ca2+. In both cases, soil properties without a significant effect were excluded from the model. Moreover, the biochar effect on soil exchangeable K+ and Mg2+ was not presented due to the insignificance of the whole model.

Table 7 Regression model for CEC and exchangeable Ca2+ including soil parameters and biochar addition rate at 12th week of incubation for seven soils, which have lower exchangeable Ca2+ than added biochar; regression coefficients are displayed
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Table 8 Regression model for CEC and exchangeable Ca2+ including soil parameters and biochar addition rate at 12th week of incubation for three soils, which have higher exchangeable Ca2+ than added biochar; regression coefficients are displayed
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4 Discussion

4.1 Effect of biochar on soil pH

The addition of biochar effectively increased soil pH (Table 2). Further, pH increment clearly depended on soil type and biochar application rate. In acidic to slightly acidic soils, a significant increment of pH began from a 2% biochar addition rate, while 8% biochar rate induced a significant pH increment in all soils. The increment of soil pH due to biochar addition is consistent with findings in other studies (Jien and Wang 2013; Masud et al. 2014; Wang et al. 2014; Gul et al. 2015; Hilioti et al. 2017; Kameyama et al. 2017; Al-Wabel et al. 2018; Mandal et al. 2018). For example, Jien and Wang (2013) reported an increment in soil pH after the addition of 2.5%, and 5% (w/w) biochar (from 3.9 in the control samples up to 5.1 after the addition of 5% biochar) produced from waste wood of white lead trees (Leucaena leucocephala). The increment in soil pH of up to 0.4 units (from 7.3 up to 7.7) was also reported after the addition of poplar leaf–based biochar produced at 650 °C (Bai et al. 2017). One of the mechanisms for the pH increment after biochar addition is likely due to the considerable amount of ash and base cations in biochar. This mechanism is evidenced by Li et al. (2018), where high-temperature biochar with high pH increased the pH of soils with its considerable ash content. The alkalinity of biochar, and the subsequent release of base cations, especially Ca2+ and K+, and the replacement of soil exchangeable Al3+ and H+ by these cations on the soil’s negatively charged sites could greatly increase soil pH (Masud et al. 2014). In addition, negatively charged functional groups (e.g., phenolic, carboxylic, and hydroxyl) present at the surface of biochar could also contribute to the increment of soil pH by binding the surplus H+ ions present in the soil solution (Gul et al. 2015). Another possible mechanism is the decarboxylation of organic anions initiated by biochar addition, owing to increasing attack of organic anions by microbes, which may consume surplus H+ from the soil solution, and thus increase soil pH (Wang et al. 2014). Based on the multivariate analysis of variance (Table 4), the effect of soil type on the amount of biochar-induced pH rise was greater than the effect of incubation period, biochar addition rate, or their interaction effect. Our correlation analysis (Table 5) also confirmed the dependence of biochar effectiveness on original soil properties. Overall, the results suggest that the effectiveness of biochar amendment for increasing soil pH is greater in soils with low pH, CEC, exchangeable Ca2+ content, and clay fraction, and in soils with a higher sand fraction. This is due to the greater buffering capacity of soils with higher CEC than soils with lower CEC (Xu et al. 2013). The low increment in soil pH in soils with high clay contents is similarly due to the high buffering capacity of clay soils compared with sandy soils (Jones Jr 2012). Our regression model (Table 6) is applicable for testing the effect of biochars with similar properties (high-temperature wood biochar produced at 700 °C) on soil pH.

4.2 Effect of biochar on soil CEC and exchangeable cations (Ca2+, K+, Mg2+)

The multivariate analysis of variance (Table 4) indicated that biochar application rate and the interaction of biochar rate with both soil type and incubation period induced significant differences in soil CEC and exchangeable Ca2+ content. There was also clear evidence that the exchangeable Ca2+ content of soil was the most consequential factor for determining the magnitude of biochar-induced changes to soil CEC and exchangeable Ca2+. We found no studies supporting or contradicting this generalization. However, the base of this generalization is that the increment in soil CEC and exchangeable Ca2+ was in seven soils (Poděbrady, Hněvčeves, Červený Újezd, Lhota, Humpolec, Žamberk, and Lukavec), which had higher exchangeable Ca2+ content than the biochar added. The more general finding that increment in soil CEC and exchangeable Ca2+ after biochar addition is in agreement with previous studies (Jien and Wang 2013; Chintala et al. 2014; Martinsen et al. 2015; Hilioti et al. 2017; Al-Wabel et al. 2018; Cornelissen et al. 2018). The primary mechanisms for biochar additions increasing CEC are likely mediated through the greater surface area, negative surface charge, and charge density of biochar (Liang et al. 2006; Li et al. 2018) and the release of surplus Ca2+ content from biochar, which accounted 176 mmol kg−1 (Table 1). The presence of oxygenated (acid) functional groups on biochar surfaces can also increase soil CEC (Glaser et al. 2003; Sohi et al. 2010). Another potential mechanism is the adsorption of oxides (Al and Fe) by biochar and the subsequent decline in soil point of zero charge (pzc) leading to an increase in soil CEC (Trakal et al. 2016). In addition, the increment in pH of acidic and weakly acidic soils after the addition of biochar could result in the deprotonation of functional groups from minerals, such as kaolinite, resulting in the development of more negative charges that contribute to the development of higher CEC (Sparks 2003). A decrease in CEC and exchangeable Ca2+ was observed in three soils (Malín, Suchdol, and Kbely), each of which had lower exchangeable Ca2+ content than the added biochar. A decline in soil CEC was reported previously. Prommer et al. (2014) observed a decline of soil CEC from 225 to 208 mmol(+) kg−1 after application of biochar (produced at 500 °C with pH value of 7.5 and an unreported CEC) to soil with a pH of 7.5 and CEC of 225 mmol(+) kg−1. In our soils, the first cause for the decline in CEC and exchangeable Ca2+ content of the three soils (Malín, Suchdol, and Kbely) likely arose from their high exchangeable Ca2+ contents. Thus, the release of surplus exchangeable Ca2+ from biochar into the soils, which already had high exchangeable Ca2+ and organic matter contents, possibly led to the formation of aggregates between exchangeable Ca2+ and soil organic matter. Consequently, this formation of aggregates can lead to reductions of both CEC and exchangeable Ca2+ (Clough and Skjemstad 2000). This mechanism is in agreement with the report of Glaser et al. (2002), where they reported the binding of highly available calcium with soil organic matter (SOM). In another study, the reduction of Ca2+ extractability in soils containing large amounts of calcium was reported and resulted from Ca2+–organic matter bridging (Shen 1999). The presence of high Ca2+ content in soil may also protect biochar from oxidation and decomposition (Clough and Skjemstad 2000). A second mechanism for the decrease in CEC of these three soils (Malín, Suchdol, and Kbely) could be their high organic matter content. SOM, which is a rich source of surface negative charges, could be adsorbed by biochar resulting in a decline of exchange sites on both SOM and biochar surfaces (Clough and Skjemstad 2000). This phenomenon was reported as a possible mechanism for the decline of soil CEC and exchangeable Ca2+ (Kwon and Pignatello 2005). Third, the humic and fulvic acids from organic matter-rich soils could block the inner pores of biochar, rendering them inaccessible for further physical adsorption and thus reducing CEC (Pignatello et al. 2006). Based on the Pearson correlation coefficients (Table 5), the effectiveness of biochar to increase soil CEC and exchangeable Ca2+ was high in soils with lower original exchangeable Ca2+, pH, CEC, clay fraction, and exchangeable Mg2+. The final content of soil exchangeable Ca2+ and soil CEC was estimated based on our regression model (Table 7 and Table 8). This model provides a first approximation of what to expect after the addition of biochars with different properties. In our model, we clearly presented which model should be used for which type of soil. Here, we would like to emphasize that it is important to use a specific model for soils with differing exchangeable Ca2+ contents.

Results from the multivariate analysis (Table 4) indicated that the paramount factor affecting exchangeable K+ content was soil type followed by the biochar application rate. This could simply indicate that there was considerable variability in the K+ content between incubated soils, and that biochar also significantly affects the overall soil exchangeable K+ content. Generally, biochar was effective at increasing the content of soil exchangeable K+ regardless of the soil properties. This was likely due to the high content of exchangeable K+ in our biochar, which was almost five times higher than the content of soil exchangeable K+ and its direct release into the soil. A high availability of K (even up to 80% of total K) was reported in biochar derived from manure, crop residue, and municipal waste (Zornoza et al. 2016). Similar results were reported by Kongthod et al. (2015), where cassava (Manihot esculenta) stem–based biochar-amended soils released up to 148 mg kg−1 of K after 7 days of incubation, and rice husk biochar treatments released up to 188 and 186 mg kg−1 of K after 1 and 3 days of incubation, respectively. An increment of soil K content following the application of peanut (Arachis hypogaea) hull and pine (Pinus spp.) chip biochar to the top soil was reported (Gaskin et al. 2010). In an additional study, an increment in K content of soil from 39.3 to 48.5 mg kg−1 was reported following the application of Conocarpus spp. wood waste biochar at the rate of 10 mg kg−1 (El-Naggar et al. 2015). Regarding factors controlling the amount of increment in soil exchangeable K+, the Pearson correlation (Table 5) revealed the negative contribution to soil initial exchangeable cations and clay fraction on the increase of soil exchangeable K+ after biochar application. The low effectiveness of biochar to increase soil exchangeable K+ in soils with high proportions of clay could be due to the fixation or trapping of K+ between the layers of clay minerals. The high rate of K fixation in soils with relatively high activities of K is in agreement with finding of Matthews and Sherrell (1960). On the other hand, following the addition of biochar with K+, higher increments of K+ in soils with relatively lower original K+ compared with the increment in soils with higher original K+ content can be expected.

Although no effect of biochar was observed on exchangeable Mg2+ content in most soils (Table S3—ESM), the multivariate analysis of variance (Table 4) did indicate a significant effect of biochar application rate on exchangeable Mg2+ content. However, the higher variability in soil exchangeable Mg2+ content arises due to soil type and period of soil incubation. Here, the results indicated an effect of biochar on soil exchangeable Mg2+ content. The multivariate analysis of variance considers the overall effect by taking the average of all soils and summing it. Focusing on individual soils, the increment of exchangeable Mg2+ was significant in only two soils with very low original exchangeable Mg2+ contents. This suggests that Mg was released from biochar when in soils with relatively low available Mg content. These findings accord with the report of Wu et al. (2011). Similarly, the application of peanut hull biochar increased soil Mg content (Gaskin et al. 2010). The release of Mg in acidic soils from biochar was also reported (Xu et al. 2013). Other studies have shown insignificant change in soil Mg content following the addition of biochar produced from a mixture of hardwood (primarily oak, Quercus spp. and hickory, Carya spp.) (Laird et al. 2010). Even if the substantial increment in soil exchangeable Mg2+ was significant only in two soils, the percentage of change relative to the control was significantly correlated with soil initial exchangeable Mg2+ content and the clay fraction of soils (Table 5). This implies that the initial exchangeable Mg2+ and clay fraction comprise the main limiting factors for biochar-induced change in soil exchangeable Mg2+ content. This principle is identical with exchangeable K+ content in soils. The amount of Mg2+ added from biochar (containing certain amounts of Mg) will be high in soils with relatively lower original exchangeable Mg2+ than in soils with higher original exchangeable Mg2+ content. In addition, the low increment of exchangeable Mg2+ in soils with high amounts of clay could be due to the trapping of Mg2+ between the layers of clay minerals.

5 Conclusions

The main findings of this study investigating a wide range of soils with different properties treated with elevated rates of high-temperature pyrolysis biochar (700 °C) can be summarized as follows: (i) Greater biochar addition yields a greater rise in pH and 8% biochar addition is the most effective rate. The increment is higher in soils that have relatively low original pH, CEC, exchangeable Ca2+ content, and clay percentage. (ii) The addition of the same type of high-temperature produced biochar to different types of soils can result in both a decrease and an increase of soil CEC and soil exchangeable Ca2+ content, where the effect differs according to specific soil properties. The percentage of biochar-induced changes in both CEC and exchangeable Ca2+ is high in soils with low initial pH, CEC, exchangeable Ca2+ and Mg2+, and a low percentage of clay. (iii) The addition of biochar increases exchangeable K+ content in a range of soils, where the size of increment is higher in soils with lower original contents of soil exchangeable K+ and Mg2+, and lower percentage of clay.

Our findings clearly demonstrate that knowledge of biochar properties as well as soil properties is useful for predicting the efficiency of biochar application to alter soil properties. Among soil properties, soil exchangeable Ca2+ content is the salient factor controlling the magnitude of biochar-induced changes to soil CEC and exchangeable Ca2+ content.