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\begin{document}

\title{Cavitated charcoal -- an innovative method for improving the biochemical
properties of soil}



\author[1]{Krzysztof Gondek}%
\author[1]{Monika Mierzwa-Hersztek}%
\author[2]{Wojciech Grzymała}%
\author[1]{Tomasz Głąb}%
\affil[1]{Uniwersytet Rolniczy im. Hugona Kollataja w Krakowie}%
\affil[2]{BirkoProject}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
The study was carried out with cavitated charcoal that were introduced
into loamy sand and clay at rates of 1.76\%, 3.5\%, 7.0\%, and 14.0\%.
The highest introduction rate of the material (14.0\%) increased the
content of total carbon (CTot) by 197\% in the loamy sand and by 19\% in
the clay compared to that in the control treatments. The application of
cavitated charcoal did not significantly change the total content of
heavy metals. Regardless of the element and the soil used, the
application of cavitated charcoal reduced the content of the
CaCl2-extracted forms of heavy metals. Following the application of
cavitated charcoal, the loamy sand soil presented an even lower content
of the most mobile forms of the studied elements. It should also be
noted that regardless of the soil texture, mobile forms of the elements
decreased with the increased cavitated charcoal rate. The respiratory
activity values of the soils into which cavitated charcoal was
introduced were low, which indicates a large number of dormant
microorganisms. Additionally, the results of dehydrogenase and urease
activity indicated the low metabolic activity of the microbial
population in the soils, especially with the relatively high rates
(7.0\% and 14.0\%) of cavitated charcoal. However, the cavitated
charcoal used in the study showed a significant, positive effect on the
amount of biomass Sorghum saccharatum (L.), and its application
significantly reduced the heavy metal content in the biomass of Sorghum
saccharatum (%
\end{abstract}%



\sloppy


\textbf{Abstract}

Thermal biomass transformation products are considered to be one of the
best materials for improving soil properties. The use of cavitated
charcoal is not described in the literature. The study was carried out
with 10\% aqueous charcoal mixture that were introduced into loamy sand
and clay at rates of 1.76\%, 3.5\%, 7.0\%, and 14.0\%. The application
of cavitated charcoal reduced the acidification of both soils. The
highest introduction rate of the material (14.0\%) increased the content
of total carbon (C\textsubscript{Tot}) by 197\% in the loamy sand and by
19\% in the clay compared to that in the control treatments. The
application of cavitated charcoal did not significantly change the total
content of heavy metals. The effect of the application of cavitated
charcoal on the bioavailability of heavy metals was tested after
extracting the most mobile forms of Cu, Cd, Pb, and Zn with a 0.01 mol
dm\textsuperscript{-3} solution of CaCl\textsubscript{2}. Regardless of
the element and the soil used, the application of cavitated charcoal
reduced the content of the CaCl\textsubscript{2}-extracted forms of
heavy metals. Following the application of cavitated charcoal, the loamy
sand soil presented an even lower content of the most mobile forms of
the studied elements. It should also be noted that regardless of the
soil texture, mobile forms of the elements decreased with the increased
cavitated charcoal rate. The respiratory activity values of the soils
into which cavitated charcoal was introduced were low, which indicates a
large number of dormant microorganisms. Additionally, the results of
dehydrogenase and urease activity indicated the low metabolic activity
of the microbial population in the soils, especially with the relatively
high rates (7.0\% and 14.0\%) of cavitated charcoal. However, the
cavitated charcoal used in the study showed a significant, positive
effect on the amount of biomass \emph{Sorghum saccharatum} (L.), and its
application significantly reduced the heavy metal content in the biomass
of \emph{Sorghum saccharatum} (L.).

\textbf{Keywords:} cavitation, charcoal, soil, enzymatic activity,
plant, heavy metals

1. \textbf{Introduction}

The appropriate use of waste biomass is one of the ways to achieve
sustainable agriculture in the 21\textsuperscript{st} century. This
approach will help create a product capable of improving soil
characteristics in terms of its chemical and biological properties, as
well as the quantity and chemistry of obtained plant biomass (Glaser,
2007). Significant amounts of waste biomass are generated in the world,
and the instability of this waste makes its transformation often a key
problem for the environment. The product of thermal biomass
transformation is charcoal with properties similar to biochar. Solid
products of thermal biomass transformation, due to their multiple
specific properties, are often referred to as environmentally friendly
materials (Deenik et al., 2010; Oliveira et al., 2017). The beneficial
effects of these materials on soil water retention capacity, cation
exchange capacity, and improvement of soil function as a carbon
reservoir is noted here because of the high C content in these
environmentally friendly materials and their resistance to microbial
degradation (Cheng et al., 2008; Tan et al., 2017). Despite many studies
on the effect of charcoal and biochar on the environment, it was not
possible to clearly define the mechanisms of their action
(Dieguez-Alonso et al., 2018). The increased number of tests carried out
have proven that the effect of thermally transformed organic materials
on soil and plant properties is varied. Their effect is also conditioned
by, among others, the type of feedstock, production conditions, material
rate, location of tests and type of plant cultivated (Deenik et al.,
2010; O'Connor et al., 2018). Deenik et al. (2010) demonstrated that the
independent application of thermally transformed organic materials
worsened the conditions for plant growth and development, probably due
to limited nitrogen access and stimulation of soil microorganismal
growth. The study of Kloss et al. (2013) indicated a similar plant
response, especially in the first two years after the application of
biochar. In addition, these authors noted a decrease in the content of
some trace elements in plant biomass. According to Kloss et al. (2013),
even with additional mineral fertilisation, the use of thermally
transformed organic materials creates the risk of a short-term reduction
in plant growth and development.

Previous studies on charcoal and biochar were focused not only on
determining the structure and chemistry of these materials but also on
searching for new solutions aimed at enriching them with various
components and modifying their production process (Jassal et al., 2015;
Gondek et al., 2018). Given the properties of charcoal and the way it is
applied, as well as the resulting environmental problems, there is a
need to identify alternative production methods (form) or preparations
for use (Montanarella and Lugato, 2013). Due to the nature of the
cavitation process, it is possible to homogenise cavitated material with
the implosion of gas bubbles. In the case of hydrodynamic and acoustic
cavitation, cavitation bubbles are present in liquid as a result of
local ruptures of the continuous medium caused by high tensile forces.
These forces arise from local sudden pressure decreases that may occur
either in hydrodynamic processes or in a high-intensity ultrasonic field
(20 kHz -- 1 MHz) (Lenik and Ozonek, 2012). The cavitation process can
be modified by adding various types of chemical substances or by
changing the physical parameters of the process (Nakashima et al.,
2016).

The aim of the study was to assess the effect of charcoal after
cavitation on the chemical and biochemical properties of soil, as well
as on the quantity and chemistry of \emph{Sorghum saccharatum} (L.)
biomass.

\textbf{2. Materials and methods}

\emph{2.1. Cavitation of charcoal}

The study was carried out on charcoal obtained from an industrial
installation located in the Bieszczady Mountains (southeastern Poland).
The production process was performed using deciduous wood. Before
starting dry distillation, feedstock was ground and treated with drying
gases at 160-180 \selectlanguage{ngerman}°C. Then, the material was placed in a retort. After
closing the retort, the residual water evaporates, and the thermal
decomposition of the wood begins. It is initiated by hot gases of
250-300 °C, moving from the central part of the retort (annealing zone).
The process is exothermic. The organic mass of wood is charred. The
partially charred wood mass has a reduced volume; hence, it moves down
the retort and enters the annealing zone. Here, at 500-550 °C, the final
form of charcoal is formed (Lewandowski and Milchert, 2011).

The cavitation process was carried out in 10\% w/w aqueous charcoal
mixture (particle size of 0-0.5 mm). The suspension was placed in a tank
to which a 2.5 kW cavitation pump was connected. After passing through
the pump, the suspension was returned to the tank (closed circuit). The
micronisation time for 150 dm\textsuperscript{3} of the suspension was 2
hours, and the temperature increase during the cavitation process was
from 40 °C (initial stage) to 60 °C (final stage).

\emph{2.2. Chemical composition of charcoal after cavitation}

To characterize the properties of the charcoal suspension after
cavitation (CHAR-C), the following properties were determined in the
material: electrical conductivity -- conductometrically, pH --
potentiometrically, and the content of N-NH\textsubscript{4} and
N-NO\textsubscript{3} -- using ion selective electrodes. The contents of
macroelements (P, K, Ca, and Mg) and trace elements (Cu, Fe, Mn, and Zn)
were determined after mineralising the sample in a mixture of
concentrated HNO\textsubscript{3} and HClO\textsubscript{4} (3:2). The
contents of the studied elements were determined in the obtained
solutions by inductively coupled plasma optical emission spectrometry
(ICP-OES, Perkin Elmer Optima 7300 DV, Waltham, USA) (Oleszczuk et al.
2007). The material chemical composition is presented in Table 1.

\emph{2.3. Growth experiment}

The experiment was carried out on two soil types: loamy sand (LS) and
clay (C) collected from the 0-0.2 m layer in southern Poland. The
properties of the soils used in the experiment are presented in Table 2.

The growth experiment was conducted in containers with a capacity of 500
g of soil dry mass. The experimental scheme was the same for LS and C
soils and included the following treatments: the control with no
cavitated charcoal (LS-0; C-0), a treatment with a 1.76\% addition of
cavitated charcoal (LS-1; C-1), a treatment with a 3.5\% addition of
cavitated charcoal (LS-2; C-2), a treatment with a 7.0\% addition of
cavitated charcoal (LS-3; C-3), and a treatment with a 14.0\% addition
of cavitated charcoal (LS-4; C-4). After introducing cavitated charcoal,
distilled water (up to 45\% water capacity) was added to the soils and
thoroughly mixed with the material. Subsequently, soils with and without
the addition of charcoal were placed in PVC containers. After 24 hours,
seeds of \emph{Sorghum saccharatum} SOS~101116 were sown. During plant
growth, the soil moisture content was maintained at 60\% of the water
capacity. The experiment was carried out under laboratory conditions at
25±3 °C for 156 days. Then, plant aboveground biomass was collected, and
plant roots were separated from the soil block and thoroughly washed
with distilled water. The collected biomass was dried to a constant
weight at 105 °C, and then, the amount of biomass was determined. To
determine the chemical composition, the plant material was ashed in a
chamber furnace at 450 °C for 12 hours, and the residue was dissolved in
diluted (1:2) (v/v) nitric acid. The contents of the studied trace
elements were determined by inductively coupled plasma optical emission
spectrometry (ICP-OES, Perkin Elmer Optima 7300 DV, Waltham, USA)
(Oleszczuk et al., 2007).

\emph{2.4. Biochemical properties of soil}

The soil biological activity measurements included enzymatic activity
and soil respiration. Dehydrogenase activity (DhA) was determined by the
method in Thalmann (1968) using triphenyltetrazolium chloride (TTC) as
the electron acceptor. Samples were incubated at 37±2 °C for 24 hours.
The enzymatic activity of the dehydrogenases was determined by
colourimetry using a Backman DU 640 spectrophotometer (Beckman Coulter,
Woonsocket, USA) at a wavelength of 546 nm.

Urease activity (Ure) was determined by the Zantua and Bremner (1975)
method, with urea as a substrate, after a 18 hour incubation at 37 °C.
The enzymatic activity of ureases was determined by colourimetry using a
Backman DU 640 spectrophotometer at a wavelength of 470 nm.

Basal respiration (BR) was determined according to the ISO 16072
(International Organization for Standardization, 2002) method. Moist
soil subsamples (20 g) were incubated at 20±2 °C for 24 hours. The
released CO\textsubscript{2} was absorbed in a solution of 0.05 M NaOH
and precipitated as barium carbonate with the addition of a 0.5 M
BaCl\textsubscript{2} solution. The unconsumed sodium hydroxide was
titrated with 0.1 M HCl in the presence of phenolphthalein as an
indicator, and the amount of CO\textsubscript{2} was calculated (ISO
16072: 2002). Substrate-inducted respiration (SIR) was determined by the
quantification of carbon dioxide evolution shortly (6 hours) after
glucose addition (aqueous solution equivalent to 10 g glucose per kg of
soil) (ISO 14240-1: 1997). The respiratory-activation quotient (QR) was
calculated by dividing the BR rate by the SIR rate according to ISO
17155 (2012).

\emph{2.5. Chemical properties of soil}

In 1 mm of dried, sieved soil samples, the following parameters were
determined: pH -- potentiometrically, in the suspension of soil and
water as well as soil and 1 mol\selectlanguage{english}[?]dm\textsuperscript{-3} solution of KCl
(soil:solution = 1:2.5) and electrical conductivity (EC) --
conductometrically. The total carbon (C\textsubscript{Tot}) and total
nitrogen (N\textsubscript{Tot}) contents were determined with a CNS
analyser Vario MAX Cube (Elementar, Langenselbold, Germany) (Elementar
Analysensysteme, 2013). Bioavailable trace elements were extracted from
the soil for 2 hours with a 0.01 mol~dm\textsuperscript{-3} solution of
CaCl\textsubscript{2} (soil:solution = 1:10) (Houba et al., 2000). In
the obtained extracts, the studied elements were determined by
inductively coupled plasma optical emission spectrometry (ICP-OES,
Perkin Elmer Optima 7300 DV, Waltham, USA) (Oleszczuk et al., 2007).

\emph{2.6. Statistical analysis}

The experiment was carried out in three replicates. An analysis of
variance for a randomized design was performed using the statistical
software package Statistica v. 13.3 (StatSoft Inc., Tulsa, OK, USA)
(Table S1 in Supplementary Materials). The distribution of the data was
checked for normality using the Shapiro-Wilk test. The homogeneity of
variance was checked using Levene's test. The multiple means comparison
was conducted using a Duncan test with a P\textless{}0.05 level of
significance. Pearson's correlation analysis was used to analyse the
correlation between the CHAR-C rate and biochemical parameters and
biomass productivity (Table S2). For data where significant values of
correlation coefficients (\emph{r} ) were observed, the results of the
linear regression models are presented in graphical form, together with
regression equations and coefficients of determination
(\emph{R\textsuperscript{2}} ).

\textbf{3. Results and discussion}

\emph{3.1. Properties of cavitated charcoal and soils}

The cavitated charcoal (CHAR-C) used in the study had an alkaline pH and
high electrical conductivity (EC). The content of macronutrients, e.g.,
calcium and potassium, varied in the studied material (Table 1). The
content of heavy metals was lower than that observed in biochar produced
from plant biomass (Gondek et al., 2017). The LS and C soils used in the
study had different granulometric compositions and chemical properties
(Table 2). Both soils were characterised by the natural content of heavy
metals examined in this study.

\emph{3.2. Effect of the addition of cavitated charcoal on selected
chemical properties of soils}

The application of CHAR-C reduced the acidification of the LS and C soil
(Table 3). Due to the greater buffering of the C soil than of the LS
soil, the efficiency of the deacidification effect of CHAR-C was lower.
According to the literature, the application of charcoal in soil reduces
soil acidification as a result of active compounds and alkaline elements
accumulated in this material. The durability of the charcoal
deacidifying effect depends on soil and climatic conditions and, as
indicated by the published data, on the rate of alkaline cation leaching
to the deeper layers of the soil profile (Hardy et al., 2016;
Mastrolonardo et al., 2019).

An increase in the electrical conductivity (EC) value was noted in
proportion to the amount of CHAR-C introduced into the LS soil; however,
the parameter increase was significant only in the LS-4 treatment with
the highest CHAR-C rate (Table 3). Introduction of the 7.0\% (C-3) and
14.0\% (C-4) rates of CHAR-C into the C soil significantly increased the
EC values by 80\% and 109\%, respectively, compared to in the control
soil (C-0). Due to the lack of data in the literature on the effect of
cavitated charcoal on soil EC values, the obtained results were
referenced in terms of the effect of applied biochar to the soil on the
parameter value. The literature provides examples of increases in EC
values after the application of biochar. The EC increase was generally
proportional to the rate of the material used. This phenomenon results
from the release of salts from organic connections that easily pass into
the soil solution, increasing the pool of ionic forms that actively
conduct an electrical charge (Gondek and Mierzwa-Hersztek, 2016).

Significant differences in the total carbon (C\textsubscript{Tot})
content in the soils were found after using the same rates of CHAR-C
(Table 3). Considering only the smallest rate of CHAR-C introduced into
the LS soil, an over 40\% increase in the C\textsubscript{Tot} content
was noted, while this content in the C soil decreased by over 8\%
compared to that in the control. The highest rate of CHAR-C resulted in
a 197\% increase in the C\textsubscript{Tot} content in LS and a 19\%
increase in C compared to that in the control treatments. The trend in
the increased C\textsubscript{Tot} in this study is confirmed by the
literature on the charcoal effect on the carbon content in soil (Hardy
and Dufey, 2017; Mastrolonardo et al., 2019). However, it should be
noted that there are large discrepancies in the values of the increased
C\textsubscript{Tot} content in soils, which are probably dictated by
the type of charcoal used, its rate, and climatic conditions (Kerre et
al., 2016).

The total nitrogen content differed significantly but only between soils
(Table 3). No significant changes in the total nitrogen
(N\textsubscript{Tot}) content were noted after the application of
CHAR-C. The literature lacks information on the effect of the tested
product on the N content in soil. Hardy et al. (2016) did not report
significant differences in the nitrogen content between the soil
fertilised with charcoal and the control soil. Hirsch et al. (2017)
discovered even lower N content in soils into which charcoal was
introduced. However, it should be noted that charcoal contains a
nitrogen pool in a heterocyclic form that is practically inaccessible to
plants, creating the need to include fertilisation with mineral forms of
this element.

\emph{3.3. Effect of the addition of cavitated charcoal on the content
of selected heavy metals in soils}

Regardless of the soil (LS or C), the applied CHAR-C rates did not
significantly change the total contents of the studied heavy metals
(Table 4). Significant differences in the studied element contents were
dictated by the type of soil used.

The effect of the application of CHAR-C on the bioavailability of heavy
metals was tested after extracting the most mobile forms of Cu, Cd, Pb,
and Zn with a 0.01 mol\selectlanguage{english}[?]dm\textsuperscript{-3} solution of
CaCl\textsubscript{2} (Table 4). Regardless of the element and soil
used, the application of CHAR-C reduced the content of
CaCl\textsubscript{2}-extracted forms of heavy metals. Following the
application of CHAR-C, the LS soil presented a relatively low content of
the most mobile forms of the studied elements. It should also be noted
that, regardless of the soil, mobile forms decreased with an increased
CHAR-C rate.

Recent studies indicate that thermally transformed organic materials,
including charcoal and biochar, exhibit sorption capacity for heavy
metal ions (Miura and Shiratani, 2018; Mierzwa-Hersztek et al., 2019).
These materials have a large specific surface area, i.e., a surface on
which there are functional groups that are active in the sorbing of
heavy metal ions. The sorption capacity of charcoal and biochar is
determined, among others, by the concentration of heavy metals in the
soil solution. In unpolluted soils, the natural consequence of using
these materials is the reduction in the content of the most available
element forms (Miura and Shiratani, 2018; Mierzwa-Hersztek et al.,
2019). In our study, the increase in soil pH after the use of CHAR-C was
one of the important factors reducing the content of the most available
heavy metals (Gondek and Mierzwa-Hersztek, 2016).

\emph{3.4. Effect of the addition of cavitated charcoal on the value of
selected biochemical parameters of soils}

The respiratory activity of microorganisms in soil depends on their
physiological state and environmental conditions. Despite the large
variability in the soil parameters, such as humidity, pH, temperature,
availability of nutrients, and structure, it is believed that soil
respiratory activity is a very sensitive parameter closely related to
other soil biological properties (Han et al., 2007; Mierzwa-Hersztek et
al., 2018). Values of basal respiration (BR) determined in our study
differed significantly between the soils. The largest significant
increase in the BR value compared to that in the control was found in
treatments with 7.0\% (LS-3 and C-3) and 14.0\% (LS-4 and C-4) CHAR-C
(Table 5). Compared to the control treatments, in LS-3 and LS-4, the BR
increase was 38\% and 58\%, respectively, and in C-3 and C-4, it was
20\% and 19\%, respectively. Linear regression for the relationship
between BR and the CHAR-C rate is presented in Figure 4.

The addition of glucose to the analysed soil samples significantly
increased the respiration rate due to the presence of a source of easily
available carbon in the microorganism habitat. Additionally, it can be
concluded that there were no inhibitory substances in these soils or
they were in concentrations that did not inhibit the viability of
dormant microorganisms. Similar to BR, the largest increase in the SIR
parameter was determined in soils with the highest CHAR-C rates. In both
soils, the smallest rate of CHAR-C (LS-1, C-1) reduced the SIR value
(Table 5).

The respiratory activity coefficient quotient (QR) determined in the
study, which illustrates the number of dormant or active microorganisms,
ranged from 0.11 to 0.14 for the LS soil and from 0.11 to 0.22 for the C
soil (Table 5). As reported by Eisentraeger et al. (2000), a respiratory
activity coefficient ranging from 0.1 to 0.3 is low and indicates a
large amount of dormant microorganismal biomass as well as a low rate of
bioremediation with potential toxic substances in the soil. From this
point of view, a high BR value is most desirable for soil. Considering
the observations of the cited authors, the poorest microbiological
parameters were found in soils without CHAR-C (LS-0, C-0) and with 8.8
ml (LS-1, C-1) and 17.5 ml (LS -2, C-2) of CHAR-C.

Biological processes affecting soil fertility and productivity are
mainly associated with soil microbial activity, which translates into
the number of enzymes produced (Beheshti et al., 2018). Table 5 presents
the activity of dehydrogenases (DhA) associated with the transformation
of carbon compounds and ureases (Ure) involved in the transformation of
nitrogen compounds. The highest DhA and Ure activity occurred in the
control treatments. Regardless of the soil and rate, the application of
CHAR-C reduced the DhA activity (Figure 4). Compared to the control, the
treatment with the highest rate of CHAR-C (70 ml) reduced the DhA
activity by 57\% in the LS soil and by 24\% in the C soil.

The lowest CHAR-C rates (1.76\% and 3.5\%) significantly increased the
Ure activity in all soils. The application of 7.0\% and 14.0\% rates
drastically reduced the Ure activity in the LS soil, making the
parameter value equal to that determined in control soil. The Ure
activity in the C soil was also reduced under the influence of the same
CHAR-C rates, but the decrease was not as rapid.

Enzymatic activity is a very sensitive indicator of changes in soil
after applying various materials. The literature presents both positive
and negative effects of introducing thermally transformed organic
materials into the soil (Tian et al., 2016; Vithange et al., 2018;
Lammirato et al., 2011; Ameloot et al., 2013). The varied activity of
individual enzymes can occur for many reasons. The most commonly
indicated reasons in the literature are the content of C and N and the
relationship between these components. Equally important is the
available content of both elements (Paz-Ferreiro et al., 2012). One
should also remember that the introduced rate of material is of key
importance in maintaining optimal enzymatic activity in the soil. Our
studies clearly showed that higher CHAR-C rates had adverse effects on
both DhA and Ure activities. The cause could have been, inter alia,
increased EC values and limited availability of other nutrients. The
study results published by Mierzwa-Hersztek et al. (2019) indicated that
the interaction of many factors significantly affects the enzymatic
activity and microbiocenotic composition of soil.

\emph{3.5. Effect of the addition of cavitated charcoal on the amount
and content of selected heavy metals in} Sorghum saccharatum \emph{(L.)
biomass}

Significantly higher amounts of \emph{Sorghum saccharatum} (L.)
aboveground biomass compared to that in the control occurred in the LS
soil after applying 3.5\% (LS-2), 7.0\% (LS-3), and 14.0\% (LS-4) rates
of cavitated charcoal (Figure 1). For the C soil, the significant
increase in the amount of \emph{Sorghum saccharatum} (L.) aboveground
biomass occurred with the 7.0\% (C-3) and 14.0\% (C-4) rates. The
largest amounts of biomass collected in the LS-4 and C-4 treatments were
25\% and 145\% higher, respectively, compared to that collected in the
control treatments (LS-0 and C-0). The relationship between the
aboveground biomass and CHAR-C rate is presented in Figure 2.

The study revealed different trends in the amount of \emph{Sorghum
saccharatum} (L.) root biomass in the LS and C soils. For all CHAR-C
rates, the amount of \emph{Sorghum saccharatum} (L.) root biomass was
higher in the LS soil than in the control (Figure 3). On the other hand,
when compared to that in the control soil, the root biomass in the C
soil was lower after applying two lower rates (treatments C-1 and C-2)
and higher after applying two higher rates (treatments C-3 and C-4).

There are numerous accounts in the literature on the significant
fertilisation potential of thermally converted organic materials
(charcoal, biochar). Some studies also indicate the sorption capacity of
these materials in relation to nutrients. According to Yao et al.
(2012), thermally converted organic materials can effectively sorb
nitrate nitrogen, ammonium nitrogen or phosphates; however, the
properties of such materials, including pH, surface acid groups, and ion
exchange capacity, can have a great impact on the ability to adsorb
nutrients (Yao et al. 2012, Morales et al. 2013, Mierzwa-Hersztek et al.
2019). Our results indicate that in addition to improving soil
properties, there is also a risk of limiting the availability of
nutrients after applying charcoal or biochar to the soil. In practise,
it seems reasonable to combine charcoal or biochar applications with
mineral fertilisers that meet the nutritional needs of plants, at least
in the initial periods of their growth (Deenik et al. 2010). Although
there are only a few field studies on using biochar as a slow-release
fertiliser, many laboratory studies describe the application of biochar
in the context of nutrient availability. It is necessary to better
understand not only sorption but also desorption of nutrients because
these are processes that, together with mineralisation of nutrients,
control their concentration in soil solution. Consideration should be
given to factors affecting the desorption of nutrients, such as soil
types, feedstocks, conditions for thermal conversion, and organic
material rates.

The contents of the tested heavy metals in the aboveground biomass and
roots of \emph{Sorghum saccharatum} (L.) differed mainly due to the soil
type used (Table 6). The analysis of the contents of Cd, Pb, Zn, and Cu
in the aboveground biomass and roots of \emph{Sorghum saccharatum} (L.)
revealed that the heavy metal content was lower with a higher CHAR-C
rate in all soils (Figure 4). This trend was clearer for the aerial
parts. However, it should be noted that the significantly higher
contents of Cd, Zn, and Cu in \emph{Sorghum saccharatum} (L.) biomass in
the C soil than in the other soils resulted from the greater
acidification of that soil. The determined values of the tested heavy
metals in \emph{Sorghum saccharatum} (L.) reflected changes in their
availability in the soil.

\textbf{4. Conclusions}

Cavitated charcoal has great potential to deacidify soils and increase
the C content. The application of cavitated charcoal does not increase
the total content of heavy metals; however, it substantially reduces
their mobility. The determined values of the base respiratory activity
of the soils into which CHAR-C was introduced were low, which indicates
a large number of dormant microorganisms, as confirmed by the values of
induced respiratory activity. The obtained results of DhA and Ure
activity showed the low metabolic activity of the microbial population
in the soils, especially at high rates (7.0\% and 14.0\%) of CHAR-C. The
cavitated charcoal dust used in the study showed high-yielding
properties, and its application significantly reduced the heavy metal
content in the biomass of \emph{Sorghum saccharatum} (L.).

\textbf{Acknowledgements}

The research was financed by the Ministry of Science and Higher
Education of the Republic of Poland.

\textbf{Conflict of interest:} The authors declare that they have no
conflicts of interest.

\textbf{Data Availability Statement:} Data available on request from the
authors

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