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.
3.5. Effect of the addition of cavitated charcoal on the amount
and content of selected heavy metals in Sorghum saccharatum (L.)
biomass
Significantly higher amounts of 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 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 Sorghum
saccharatum (L.) root biomass in the LS and C soils. For all CHAR-C
rates, the amount of 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 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 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 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 Sorghum saccharatum (L.) reflected changes in their
availability in the soil.