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.