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 (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. 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-3 solution of CaCl2. 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 (L.).
Keywords: cavitation, charcoal, soil, enzymatic activity, plant, heavy metals
1. Introduction
The appropriate use of waste biomass is one of the ways to achieve sustainable agriculture in the 21st 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 Sorghum saccharatum (L.) biomass.