Environmentalists around the globe have been searching for ways to reverse the environmental destruction by putting back coal into the soil which to the date has been dug up from the earth in massive quantities. The pyrogenic compound biochar, when applied to soil, acts as a carbon sink for the atmospheric CO2 and as a carbon source for the soil. Studies have shown that the application of biochar in the soil can reduce global greenhouse gas emissions by 12%. Thus, it is important to understand how does biochar influence soil characteristics?
The physiochemical characteristics of biochar are responsible for changes in soil microbial activities. Attributed to its highly porous structure, biochar acts as a shelter for soil microbes. There are several ways in which these properties can help achieve agricultural sustainability when integrating the soil with biochar.
Thermal decomposition during pyrolysis of biomass results in free-electron generation on biochar and these electrons when transferred to soil microbes acts as a medium to destroy the soil contaminants. Various techniques are used to identify the interaction of biochar with soil microbes. These techniques include such as Electron microscopy, ergosterol extraction, denaturing gradient gel electrophoresis, etc.
Different soil varieties react differently with all the biochar. During pyrolysis, the biochar is optimized according to the characteristics of the soil such as pH, nutrient availability, etc. To increase the specific surface area of biochar, it is chemically oxidized using various oxygen containing functional groups such as –OH, -COOH, HNO3, etc. The selection of any functional group depends on the type of contaminants to be destroyed from the soil. For example, if the soil has high lead contaminants in it, the biochar to be utilized is oxidized using H3PO4 to obtain maximum results. The presence of phosphate in H3PO4 increases the specific surface area of biochar which is further attributed to the increased adsorption capacity of biochar toward the lead.
The eradication of heavy metals from the soil is performed in two ways; first by adsorption of heavy metals in the pores of biochar and second by ion exchange method between biochar and heavy metals. Ion exchange between biochar and heavy metals transforms the latter into low-valent states, thus decreasing the toxicity. In addition to this, biochar affects the soil quality by changing the enzyme activity in the soil. The uncharged surface of biochar reacts with the uncharged surface of protein by non-columbic force and this attributes to the sorption of enzymes on the biochar surface. As a result of this, the activation energy of enzymes reduces which further increases carbon sequestration in the soil.
Further, the integration of biochar with inorganic fertilizers also results in improved crop yield in the soil under consideration. A study done on two types of biochar produced at different temperatures shows about the same increase in yield of Radish farm by about 42% to 96%. However, the same biochar showed an additional increase in the yield when applied on the farm after integrating with inorganic nitrogen fertilizers. The pyrogenic nature of biochar allows it to act as a carbon sink in the soil and the carbon present in it does not react with other compounds in the soil to emit CO2. This attributes to a very slow rate of microbial decomposition and chemical transition of biochar.
To further examine the effect of biochar on soil water holding capacity, texture, particles, etc. several studies are ongoing to quantify the microstructure of biochar and compare the porosity with the soil under consideration. The complex aromatic and graphitic nature of biochar attributes to its high resistant capacity to environmental stresses and this allows it to remain inland for more than 5000 years. It has also been found that at different pyrolysis temperatures biochar with varying physicochemical properties such as crystalline structure, cation exchange capacity, pH, etc. can be obtained. As a result, to achieve maximum soil fertility, quality, and yield it has become easier to modify these characteristics according to the soil under the application.
The massive quantity of stubble burnt every year across the world by the farmers can be used to produce biochar, thus reducing the atmospheric CO2. Many studies have addressed challenges occurring in adopting this technology. Some of these challenges include the cost incurred, transportation and storage of crop residue, quantification of biochar structure and comparing it to different soil quality, optimizing the conditions of the pyrolysis process, and completely understanding the process by which biochar ameliorates Nitrogen content, and retains nutrient leaching in the soil. Several research studies have examined the efficacy of biochar on crop yield when applied with inorganic fertilizers.