Long-term cultivation results in poor soil quality, decreasing fertility by depletion of organic matter and increased erosion. This has had a significant adverse effect on agricultural production. Researchers have focused on improving sustainable agricultural systems to help improve crop production and enhance weak rural economies. Biochar is considered a renewable and promising resource for soil fertility management.
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Composition and Development of Biochar
Biochar is a charcoal-like substance obtained by burning organic materials, i.e., forest waste (biomass) and agriculture, via a controlled process known as pyrolysis. Biochar is a black, highly porous, fine-grained, and lightweight substance with a large surface area. This compound is composed of 70% carbon (C).
Biochar also contains nitrogen (N), hydrogen (H), oxygen (O), calcium (Ca), potassium (K), and many other elements. The high surface area of biochar and the presence of several polar and nonpolar groups increases its affinity towards inorganic ions, such as heavy metal ions, phosphate, and nitrate.
The chemical composition of biochar varies based on the feedstocks used, the pyrolysis method, and the temperature used for its production. The carbon content increases with an enhancement in the pyrolysis temperature from 300 to 800 °C. However, an increase in temperature leads to a decrease in the other elemental content including N and H.
The process of pyrolysis involves burning organic materials, such as wood chips, leaf litter, or dead plants, in a container comprising a limited amount of oxygen.
One of the main advantages of this process is that it does not release contaminating fumes. The organic components are converted into biochar via pyrolysis, which is a stable form of carbon that is not released into the atmosphere. The heat created during this process can be captured and used as clean energy.
The Role of Biochar in Soil Improvement
Two of the key effects of biochar include:
- Improvement in the chemical and physical properties of soil
- Increase in the microbial properties of the soil
The combination of biochar with soil improves the soil structure and enhances the aggregation of soil particles, porosity, and water retention.
Biochar also enhances the soil cation exchange capacity by 20% and electrical conductivity by 124.6%.
Biochar reduces soil acidity by 31.9% and enhances soil’s biological diversity and microbial biomass by 125%.
The application of biochar has led to an increment in basal respiration by 30.1% carbon dioxide, after 35 hours of substrate addition.
Researchers have focused on applying biochar in nutrient-deficient soil for ecological restoration, including sequestering carbon.
Several mechanisms are associated with the increasing availability of plant nutrients in nutrient-limited agroecosystems, for instance, the addition of soluble nutrients contained in the biochar and mineralization of the labile fraction of biochar containing organically bound nutrients.
Other mechanisms include a decrease in nutrient leaching due to the unique physicochemical properties of biochar.
Retention of nitrogen (N), phosphorus (P) and sulfur (S) leads to an increase in biological activities and a shift in microbial communities.
During field trials, several researchers have reported that biochar application improved the soil quality and promoted crop yield and growth.
A study reported that compared to the control group, maize grains grown in soil treated with an optimal concentration of biochar showed an enhancement in yield by 98%.
Elemental Analysis of Biochar
Scientists stated that tetracycline, glycine and cellulose content are decreased systematically with increasing pyrolysis temperature.
At temperatures between 300 and 400°C, polyethylene glycol decreases abruptly.
Cellulose exhibited the lowest biochar mass yield for all pyrolysis temperatures. This may be because cellulose does not contain aromatic C and possess a high concentration of structural OH groups, which are easily volatilized during pyrolysis.
It is important to perform a complete elemental characterization of biochar samples before applying them to agricultural practices. It is challenging to analyze biochar due to technical limitations, such as difficulties associated with achieving total digestion of the samples for determining and estimating elements of interest.
Several analytical tools and methods have been designed to overcome the limitations associated with the elemental analysis of biochar. For instance, researchers use X-ray fluorescence (XRF) spectrometry for rapid estimation of the total elemental composition of biochar.
In many studies, XRF has been used to analyze the chemical composition of biochar-based fertilizers, which typically contain magnesium (Mg), sodium (Na), aluminum (Al), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). The biochar samples are digested using a modified dry-washing method (MDA) and are characterized using wavelength dispersive X-ray fluorescence (WDXRF).
Although C is the key element in condensed aromatic structures, which dominate the organic phase of biochar, O is the primary element in many polar organic functional groups that plays an important role in the biochar reactivity in soil environments.
Analyzing methods are modified based on the element to be analyzed. For instance, an increase in temperature results in a larger loss of H and O compared to C.
Dehydrogenation of methyl radical (CH3) owing to thermal induction manifests a change in the biochar recalcitrance.
Biomass material is typically composed of labile and recalcitrant O fractions. During initial heating, the labile fraction is lost, while the recalcitrant O fractions are retained in the char of the final product.
Treatment of biomass at a high temperature reduces the H /C and O /C ratios, due to dehydration and decarboxylation reactions.
Researchers stated that pXRF could be used for the rapid and environmentally-friendly characterization of biochar-based fertilizers.
Scientists also use an elemental analyzer to determine C, H and N present in biochar. The O content has been determined by Vario El cube, Elementar Analysensysteme GmbH.
Biochar is also analyzed using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS) analysis.
References and Further Reading
Pandey, D.S. et al. (2021) Structural and elemental analysis of biochars in the search of a synthetic path to mimetize anthropic Amazon soils. Journal of Environmental Management. 279. https://doi.org/10.1016/j.jenvman.2020.111685.
Gomes deFaria, J.A. et al. (2021). Elemental analysis of biochar-based fertilizers via portable X-ray fluorescence spectrometry. Environmental Technology and Innovation. 23. https://doi.org/10.1016/j.eti.2021.101788.
Bakshi, S. et al. (2020) Estimating the organic oxygen content of biochar. Scientific Reports. 10, 13082. https://doi.org/10.1038/s41598-020-69798-y
Ding, Y. et al. (2016) Biochar to improve soil fertility. A review. Agronomy for Sustainable Development. 36, 36. https://doi.org/10.1007/s13593-016-0372-z
Ścisłowska, M. et al. (2015) Biochar to Improve the Quality and Productivity of Soils. Journal of Ecological Engineering. 16(3). pp. 31–35. DOI: https://doi.org/10.12911/22998993/2802
Jindo, K. et al. (2014) Physical and chemical characterization of biochars derived from different agricultural residues. Biogeosciences. 11. pp. 6613–6621. https://doi:10.5194/bg-11-6613-2014
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