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Biochar – biomass recovering as part of a circular economy and a closed-loop system

The use of biochar amendments on soil could address the issues of soil fertility, water holding capacity, and climate change mitigation via carbon sequestration

What is biochar?

Biochar is a form of charcoal and a heterogeneous by-product of pyrolysis, a thermochemical process that decomposes organic material like biomasses through anaerobic heating. Biochar has a high carbon content, beneficial effects on the soil phosphorus (P) cycle, and the potential to alter the composition and microbial biomass of soils, promoting microbial communities’ diversification and stimulating soil microbial activity. The use of biochar is considered a possible manner to address site-specific goals and the global issues of soil fertility, water holding capacity and climate change mitigation via carbon sequestration while increasing agronomic productivity. Research areas that require further assessments to ensure a safe and sustainable use of biochar include understanding the interactions among common agricultural chemicals like Glyphosate (GPS) and biochar, the molecular and cellular mechanisms related to the effects of biochar amendments on plant resistance to biotic factors and the impact of aging on biochar and the influence of pyrolysis conditions and feedstock type.   

The fertility of Amazonian Dark Earths

Amazonian Dark Earths (ADEs) are fertile soils located in the Brazilian Amazon region that present elevated concentrations of pyrogenic carbon in the form of microscopic charcoal particles. The existence of Amazonian Dark Earths was reported by Smith in 1879 and Canadian geologist Charles F. Hartt in 1885. Their origin has been at the center of conflicting theories in the twentieth century, which include ADEs being a result of fallouts from volcanoes in the Andes (Camargo 1941), the sedimentation in recent ponds (Cunha-Franco, 1962), or Tertiary lakes (Falesi, 1974). The fertility of Amazonian Dark Earths, their elevated pyrogenic carbon content, and the presence of pre-Columbian indigenous artifacts were seen as evidence of their Anthropogenic roots, bringing about their classification as anthropic soils or Pretic Anthrosols. A recent study highlighted that the presence of stable isotope ratios of neodymium, strontium, and radiocarbon activity of microcharcoal particles in Amazonian Dark Earths could be a sign of ADEs pre-dating human habitation, suggesting that indigenous peoples didn’t trigger the genesis of ADEs, but utilized these areas of high soil fertility in an effort to harness the natural processes of landscape formation.

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Carbon-rich biochar piled on the forest floor, where it will later be scattered around tree trunks to improve soil health. Ph US Department of Agriculture

Soil carbon’s CO2 sequestration

Between 1750 and 1999, the atmospheric CO2 went from 280 ppm to 367 ppm. Today’s global average atmospheric carbon dioxide measured at Mauna Loa Observatory in Hawaii amounts to 416.75 ppm. Today’s Carbon dioxide levels are higher than at any point in at least the past 800,000 years. Soil organic carbon (SOC) represents a substantial portion of the carbon (C) in most terrestrial ecosystems (Lal, 2008a). With the total amount of organic carbon in the soil amounting to 2500 gigatons and the entire C in terrestrial ecosystems comprising about 3170 gigatons, around eighty percent of the terrestrial carbon is found in the soil (Lal 2008). Soil carbon has the ability to sequestrate Carbon Dioxide (CO2) through a process in which CO2 is confiscated from the atmosphere and stored in the soil carbon pool, hence the goal of increased carbon storage in the soil. The conversion from natural ecosystems to agricultural use has caused a drop in SOC levels, emitting 50 to 100 GT of carbon from soil into the atmosphere (Lal 2009), creating an opportunity to store carbon in the soil through a plethora of land management practices. Amazonian Dark Earths have a carbon content of up to 150 g C/kg soil, while the surrounding soils have a carbon content of 20-30 g C/kg soil. The organic matter levels in the dark earths remained stable overtime: tests detected high carbon contents after hundreds of years of abandonment. Black carbon (BC) generated by the incomplete combustion of biomasses that composes up to thirty percent of the SOM of Amazonian Dark Earths was identified as a possible cause for Ades’s high carbon content stability (Glaser et al., 2000). Based on the similarities between the organic matter in Amazonian Dark Earths and biochar, scientists hypnotized that presence of organic carbon from incomplete combustion could have been the primary cause of the fertility and high carbon content of ADEs soils (Glaser et al., 2001), as theorized by American professor and geographer Nigel J. H. Smith in 1980.

The growing use of biomass

The development of the global agro-industries has generated income and employment, contributed to economic growth, and supplied goods and services. Agro-industries generate waste and organic agricultural by-products, including crop residues, straw, branches, buds, and roots removed during the pruning process. As renewable organic materials that come from plants, agricultural waste can be utilized as a biomass feedstock. Until the mid-1800s, biomass was the largest source of total annual energy consumption in the United States. The use of biomass for electricity generation and biomass fuels for transportation is growing as a means of avoiding the generation of carbon dioxide emissions from the use of fossil fuels like coal and petroleum. Between 2002 to 2013, biomass energy consumption in the United States grew more than sixty percent. Biomass generated about five quadrillion British thermal units (Btu) and around five percent of total primary energy use in the United States in 2019.  Of the whole five percent, wood and wood-derived biomass provided around forty-six percent, the remaining forty-five percent was from biofuels like ethanol, while biomass from municipal wastes generated nine percent of the total. In the same year, the global capacity of bioenergy hit over 123.8 gigawatts.

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Biochar is found in soils around the world as a result of vegetation fires and historic soil management practices, International Biochar Initiative

Biomass’ circular economy

The chemical energy found within biomass comes from the sun: plants absorb the sun’s energy through photosynthesis, a process that allows photoautotrophs to convert light energy into chemical energy that plants utilize to power their metabolic activities. Biomass energy sources include human sewage, biogenic materials in municipal solid waste like cotton, paper, yarn, food, wood, and wood processing wastes like lumber and furniture mill sawdust and waste, firewood, wood pellets, and wood chips, and black liquor from pulp and paper mills. Biomass can be utilized for heating buildings and water through direct combustion or converted to renewable liquid and gaseous fuels through thermochemical, chemical and biological processes. Agricultural biomass includes agricultural crops and waste materials like soybeans, sugar cane, corn, switchgrass, algae, woody plants, and crop and food processing residues. From the biomass from agro-food production, we can obtain energy and organic substances for conventional and organic agriculture by applying biotechnologies for the bioremediation of soils, achieve an increase in yields and a reduction in the need for mineral fertilizers, soil improvers and conditioners. In a circular economy, the production and consumption systems generate products that can be re-utilized as feedstock, leading to a cycle of reuse, recycling, and recovery, repair, refurbishment and remanufacturing that minimizes the use of resource inputs and the creation of waste. The recovery of organic biomass is significant to establishing a circular economy and a closed-loop system, as it can provide energy and materials.

Pyrolysis process

Biochar is a vegetable charcoal obtained from the pyrolysis of different types of vegetable biomass, including agricultural residues and by-products, corn or wheat stubble, rice husk, almond husk and dry foliage. Pyrolysis is the thermal decomposition of organic material, such as biomass, in an inert atmosphere in the absence of oxygen. Due to the lack of oxygen, the material does not combust, but the chemical compounds like cellulose, hemicellulose, and lignin that compose the feedstock decompose into combustible gases and charcoal thanks to the elevated temperature. The majority of these combustible gases are condensed into a combustible liquid and synthetic fuel called pyrolysis oil or bio-oil. Gasses like CO2, CO, H2 and light hydrocarbons linger at the end of the process. The three products of the pyrolysis of biomass are a liquid, the bio-oil, a solid, the biochar, and a gas, the syngas. Several factors, including the process parameters and the feedstock’s composition, influence these products’ proportion. The syngas produced during pyrolysis is a gas with a calorific value similar to methane that can be used in the production processes that require heat, like electricity generation and drying processes. Biochar addition to arsenic (As)-contaminated soils causes a reduction in its passage from soil to roots and a decrease in the transfer from the roots to leaves and fruits. Biochar amendment causes an overall amelioration of soil quality and fertility, but the use of biochar is unlikely to cause yield enhancements in species like tomatoes that have indeterminate growth habits. Low biochar applications result in high seedling survival rate on tropical tree species, showing biochar’s potential as a tool for reforestation and afforestation projects. Biochar can be added to mine waste, which is dangerous due to its heavy metal content (cadmium and lead), as its addition prevents plants from accumulating these metals while trapping them in the soil, preventing the heavy metals from contaminating water bodies. The land application of biochar can decrease greenhouse gas emissions by storing atmospheric carbon in the soil. Biochar can have variable agronomic and soil effects that vary depending on farmland management practices, soil types, and crop species.  

IMAGE GALLERY

Biochar

is obtained from the carbonization thermochemical conversion of biomass in an oxygen-limited environment. This process mirrors the production of charcoal, but unlike charcoal biochar is utilized as a soil amendment with the intention to improve soil functions and to reduce emissions. 

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