August 25, 2021. By Kolemann Lutz
Biochar is charcoal that is produced by pyrolysis or heating of biomass in the absence of oxygen. Biochar or granualirized charcoal is made from wood chips, animal manure, sludges, compost, urban, agriculture, and forestry waste to produce bio oils such as bio-polymers, heating oils, as well as transportation fuels, biochemicals, and syngas.
After applied to soil, the carbon–rich biochar can help remediate the soil, which is the largest market for biochar, by increasing soil fertility, water holding capacity, biological adaptation, and crop productivity. By increasing pH, porosity and water availability, biochars can create favorable conditions for root development and microbial functions.
Researchers synthesized 20 years of research on biochars in a study published in the GCB Bioenergy Journal that found that biochar helps build organic carbon in soil by up to 20 percent (average 3.8 percent) and can reduce nitrous oxide (N2O) emissions from soil by 12 to 50 percent, which increases the climate change mitigation benefits of biochar.
“Biochar can draw down carbon from the atmosphere into the soil and store it for hundreds to thousands of years,” mentions lead author Stephen Joseph, Visiting Professor in the School of Materials Science and Engineering at UNSW Science.
“The intergovernmental panel on Climate Change found that globally, biochar could mitigate between 300 million to 660 million tonnes of carbon dioxide per year by 2050,” Prof. Joseph says.
Biochar could absorb 500million tons of CO2 Australia produces each year. Currently, the largest producers of biochar are the US, producing about 50,000 tonnes a year, while China is producing more than 500,000 tonnes a year.
Results from experiments demonstrate that the average crop yield increases by 10-42% with greatest increases due to increase in nutrient retention and water holding capacity. Meta-analyses found that biochars increase Phosphorus availability by 4.6X on average; decrease plant tissue concentration of heavy metals by 17-39%; build soil organic carbon through negative priming by 3.8% (range -21-20%); and reduce non-CO2 greenhouse gas emissions from soil by 12-50%.
“Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development and increase resilience to disease and environmental stressors.” The AM fungal hyphae could access microsites within biochar that are too small for most plant roots to enter.
Three stages of reactions of biochar in soil include: dissolution (1-3 weeks) to stimulate seed germination; reactive surfaces are created on biochar particles (1-6 months); and aging (beyond 6 months) to form microaggregates that protect organic matter from decomposition.
The main factors that determine whether biochar impacts seed germination include: (i) release of salts from biochar to the soil (or germinating) solution; (ii) release of phytotoxins to the soil solution; (iii) release of germination-inducing hormones or karrikins; (iv) change in water holding capacity and porosity of the soil.
As soil acidity is the main factor regulating microbial composition, low temperature alkaline biochars have also been proven to increase hydrophobic compounds in soils to favor fungi activity. Biochar increased microbial carbon biomass and the activities of the enzymes urease, alkaline phosphatase and dehydrogenase by 22%, 23%, 25% and 20%. Water entering biochar pores dissolves soluble organic and mineral compounds (Pokharel et al., 2020). Arbuscular mycorrhizal fungi (AMF) invade the pores of biochar, especially biochars with high P content on the pore surface. Blackwell et al. (2015) found that a phosphorus-enhanced biochar compound fertilizer (BCF) increases root colonization to 75% compared with 20% in mineral fertilizer and unfertilized control, and increased P uptake efficiency.
Prof. Joseph says the study found the greatest responses to biochar were in acidic, dryland, sandy soils where biochar had been applied together with fertiliser. Although Martian regolith has a pH between 8 and 9 with magnesium, sodium, potassium and chloride minerals, Mars is covered with vast expanses of sand and dust with small amounts of H2O in an arid environment.
On Mars, a lack of available organic nutrients and carbon in Martian regolith imposes challenges for soil remediation with AMF fungi, crop growth, and cyanobacteria. Compared to "typical" soil on Earth which is made of about 5% organic matter, this is quite high in iron and magnesium, and lower in silicon, aluminum, potassium and sodium.
By modifying pyrolysis conditions and matching the properties of a Martian biochar simulant to soil constraints and plant nutrient requirements, botanists and systems could remediate soils with the heated biomass, in addition to compost. As an ecological construction building block, biochar can help reduce toxins in soil and remediate 17 biocidal factors in Martian soil such as the globally distributed oxidizing soils, extremely high salt levels (e.g., MgCl2, NaCl, FeSO4, and MgSO4), and high concentrations of heavy metals. Bioremediating microbes or nano-powders could be mixed for soil remediation before the dissolution stage to absorb perchlorate salts and metals in regolith.
On Earth, researchers demonstrated that a 2 kW microwave pyrolysis reactor, electric arc or solar furnace on the surface could burn organic waste in an oven at 450°C and 800°C to achieve biochar yields up to 61 by wt%. Microwave-assisted pyrolysis (MWP) nearby waste management facilities and not too far from greenhouses or soil remediation sites in a pressurized inert atmosphere.
If biochar could be mass produced for rocky planets such as Moon and Mars, the heated biomass could become a potential strategy for paraterraforming to supplement microbial growth in pressurized and perhaps terraforming in unpressurized environments. Biochar can help mitigate climate change and support food security on Earth while advancing soil bioremediation throughout the Solar system.
Reference: “How biochar works, and when it doesn’t: A review of mechanisms controlling soil and plant responses to biochar” by Stephen Joseph, Annette L. Cowie, Lukas Van Zwieten, Nanthi Bolan, Alice Budai, Wolfram Buss, Maria Luz Cayuela, Ellen R. Graber, Jim Ippolito, Yakov Kuzyakov, Yu Luo, Yong Sik Ok, Kumuduni Niroshika Palansooriya, Jessica Shepherd, Scott Stephens, Zhe (Han) Weng and Johannes Lehmann, 27 July 2021, GCB Bioenergy.
