By Harry Short
Biochar (otherwise known as charcoal or black carbon) is a residue of the incomplete combustion of biomass. Scientists are excited by biochar as it could represent a way to significantly increase stable soil carbon in agricultural land, the land use type with the smallest carbon pool. Biochar breaks down to carbon dioxide slowly and when applied to cropland increases fertility and crop yields. The production of biochar also generates surplus bioenergy, which could be used to offset fossil fuels. Thus, biochar represents a synergistic solution to multiple problems: it improves agricultural output, while sequestering carbon and providing renewable energy.
How is biochar made?
Biochar is a naturally produced in wildfires, though the percent of carbon from the original biomass converted to biochar is quite modest (roughly 3%) (Lehman et al. 2006). However, in controlled burnings, where oxygen is limited or absent, roughly 50% of the carbon in the original biomass can be converted to biochar. This process is called pyrolysis, and by varying the temperature, pressure, and parent material, the amount of biochar produced can be altered. Though biochar is complex chemically, it possesses fewer oxygen-containing functional groups and is enriched for aromatic C residues compared to the parent material (Czimiczik et al. 2002).
Once produced, biochar is normally used as an agricultural amendment, but may also be used as a construction material or as a filter for exhaust streams (Okimori et al. 2003). Pyrolysis also produces bio-oil and syngas, which may be captured to generate energy. However, the ability of pyrolysis to do this, while also sequestering a net amount of carbon, is most intriguing from a global change perspective (Gaunt & Lehmann, 2008). Moreover, if the bioenergy produced during pyrolysis leads to displacement of fossil fuels, it could have an even greater impact on reducing greenhouse gas emissions.
Role in Carbon Mitigation
Biochar mineralizes at a very slow rate and represents a potentially significant sink for carbon. What is exciting for policy makers interested in arresting global climate change is that carbon sequestration via biochar seems relatively independent of human action once the biochar is applied to agricultural soils. Other forms of carbon storage depend on adherence to the original policy (reforestation, adding crop residues to land, etc), which may change as politics evolve in a particular jurisdiction. Since biochar is so chemically recalcitrant, once biochar is mixed with soil, its carbon is safely stored.
But for how long? The exact mineralization rate is difficult to determine for several reasons: (1) methodological constraints: decay constants determined in the laboratory fail to describe processes in the field and errors become significant when extrapolating from a small time period to a large time period, and (2) the diversity of parent materials for biochar production and the diversity of natural soils and climate patterns make the determination of a single rate impossible. For example, Glaser et al (2009) cite mean residence times of 2,000 years, whereas other work suggests means residence time of roughly a decade (Nguyen et al. 2008). Part of this discrepancy may result from differing soil types, climate, and the type of biochar formation. For example, biochar generated by humans is highly stable in soils from the Amazon basin (Liang et al. 2008), whereas black carbon generated from forest fires in African savannah is more labile (Nguyen et al. 2008).
The point is not that biochar fails to decay. Rather a fraction (98% to 30%) of the original biochar is incredibly resistant to mineralization (Preston & Schmidt, 2006; Nguyen et al.2008; Liang et al. 2008), so resistant that it may remain in the soil after centuries if not millennia. Thus, this portion represents a way to hedge the risk that a contemporary carbon sink might become a future carbon source.
It is important to note that most of the work on biochar has been with agricultural soils or soils that are already highly weathered. Other soil types and land uses may see different soil chemistry dynamics once biochar is added. For example, Wardle et al (2008) found that carbon release from boreal forest humus was increased with addition of large quantities of charcoal. They demonstrated that microbial activity was spurred by the addition of charcoal to humus. This research suggests that charcoal may enhance the carbon release in boreal forest soils, by increasing soil microbial biomass, thus enhancing the degradation of the labile carbon pool.
However, this study is limited in two respects: carbon measurements were taken of a mesh bag containing the original charcoal and humus, therefore soluble C fractions that leached into the soil were counted as ‘carbon release’, though the soluble C may have remained immobilized in the subsoil. Secondly, additions of 50% charcoal (as used in the study) are not realistic or recommended (this amount represents at least five times the maximum mass derived from agronomical experiments (Rondon, 2007).
More significant concerns about the use of biomass in forest soils deal with the ability of subsequent fires to mineralize biochar in the soil, thereby negating any sequestration (Preston & Schmidt, 2006). For this reason, it would be unwise to apply biochar in forests with recurring fires. Therefore, limiting biochar application to agricultural soils and biochar production feedstocks to crop residues seems justified until more research can be undertaken. However if concerns can be allayed, using forest residues from timber production along with biomass from abandoned agriculture lands could allow for the sequestration of 10% of US annual fossil fuel emissions (Lehmann 2007).
An Agriculture Amendment
Excitement around biochar as a soil amendment in tropical agriculture preceded our current focus on carbon storage and climate mitigation (Marris 2006). Wim Sombroek, a pioneer in the field, studied the famous terra preta soils of the Amazon basin. In contrast to surrounding soils, the terra preta anthrosols were noted for their dark color (due to having upwards of 10% carbon by mass) and drew attention for their high agricultural productivity. Due to high rainfall in the tropics, significant leaching of soils occurs, such that slash-and-burn agriculture requires years of fallowing to regenerate soil fertility. High rainfall also limits the productivity gains from exogenous application of fertilizer for these same reasons. However, terra preta soils resist leaching to an incredible degree (Lehmann et al. 2003). By adding charred matter to soils along with other refuse, the ancient residents of the Amazon, were able to produce soil that retained its fertility, allowing for long-term settlements to arise.
Initial interest was in transporting a slash-and-char model of agriculture to other tropical areas. Biochar improves soils, especially the weathered soils of the tropics in several important ways (Lehmann 2007b). Biochar has a high surface area for absorption and can form organometallic complexes, so it retains dissolved nutrients like ammonium and potassium well. It increases the pH, decreases Al saturation, and enhances the cation exchange capacity of soils (Glaser et al. 2002). Since biochar also contains a certain amount of ash, it also acts as a fertilizer in soils low in K, Ca, or Mg. Depending on the original texture of the soil, adding biochar may also improve the water retention of the soil, though this effect is mostly seen in very sandy soils.
Some caution must be offered. Though the availability of many inorganic ions is higher in soils with high biochar content, nitrogen availability is reduced in such soils, especially those in which the charcoal has aged (Lehmann et al. 2003). Though this may seem to suggest the need for higher fertilizer inputs, the complexing of nitrogen with fresh biochar and its slow release may actually require less fertilizer as plants will have a lower but continuous supply of nitrogen for growth. More research is needed here. Secondly, maximas exist in biochar application; depending on crop and original soil type, excess biochar application reduces biomass production and ultimately yields. As a result, caps on biochar applications should be erected.
Still, the literature is clear on the benefits to plant growth (above and below ground), crop yield, and foliar tissue nutrient concentrations for P, S, K, Ca, Mg, B, and Mo of moderate applications of biochar (~60g of bochar kg-1 of soil) to cropland (Rondon et al. 2007; Lehmann 2003). Many soil types and crop types could benefit from biochar.
Finally, charcoal production is a low-technology craft (prior to the discovery of coal, much of the pig iron in colonial America was forged using biomass-derived charcoal). Since the technology involved is relatively simple, advocates for slash-and-char agriculture argue that food production in tropical areas can be made more productive, while enhancing carbon sequestration without large investments of capital. Other than education about charcoaling, few barriers seem to exist. Mobilization of the world’s farmers seems more feasible with such a low-technology solution.
Can Biochar Be a Significant Carbon Sink?
Recognizing that an upper limit of application for biochar exists (approximately 60g kg-1 soil) and assuming the average density of soil is 2500lbs/ cubic yard (Hall, 1980), one can determine the amount of biochar that could safely be applied to a cubic yard of soil (approximately 68kg). Assuming the United States has approximately 350 million acres of cropland (excluding pasture and idled cropland) (Vesterby 2002), and that biochar will only be added to the top six inches of the soil, one estimates a total volume of 282 billion cubic yards of soil in the US. Assuming that 80% of the soil is enriched with biochar, we arrive at a maximum total biochar application amount of 15.4 GtC. If we take global cropland to be 1.5 billion hectares, this figure jumps to 163GtC, or approximately a fifth of the current carbon in the atmosphere or 8% of the soil carbon stored globally. This amount represents a ceiling of total carbon that could be stored in agricultural soils as biochar with no harm to those soils.
But how much crop residue is available to generate the biochar to be stored (assuming a massive mobilization)? Annual crop residues in the US total about 500 million tones, of which 203 million tonnes is carbon (Lal, 1998). Following recommendations that only 30% of the crop residue be removed to prevent erosion (SQNTDT, 2006), of which approximately half the carbon will remain as biochar after pyrolysis, one arrives at 30.5 MtC/yr in the US or approximately 322 MtC/yr for the world, assuming similar rates of crop residue production as in the United States. Since carbon is accumulating in the atmosphere at a rate of 3.3 GtC/yr (Bolin & Sukumar, 2000), global use of biochar could represent a 10% reduction in atmospheric carbon accumulation per year (assuming the accumulation rate does not increase significantly beyond that in 2000). Not a magic bullet, but certainly a wedge in the overall solution.
Bioenergy v. Biochar
Production of biochar involves deliberate inefficiency. Not all of the carbon is combusted and thus a significant amount of bioenergy is not released. Competing users of biomass might claim this to be a waste of crop residues. However, Gaunt and Lehmann (2008) show that a focus on efficiency, on oxidizing all the carbon in a unit of biomass, neglects to consider the total inputs to agricultural lands and the carbon mitigation benefits of returning biochar to the land. They calculate that total avoided GHG emissions from displacing natural gas for a bioenergy (complete combustion) approach to be roughly one third of the avoided emissions reaped from a biochar approach. Moreover, over 90% of the avoided omission for a bioenergy approach depends on displacement of fossil fuels, which may not actually occur. Whereas for a biochar strategy, only 14% of the avoided emissions depends on such displacement. The rest is derived from stabilization of carbon in soil, reduced nitrous oxide and methane emissions, and reduced fertilizer requirements. For example, if avoided nitrous oxide and methane release is converted to carbon dioxide equivalents, we see substantial emissions reductions (3867 kg CO2 ha-1 y-1 for wheat and 4126 kg CO2 ha-1 y-1 for corn) from application of biochar to croplands (Gaunt & Lehmann, 2008). Gaunt and Lehmann also estimate that between 90 to 100 kg CO2 ha-1 y-1 are avoided from reduced fertilizer use (and thus, manufacture). This suggests that policy makers and industry should take new interest in biochar.
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