Beware the Biochar Initiative

Turning bioenergy crops into buried charcoal to sequester carbon does not work, and could plunge the earth into an oxygen crisis towards mass extinction

by Dr. Mae-Wan Ho

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The story goes that charcoal buried in the soil is stable for thousands if not hundreds of thousands of years and increases crop yields. The proposal to grow crops on hundreds of millions of hectares to be turned into buried ‘biochar’ is therefore widely seen as a “carbon negative” initiative that could save the climate and boost food production.

That story is fast unravelling. Biochar is not what it is hyped up to be, and implementing the biochar initiative could be dangerous, basically because saving the climate turns out to be not just about curbing the rise of CO2 in the atmosphere that can be achieved by burying carbon in the soil, it is also about keeping oxygen (O2) levels up. Keeping O2 levels up is what only green plants on land and phytoplankton at sea can do, by splitting water to regenerate O2 while fixing CO2 to feed the rest of the biosphere [1] (Living with Oxygen, SiS 43).

Climate scientists have only discovered within the past decade that Ois depleting faster than the rise in CO2, both on land and in the sea [2, 3] (O2 Dropping Faster than CO2 Rising, and Warming Oceans Starved of Oxygen, SiS 44). Furthermore, the acceleration of deforestation spurred by the biofuels boom since 2003 appears to coincide with a substantial steepening of the O2 decline. Turning trees into charcoal in a hurry could be the surest way to precipitate an oxygen crisis from which we may never recover.


Burying charcoal to save the climate

The International Biochar Initiative (IBI), according to its website [4], was formed in July 2006 at a side meeting of the World Soil Science Congress at Philadelphia, Pennsylvania, in the United States, by people from academic institutions, commercial ventures, investment banks, non-government organizations and federal agencies around the world, dedicated to research, development, demonstration, deployment, and commercialisation of biochar on a global scale.

IBI has introduced biochar into the 2008 US Farm Bill, so it now counts among a handful of “new, high-priority research and extension areas”. IBI is also working with the United Nations Convention to Combat Desertification to promote biochar in the post-Kyoto climate agreement. And the United Nations Framework Convention on Climate Change has already included biochar in a section entitled: “Enhanced Action on Mitigation” to serve as basis for negotiations during pre-Copenhagen meetings [5].

Biochar is just charcoal, produced by burning organic matter such as wood, grasses, crop residues and manure, under conditions of low oxygen (pyrolysis). A number of different pyrolysis techniques exist depending on temperature, speed of heating, and oxygen delivery [6, 7], resulting in different yields of biochar and co-products, “bio-oil” (with energy content value approx 55 percent that of diesel fuel by volume) and “syn-gas” (a mixture of hydrogen, carbon dioxide, carbon monoxide, and hydrocarbons), which can be used to generate electricity, or as low-grade fuel for ships, boilers, aluminium smelter and cooking stoves.

IBI has encountered strong criticism as a “new threat to people, land and ecosystem” in a declaration signed by more than 155 non-profit organisations worldwide [8]. But patent applications have been made, and companies formed for commercial exploitation of biochar production. Intense lobbying is taking place for biochar to be included in the Kyoto Protocol’s Clean Development Mechanism for mitigating climate change [9, 10], so people implementing that technology would be able to sell certified emission reduction (CER) credits.

Things have moved forward so fast with so little public awareness and debate that critics are alarmed, especially over the proposal from some prominent advocates that 500 million hectares or more of ‘spare land’ could be used to grow crops for producing biochar [11, 12], mostly to be found in developing countries; the same as was proposed in the biofuels initiative several years earlier.

Biofuels proving disastrous

The biofuels ‘boom’ has already exacerbated climate change by speeding up deforestation and peatland destruction, loss of habitats and biodiversity, depletion of water and soil, and increased the use of agro-chemicals. Above all, it has generated poverty, land grab, land conflicts, human rights abuses, labour abuses, starvation and food insecurity as documented by BiofuelsWatch and 10 other groups [13, 14] (see also [15] (Biofuels: Biodevastation, Hunger & False Carbon Credits, SiS 33).  Calls for moratorium on biofuels came from Africa, the US, and the United Nations [16] (UN ‘Right to Food’ Rapporteur Urges 5 Year Moratorium on Biofuels, SiS 36).

Biofuel production – mainly bioethanol and biodiesel –  more than doubled between 2003 and 2008, driven by rising oil prices; while food prices rose 70 percent between 2005 and 2008 [17], according to data compiled by the international Monetary Fund. The UN declared 2008 the year of the Global Food Crisis (see [18] Food Without Fossil Fuels Now, SiS 39); food riots and fuel protests were rife. UK’s Environment Audit Committee joined the call for moratorium in January 2008 [19], and reiterated it in May 2008 [20].

Biochar is widely seen as the successor to biofuels on grounds that it will sequester carbon and improve soil fertility while also producing energy. Biochar is not just carbon neutral; it is “carbon negative”, according to its proponents, because buried biochar is stable for thousands, if not hundreds of thousands of years.

A lifecycle analysis published in 2008 [21] by John Gaunt and Johannes Lehmann, principal biochar proponent at Cornell University, New York, in the United States, considered both purpose grown bioenergy crops (BEC) and crop wastes (CW) as feedstock. The BEC scenario involves a change from growing winter wheat to miscanthus, switchgrass, and corn as bioenergy crops. The CW scenario considers both corn stover and winter wheat straw as feedstock. The energy balance is much more favourable than the production of biofuels such as ethanol from corn. The avoided emissions are between 2 and 5 times greater when biochar is applied to agricultural land than used solely for energy in fossil energy offsets. Some 41–64 percent of emission reductions are related to the retention of C in buried biochar (so the stability of biochar is important), the rest due to offsetting fossil fuel use for energy, fertilizer savings, and avoided soil emissions of N2O and CH4, as additional effects of biochar.  Unfortunately, the analysis is largely based on assumptions. Biochar is now found to be not quite as stable as claimed and can speed up litter decomposition in the soil (see below). The energy balance of pyrolysis is taken as that reported by one company; and there is lack of conclusive evidence in support of the supposed significant N2O reduction for at least ten years [6, 11].

Biochar is not ‘terra preta’

The biochar initiative was inspired by the discovery of ‘terra preta’ (black earth) in the Amazon basin [22, 23], at sites of pre-Columbian settlements (between 450BC and 950AD), made by adding charcoal, bone, and manure to the soil over many, many years (see Fig. 1). Besides charcoal, it contains abundant pottery shards, plant residues, animal faeces, fish and animal bones. The soil’s depth can reach 2 metres, and is reported capable of regenerating itself at the rate of about 1 cm a year. Similar sites are found in Benin and Liberia in West Africa, in the South African savannahs, and even in Roman Britain. According to local farmers in the Amazon, productivity on the terra preta is much higher than surrounding soils.

Investigations in the laboratory revealed that terra preta soils are rich in nutrients such as nitrogen, phosphorus, calcium, zinc, and manganese, and have high levels of microbial activities. Terra preta contains up to 70 times more black carbon (BC) than the surrounding soils. Due to its polycyclic aromatic structure, black carbon is believed to be chemically and microbiologically inert (but see later) and persists in the soil for centuries, if not thousands of years. During this time, oxidation produces carboxylic groups increasing its nutrient-holding capacity. Bruno Glaser and colleagues at the University of Bayreuth concluded that [24] “black carbon can act as a significant carbon sink and is a key factor for sustainable and fertile soils, especially in the humid tropics.”

Similarly, BC derived from terra preta sites in central Amazon differing in age from 600 to 8 700 years were chemically, biologically and spectroscopically indistinguishable, as consistent with their “extremely slow” rate of decomposition [25].

However, BC collected from 11 historical charcoal blast furnace sites from Quebec Canada to Georgia USA, were quite different from BC newly produced using rebuilt historical kilns [26]. The historical BC samples were substantially oxidized after 130 years in soils compared to the new BC, or new BC incubated for one year at 30 C or 70 C. The major alterations were an increase in oxygen from 7.2 percent in new BC to 24.8 percent in historical BC; a decrease in carbon from 90.8 percent to 70.5 percent; formation of oxygen-containing function groups, particularly carboxylic acid and phenolic functional groups; and disappearance of surface positive charge, to be replaced entirely by negative charges. New BC incubated at 30 C or 70 C for 12 months increased in oxygen concentrations to 9.2 and 10.6 percent respectively; and also had complete replacement of surface positive charges by negative charges.

These findings show that BC is a substantial oxygen sink, and could deplete atmospheric O2 fairly rapidly if massive amounts are produced in a hurry!

The main factor accounting for the changes was mean annual temperature, which was highly correlated with degree of oxidation. BC oxidation was increased by 87 nmoles/kg C / degree Celsius increase in mean annual temperature. BC oxidation to carboxylic groups accounts for the high cation exchange capacity of natural BC in the soil that the authors suggest is the basis of the enhancement in soil fertility.

So charcoal is not the same as terra preta that has been created over thousands of years by human intervention and natural geochemistry. The claim that biochar is a “stable carbon pool” in the soil that does not degrade for thousands of years is not borne out by the study, nor by a number of other studies (see below).

Naturally occurring black carbon has a far more complex relationship with the soil and the earth as a whole, as recent research is revealing. Moreover, black carbon pollution from fossil fuel and biomass burning associated with deforestation contribute as much to global warming as CO2, and climate scientist are proposing a reduction of black carbon emissions as a way of cooling the planet [27] (see Black Carbon Warms the Planet Second Only to CO2, SiS 44). That’s another reason the biochar initiative will spoil the climate, by increasing BC emissions.

Biochar increases loss of organic carbon from humus

A ten-year trial in Swedish forests showed that buried charcoal appear to promote the breakdown of humus, the decomposing plant matter on the forest floor [28], thus completely offsetting the carbon sequestered in the charcoal.  

David Wardle and colleagues at Umeå University started their experiment to investigate the effect of forest fires on soil ecology. They buried hundreds of litter bags containing humus, charcoal, or a 50–50 mixture of the two in several sites in the Swedish boreal forest.

Periodically, they weighed the bags and measured the concentration of carbon and nitrogen.  After just one year, they began to see an unexpectedly large decrease in mass from the bags containing the humus–charcoal mixture: 17 percent (the expected was 9 percent), compared to 18 percent in the bags with only humus and 2.5 percent in the bags with only charcoal Over ten years, the bags with mixed humus and charcoal released just as much carbon as did those containing only humus (130 mg per g initial mass), instead of only half as much as would be expected if charcoal had no effect on the loss of carbon from humus. The bags with charcoal had lost a small amount of its carbon (less than 5 mg per g initial mass) but gained about the same in nitrogen and microbial activity. The mixture did not gain or lose any nitrogen while humus released 2 mg N per g initial mass. 

The results show that burying charcoal can speed up the decomposition of forest humus during the first decade, thus offsetting nearly all of the carbon sequestered in the charcoal itself.

Biochar may not be a stable carbon pool

Caroline Masiello, marine chemist at Rice University Houston, Texas, in the United States, pointed to apparent discrepancy in the production and deposition of of BC on both sea sediment and on land [29]. BC production globally was previously estimated at 0.05 to 0.27 Gt/y [30], representing 1.4 to 1.7 percent carbon exposed to fire that’s converted to BC. The only documented loss process for BC is deposition in ocean sediments. However, the rate of total organic carbon deposited on the seafloor is only 0.16 Gt/y. Even assuming the lower end of the BC production rate, 0.05 Gt/y, would mean that BC should be 30 percent of ocean sediment organic carbon; but the actual measured amount is 3-10 percent.

Furthermore, isotope studies of highly refractory BC detected 14C graphite BC in sediment from the Northeast Pacific coastal transept. This was not a product of fossil fuel combustion but the result of erosion of very old graphite from rocks and deposited into the ocean, which is at least in part derived from petrogenic graphite. If BC deposited in ocean sediments comes both from biomass burning and from recycled petrogenic graphite, even less of the annually produced BC can be accounted for in ocean sediments. So where does the rest of the earth’s annually produced BC go?

The same applies to BC on land. If BC has been produced since the last glacial maximum from biomass burning at the same rate as it is now produced, and if it is as stable as assume, it should account for 25 – 125 percent of total soil organic carbon pool. Instead, only a few measurements of BC or soil organic carbon ever reach 25 percent. A study of BC production during Siberian boreal forest fires made clear that not enough BC remains even after 250 years to account for all the BC produced during a fire [31] – estimated at 0.7 -0.8 percent of organic carbon – due to a combination of in situ erosion and translocation within the soil profile, with in situ degradation being the most likely.

In a later study, the amount of BC in organic carbon was compared in soils of three Siberian Scots pine forests with frequent, moderately frequent, and infrequent fires [32]. The researchers concluded that BC did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. They found 99 percent of BC in the organic layer, with a maximum stock of 72 g/m2. Less intense fires consumed only parts of the organic layer and converted some organic matter to BC, whereas more intense fires consumed almost the entire organic layer.

But appreciable degradation of BC can also occur in the absence of fires, by microbial action or photo-degradation. The stability of BC was investigated in a sandy savannah soil at Matopos in Zimbabwe, where some soil plots have been protected from fire for the past 50 years [33]. The abundance of BC in these plots was compared to plots that have continued to be burnt. The plots protected from fire had 2.0+5 mg/cm2 BC, about half of the 3.8+0.5 mg/cm2 found in plots burnt every 1-5 years. The half-life of BC at a depth of 0-5 cm of the soil protected from fire was estimated at < 100 years, and that of large particles <50 years. The results suggest that in well-aerated tropical soil environments, charcoal and other BC can be significantly degraded in decades to a hundred years.    

BC is best understood as a continuum of combustion products, ranging from slightly charred, degradable biomass to highly condensed refractory soot [28]. All components of this continuum are high in carbon content, chemically heterogeneous and dominated by aromatic structures. The reactivity of BC also varies along the combustion continuum. Charcoal decomposes much more rapidly than soot when exposed to chemical oxidants, such as acid dichromate, in the lab [33].

The results are also complicated by the different ways of producing charcoal and different methods of quantifying BC [28]. In studies on the National Institute of Standards and Technology reference materials, the values varied by a factor of 500, depending only on the method used in quantification.         

Research in the atmospheric chemistry community has shown that even soot, the most inert part of the combustion spectrum, can be chemically altered on a very short timescale through reaction with atmospheric oxidants. Reaction with ozone and other atmospheric oxidants create hydrophilic carboxylic acid groups on its exterior These reactions are so rapid that solubilisation of soot particles can occur in 30 min in the presence of 50 ppb (parts per billion) ozone, making it possible to dissolve soot in a solution of distilled water. Ozone concentration in rural air in the US ranges diurnally from 20 to 70 ppb. So soot can enter some of the Earth’s dissolved organic carbon pools.

BC has been measured by thermal techniques to be 5 to 12 percent of dissolved organic carbon in Chesapeake Bay, the Delaware Bay, and in adjacent Atlantic Margin. Another electrospray ionization with high resolution mass spectrometry applied to dissolved organic matter from a small stream in New Jersey and Rio Negro detected BC degradation products that were assigned chemical structures.

Biochar effects on soil fertility not always positive

Experiments carried out so far have yielded equivocal results on the ability of biochar to increase productivity. There have been positive effects claimed, at least in the short term, but also some negative impacts, at least partly due to nitrogen limitation [34]. In a small scale lab experiment, biochar appeared to increase nitrogen fixation by legumes, principally by increasing the availability of trace elements boron (B) and molybdenum (Mo), and to a lesser extent, K, Ca, and P, while lowering N availability and Al saturation. The results on productivity were not statistically significant, however.     

A report published in 2007 presented results on crop yields over four seasons [35]. Researchers at the University of Bayreuth in Germany, and EMBRAPA Amazonia Occidental Manaus in Brazil carried out a field trial near Manaus on cleared secondary forest with 15 different amendment combinations of chicken manure (CM), compost (CO), forest litter, chemical fertilizer (F), and charcoal (CC) applied once on rice and sorghum, and followed over four cropping cycles (see Fig. 2).

Chicken manure gave by far the highest yield over the four cycles (12.4 tonne/ha). Compost application came second at about half the yield, but was still four times higher than chemical fertilizer. The control, leaf litter (burnt and fresh), and charcoal treatments gave no grain yields after the second season, and were discontinued.

In combination with compost, charcoal amendment decreased yield by about 40 percent compared to compost alone, and only improved yield in combination with chemical fertilizer. The charcoal was derived from secondary forest wood bought from a local distributor, and applied at the rate of 11 tonne/ha. This corresponded to the amount of charcoal C that could be produced by a single slash-and-char event in a typical secondary forest on the dry iron-rich soil of central Amazonia.

The highest yields for all treatments were obtained at the first harvest, and except for chicken manure, yields declined rather sharply by the second harvest.

A second fertilization with chemicals was applied after the second harvest to all remaining treatments, but that did not improve the yields.

Plants fertilized with chicken manure had the highest nutrient contents followed by plants that received compost and/or chemical fertilizer. Chicken manure significantly improved the K and P nutrition compared to all other treatments, while charcoal applications did not show a significant effect on nutrient levels. Most importantly, surface soil pH, phosphorus, calcium and magnesium were significantly enhanced by chicken manure. Plots fertilized by chicken manure had pH higher than 5.5 and increased cation exchange capacity.

These results are disappointing for those looking to promote ‘biochar’ as a means of improving the yield of crops at the same time as sequestering carbon, which also turns out to be illusory.

The potential for an oxygen crisis is real

It is clear that biochar has not lived up to its promises as a stable C repository or enhancer of crop yields. On the other hand, the risk of oxygen depletion is real [1-3]. Biochar itself is an oxygen sink in the course of degrading in the soil [24. 32]; adding to the depletion of oxygen that cannot be regenerated because trees have been turned into biochar for burial. And worse, as in the biofuels boom that has already apparently speeded up deforestation and oxygen depletion since 2003 [2], if biochar is promoted under the Clean Development Mechanism, it will almost certainly further accelerate deforestation and destruction of other natural ecosystems (identified as ‘spare land’) for planting biochar feedstock, and swing the oxygen downtrend that much closer towards mass extinction.    

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