Carbon sequestration potential of biochar

Biochar for carbon sequestration

      Globally, soil is estimated to hold more organic carbon (1,100 Gt; 1 Gt=1,000,000,000 tones) than the atmosphere (750 Gt) and the terrestrial biosphere (560 Gt) (Post et al., 1990; Sundquist, 1993). In the Kyoto Protocol on Climate Change of 1997, which was adopted in the United Nations Framework Convention on Climate Change, Article 3.4 allows organic carbon stored in arable soils to be included in calculations of net carbon emissions. It speaks of the possibility of subtracting the amounts of CO2 removed from the atmosphere into agricultural sinks, from the assigned target reductions for individual countries. SOC sequestration in arable agriculture has been researched (Schmidt et al., 2000; Freibauer et al., 2002; West and Post, 2002; Sleutel et al., 2003; Janzen, 2004; King et al., 2004; Lal, 2004) against the background of organic carbon (OC) credit trading schemes (Brown et al., 2001; Johnson and Heinen, 2004). However, fundamental knowledge on attainable SOC contents (relative to variation in environmental factors) is still in its infancy, and it is mostly approached by modeling (Falloon et al., 1998; Pendall et al., 2004).

The principle of using biochar for carbon (C) sequestration is related to the role of soils in the C-cycle. As figure 4 shows, the global flux of CO2 from soils to the atmosphere is in the region of 60 Gt of C per year. This CO2 is mainly the result of microbial respiration within the soil system as the microbes decompose soil organic matter (SOM). Components of biochar are proposed to be considerably more recalcitrant than SOM and as such are only decomposed very slowly, over a time frame which can be measured in hundreds or thousands of years. This means that biochar allows carbon input into soil to be increased greatly compared to the carbon output through soil microbial respiration, and it is this that is the basis behind biochar’s possible carbon negativity and hence it is potential for climate change mitigation.

Although the Diagram of the carbon cycle figure is clearly a simplification of the C-cycle as it occurs in nature, the numbers are well established (NASA, 2008) and relatively uncontroversial.

Diagram of the carbon cycle 

Legend

Units: Petagrams (Pg) = 10 ^˄ 15 gC

Pools: Pg

Fluxes: Pg/year

     The carbon cycle refers to the continuous movement of carbon, the most abundant element on the planet, through the oceans, land, atmosphere, fossil fuels and all life on Earth. Learning about the existence of carbon at each of these levels of the environment helps us understand the importance of keeping the cycle stable and the consequences of an unbalanced cycle (NASA, 2008).

     A calculation of the fluxes, while being more a ‘back of the envelope’ calculation, than precise mathematics, is highly demonstrative of the anthropogenic influence on atmospheric CO2 levels. When all of the sinks are added together (that is the fluxes of CO2 leaving the atmosphere) the total amount of C going into sinks is found to be in the region of 213.35 Gt per year. 

   Conversely, when all of the C fluxes emitted into the atmosphere from non-anthropogenic (natural) sources are added, they total 211.6 Gt per year. This equates to a net loss of carbon from the atmosphere of 1.75 Gt C. It is for this reason that the relatively small flux of CO2 from anthropogenic sources (5.5 Gt C per year) is of such consequence as it turns the overall C flux from the atmosphere from a loss of 1.75 Gt per year, to a net gain of 3.75 Gt C per year. This is in relatively close agreement with the predicted rate of CO2 increase of about 3 Gt of C per year (IPCC, 2001). It is mitigation of this net gain of CO2 to the atmosphere that biochar’s addition to soil is proposed for. Lehmann et al. (2006) estimates a potential global C-sequestration of 0.16 Gt yr-1 using current forestry and agricultural wastes, such as forest residues, mill residues, field crop residues, and urban wastes for biochar production. 

     Using projections of renewable fuels by 2100, the same authors estimate sequestration to reach a potential range of 5.5-9.5 Gt yr-1, thereby exceeding current fossil fuel emissions. However, the use of biochar for climate change mitigation is beyond the scope of this proposal that focuses on the effects of biochar addition to soils with regard to physical, chemical and biological effects, as well as related effects on soil and ecosystem functioning.  

Greenhouse gases

Carbon dioxide is not the only gas emitted from soil with the potential to influence the climate. Methane (CH4) production also occurs as a part of the carbon cycle. It is produced by the soil microbiota under anaerobic conditions through a process known as methanogenesis and is approximately 21 times more potent as a greenhouse gas than CO2 over a time horizon of 100 years.  Nitrous oxide (N2O) is produced as a part of the nitrogen (N) cycle through process known as nitrification and denitrification which are carried out by the soil microbiota. Nitrous oxide is 310 times more potent as a greenhouse gases than CO2 over a time horizon of 100 years (U.S. Environmental Protection Agency, 2002).  Whilst these gases are more potent greenhouse gases than CO2, only approximately 8% of emitted greenhouse gases are CH4 and only 5% are N2O, with CO2 making up approximately 83% of the total greenhouse gases emitted. 

     Eighty percent of N2O and 50% of CH4 emitted are produced by soil processes in managed ecosystems (U.S. Environmental Protection Agency, 2002). It should be noted that these figures detail total proportions of each greenhouse gas and are not weighted to account for climatic forcing. In a study by Rondon et al. (2007), biochar addition to soils has been shown to reduce the emission of both CH4 and N2O. They reported that a near complete suppression of methane upon biochar addition at an application rate of 2% w/w-1 to soil. It was hypothesized that the mechanism leading to reduced emission of CH4 is increased soil aeration leading to a reduction in frequency and extent of anaerobic conditions under which methanogenesis occurs. 

     A reduction in N2O emissions of 50% in soybean plantations and 80% in grass stands was also reported (Rondon et al., 2007). It is also possible that the N that exists within the biochar is not bioavailable when introduced to the soil as it is bound up in heterocyclic form. Yanai et al. (2007) measured N2O emissions from soils after rewetting in the laboratory and found variable results, i.e. an 89% suppression of 2 emissions at 73-78% water-filled pore space contrasting to a 51% increase at 83% water-filled pore space. These results indicate that the effect of biochar additions to soils on the N cycle depend greatly on the associated changes in soil hydrology and those thresholds of water content effects on N2O production may be very important and would have to be studied for a variety of soil-biochar-climate conditions.

     Furthermore, if biochar addition to soil does slow the N-cycle, this could have possible consequences on soil fertility in the long term. This is because nitrate production in the soil may be slowed beyond the point of plant uptake, meaning that nitrogen availability, often the limiting factor for plant growth in soils, may be reduced leading to concurrent reduction in crop productivity. 

     Yanai et al. (2007) reported that this effect did change over time, but their experiment only ran for 5 days and so extrapolation of the results to the time scales at which biochar is likely to persist in soil is not possible. Therefore, further research is needed to better elucidate the effects and allow extrapolation to the necessary time scales.