Leonardo da Vinci (1452-1519) once said: “We know more about the movement of
celestial bodies than about the soil underfoot” (Montgomery, 2007). The challenge of
understanding how soil, microbes, nutrients, and plant roots interact with each other is expanded when adding biochar into the equation. It will require many different tests under various conditions at a wide range of scales and probably many years (if not decades) to solve it.
The characterisation of biochars is a topic of relevant discussion in the literature. Seven physical properties have been identified to measure the quality of biochar: pH, volatile compound content, water holding capacity, ash content, bulk density, pore volume, and specific surface area (Sohi et al., 2009). The IBI (2013a) has formalised these and other properties into a proposed reporting standard.
The characteristics of feedstock and production parameters determine the physico-chemical properties and nutrient content of biochar (Demirbas, 2006; Lehmann, 2007; Tagoe, 2008; Sohi et al., 2009; Camps Arbestain et al., 2009; Van Zwieten et al., 2010). Based on surface area, pH, and cation exchange capacity (CEC), Lehmann (2007) proposed a temperature between 450-550oC to optimise the characteristics of biochar (Fig. 1) and doubted that the use of biochars produced below 400oC would improve soil fertility.
Fig. 1. Temperature effects on carbon recovery, cation exchange capacity (CEC; measured at pH 7), pH, and surface area for dried wood from Robinia pseudacacia L. (Lehmann, 2007).
Balancing parameters depend on what is desired. For example, the higher the process temperature the less biochar produced (less soil amendment and less C sequestered) but the higher its carbon stability (safer C sequestration) and co-products yield (more energy). Since biochar science is relatively new, biochar publications correspond to short-term laboratory, greenhouse, and small plot experiments. Because of its origins, early studies focused on ancient TP soils and compared them to nearby soils in the Amazon Basin (Glaser et al., 2001; Lehmann et al., 2003). TP soils were found to have biochar that increased CEC (Liang et al., 2006), phosphorous nutrition and uptake, and decreased leaching of applied fertiliser N (Lehmann et al., 2003). Moreover, the latter showed that the amount of biochar in the soil is critical for the effects on plant growth and nutrition.
The porosity of biochar offers pore networks for water retention and microorganisms to thrive, but these can accelerate the decomposition of SOM and the biochar itself. Hamer et al. (2004) conducted a 60-day laboratory experiment, where they added glucose to different biochar-soil samples. After 60 days 0.78%, 0.72%, and 0.26% of carbonised maize, rye,
23 and wood were respectively mineralised in the controls and glucose additions promoted the decomposition of black carbon by 58%, 72%, and 115% relative to the controls. The biochar made of wood at less than 200oC proved to be more easily degradable because of its lower content of aromatic (chemically stable) carbon.
Liang et al. (2006) suggested that oxidation of biochars may not only mineralize organic C in soil but may also create negatively-charged surfaces causing higher CEC and nutrient retention in soil. A 120-day laboratory experiment (Cheng et al., 2006) compared
uncrushed particles with finely-ground biochar, with and without inoculation with microbes or manure incubated at 30o and 70oC. The results showed that aliphatic (labile) C
compounds were abiotically oxidized to CO2 and while CEC increased, the pH decreased
and aluminium saturation increased. This result contradicts the general assumption that CEC increases with higher pH (Lehmann, 2007).
According to Cheng et al. (2006), biochar contains a small fraction of labile compounds that degrade over time. Cheng et al. (2006) went on to explain that the abiotic processes were more relevant than biotic oxidation for fresh biochar. The rate of oxidation of biochar is surface dependent and finely ground biochar may oxidise faster. Long-term field studies are needed to elucidate this.
More recently, an aerobic incubation experiment showed that ageing (changes in biochar properties) of biochar can occur in any soil climate (-22o to 70oC) within a short period of 12 months (Cheng and Lehmann, 2009). Whether such changes will become more
important over longer periods of time is not certain.
One of the longest field experiments in the literature illustrated that biochar in soils could decrease organic C in the form of humus in boreal forests in Northern Sweden (Wardle et al., 2008a), and therefore increase CO2 emissions. The experiment compared mesh bags
filled with (i) humus from the forest, (ii) biochar, and (iii) a 50:50 mixture of humus and biochar. The bags were buried in three contrasting boreal forest sites and monitored over a 10 year period. The results indicated a considerable loss (of over 20%) of mass and humic
C in the mixture bags. The humus decomposition was attributed to the higher amount of microbial activity caused by adding biochar to the soil. Interactions of this type are typically described as priming, with increased or decreased turnover rates of native soil organic carbon referred as positive or negative priming, respectively. Both positive and negative priming effects due to biochar application have been reported in the literature (Woolf and Lehmann, 2012).
In response to Wardle et al. (2008a), Lehmann and Sohi (2008) commented that the
decrease of mass and C in the mixture bags could have come also from the labile fraction of biochar and not from the humus alone. However, Wardle et al. (2008b) pointed out that the bags buried with only biochar were in close contact with humus from the forest for 10 years but presented negligible mass loss. And the high amount of mass lost in the mixture bags was incomparable to the ones observed elsewhere (Cheng et al., 2006). Wardle et al.
(2008b) concluded that the strong advocacy to include biochar in carbon markets remains premature.
Based on the literature review, further characterisation of biochar-soil dynamics is required and could include analysis of the following factors:
x scale, baseline and test conditions, duration, and purpose of the study (eg. soil remediation, crop production, carbon sequestration);
x type of climate (eg. temperate, tropical, rainfall);
x type of soil (eg. sandy, clay, ferrosol, anthrosol);
x type of crop (eg. pine, corn, wheat, kiwifruit, apples, grapes);
x feedstocks for producing biochars (eg. orchard and vineyard prunings, logging residues, cereal straw, chicken and/or cow manure, sewage sludge);
x type of production process (eg. HTC, slow or fast pyrolysis) and respective parameters (eg. temperature, heating rate, residence time, pressure);
x post-production treatment (eg. inoculation with water, urine, manure, compost, synthetic fertilisers, microbes, lime, minerals);
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x follow-up and management (eg. watering, more fertilisers, extra biochar); and
x effects (eg. water holding capacity, pH, CEC, turnover rate, soil temperature, microbial activity, plant growth, nutrient leaching, toxins immobilization, GHG emissions, albedo).