Characterisation of biochar-soil dynamics

Several 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. The IBI (2015) 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. Based on surface area, pH, and cation exchange capacity (CEC), Lehmann (2007) proposed a temperature between 450-550oC to optimise the characteristics of biochar for use as soil amendment (Figure 1).

Figure 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).

Figure 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 (longer C sequestration) and co-products yield (more energy). It is important to note that the structure of biochar offers pore networks for water retention and microorganisms to thrive. Further characterisation of biochar-soil dynamics is required and could include analysis of the following factors:

  • scale, baseline and test conditions, duration, and purpose of the study (eg. soil remediation, crop production, carbon sequestration);
  • type of climate (e.g. temperate, tropical, rainfall);
  • type of soil (e.g. sandy, clay, ferrosol, anthrosol);
  • type of crop (e.g. rice, corn, wheat, soybean, coffee, chillies);
  • feedstock for producing biochar (e.g. rice husk, corn stover, animal manure);
  • type of technology and respective parameters (e.g. temperature, heating rate, residence time);
  • post-production treatment (e.g. combine with water, urine, manure, compost, synthetic fertilisers, microbes, lime, clay, minerals);
  • application into soils (e.g. application method and rate, depth, particle size);
  • follow-up and management (e.g. watering, more fertilisers, extra biochar); and
  • effects (e.g. on water holding capacity, pH, CEC, turnover rate, soil temperature, microbial activity, plant growth, nutrient leaching, toxins immobilisation, GHG emissions, albedo).