Biochar is charred (pyrolysed) biomass that is engineered to have specific physicochemical properties that make it both more recalcitrant to decomposition and beneficial to soil functioning. Recalcitrance is increased by the transformation of the mainly aliphatic molecular structure of biomass to the mainly aromatic molecular structure of biochar during pyrolysis, i.e. the thermal degradation/transformation of biomass in the absence of oxygen. The estimated residence time of biochar-carbon is in the range of hundreds to thousands of years while the residence time of carbon in plant material is in the range of decades. Consequently, the CO2 captured from the atmosphere into plant biomass during photosynthesis and transformed into biochar during pyrolysis, is retained in the soil for longer, thereby increasing the soil carbon pool and decreasing the atmospheric soil carbon pool. If emissions of other greenhouse gasses (e.g. CH4, NO2) are not increased (or even reduced), and the biochar production/transport system is efficient, biochar has been shown to sequester carbon in Life C ycle Assessment modelling studies.

 

Biochar is further thought to substantially improve soil properties relevant to agronomic and environmental performance. The underlying mechanisms are thought to be those of increased nutrient as well as water retention, provided by the electrostatically charged surface area of biochar. Furthermore, biochar was found to directly affect soil microorganisms and their activity. Nonetheless, the exact role of these mechanisms remains poorly elucidated. Biochar research is in its relative infancy and much more data are required to allow robust predictions of the effects of biochar application to soils. The effect of biochar on crop yield was studied already back in the 1800s. Even disease suppression and hydric stress effects of biochar were investigated in the 1800s and early 1900s, albeit to a limited extent and with variable results.

 

Thus, entrepreneurial efforts in charcoal production for agricultural use in the 1800s were thwarted by the lack of consistent results, which resulted in charcoal being regarded as a very expensive amendment and typically not worth the cost to farmers. Interest in biochar and its effects on crop productivity has been increasing rapidly in recent years, with a growing body of data published in the primary literature. It now looks like we are perhaps ready to reconcile the differences that Durden mentioned in 1849 and move towards a mechanistic understanding of the effects of biochar when applied to soil. The recent work on biochar mainly focused on crop yield effects.