Biochar Application to Soils - A Critical Scientific Review of Effects on Soil Properties, Processes and Functions

Biochar application to soils is being considered as a means to sequester carbon (C) while concurrently improving soil functions. This report reviews the evidence for potential effects of biochar incorporation into soils on soil properties and processes with implications for agronomic and environmental functions. Further to this, while beyond the main scope of this report, an overview of the main environmental and occupational health issues associated to biochar production and handling, and the socio-economic implications is given. Extrapolation of reported results must be done with caution, owing to the relatively small number of studies reported in the primary literature, when compared to possible instigation of biochar application to soils on a national or European scale. To try and bridge the gap between small scale, controlled experiments and large scale implementation of biochar application to a range of soil types across a range of different climates, a statistical meta-analysis was undertaken. A full search of the scientific literature led to a compilation of studies used for a meta-analysis of the effects of biochar application to soils and plant productivity. Results showed a small overall, but statistically significant, positive effect of biochar application to soils on plant productivity. The greatest positive effects were seen on acidic free-draining soils with other soil types, specifically calcarosols showing no significant effect (either positive or negative). This suggests that one of the main mechanisms behind the reported positive effects of biochar application to soils on plant productivity may be a liming effect although further research is needed to confirm this hypothesis. There is currently a lack of data concerning the effects of biochar application to soils on other soil functions. This means that although these are qualitatively and comprehensively discussed in this report, a robust meta-analysis on such effects is not possible currently. Effects that are expected (both positive and negative) include nutrient leaching to ground/surface waters, soil water retention, soil water repellency, N2O and CH4 emissions, albedo and soil biological activity although insufficient data are available at present to accurately quantify any of these effects or to report a consensus on their direction (either positive or negative). evidence has also been reported concerning a possible priming effect whereby accelerated decomposition of soil organic matter occurs upon biochar addition to soil. This has the potential to both harm crop productivity in the long term due to loss of soil organic matter, as well as releasing more CO2 into the atmosphere as increased quantities of soil organic matter is respired from the soil. This is an area which requires urgent further research and is discussed in more detail in this report.