The biochar effect: plant resistance to biotic stresses

Abstract

Biochar (charcoal) is the solid co-product of pyrolysis, the thermal degradation of biomass in the absence of oxygen. Pyrolysis also yields gaseous and liquid biofuel products. There is a growing interest worldwide in the pyrolysis platform, for at least four reasons: (i) pyrolysis can be a source of renewable biofuels; (ii) many biomass waste materials can be treated by pyrolysis and thus converted into a fuel resource; (iii) long-term sequestration of carbon dioxide which originated in the atmosphere may result from adding biochar to soil; and (iv) biochar soil amendment contributes to improved soil fertility and crop productivity. Currently, however, very little biochar is utilized in agriculture, in part because its agronomic value in terms of crop response and soil health benefits have yet to be quantified, and because the mechanisms by which it improves soil fertility are poorly understood. The positive effects of biochar on crop productivity under conditions of extensive agriculture are frequently attributed to direct effects of biochar-supplied nutrients and to several other indirect effects, including increased water and nutrient retention, improvements in soil pH, increased soil cation exchange capacity, effects on P and S transformations and turnover, neutralization of phytotoxic compounds in the soil, improved soil physical properties, promotion of mycorrhizal fungi, and alteration of soil microbial populations and functions. Yet, the biochar effect is also evident under conditions of intensive production where many of these parameters are not limited. Biochar addition to soil alters microbial populations in the rhizosphere, albeit via mechanisms not yet understood, and may cause a shift towards beneficial microorganism populations that promote plant growth and resistance to biotic stresses. In addition to some scant evidence for biochar-induced plant protection against soilborne diseases, the induction of systemic resistance towards several foliar pathogens in three crop systems has been demonstrated. There are indications that biochar induces responses along both systemic acquired resistance (SAR) and induced systemic resistance (ISR) pathways, resulting in a broad spectrum controlling capacity in the canopy. This review examines the effects of biochar soil amendment on the different soil-plant-microbe interactions that may have a role in plant health. Improvement of plant responses to disease can be one of the benefits gained from applying biochar to soil.