A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis

Increased crop yield is a commonly reported benefit of adding biochar to soils. However, experimental results are variable and dependent on the experimental set-up, soil properties and conditions, while causative mechanisms are yet to be fully elucidated. A statistical meta-analysis was undertaken with the aim of evaluating the relationship between biochar and crop productivity (either yield or above-ground biomass). Results showed an overall small, but statistically significant, benefit of biochar application to soils on crop productivity, with a grand mean increase of 10%. However, the mean results for each analysis performed within the meta-analysis covered a wide range (from −28% to 39%). The greatest (positive) effects with regard to soil analyses were seen in acidic (14%) and neutral pH soils (13%), and in soils with a coarse (10%) or medium texture (13%). This suggests that two of the main mechanisms for yield increase may be a liming effect and an improved water holding capacity of the soil, along with improved crop nutrient availability. The greatest positive result was seen in biochar applications at a rate of 100 t ha⁻¹ (39%). Of the biochar feedstocks considered and in relation to crop productivity, poultry litter showed the strongest (significant) positive effect (28%), in contrast to biosolids, which were the only feedstock showing a statistically significant negative effect (−28%). However, many auxiliary data sets (i.e. information concerning co-variables) are incomplete and the full range of relevant soil types, as well as environmental and management conditions are yet to be investigated. Furthermore, only short-term studies limited to periods of 1 to 2 years are currently available. This paper highlights the need for a strategic research effort, to allow elucidation of mechanisms, differentiated by environmental and management factors and to include studies over longer time frames.