Influences of soil and biochar properties and amount of biochar and fertilizer on the performance of biochar in improving plant photosynthetic rate: A meta-analysis

Most studies found the benefit of biochar in increasing plant photosynthetic rate (PR), but how experimental conditions affect PR response to biochar application remains unclear. Therefore, we performed a meta-analysis by including 965 pairwise comparisons from 135 studies. Generally, biochar application enhanced PR by 23 % on average, a statistically significant increase relative to the control. However, a large variation of percent change in PR ranging from -59 to +1267 % was also observed under different experimental conditions. Specifically, biochar properties including contents of potassium (K), phosphorus (P), sodium (Na), carbon (C) and ash and their determining factors (feedstock and pyrolysis temperature) significantly influenced PR responses to biochar addition, while biochar pH could not be used alone as an effective predictor of PR increases after biochar treatment. There was a potential match between soil pH and biochar pH, determining the performance of biochar in improving crop PR. However, the complementarity between soil pH and biochar pH was not sufficient to achieve the maximum increase in PR, appropriate biochar application rate was needed at the same time. The most suitable application rate of biochar was 10.1-20 t ha-1 or 2.01-4 %. Moreover, it is essential to take the compatibility and complementarity between biochar, soil texture and management factors such as Nitrogen (N) application rate and growing environment into consideration. Overall, adding biochar into coarse textured soils or heavy metal contaminated soils is highly recommended for increasing plant PR. Co-application of biochar with nitrogen fertilizer is also worth being advocated. When the N application rate was 101-200 kg ha-1 or <= 0.015 %, the maximum increase in PR was obtained. These findings can provide suggestions for biochar application in agricultural production to optimize PR benefits.