Meta-analysis of the effect of biochar application on the yield and quality of tomatoes in China

This study aims to quantitatively analyze the space-time effect and influence mechanism of biochar application on tomato yield and quality. Meanwhile, an optimal application amount of biochar was determined in tomato production. A meta-analysis was carried out to evaluate the effects of biochar characteristics, soil conditions, field measures, and spatiotemporal patterns on the tomato yield and quality under biochar application. Literature was searched to integrate the published field data from the relevant experiments. Meanwhile, path analysis was constructed to explain the interaction among the factors. Non-biochar application was used as the control treatment. 24 valid cases were screened from the collected literature to obtain 138, 66, 54, 41, 52, and 14 datasets on the yield, Vc, soluble sugars, soluble solids, sugar-acid ratio, and nitrate content of tomatoes, respectively. The experiments lasted from 2012 to 2021. The effect size was determined as the indicator in the meta-analysis. Important factors were selected for the effectiveness of biochar production. The top three factors were identified in the descending order of direct flux coefficients. The results showed that the biochar application significantly increased the tomato yield, fruit Vc, soluble sugar, and soluble solids content by 30.7% (95% CI: 27.6%-33.9%), 13.8% (95% CI: 10.8%-16.9%), 6.7% (95% CI: 3.3%-10.2%), and 10.7% (95% CI: 5.8%-15.8%), respectively, whereas, significantly reduced the tomato fruit nitrate content by 3.1% (95% CI: -6.7%-0.8%), and there was the non-significant effect of 2.3% (95% CI: -3.0%-7.9%) on the increase in the tomato fruit sugar to acid ratio. No publication bias was found, because both yield and quality data were linearly correlated by the publication bias test, indicating that the data conformed to the normal distribution ( P <0.01). The groups of data with and without biochar application were almost always distributed within the confidence interval of the normal percentile plot. Subgroup analysis of tomato yield showed that the effect of biochar on tomato yield had a significant temporal and regional pattern. The effect of biochar application on tomato yield showed a fluctuating growth trend. The increasing tomato yield was found in North China, where there was no obvious in Northeast China. There was a positive correlation between soil organic matter content and tomato yield increase rate, whereas, a negative correlation between soil pH value and tomato yield increase rate. The tomato yield increase rate increased first and then decreased with the increase in the soil's total nitrogen and potassium nitrate content. Field management measures on tomato yield showed that a yield increase of 37.8% (95% CI: 33.5%-42.1%) was achieved when biochar was applied at a rate of ＞40-80 t/hm². The optimal combination was achieved in the biochar pyrolysis temperatures of 400-600 °C and biochar pH values of ＞8-9. The application amount of biochar, pH value of biochar, and yield year were the three main factors on the yield-increasing effect of biochar. The finding can also provide theoretical support for the optimal biochar application rates in tomato production.