[Influence of Biochar Application on Soil Nitrate Leaching and Phosphate Retention: A Synthetic Meta-analysis]

How to control non-point source pollution caused by leaching of soil nitrate and phosphate from agricultural land is currently an extremely important global environmental problem facing human society. Biochar, a carbon-rich material produced from various organic feedstocks using thermochemical technologies, has attracted much attention because of its great potential in soil improvement. Many studies have been carried out to investigate the effects of biochar application on the retention, utilization, and use efficiency of soil nutrients. Unfortunately, the results from individual experimental studies regarding the effects of biochar on soil nitrate leaching and phosphate retention differed greatly. Consequently, the underlying mechanisms related to reduction in nitrate and phosphate leaching/retention by biochar application, as well as the appropriate preparation conditions (or biochar type), remain unclear. In this study, the effects of biochar application on soil nitrate leaching and phosphate retention were systematically examined using the method of Meta-analysis (MA); based on these results, the inhibition mechanisms for nitrate leaching and enhancement mechanisms for phosphate retention were also explored. In total, 149 paired datasets from 41 articles and 180 paired datasets from 36 articles were collected for nitrate and phosphate, respectively. The MA results demonstrated that, regardless biochar and soil properties, biochar application could significantly reduce soil nitrate leaching by 37.1% and increase soil phosphate retention by 20.8%. Furthermore, the C/N ratio of biochar, heating treatment temperature, and biochar application amount indicated a significant effect on the response of soil nitrate leaching to biochar application. The specific surface area of biochar, heating treatment temperature, and soil organic carbon content had a significant effect on the response of soil phosphate retention to biochar application. Based on the results from MA, the potential mechanisms of soil nitrite leaching reduction and phosphate retention enhancement were further explored from different perspectives. Lastly, the biochars prepared from straw or wood materials and pyrolyzed at a medium temperature (400-600℃) or high temperature (>600℃) were recommended for reducing soil nitrate leaching and improving soil phosphate retention, respectively. In sum, the results presented in this study can provide a scientifically theoretical basis for the practical application of biochar in the control of soil non-point source pollution of nitrate and phosphate.