Impacts of biochar application on soil nutrient availability and exchangeable based cations: a meta-analysis

In order to accurately assess the effects of biochar application on soil nutrient availability and exchangeable based cations, a total of 2000−2020 published literature was collected to obtain 648 matched data for no biochar application (blank or no addition) and single biochar application, and 430 matched data for no biochar application and biochar co-application with fertilizer. A meta-analysis was performed to quantify the effects of different biochar applications on soil N and P availability (total N, NH₄⁺-N, NO₃^–-N, Olsen-P), salt-based ions contents (K⁺, Ca²⁺, Na⁺, and Mg²⁺), and cation exchange capacity (CEC). Results showed that the application of biochar (alone or combined with chemical fertilizers) significantly increased soil N and P content by 14.0%−128.1%, and salt-based ions contents by 22.5%−270.2%, respectively. By comparing the effects of different biochar application, it was found that the increasing of soil N and P availability in chemical fertilizer combination treatment was higher than that in biochar application alone, while the increasing of salt-based ions contents in biochar application alone was higher than that in chemical fertilizer combination. Further analysis showed that when the pH of biochar was higher than 8, the application of biochar alone significantly increased soil Olsen-P content by 10.3%−58.5%. When the biochar pyrolysis temperature was higher than 500℃, the application of biochar alone increased the soil salt-based ions contents by 33.9%−384.7%. When the application rate of biochar was less than 10 t∙hm⁻², soil Olsen-P content increased higher under biochar combined with chemical fertilizer (374.1%) than that of biochar application alone (2.1%). In addition, applying biochar to soil with pH<6.5 could effectively increase soil N, P and Ca²⁺ contents, with Olsen-P content and CEC increased by 45.0% and 17.9%, respectively, under biochar application alone. Therefore, the application of biochar could effectively improve soil nutrient availability and ion exchange properties, and reducing environmental risks. In practical applications, biochar can be applied alone or in combination with fertilizer depending on the specific aims. Taking into account the characteristics of biochar, application rate, and soil conditions, the effective use of biochar to improve soil fertility will be the priority direction of the high-quality agricultural development in the future.