Objectives The paper summarized the effects of biochar and phosphate-solubilizing biofertilizers (PSBF) on soil phosphorus availability and crop growth across China.

Methods Fifty research papers published between 2014 and 2024 were collected, and a total of 233 datasets were extracted, including soil total and available P content, and phosphatase activity, and crop growth parameters such as plant height, above-ground biomass, and under-ground biomass. A meta-analysis method was used to elucidate the impacts of various application ways of biochar and PSBFs. Additionally, regression analysis was conducted to examine the relationships between the application rates of biochar and PBSFs and the soil and crop indicators.

Results 1) The increment of soil AP resulted by application of biochar, PSBF, and both together were 46.44%, 31.88%, and 138.11%, indicating the better effect of combined application. 2) All types of PSBFs (containing phosphate-solubilizing bacteria, fungi, and together) increased soil AP and crop growth notably, and the combined application of PSBFs with biochar resulted in a 15-fold higher effect in AP than the single application of PSBFs, achieving an effect size of 395.50%. 3) Biochar types showed significantly varied effects on soil phosphorus availability and crop growth, and the mixed application of biochar derived from different sources exhibited the most pronounced effects. Furthermore, the integrated application of biochar and PSBFs had synergistic effect on increasing soil AP and crop height. 4) The effects of biochar and PSBFs on soil AP varied across various cropping systems and regions in China. Biochar alone significantly increased AP in forage and cash crop soils by 52.79% and 47.67%, PSBF alone enhanced AP by 52.98% in grain crop soils. Greater increases in soil AP were achieved in northern China than in southern China, with differential effects of 65.24% (biochar alone), 16.04% (PSBF alone), and 78.95% (PSBF and biochar together). 5) Within the reported ranges, positive correlations were observed between the application rates of biochar and PSBF and soil AP, phosphatase activity, and crop height.

Conclusions The application of biochar and phosphate-solubilizing microbial fertilizers shows general effect in enhancing soil phosphorus availability and promoting crop growth, and their combined application exhibits a notable synergistic effect. Composite microbial fertilizers containing a combination of phosphate-solubilizing bacteria and fungi outperform those with a single microbial species. Whether applied individually or in combination, biochar and phosphate-solubilizing microbial fertilizers demonstrate a greater effectiveness in increasing the available phosphorus content in northern soils compared to southern soils. Within the reported application rates of biochar and phosphate-solubilizing microbial fertilizers, there is a significant positive correlation between the application amount and soil available phosphorus content, phosphatase activity, as well as plant height.