Biochar Application Can Improve Most of the Chemical Properties of Acidic Soils: A Global Meta-Analysis

Biochar has been widely used to improve the properties of acidic soil properties. However, the effectiveness of biochar in acidic soil properties was debated and has been less focused on in previous reviews. The present meta-analysis aimed to determine the importance of the biochar pyrolysis temperature (BPT) and the effect of biochar properties on acidic soil properties. The Pearson correlation and a meta-analysis involving the data obtained from 50 peer-reviewed publications showed that BPT has a significant positive correlation with biochar pH (BpH), biochar total carbon, biochar ash (BAs), and the biochar carbon to nitrogen ratio (BC/N), and a significant negative correlation with biochar total nitrogen (BN). The biochar feedstock straw was more effective in increasing the acidic soil pH (SpH), soil electrical conductivity (SEC), soil available phosphorus (SAP), and soil organic matter (SOM). The biochar application rates ≥ 80 t ha–1 significantly increased the acidic SpH, soil available nitrogen (SAN), soil available potassium (SAK), and SOM. The BpH ≥ 10 increased all the observed acidic soil properties except SEC. The BC/N 30–45 significantly increased SAN, SAP, and SOM, while the BC/N ≤ 30 was more effective at increasing SpH and SAK. The relationship of BN, biochar total phosphorus, and biochar total potassium with soil properties was inconsistent except that BN ≥ 1% not only increased the SAN but also increased the SAP and SAK. Except for SAK, BAs ≤ 15 significantly increased all observed soil properties. According to the PLS-SEM, BPT affects biochar properties and then affects soil properties (P < 0.05). Generally, the application of biochar in acidic soil increases all the observed soil properties according to the grand mean total effect size. This article gives us a clear image that if the SpH < 7, then BPT ≥ 800, BF straw, and BpH > 10 can be used to improve the acidic soil properties.