Biochar effects on salt-affected soil properties and plant productivity: A global meta-analysis

Biochar has been recognized as a promising practice for ameliorating degraded soils, yet the consensus on its effects remains largely unknown due to the variability among biochar, soil and plant. This study therefore presents a meta-analysis synthesizing 92 publications containing 987 paired data to scrutinize biochar effects on salt-affected soil properties and plant productivity. Additionally, a random meta-forest approach was employed to identify the key factors of biochar on salt-affected soil and plant productivity. Results showed that biochar led to significant reductions in electrical conductivity (EC), bulk density (BD) and pH by 7.4%, 4.7% and 1.2% compared to the unamended soil, respectively. Soil organic carbon (by 55.1%) and total nitrogen (by 31.3%) increased significantly with biochar addition. Moreover, biochar overall enhanced plant productivity by 31.5%, and more pronounced increases in forage/medicinal with higher salt tolerance than others. The results also identified that the soil salinity and biochar application rate were the most important co-regulators for EC and PP changes. The structural equation model further showed that soil salinity (P < 0.001), biochar pH (P < 0.001) and biochar specific surface area (P < 0.01) had a significant negative effect on soil EC, but it was positively impacted by biochar pyrolysis temperature (P < 0.05). Furthermore, plant productivity was positively affected by biochar pH (P < 0.001) and biochar feedstock (P < 0.01), while negatively influenced by biochar pyrolysis temperature (P < 0.01). This study highlights that woody biochar with 7.6 < pH < 9.0 and pyrolyzed at 400-600 °C under 30-70 t ha^-1 application rate in moderately saline coarse soils is a recommendable pattern to enhance forage/medicinal productivity while reducing soil salinity. In conclusion, biochar offers promising avenues for ameliorating degradable soils, but it is imperative to explore largescale applications and field performance across different biochar, soil, and plant types.