Biochar is widely applied as a soil amendment to facilitate plant growth, but the underlying mechanisms in coastal saline soils across seasons are still unclear. Greenhouse pot experiments were conducted to investigate the growth responses of two halophytes to different biochar doses (1–8%, w/w%) and identify the possible influencing factors in the Yellow River Delta, China. Seepweed (Suaeda salsa [L.] Pall.) and hybrid sorrel (Rumex patientia L. × Rumex tianschanicus A. Los. cv. K-1) were sequentially grown in the same pots over two consecutive seasons. Plant total biomass increased in up to 4% biochar and decreased at 8% biochar dose, similar to the trend of superoxide dismutase activity in the leaves. Soil pH and leaf malondialdehyde content both decreased, while soil nutrient contents and leaf catalase and peroxidase activities increased with increases in biochar application. Despite the distinct improvement of soil fertility with 8% biochar, electrical conductivity was more than twice that in the control group. At the end of the second season, higher biochar doses enhanced bacterial diversity (mainly species richness) and shifted the community composition towards potentially beneficial taxa (e.g., Firmicutes and Proteobacteria). Biochar application enhanced plant biomass accumulation mainly by promoting root volume growth, and this effect was achieved via increasing soil organic carbon, enriching rhizosphere Firmicutes, and alleviating leaf oxidative damage. The results of this study indicate that 4% biochar (equivalent to 5.4 t ha−1) can be used to facilitate plant growth of seepweed and hybrid sorrel in the coastal saline soil.
