Wood biochar in soil enhances the promotion of KNO on sulfur accumulation in apple trees by regulating root sulfate assimilation.

To determine how potassium nitrate (KNO) effect apple roots and sulfate assimilation in the presence of wood biochar in soil, KNO was applied to the root-zone soil without or with 150-day naturally aged wood biochar (1% w/w) in soil. Soil properties, root architecture, root activity, the accumulation and distribution of sulfur (S), enzyme activity, and gene expression related to sulfate uptake and assimilation in apple trees were analyzed. Results showed that KNO and wood biochar application exhibited synergistic effects on improving S accumulation and root growth. Meanwhile, KNO application increased the activities of ATPS, APR, SAT, OASTL and upregulated the expression of ATPS, APR, Sultr3;1, Sultr2;1, Sultr3;4, and Sultr3;5 in both roots and leaves, and the positive effects of KNO addition on both genes and enzyme activity were enhanced by wood biochar. Wood biochar amendment alone promoted the activities of enzymes described above, upregulated the expression of ATPS, APR, Sultr3;1, Sultr2;1, Sultr3;4, and Sultr4;2 in leaves, and enhanced S distribution in roots. KNO addition alone decreased S distribution in roots and increased that in stems. In the presence of wood biochar in soil, KNO application decreased S distribution in roots but increased that in both stems and leaves. These results indicated that the wood biochar in soil enhances the effect of KNO on S accumulation by promoting root growth and sulfate assimilation in apple trees.