Objectives  Apple orchard soil is least disturbed after transplanting of apple trees, so the soil often becomes compacted after years’ growth. We studied the effects of rice husk biochar on reversing the adverse impaction.

Methods  Two-year old potted apple trees (Malusdomestica ‘Red Fuji’) were used as the test material, with two types of rootstocks of Malushupehensis var. pingyiensis Jiang and Malusrobusta Rehd, respectively. The pot soil was subjected respectively to four treatments, mixed with or without rice husk biochar, compacted or not compacted. After the apple trees grew for 60 days, the soil porosity, oxygen concentration, sulfur fraction contents were analyzed. The apple root sulfate and H₂S content and the sulfur metabolism related enzyme activities, and root activity and root architecture were measured.

Results  Soil compaction significantly decreased the content of water soluble sulfur, sulfate and H₂S, and the activities of ATP sulfurylase (ATPS), O-acetylserine (thiol) lyase (OASTL) and L-cysteine desulfhydrase, D-cysteine desulfhydrase (L-CD, D-CD), and decreased apple root activity, root length density, root length, root volume and root fractal dimension. The decreases for the roots of M. hupehensis var. pingyiensis as the rootstock were higher than those of the rootstock of M. robusta. Compared with the treatment without rice husk biochar, the soil porosity, soil oxygen concentration, contents of water soluble sulfur, sulfate and H₂S in roots, and the activity of ATPS, OASTL and L-CD, D-CD in roots, root activity, root length, root length density and root fractal dimension were all improved in treatment with rice husk biochar under the normal or compact soils. The increases for the rootstock of M. robusta were higher than M. hupehensis var. pingyiensis under the compact soils, and that of the young apple of two rootstocks in the compact soil was greater than in the normal soil.

Conclusions  Soil compaction significantly decreases the content of water soluble sulfur in soil and inhibites apple root sulfur metabolism and growth. Addition of rice husk biochar can significantly alleviate the adverse impaction, and improve root growth. Therefore, it is recommended to add rice husk biochar in soil before transplanting of apple trees.