Straw return accompany with low nitrogen moderately promoted deep root

•Straw return treatments were more likely to improve soil structure, not only in topsoil layers but also in subsoil layers, especially at plough pan.•Nitrogen deficiency moderately significantly induced larger root length density of subsoil and its proportion.•The level of significance between soil bulk density of 20–40 cm soil layer and root length density in 0–120 cm soil layer reached a very significant level.•Suitable amount of straw returned back to field, combining with nitrogen deficiency moderately, achieved higher nitrogen efficiency dramatically, on the basis of sacrificing grain yield slightly.