Objectives Because of the long-term practice of shallow rotary tillage, subsoil has become compact and organic matter content has decreased in the northern Huang-Huai-Hai region, where wheat-maize rotation system dominates. This paper explores the effects of different amount of deep incorporation of crushed straw on subsoil fertility to find a feasible method to improve soil fertility in this region.

Methods Using micro plots, a one-time deep-buried straw incorporation experiment was set up with low (6000 kg/hm², T1), medium (12000 kg/hm², T2), high (18000 kg/hm², T3) rates, along with the conventional rotary tillage non-straw returning (CK). The effects on soil water storage (SW) and soil compactness (SC), soil organic carbon (SOC) and total nitrogen content (TN), available nitrogen (AN), available phosphorus (AP) and readily available potassium (AK), crop root biomass and grain yield were studied during 2013–2016.

Results 1）The SW of the 0–40 cm depth increased with the straw dosage increasing, and the SW of the T3 and T2 were increased by 50.94% (P < 0.05) and 59.77% (P < 0.05) on average in the winter wheat season (P < 0.05). The increase of SW in the summer maize season was lower than that in the winter wheat season, which indicated that increasing straw dosage was more beneficial to soil water conservation in dry season. Higher returning rate could decrease the SC of the subsoil by 60%, and the soil pH tended to be neutral. 2) Soil nutrients in the subsoil were also significantly increased with straw dosage increasing. Under T2 and T3 treatments, SOC was increased by 7%–20% (P < 0.05), TN increased by 7%–18% (P < 0.05), available nutrients increased by 10%–30% (P < 0.05), C/N and nutrient storage capacity of subsoil were also increased. 3) Principal component analysis showed that soil fertility of T3 was slightly higher than T2, while that of T1 was the lowest. One-time incremental-deep straw returning could maintain higher soil fertility for a long time. 4) The grain yield and root biomass of wheat and maize were significantly increased by medium and high rates of straw application. The grain yield of T2 was the highest during 3 years and increased by 7.02% (wheat) and 5.11% (maize) (P < 0.05). The root biomass of winter wheat was increased by 21.9% and 16.0% (P < 0.05) under T2 and T3, and for summer maize they were 18.4% and 19.5% (P < 0.05), respectively. However, the straw biomass was not significantly different between T2, T3 and CK. T2 had a higher dominance than T3 in the early stage.

Conclusions One-off bury of straws at 40 cm deep can significantly improve subsoil structure, increase soil nutrient storage capacity, and raise root biomass and grain yield. The fertility of subsoil can be significantly improved by burying 12000–18000 kg/hm² straw in the northern Huang-Huai-Hai region.