Effects of subsoiling depth on topsoil properties, crop yield and water use efficiency in Lime Concretion Black soil

Lime Concretion Black soil is widely distributed in the south of the Huang-Huai-Hai Plain with area of 3.7 million hm², which is one of the main middle-to-low-yielding soil types in the region. Currently, the long-term continuous farming system is made possible by using small-sized 4-wheel tractors. However, owing to the years of over exploitation and improper mechanical plough, the effective depth of topsoil has gradually decreased and the plow pan thickened. The problem of shallow, solid and little topsoil in the plough layer has limited the ability of storage and release of fertilizer and continuous increase in crop yield in the Lime Concretion Black soils. Subsoiling is one of the main technologies in conservation tillage and the area with sub-soling in China has exceeded 10 million hm². Studies have shown that subsoiling can improve soil properties in plough layer by reducing soil bulk density and penetration resistance, increasing soil porosity, hydraulic conductivity and infiltration rate, and creating more favorable soil environment for root growth and crop production than rotary tillage. However, most studies have been carried out only on the 30 cm depth of subsoiling and little research has been focused on the effects of different depths of subsoiling on soil characteristics, root growth, crop yield and water use efficiency under wheat-maize cropping system in Lime Concretion Black soils. The objective of this study was to determine the effects of the depth of subsoiling on topsoil properties, crop yield and water use efficiency and to build the basis for establishing suitable depth of subsoiling in Lime Concretion Black soils. To that end, a multi-year experiment with four depths of subsoiling30 cm (SS30), 40 cm (SS40), 50 cm (SS50) and 60 cm (SS60) and rotary tillage (RT) as the control was carried out to study the effects of different depths of subsoiling on soil penetration resistance, soil three-phase (R) value, root growth, crop yield and water use efficiency. The results showed that increasing depth of subsoiling significantly reduced soil compaction, created more suitable soil three-phase (R) value and thus promoted crop root growth. In the four depths of subsoiling, decrease in soil penetration resistance and soil three-phase (R) value were the highest under SS60 treatment, root weight of winter wheat was the highest under SS40 treatment and root weight of summer maize the highest under SS50. Subsoiling not only increased crop yield, but also increased water use efficiency. Annual crop yield was the highest under SS30 treatment, 12.2% higher than that under RT. There was no significant difference in annual crop yield between SS30 and SS40 treatments. Annual water use efficiency was the highest under SS50 treatment, which was 12.7% higher than that under RT. Also there was no significant difference in annual crop yield among SS30, SS40 and SS50 treatments. Annual water use efficiency was respectively 9.1% and 8.8% higher under SS30 and SS40 treatments than that under RT. Therefore, subsoiling at the 30-40 cm depth was a suitable depth in Lime Concretion Black soils.