Effects of deep tillage and straw return on soil structure of high-yield spring maize field

There are some structural issues of farmland soils in the irrigated Inner Mongolia Plain, such as hard (bulk density of 1.55-1.62 g·cm^-3), shallow plough layer (0-16 cm) and thick plow pan (45 cm). To solve these problems and achieve optimum tillage depth of 22 cm and soil bulk density of 1.1-1.3 g·cm^-3 of the topsoil layer, a test with 1, 2, 3 and 4 years of continuous straw return plus deep tillage was conducted in a high-yield spring maize field in the Science-Technology Demonstration Garden of Inner Mongolia Agricultural University. The full maize straw (20 034. 97 kg·hm^-2) was crushed after autumn harvest and returned to soil combined with 40 cm deep tillage. Four treatments for 1-4 years of straw return plus deep tillage (SF1-SF4) were set and no deep tillage with maize straw return was the control (CK). The soil bulk density, hardness, aggregates and their stability, fertility and pH value were studied. The results showed that:1) in 0-40 cm layer, soil bulk density and hardness significantly decreased under SF1-SF4 treatments compared with CK. 2) In 0-20 cm soil layer, the proportion of aggregates > 0.25 mm (R_0.25), mean weight diameter and geometric mean diameter of SF4 significantly dropped compared with CK. The percentage of aggregate disruption under SF1 significantly decreased by 9.56% compared with CK, the sabotage water-stable aggregates decreased significantly with years of the experiment. The fractal dimension of aggregates of SF4 significantly increased by 7.30% compared to CK. 3) In 20-40 cm layer soil, the R_0.25 of SF1 and SF2 were significantly increased by 13.69% and 17.83%, respectively, compared with CK; the mean weight diameter and geometric mean diameter of aggregates of SF2 were significantly increased by 23.92% and 53.38%. Then percentage of aggregate disruption of soil aggregates significantly decreased by 9.20% (SF1), 3.02% (SF2), 8.38% (SF3) and 3.16% (SF4) compared with CK. Sabotage water-stable aggregates significantly increased by 13.58% (SF1), 16.49% (SF2), 22.67% (SF3) and 25.42% (SF4) compared with CK. Fractal dimension of aggregates of SF2 significantly decreased by 7.39% compared with CK. 4) Soil organic matter content significantly increased by 16.32% (SF1), 24.78% (SF2), 25.07% (SF3) and 25.56% (SF4) compared with CK. Also, available nitrogen, phosphorus and potassium significantly increased compared with CK. Soil pH of SF3 and SF4 significantly decreased respectively by 1.95% and 1.73% compared with CK. In conclusion, the 2-year deep tillage with maize straw return was most suitable for improving the structure of soil plow pan. The 3-year and 4-year deep tillage with maize straw return were suitable for improving the structure of topsoil layer. The application of deep tillage with maize straw return not only improved soil structure of plough layer, but also increased soil organic matter, decreased the percentage of aggregate disruption of soil aggregates and the sabotage water-stable aggregates, and enhanced tillage layer fertility.