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Black soil in northeast China is well known as high natural fertility for grain production, but the growth and yield of crop can be limited due to the high clay content of black soil can deteriorate physical properties. In this case, both tillage and straw return can be expected to serve as the important agronomic practices, in order to efficiently improve soil physical properties. However, the optimal depth of tillage and straw return into soil for favorable soil physical properties still remained unknown, particularly in black soil region. In this study, a field experiment, referred to the depth of tillage and straw return into soil, was conducted for six years in the black soil region in northeast China, in order to investigate the effects of incorporation depth of tillage and straw return into soil on soil physical properties of black soil. Seven treatments consisted of four replicates and random block design, including no tillage (D0), shallow tillage (0-20 cm) without or with straw return (D20 or D20S), deep tillage (0-35 cm) without or with straw return (D35 or D35S), super deep tillage (0-50 cm) without or with (D50 or D50S) straw return. In the treatments with straw return, a straw rate of 10 000 kg/hm2 was set to return into the corresponding tilled soil layer. Bulk density and water holding capacity were measured using cutting ring method, while the hydraulic conductivity was determined using constant head method, and the aggregate was measured using wet sieving method. The results showed that: 1) Compared with initial soil values, D0 treatment significantly increased soil bulk density, while decreased soil porosity, water holding capacity, hydraulic conductivity and >0.25 mm water stability aggregate (WAS>0.25) within 0-20 cm soil layer (P<0.05). However, there was no significantly impact on those indicators in soil layer within 20-50 cm (P>0.05). 2) In D20, D35 and D50 treatments, there was no obvious impact on the soil physical properties within 0-20 cm soil layer with exception for WAS>0.25 in D50 treatment, but D20S and D35S treatments remarkably all indicators of soil physical properties within 0-20 cm soil layer. 3) D35, D35S, D50 and D50S treatments significantly improved soil physical properties within >20-35 cm soil layer with exception of bulk density in 2014, while D50 and D50S treatments dramatically enhanced soil physical properties within in >35-50 cm soil layer (P<0.05). Specially, aeration porosity and hydraulic conductivity increased significantly, within the corresponding soil layers, while the higher effect was found in the treatment with straw return, compared with only tilled treatments. The comprehensive evaluation also showed that D35S and D50S treatments reached the best effect for improving soil physical properties. Therefore, the findings demonstrated that deep tillage or super deep tillage with straw return can efficiently improve soil physical properties within the whole tilled soil layers, concurrently to increase the depth of tilled soil layer, and thereby to enhance the water infiltration rate and soil water storage in black soil with heavy clay. In view of the positive effect of deep tillage and super tillage with straw return on soil physical properties and the limit of no tillage, the combined tillage practices of deep tillage (or with straw return) and no tillage were proposed for the black soil with high clay in northeast China and other regions. [ABSTRACT FROM AUTHOR]

The effect of high concentration sodium salt solution infiltration in reclaimed water on soil hydraulic characteristics is one of the key problems of soil ecological environment degradation caused by long-term low-quality water irrigation. In this paper, the hydrodynamic parameters of 5 kinds of subtropical soil, such as clay fluvo aquic soil, sandy fluvo aquic soil, red soil, paddy soil and purple soil, were measured by constant head penetration method and one-dimensional horizontal soil column imbibition method. The influence of sodium salt solution concentration on the hydraulic properties of soil was investigated, and the mechanism of the physical and chemical properties of soil affecting the hydraulic conductivity was analyzed. The results showed sodium slat solution infiltration could enhance soil wet front migration rate of clay fluvo aquic soil, sandy fluvo aquic soil and paddy soil maximally by 33.0%, 32.5% and 93.5%, lower that of red soil maximally by 34.8%. The soil clay content, exchangeable iron and aluminum content mainly inhibited soil water movement, while silt content, exchangeable calcium and magnesium content promoted soil water movement. Sodium salt accelerated water movement in clay fluvo aquic soil, sandy fluvo aquic soil and paddy soil. Relative soil water diffusivity increased by 22.0%, 37.3% and 39.7% respectively. Sodium salt inhibited the water diffusion of red soil and purple soil. The higher salt concentration could result in the smaller relative soil water diffusivity. At 15 g/L salt concentration, the soil water diffusivity decreased by 30.5% for red soil and 42.1% for purple soil, respectively. Relative soil water diffusivity of clay fluvo aquic soil, sandy fluvo aquic soil, red soil and purple soil fluctuated with the increase of soil water content, while the relative soil water diffusivity of paddy soil decreased with the increase of soil water content. Relative soil water diffusivity of paddy soils showed a good parabolic relationship with salt concentration (R²=0.987), while red soils and purple soils showed a good linear relationship with salt concentration (R²>0.933). The saturated hydraulic conductivity of clay fluvo aquic soil treated with sodium salt solution increased most obviously, with a maximum increase of 77.3%, followed by paddy soil, with a maximum increase of 25.1%. With the increase of salt solution concentration, the relative saturated hydraulic conductivity of soil decreased first and then increased. In the range of 1-10 g/L salt concentration, the saturated hydraulic conductivity of soil had a good parabolic relationship with the sodium salt concentration (R² was not less than 0.807), and the saturated hydraulic conductivity of soil was the smallest when the concentration of each extreme point of soil was about 5 g/L. Therefore, the salt concentration of reclaimed water should be moderately controlled to a level lower than the extreme point concentration. [ABSTRACT FROM AUTHOR]