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Soil structure is a key factor to determine the different functions of soil, such as water infiltration and retention, soil permeability, and nutrients availability. The dynamic behavior of soil also depends mainly on the tillage, crop roots, and soil organic matter return. Black soil is one of the most important arable soil with high clay content, but hard soil structure is one of the main factors limiting crop growth. It is still lacking on the effect of organic amendment on soil pore structure in the black soil. In this study, a field experiment was carried out in the black soil region located in Hailun City, Heilongjiang Province of China, in order to clarify the effects of organic amendment and depths on soil physical properties and pore structure before maize sowing in 2019. Four treatments were conducted using random block design with four replicates, including shallow tillage with maize straw return in 0-15 cm soil layer (T1), deep tillage with maize straw return (T2), or combined maize straw and organic manure return (T3) in 0-35 cm soil layer, conventional tillage without organic amendment as control. Maize straw and organic manure were also incorporated into the soil layers by shovel using manual work, where the application rates were 10 000 and 30 000 kg/hm2, respectively. CT scanning was adopted to visualize and quantify soil pore structure after maize harvesting. The constant head method was used to measure the saturated hydraulic conductivity. The results demonstrated that the bulk density under T1, T2, and T3 treatments significantly decreased 3.77%-5.66%, compared with CK, while the saturated hydraulic conductivity increased by 2.2-5.00 times, and the field water capacity increased by 6.91%-11.01% within 0-15 cm soil layer (P<0.05). T2 and T3 treatments also improved the bulk density, saturated hydraulic conductivity, and field water capacity within >15-35 cm soil layer after one maize growing season. Furthermore, the number of soil pores and porosity of >500 μm aperture under T1, T2, and T3 treatments significantly increased by 18.1%-180% and 69.2%-256% (P<0.05) in 0-15 cm soil layer, respectively, compared with CK treatment (P<0.05). Specifically, the soil pore number and porosity of >1 000 μm aperture were more 6.06%-150% and 17.4%-196% under T2 treatment than the rest. It infers that the treatments with organic amendment increased the number of soil pores with optimal distribution. In addition, the complexity and connectivity of soil pore also increased significantly, in presence of connected and sender pore. The Euler number values under T1, T2, and T3 treatments were decreased by 21.7%-54.3%, whereas, the fractal dimension under T1 treatment increased by 1.5% in 0-15 cm soil layer (P<0.05), compared with CK treatment. T2 and T3 treatments improved the soil pore structure in >15-35 cm soil layer, as shown in that the Euler decreased by 65.9%-70.5%, anisotropy, fractal dimension, and circularity increased by 50.0%-110%, 12.8%-13.9%, and 12.2%-14.9%, respectively, compared with CK treatment (P<0.05). The Euler and porosity of >1 000 μm aperture presented the largest contribution both for the bulk density and saturated hydraulic conductivity in 0-35 cm soil layer, whereas, the anisotropy and Euler presented the largest contribution for the field capacity in 0-15 cm and >15-35 cm soil layers. Pearson correlation analysis indicated that the extremely significant negative relationship was found between bulk density and saturated hydraulic conductivity, field water capacity, soil porosity within 0-15 cm soil layer (P<0.01), while the extremely significant or significant correlation was found between bulk density and all indicators, excepting for the anisotropy and Euler. Consequently, the improved physical properties of soil were attributed to the fact that the organic amendment regulated soil structure, including optimal distribution of soil pore, complex, and connectivity of pore structure. Correspondingly, the organic amendment can be expected to serve as an effective approach for the pore structure of black soil. It is also necessary to consider the response of soil pore structure in the decomposition of returned organic manure in the future. [ABSTRACT FROM AUTHOR]

Calcic vertisol is a typical kind of low-yield field with a total area of about 4 million hm2 in China. Calcareous concretion with particle size larger than 2 mm has been one of the representative characteristics of calcic vertisol. The limitation of soil available water is an important reason for crop failure in this region. Therefore, the curve of soil water retention dominates the estimation of available water content. However, most studies focused on the effects of calcareous concretion on soil water retention of calcic vertisol mainly under laboratory conditions. It is lacking that under field conditions. In this study, both field surveys and laboratory experiments were conducted to explore the effect of calcareous concretion distribution in 0-1 m soil profile on soil water retention. Firstly, the spatial distribution of calcareous concretion was investigated in the study plot with 150 m length, 50 m width, and 1 m depth. After then, soil profiles (0-1 m soil depth) were chosen to divide into 5 layers with a 20 cm interval. Disturbed and undisturbed soil samples were collected in each layer. Physicochemical properties were evaluated to measure the distribution of soil particle size, size and content of calcareous concretion, bulk densities of soil, and calcareous concretions. Meanwhile, the curves of soil water retention were determined using high speed centrifuge. A pressure plate method was conducted to measure water retention curves in calcareous concretion. The results showed that the mass contents of clay, silt, and sand were 30.56%-39.75%, 39.88%-45.34%, and 18.67%-24.45%, respectively, which belonged to silty clay soil in the classification standard of the United States Department of Agriculture (USDA). Clayey particles decreased, but sandy particles increased significantly with the increase of soil depth. Calcareous concretion was mainly distributed in the soil depth of >20-100 cm, where the content and size increased as soil depth increased. Calcareous concretions content in the surface soil (0-20 cm) was very low (with mass content 0.02%), where only a few calcareous concretions of 2-5 mm were discovered. The maximum content of calcareous concretion was detected at > 80-100 cm (up to 11.42%), where the calcareous concretion content was 8.04% in the size of >8-30 mm. Soil bulk density ranged from 1.23-1.61 g cm-3. The densities of calcareous concretions were 2.01, 2.21, and 2.23 g cm-3 with the size of 2-5 mm, >5-8 mm, and >8-30 mm, respectively. Soil bulk density increased along with the calcareous concretion content increasing. Water retention curves of soil and calcareous concretion were well fitted by the van Genuchten model, with the determination coefficient larger than 0.95. The saturated water content of surface soil (0-20 cm) was significantly higher than that of 20-100 cm soil. Nevertheless, the Gravitational-water content in the surface soil (0-20 cm) was much larger than that in the soil of > 20-100 cm, which was related to macropores formed during tillage. In the subsurface (>20-100 cm), soil gravitational-water content and available water-holding capacity increased with the increase of soil depth, but the field capacity and wilting point decreased. Calcareous concretion maintained a non-negligible amount of water, 0.25, 0.22, and 0.20 cm3 /cm3 in the particles with the size of 2-5 mm, >5-8 mm, and >8-30 mm, respectively. But the water-holding capacities of calcareous concretion were significantly lower than that of soil. The effects of calcareous concretion on water-holding capacity mainly occurred in the subsurface soil. Soil saturated water content, field capacity, and wilting point decreased with increasing calcareous concretion. Interestingly, calcareous concretion content was positively correlated with the soil sand content, but negatively correlated with the soil clay content. Calcareous concretion can bind soil particles together to prevent sand weathering, and thereby effectively improve soil texture. More calcareous concretion led to higher sand contents, and thus increased the available water-holding capacity in soil. This finding can provide a theoretical basis to accurately assess the soil water-holding capacity in calcic vertisol for precision irrigation and high crop productivity. [ABSTRACT FROM AUTHOR]

In order to obtain the interaction parameters which can be used for discrete element simulation between the soil-engaging components and clayey black soil in northeast China, the Hertz-Mindlin with JKR Cohesion contact model in EDEM2.7 was used to simulate the clayey black soil. Aiming at the actual working environment with moisture content of 10%-20%, 2 kinds of clayey black soil with moisture content of 12.46%±1% and 17.15%±1% were prepared respectively. The rolling friction coefficient, recovery coefficient, JKR surface energy parameter and the static friction coefficient between the soil particles were used as the model calibrated parameters. The soil repose angle simulated under the simulation parameters was set as the response value. The regression model of the soil repose angle was established based on the Box-Behnken response surface method, and the regression model was optimized by using the soil repose angles under 2 moisture contents obtained by physical experiments. The optimal solution of the contact model parameters of the clayey black soil particles with 2 moisture contents was obtained. The parameter range of the discrete element contact model of clayey black soil with moisture content between 10%-20% were given. The soil repose angle and stacked shape obtained by the simulation experiment with the optimized solution had a high similarity with that of the physical test. At the same time, the static friction coefficients of 3 kinds of soil contact materials such as 65Mn plate, UHMW-PE plate and PTFE plate were tested under the conditions of 4 kinds of moisture content, which were 0, 12.46%, 17.15% and 23.5% respectively. Then for the 2 typical soil contact materials such as 65Mn and PTFE, the slope physical test bench were constructed for the clayey black soil with moisture content of 17.15%±1%. The slope simulation models of 65Mn and PTFE materials were established under the EDEM2.7 software platform. Based on the critical simulation parameters such as JKR surface energy parameter, static friction coefficient, rolling friction coefficient and recovery coefficient between soil particle and soil contact material, the simulated rolling distance of the soil ball on different soil contact materials was set as the response value. The excellent fit regression model of rolling distance for the 2 kinds of typical soil contact materials were established based on the response surface optimization method of Box-Behnkende. The 2 regression models were optimized by the sliding distance obtained by the physical experiment. Then the optimized solution of the contact model parameters of different materials was obtained. The simulation results showed that when the surface energy of JKR between soil and 65Mn was 5.5 J/m2, the recovery coefficient was 0.61, the static friction coefficient was 0.57 and the rolling friction coefficient was 0.056, the rolling distance simulation result was 153.56 mm, which was close to the average rolling distance obtained from physical experiments of 155.93 mm, and the relative error was 1.52%; When the surface energy of JKR between soil and PTFE was 4.08 J/m2, the recovery coefficient was 0.6, the static friction coefficient was 0.52 and the rolling friction coefficient was 0.045, the simulation result of rolling distance was 269.35 mm, which was close to the average rolling distance of 269.55 mm obtained from physical experiment, and the relative error was 0.07%. And the optimized parameters can be used to simulate the discrete parameters between the clayey black soil and the soil-engaging components. The study provides credible basic data for the design and simulation of agricultural machinery under clayey black soil conditions. [ABSTRACT FROM AUTHOR]