Soil Physics, Erosion and Conservation
Sahar Akhavan; Soheila Ebrahimi; Maryam Navabian; Mahmoud Shabanpour; Alireza Movahedi; Ali Mojtahedi
Abstract
Introduction Soil macropores are the prominent factor in the transfer of wastewater, fertilizers, and microorganisms, including fecal bacteria to deeper soils and even underground waters. On the other hand, a vast majority of land in Iran is located in arid and semi-arid regions. Therefore, the use of ...
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Introduction Soil macropores are the prominent factor in the transfer of wastewater, fertilizers, and microorganisms, including fecal bacteria to deeper soils and even underground waters. On the other hand, a vast majority of land in Iran is located in arid and semi-arid regions. Therefore, the use of salty and unconventional waters has recently gained considerable importance. The aim of this study is to investigate the preferential transportation and storage of Escherichia coli (as an important bacterium in commonly used fertilizers) under the condition of saline water application. Materials and methods The laboratory studies were conducted in a preferential flow system with artificial macropores with different diameters (1 and 2 cm) and varying salinity treatments (1, 2 and 4 dsm-1) under a saturated flow condition. The leaching test was performed at 20°C within several phases. Microbial transfer tests were carried out in two phases with boundary conditions and flow velocities similar to the zero-phase condition. In order to evaluate the bacteria in the soil profile, after the end of the bacterial transfer test, the soil column was divided and cut into 3 layers. Two samples were collected from 3 depths and at macropore and matrix zones. The experiment was conducted in a factorial format and completely randomized design with three replications. The results showed that the mutual effect of salinity and macropore diameter was significant (at 5%) on mean output concentration (Cav), soil filtration coefficient (fλ), relative absorption index (SR), and maximal predicted depth of bacteria transfer (Zmax). Results The results indicated that the bacteria were affected by the treatments during the transfer, so that with increasing the salinity and reducing the diameter of macropores, the average bacterial concentration output decreased. The presence of macropores and the integrity of pores in a column with a diameter of 2 cm accelerated the bacterial movement and increased the pollutant outflow index due to high porosity; therefore, more bacteria passed (compared to the control column without macropores). The salinity treatment, however, served as an inhibitor and hindered further transmission of bacteria. Moreover, The macropore-free column with a salinity of 4 dsm-1 exhibited a higher refining coefficient (0.85 cfuml-1) compared to other treatments. A salinity treatment involving a 1 dsm-1 salinity and a pore diameter of 2 cm showed the least filtration coefficient (0.82 cfuml-1), so by doubling the ionic strength, 30% reduction can be seen in the bacterial filtration coefficient. Increasing the salinity up to 2 dsm-1 and decreasing the macropores diameter increased the relative absorption index. The macropore-free treatment with a 2 dsm-1 salinity showed the highest relative sorption index (0.92). Although the bacterial growth and mortality are unknown during the bacterial transfer process, according to the results, it is expected that the bacterial mortality rate increases by the salinity enhancement from 2 to 4 dsm-1 and the relative adsorption index reduction which may result in lower surface sorption. The significant treatment for the maximum predicted depth of bacterial transfer was the mutual effect of salinity and diameter at a probability level of 5%, which confirmed the significant impact of salinity on the bacterial filtration and transfer. The maximum depth of predicted bacterial transfer was obtained in the macropore-free treatment with the salinity of 1 dsm-1 (16.81 cm). The role of the underlying layers in the bacterial refinery seems to be more profound compared to the surface layer. Conclusion Overall, the results showed that the main source of transmission of bacteria is the preferential flow due to the macropore continuity. However, the salinity reduced the amount of bacterial refining by increasing the ionic strength of the soil solution. The salinity had a significant effect on the average output bacterial concentration, bacterial refining coefficient, relative sorption index, and maximum predicted bacterial transmission depth. The results of this study revealed that increased ionic strength of soil solution can enhance the bacterial refining and the further elimination of bacteria which can be effective in controlling the pollution of underground water by saline irrigation management. Regarding the quantitatively and qualitatively critical water status in the country, conditions can be provided for the use of unconventional water sources, without threatening the environment and contaminating the underground water.
M Aalipour Shehni; A Farrokhian Firouzi; A Koraie; H Motamedi
Abstract
Introduction: Preferential flow is one of the major processes influencing the rapid movement of pollutants to ground water. Macropores created by plant roots provide pathways for rapid transport of pollutants in a soil profile. The growth of plant roots into soil causes creation of big pores that ...
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Introduction: Preferential flow is one of the major processes influencing the rapid movement of pollutants to ground water. Macropores created by plant roots provide pathways for rapid transport of pollutants in a soil profile. The growth of plant roots into soil causes creation of big pores that improve water movement and solute transport through soil profile. Field soils or undisturbed soils have many different types of macropores. These macropores may contribute to preferential flow. Therefore, to better evaluate the macropores that are created by plant root in preferential flow, it is essential to isolate the macropore and examine that macropore individually. The main objective of this study was quantitative investigation of the effect of plant root on chloride transport through soil profile under a saturated condition. Materials and methods: In order to investigate the influence of corn root system on soil hydraulic properties and chloride transport in soil an experiment was conducted in completely randomized design. The treatments were prepared as bare soil (control), soil with corn (Zea mays L.) root and soil with corn root 3 months after harvesting in 9 soil columns packed uniformly with loamy sand-textured soil (Bulk density=1.48 g/cm3). The particle size distribution and organic carbon of soil were determined. Saturated hydraulic conductivity was measured for each soil column using constant head method. The breakthrough curves of chloride were measured under saturated condition (constant head method). Before starting the displacement experiment, the soil columns were subjected to capillary saturation from the bottom with 0.01 M CaCl2 for two consecutive days. In order to establish steady state flow conditions, the soil columns were irrigated with a 0.01 M CaCl2 solution at a constant rate and less than 0.5 cm of water was ponded above the soil surface. The chloride concentration in the outflow samples was measured using an electrical conductivity sensor. For measuring the chloride breakthrough curves (BTCs), the 0.01 M CaCl2 solution was replaced by a 0.05 M CaCl2 solution. The chloride transport in the soil columns was simulated using CXTFIT Convection-Dispersion Equation (CDE) and Mobile-Immobile Model (MIM). A nonlinear least-squares program was used to fit the convection-dispersion equation (CDE) and the physical nonequilibrium model (MIM) to the experimental data. Results and Discussion: The research result showed that macropores created by growing and remaining of plant root (Zea mayz L.) have a significant effect on soil hydraulic properties and solute transport.The results indicated that there is significant difference between soil hydraulic properties (saturated hydraulic conductivity and Darcy's flux density) in different treatments (p<0.05). Darcy's flux density indices in soil columns were 1.23 and 1.31 times more than control treatment in plant root and plant root 3 months after harvesting treatments, respectively. The two models (CDE and MIM) fit the BTCs curve data well. Models fits were excellent with R2 values from 0.85 to 0.97. The CDE parameters (D and ν) in treatments had significant difference (p<0.05). Dispersion coefficient (D) values were 2.65 and 3.71 times more than control treatment in plant root and plant root after 3 months harvest treatments, respectively. Pore water velocity (ν) values were 1.36 and 1.52 times more than control treatment in the mentioned treatments.The breakthrough curves of soil with corn (Zea mays L.) root and soil with corn root after 3 months harvest treatments were asymmetrical in shape (asymmetrical with respect to the C/C0=0.5 point on the BTCs). The relative concentration C/C0 in the effluent is obtained before one pore volume of chloride is passed through the soil column. Conclusion: High flow velocity, saturated hydraulic conductivity (Ks) and dispersion coefficient (D) of the soil columns treated with plant root or with plant root after 3 months harvest indicated the presence of macropores in the soil that is created by deep corn root system. The early breakthrough of chloride BTCs reveals the existence of preferential flow, suggesting that a portion of chloride moves through soil macropores. The occurrences of preferential flow were attributed to well-connected macropores created by plant roots and decayed corn root 3 months after harvesting. Furthermore, the results of this research indicated that when considering solute transport in agricultural soil the effect of plant root needs to be considered.
