Effect of Plant Roots on Chloride Transport in a Soil under Saturated Condition

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. 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. 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. 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.