Changes of soil moisture profile in the application of biochar and water with microbial pollution under subsurface drip irrigation

Reuse of wastewater as an important and constant water resource for agriculture that as the world's largest water consumption has gained incredible attention. Wastewater contains a large number of microorganisms of which some can lead to microbial pollution for the environment. There are different management ways to reduce soil surface pollution such as using subsurface drip irrigation and some amendments. Subsurface drip irrigation could reduce the movement of pathogens in the soil. Moreover, Biochar is a carbon material that has gained more attention for removing various pollutions in soil. So, it is necessary to investigate water movement and distribution in conjunction with using tools and methods for preserving water in water scarcity situation and reducing subsequent problems of wastewater use in the soil. The main objectives of this study were (i) to assay the effect of irrigation management, different flow rates, and adding biochar into the soil on water distribution, under subsurface drip irrigation; and (ii) to validate the HYDRUS model for modeling distribution of water and its movement in different conditions in subsurface drip irrigation. The study was conducted in outdoor glasses’ lysimeters (70×60×17 cm) in the research farm of Shahrekord University, Shahrekord. First, the lysimeters' walls were treated with grease in order to prevent preferential flow along the walls. Then, the lysimeters were filled with air-dried clay loam soil. The dripper was installed at the depth of 20 cm below the soil surface. Treatments included two discharge drippers (Q2 and Q4), three levels of maximum allowable depletion (MAD) (30, 50, and 70%), and three application rates of biochar (0, 0.5, and 1%). Three irrigations were done for each lysimeter based on the defined MAD by polluted water containing fecal coliform bacteria. The soil moisture content in lysimeters was measured using a moisture meter device at different times including 1, 2, 4, 6, 8, 10, and 24 hr after ceasing irrigation. Also, the HYDRUS-2D/3D was applied for simulating water movement and soil moisture content in various treatments under subsurface drip irrigation. Water distribution and moisture content depends on flow rate and soil hydraulic properties. In the current study, the biochar was an amendment that changed moisture distribution in soil due to creating various soil hydraulic properties, and it was more effective than MAD. Additionally, biochar affected the soil water content of a clay loam soil at various matric suctions especially between saturated content and field capacity. However, the moisture content at the permanent wilting point of any biochar rates did not change significantly. The horizontal direction of the wetting front in treatments mixed with biochar was smaller than the control. Treatments with a smaller discharge rate (2 l h-1) created the higher wetted area; because those treatments delivered the total amount of irrigation water in a long time so water had enough time to distribute in soil, in comparison with treatments with higher discharge rate (4 l h-1). Also, the irrigation interval in the biochar amended soils was higher than the counterparts. For all times, a higher soil water content around the dripper was observed at a higher discharge rate. Also for all times and both discharge rates, downward soil water movement was greater than upward soil water movement because capillary forces are small compared with gravity forces, and it can be the main point for soil and human health. It should be mentioned that the amount of moisture in soil surface in treatments mixed with 0.5 and 1 % biochar was less than 0% biochar; however, there is no significant difference between all treatments. The lowest and highest moisture content on the soil surface was observed in Q2B1 and Q4B0 treatments, respectively. MAD 30% with reducing irrigation interval, decreased the moisture fluctuations in soil surface than MAD 50 and 70%. Moreover, HYDRUS-2D/3D has an acceptable ability in simulating spatial-temporal changes in moisture under subsurface drip irrigation. The R2 and RMSE ranged from 0.62 to 0.78 and 0.037 to 0.053, respectively. Overall, the addition of biochar into the soil can reduce the risk of soil surface pollution resulting from using wastewater due to less moisture in the soil surface in arid and semi-arid regions with water scarcity. The porous structure of biochar improves water holding capacity and decreases evaporation from soils. Also creating a bigger saturated zone, using drippers with high flow rates can be suitable for short root plants.