The effect of the combined use of biochar and bentonite on evaporation and soil moisture

Introduction
Due to climate change in recent years, the water crisis has become much more serious than before. Therefore, water shortage is one of the most important problems in the world in the last century. Evaporation from the soil surface can be considered the most important component in the water balance. Using soil amendments is a strategy to reduce the adverse effects of drought stress on plants. Water-absorbent polymers can be used as a modifier to improve soil health, which improves water holding capacity. Natural or artificial absorbents superused in agriculture are hydrophilic compounds that increase the water-holding capacity in the soil, reduce the leaching of soil nutrients, reduce the amount of evaporation from the soil surface, and increase the soil ventilation, causing better growth and development of plants and increased yield in drought stress conditions. These polymers can absorb water several times their weight in their building and gradually return water to the soil by reducing the water in the surrounding soil, so the soil is moist for a longer period without re-irrigation. Biochar and bentonite are among the important natural superabsorbents, whose effect on increasing the water-holding capacity in the soil has been proven. Despite many studies in this field, the impact of combining these two absorbents super has not been studied, so in the current study, the simultaneous effect of these two absorbents super on evaporation from the soil and the amount of residual moisture in the soil was investigated.
 
Materials and Methods
This research was conducted at Shahrekord University. To prepare biochar, thin and dry walnut tree branches were first collected, and after chopping, the wood was placed in cylindrical metal containers with lids and low oxygen conditions at two temperatures of 400 and 600 ̊C in an oven for two hours. The bentonite was also purchased from the market and its purity was checked in the laboratory. This research was a factorial experiment using a completely randomized design and three replications. The treatments include the combination of bentonite (Be) at three levels of two, five, and 10 % by weight with biochar (B) prepared at two temperatures of 400 and 600 ̊C at two levels of two and three percent by weight and a control treatment (a ClayLoam soil). In this research, the water balance method was used to obtain the amount of evaporation. In this method, the amount of water entering and leaving the lysimeters (pots) is measured using a graduated container, and since no plants are grown in the pots, the amount obtained from the balance equation is the amount of evaporation from the soil surface. The humidity of the treatments was also measured by the SM150 hygrometer at the surface and 15 cm depth of the pots. The analysis of the data obtained from this experiment, including analysis of variance and means comparison, was done in Statisca software and drawing figures in Excel software.
 
Results and Discussion
The results showed that the changes in the amount of evaporation from the soil surface in all the investigated months as well as the total evaporation of the entire period are significant at a significant level of 0.01. The comparison of the means for the total amount of evaporation in different treatments showed that the amount of evaporation in all the investigated treatments was lower than in the control treatment. The lowest rate of evaporation is related to the soil mixed with two percent bentonite and biochar produced at 600 ̊C. The highest evaporation from the soil surface after the control treatment (432 mm and equivalent to 3.1 mm per day) was observed in the soil treatment mixed with 10% bentonite and two percent biochar produced at a temperature of 400 ̊C. In total, the amount of evaporation from the soil in different treatments has decreased between seven and 14 % compared to the control treatment. The amount of soil moisture in all the investigated treatments has increased significantly compared to the control treatment. In other words, the combination of biochar and bentonite has helped to maintain moisture in the soil. The highest amount of soil moisture was obtained in the treatment of three percent biochar produced at 600 ̊C and 10 % bentonite and it was 25.4 %, which is 18 % more than the moisture content of the control treatment. Increasing the amount of bentonite added to the soil has also increased soil moisture.
 
Conclusion
The results showed that the soil evaporation in the simultaneous use of three percent biochar with different levels of bentonite is higher than the use of two percent biochar. Therefore, it can reject the use of a three percent biochar composition to reduce evaporation from the soil. Based on this, the best treatment should be sought among the two percent biochar treatments. Among the treatments of two percent biochar, the greatest effect on reducing evaporation occurred in the two treatments of two percent biochar produced at 600 ̊C and two percent bentonite and the treatment of two percent biochar produced at 400 ̊C and five percent bentonite. Choosing one of these two treatments for use in large areas requires an economic study of the prevailing conditions in the region and the availability of bentonite minerals and raw materials for biochar production. In one treatment, the temperature of biochar production is higher (B2-Be2-T600), and in the other, the amount of bentonite used is higher (B2-Be5-T400). Optimum use of water in the field and scientific management of water, require appropriate tools that, in addition to being available, do not impose large costs on farmers. In addition to all the things mentioned above, not causing pollution to the soil and plants is also a very important parameter in this field. Therefore, according to the results of this research, it is recommended to use a combination of two or five bentonites with two percent biochar for use in fields.