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

Introduction

Due to the 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. Because it significantly affects the amount of agricultural production. Also, due to the arid and semi-arid climate in most regions of Iran, the amount of evaporation from the soil surface is also high. Using soil amendments is a strategy to reduce the adverse effects of drought stress in plants.

Natural or artificial absorbents super used 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 types of polymers can absorb water several times their own 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 of time without the need for re-irrigation.

The 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 effect 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

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 in low oxygen conditions at two temperatures of 400 and 600 degrees Celsius in an oven for 2 hours. The bentonite was also purchased from the market and its purity was checked in the laboratory.

This research was conducted as a factorial experiment in the form of a completely randomized design and in three replications. The treatments include the combination of bentonite (Be) at three levels of 2, 5 and 10% by weight with biochar (B) prepared at two temperatures of 400 and 600 degrees Celsius at two levels of 2 and 3% by weight and a control treatment.

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 of variance analysis of the treatments 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 the control treatment. In other words, adding different percentages of studied biochar and bentonite to the soil has been able to significantly affect the rate of evaporation from the soil surface.

The lowest rate of evaporation is related to the soil mixed with 2% bentonite and 2% biochar produced at 600 degrees (B2-Be2-T600) and is 398 mm (2.6 mm/day). 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 2% biochar produced at a temperature of 400 degrees and equivalent to 431 mm (2.8 mm per day). In total, the amount of evaporation from the soil in different treatments has decreased between 7 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 3% biochar produced at 600 degrees 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 of this research showed that the soil evaporation in the simultaneous use of 3% biochar with different levels of bentonite is higher than the use of 2% biochar. Therefore, it can reject the use of 3% biochar composition to reduce evaporation from the soil. Based on this, the best treatment should be sought among the 2% biochar treatments. Among the treatments of 2% biochar, the greatest effect on reducing evaporation occurred in the two treatments of 2% biochar produced at 600 degrees and 2% bentonite and the treatment of 2% biochar produced at 400 degrees and 5% 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 mineral and raw material 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).