Evaluating the Effect of Soil Organic Matter on Leaching Depth of Imazethapyr

Application of herbicides in modern intensive agriculture to control weeds has increased dramatically, which leads to the serious environmental problems due to transferring and leaching of herbicides to non-target locations; and their negative effects on non-target organisms need to be reduced. In other words, leaching and transferring of herbicides not only reduce its efficiency but also leads to groundwater pollution. Among the herbicide transfer processes in the soil, leaching is the most important processes because of its potential to contaminate groundwater. Imazethapyr is extensively used in the country's arable lands, and the present experiment was conducted to investigate the effect of soil organic matter on the leaching depth of this herbicide.

A factorial pot experiment was conducted in 2017 based on a completely randomized design. The first factor was soil type (including 1- without adding organic matter and without herbicide application (S1), 2- without adding organic matter + herbicide (S2), 3- 10% organic matter + herbicide (S3), 4 - 25% organic matter + herbicide (S4) and 5-50% organic matter + herbicide (S5)), and leaching depths of 2, 4, 6, 8, 10, 12 and 14 centimeters was considered as the second factor. Imazethapyr herbicide was applied based on its recommended dose. After applying the herbicide in the soil columns, no cultivation operation was carried out for 48 hours so the herbicide had enough time to transfer to different depths of the soil. It was closed under each pot with a flat plate and then five cotton seeds were planted in each pot. After planting, the pots were not irrigated for 48 hours to absorb the herbicide by seeds and then irrigation was done according to the needs of the plant with a suitable sprinkler. All pots were cared for in the greenhouse for 30 days and then emergence percentage, stem height, root length, stem dry weight and root dry weight were measured and recorded. To measure the dry weight, a digital scale with an accuracy of one hundredth was used and to dry the plant components, all samples were placed in an oven at 75 ° C for 24 hours.

The results of analysis of variance related to the measured traits of cotton crop indicated that the simple and interaction effects of soil type treatments and leaching depth had a significant effect (p-value < 0.01) on stem length, root length, stem dry weight and weight dried cotton roots. Due to the significant interaction between soil type and leaching depth, physical cutting was performed based on the soil type. Emergence percentage trait was not affected by soil type but leaching depth treatments and the interaction of soil type leaching depth showed a significant effect on this trait (p-value < 0.05 and p-value < 0.01, respectively). Mean comparisons related to the emergence percentage of cotton seeds showed that in almost all soil treatments with increasing the leaching depth, the emergence percentage of cotton increased significantly, which indicates a decrease in the amount of imazethapyr herbicide leaching in more soil depths. In other words, with increasing soil depth, the amount of leached herbicide was also reduced. The average emergence percentage in each soil depth shows that the highest emergence percentage was obtained from a depth of 14 cm at 78.85 and the lowest was obtained from a depth of two centimeters at 42.66. The minimum stem length was obtained from depths of 2 and 4 cm. At depths of 6, 8, 10, 12, and 14 cm, stem length increased with increasing leaching depth and decreasing the concentration of imazethapyr herbicide. This could be due to the accumulation of washed herbicides at the bottom of the column, because after applying the herbicide, the soil columns were left standing for 48 hours to allow the herbicide to be transported deeper, and possibly after the herbicide reaches the end of the column and the drainage rate is low in this area, the accumulation of imazethapyr at low depths, especially at a depth of 14 cm, has reduced the stem length at this depth. In addition to the possibility of herbicide uptake by soil organic matter, plant growth may also be affected by more soil organic matter and be effective in compensating for the negative effect of herbicides. In herbicide treatments, the root length of cotton increased with increasing the leaching depth. Stem and root dry weight also increased significantly in S2, S3, S4, and S5 treatments with increasing the leaching depth. It seems that after absorption by cotton seedlings, imazethapyr stops the production of amino acids and reduces their levels such as valine, lysine and isoleucine, causing cell death and ultimately reducing the growth of cotton.

 The percentage of organic matter was effective on the penetration depth of the herbicide. In treatments with higher percentages of organic matter at depths greater than 4 cm, cotton traits were not zero and the presence of higher organic matter, in addition to helping plant growth, by absorbing more herbicides, caused its leaching damage. It is possible to consider a significant increase in the parameters studied in cotton crop by increasing the amount of organic matter at different depths of the leaching of imazethapyr herbicide from two aspects. First, an increase in organic matter is likely to increase the degradation of the imazethapyr herbicide. Second, the increase in organic matter has improved the metabolic reactions in the plant and therefore the increase in organic matter has been associated with the improvement of cotton growth indices.