taher ajami

The herbicides environmental pollution caused by their over-use is one of the most critical threats to ecosystems and human societies' health. Therefore, understanding the behavior of pesticides in the environment is necessary to reduce the adverse effects of the environment and optimize agricultural activities. The goal of this experiment was to evaluate the effect of manure level on the leaching depth of 2,4-D via soybean and cucumber seedling bioassay.

This factorial experiment was performed based on a RCBD with three replications. The first factor was livestock manure including control (without manure and herbicide), 0, 10, 25 and 50% of rotted manure all with herbicide and the second factor of soil layers considered as 0-2, 2-4, 4-6, 6-8, 8-10 and 10-12 cm. 2,4-D herbicide (72% L, 2 L.ha-1), 24 hours after soil saturation was applied. 48 hours after herbicide application, five seeds of the plant were planted in each pot and by the end of 30 days, shoot and root length and dry weight of shoot and root were measured.

The results showed that the 2,4-D leaching depth was 10 cm in soybean and 12 cm in cucumber without manure. Adding 10 and 25% manure did not show a significant effect on the 2,4-D leaching reduction, but treatment of 50% manure diminished the leaching depth in soybeans and cucumbers up to 8 and 10 cm. With 50% increase in manure due to increased herbicide adsorption in 2 and 4 cm layers, more inhibition was observed than other layers and as a result, less herbicide was transferred to the next layers. On the other hand, cucumber showed more sensitivity to different amounts of 2,4-D herbicide leaching than soybean, so that up to 8 cm layer, all treatments except herbicide-free control treatment indicated a significant reduction in measured traits. It seems that the increase of soil organic matter in the treatment of 50% manure in layers larger than 8 cm, improved the growth of measured traits close to the control treatment. According to the results of this experiment, cucumber can be used as an index plant to monitor the leaching depth of 2,4-D herbicide.

The more sensitive the plant is, the more it reacts to the herbicides, and as a result, the effective depth of leaching increases for that plant. At the present study, the effective leaching depth in cucumber and soybean plants was different. In soybean, it was between 6 and 10 cm, and in cucumber, depending on the measured trait sensitivity, it was between 10 and 12 cm. Cucumber showed high sensitivity to different amounts of leaching, so that up to a depth of approximately 6 cm in all treatments a significant reduction in measured traits occurred. Comparing the reaction of soybean and cucumber crops to the depth of 2,4-D leaching, it can be concluded that cucumber is more sensitive than soybean; So that in soybean from a depth of 6 cm and after, no sharp decrease in the studied traits is observed, but in cucumber up to a depth of 10 cm, the decrease was noticed. It is noteworthy that the addition of manure may reduce the leaching of herbicides, but at shallow depths due to the adsorption of high amounts of herbicides, and its gradual application will cause plant burns. Also, according to the results of this experiment, cucumber can be used as a benchmark in monitoring the amount of 2,4-D herbicide leaching.

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.