Evaluation of 2,4-D leaching potential via soybeans and cucumbers bioassay at different levels of manure

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.