Document Type : Research Article
Authors
1
Department of Agronomy and Plant Breeding, Faculty of Agriculture, Bu-Ali Sina University, Hamedan, I.R. Iran
2
Bu-Ali Sina University
Abstract
Introduction
The world's population continues to grow, and agriculture must keep pace with increasing demand for food production. Many challenges threaten crop yields, such as herbivorous insects, plant pathogens and weeds, the occurrence or risk of each one often requires the use of pesticides. Despite the usefulness of pesticides in crop protection, their excessive and irrational use can endanger human health and the environment. On the other hand, the very costly application of chemical fertilizers, especially nitrogen, which is a key element in plant nutrition under insufficient soil fertility conditions, can cause groundwater pollution with nitrate and air pollution with nitrogen oxide (21). Although herbicides are intended to protect crops, they can potentially pose a threat to the activity of rhizobium that symbiosis with legumes, thereby reducing the nitrogen fixation of symbionts. If symbiotic nitrogen fixation is adversely affected, crop yield will subsequently be adversely affected (6). All previous studies on the effect of herbicides on crop-rhizobium symbiosis under a certain soil pH have been evaluated. Therefore, it is important to understand the interaction between soil pH and the toxicity severity of herbicides on crop-rhizobium symbiosis, because soil acidification or alkalization (4) can occur over several years in intensive agricultural ecosystems. Therefore, this study aimed to investigate whether the severity of herbicide toxicity on crop-rhizobium symbiosis in different soil pH conditions can be attributed to variations in the electrical charge properties of herbicides. Based on this hypothesis, three herbicides were selected: ethalfluralin, a non-ionic herbicide, imazethapyr, an acidic herbicide, and metribuzin, a basic herbicide (23). The objective was to examine their toxicity on soybean-rhizobium symbiosis under three soil pH levels.
Materials and Methods
The soil required for this experiment was prepared from the educational farm of Bu-Ali Sina University of Hamedan, which had a pH of 7.2. This soil is considered as natural soil. Based on a pre-test results on natural soil, adding and mixing 0.2 g sulfur and 5.5 g lime with each kg natural soil could create artificial soils with pH of 6.4 and 8, respectively. The pot experiment was performed in a completely randomized factorial design under open-air conditions. Herbicidal factor included control, pre-planting application of 990 g ethalfluralin ha-1, post-planting application of 450 g metribuzin ha-1and post-emergence application of 108 g imazethapyr ha-1. Soil pH factor was 6.4, 7.2 and 8. Soybean seeds (cv. Hobbit) were disinfected with 1% sodium hypochlorite for five min and washed and dried with water. They were then immersed in commercial soybean inoculum (BiosoyTM) for five min and dried again. Inside each pot, four seeds inoculated with bacteria were planted at two cm depth. Separately, an inoculated seed treatment under natural soil conditions without herbicide application was considered to investigate the effects of seed inoculation. Growth parameters including height, dry weight of stem and root, number and dry weight of nodes formed on root, the nitrogen content of stems and roots were measured and analyzed statistically using SAS software. The means were compared with the LSD test at the level of 5% probability.
Results and Discussion
In comparing the two treatments of seed inoculation and non-inoculation with a commercial soybean inoculum and their cultivation in natural soil, it was found that seed inoculation had a significant positive impact on various growth parameters. Specifically, it led to increased plant height, dry weight of shoots and roots, as well as nitrogen content in both shoots and roots. Additionally, there was a notable decrease in the shoot-to-root dry weight ratio. Previous studies have also reported similar results, indicating changes in the photosynthetic response pattern due to bacterial seed inoculation in different soybean genotypes (16). In treatments where no herbicide was used (control), soybean nodulation and certain growth parameters were significantly influenced by soil pH. The highest level of nodulation was observed at soil pH values of 7.2 and 8.The lowest number of nodes (21.3 node plant-1) and the lowest dry weight of nodes (491.8 mg plant-1) were also observed in soil with a pH of 6.4. Poor nodulation observed at acidic soil pH may be associated with hydrogen ion toxicity, which may prevent the onset of nodulation, as reported in previous studies (3). The results showed that the toxicity severity of imazethapyr and metribuzin on nodulation (number and dry weight of nodules) depended on soil pH. As the pH of the soil increased, the toxicity of imazethapyr decreased, but the toxicity of metribuzin increased. While the toxicity severity of ethalfluralin on nodulation was not affected by soil pH.
Conclusion
Despite the advantages of herbicide application, it can have negative effects on soybean-rhizobium symbiosis, limiting the capacity of rhizobium for nitrogen fixation. Consequently, the reliance on chemical fertilizers increases to meet the nutritional needs of soybeans. Additionally, in acidic soils where herbicides are less absorbed by soil particles, further disruption of the soybean-rhizobium symbiosis can occur. Our experiment revealed that the use of metribuzin in alkaline pH soil or imazethapyr in acidic pH soil can cause severe damage to the soybean-rhizobium symbiosis. As soil acidity or alkalinity can change rapidly in agricultural ecosystems due to conventional farming practices such as lime or sulfur addition to adjust soil pH, it is crucial for farmers to consider the electrical charge of herbicides and the pH of their soil. This awareness can help minimize the herbicide toxicity on soybean-rhizobium symbiosis.
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