اثر زمان خواباندن و سطوح مختلف ورمی‌کمپوست بر شکل‌های شیمیایی نیکل در یک خاک آهکی

Background and Objectives
One of the major environmental problems is soil and water pollution with heavy metals in human societies. One of the main reasons for increasing the availability of heavy metals in the soil is the excessive use of chemical fertilizers, pesticides, as well as the entry of municipal and industrial wastewater into the soil environment. Nickel is one of the heavy but essential elements for plants. Because this metal is part of urease and plays an important role in nitrogen metabolism. But its excessive accumulation reduces the nutritional value of plants. Heavy metals show different behaviors with the passage of time according to the type of soil and its components. So that with the passage of time, their availability for absorption by plants is reduced and they become less soluble. Heavy metals are observed in the soil in the form of soluble, exchangeable complexes, connected to carbonates, oxides and hydroxides of iron and manganese, organic substances and in the form of a part of the structure of minerals. Determining the chemical forms of heavy metals using successive extractors helps to relate their chemical forms in the soil, evaluate their availability and leaching. In fact, it shows the destructive and environmental effects of heavy metals in the soil.
Nowadays, the use of organic fertilizers has increased in reducing the bioavailability of heavy metals and converting them into less soluble forms, as mentioned, nickel is an essential element for plants, humans and animals, but its high concentrations can cause toxicity in these organisms. Therefore, it seems necessary to study the change of shape and its transformation over time in the environment on the growth and health of plants and animals. Therefore, in this report, we investigate the effect of time changes on the forms and chemical components of nickel element in a calcareous soil under the influence application of vermicompost.
Methodology
In order to investigate the effect application of vermicompost on the distribution of chemical forms of nickel in the soil, a factorial experiment was conducted in the form of a completely randomized design in three levels of vermicompost (0, 5 and 10% by weight), 2 levels of nickel (0 and 100 mg kg-1) and 2 times (1 and 2 months) were done with three repetitions in greenhouse conditions. In order to contaminate the soil, first, 500 grams of soil was weighed for each pot, then different levels of nickel nitrate were added to the soil samples by spraying.
After drying, the pot soil samples were completely mixed and the desired levels of vermicompost were added to them. During the test period, the samples were incubated at 25 °C and the humidity of the sample were also kept within the agricultural capacity. In order to ventilate and create aerobic conditions, a hole with a diameter of several millimeters was installed on the lid of the samples. In the first stage, one month after contaminating the target soil with nickel nitrate, 1 gram of soil was removed from each pot and the chemical forms of nickel (solution + exchange, carbonate, organic, bound to manganese and iron oxides) were measured. In the second stage (2 months after contaminating the soil), the steps of the first stage were repeated. Finally, SAS software and Duncan's test were used to analyze the data.
Findings
The results of this research showed that vermicompost had different effects on the chemical forms of nickel over time, so that in both times (1 and 2 months) the application of 5 and 10% by weight of vermicompost caused a decrease in the form of soluble + exchangeable, carbonate and residual nickel, while it caused an increase in the organic form, bound to manganese oxide, amorphous iron oxide and crystalline iron oxide.
Conclusion
Considering the results of this research and the positive effects of vermicompost in reducing the bioavailability of soil nickel over time, it can be said that this fertilizer can reduce the environmental risks of this element in soils contaminated with heavy elements, including with nickel. Also, considering the short period of time used in this research, it is suggested to investigate the long term effect of vermicompost on the bioavailability of nickel and other heavy elements in longer periods of time.