Effect of incubation time and different levels of vermicompost on nickel chemical forms in a calcareous soil

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 wastewaters into the soil environment. Nickel (Ni) is one of the heavy metals but essential elements for plants. This metal is a part of urease enzyme 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 plants root 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 can show 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. Nickel is a nutrient 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 study, we investigate the effect of time duration on the forms and chemical components of Nickel in a calcareous soil under the different levels of vermicompost application.

In order to investigate the effect of vermicompost application on the distribution of chemical forms of nickel in the soil, a factorial experiment was conducted on the basis of a completely randomized design with three factors of vermicompost at three levels (0, 5, and 10 % by weight), nickel at two levels (0 and 100 mg Ni/kg), and time duration at two levels (1 and 2 months) 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 moisture of the samples were also kept at field capacity. In order to ventilate and create aerobic conditions, a hole with a diameter of several millimeters was created on the lid of the samples. In the first stage, one month after contaminating the studied soil with nickel, 1 gram of soil was removed from each pot and the chemical forms of nickel including solution + exchange, carbonate, organic, and 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 multiple range test were used for statistical analysis of the data.

The results of this research showed that vermicompost application 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 % of vermicompost caused a decrease in the form of soluble + exchangeable, carbonate, and residual nickel, while it increased the nickel concentration in the forms of organic-bound, and bounded to manganese oxides, amorphous iron oxides, and crystalline iron oxides.

Considering the results of this research and the positive effects of vermicompost application in reducing the bioavailability of soil nickel over time, it can be concluded that application of this organic fertilizer can reduce the environmental risks of this heavy metal in contaminated soils. 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.