Mineralization of 0rganic Carbon and Nitrogen of Wheat strawresidue in Cadmium Contaminated Soils

Dynamics of organic carbon and nitrogen are controlled by several factors, including physical, chemical and biological properties of soil. Heavy metals contaminate soils and change soil properties and affect organic carbon and nitrogen dynamics. Since toxicities of heavy metals are different and organic carbon and nitrogen dynamics are affected by available concentrations of these metals, the aims of this experiment were to assess the effects of different levels of soil cadmium on mineralization of organic carbon and nitrogen.
To assess the effects of different levels of soil cadmium on mineralization of organic carbon and nitrogen, a factorial pot experiment was conducted using litter bag method. The factors examined were different levels of soil cadmium (0, 10, 20, 40, and 80 mg kg -1soil) and incubation periods (1, 2, 3 and 4 months) that were applied in three replications. Soil samples were artificially contaminated with cadmium to desirable levels using cadmium sulfate and the samples were placed in plastic pots and the pots incubated at constant moisture and temperature for one month. Then litter bags containing 15 g wheat residues were buried in pots and incubated for different periods of time. At the end of incubation periods, the remaining amounts of plant residues were measured and analyzed for organic carbon and nitrogen concentrations using Walkley and Black and Kjeldahl methods respectively. The decomposition rate constants of organic carbon and nitrogen were calculated using Mt = M0 e –kt equation. Organic carbon and nitrogen losses were calculated by subtracting the remaining amounts of organic carbon and nitrogen in one incubation time interval from those of former one.
The results showed that the effects of soil cadmium levels and incubation periods were significant on organic carbon and nitrogen mineralization. The losses of organic carbon and nitrogen from wheat residues decreased as the levels of soil cadmium increased. The highest and the lowest organic carbon and nitrogen losses were measured in control and treatments with 80 mg Cd kg -1 soil respectively. Increase in soil cadmium levels decreased the losses of organic carbon and nitrogen from wheat residue. The losses of organic carbon for a period of four months were 37.54, 37.21, 36.11, 35.12 and 33.69 (%) in treatments with soil cadmium levels of 0, 10, 20, 40 and 80 mg kg -1 respectively. The loss of organic carbon in the first month of incubation was (30.78%) and in the other three months of incubation was (9.74%) with a sum of (40.52%) for a period of 4 months. Similarly, the loss of organic nitrogen in the first month of incubation was 23.69% and in the other three months of incubation was 8.56% with a sum of 32.25 (%) for a period of 4 months. The highest losses of organic nitrogen from wheat straw residue were measured in treatment of control cadmium (31.64 percent) and lowest losses of organic nitrogen (23.86percent) related to treatment with 80 mg of cadmium / kg of soil. The losses of organic nitrogen, after 4 months were 31.64, 30.69, 28.68, 26.25, and 23.86 (%) when treatment of cadmium contamination of soil was 0, 10, 20, 40 and 80, respectively. The decomposition rate constants for organic carbon were 0.0076, 0.0075, 0.0073, 0.0070 and 0.0066 day -1 when soil cadmium levels were 0, 10, 20, 40, and 80 mg kg -1 respectively. The rate constants for organic nitrogen at the mentioned soil cadmium levels were also 0.0061, 0.0059, 0.0054, 0.0048 and 0.0044 day -1 respectively.
The results of this research indicate that contamination of soils by heavy metals increases the residence time of organic carbon and nitrogen in soils and slows down the cycling of these elements. The mineralization rate of organic nitrogen was affected by soil cadmium levels more than that of organic carbon. The amounts of organic carbon and nitrogen losses are higher in the first month of incubation than those of other months and decomposition of wheat residue had a fast and a slow stage. The results of this study indicate that due to the adverse effects of heavy metals on soil organisms, mineralization rate of plant residue carbon is slower in polluted soils compared with non polluted soils.