A pot experiment was carried out in the roof top of a building of Asian University for women in Chittagong, Bangladesh. A sandy loam surface soil (0 - 15 cm) was used for this experiment. Soil sample was air dried and passed through a 4-mm sieve for using in the pots for plant growth. A sub sample was air dried and passed through a 2-mm sieve and stored for laboratory analysis. Physical and chemical properties of the soil are presented in Table 1. Soil pH was measured at 1:2.5 soil to water ratio, soil organic carbon was measured by Walkley and Black [16] and cation exchange capacity (CEC) was measured with 1 M NH₄OAc extraction [17] . The hydrometer method [18] was used for the particle size distribution. The available Fe, Mn, Zn, Cu and Cd contents of soil were extracted by 0.005M DTPA [19] and measured by atomic absorption spectrophotometer 240 AA (Agilent Technologies, Australia) (Table 1).

Moist soil equivalent to 2 kg dry mass was placed in plastic pot (20 cm height and 15 cm diameter) after mixing with Cd at levels of 0, 2.5, 5 mg∙kg⁻¹ and lherzolite at levels of 0%, 2.5% and 5% on dry weight basis and their combinations. The source of Cd was CdSO₄ (ACS grade, Sigma-Aldrich Co.) The lherzolite used in this experiment was collected from the Laboratory of Plant Physiology and Nutrition, Faculty of Agriculture, Iwate University, Japan. Each pot received a uniform basal dose of NPK fertilizer (N-P-K = 137-32-70 kg∙ha⁻¹ as recommended by Soil Resources Development Institute, Bangladesh [20] . The pots were arranged in a completely randomized design (CRD) with three replications.

Six seeds of Chinese spinach (Spinacia Oleracea L.) were sown in each pot and water was applied to field capacity. After 12 days of emergence, 3 healthy seedlings were kept in each pot. The plants were harvested at 45 days of growth and separated into shoot and roots. The plant parts were dried at room temperature to remove excess water prior to oven dry at 65˚C for 72 h and dry mass was recorded. Plant samples were digested with nitric-perchloric acid mixtures (3:1) [21] and the Cd concentration in the digested solutions was analyzed using atomic absorption spectrophotometer 240 AA (Agilent Technologies, Australia). Reagent blanks were also processed. All results are presented on dry weight (DW) basis.

After the harvest, soil samples were collected from each pot, air-dried, and ground to pass a 2-mm sieve. Soil pH was measured the same way as mentioned above. The same soil was used to measure microbial activity or Basal respiration. The most widely used technique to quantify microbial activity is the evaluation of soil basal respiration. Soil respiration was measured by trapping the CO₂ evolved from the soil during incubation period (11 days) in a closed system [22] . For this purpose, 100 g (oven dry basis) soil at 50% of water holding capacity (optimum moisture content for microbial activity) was placed in 1 litre capacity incubation jars and then 20 ml of 0.5 M NaOH in 100 ml capacity beakers were placed in each jar to trap the evolved CO₂. Three jars with 0.5 M NaOH without soil were used as control. Incubation of all jars was kept at room temperature (25˚C). Carbon dioxide absorbed in 0.5 M NaOH after 11 was analysed. The trapped CO₂ was determined by measuring electrical conductivity (EC) of the NaOH traps were measured and CO₂ absorbance was analysed by calibration curve.

Microsoft Excel and Minitab program [23] were used for analysis of variance (ANOVA).

The growth of spinach was significantly affected by application of lherzolite and cadmium. The height of the plant after 45 days of growth in the control pot (without Cadmium and without lherzolite application) was 29.5 cm. The height of plant increased significantly (p < 0.05) with the application of lherzolite at both 2.5% and 5% levels compared to the application of Cd at 5 mg∙kg⁻¹ level (Figure 1(a)).

The shoot dry weight was the highest in the control pot (without Cd and lherzolite) and was the lowest in 5 mg∙kg⁻¹ Cd treated pot (without lherzolite). Dry weight of shoot increased gradually with the increase of the doses of lherzolite with and without Cd application in soil. Dry weight of shoots of spinach decreased from 3.33 g∙pot⁻¹ in control pot (without Cd) to 0.25 g∙pot⁻¹ with the application of 5 mg∙kg⁻¹ Cd. Shoot dry weight in the pot of 5 mg∙kg⁻¹ Cd (0.25 g) increased (1.67 g) by 7 folds with the application of 5% lherzolite in the same 5 mg∙kg⁻¹ Cd treated pot. Application of 5 mg∙kg⁻¹ Cd decreased dry weight of shoot by 13 folds but 55% of its reduction was recovered by the application of 5% lherzolite compared to control indicating further cultivation of plants will increase the efficiency of growth enhancement and decrease Cd tolerance to plants (Figure 1(b)). Dry weight of shoots of Chinese spinach was negatively correlated (−0.874, p < 0.001) with the concentration of Cd of the shoots.

Like shoot dry weight, root dry weight was influenced by lherzolite and Cd application. Lherzolite enhanced root growth both in Cd treated and untreated

soil as did for shoot growth. Root dry weight decreased from 0.51 g in control pot (without Cd and lherzolite) to 0.06 g in 5 mg∙kg⁻¹ Cd added pot. In one hand, Cd application decreased root growth. On the other hand, it increased gradually with the application of lherzolite. Application of 5% lherzolite increased root dry weight (0.06 g to 0.29 g) by 4 folds in 5 mg∙kg⁻¹ Cd treated pot which indicates reduction of Cd toxic effect and enhanced root growth (Figure 1(c)).

Cadmium concentration was measured only in the shoots of Chinese spinach because less amount of dry matter of roots to digest. Plants grown in the control (0%) and lherzolite (2.5% and 5%) amended soil without Cd addition, concentration of Cd was below the detection limit indicating no Cd was added from the amendments of lherzolite as contaminates. Shoot Cd concentration was the highest of 301 mg∙kg⁻¹ in the plants grown in 5 mg∙kg⁻¹ Cd treated soil pot and of 101 mg∙kg⁻¹ in the plants grown in 5 mg∙kg⁻¹ Cd treated soil amended with 5% lherzolite that resulted 3 folds (70%) reduction of Cd uptake in shoots of Chinese spinach due to lherzolite addition. Concentration of Cd decreased from 191 mg∙kg⁻¹ to 73 mg∙kg⁻¹ when 5% lherzolite was added in the 2.5 mg∙kg⁻¹ Cd treated pot indicates higher reduction of Cd uptake due to lherzolite application (Figure 2).

Soil pH measured after the plant harvest, pH in the control soils was 4.4. Soil pH increased with the rates of lherzolite up to 5.92. In the 5% lherzolite application pot, soil pH was about 1.5 units higher than the control pot. Soil pH showed significant negative relationship (p < 0.05) with shoot Cd concentration and positive relationships with CO₂ respiration and shoot dry weight (Figure 3).

Similar to shoot and root dry weights of spinach, CO₂-C respiration was influenced negatively by doses of Cd and positively by the application of lherzolite. During the first day of incubation, CO₂-C respiration increased with the rate of lherzolite in Cd contaminated soil. The rate effect of lherzolite was negligible in the Cd non contaminated soil. The CO₂-C respiration decreased from 60 mg∙kg⁻¹ (in the control) to 20 mg∙kg⁻¹ (5 mg∙kg⁻¹ Cd treated pot) soil per day. Highest dose (5%) of lherzolite in 5 mg∙kg⁻¹ Cd (49 mg∙kg⁻¹ soil per day) treated soil increased CO₂-C respiration by 2.5 folds compared to 5 mg∙kg⁻¹ Cd treated soil without lherzolite application (Figure 4).