The pot experiment was conducted at micronutrient experimental field, Soil Science Division, Bangladesh Agricultural Research Institute (BARI), Joydebpur Gazipur on 31st December 2015 with a view of study the effect of P addition to As-contaminated soils and the consequences on As uptake of rice/maize plants. The sandy clay loam soil was used in pot. The physical and chemical properties of the initial soil which was used in pot are presented.
The experiment consisted of a total of 42 pots, each pot of 28 cm deep and 27 cm diameter plastic pot, containing 10 kg air dried soil. Twenty one plastic pots were used for rice and rests of 21 pots were used for maize. All the pots were fertilized two days before sowing/planting with N: 90 mg kg-1 soil, K: 140 mg kg-1 soil, S: 30 mg kg-1 soil, Zn: 2 mg kg-1 soil, B: 1 mg kg-1 soil. Urea, Muriate of potash (MoP), Gypsum, Zinc Sulphate Monohydrate (ZnSO4 H2O) and Boric acid were used as a source of N, K, S, Zn and B, respectively. Nitrogen was added in three splits, the first split during final land preparation and the remaining splits at 30 days and 50 days after sowing/transplanting. Arsenic was added to the pots at the rates of 0, 20 and 30 mg kg-1, and P at 0, 30 and 60 mg kg-1. Thus there were seven treatment combinations, i.e., As0P0, As20P0, As30P0, As20P30, As20P60, As30P30, and As30P60, in three replications for each plant. Arsenic and P were added as As2O3 and Ca(H2PO4)2, respectively, in solutions. All fertilizers were added as solutions and thoroughly mixed with the soil.
Maize (Zea mays var. BARI hybrid Maize-7) seeds were sown directly in pots at a density of 6 seeds per pot. Fourteen days after sowing the seedlings were thinned to 2 plants per pot. Rice cultivar, BRRI dhan 28 was planted in pot. Two healthy and vigor seedlings were transplanted per hill. Accordingly, two hills were received in each pot. Water requirement was monitored visually. Intercultural operations such as weeding and irrigation were done whenever required. The crop was harvested at 80 days following seeding/planting of maize and rice, when it had attained reproductive maturity (before flowering). Soil was removed from the roots by careful and repeated washing in water. Shoots and roots were washed with tap water and then rinsed twice with deionized water. Arsenic and P concentrations were measured in the roots and shoots samples.
Preparation and preservation
After delivery to the laboratory, all samples were washed in fresh running water to eliminate dust, dirt, possible parasites or their eggs and then were again washed with deionized water. The clean samples were air-dried and placed in an electric oven at 65 °C for 72–96 h depending on the sample size. The dries samples were homogenized by grinding using a ceramic coated grinder. The final samples were kept in labeled polypropylene containers at ambient temperature before analysis.
Digestion and determination of phosphorus
One gram of dry matter was weighed into 50-ml beakers, followed by an addition of 10 ml mixture of analytical grade acids HNO3: HCIO4 in the ratio 5:1. The digestion was performed at a temperature of about 190ºC for 1.5 h. After cooling, the solution was made up to a final volume (50 ml) with distilled water in a volumetric flask. The P was measured calorimetrically by ascorbic acid method using a Varian spectrophotometer. Analysis of each sample (on a dry matter basis) was carried out three times to obtain representative results.
Digestion and determination of Arsenic
The total plant as concentration was determined digesting the plant sample (1 g) with concentrate HNO3 of 10 ml and H2O2 having a ratio of 2:1. After cooling, the solution was made up to a final volume (50 ml) with distilled water in a volumetric flask. Reduction with KI and ascorbic acid in presence of NaBH4 and NaOH (Loeppert and Biswas, 2002). Total As concentration of extract was determined by Hydride Generation Atomic Absorption Spectroscopy. Analysis of each sample was carried out three times to obtain representative results and the data are reported in mg kg-1 (on a dry matter basis).
Statistical analysis
The experiment was designed in completely randomized (CRD) with 7 treatments and three replications. Treatment effects were determined by analysis of variance with the help of statistical package STATISTIX-10 and mean separation was tested by Tukey HSD. Linear correlation coefficients were also computed for various parameters using Microsoft Excel.