The experiment was conducted in a net house at Bangladesh Agricultural University (BAU), Mymensingh, during January 2004 to January 2006. Two high As soils (>13 mg As kg−1), Faridpur-1 and Faridpur-2, were collected from Faridpur district (Ganges River Floodplain) in south-central Bangladesh, as well as two low As soils (<3 mg As kg−1), BAU-1 and BAU-2, from the BAU farm, Mymensingh (Old Brahmaputra Floodplain). Twelve undisturbed soil cores (30-cm diameter by 40-cm height) were collected in PVC pipes. Filter paper (Whatman No. 1), glass wool and a 4-cm layer of acid-washed silica sand (sieved to obtain a 1–2 mm particle diameter) were placed at the bottom of the plastic container that served as the base for the PVC soil core. Two holes in the plastic base were connected by means of polypropylene tubes and a Ttube to a N2-filled, air-tight conical flask that was used to collect column leachate. Boro rice (a term used to describe winter dryseason, flood-irrigated rice in Bangladesh and vicinity) and T. Aman rice (a term used to describe summer monsoon, rain-fed, flooded, transplanted rice) were planted in the columns in succession during the 2004 and 2005 Boro and T. Aman seasons. The varieties of Boro and T. Aman rice were BRRI dhan 29 and BRRI dhan 33, respectively. The total crop sequence was as follows: Boro rice 2004 (15 Feb. to 25 May 2004)– Fallow 1 (no crop; 26 May 2004 to 24 July 2004)–T. Aman rice 2004 (25 July 2004 to 15 Nov. 2004)– Fallow 2 (no crop; 16 Nov. 2004 to 8 Feb. 2005)–Boro rice 2005 (9 Feb. 2005 to 7 May 2005)–Fallow 3 (no crop; 8 May 2005–17 July 2005)–T. Aman rice 2005 (18 July 2005 to 12 Nov. 2005). The top soil was puddled before transplanting each crop. Irrigation treatments (0, 1 and 2 mg As L−1 as Na2HASO4·7H2O in distilled water) were applied to the soil cores during the two Boro seasons; the T. Aman rice was grown under normal rainfall with supplemental distilled water added when rainfall was insufficient to maintain a flooded system. The soil was irrigated and flooded during the fallow with distilled water. Measured quantities of irrigation water were added to each core to maintain a 3–5 cm water depth for the duration of the four cropping seasons, and a 1 cm water depth was maintained during the three fallow periods. For both Boro and T. Aman rice, cores received equal amounts of fertilizer at the rates of 100 mg N kg−1 as urea, 25 mg P kg−1 as KH2PO4, 40 mg K kg−1 as muriate of potash (KCl) and 25 mg S kg− 1 as gypsum (CaSO4⋅2H2O). The full amounts of P, K, S were applied during puddling of the soil 1 day before seedling transplanting; N was applied in three equal splits, onethird with the other nutrients and the remaining two thirds topdressed once at the active tillering stage and once 1 week before panicle primordia initiation. Leachate samples were collected at 5-day intervals during cropping seasons and fallow periods in the following sequence: Boro rice 2004–Fallow 1–T. Aman rice 2004–Fallow 2–Boro rice 2005–Fallow 3–T. Aman rice 2005. The leachate samples were analyzed for pH, Eh, and dissolved arsenate, arsenite, P, Fe andMn concentrations. Upon collection, leachates were filtered immediately using 0.45-μm pore-size membrane filters to remove any fine suspended particulates. Eh and pH were measured using an AP 6 pH/Eh meter (Fisher Scientific). For determination of As(V), As(III), Fe, Mn and P in leachates, the samples were acidified to pH 2 with HCl and stored in a refrigerator at 2°C until analysis. The grain, straw and husk of the four crops were collected for subsequent As analysis. Oven-dried samples of husked rice grain, husk and straw were ground separately. After harvest of the third crop, a small amount of soil was collected from the 0–5 cm depth and analyzed for total and NH4-oxalate extractable As and Fe. After harvest of the fourth crop, soil samples at 5-cm depth intervals, as well as sand samples from the lower sand barrier, were collected for analysis of total-As concentration. The soil, sand, grain, straw and husk samples were digested using HNO3–H2O2 at 95±5°C. The digests were cooled, diluted to 50 mL with deionized water and filtered through Whatman No. 42 filter paper into acidwashed plastic bottles. Arsenic in the digests and leachates was determined by flow-injection hydride-generation flame-atomicabsorption spectrophotometry (FI-HG-FAAS) using a UNICAM model 969 and MHS-10 hydride-generator assembly, using matrix matched standards. Analyses of As(III) and As(V) in the leachate samples were performed using a pH-selective hydride-generation method. Sodium phosphate buffer (0.2 M, pH 3) and NaBH4 (1.5% NaBH4 in 0.5% NaOH solution), to give a reaction pH of approximately 6.5 in the mixing chamber, were used for As(III) determination in the flow-injection system. For total As [As(III) + As(V)] determination, 5 M HCl and NaBH4 were used to give a reaction pH<1. NIST SRM 1568 rice flour was used as a certified reference material for As analysis, with a recovery of 88±6% (n=10). The data has not been corrected for these recoveries. Fe and Mn were analyzed by FAAS using a UNICAM model 969 spectrophotometer.