Abstract
Pot and field experiments were conducted to examine the effects of water management (WM),arsenic (As) contaminated soil-water and P rate on As uptake, growth, yield and yield attributesaof boro and aman rice. In the pot experiment, there were 18 treatments, which were
combination of three As rates (0, 20 and 40 mg kg soil), three P rates (0, 12.5 and 25 mg kg
soil) and two WM (aerobic and anaerobic) strategies on aman (var. BRRI dhan 32) in 2003 andboro rice (var. BRRI dhan 29) in 2004 at the Wheat Research Centre (WRC), Nashipur,Dinajpur, Bangladesh. In the field experiment, there were 6 treatment combinations consisting oftwo tillage options [Permanent raised bed-PRB (aerobic WM) and conventional till on flat-CTF(anaerobic WM)] and three P levels (0%, 100% and 200% of recommended P) using same ricevarieties as that of pot experiment, in an As-contaminated field at Dharaikandi, Gobindagonj,Gaibandha, Bangladesh in 2004 and 2005. The yield contributing characters decreasedsignificantly with increasing arsenic rates in the pot study. Grain yield decreased from 63.0 to 7.7g pot in boro and from 35.0 to 16.5 g pot in aman rice with increasing As rate. Aerobic WM
reduced As toxicity compared to anaerobic WM for all P levels. Phytotoxic symptoms like blacklesions on the stem sheaths and straighthead (As toxicity) developed when rice plants wereexposed to the highest As rates under anaerobic WM in pots. Straighthead drastically reducedgrain yield in boro rice. Arsenic availability was less under aerobic WM compared to anaerobicWM (flooded). In the pot experiment, the highest total grain (husked) As content (2.23 and 0.623ppm in boro and aman rice, respectively) was found in T6 (P12.5AS40-anaerobic: Eh = -53 mV),which was significantly higher (38% to 55%) than in the same As and P treated pot underaerobic conditions (Eh = +135 mV). The As content in rice straw (up to 24.7 ppm in boro, 17.3ppm in aman rice with the highest As level) suggested that As can more easily be translocated to
the shoots under anaerobic conditions than aerobic condition. Under aerobic WM, P soilamendments reduced As uptake. The regression values showed that As content in grain andstraw increased progressively with the increase of As levels and As content in grain decreasedwith the increase of P level under aerobic pot culture. Significantly, the highest grain yields (6.65and 7.12 t ha¹ in boro 6.36 and 6.40 t ha¹ in aman in both the years' trials) were recorded inPRB (aerobic WM: Eh = +360 mV) plus 100% P amendment in field study. There was a 14%
yield increase over CTF (anaerobic WM: Eh = -56 mV) at same P level. The As content in grainand As content in straw were about 3 and 6 times higher in CTF compared to PRB, respectively.The highest total As content (0.646 ppm in grain and 10.931 ppm in straw) was recorded underCTF, and the lowest total As content (0.247 and 1.554 ppm in grain and straw, respectively) was
recorded under PRB (aerobic WM). The furrow irrigation approach of the PRB treatmentsconsistently reduced irrigation input by 29-31% for boro and 27-30% for aman rice relative toCTF treatments in 2004 and 2005, respectively, thus reducing the amount of As added to the soilfrom the As-contaminated irrigation water. Yearly, 30% less As was deposited to the soilcompared to CTF system through irrigation water during boro season. High As concentrations ingrain and straw in rice grown using CTF in the farmers' field, and the fact that using PRB
reduced grain As concentrations to value less than half of the proposed food hygiene standard.Higher yields with reduced water have important implications for rice production. Watermanagement is a potential tool to reduce As contamination of rice and straw to levels safe forhuman and cattle consumption in the As affected areas of Bangladesh