Application of. SiO4 found effective in reducing the rice grain arsenic content by 23% over the control counterparts. Competitive oxyanions like NO3. -. , PO4. =.
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Proceedings of the International workshop on Arsenic in Food Chain: Cause, Effect and Mitigation, 20th February, 2012, Kolkata, India, DNGM Research Foundation, Kolkata, p. 2012
Comprehensive overview of mitigation options on arsenic entry in foodchain (Arsenic Mitigation in Food Chain)
S. Sarkar, K. Bhattacharya, S. Bhattacharya, S.Mondal, S.C.Kole, C.K. Kundu, P.K.Patra, A. Mukherjee and D. Mukhopadhyay
Abstract
Arsenic (As) concentration in aquifer water in 87 blocks of West Bengal is above the permissible limit (0.05 mg L-1). Use of such water to grow summer rice adds huge quantity of As in to the soil and convert soil as a secondary source of As toxicity. As in irrigation water and soil enters into plant and finally increase its concentration in edible parts. Finally through food grain As enters into human body. Due to favourable soil environment As uptake in rice is 10 times higher over other crops. During 2007-2012 scientists of the two agricultural universities tried / developed different strategies to reduce As load in rice grain. Experiments were carried out in As contaminated villages of Nadia and Maldaha districts of West Bengal. Baseline survey shows that, rice genotypes like Lalswarna and Nayanmoni contains relatively low As in grain (0.22 – 0.51 mg kg-1). It was moderate in Shatabdi and khitish (0.33 – 0.61 mg kg-1). Cultivation of rice in medium and low land topo-sequence reduced grain As load by 33 and 86% respectively over medium-upland. Adoption of intermittent ponding or keeping the soil under saturation state during vegetative stage of rice reduce the grain As content by 33 and 36% over farmers continuous ponding (followed by the farmers) without insignificant variation in yield. Organic amendments like vermicompost, farm yard manure and municipal sludge reduce the grain As content of rice by 19, 5 and 9% over non amended soil. The efficiencies of different oxy-anions in reducing As content of rice grain appeared in an order of: NO3 >PO4>SO4>MoO4 ≈ BO3. Speciation study shows that beside As-V, phosphate also reduces the concentration of As-III in rice grain. Application of SiO4 found effective in reducing the rice grain arsenic content by 23% over the control counterparts. Competitive oxyanions like NO3-, PO4= inhibit uptake of As+5, SiO4= and BO33inhibit uptake of As+3 by rice plant. However SO4= restricts transport of both As+5 and As+3 from root to shoot.Different bacterial strains can transform 20 to 90% of the more toxic form
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(As-III) of As to less toxic form (As-V). Besides, some bacterial strains can volatilize arsenic under both aerobic and anaerobic conditions. Identification of genes responsible for As uptake and its transport in rice will help to breed rice varieties with low arsenic content in grain and straw. Few numbers of rice markers are already been identified for such purpose. Identification of rice genotypes, genes responsible for entry and transport of As in plant, bacterial strains along with suitable water and soil management in a proper cropping sequence can reduce As load in soil-plant system and thus minimise entry of As into human system through food-chain. Key Words: Arsenic,oxyanions,genotype,food chain.
Introduction Arsenic (As), a number one carcinogen, exists in different inorganic and organic forms in nature [1]. Inorganic forms are mainly comprised of the trivalent As-III, while pentavalent As-V dominates the aquifer water [2]. In oxidizing environment the As-V species is predominant, which is less toxic and soluble. On the other hand, As-III is the more toxic and soluble form which predominates in reducing environment. Reduced soil environment forced rice to accumulate 10 folds higher As over other cereals [3]. In West Bengal, India, source of As pollution is geogenic [4]. Reductive dissolution of iron and manganese oxy-hydroxides is by far the most important geochemical mechanisms for As pollution in South and South-East Asia [5]. The aquifer water of the Lower Gangetic Plain (LGP) of West Bengal used for irrigation purpose has total As content in the range of 0.1 to 0.35 mg L-1 [6]. Continuous use of high As contaminated water for irrigation also increased the As load of the soil and transform the soil from sink to secondary source [7]. Cultivation of summer rice leads to the addition of 150 to 400 mg As m-2 soil season-1. Crops grown on As polluted soil and irrigated with As contaminated water, take up this toxic metalloid in a higher concentration [8]. Transport of As from water and soil to the plant increased the As load in edible and other parts of both rice and other arable crops [9-11]. In West Bengal, it has been noted that rice constitutes a major source of inorganic As in the diet [12-13]. Due to this millions of people in West Bengal, exposed to the threat of arsenic-related health hazards [14].Thus there is a need to assess the efficiency of various agricultural management practices to reduce the As load in edible part of the crops. With this background scientists of two agricultural Universities of West Bengal, India assessed the possibilities of various mitigation options for As contamination in rice grain and straw. Work programme In terms of ground water arsenic toxicity level, Nadia is one of the worst affected districts in West Bengal, India. A base line survey was carried out in five villages of the district (Fig. 1) to assess the total arsenic status of aquifer water and surface (0 – 150 mm) soil. Database
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generated through the survey shows that the total As content of the ground water was in the range of 0.058–0.573 mg L−1 with a mean value of 0.231 mg L−1, which is 23 times higher than the safe limit of arsenic toxicity as defined by WHO for India and Bangladesh. Initially
Fig. 1. Different villages of Nadia District where the base line survey and different field study were carried out to reduce arsenic load in rice grain
soils of the contaminated area acts as a sink and arrest the entry of As to plant system through roots. However, continuous application of polluted water built up the soil As content in the range of 13.84 to 21.88 mg Kg-1 and made the soil as secondary source of As pollution which contributes to substantial As uptake in grains of maize, coriander, onion, lentil, mustard, sesame and potato to the tune of 0.25, 0.36, 0.23, 0.34, 0.37 and 0.68 mg kg-1 respectively . Effective mitigation options Screening for suitable rice genotypes During the baseline survey, arsenic content of irrigation water-soil-rice root- straw-rice grain were analysed across three consecutive years during the season of summer (boro) rice cultivation. Samples were collected from 364 farmers field followed rice-rice cropping sequence. As our main concern is to assess the As uptake pattern under different genotypes only the As content of root, straw and grain have been reported in this paper, though As content of respective irrigation water and soil are also available with us. In this region farmers are cultivating 14 genotypes; out of them six are predominant, cultivated at least by 20+ farmers. Though lowest As content (0.22-0.48 mg kg-1) has been found in khitish, this genotype was confined in the non toxic area. Data from Table 1 shows that grain As content is relatively low for Naraminikit and Khitish, medium for shatabdi and high for Nayanmoni and GS-3. Interestingly, straw As content of shatabdi is also at a lower side. Though considering grain As load one can advocate the cultivation of Khitish. But considering grain quality, yield and market price (double over Khitish) we recommend shatabdi as most
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suitable rice genotype of the region. Rather one should try other non-biological measures like water and soil management towards further reduction in grain As load of shatabdi.
Table 1. As content of important rice genotypes of Nadia district(Unpublished data) Variety Nayanmoni Gs-3 Satabdi Naraminikit Khitish Lal swarna
As mg kg-1 ROOT STRAW GRAIN 7.37 - 9.71 1.20 – 2.35 0.46 – 0.93 7.52 – 11.64 1.79 – 1.93 0.47 -0.68 7.26 – 9.52 1.22 – 1.64 0.43 – 0.61 6.21 – 8.92 1.12 – 1.94 0.33 – 0.61 6.49 – 8.42 1.02 – 1.65 0.38 -0.51 6.94 – 9.36 1.18 – 2.30 0.22 – 0.48 As concentration of irrigation water: 0.148 – 0.345 mg L-1
Selection of appropriate site for rice cultivation In any irrigation command area, it has been found that textural status of soil in up and low lands are coarse and fine respectively. Dominance of sand with deep water table is responsible for higher percolation rate from upland rice field. In lowland, shallow water table depth clubbed with higher clay percent of soil made the rate of percolation low. Decrease in percolation rate from medium-upland to low land also decreased the total amount of water irrigated to rice (Table 2).Growing rice in appropriate place can reduce the grain and straw As content by 44 and 86% with an increase in 36% grain yield. Table 2. As load and grain yield of rice in different topo-sequences (Unpublished data) Land situation
Med-up land Medium land Low land CD (P=0.05)
Total amount of irrigation, mm
1400 950 500
As content mg kg-1 Straw
Grain
Grain yield Mg ha-1
2.91 2.63 2.01 0.24
0.67 0.51 0.36 0.12
4.05 5.13 5.56 0.41
Irrigation Management Irrigation water is the carrier of As from the aquifer to the soil-plant environment. In the present study site, farmers are adding 80 to 820 mg As per m2 soil each year through cultivation of summer rice. It is already established that the vegetative phase (16 to 40 Days after transplanting, DAT) is the water stress allowable stage of rice crop [15]. Imposition of intermittent pounding during 16 to 40 DAT reduced the As content of straw, husked grain
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and unhusked grain respectively by 22, 33 and 36% with insignificant reduction in grain yield (Table 3). Exposure of the crop to saturated or aerobic soil environment during that period caused further reduction in As load of different plant parts. However, grain yield under saturated or aerobic condition significantly reduced the grain yield of rice respectively by 19.6 and 28.5% over continuous pounding. Therefore, considering both arsenic reduction and Table 3. Impact of deficit irrigation on arsenic load in soil and different parts of rice as well as yield of rice grain (As content of irrigation water 0.163 mg L -1) [6] Irrigation regimes
As added, mg m-2 soil
Soil
CP IP SAT AER CD (p = 0.05)
171.6 143.0 125.84 115.83 1.98
18.74 18.16 17.75 16.22 0.13
Total As, mg Kg-1 Straw Husked Grain 4.20 0.56 3.42 0.42 3.96 0.53 3.51 0.46 0.08 0.04
Unhusked grain 0.26 0.19 0.21 0.19 0.03
Grain yield, Mg ha-1 4.69 4.43 3.92 3.65 0.65
grain yield we recomme nded in position of intermitt ent
pounding as a suitable irrigation option towards As load in rice plant.
Application of organics Application of organics initially enhances the organic carbon level of the soil and thereby improves hydro-physical and chemical properties of soil. Release of humic and fulvic acids from the organics make chelates with different metals/metalloids and decreased their availability in the soil solution. Although the organic matters used for such studies contain considerable amount of As itself, they still reduced the availability of As in the soil solution and thereby reduce As uptake in edible part of different crops. A field study conducted by our student at Ghentugachi shows that application of organics reduced the availability of As to the tune of 3.45 to 19.31% (Fig. 2) over the corresponding control counterparts. The ratio of humi-fulvic acids of an organic material play significant role in reducing the available As in soil solution by that particular organic matter. Present study shows vermicompost is the most effective organic amendment in reducing As load of grain followed by municipal sludge. The
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municipal sludge can be used for different field crops. Under controlled condition study the extractable As was found to be lowered by the applied vermicompost and farmyard manure at different period of incubation [16].
Per cent reduction in As over control
20
15
10
19.31
5
8.97 4.83
3.45
0 FYM
Vermicompost
Sludge
Mustard cake
Fig. 2. Per cent reduction in grain arsenic content in rice (genotype Shatabdi) through organic amendments [17].
Influence of oxianions The influences of common oxyanions (i.e. nitrate, molybdate, sulfate, Borate and phosphate) and vermicompost on the sorption of arsenate have been studied. Type and concentration of the co-existing species influence the absorption of As. So far the concentration of As in the Vermicompost-Fulvic acid complex, the co-existing common oxyanions were ranked in the order of sulphate< molybdate < nitrate < phosphate
