Uptake and Bioaccumulation of Heavy Metals in Rice Plants as Affect by Water Saving Irrigation

To reveal the impact of Non-Flooding controlled Irrigation (NFI) on the bioavailability and bioaccumulation of metals (Cu, Pb, Cd and Cr) in rice fields, metals concentration in different organs of rice plant growing under both Flooding Irrigation (FI) and NFI were measured. It indicated that metals concentrations in root are always the highest one among all the plant organs and in the spike is the lowest. Compared with FI rice, NFI resulted in higher metal concentrations, bioaccumulation factors and metals uptakes of Cd, Cu and Pb. That might ascribed to the higher solubility and bioavailability of metals and the higher rice root absorbent ability under dryingwetting condition. But for Cr, NFI resulted in lower Cr concentration and uptakes in rice root than FI. It indicated that the Cr bound to the Fe and Mn oxides which were more stable under NFI condition may play a more important role in determining the bioavailability of Cr in paddy soil, but metals bound to organic matter which were more likely released may play a more important role for metals of Cu, Cd and Pb. When the soil is free from metals polluted, NFI can help to improve the availability of Cu as a micronutrient and reduce soil metals accumulation by drawing more metals out of the soil by plant uptakes. If the soil is metal polluted, NFI might result in the higher risk of food metals pollution in short-term. But long-term use of NFI will result in less metals accumulation in soil and finally resulted in reduced crop metals uptakes.


INTRODUCTION
Heavy metals contamination has become a severe issue in agricultural production system around the world in the past few decades as a result of anthropogenic activities such as mining or industrial activities and the improper use of metal-enriched materials (Arao et al., 2010;Wu and Zhang, 2010;Zheng and Zhang, 2011).The accumulation of metals in soil-plant system endangered the food security and people health.Rice is one of the most important cereal crops in monsoon Asian, which was also reported be confronted with metals pollution.The absorption and uptakes of metals by rice were determined simultaneously by the bioavailability of metals in soil and the rice root activity (Zheng and Zhang, 2011).The bioavailability of soil metals were always determined by the binding forms of metals in the soil (Zalidis et al., 1999;Aydinalp and Marinova, 2003), which were associated with soil properties, including pH, cation exchange capacity, redox potentials (Eh) and contents of organic matter, clay minerals, calcium carbonate, Fe and Mn oxides (Kashem and Singh, 2001a, b;Kabra et al., 2007;Usman, 2008).The absorption ability of rice roots are determined by soil aerobic condition, crop variety and soil mechanics conditions (Yoshida and Haseigawa, 1982;Iijima et al., 1991;Kirk, 2003;Liu et al., 2006Liu et al., , 2007;;Adachi et al., 2010;Mishra and Salokhe, 2011).
With the development of rice Water Saving Irrigation (WSI) technique, paddy soils were frequently subjected to multi-wetting-drying cycles and high Eh conditions (Mao, 2002;Bouman et al., 2007;Mishra and Salokhe, 2011).Compared with the anaerobic saturated condition in traditional flooding rice field, which resulted in more stable or less liable fraction of metals in soil (Schwab and Lindsay, 1983;Patrick and Jugsujinda, 1992;Arao et al., 2010), the wetting-drying and high Eh conditions in WSI fields led to change in the transformation and repartition of heavy metals in soil.At the same time, rice root growth was enhanced under WSI condition (Yang et al., 2009;Liu et al., 2010).Thus, wetting-drying cycle in flooding rice fields has been reported lead to change in both metals solubility and plant uptakes (Makino et al., 2000;Zhang et al., 2006;Liu et al., 2010;Zheng and Zhang, 2011).But information on the bioaccumulation of metals in rice organs under WSI condition is still very little.
In current research, with Non-Flooding controlled Irrigation (NFI) as a case of WSI technique, metals (Cu, Pb, Cd and Cr) uptakes and bioaccumulation were analyzed to reveal the influence of NFI on the bioavailability and bioaccumulation of metals in rice fields.

MATERIALS AND METHODS
The study was conducted in 2010 and 2011 in the rice fields at Kunshan irrigation and drainage experiment station (31° 15′ 15′′ N, 120° 57′ 43′′ E) in the Tai-lake region of China.The paddy soil is Stagnic Anthrosol, developed from alluvial deposits.The soil texture in the 0-20 cm is clay, with pH of 6.84 (soil: water, 1:2.5).Contents of organic matter, total nitrogen, total phosphorus and total potassium in 0-20 cm soil are 21.9, 1.03, 1.35 and 20.9 g/kg.The saturated water content (v/v) in 0-20, 0-30 and 0-40 cm soil layers are 54.4,49.7 and 47.8%, respectively.The total Cu, Pb, Cd and Cr contents in 0-20 cm soil are 28.04,15.69, 0.799 and 36.45 mg/kg.Those soil metals contents are less than the limit for soil pollution according to the Environmental quality standard for soils (State Environment Protection Administration of China, 1995), except for metal of Cd.The variety of rice is Japonica Rice NJ 46.It was transplanted with 13×25 cm hill spacing later June and harvested in later October.The same doses of fertilizers for each split were applied to each rice field, in agreement with the local conventional fertilizer application.
There were two irrigation treatments: Flooding Irrigation (FI) and NFI.A randomized complete block design was established with three replications in six plots (7×20 m).The ridges between plots were covered with plastic membranes which were inserted into the soil plough layer to a depth of 50 cm to avoid hydraulic exchange of water and nutrients between adjacent plots.The water management practice in both FI and NFI paddies were reported by Xu et al. (2013).Soil moisture during non-flooding period and water depth during flooding period were measured with three replicates for irrigation practice, by using Time-Domain Reflectometry equipment (TDR, Soil Moisture, USA) and vertical rulers fixed in the fields.
Plant samples were randomly taken from the plots at harvest and then divided into roots, sheaths, stems, leaves and spikes, before those were oven dried to a constant weight at 65°C.The dry weights were recorded and the dried subsamples of plant were ground to pass through a 1 mm sieve.Then dried crop samples (0.5 g) were digested in a polyvinyl-fluoride crucible with 4 mL of concentrated nitric acid (HNO 3 ).The Cu, Pb, Cd and Cr contents in the digested solution were measured using ICP-OES (Thermo ICAP 6000 duo, Thermo Scientific).Plant metals uptakes were calculated based on biomass and metals contents in each organ of the plants.The bioaccumulation factors were calculated as the ratios of metals concentrations in plant organs to the concentration in 0-20 cm paddy soils (Rezvani and Zaefarian, 2011).

Metals concentrations in plant organs:
The metals concentrations in root are always the highest among all the plant organs and in the spike is the lowest (Table 1).For metals of Cd, Cu and Pb, NFI resulted in higher metal concentrations than FI in different organs of rice plant, but the different between NFI and FI treatments were mostly insignificant.For metal Cr, the concentrations in aboveground organs were almost the same between both irrigation treatments and FI resulted in higher Cr concentration than NFI in rice root.
Metal contents in spike are relevant to food security.According to the national standard for safety milled rice criteria (Ministry of Health of China, 2005), the Maximum Allowable Concentrations (MAC) of Cd, Pb and Cr are 0.20, 0.20 and 1.0 mg/kg, respectively.There was a certain metals pollution risk in rice for Cr and Pb, although the soil metals levels were safe except for Cd (State Environment Protection Administration of China, 1995).High risk in Cr and Pb pollutions for rice was also reported by Fu et al. (2008).But Cd level in spike is safe in current research, that might partially ascribed to change of binding forms of metals in soil under drying-wetting cycles condition.Assuming that the metals concentration in current research were determined in per weight of spike biomass (including null and rice) and the standards is mainly regarding to the polished rice, we cannot ensure the metals contents in polished rice is higher than the maximum level, because the metals in null always much higher than in polished rice (Cao and Hu, 2000;Fu et al., 2008).
Plant metals uptakes: Metal uptakes in total and aboveground plant biomass were calculated and listed in Table 2.As a result of increased metals concentrations in different organs in NFI field, metals uptakes in NFI rice plant were mostly higher than in FI rice.The total uptakes of Cd, Cu and Pb were 2.30 and 5.70 and 10.39 g/ha higher in NFI field than in FI field in 2010 and 1.41, 3.64 and 3.23 g/ha higher in NFI field than in FI field in 2011.For the uptakes in root and aboveground organs of rice, uptakes of Cd, Cu and Pb were also higher in NFI fields than in FI field.But for metal of Cr, NFI resulted in fewer uptakes in root and more uptakes in aboveground organs.Root uptakes of Cr in NFI field were 1.37 and 0.09 g/ha lower than in FI in 2010 and 2011, uptakes of Cr in aboveground organs in NFI fields were 3.11 and 3.15 g/ha higher than in FI.

Bioaccumulation factors of metals:
Bioaccumulation factors of metals in different rice plant organs under different irrigation treatment were listed in Table 3.It indicated root accumulated more metals and the spike accumulated fewer metals than other organs.Compared with rice under FI condition, rice under NFI irrigation accumulated more metals (Cd, Pb and Cu) in the most organs, although only a few comparison are significant at p = 0.05 level.But for metal Cr, FI resulted in higher Cr accumulation than NFI in rice root.That shows the same phenomena to the comparison of metals concentration in rice plant organs.

DISCUSSION
Why NFI resulted in higher uptakes and enrichment of metals (Cd, Pb and Cu) in rice plant organs than FI.That might ascribed to two main factors.The first is that the drying-wetting conditions led to higher solubility and bioavailability of metals in NFI fields.That was confirmed on different metals in soils and sediments (Kelderman and Osaman, 2007;Vandecasteele et al., 2007;Zhang and Meng, 2008;Zheng and Zhang, 2011;Frohne et al., 2011;Xu et al., 2013).Another reason is that the rice root under watersaving irrigation was frequently reported with higher absorbent ability than rice under flooding condition (Yang et al., 2009;Liu et al., 2010).In current research, NFI rice root dehydrogenase activity measured by the TTC reduction method were 336.4, 181.3, 137.5, 77.2 and 59.6 ug/g/h in later tillering, jointing and booting, earing and sprouting and milk maturity stages, it was increased by 25.8-99.5% than in FI rice root.
But for metal Cr, it was different.That might ascribed to the different in the change of binding forms of metals in soils under the NFI condition.The dryingwetting cycles enhanced the transform of Fe and Mn from amorphous oxides to more crystalline forms which can immobilize trace metals (Zhang et al., 2006;Tack et al., 2006;Koopmans and Groenenberg, 2011) and consequently led to a stronger binding of the metals and an increase of metals binding with the Fe and Mn oxides.But at the same time, it also led to increase in soil dissolved organic matter which resulted in the release of metals which were bounded to sulfurs and organic matters (Arao et al., 2010;Tang et al., 2011;Koopmans and Groenenberg, 2011).That implies that the change of the transformation of metals Cr is different from other metals.The Cr bound to the Fe and Mn oxides may play a more important role in rice field, but metals bound to organic matter may play a more important role for metals of Cu, Cd and Pb.
Some of the metals are also micronutrients to crops, such as Cu and Zn, Some metals are toxic pollutant, such as Pb, Cd, Cr, As and Hg.In current research, NFI can help to improve the availability of Cu as a micronutrient.That means NFI may play an active role in improving micronutrient availability when the soil is free from Cu pollution.But long-term application of NFI might lead to soil Cu deficit.When the soil is metal polluted, NFI is help to reduce soil metals accumulation by drawing more metals out of the soil by plant uptakes, but might result in the higher risk of food metals pollution in short-term.If the plant is hyperaccumulation and not for food, which can be used in phytoremediation, NFI might be the suitable water management for phytoremediation of metal polluted paddy soil.And in long-term NFI will result in les metals accumulation in soil and finally resulted in reduced crop metals uptakes.

CONCLUSION
For metals of Cd, Cu and Pb, NFI resulted in higher metal concentrations and bioaccumulation factors than FI in different organs of rice plant.As a result, Cd, Cu and Pb uptakes in NFI rice plant were mostly higher than in FI rice.But NFI resulted in lower Cr concentration and uptakes in rice root than FI.The higher uptakes and enrichment of Cd, Pb and Cu in NFI rice plant might be ascribed to the higher solubility and bioavailability of metals and the higher rice root absorbent ability under drying-wetting condition.It also indicated that the Cr bound to the Fe and Mn oxides which were more stable under non-flooding condition may play a more important role in determining the bioavailability of Cr in paddy soil, but metals bound to organic matter which were more likely released may play a more important role for metals of Cu, Cd and Pb.When the soil is free from metals polluted, NFI can help to improve the availability of Cu as a micronutrient and reduce soil metals accumulation by drawing more metals out of the soil by plant uptakes.If the soil is metal polluted, NFI might result in the higher risk of food metals pollution in short-term.But longterm use of NFI will result in less metals accumulation in soil and finally resulted in reduced crop metals uptakes.

Table 1 :
Metal concentrations in rice plant (unit: mg/kg)