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Fate and Transformation of CuO Nanoparticles in the Soil-Rice System during the Lifecycle of Rice Plants Cheng Peng, Chen Xu, Qinglin Liu, Lijuan Sun, Yongming Luo, and Jiyan Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05882 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 6, 2017
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Fate and Transformation of CuO Nanoparticles in the
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Soil-Rice System during the Lifecycle of Rice Plants
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Cheng Peng,†,‡ Chen Xu,†,§ Qinglin Liu,† Lijuan Sun,† Yongming Luo,|| Jiyan Shi†,⊥,*
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†
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Sciences, Zhejiang University, Hangzhou 310058, China
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‡
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Engineering, Donghua University, Shanghai 201620, China
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§
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Limited, Hangzhou 310015, China
Department of Environmental Engineering, College of Environmental and Resource
Department of Environmental Science, College of Environmental Science and
Zhejiang Bestwa Environmental Protection Science and Technology Company
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||
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Zone Research, Chinese Academy of Sciences, Yantai 264003, China
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⊥
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Province, Hangzhou 310058, China
Key Laboratory of Coastal Zone Environmental Processes, Yantai Institute of Coastal
Key Laboratory for Water Pollution Control and Environmental Safety, Zhejiang
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*
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Jiyan
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(+86)-571-8898-2019.
Corresponding author: Shi.
Email:
[email protected];
phone:
(+86)-571-8898-2019;
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Abstract:
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Agricultural soil is gradually becoming a primary sink for metal-based nanoparticles (MNPs). The uptake and accumulation of MNPs by crops may contaminate food chain and pose unexpected risks for human health. Here, we investigated the fate and transformation of CuO nanoparticles (NPs) in the soil-rice system during the rice lifecycle. The results show that at the maturation stage, 1000 mg/kg CuO NPs significantly decreased redox potential by 202.75 mV but enhanced electrical conductivity by 497.07 mS/cm compared to controls. Moreover, the bioavailability of highest CuO NPs in the soil was reduced by 69.84% along with the plant growth but then was significantly increased by 165% after drying-wetting cycles. Meanwhile, CuO and Cu combined with humic acid were transformed to Cu2S and Cu associated with goethite by X-ray absorption near edge structure analysis. Additionally, CuO NPs had an acute negative effect on the plant growth than bulk particles, which dramatically reduced the fresh weight of grain to 6.51% of controls. Notably, CuO NPs were found to be translocated from soil to plant especially to the chaff and promoted the Cu accumulation in the aleurone layer of rice using micro X-ray fluorescence technique, but could not reach the polished rice.
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Keywords:
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Metal-based nanoparticles, Bioavailability, Speciation, Accumulation, Drying-wetting cycles
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TOC Abstract:
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Introduction
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The rapid development of nanotechnology has ultimately led to mass production of
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diverse engineered nanoparticles (NPs) worldwide. As a typical metal-based NPs
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(MNPs), CuO NPs have various and important applications such as energy transfer,1
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degradation of dyes,2 sensitive gas sensors in agriculture,3 and antibacterial agent in
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daily necessities.4 Incomplete statistics show that the annual global production of Cu
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NPs and CuO NPs has reached 200 tons/year as early as 2010 and it is still
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dramatically increasing at present.5 However, most MNPs can be introduced into
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agricultural soil by the reuse of sewage sludge from wastewater treatment plants and
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the usage of nano-related fertilizers, herbicide, and pesticide.6, 7 Also, NPs can be
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released from applications of nanosensors, in-situ environmental remediation, and
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wastes of nanotech products.8 Besides, atmospheric deposition and accidental
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discharge from transport may also contribute to this pollution.9 Thus, the potential
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pollution of these MNPs in agricultural soil may pose a threat to the environment and
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ecosystem.
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Once be released into the soil, MNPs may contact soil particles, natural organic
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matter (NOM), and organisms. The actual environmental risks posed by MNPs are
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mainly determined by the species and bioavailability of MNPs in soil. Many studies
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have demonstrated that several environmental factors such as pH, ionic strength, and
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NOM, affect the MNP behavior in the aquatic environment.10 Nonetheless, the
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knowledge on the fate and transformation of MNPs in soil is still limited because of 4
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the uncertainty and complexity of dynamic characteristics in soil system. Compared
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with the dry farmland, the wet land such as paddy soil has a waterlogged condition.
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The specific water management in paddy soil may impact the mobility, solubility,
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bioavailability, and toxicity of MNPs. Moreover, the interaction among MNPs,
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microorganisms, and plants may change the MNP speciation. Hence, it is critical to
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study the fate of MNPs in paddy soil, which is beneficial to control risks from the
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translocation of MNPs to agricultural products and even underground water body.
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Plants as a vital component in the ecosystem also play an important role in the
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fate of MNPs. Our previous studies have shown that the Cu-tolerant plant Elsholtzia
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splendens and rice can directly absorb, accumulate, and translocate CuO NPs from
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root to shoot under hydroponic conditions,11,
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dissolution, and redox.10 In fact, the environmental behavior of MNPs greatly depends
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on MNP properties, plant species, and medium. Early studies have reported there is no
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translocation of Fe3O4 NPs and Al NPs in soil-plant system,13, 14 but recent studies
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have demonstrated that TiO2 NPs from soil were absorbed and accumulated in wheat
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roots,15 translocated to leaf trichomes of cucumber,9 and even into cucumber fruits.16
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Likewise, the translocation of CeO2 NPs was found from soil to corn roots,17 soybean
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roots and root nodules,18 and soybean pods.19 Dimkpa et al. have investigated the
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translocation and fate of CuO NPs from sand to wheat seedlings,20 but there is still
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much lacking of plants response to CuO NPs and the transformation of CuO NPs in
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plants over longer-term timescales (i.e., during the whole lifecycle of plant).
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involving adsorption, aggregation,
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Rice (Oryza sativa L.) can provide more calories than any other single food as a
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popular staple for the majority of the world's population.21 Compared with dryland
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crops such as wheat and bean, as an aquatic plant, rice is more likely to be suffered
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from MNPs due to the possible increased solubility and bioavailability of MNPs
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under changed soil moisture condition. Moreover, root exudates and the feature of
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oxygen secretion by roots may strengthen the impact on MNPs. Thus, whether MNPs
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can be translocated and accumulated in the edible part of rice plant is very important
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to assess the risk of MNPs for human health.16
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The main aims of this study were to (1) investigate the dynamic effect of MNPs
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on soil properties; (2) explore the bioavailability and speciation of MNPs in the paddy
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soil with its specific water management; (3) probe the influence of MNPs on the plant
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growth during the whole lifecycle and the translocation and transformation of MNPs
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in the plant. This work would support the assessment of environmental and ecological
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risks of MNPs and their impact on the safety of agricultural products.
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Materials and Methods
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CuO Particles
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CuO NPs and bulk particles (BPs) were characterized in our previous studies.11, 12
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CuO NPs have a primary size of 43 ± 9 nm with a spherical shape, a hydrodynamic
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diameter of 240 ± 23 nm in deionized water, a specific surface area of 131 m2/g, and a
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purity over 99.9%. CuO BPs have a reported particle size of over 1000 nm.
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Plant Culture and Treatment 6
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Rice (Oryza sativa L.) seeds were obtained from the Wuwangnong group in China.
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The specific methods of solution culture were described previously.12 In brief, seeds
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were disinfected by sodium hypochlorite and germinated on moist gauze for a week
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under dark condition at 25 °C, then were transferred into the nutrient solution (Tables
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S1 and S2) for two weeks.22 The rice seedlings grew in a phytotron with a relative
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humidity of 60-70 % at 25 °C in the day (16 h) and 20 °C at night (8 h).
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The surface soil was collected from a site (119°51´E, 30°22´N) in Jingshan town,
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Hangzhou, China. The methods of soil characterization and mixture of soil and CuO
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particles were described in the supporting information (SI). The soil had a pH of 5.30,
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4.15% organic matter, 6.71 cmol/kg cation exchange capacity. The sand, silt and clay
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contents were 11.1%, 57.0% and 31.9%, respectively. The concentration of total Cu in
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soil was 11.43 mg/kg (Table S3). After air-dried, the collected soil were sieved to less
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than 2 mm. CuO particles were added as powder into the air-dried soil for each
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treatment and stabilized for 24 h before planting.18 The target concentrations were 50,
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100, 500, and 1000 mg/kg (dry mass/air-dried soil mass basis) for CuO NPs, and 1000
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mg/kg for CuO BPs. These levels of CuO particles were chosen based on previous
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studies20, 23, 24 and the likely low mobility of NPs in the paddy soil. The unspiked and
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air-dired soil was set as control. Each mixture of CuO and air-dried soil was placed in
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the polyvinyl chloride (PVC) container (16 cm diameter × 30 cm height), and then an
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uniformed rice seedling was transplanted (Figure S1). The fertilizers (60 mg N/kg
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from CO(NH2)2, 100 mg P2O5/kg, and 100 mg K2O/kg) were added as solution at the 7
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beginning of the soil culture for plant growth. The additional urea (45 mg/kg) was
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supplied during tillering stage and heading stage. The plant culture was conducted in a
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greenhouse with natural light and a range of temperature from 20 °C to 32 °C. We
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used 72 pots (6 treatment groups × triplicate × 4 growth stages) in this experiment.
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The pots were irrigated with deionized water every 48 h to achieve water-saturated
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soil and maintain the depth of the water layer over the soil surface for 4 cm. And the
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operation pattern of alternate drying-wetting was adopted following the heading stage.
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Soil moisture in planted pots underwent 4 drying-wetting cycles, each drying-wetting
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cycle lasted 7 days. The drying process was gradually and naturally performed by
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water loss through evapotranspiration, while the wetting process was manually
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conducted by the addition of deionized water to the soil surface. During the
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drying-wetting cycles, soil moisture in the pots varied between 40 % and 15 %.
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Based on plant growth status, plant and soil samples were collected on Day 7, 21,
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60, and 88, which were at the seedling stage, tillering stage, heading stage, and
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maturation stage, respectively. Meanwhile, the values of soil pH, redox potential (Eh),
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and electrical conductivity (EC) were measured in-situ using an ion analyzer
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(Thermo-Orion, Beverly, MA, USA) equipped with a pH electrode (ROSS), an
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oxidation reduction potential electrode, and a conductivity meter.
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Single Chemical Extraction of Cu from the Soil
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After pre-frozen in a -70°C lab freezer overnight, soil samples were lyophilized at
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-56 °C and 0.280 mbar pressure for 48 h in a freeze-dryer (Alpha1-4LSC, Marin 8
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Christ Ltd., Germany). Table S4 shows the detail on the single chemical extraction
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method applied to determine the bioavailability of CuO NPs in soil. After shaking,
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mixtures of soil and reagent were centrifuged for 10 min at 12,000 g, and then filtered
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through quantitative filter papers (1-3 µm pore size, Whatman Xinhua Limited
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Company, China). The Cu concentration in the collected liquid was determined by an
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atomic absorption spectrometer (AAS, MKII M6, Thermo Electron, USA).
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Determination of Plant Biomass and Cu Content
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At each growth stage, the collected fresh plants were washed with deionized water to
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remove the soil on roots. Then shoots and roots were separated, and their height, fresh
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weight, dry weight, and water content were measured. The fresh weight and number
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of filled grains and unfilled grains was recorded. The Cu content determination in
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plants was described previously.12 Briefly, plants were dried, ground, and digested
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with a mixture of HNO3-H2O2 (1:4, V/V) using a microwave digestion unit (MARS5,
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CEM Microwave Technology Ltd., USA). The Cu content in digested plants was
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analyzed by an inductively coupled plasma-optical emission spectrometer (ICP-OES,
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iCAP 6300 DUO, Thermo, USA). The information of quality assurance/quality
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control (QA/QC) of ICP was described in SI.
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Bulk X-ray Absorption Near Edge Structure (XANES) Analysis
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Bulk-XANES was performed to characterize Cu chemical forms in the soil and plant.
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Soil and plant samples were pre-frozen at -70°C overnight and lyophilized at -56 °C
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and 0.280 mbar for 72 h, then were ground to powder, pressed into slice, covered with 9
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tape, and placed on sample holders. The Cu K-edge XANES spectra of samples and
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references were mainly recorded on the beamline 1W1B at the Beijing Synchrotron
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Radiation Facility (BSRF, Beijing, China). The rest XANES spectra were collected on
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the beamline 14W1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai,
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China). The reference samples of Cu adsorbed on humic acid and Cu adsorbed on
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goethite were prepared according to the Strawn’s method.25 The SI presents details on
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bulk-XANES analysis. The IFEFFIT Athena software was used to process XANES
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data.11
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Micro X-ray fluorescence (µ-XRF) Analysis
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µ-XRF analysis was conducted to characterize the element distribution in the grain.
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Rice samples were pre-frozen at -70 °C overnight, then were lyophilized (-56 °C,
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0.280 mbar) for 48 h. The polished rice was bisected along its latitudinal axis, and a
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thin slice of the half grain without germ was stuck on tape (Scotch 810, 3M, USA)
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and fixed on the sample holder for the experiment.26 The µ-XRF experiment was
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proceeded at the beamline 4W1B in BSRF, which runs 2.5 GeV electron with a
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current ranging from 150-250 mA. The excitation energy was monochromatized by a
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W/B4C double-multilayer-monochromator at 15 keV. Then the incident beam was
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focused to the size of 50 µm × 50 µm by a Kirkpatrick-Baez mirror and a
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polycapillary lens. The 2D mapping was acquired as follows: the sample holder was
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fixed on a precision motor-driven stage, and then the chosen site was scanned with a
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step size of 500 µm. The Si(Li) solid state detector was applied to detect XRF 10
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emission lines with a live time of 60 s. The reduction and process of µ-XRF data were
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performed using PyMCA package.27
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Statistical Analysis
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The data were reported as the mean value ± standard deviation (SD) of triplicates for
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each treatment group. One-way analysis of variance (ANOVA, SPSS Version 16.0,
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SPSS Inc., USA) followed by the Tukey-HSD (honestly significant difference) test
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was applied to analyze the statistical significance among values in the experiment
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including the determination of pH, Eh, EC, and Cu content in the soil, Cu content in
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the plant, plant biomass, water content, and grain yield. P
