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Soil-incorporation of silica-rich rice husk decreases inorganic As in rice grain Angelia L. Seyfferth, Andrew H. Morris, Rattandeep Gill, Kelli A. Kearns, Jessica N. Mann, Michelle Paukett, and Corey Leskanic J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b01201 • Publication Date (Web): 24 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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Journal of Agricultural and Food Chemistry

TITLE: Soil-incorporation of silica-rich rice husk decreases inorganic As in rice grain AUTHORSHIP: Angelia L. Seyfferth1*, Andrew H. Morris1,‡, Rattandeep Gill1,†, Kelli A. Kearns1, Jessica N. Mann1, Michelle Paukett1, §, Corey Leskanic1 1

Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716 Present Address: Department of Ecosystem Science and Management, The Pennsylvania State University, State College, PA 16802 † Present Address: Department of Plant Agriculture, University of Guelph, Guelph, ON, Canada, N1G 2W1 § Present Address: Department of Agricultural Economics, Texas A & M University, College Station, TX, 77843 ‡

*Corresponding Author: Angelia L. Seyfferth, Email: [email protected], telephone: (302) 831-4865, fax: (302) 831-0605

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ABSTRACT

Arsenic decreases rice yield and inorganic grain As threatens human health; thus,

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strategies to decrease rice As are critically needed. Increased plant-available silica (Si) can

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decrease rice As, yet the source of Si matters. Rice husk, an underutilized and Si-rich by-product

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of rice production that contains less labile C and an order of magnitude less As than rice straw

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may be an economically-viable Si resource to decrease rice As, yet the impact of rice husk

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incorporation on As in the rice-soil nexus has not been reported. This proof-of-concept study

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shows that rice husk incorporation to soil (1% w/w) decreases inorganic grain As by 25-50%

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without negatively affecting grain Cd, yield, or dissolved CH4 levels. Rice husk is a critical yet

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perhaps overlooked resource to improve soil quality through enhanced nutrient availability and

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attenuate human health risks through consumption of As-laden grain.

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KEYWORDS: biocycling; cadmium; Oryza sativa L.; methane; silicon; sustainable rice

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production

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INTRODUCTION Arsenic lowers rice yield1-3, and impacts human health via consumption of inorganic As

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(As(V) and As(III), Asi) in grain4-6. While Asi species are more acutely toxic to humans than

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mono- or dimethylated As(V) (MMA and DMA, Aso)7 all four forms are absorbed by rice roots

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through their chemical similarities to needed plant nutrients of phosphate (As(V))8, 9 or silicic

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acid (As(III), DMA, MMA)10, 11 and are detected in grain12. Under typical flooded rice

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cultivation, As is mobilized through reductive dissolution of Fe (oxyhydr)oxides and the

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reduction of As(V) to the more soil-mobile As(III)13, 14. Thus, alternative water management that

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leads to periods of oxic soil conditions and consequently soil-retention of As can decrease As

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uptake in rice. Rice grown in non-flooded or aerobic conditions has low grain As15, but also has

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lower yield than intermittently flooded or flooded rice15-17. Intermittent flooding minimally

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impacts yield18 and decreases total grain As, but may or may not decrease grain Asi15, 17, 19.

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Moreover, non-flooded or intermittently flooded rice cultivation may increase grain Cd when

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aerobic soils are acidic17, 18. Decreasing As at the expense of increasing Cd is not a viable

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solution, since Cd is also toxic to humans20. Additional strategies to decrease grain As that do

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not increase grain Cd or decrease yield are critically needed.

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Increasing plant-available Si, an important nutrient for rice21 that is typically poorly

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plant-available in well-weathered rice soils22 can decrease grain As 15, 23, 24 because As(III), the

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dominant As species in flooded rice paddies25 shares a root-uptake transporter with Si10;

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however, the source of Si matters. Si-rich materials that increase pore-water Si to near saturation,

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such as Si gel, decrease grain As15, 24. In contrast, Si-rich materials that moderately increase

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pore-water Si may increase grain As24 via competition between Si and As(III) for soil-mineral

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sorption26. Silica fertilizers such as CaSiO3 are used in in Japan for rice cultivation22, and may

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decrease As in rice seedlings27, 28, yet CaSiO3 slag may contain trace metals and thus negatively

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impact rice29. Because most of the world's rice is grown in well-weathered, Si-depleted soil in

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tropical developing countries22, 30 (i.e., South and Southeast Asia) that are projected to

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experience devastating impacts on rice yield in the next 40 years31, a Si-rich source to attenuate

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As uptake and stress in rice is critically needed.

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Rice husk is an underutilized and Si-rich by-product of rice production that may aid in

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improving rice and human health, but the impacts of rice husk incorporation to soil on As uptake

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and grain speciation has not been reported. Rice husk contains less labile carbon32, an order of

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magnitude less As24, 33 and upon incorporation to plant-less soil leads to 50% more pore-water

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Si33 than rice straw, and may be an economically-viable resource to improve rice nutrition22, 34

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and decrease rice As. To test this, we grew three cultivars of rice in well-weathered soil

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amended with Si-rich rice husk residues and report the impacts on inorganic As and Cd in grain,

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rice yield, and CH4 production compared to nonamended controls.

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MATERIALS AND METHODS

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certified 99% purity or higher, and all reagents were prepared using 18 MΩ cm water or better.

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Trace-metal grade HNO3 was used for plant digestions, and ACS certified NaOH was used for Si

Reagents and biological materials. All salts used to prepare reagents were ACS ●

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dissolution. Standards used for total elemental quantification with ICP-OES and ICP-MS were

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certified (SPEX CertiPrep for As, Cd, and Zn; Fisher Scientific for Si and other plant nutrients).

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Arsenic speciation standards for MMA, As(V), and As(III) were prepared by serial dilution after

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dissolving ACS certified sodium salts with 18 MΩ cm or better water, and the acid form was

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used for DMA.

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The three cultivars of rice used in this study consisted of M206 (medium-grain Calrose

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rice, japonica), IR66 (indica) and Nipponbare (japonica). M206 is a Californian cultivar, IR66

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was developed by IRRI and is grown in Cambodia, and Nipponbare is the model rice cultivar for

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which the rice genome is known. M206 was obtained from the Rice Experiment Station in

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Biggs, CA, USA; IR66 was obtained from the International Rice Research Institute, and

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Nipponbare was obtained from the laboratory of Nicole Donofrio at the University of Delaware.

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Soil collection and characterization. To more closely capture edaphic conditions, soil-

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based experiments were utilized in plant-uptake experiments. Soil was collected from the

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University of Delaware Newark Farm (UD Farm) from the upper 30 cm of the profile, after

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removal of the organic horizon (sod) and utilized for rice experiments. This soil is of the

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Elsinboro series (23% sand, 61% silt, 16% clay) and is classified as a fine-loamy, mixed,

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semiactive, mesic Typic Hapludults according to the US Soil Taxonomic description, or an

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Acrisols according to the Food and Agriculture Organization classification system and was

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chosen because of its similarities (available Si, total As content) to well-weathered soils in

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Southeast Asia that are used for rice production35. Soil collected from several shallow pits was

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combined into a composite sample using large plastic tubs with care to preserve soil structure as

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much as possible. Subsamples of the composite soil were obtained and utilized for soil

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characterization and the remainder was utilized in pot experiments. A detailed explanation of the

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soil characterization procedures and results was previously published33 and are briefly given

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here: the soil pH (1:1) was initially 6.2, the acetic acid-extractable Si was 14 mg kg-1, the total

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As (acid-digestible) was 17 mg kg-1 and organic matter (loss on ignition) was 2.5%. The

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elevated As in these soils compared to background As concentrations in the U.S. may have been

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the result of As-based herbicides dating back to the early 1900s, as this plot was an orchard at

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that time.

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Experimental design. To determine the impact of Si-rich rice husk incorporation to soil

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on As uptake by rice, plants grown in soil amended with two Si-rich rice residues were compared

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to non-amended controls. To each opaque, HDPE 4-liter acid washed pot, 3.5 kg of air-dried soil

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were added. Based on previous work33, powdered fresh rice husk (FH) or rice husk ash (RHA)

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were utilized as Si amendment treatments at a rate of 1% (w/w) of rice residue:soil. While this

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level of amendment is higher than feasible on a large-scale (i.e., husk availability may be 0.1%

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per crop cycle), here we aimed to conduct a proof-of-concept study that was consistent with

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previous work33, 36 and that would not have an initial difference in pore-water As levels between

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Si treatments33. Treatments as well as a nonamended control were conducted in triplicate. After

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gently hand mixing each amendment into soil, pots were flooded to 4 cm above the soil surface

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using deionized water. At this stage, Rhizon samplers (Soil Moisture Corp., Goleta, CA, USA)

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were inserted at a downward 45° angle for subsequent pore-water analysis; these 10-cm long

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samplers were able to capture pore-water in the rooting zone of the plants.

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Rice seedling germination and plant growth. To determine the impact of Si-rich rice

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husk addition on As uptake by rice, three cultivars of rice (Oryza sativa L., cvs. M206, IR66, and

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Nipponbare) were utilized in uptake experiments. Surface-sterilized seeds were soaked in

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deionized water for 24 hours and then transplanted to germination cubes that contained

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nonamended UD Farm soil. Seedlings were allowed to germinate for 7 days prior to

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transplantation into treatment pots. Each pot contained three seedlings of uniform height, and

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each treatment or control was replicated three times at the pot level.

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Pots were transferred to a controlled environment chamber under LumiBar LED strip

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lights, which are designed to deliver the maximum light density by enhancing the spectral output

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covering the full PAR region (400 – 700 nm) (LumiGrow, Novato, CA, USA) under which light

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intensity was maintained > 350 µmol m-2 s-1. Day/night relative humidity and temperature were

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maintained at 80/60 % and 28/26 °C, respectively. Pots were kept flooded at 4 cm above the soil

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surface with reverse-osmosis deionized water that did not contain ICP-MS-detectable As or Si,

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and watering ceased one week prior to harvest. No exogenous As was added; As available to

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rice roots for uptake resulted from the ensuing reducing soil conditions due to flooding aimed to

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simulate conditions in, e.g., Cambodia or California, where As-contaminated water is not

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extensively used for rice irrigation.

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Pore-water collection and analysis. To monitor the impacts of Si amendment on pore-

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water chemistry, pore-water was collected and analyzed weekly. Pore-water was collected from

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Rhizon samplers using a needle-stop cock assembly into evacuated, acid-washed vials that had

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been crimp sealed under an oxygen-free atmosphere using methods published previously24.

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Briefly, water was first collected into glass vials and used for pH and redox potential (Eh)

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measurements using calibrated probes, and then collected into plastic vials for all other analyses

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including total As using ICP-MS, Si and other plant nutrients using ICP-OES after sample

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dilution and acidification with trace-metal grade nitric acid24. Based on results of total As, M206

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pore-waters were analyzed for As species As(III), As(V), DMA, and MMA using HPLC-ICP-

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MS using the chromatography conditions reported in Maher et al.37. For Nipponbare only, Eh

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was not measured. Dissolved CH4 concentrations were additionally measured for IR66 using the

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headspace equilibration technique38 with subsequent analysis with gas chromatography as

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described previously33.

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Plant harvesting, digestion, and analysis. Rice plants were grown to maturity (ca. 90 –

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120 days, depending on cultivar) under flooded conditions to determine the impact of Si-rich rice

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husk residue amendment on grain As and yield. Prior to harvest, plant height and the number of

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productive tillers were measured. Panicles were then cut and placed on foil to air dry. The

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number of grains and unfilled grains were counted and rough rice biomass (i.e., weight of filled

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grains with husk intact) was measured. Rice straw was cut 1 cm above the water line and was

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placed onto foil. After drying in a 65 °C oven, rice straw biomass was measured. Roots were

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removed from the soil manually and thoroughly washed with tap water and then with deionized

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water to remove adhered soil. Roots were patted dry and left to air dry on foil in the laboratory

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air. Once dry, roots were hand shaken and then rewashed to ensure the removal of soil particles

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and left to air dry. Once dry, root biomass was obtained. Roots were then bisected

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longitudinally: one half was archived, and the other half was subject to dithionite-citrate-

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bicarbonate extraction at room temperature to remove Fe plaque from root tissues according to

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prior work39, 40. Plaques were analyzed for total As, Fe, Si and P using ICP-OES24. After plaque

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removal, roots were again washed and then dried in a 65°C oven.

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Rough rice was dehusked using a benchtop dehusker, but rice grain could not be polished

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using the laboratory polisher due to small sample sizes. A subset of 5 dehusked brown rice

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grains (i.e. with bran intact) per pot were stored for later spectroscopic analysis, and the

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remaining grain, husk, straw, and plaque-free roots were separately ground into fine powders.

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Powdered plant fractions were weighed into Teflon digestion vessels (CEM Corporation,

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Matthews, NC, USA) and were subject to microwave-assisted acid digestion with HNO3 (MARS

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6, CEM Corporation) according to previous work24, 35. Total As, Cd, and Zn were analyzed in

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the HNO3 digest after dilution using ICP-MS and plant nutrients using ICP-OES24. NIST

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Certified Reference Material (CRM) 1568a rice flour was used to validate the analysis and gave

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excellent recoveries for As, Cd, and Zn of (n = 3) 103 ± 8, 105 ± 6, and 92 ± 6 %, respectively.

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Due to the high Si content of husk and straw, the Si-gel residue remaining after acid digestion

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was washed with deionized water after centrifugation and subject to 2 M NaOH digestion using a

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modified method of Derry et al.41. Silicon was determined colorimetrically on the NaOH digest

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using the methods of Kraska and Breitenbeck42. Two CRMs with indicative Si values (WEPAL

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IPE 883, Carnation Straw, Si = 3250 – 9050 mg kg-1 and ERM-CD281, Rye Grass, Si = 1100 ±

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1500 mg kg-1) were used to assess Si recovery and values obtained (3503 ± 1034, n = 3 and 1088

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± 20, n = 2) were within the reported ranges.

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As speciation in rice grain. Powdered brown rice grain was subject to dilute nitric acid

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extraction and analysis with HPLC-ICP-MS using a PEEK PRP-X100 anion exchange column

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(250 mm x 4.6 mm, 10 µm) according to the methods of Maher et al.37 to quantify inorganic As

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species (As(V) and As(III)) and organic As (MMA and DMA) species. Repeated extractions and

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analyses (n = 3) of CRM 1568a rice flour yielded recoveries similar to reported values37 for

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As(III), As(V), DMA, and MMA of 0.086 ± 0.005, 0.037 ± 0.003, 0.152 ± 0.018, 0.011 ± 0.001

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mg kg-1, respectively.

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Statistical analyses. The concentration of As, Cd, Zn and the percent inorganic As in

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grain and yield parameters were compared among FH, RHA and nonamended controls for each

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cultivar separately with ANOVA and Tukey HSD tests after verifying the data complied to the

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normality and homogeneity of variance assumptions required of ANOVA using the Shapiro-

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Wilks and Levene’s tests, respectively. For each variety separately, regressions were performed

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between straw Si or As and grain or straw As concentrations and for time-averaged pore water Si

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or As and grain As concentrations using data for all treatments and control together. All

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statistical tests were conducted using SPSS v. 21 software. All pore-water data is averaged and

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plotted ± the 95 % confidence interval; non-overlapping error bars thus represent statistical

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differences between treatments at the 95% confidence interval.

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RESULTS AND DISCUSSION

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decreased either inorganic or total As in rice grain, husk, and straw without compromising yield

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or negatively affecting grain Cd or Zn (Fig. 1, Fig. S1-S2, Table 1), but differences were

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observed between FH and RHA amendments and cultivars. In all cultivars, FH decreased toxic7

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grain Asi by 25 – 50% and straw As by at least 50%, and increased straw and husk Si by 25 –

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60% (Figs. 1, S1, S3) without affecting yield (Table 1). In contrast, RHA amendment marginally

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improved yield and decreased total grain As, but only decreased grain Asi for IR66 (Fig. 1, Table

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1). Prior work with RHA in Sir Lanka showed that yield improvements were achieved with

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RHA amendment 43, which corroborates our findings. RHA also decreased husk As for M206,

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straw As for M206 and Nipponbare, and husk Si for all cultivars (Figs. S1, S3). The FH-induced

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decreased grain Asi we observed is noteworthy since Asi is considered more toxic to humans

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than pentavalent MMA and DMA)7; thus, FH incorporation may hold promise to decrease As

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risk from rice consumption.

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Impacts on grain As, Cd, and yield. Soil incorporation of Si-rich rice husk residues

We observed As values in brown rice ranging from 0.6 to 0.95 mg kg-1 for M206 and

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IR66, depending on amendment, and ~ 0.45 mg kg-1 for all Nipponbare plants (Fig. 1). These

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values are higher than those found in most market-basket surveys of polished rice worldwide12, 44

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but are on par with unpolished grain As in pot and field studies conducted previously17, 18, 35, 45.

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Higher values in brown rice (unpolished) are expected since most grain As is found in bran46, 47

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which is removed during polishing. Nipponbare grew for a longer period of time and had much

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higher overall biomass compared to M206 and IR66 (Table 1), which likely had a dilution effect

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in planta.

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We observed grain Cd levels of ~ 0.01 mg kg-1 for Nipponbare and M206 irrespective of

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amendment, and 0.03 to 0.075 mg kg-1 for IR66 (Fig. S2); these are similar to those obtained

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from field studies in China, India, and Bangladesh48. IR66 grain Cd was nearly 50% lower in FH

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or RHA-amended soil compared to nonamended soil and was higher than in M206 and

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Nipponbare. IR66 pots remained oxidized for a longer period of time and took longer to reach

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Fe-reducing conditions compared to M206 and Nipponbare experiments (Fig. S4), which may be

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due to cultivar differences in radial oxygen loss from roots that affects metal(loid) cycling49.

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Because Cd availability and uptake by rice is enhanced under more aerobic conditions17, 18, the

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cultivar-induced difference in redox potential is likely responsible for higher Cd levels in IR66

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compared to M206 and Nipponbare that we observed.

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Pore-water, plant and grain As relations. All cultivars converge to their lowest grain

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Asi levels at maximum pore-water Si (Fig. 2), which was achieved with FH (Fig. S5). In

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contrast, no relation was observed between grain and pore-water As (Fig. S6). Incorporation of

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Si-rich amendments had little effect on other measured pore-water parameters (Fig. S4). Pore-

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water Si explains 74% of grain Asi variation for M206, 62% for IR66 (Fig. 2) and 30% for all

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cultivars together (Fig. 2, P < 0.001, trend line not shown). The highest pore-water Si, achieved

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with FH amendment, was 2 – 10 times higher than nonamended control soils, yet FH amendment

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also had the highest pore-water As (Fig. S5). Pore-water and straw Si (Figs. S3, S5) were

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strongly negatively correlated with straw and grain As (Figs. 2, 3) and were stronger predictors

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of grain Asi than plant or pore-water As (Figs. S5, S7).

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Several mechanisms may explain the decreased plant As due to FH or RHA amendment.

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Husk residues enhanced Si release to pore-water (Fig. S4), which limited As(III) uptake by 1)

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downregulation of the expression of Lsi1 and Lsi2 in roots thus limiting the transporters

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available for As(III) uptake50, and 2) enhanced Si competition with As(III) for uptake23, 24. As a

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result of less As being taken up by roots, and less adsorption of arsenite to soil solids via

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competition with added Si26, more As remained in pore-water in FH and RHA amended pots

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(Fig. S5). If this strategy were applied under field conditions in Southeast Asia, liberated pore-

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water As could be subject to monsoon flushing and thus removal of As from paddy soils over

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successive crop cycles51. If this strategy were applied in other fields where As is less of a

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concern (e.g., California), increased Si from FH incorporation could help rice nutrition and

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disease resistance52.

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FH may have decreased grain Asi levels by altering the functional diversity of the

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microbial community. In particular, we suggest that FH amendment increased the activity of soil

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microbes that express the As methyltransferase gene, arsM, and are able to methylate Asi, which

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plants are not known to do53. For M206, we observed an average of 5% (2 – 14%) of pore-water

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As as DMA with FH treatment and only 2% (n.d. – 5%) with control and RHA treatments, which

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supports this mechanism. DMA is not sequestered by sulfur-rich phytochelatins and is thus more

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mobile in planta than Asi and can be more readily transported to grain54. Ma et al.55 showed that

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arsM activity increased after rice straw incorporation and while straw resulted in a higher level

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of grain Aso, its overall impact was an increase in total grain As and no change in grain Asi. In

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contrast, husk incorporation in our study resulted in either no change or a decrease in total grain

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As and a 25 – 50% decrease in grain Asi (Fig. 1); these contrasting findings between straw and

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husk incorporation indicate that the type of Si-rich residue matters. In experiments without

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plants, we previously showed33 that incorporation of Si-rich rice residues altered the soil

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microbial community, where rice straw promoted more reductive dissolution of iron

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(oxyhydr)oxides and pore-water As release than rice husk due to differences in inherent carbon

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lability32 between the two residues. Excess silica from husk dissolution could have mobilized

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adsorbed soil DMA and promoted its root-uptake56. Thus, FH amendment in our study likely

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resulted in the conversion of Asi to organic Aso in soil/pore-water and enhanced DMA

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desorption. This DMA was then taken up and transferred effectively to the grain, resulting in no

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change or a decrease to total but increased grain Aso (Fig. 1).

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Impacts of residues on CH4 production. The potential of husk amendments to decrease

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grain Asi are not at the expense of CH4 production (Fig. 4). Despite enhanced labile carbon due

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to FH amendment33, there was no difference in dissolved CH4 between treatments, which are

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relatable to CH4 emissions. We33 and others57-59 previously showed that rice straw incorporation

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enhances CH4 emissions from flooded soils, and it also releases more pore-water As and less Si

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than husk or ashed residues with consequent drawbacks as an amendment33. In most rice-

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growing regions, rice straw and husk are typically removed from the field and used for various

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purposes including animal fodder, fuel for household stoves, or burned as a means to power rice

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milling machines, leaving behind a Si-rich ash22. If the ashed residues are re-incorporated rather

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than discarded it might be feasible to recycle Si from already combusted husks, despite the CO2

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emissions during ashing. Since most rice Si is contained within straw and husk, rice residue

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removal each crop cycle exacerbates soil desilication by circumventing biocycling60.

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Implications. Biocycling Si-rich rice husk residues is a holistic strategy to increase plant-

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available Si which, we show, has promise for decreasing grain Asi by 25 – 50% without

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negatively impacting grain Cd, CH4 production or yield. Rice husk is advantageous over straw

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because it contains 10-fold less As, less labile C, and provides 50% more Si to pore-water33. In

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this proof-of-concept study, we utilized a relatively high incorporation rate of 1%, which is 10-

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fold higher husk amount than would be available each crop cycle. While this incorporation rate

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is not feasible on a large-scale each crop cycle, prior work indicates that Si release from 1% rice

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straw incorporation benefits multiple crop cycles because the material acts as a slow-release

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fertilizer36. Because husk is more recalcitrant than straw32, it may be an even slower-release

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fertilizer and thus one application may provide multiple years of benefit. Furthermore, positive

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impacts of rice husk incorporation shown here may be achieved at lower incorporation rates and

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further research in this area is warranted.

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ABBREVIATIONS: FH = fresh husk; RHA = rice husk ash

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ACKNOWLEDGEMENT

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We thank Caroline Golt and the UD Soil Testing Laboratory for analytical assistance, Izzy

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Sibbers and Gang Li for sampling assistance and Michael V. Schaefer for assistance with residue

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collection. This project was funded by the 2014 University of Delaware Research Foundation

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grant No. 14A00765, and the National Science Foundation grant No. 1338389 and No. 1350580

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awarded to ALS.

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SUPPORTING INFORMATION AVAILABLE

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Additional data as cited in the manuscript are presented. This information is available free of

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charge via the Internet at http://pubs.acs.org/.

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rice (Oryza sativa L.) irrigated with contaminated water. Plant Soil 2002, 240, (2), 311-319.

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Communications in Soil Science and Plant Analysis 2008, 39, (1-2), 302-313.

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Duxbury, J. M., The effects of iron plaque and phosphorus on yield and arsenic accumulation in

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rice. Plant Soil 2009, 317, (1-2), 167-176.

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Trends in Plant Science 2004, 9, (9), 415-417.

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515

FIGURE CAPTIONS

516

Figure 1. Total and speciated grain As concentrations in unpolished grain from three cultivars

517

of rice grown in nonamended soil or soil amended with 1% (w/w) of either fresh rice husk (FH)

518

or rice husk ash (RHA). Grain Asi is depicted by the lower solid bars and grain Aso is depicted

519

by the upper dashed grey bars, for which only DMA was detected. Total As or *Asi is

520

significantly different (P < 0.05) from the nonamended control for that rice cultivar.

✪

521 522

Figure 2. Logarithmic relationships between inorganic grain As concentrations and time-

523

averaged concentrations of pore-water Si concentrations (± standard error, n = 36) in M206,

524

IR66, and Nipponbare rice grown in a well-weathered soil amended with fresh rice husk (FH) or

525

rice husk ash (RHA) compared to a nonamended control soil. **P < 0.01, IR66 = medium dash;

526

M206 = long dash; Nipponbare = n.s. and no trend line shown.

527 528

Figure 3. Relationships between straw Si concentrations and inorganic grain As (A) or straw As

529

(B) concentrations in M206, IR66, and Nipponbare rice grown in a well-weathered soil amended

530

with fresh rice husk (FH), rice husk ash (RHA), or nonamended control soil. *Linear regressions

531

are shown only for significant relationships within each cultivar. Nipponbare = short dash; IR66

532

= medium dash; M206 = long dash; P < 0.05, **P < 0.01, ***P < 0.0001.

533 534

Figure 4. Weekly pore-water CH4 concentrations of cultivar IR66 grown in a well-weathered

535

soil amended with fresh rice husk (FH) or rice husk ash (RHA) compared to a nonamended

536

control soil. Values are averages ± 95% confidence intervals, n = 3.

537

23



538 539 540

Page 24 of 29

Table 1. Yield and harvesting parameters for three rice cultivars grown in a well-weathered soil amended with either fresh rice husk (FH) or rice husk ash (RHA) at 1% w/w and a nonamended control under flooded conditions. Values are mean ± standard deviation, n = 3. *significantly different from the nonamended control. Rice Variety

Total Treatment

% unfilled grains

Rough rice biomass (g)

Plant height (cm)

Number of Panicles

straw biomass (g)

7±4 6±3 8±4

7.6 ± 0.8 7.5 ± 0.2 8.7 ± 1.1

100 ± 5 100 ± 4 93 ± 6

5.0 ± 0 4.3 ± 1.2 6.0 ± 1.0

9.8 ± 1.1 7.7 ± 1.2 10.3 ± 0.5

M206

nonamended FH RHA

filled grains 279 ± 26 281 ± 49 319 ± 35

IR66

nonamended FH RHA

313 ± 63 328 ± 39 290 ± 52

12 ± 6 8±4 13 ± 4

6.8 ± 1.5 6.9 ± 0.8 6.7 ± 1.4

80 ± 7 84 ± 5 96 ± 6

7.0 ± 1.0 5.3 ± 0.6 5.0 ± 1.7

6.1 ± 1.0 5.5 ± 1.3 5.3 ± 0.8

Nipponbare

nonamended FH RHA

185 ± 57 145 ± 56 258 ± 33

47 ± 16 59 ± 10 27 ± 10*

4.7 ± 1.5 4.5 ± 1.3 7.0 ± 1.1*

88 ± 3 91 ± 6 89 ± 4

29 ± 6.4 32 ± 1.5 19 ± 2.3

27.6 ± 0.8 33.4 ± 2.4 31.2 ± 1.9

541 542 543

24


544

Page 25 of 29


Figure 1. Total and speciated grain As concentrations in unpolished grain from three cultivars of rice grown in nonamended soil or soil amended with 1% (w/w) of either fresh rice husk (FH) or rice husk ash (RHA). Grain Asi is depicted by the lower solid bars and grain Aso is depicted ✪

by the upper dashed grey bars, for which only DMA was detected. Total As or *Asi is significantly different (P < 0.05) from the nonamended control for that rice cultivar.



Figure 2. Logarithmic relationships between inorganic grain As concentrations and timeaveraged concentrations of pore-water Si concentrations (± standard error, n = 36) in M206, IR66, and Nipponbare rice grown in a well-weathered soil amended with fresh rice husk (FH) or rice husk ash (RHA) compared to a nonamended control soil. **P < 0.01, IR66 = medium dash; M206 = long dash; Nipponbare = n.s. and no trend line shown.


Page 26 of 29

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Figure 3. Relationships between straw Si concentrations and inorganic grain As (A) or straw As (B) concentrations in M206, IR66, and Nipponbare rice grown in a well-weathered soil amended with fresh rice husk (FH), rice husk ash (RHA), or nonamended control soil. *Linear regressions are shown only for significant relationships within each cultivar. Nipponbare = short dash; IR66 = medium dash; M206 = long dash; P < 0.05, **P < 0.01, ***P < 0.0001.



Figure 4. Weekly pore-water CH4 concentrations of cultivar IR66 grown in a well-weathered soil amended with fresh rice husk (FH) or rice husk ash (RHA) compared to a nonamended control soil. Values are averages ± 95% confidence intervals, n = 3.


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TOC Art.


Soil Incorporation of Silica-Rich Rice Husk ... - ACS Publications

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