Effects of pretreatment and solution chemistry on solubility of rice straw phytoliths

J Plant Nutr Soil Sci 2014, 177, 349–359 DOI: 10.1002/jpln.201300056 349 Effects of pretreatment and solution chemistry on solubility of rice-straw phytoliths Minh Ngoc Nguyen1*, Stefan Dultz2, and Georg Guggenberger2 Department of Pedology and Soil Environment, Faculty of Environmental Sciences, VNU University of Science, Vietnam National University, Hanoi 334–Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Institute of Soil Science, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany Abstract Rice is a Si-accumulator plant, whereby Si has physio-chemical functions for plant growth Its straw contains high shares of plant silica bodies, so-called phytoliths, and can, when returned to the soil, be an important Si fertilizer Release of Si from phytoliths into soil solution depends on many factors In order to improve prognosis of availability and management of Si located in phytoliths, in this study we analyzed the effect of pretreatment of rice straw by dry and wet ashing and the soil-solution composition on Si release Dry ashing of rice straw was performed at 400°C, 600°C, and 800°C and wet ashing of the original straw and the sample from 400°C treatment with H2O2 To identify the impact of soil-solution chemistry, Si release was measured on separated phytoliths in batch experiments at pH 2–10 and in presence of different cations (Na+, 2À K+, Mg2+, Ca2+, Al3+) and anions (Cl–, NOÀ , SO4 , acetate, oxalate, citrate) in the concentration range from 0.1 to 10 mmolc L–1 After burning rice straw at 400°C, phytoliths and biochar were major compounds in the ash At an electrolyte background of 0.01 molc L–1, Si released at pH 6.5 was one order of magnitude higher than at pH 3, where the zeta potential (f) was close to zero Higher ionic strength tended to suppress Si release The presence of cations increased f, indicating the neutralization of deprotonated Si-O– sites Monovalent cations suppressed Si release more strongly than bivalent ones Neutralization of deprotonated Si-O– sites by cations might accelerate polymerization, leading to smaller Si release in comparison with absences of electrolytes Addition of Al3+ resulted in charge reversal, indicating a very strong adsorption of Al3+, and it is likely that Si-O-Al-O-Si bonds are formed which decrease Si release The negative effect of anions on Si release in comparison with deionized H2O might be due to an increase in ionic strength The effect was more pronounced for organic anions than for inorganic ones Burning of rice straw at low temperatures (e.g., 400°C) appears suitable to provide silicon for rice in short term for the next growing season High inputs of electrolytes with irrigation water and low pH with concomitant increase of Al3+ in soil solution should be avoided in order to keep dissolution rate of phytoliths at an appropriate level Key words: rice straw / phytolith / dry ashing / solution chemistry / Si release / zeta potential Accepted July 30, 2013 Introduction Rice (Oryza sativa) belongs to a plant group known to take up monosilicic acid (Si(OH)4) by their roots resulting in an Si content of 5%–10% in plant dry matter (Marschner, 1995) By deposition in inter- and intracellular spaces throughout their leaf and stem, silicified structures are formed consisting of biogenic silica, so-called phytoliths (Parr and Sullivan, 2005) Within each growing period of rice relatively large amounts of Si are taken up from the soil solution and are cycled through the crop back into the soil (Wickramasinghe and Rowell, 2006) In the soil Si located in phytoliths is an important pool for supplying Si (Sommer et al., 2006) The following crops can benefit from this pool, and it is of particular interest for cultivation safety of rice to know the decisive factors for dissolution of phytoliths and release of Si The function of phytoliths in the rice plant can be deduced from the principal arrangement of silicified structures and organic matter (OM) in the plant material, which is shown for a vascular bundle in a rice leaf in Fig Between the bundle sheath and the leaf surface tightly packed bundle-sheath cells and more loosely arranged mesophyll cells form a protective cover on leaf veins stabilized by silicified structures in inter cellular spaces Through the deposition of silica in the cell walls the mechanical strength of leaves and stem is increased, which prevents plants from lodging in heavy wind Also transpiration rate of rice is reduced, and thus, sufficient Si supply contributes to the reduction of drought stress (Chen et al., 2011) Reduction of excessive transpiration and enhanced light interception promotes also photosynthesis (Kato and Owa, 1997) Silicon fertilization of soils for rice cultivation increased the resistance to fungal stress (Kato and Owa, 1997) and might also increase resistance to insect pests Recently an active impact of Si on rice root anatomy enhancing suberization and lignifications in roots was ob- * Correspondence: Dr Minh Ngoc Nguyen; e-mail: minhnn@hus.edu.vn  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.plant-soil.com 350 Nguyen, Dultz, Guggenberger _ 20 µm served (Fleck et al., 2011) Ma (2004) summarized the effects of Si on rice, and the general conclusion was that the resistance of plants to various biotic and abiotic stresses is enhanced Paddy soils with a high content of plant-available Si induce low As contents in rice plants (Bogdan and Schenk, 2008) In presence of Si, the uptake of As by paddy rice is decreased markedly This is of special importance for rice, as many paddy fields, e.g., in Bangladesh show geogenic arsenic contamination (Meharg and Rahman, 2003) On-site burning after harvesting is the primary method of handling rice straw to return nutrients to the soils In recent decades, burning of rice straw has been predominant because it is a cost-effective method of straw disposal, avoids interferences with soil preparation, and helps to reduce pest and disease populations resident in the straw biomass (Dobermann and Witt, 2000) Although burning of rice straw causes significant emission of CO2, almost complete loss of N and S, and contributes to air pollution, it is the easiest way of returning most nutrients to the soils, and at present rice growers have little incentive to quit burning Considering the large amount of Si accumulated in rice straw, products of straw burning are an interesting pool to serve as a silicon source for plants Due to a relatively low ignition temperature, burning of straw is observed at > 300°C (Babrauskas, 2003) Burning of biogenic silica, e.g., of the rice husk at higher temperatures > 700°C can lead to the formation of crystalline SiO2, where amorphous silica is transformed to more stable tridimite or cristobalite (Kordatos et al., 2008) The apparent reduction in reactivity of biogenic silica is associated with changes in the surface chemical  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim J Plant Nutr Soil Sci 2014, 177, 349–359 Figure 1: Three-dimensional image of the principal arrangement of silicified structures and organic matter for a vascular bundle in a dried leaf of a rice plant The visualization was performed by X-ray tomographic microscopy using the 3D segmentation and visualization software YaDiV (Friese et al., 2013) for analysis of the dataset Phytoliths appear gray and organic matter dark gray The pixel width is 0.37 lm structure, and in particular with progressive loss of reactive surface sites (Dixit and Van Cappellen, 2002) Crystalline forms are very inactive in the soil and their potential to serve as a Si source is almost lost For this reason, the relation of burning temperatures and dissolution of rice-straw-burned products has to be considered, but not much literature is available on this issue Burning temperature of rice straw can be decreased, when the straw is more compacted and the water content is high It is generally accepted that the dissolution of silica in aqueous solutions occurs via hydrolysis of Si–O–Si bonds of the SiO2 network Water itself is a strong promoter by means of its molecules oriented with their electronegative oxygen towards the Si atom, leading to a transfer of electron density to the Si–O–Si bond, thereby increasing its length and eventually breaking it (Dove and Crerar, 1990) pH is understood as an important factor driving silica-dissolution kinetics (Fraysse et al., 2006; Loucaides et al., 2008) In particular, the acceleration of the dissolution rate with increasing pH is explained by the increase in concentration of deprotonated ≡Si–O– sites at the solid’s surface (Brady and Walther, 1990) The negatively charged sites promote dissolution kinetics, either by enhancing the nucleophilic properties of water (Dove, 1994) or polarizing, and thus weakening, surface Si–O–Si bonds (Brady and Walther, 1990) It seems likely that anions can attack Si–O–Si bonds in a similar way as OH– Additives containing chemical groups that are strongly anionic, such as –COO– and –PO2À , may react with Si centers in Si–O–Si bonds of biogenic silica (Ehrlich et al., 2010) Several studies have highlighted the desilification of silica under alkaline conditions (Sauer et al., 2006; Saccone et al., 2007) However, in this way, an investigation on anion effects with aqueous solutions close to realistic pH conditions of www.plant-soil.com J Plant Nutr Soil Sci 2014, 177, 349–359 paddy soils, which are in the Red River Delta from pH 5–6 (Nguyen et al., 2009) is a necessity Effect of pretreatment and solution chemistry 351 Materials 2.1 Sample production Deprotonation of the silanol groups (Si–OH) on the phytolith surface can facilitate the water molecules to attack Si–O–Si bonds, which is known as a first step for desilification (Dove and Elston, 1992; Fraysse et al., 2006) On the other hand, adsorption of cations from aqueous solution onto deprotonated ≡Si–O– sites might occur and accelerate polymerization (Weres et al., 1981) The surface of phytolith might be, therefore, strengthened to resist dissolution Under reducing conditions of paddy soils, the release of bivalent cations such as Fe2+ and Mn2+ from dissolving oxides and in consequence Ca2+ and Mg2+ desorption from exchange sites is pronounced (Nguyen et al., 2009) The reaction of these cations with phytoliths may have a marked effect on their solubility and can be an important factor for Si release Considerable amounts of electrolytes are added to paddy fields if irrigation is performed close to the coastline with brackish water A strong decrease in pH is observed in paddy fields when the water table is lowered before harvesting Hence, Al3+ occurs in soil solution Al3+ is known to react with biogenic silica and to reduce its solubility remarkably (Wilding et al., 1979; Van Bennekom et al., 1991) These studies indicate that Al3+ has a strong effect on phytolith dissolution, which has to be considered for the management of silicon in paddy soils Usually, dry and wet ashing techniques used for the extraction of phytoliths from plant material (Parr et al., 2001) not remove all OM present in rice-straw samples There are still certain amounts of OM remaining in ashes (Lai et al., 2009) The effect of OM created by pyrolysis of biomass, so called biochar, on dissolution of phytoliths is not yet fully known The organic matrix may act as a protective barrier against hydrolysis of the silica Like phytoliths, biochar has variably charged surface sites, and both compounds contribute to the total net charge of the burned products The question arises if surface-charge properties can be used as a parameter for predicting phytolith dissolution in presence of unburned OM In this study, the effect of different parameters of solution chemistry, including pH, ionic strength, valency, and size of cations and anions on the solubility of phytoliths from rice straw was determined in order to make the prediction of Si release more reliable The mode of pretreatment of rice straw, burning at different temperatures, and wet ashing using H2O2 resulting in, e.g., different degrees of dehydroxilation of biogenic silica and OM contents was also investigated The apparent reduction in reactivity of biogenic silica goes along with changes in surface chemical structure, and in particular loss of reactive surface sites Thus, besides batch experiments for quantifying Si release also zeta potential (f), the key electrochemical parameter of the solid–liquid interface providing information about the interfacial double layer between the solution and the stationary layer of fluid attached to the phytoliths, was determined f indicates ion adsorption and ionization of surface functional groups, and thus provides important information on dissolution kinetics of the rice-straw phytoliths  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Rice straw was collected from a paddy field of a research station close to Tay Mo commune in the rice-growing area in the central part of the Red River Delta (105°44′17″ E, 20°59′57″ N) directly after harvesting in spring 2011 The rice straw was air-dried, ground in a blade grinder (Clatronic KSW 3306, Kempen, Germany), and passed through a 1.0-mm sieve The rice straw had 42.1 g kg–1 (d.w.) Tiron-extractable (Guntzer et al., 2010) Si, 387 g kg–1 C, and 13.2 g kg–1 N as determined by an Elementar Vario EL (Hanau, Germany) elemental analyzer with a respecitve C : N ratio of 29 Dry ashing of rice straw was performed by heating finely ground air-dried rice straw in an oven at 400°C, 600°C, and 800°C, respectively, for h To avoid strong exothermic reactions during dry ashing the weight of sample was limited to g For comparison, OM in air-dried finely chopped rice straw was treated by wet ashing with H2O2 until the end of reaction For wet ashing 25 mL of a 15% H2O2 solution were added to g straw, stirred, and kept in a water bath at 80°C 2.2 Sample properties The different pretreatments of straw changed organic-C content drastically (Table 1) Organic C was most completely removed by heating at 800°C, whereas treatments of rice straw with H2O2 in a water bath at 80°C alone removed only less than 1.5% of total organic C, showing a high resistance of rice straw against this oxidant Consequently, also the Tiron-extractable Si varies from 42 g kg–1 in the original ricestraw sample to 193 g kg–1 in the dry-ashed produced at 600°C (Table 1) Treatment of samples with H2O2 resulted in slight increase in Tiron-extractable Si only The ashes of the dry-ashing treatment had an alkaline reaction (pH 10–11) Soluble anions and cations of the dry-ashed rice-straw samples were determined by anion chromatography (Dionex, ICS-90) and ICP-OES (Varian, 725-ES) in a 1:10 extract with deionized water In solution, K+ was the most abundant cation but also marked amounts of Na+, Ca2+, and Mg2+ were observed (Table 1) For the anions, besides Cl– and SO2À also PO3À was found in solution A marked decrease of the specific surface area (SSA) determined by the N2-adsorption method (Quantachrome, NOVA 4000e, Boynton Beach, FL, USA) was obtained with increasing heating temperature The SSA of the sample heated at 400°C, 600°C, and 800°C were found to be 68.6, 19.8 and 1.0 m2 g–1, respectively, indicating a strong condensation of silica structures Temperatures > 700°C are known to inherit formation of crystalline SiO2 phases such as tridimite or cristobalite (Kordatos et al., 2008) Because of the severe decrease of SSA at higher burning temperatures strongly decreasing the amount of active surface sites, for further analyses focus was given on the straw sample ashed at 400°C For the original sample, SSA determination by the N2-adsorption method failed because N2 did not enter the micropores of OM www.plant-soil.com 352 Nguyen, Dultz, Guggenberger J Plant Nutr Soil Sci 2014, 177, 349–359 Table 1: Specific surface area (SSA), Si and C content of the original rice-straw sample (1), dry-ashed samples treated at 400°C, 600°C, and 800°C (2–4), wet-ashed sample treated with H2O2 (5), and combined treatment of dry ashing at 400°C and subsequent wet ashing by H2O2 addition (6) Soluble cations and anions were analyzed for the heat-treated samples alone Treatment SSA / m2 g–1 (1) original sample – Si / g kg–1 42.1 C / g kg–1 Soluble ions / mg kg–1 387 n.a K+ (2) 400°C 68.6 166 95 1.06 · (3) 600°C 19.8 193 19 > 104 104 104 Na+ Ca2+ Mg2+ Cl– SO2– PO3– n.a n.a n.a n.a n.a n.a 505 220 180 19.9 13.7 3.5 275 275 55 18.7 16.3 3.7 (4) 800°C 1.0 112 455 65 145 6.9 11.5 1.7 (5) H2O2 20.2 50 374 n.a n.a n.a n.a n.a n.a n.a (6) 400°C/H2O2 71.2 173 n.a n.a n.a n.a n.a n.a n.a > Electron micrographs of the different samples were made on a Fei Quanta 200 (Hillsboro, OR, USA) For this purpose, specimen were mounted on a double adhesive tape and sputtered with gold Back-scattered-electron images on ground original rice straw revealed that the fragments had an intact outer cell wall (Fig 2a), whereas dry ashing even at 400°C resulted in a strong degradation of the rim of straw fragments (Fig 2b) In all samples from dry ashing silicified cell structures were clearly detectable (2005) stated that the absorption band at 950 cm–1 of fumed silica only becomes visible for samples with a SSA > 200 m2 g–1 Dehydroxilation of OH groups upon burning of straw samples might be another reason for weakened Si–O stretching vibration of Si–OH groups Methods 3.1 Determination of dissolution kinetics Functional groups in the samples were determined with an FTIR spectrometer (Bruker, Tensor 27, Karlsruhe, Germany) using the attenuated total reflectance (ATR) mode at ambient conditions The bands at ≈ 1100 cm–1 and 800 cm–1, attributed to the stretching vibration mode of the SiO4 tetrahedron and the bending vibration mode of intertetrahedral Si–O–Si bonds, were obvious in all modes of pretreatment of rice straw (not shown) The band at 800 cm–1 proposes a full condensation of Si surrounded by four Si–O–Si linkages, whereas the band at 950 cm–1, representing the Si–O stretching vibration of Si–OH groups, is missing The absence of this band might be due to the relatively low SSA of the samples under investigation, which is up to 71 m2 g–1 Gun’ko et al 20 µm For determination of effects of solution chemistry on Si release from phytoliths, soluble salts from the ashes were removed by washing with deionized water for two minutes followed by centrifugation and decantation The procedure was repeated twice, and finally samples were freeze-dried As C analysis revealed, that there were still marked amounts of organic C in the samples, ranging from 95 g kg–1 for the 400°C treatment, 19 g kg–1 for 600°C treatment to g kg–1 for the 800°C treatment (Table 1), subsequent wet ashing with H2O2 of the sample treated at 400°C was carried out as suggested by Parr et al (2001) The relatively high stability of OM matter in rice straw against burning is thought to be 20 µm Figure 2: Back-scattered-electron images of a leaf in the original dried and hackled rice straw (a) and leaf fragment in dry-ashed rice-straw sample treated at 400°C (b)  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.plant-soil.com J Plant Nutr Soil Sci 2014, 177, 349–359 related with the phytolith “coating” of OM in plant residues (Fig 1) Subsequent treatments with H2O2 (5 g ash from the 400°C treatment, 25 mL of a 15% H2O2 solution, 24 h in a water bath at 80°C) were performed until the end of the reaction and resulted in a marked decrease of C content, which was g kg–1 in the freeze-dried material We analyzed effect of pH, ionic strength, and different cations and anions on the dissolution kinetics of phytoliths in the 400°C-treated rice-straw sample by monitoring Si release into solution In all experiments, 50 mg of sample was mixed with 100 mL of solution in 250-mL polypropylene tubes Based on results for the specific surface area and preliminary experiments phytoliths obtained by the 400°C treatment were used for most of the analyses Suspensions were gently shaken by hand directly after mixing and allowed to stand for 24 h at room temperature Some of the batch experiments were extended up to days with sampling at 24 h intervals The experiments were terminated by filtration of the suspension through a 0.45-lm pore-size cellulose acetate filter (Macherey-Nagel, Düren, Germany) Silicon in solution was determined in duplicate using the molybdate-blue method (Mortlock and Froelich, 1989) and an UV-VIS spectrophotometer (Agilent/Varian Cary-50 Scan, Böblingen, Germany), whereby the detection limit of the method was 0.1 mg Si L–1 In detail, we performed the following experiments: Experiment 1: Determination of the effect of pH on solubility of Si and f of phytoliths To identify the effect of pH on Si solubility, the solution was adjusted to pH 3.0, 4.5, 6.0, and 6.5 with 0.1 M HCl The dissolution experiment lasted d, and pH, f, and electrical conductivity were controlled every 24 h In case of an increase in pH small amounts of 0.1 M HCl were added under continuous stirring to adjust the scheduled pH f was measured to get information about the interfacial contact zone between the solution and phytoliths Electrical conductivity was recorded in order detect possible changes in ion concentration during the experiment Experiment 2: Evaluation of the effect of ionic strength on Si solubility Solutions with an electrolyte background (EB) of 10 and 50 mmolc L–1 NaCl were prepared pH values from to 10 were adjusted according to Fraysse et al (2009) by adding corresponding amounts of 0.01 or 0.05 M HCl and NaOH, respectively f was measured in order to get information about the underlying process Experiment 3: Assessment of cation effects on f and Si solubility Solutions of different cation composition and concentrations were prepared in the concentration range of 0.5–2.5 mmolc L–1 for Al3+ and 1.0–20 mmolc L–1 for Ca2+, Mg2+, K+, and Na+ from pure analyzed chloride salts Experiments were started with an initial pH of 3.5 in suspension adjusted with 0.01 M HCl The pH of 3.5 was adjusted to avoid precipitation of Al hydroxides which would affect f The suspension was sampled after 24 h for f measurement, and pH was controlled again Cation effects on the release kinetics of Si were determined at pH 5, and a fixed concentration of all cations under investigation of 10.0 mmolc L–1 for Na+, K+, Mg2+, and Ca2+ and 1.0 mmolc L–1 for Al3+ over a time course of d In order to specify the effect of Al3+ on Si  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Effect of pretreatment and solution chemistry 353 release and f, in a modification of this experiment the effect of five different concentrations of Al3+ (0.1, 0.25, 0.5, 1.0, and 2.0 mmolc L–1) on Si release was determined at pH 3, 4, and Experiment 4: Analysis of anion effects on solubility of Si 2À and f Solutions with 10 mmolc L–1 of Cl–, NOÀ , SO4 , acetate, oxalate, and citrate were prepared from pure analyzed Na salts The pH of the solutions was adjusted to pH by dropwise addition of 0.01 M solutions of the respective acids (HCl, HNO3, H2SO4, CH3COOH, H2C2O4, and C6H8O7) pH and Si concentration were determined every 24 h over a time course of d In case of an increase of pH, small amounts of the acids were added under continuous stirring to adjust pH Experiment 5: Determination of the effect of pretreatment of rice straw on Si release and its relation to changes of f Rice straw samples from dry ashing at 400°C, combined treatment of dry ashing at 400°C and subsequent wet ashing by H2O2 addition, and wet ashing treated with H2O2 were extracted with deionized H2O adjusted to pH pH and Si concentration were determined every 24 h over a time course of d 3.2 Zeta potential measurements The zeta potential (f) was determined for the rice-straw samples from dry and wet ashing in suspension to characterize properties of the solid–liquid interface as a function of pH, cation and anion concentration, and time as described in section 3.1 for experiments 1–5 After gentle shaking of the suspension, 1.6 mL of the suspension was sampled with a pipette and transferred in a cuvette for measurement in the zeta potential analyzer (ZetaPALS, Brookhaven, Holtsville, NY, USA) Here, f was determined using phase-analysis light scattering (PALS), allowing measurement of particles of very low mobility, i.e., particle movement of a fraction of their own diameter is sufficient to obtain a good reproducibility of data Measurement of f was performed with each 10 runs partitioned in 20 cycles, whereby the mean is given in the figures Sampling of suspensions was performed simultaneously for analysis of zeta potential and determination of Si concentration In addition, surface charge was quantified by polyelectrolyte titration for a dry-ashed rice-straw sample (400°C treatment) and a dry-ashed sample with subsequent H2O2 treatment in a particle-charge detector (PCD 03, Mütek, Herrsching, Germany) according to the procedure described in Nguyen et al (2009) in order to determine the effect of included residues of OM after pyrolysis in the sample on surface net charge Results and discussion 4.1 Dependency of Si release on pH and ionic strength For the dry-ashed rice-straw sample heated at 400°C the increase of pH from 3.0 to 6.5 resulted in an increase of Si release (Fig 3) Extraction during d at pH 6.5 resulted in a Si concentration of 40 mg L–1, which is equivalent to ≈ 46% of the total Si introduced in the experiment This result is in bewww.plant-soil.com 354 Nguyen, Dultz, Guggenberger J Plant Nutr Soil Sci 2014, 177, 349–359 tween the findings of Wickramasinghe and Rowell (2006) and Wilding et al (1979), who measured a Si extractability of 20%–38% and 50%–75%, respectively The Si concentration observed at pH 6.5 is one order of magnitude higher than that at pH 3, with the other pH values showing intermediate Si solubilites Such strong pH dependency was also observed in other studies (Fraysse et al., 2009) According to Ehrlich et al (2010), the strong pH dependency is a result of increasing pH deprotonation of Si–OH groups resulting in a H-bonded H2O adsorption on the negatively charged Si–O– surface We further suppose that a negatively charged fivefold coordinated Si species is formed Consequently, Si–O bonds are weakened and Si release is facilitated at higher pH 40 Si in solution / mg L -1 pH: 6.5 30 6.0 20 4.5 10 3.0 0 Days Figure 3: pH dependency of Si release from ashed rice straw treated at 400°C determined in batch experiments at pH 3.0, 4.5, 6.0, and 6.5 in a time sequence up to d It can also be deduced from Fig that the time to reach close-to-equilibrium conditions in the suspensions is also depending on pH At pH 3.0 and 4.5, the steady state in Si concentration was reached after d, whereas in the supernatants at pH 6.0 and 6.5 marked increases of soluble Si were observed up to d Deduced from data on quartz (Dove and Elston, 1992) and bamboo phytoliths (Fraysse et al., 2006) Si re le as e Si in solution / mg L 12 10 -20 Electrolyte background: NaCl / mmolc L-1 -40 10 50 Zet ap ote ntia l -60 Zeta potential / mV -1 14 -80 10 pH Figure 4: Effect of ionic strengths on Si release and zeta potential of ashed rice straw treated at 400°C, determined by batch experiments at pH 2–10 in 0.01 and 0.05 mol L–1 NaCl solutions and 24 h reaction time  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim dissolution requires deprotonation of Si–OH groups and polymerization of Si–O–Si bonds before Si is released by nucleophilic attack by OH– groups For this reason, increasing numbers of OH– in solution at higher pH result in more extensive Si release In the experiments with an EB it was shown, that Si solubility, in comparison of experiments at the same pH, decreased for the higher EB introduced (Fig 4) This can clearly been observed from pH 4–10, whereas at pH and at very low Si solubility, no marked differences were obtained The increased solubility of phytoliths at higher pH, already shown for deionized H2O in Fig 3, can also clearly be deduced in the experiments with an EB of 10 and 50 mmolc L–1 At pH and 3, the amounts of Si in solution were low with ≈ mg L–1 after 24 h reaction time for both EBs, and with increasing pH, Si release increased from ≈ at pH to 14.2 mg L–1 at pH f was more negative for the EB at lower NaCl concentration, indicating a higher number of deprotonated silanol groups (≡Si–O–) in these samples An increase in pH from to led to progressive decrease of f from –2 to –76 and ≈ to –49 mV for the EB of 10 and 50 mmolc L–1, respectively The observations on the principal course of f referring to pH are in line with the results of Fraysse et al (2006) on phytoliths separated from bamboo where the OM was removed by combustion at 450°C for h No clear changes in f were found between suspension of pH and 10 At the EB of 50 mmolc L–1, higher values for f might indicate a more extended adsorption of positive charges onto deprotonated silanol groups of the silica surface (Fig 4) The formation of such siloxane groups is thought to be the rate-limiting step for the dissolution process (Bickmore et al., 2006) However, the observed differences in values of f can also be assigned to other factors The increased ionic strength at higher EB can shift f to higher values because of compression of the electrical double layer It is also probable that at high EB more Na+ was adsorbed on deprotonated silanol groups which would contribute to charge neutralization Differences in f between the two EB were most pronounced at high pH f of the dry-ashed rice-straw sample was close to zero at pH indicating that the point of zero charge (pzc) was near pH The progressional decrease of f with increasing pH indicated the presence of variably charged functional groups of inorganic and organic compounds in the burned ash At pH 2, these were almost completely protonated and the pzc was almost reached 4.2 Cation effects on the release of Si Increasing concentrations of monovalent and bivalent cations at pH resulted in some increase of negative f values of the dry-ashed rice straw sample; i.e., raising the concentrations of Ca2+, Mg2+, K+, and Na+ from to 20 mmolc L–1 led to a change of f from –26.7 mV in deionized water to –6.3 (Ca2+), –7.0 (Mg2+), –10.0 (K+), and –11.5 mV (Na+) (Fig 5) Again, it has to be considered that the shift of f to higher values with increasing concentrations cannot be attributed to increasing sorption of ions on the phytoliths only because of concentration-dependent effects on the thickness of the electrical douwww.plant-soil.com J Plant Nutr Soil Sci 2014, 177, 349–359 Effect of pretreatment and solution chemistry 355 ble layer and f Al3+ was most effective in increasing f resulting in charge reversal, whereas di- and monovalent cations showed similar behavior with a higher preference for divalent cations The strength of different cations appeared to be controlled first of all by the valency and secondly by ionic radius/ hydrated-ion size 30 Zeta potential / mV The Si-release pattern at presence of K+ was closest to that of Al3+, whereas the Si-release pattern at presence of Ca2+ was closer to that of deionized H2O Amounts of Si released in presence of Mg2+ and Na+ were similar, with Si concentrations of 19 mg L–1 being obtained after d Hence, the effect of cations on depressing Si release decreased in the order: Al3+ > K+ > Na+ ≥ Mg2+ > Ca2+ Na+ K+ Mg2+ Ca2+ Al3+ 20 10 supernatant after d was observed for deionized water This Si concentration was considerably smaller in the suspensions with added electrolytes, being most pronounced for Al3+, where after d the Si concentration in the supernatant was mg L–1 These batch experiments with different electrolytes confirm the trend of decreased solubility of phytoliths at higher EB, shown in Fig -10 -20 -30 10 20 15 -1 Cation concentration / mmolc L Figure 5: Change of zeta potential of ashed rice straw treated at 400°C due to the addition of the cations Na+, K+, Mg2+, Ca2+, and Al3+ in concentration range of 0–0.02 molc L–1, determined by batch experiments at pH 3.5 and a reaction time of 24 h In case for Al3+, the increase of f and charge reversal of the dry-ashed rice-straw sample treated at 400°C can clearly be assigned to adsorption of Al3+ (Fig 5) The pzc was reached at relatively low concentration of Al3+ of ≈ 0.4 mmolc L–1, and further addition of Al3+ resulted in a marked charge reversal At the highest Al3+ concentration applied (2.5 mmolc L–1) f was at +25 mV, which is almost the same magnitude of f in deionized water (–28 mV), indicating strong adsorption of Al3+ and exposure of positively charged sites of adsorbed Al3+ at the solid–solution interface Batch experiments at pH showed a marked effect of an electrolyte as well as the kind of cation in solution on the release of Si (Fig 6) Highest Si release of 35 mg Si L–1 in the While knowledge on the adsorption of K+ on silica has already been well established (Davies and Oberholster, 1988), not much is known about its relation to Si release The K+ ion is known to fit well with a hexagonal depression in the siloxane surface of silica (Grim, 1968) A preferential adsorption of K+ onto siloxane surface of phytoliths could explain a stronger effect in decelerating Si release over Na+, Mg2+, and Ca2+ The concentration of Al3+ in solution had a marked effect on the release of Si from dry-ashed rice-straw sample heated at 400°C (Fig 7) Batch experiments at pH revealed that at Al3+ concentrations of 0.1, 0.5, and 1.0 mmolc L–1, Si concentrations after d were 21.2, 19.0, and 7.3 mg L–1, respectively In comparison with deionized water, where a Si concentration of 35 mg L–1 was observed, clear indication was obtained that Al3+ acts as a prohibitor for phytolith dissolution, whereby the effect is enhanced by increasing Al3+ concentration Indication for a more extended adsorption of Al3+ with increasing concentration was obtained by f measurements (Fig 5) For all concentrations of Al3+ a strong increase of Si concentration within the first 72 h was obtained whereas after d Si in solution kept almost constant Also Wilding et al (1979) observed a strong reaction of Al3+ with phytoliths resulting in a reduced solubility Dixit and Van Cappellen (2002) reported 40 40 -1 H 2O 30 Ca2+ Mg 20 2+ Na + K+ 10 Al3+ Si in solution / mg L Si in solution / mg L -1 H2O 30 Al concentration / mmolc L-1: 0.1 20 0.5 10 1.0 0 Days Figure 6: Cation effects on Si release from ashed rice straw treated at 400°C in a time sequence up to d, determined by batch experiments at pH with ion concentrations of 0.01 molc L–1 for Na+, K+, Mg2+, and Ca2+, and 0.001 molc L–1 for Al3+  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Days Figure 7: Effect of Al3+ on Si release from ashed rice straw treated at 400°C, determined by batch experiments at pH as a function of time at Al3+ concentrations from to 0.001 molc L–1 www.plant-soil.com 356 Nguyen, Dultz, Guggenberger J Plant Nutr Soil Sci 2014, 177, 349–359 from investigations on silica frustules that Al3+ is structurally associated with silica, and that it is incorporated in the solid, being on fourfold coordination, and not surface-adsorbed Thus, Al3+ can prevent attacks by negatively charged electrolytes to Si centers of the tetrahedral units and thus decreases the solubility of silica We assume that Al3+ acts on phytoliths in a similar way, but the possibility that Al3+ is sorbed onto deprotonated Si–O– groups also needs to be considered effect of Al3+ in increasing f might be due to competition of H+ on the deprotonated Si–O– sites On the other hand, increases in f over the entire pH range from to showed that Al3+ was not only structurally associated with silica, but also adsorbed onto deprotonated Si–O– sites resulting in a less negative surface It is supposed that sorption of Al3+ on Si–O– sites can prohibit Si release Reduction of Si release from phytoliths by presence of Al3+ was most pronounced at pH at low Al3+ concentrations (Fig 8a) This effect was less pronounced when pH increased to and With an increase of Al3+ concentration from to 2.0 mmolc L–1, Si concentrations in the supernatant of the batch experiments at pH 3, 4, and decreased from 14.0 to 1.8, 10.9 to 0.8 and 2.5 to 0.7 mg L–1, respectively In the concentration range from 1–2 mmolc Al3+ L–1 no marked change of Si concentration in solution was obtained 4.3 Anion effects on the release of Si An increase in the Al3+ concentration from to 2.0 mmolc L–1 at pH and resulted in strong increases in f with charge reversal, whereas at pH only slightly positive f was reached at the highest Al3+ concentration introduced (Fig 8b) The pzc was reached at pH at lowest Al3+ concentration (0.3 mmolc L–1), whereas it is somewhat higher at pH (0.4 mmolc L–1) and markedly higher at pH (1.7 mmolc L–1) Indication was obtained that Al3+ was strongly adsorbed on the deprotonated ≡Si–O– sites at pH and At pH 3, a lower The fact that organic anions suppress the Si release more than inorganic ones might be related with their molecular size and reactivity of functional group According to Ehrlich et al (2010), organic anions attack the surface tetrahedral Si centers belonging to deprotonated silanol groups by using their –COO– groups in a similar way as OH– It is reasonable to assume that these carboxylate groups might not react as strongly as Cl– and SO2À A weaker effect of the Na-salt solutions in comparison with deionized water is probably due to the EB As discussed in section 4.2, the adsorption of monovalent cations such as Na+ and K+ onto deprotonated Si–O– groups of the phytoliths can prohibit the attack of water mole- 16 (a) 12 10 pH 4 pH Si in solution / mg L -1 14 0,5 1,0 The batch experiment carried out at pH and with anion concentrations of 10 mmolc L–1 revealed for all tested anions that Si concentration in the supernatant increased within the first 72 h and stayed almost constant in the time span from to d (Fig 9) After d, Si release in different aqueous solutions containing Cl–, SO2À , acetate, oxalate, and citrate was 19.0, 13.6, 6.1, 5.0, and 4.8 mg L–1, respectively, indicating that in presence of the two inorganic anions Si concentration was markedly higher than in the presence of the three organic ones Remarkably, Si in solution was highest in deionized water (34.7 mg L–1) despite the fact that anions are considered to act in a similar way as OH– ions (Ehrlich et al., 2010) It seems that anions and also cations (Fig and 6) suppress Si release from phytoliths obtained from dry-ashed rice-straw sample treated at 400°C It can be assumed that Si release from such samples under field conditions is favored when the content of soluble ions in soil solution is low pH 0,0 40 2,0 H 2O Si in solution / mg L pH 40 -1 Al3+ concentration (mmol L-1) pH Zeta potential / mV 60 1,5 20 pH -20 30 Cl 20 - SO42- 10 Oxalate (b) Acetate, Citrate -40 0,0 0,5 3+ Al 1,0 1,5 2,0 -1 concentration / mmolc L Figure 8: pH dependency of Al3+ effects on zeta potential (a) and Si release (b) from ashed rice straw treated at 400°C, determined by batch experiments for 24 h at pH 3, 4, and 5, and Al3+ concentrations of 0–0.002 molc L–1  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Days Figure 9: Anion effects on Si release from ashed rice straw treated at 400°C in time sequences up to d, determined by batch experiments at pH in 0.01 molc L–1 solutions of Na salts with Cl–, SO2À , acetate, citrate, and oxalate www.plant-soil.com J Plant Nutr Soil Sci 2014, 177, 349–359 Effect of pretreatment and solution chemistry 357 and Sullivan (2005) reported that OM strengthens the phytolith surface and its resistance against dissolution -20 Acetate Oxalate Citrate -40 -60 -80 10 pH Figure 10: Effect of pH on the change in zeta potential of ashed rice straw treated at 400°C in presence of the anions Cl–, SO2À , acetate, citrate, and oxalate (Na salts), determined by batch experiments with a reaction time of 24 h in 0.01 molc L–1 solutions at a pH range of 2–10 cules on Si–O–Si bonds in the siloxane surface and depress Si release The presence of different anions at a concentration of 0.01 mmolc L–1 showed only a minor effect in lowering f in the pH range of 2–5, whereas at higher pH different effects of anions on f were observed (Fig 10) Because of a dominance of H+ (H+ activity at pH is 100 times than that at pH 4) not much difference in f was detected at pH < In the pH range from to 10, lower f in presence of Cl– might reflect some stronger binding of this monovalent inorganic cation on phytoliths in comparison with divalent SO2À Si concentration in solution was higher at presence of Cl– than of SO2À (Fig 9) This could be explained by a more effective attack of Cl– to siloxane surfaces which facilitates Si release, but more clarification is needed for understanding differences in the affinity of these anions to the surface of phytoliths For the organic anions, the effect on f was similar and decreased in the order: citrate > oxalate > acetate No marked differences of the three organic anions on Si release were obtained It can be concluded that an increasing number of –COO– groups does not have a marked effect on dissolution efficiency of the phytoliths This conclusion is in accordance with observations reported by Ehrlich et al (2010) 4.4 Effect of pretreatment on Si release Silicon release in suspensions with deionized water at pH increased with time for rice-straw samples heated at 400°C and the 400°C-treated sample combined with subsequent treatment by H2O2 (Fig 11a) In contrast, the sample obtained by wet ashing with H2O2 only showed a low solubility throughout the experiment After d, Si release from samples was 34.7 (400°C), 17.6 (400°C, H2O2-treated), and 2.6 mg L–1 (wet ashing) Release of only small amounts of Si from the H2O2-treated rice-straw sample is in accordance with findings from other studies on Si release from unburned phytoliths For instance, Van Cappellen et al (2002) and Parr  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Comparison between the two heat treatments revealed that the sample with higher organic-C content showed a lower resistance to dissolution However, the role of OM in accelerating dissolution can still not be affirmed because the “easily dissolvable fraction of Si” of the 400°C- and H2O2-treated sample might have already been removed during wet ashing with H2O2 The sample treated by wet ashing with H2O2 only showed a high resistance against dissolution in comparison with samples treated by heating For this sample, the increase of f with time might be due to the adsorption of cations from solution onto deprotonated Si–O– sites on the external surface Despite the fact that the experiments were performed in deionized water the presence of cations in solution can be assumed as relatively high amounts of alkaline and earth alkaline cations, especially K+ were found in dry-ashed ricestraw samples (Table 1) It can be inferred that heat treatments resulted in robust destruction of the rice straw and produced a structure with low resistance to dissolution In contrast, the sample treated by wet ashing with H2O2 showed a more resistant structure, which protected the phytoliths and 40 o Treatment: 400 C (a) Si in solution / mg L-1 ClSO42- 30 20 o 400 10 Ca O2 nd H H2O2 -10 7 Treatment: 400oC and H2O2 (b) Zeta potential / mV Zeta potential / mV -20 o 400 C -30 -40 H 2O -50 -60 Days -1 C-content / g kg (ii) dry ashing at 400oC: 95 (v) wet ashing with H2O2: 374 (vi) dry ashing at 400oC/wet ashing with H2O2: Figure 11: Effect of pretreatment of rice straw on zeta potential (a) and Si-release kinetics at pH in deionized water Dry-ashed ricestraw samples treated at 400°C (ii), 400°C treatment with subsequent wet-ashing with H2O2 (v), and wet-ashing treatment (vi) www.plant-soil.com 358 Nguyen, Dultz, Guggenberger in addition occluded C within from chemical attacks In agreement with Parr and Sullivan (2005) this implies that mixing rice straw into the soil by tillage instead of burning it leads to a long-term stabilization of rice-straw phytoliths under actual soil conditions However, mixing masses of undecomposed straw into the soil would generate a very low redox potential Determination of the changes of f with time for the differently pretreated samples in deionized water revealed that within d f kept almost constant (20–24 mV) for both heat-treated samples, whereas an increase in f from –57 to –32 mV for the chemically oxidized sample was observed, indicating loss of negatively charged sites with time (Fig 11b) The dry-ashed sample combined with subsequent H2O2 treatment showed slightly higher f than the sample with single treatment (dry or wet ashing), which is probably due to strong losses of OM matter by the H2O2 treatment (Table 1) It has to be considered that charred rice straw in the ash sample might contribute to f In order to gain more insights on the effect of charred rice straw on f, surface charge of these two samples was quantified at pH by polyelectrolyte titration The results on surface charge, –12.6 mmolc kg–1 for the sample with combined treatment versus –15.5 mmolc kg–1 for the sample with dry ashing only confirm the trend observed by f measurements At all, the contribution of charred rice straw to the charge on the external surfaces appears relatively low We assume that the observed increase of negative surface charge was the result of OM removal with H2O2 Organic matter present in the sample contributes to the total negative charge of the samples Its contribution to the total net surface charge of the dry-ashed sample was calculated to be –1.47 whereas it was –0.06 mmolc kg–1 for the burned and chemically oxidized sample Here, adsorption of cations from solution onto negative surface sites of the OM may decrease neutralization of deprotonated sites of phytoliths and as a consequence, Si release is affected to a lower extent by the presence of cations Conclusions Batch experiments combined with analysis of f for getting information about the underlying process showed the importance of pretreatment of rice straw and solution chemistry on Si release In ashed samples, soluble Si was found to be up to 46% of total Si content in ashes, indicating that burning rice straw can be an important measure to make Si available for Si-accumulating crops such as rice in short term In contrast, fresh rice straw treated by H2O2 only, is highly resistant against dissolution, indicating that phytoliths can be stable on the long term in the soil when rice straw is directly mixed into the soil on site without burning Based on f measurements, we infer that cations are sorbed on deprotonated Si–O– sites and mitigate water attack on Si–O–Si bonds This leads to a decline in the dissolution rate of phytoliths at presence of cations Especially Al3+ showed a marked effect to decrease Si release Different effects of inorganic and organic anions on the dissolution of phytoliths were observed, with the latter impeding Si release more than the former Based on the relatively strong effect of organic anions in suppressing Si release, it is suggested that the presence of dissolved OM in  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim J Plant Nutr Soil Sci 2014, 177, 349–359 pore solution of paddy soils might counteract phytolith dissolution In the conducted experiments, there are no other sorption sites which may compete for the added cations or anions which is very different from applying rice straw or ash to soil Here, experiments with synthetic soil solutions and also pot experiments including analysis of Si content in dry matter of rice plants are thought to provide more valuable insights in Si management of paddy soils Our study showed that burning of rice straw can be an important measure to make Si available The practical problem is, however, that actually in all countries of SE Asia there are governmental restrictions for burning of straw and, hence, techniques for dry ashing in a more environmentally friendly way are needed Here, the use of commercial-scale systems for the production of biochar, centralized and mobile systems with temperature control being possible, should be considered Acknowledgments This research was funded by the Vietnam National Foundation for Science & Technology Development (Project 105.092010.03) An extended part of the research was supported by the German Academic Exchange Service (DAAD); grant A/ 11/00930 X-ray-tomographic microscopy was performed with skilful help by Julie Fife at the TOMCAT beamline of the synchrotron facility of the Paul Scherrer Institute, Villigen, Switzerland Great help of Sarah B Cichy and Karl-Ingo Friese for morphological characterization of phytoliths from the tomographic dataset is acknowledged We are grateful to two anonymous reviewers for constructive comments on the manuscript References Babrauskas, V (2003): Ignition Handbook Society of Fire Protection Engineers, Boston, USA, p 1116 Bickmore, B R., Nagy, K L., Gray, A K., Brinkerhoff, A R (2006): The effect of Al(OH)À on the dissolution rate of quartz Geochim Cosmochim Acta 70, 290–305 Bogdan, K., Schenk, M K (2008): Arsenic in rice (Oryza sativa L.) related to dynamics of arsenic and silicic acid in paddy soils Environm Sci Technol 42, 7885–7890 Brady, P V., Walther, J V (1990): Kinetics of quartz dissolution at low temperatures Chem Geol 82, 253–264 Chen, W., Yao, X., Cai, K., Chen, J (2011): Silicon alleviates drought stress of rice plants by improving plant water status, photosynthesis and mineral nutrient absorption Biol Trace Elem Res 142, 67–76 Davies, G., Oberholster, R E (1988): Alkali-silica reaction products and their development Cem Concr Res 18, 621–635 Dixit, S., Van Cappellen, P (2002): Surface chemistry and reactivity of biogenic silica Geochim Cosmochim Acta 66, 2559–2568 Dobermann, A., Witt, C (2000): The potential impact of crop intensification on carbon and nitrogen cycling in intensive rice systems, in Kirk, G J D., Olk, D C (eds.): Carbon and Nitrogen Dynamics in Flooded Soils IRRI, p 1–25 Dove, P M (1994): The dissolution kinetics of quartz in sodium chloride solutions at 25 degrees to 300 degrees C Am J Sci 294, 665–712 www.plant-soil.com J Plant Nutr Soil Sci 2014, 177, 349–359 Dove, P M., Crerar, D A (1990): Kinetics of quartz dissolution in electrolyte solutions using a hydrothermal mixed flow reactor Geochim Cosmochim Acta 54, 955–969 Dove, P M., Elston, S F (1992): Dissolution kinetics of quartz in sodium-chloride solutions–analysis of existing data and a rate model for 25-Degrees-C Geochim Cosmochim Acta 56, 4147–4156 Ehrlich, H., Demadis, K D., Pokrovsky, O S., Koutsoukos, P G (2010): Modern views on desilicification: Biosilica and abiotic silica dissolution in natural and artificial environments Chem Rev 110, 4656–4689 Fleck, A T., Nye, T., Repenning, C., Stahl, F., Zahn, M., Schenk, M K (2011): Silicon enhances suberization and lignifications in roots of rice (Oryza sativa) J Exp Bot 6, 1–11, DOI: 10.1093/jxb/ erq392 Fraysse, F., Pokrovsky, O S., Schott, J., Meunier, J D (2006): Surface properties, solubility and dissolution kinetics of bamboo phytoliths Geochim Cosmochim Acta 70, 1939–1951 Fraysse, F., Pokrovsky, O S., Schott, J., Meunier, J D (2009): Surface chemistry and reactivity of plant phytoliths in aqueous solutions Chem Geol 258, 197–206 Friese, K I., Cichy, S B., Wolters, F E., Botcharnikov, R E (2013): Analysis of tomographic mineralogical data using YaDiV–Overview and practical case study Comp Geosci 56, 92–103 Grim, R E (1968): Clay Mineralogy McGraw-Hill Book Company, New York Gun’ko, V M., Mironyuk, I F., Zarko, V I., Voronin, E F., Turov, V V., Pakhlov, E M., Goncharuk, E V., Nychiporuk, Y M., Vlasova, N N., Gorbik, P P (2005): Morphology and surface properties of fumed silicas J Colloid Interf Sci 289, 427–445 Guntzer, F., Keller, C., Meunier, J D (2010): Determination of the silicon concentration in plant material using Tiron extraction New Phytol 188, 902–906 Kato, N., Owa, N (1997): Evaluation of Si availability in slag fertilizers by an extraction method using a cation exchange resin J Soil Sci Plant Nutr 43, 351–359 Kordatos, K., Gavela, S., Ntziouni, A., Pistiolas, K N., Kyritsi, A (2008): Synthesis of highly siliceous ZSM-5 zeolite using silica from rice husk ash Microp Mesop Mat 115, 189–196 Lai, C H., Chen, K S., Wang, H K (2009): Influence of rice straw burning on the levels of polycyclic aromatic hydrocarbons in agricultural county of Taiwan J Environm Sci 21, 1200–1207 Loucaides, S., Van Cappellen, P., Behrends, T (2008): Dissolution of biogenic silica from land to ocean: The role of salinity and pH Limnol Oceanogr 53, 1614–1621  2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Effect of pretreatment and solution chemistry 359 Ma, J F (2004): Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses Soil Sci Plant Nutr 50, 11–18 Marschner, H (1995): Mineral Nutrition of Higher Plants 2nd edn., Academic Press, London Meharg, A A., Rahman, M D M (2003): Arsenic contamination of Bangladesh paddy firld soils: Implications for rice contribution to arsenic consumption Environ Sci Technol 37, 229–234 Mortlock, R A., Froelich, P N (1989): A simple method for the rapid determination of biogenic opal in pelagic marine sediments DeepSea Res 36, 1415–1426 Nguyen, N M., Dultz, S., Kasbohm, J., Le, D (2009) Clay dispersion and its relation to surface charge in a paddy soil of the Red River Delta, Vietnam J Plant Nutr Soil Sci 172, 477–486 Parr, J F., Sullivan, L A (2005): Soil carbon sequestration in phytoliths Soil Biol Biochem 37, 117–124 Parr, J F., Lentfer, C J., Boyd, W E (2001): A comparative analysis of wet and dry ashing techniques for the extraction of phytoliths from plant material J Archaeological Sci 28, 875–886 Saccone, L., Conley, D J., Koning, E., Sauer, D., Sommer, M., Kaczorek, D., Blecker, S W., Kelly, E F (2007): Assessing the extraction and quantification of amorphous silica in soils of forest and grassland ecosystems Eur J Soil Sci 58, 1446–1459 Sauer, D., Saccone, L., Conley, D J., Herrmann, L., Sommer, M (2006): Review of methodologies for extracting plant-available and amorphous Si from soils and aquatic sediments Biogeochem 80, 89–108 Sommer, M., Kaczorek, D., Kuzyakov, Y., Breuer, J (2006): Silica pools and fluxes in soils–a review J Plant Nutr Soil Sci 169, 310–329 Van Bennekom, A J., Buma, A G J., Nolting, R F (1991): Dissolved aluminium in the Weddell-Scotia Confluence and effect of Al on the dissolution kinetics of biogenic silica Mar Chem 35, 423–434 Van Cappellen, P., Dixit, S., Gallinari, M (2002): Biogenic silica dissolution and the marine Si cycle: kinetics, surface chemistry and preservation Oceanis 28, 417–454 Weres, O., Yee, A., Tsao, L (1981): Kinetics of silica polymerization J Colloid Interface Sci 84, 379–402 Wickramasinghe, D B., Rowell, D L (2006): The release of silicon from amorphous silica and rice straw in Sri Lankan soils Biol Fertil Soils 42, 231–240 Wilding, L P., Hallmark, C T., Smeck, N E (1979): Dissolution and stability of biogenic opal Soil Sci Soc Am Proc 43, 800–802 www.plant-soil.com ... dissolution in presence of unburned OM In this study, the effect of different parameters of solution chemistry, including pH, ionic strength, valency, and size of cations and anions on the solubility. .. Determination of the effect of pH on solubility of Si and f of phytoliths To identify the effect of pH on Si solubility, the solution was adjusted to pH 3.0, 4.5, 6.0, and 6.5 with 0.1 M HCl The dissolution... desilification (Dove and Elston, 1992; Fraysse et al., 2006) On the other hand, adsorption of cations from aqueous solution onto deprotonated ≡Si–O– sites might occur and accelerate polymerization (Weres