Straw phytolith for less hazardous open burning of paddy straw

Rice production helps feed at least half of the world’s population but generates approximately one billion tonnes of straw residue per annum. On-site open burning of rice straw after harvesting is common in recent times because there has been less demand for rice straw to use as fuel and fodder. Due to health and climate change concerns, open burning, which results in biomass losses, smog and emissions of green house gases, e.g., CO2, has been widely criticized and banned in many countries. Little is known about the positive benefits of straw burning, such as field care (eradication of biotic diseases) or nutrient cycling. Herein, we propose a new viewpoint in which the burning of rice straw followed by cycling of the burned materials, including silica material (so-called phytolith), into soil is demonstrated as a CO2-sequestration strategy via buffering the soil CO2 flux and coupling CO2 with the silicon cycle.

and eventually occlusion of OC in its structure. As a result, OC is involved in the fate of phytoliths. When plant residues are cycled into soil, phytolith-occluded OC (phytOC) can also be cycled and accumulated in soil. This phytOC is preserved by the phytolith structure and is secure from biodegradation.
To date, the occlusion of OC within the phytolith structure has been examined in many studies 7,8,15 . A recent high-end technique, i.e., X-ray microtomography, allows visualization inside the tissue organs of straw and provides a better understanding of how the silica phytolith skeleton can protect the OC occluded inside its structure (Fig. 1C). The association of phytolith silica and occluded OC can be physically and chemically strengthened. Condensation of phytolith silica via precipitation of Si from transport sap is a process that can consolidate the physical association between the phytolith and cuticle at the outer tangential epidermis cell wall 16 . Chemical bonds such as Si−C and Si−OR form on the tissue cell walls as a result of the priming effects of the chemical association followed by dehydration of silica and organic carbon 17 . This physico-chemical reaction chain strengthens the occlusion of OC within the phytolith structure. thermal resistance of phytolith-associated organic carbon. In rice paddy systems, open burning rather than incorporation is the preferential practice, and this process can alter both phytoliths and their occluded OC 18,19 . Phytoliths will be reshaped because silica is subject to condensation to form more stable phases, e.g., cristobalite or tridymite 20 , whereas OC will be transformed eventually into carbon gases, e.g., CO 2 , CO, C 1 −C 2 hydrocarbons and other volatile substances 21 . Figure 2A clearly shows the weight loss along with the decrease in the OC content. While open burning at approximately 420 °C resulted in a loss of half the OC, temperatures at >400 °C (in controlled pyrolysis experiments) led to more extensive loss of OC. The outer OC (outside the phytolith structure), which is directly subjected to fire or high temperatures, will be removed first. The inner OC (here phytOC) might be decomposed at lower rates, and the extent to which phytOC is lost by thermal decomposition is likely related to the strength of its association with the phytolith structure. If it is protected by the phytolith structure, where limited oxygen can penetrate, phytOC is believed to be more recalcitrant to thermal decomposition, and consequently black carbon can be formed to a given extent. The dark parts inside straw ash from open burning (easily observable by naked eyes) might be related to the black carbon derived from the internal anaerobic conditions of the ash phytolith structure. In ash phytoliths, black carbon is commonly observed in association with the silica phytolith structure (Fig. 2B,C). The black carbon or internal OC part is protected by the associating phytolith and stabilised by forming Si−C and Si−OR bonds with phytolith silica (as deduced from XPS analyses in Fig. 2D).
Straw-derived phytoliths are involved in buffering soil CO 2 fluxes. Phytolith makes up from 1.6 to 14.4% of rice (stem, sheath, and leaf) 9,22 , thereby hundreds of million tonnes of phytolith are recycled annually if all world rice residues are returned to the soil instead of exported or used for other purposes. Phytoliths, including fresh phytolith (from straw residue incorporated into soil), ash phytolith (obtained from straw burning) and aged phytoliths (both fresh and ash phytoliths buried in soil), might co-exist in rice field. Because CO 2 fluxes (as high as 8.62 µmol m −2 s −1 23 ) originate from the aerobic decomposition of soil OC (e.g., in root zone) and can diffuse extensively over a rice field, it is likely that CO 2 fluxes penetrate water and contact phytoliths, by which they are then transformed in different ways, as schematically described in Fig. 3.
The solubility of CO 2 is highly pH dependent, and the soluble forms of CO 2 include H 2 CO 3 , HCO 3 − and CO 3 2− . Therefore, the straw cycle and its derived phytoliths in the soil can change the soil pH and drive the CO 2 buffer capacity. Incorporation of straw into soil has been encouraged 24 , but it creates a more acidic environment because straw serves as organic matter for decomposition to form organic acids 25 . Hence, this practice, as a result, decreases the CO 2 buffer capacity and favours CO 2 emission (reaction #1). In contrast, open burning removes organic matter (transformed to gases) and produces highly alkaline materials with typical pH values ranging from 10 to 11 13 . Burning and returning burned products (ashes) can generally increase the soil pH, leading to higher solubility of CO 2 . Ashes contain not only immediately available alkali ions (Supplementary Table S1) but also phytolith-occluded alkali ions 13 . Considering these outer and inner alkali ion sources, the effect of burned materials in buffering the soil CO 2 flux can be divided into two stages involving the direct release of outer alkali ions and the "slow release" of phytolith-occluded alkali ions (reaction #2). In an experiment to simulate the release of potassium (K) accompanying the dissolution of phytoliths, Nguyen, et al. 20 demonstrated a mechanism in which the K located in the phytolith structure is slowly released when the phytoliths are destroyed. This result implies that ash phytoliths can increase the soil pH and buffer CO 2 flux via releasing and providing their internal alkali elements. Dissolution of silicate minerals has been known as a natural process that captures CO 2 6 . This CO 2 capture can be magnified for fast-reacting silicate rocks (e.g., basalt) 5,26 . Likewise, phytolith silica, which is a highly soluble material 13 , is also expected to react in the same way to mitigate CO 2 emission. Because the dissolution rate of phytoliths is controlled by various physico-chemical properties of soil solutions, e.g., pH and ionic strength 13 , the fate of phytoliths and their role in mitigating CO 2 emissions are, therefore, expected to vary from one soil/region to another.
The second CO 2 sequestration mechanism (reaction #3) relates to electro-chemical interactions between CO 2 and phytolith surfaces (Fig. 4A). Our parallel system to measure CO 2 adsorption and the electro-kinetics of phytolith suspensions (Fig. 4B) revealed the ability of phytoliths to capture CO 2 (Fig. 4C) (Supplementary Table S2) and the electro-chemical process that governs CO 2 sequestration (Fig. 4D). In a suspension, the phytolith surface develops a net negative charge primarily due to deprotonation of the surface Si−OH groups, and this process steadily increases in the pH range from 2 to 10 or even higher 13,27 . Reversibly, protonation of the negative phytolith surface might occur, and this process is likely a highly rapid, exothermic reaction with no significant  www.nature.com/scientificreports www.nature.com/scientificreports/ activation energy that occurs via the concerted motion of the protons along a chain of water molecules 27 . Because H 2 CO 3 acid has a pK a1 of 6.4, we can expect phytoliths with their alkaline nature to facilitate the deprotonation of H 2 CO 3 and attract free H + ions for transfer to their surface, by which phytoliths facilitate CO 2 conversion into HCO 3 − or even CO 3 2− and prevent reversion of CO 3 2− or HCO 3 − back to CO 2 . In our short-run experiments, the protonation due to the increasing presence of CO 2 in suspensions resulted in gradual decreases in the pH and increases in zeta potential (ζ) (Fig. 4D) (Supplementary Table S3). Differences in the variation ranges of pH and ζ between the open burning and controlled heating samples were likely because of their charge densities 28 . The CO 2 amounts that reacted with the phytolith suspensions were calculated to be ~30g CO 2 kg −1 for the open burning sample and from ~15 to 40g CO 2 kg −1 for the controlled heating samples.

Discussion
The findings on the positive aspects of on-site burning with the central role of phytoliths suggest a need for multi-dimensional views to mediate the debates and encourage new strategies. By bringing CO 2 emission from on-site straw burning and CO 2 sequestration in soils driven by straw-derived phytoliths together, a more detailed picture of the gains and losses from open burning was revealed. Straw open burning results in direct CO 2 emission; however, the burned products remaining in a field can be involved in processes that enable CO 2 sequestration via conversion of CO 2 into HCO 3 − or CO 3 2− in the presence of phytoliths. This conversion might be related to various processes, such as the dissolution of straw ash phytoliths to release alkali elements, the reaction of released alkali elements and dissolved CO 2 to form HCO 3 − , and protonation of the negatively charged surface of phytoliths that can convert H 2 CO 3 into HCO 3 − . All these processes occur concurrently and interactively; however, our study still cannot determine the order, rate and preference of each individual process.
In addition, it was evidenced that the phytolith skeleton can also protect its occluded OC by favouring an "internal anaerobic" condition, which prohibits oxidation of OC and reduces CO 2 emission from open burning. This result suggests that developments in burning techniques considering electro-chemical changes can both preserve straw OC and produce materials to enhance soil CO 2 sequestration. Overall, as straw management might involve billion tonnes of CO 2 annually released or sequestered worldwide, it is urgent to re-evaluate straw burning on the field scale at which elemental cycles can be entirely closed and other benefits from burning rice straw can also be assessed. More extensive and intensive discussion is necessary. Biomass loss and air pollution are the major reasons for exporting straw, and not returning straw's occluded nutrients to a field need to be re-considered. Straw ash phytoliths should be given a chance to demonstrate their potential to mitigate soil CO 2 emissions.
Our short-run experiments indicated possible reactions that sequester CO 2 in phytolith suspensions; however, these experiments cannot simulate or generalize the effect of phytoliths on CO 2 emissions on field, regional or larger scales, where many challenges will be raised. The heterogeneous dissolution of phytoliths due to their complicated chemical composition is one of the most difficult obstacles, and many attempts are still being made to elucidate the ageing of phytoliths and the subsequent changes in their surface properties. Under field conditions, soil biological processes, e.g., decomposition, mineralization, and respiration, which may vary from one region to another, will likely alter phytoliths and their surface reactions to sequester CO 2 at different levels. Therefore, www.nature.com/scientificreports www.nature.com/scientificreports/ the inclusion of such individual experiments from various ecoregions or biomes will allow the creation of larger and more realistic databases for the calculation of CO 2 sequestration. Last, many current cultivation initiatives regarding fertilization, irrigation, puddling, and weeding have been strategically moved towards decreasing CH 4 emissions or limiting arsenic accumulation in rice, and from now, in addition to CH 4 and arsenic issues, it is also worth incentivizing methods that can activate phytoliths to sequester more CO 2 . If all these challenges are solved, adjustment of the contributions or effects of straw phytoliths to the global CO 2 cycle can be made.

Materials and Methods
Sample preparation. Straw samples were collected at harvest time in a central part of the Red River Delta, Vietnam. Samples were air dried, oven dried at 70 °C, and then finely chopped into ~1 mm segments prior to analysis. Open burning of each 500 g of straw was performed in open-air conditions, and it took approximately 20 min for the straw samples to be burned completely. The burning temperatures, which were measured by directly injecting a thermo probe (330-1 LL, Testo) into the straw piles, were averaged to be ca. 420 °C. Because burning is exothermic depending upon the ambient air condition (e.g., wind velocity, humidity) and the manner of burning (scatter or pile up), we also treated straw samples in controlled pyrolysis conditions to evaluate the chemical composition and surface properties of the obtained straw phytoliths over a wider temperature range. The pyrolysis process was set up at a heating rate of 10 °C min controlled heating experiments was examined by the PIXE (particle induced X-ray emission) method, using the proton beam of the Tandem accelerator (5SDH-2 Pelletron accelerator system, manufactured by National Electrostatics Corporation, USA) (Supplementary Table S4). The OC content in the phytoliths was determined using the classical wet digestion method with concentrated H 2 SO 4 and 133 mol L −1 K 2 Cr 2 O 7 at 170-180 °C 29 . Thermogravimetric analysis (TGA) was conducted for straw samples using a Macro-TGA device designed by CIRAD. Specific surface area (SSA) was measured using a Gemini VII 2390p surface area analyser. Soluble ions dissolved from open-burning or controlled heating ashes were analysed by using inductively coupled plasma mass spectrometry (ICP-MS 7900, Agilent Technologies). The accuracy was assessed using the European Commission's standard reference materials BCR-277R. Recovery rates of the elements ranged from 90% to 110% (Supplementary Table S1).
The straw sample was scanned by ZEISS Xradia 520 Versa 3D X-ray microscope at the University of Kassel. The 3D X-ray microscope enables visualization of the silica phytolith structure and organic matter phase in rice leaves. The 3D phytolith and organic matter structures were assembled from 975 individual 2D image slices with 995 × 972 isotropic pixels (the pixel size was 1.14 µm) using the open-source software YaDiV 30 . The 256 grey values depict element densities: air has grey values < 50), the OC phase coloured in green has grey values from 50 to 200, and the high-density phase, i.e., phytolith, coloured in violet has grey values of approximately >200. X-ray photoelectron spectroscopy (XPS) is a particular surface-sensitive analysis technique, and the XPS-based data can be expected to visualize the integration between organic matter and inorganic phases (e.g., phytolith). The open-burning ash sample was analysed by XPS using an AXIS-NOVA spectrometer (SHIMADZU/KRATOS, Japan) with monochromatic AlKa radiation (15 kV 10 mA). electrokinetics of phytoliths and co 2 sequestration. The parallel and connected systems assembled to monitor the electrokinetics of phytoliths and CO 2 sequestration are illustrated in Fig. 4B. CO 2 gas was supplied at the rate of 0.1 L min −1 into two parallel reactors for (1) monitoring ζ changes and (2) measuring reacted CO 2 (adsorbed) at the same time. In the first block, ash phytolith suspensions (50 mg in 20 mL deionized water) were transferred into a reaction Teflon cup in the PCD-based system with a particle charge detector (PCD 05, Muetek), and the supply of CO 2 was halted when the pH stopped decreasing. Changes in pH and ζ were recorded every 10 s, and the experiments were terminated when the pH stopped decreasing. In the second block, CO 2 was consumed (adsorbed) in 20 mL of a 0.005 N NaOH solution, which represents CO 2 reacting with phytolith suspensions (in the first block), and titrated with 0.005 N H 2 SO 4 solution. All measurements were conducted in triplicate. NaOH and H 2 SO 4 were purchased from Merck, while CO 2 (99.99%) was provided by Ha Bac Nitrogenous Fertilizer and Chemical Ltd. Company.