Black carbon yields highest nutrient and lowest arsenic release when using rice residuals in paddy soils

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Fire-induced geochemical changes in soil: Implication for the element cycling

Soils play an essential role in supporting and sustaining life on this planet. In fire-impacted environments, fire causes considerable changes to the soil, especially in the various elements. The present work provides a comprehensive and up-to-date review of the effect of fire on soil geochemistry, and its impact on the cycling of different biogenic, major, minor, and trace elements in the soil. Results from both natural and experimental fires (field-scale and lab-scale) are considered in this review. The temperature at which mineral transformation occurs in the soil during fires is summarised. The review suggests that fires can significantly alter mobility and hence, the cycling of many elements in fire-affected regions. Change in speciation of elements following fires risks formation and/or increased availability of the toxic forms of elements in the soil. The unique physical, chemical, and biological conditions observed during fires make many unlikely reactions more likely. However, the information available in the literature is often fire, vegetation, and element specific. More studies on this topic by changing these three variables will improve our understanding of changes in the soil caused by fire. Hence, with fires being touted to increase global presence in the coming years, more studies on understanding their effects on soils are recommended.

Silicon as a potential limiting factor for phosphorus availability in paddy soils

Rice cultivation requires high amounts of phosphorus (P). However, significant amounts of P fertilizer additions may be retained by iron (Fe) oxides and are thus unavailable for plants. At the same time, rice cultivation has a high demand for silicic acid (Si), reducing Si availability after short duration of rice cultivation. By studying a paddy chronosequence with rice cultivation up to 2000 years, we show that Si limitation, observed as early as a few decades of rice cultivation, is limiting P availability along the paddy soils chronosequence. Using near edge X-ray absorption fine structure spectroscopy (NEXAFS) in a scanning transmission (soft) X-ray microscope (STXM) we show release of available P was linked to a Si-induced change in speciation of Fe-phases in soil particles and competition of Si with P for binding sites. Hence, low Si availability is limiting P availability in paddy soils. We propose that proper management of Si availability is a promising tool to improve the P supply of paddy plants.

Thermal induced changes of rice straw phytolith in relation to arsenic release: A perspective of rice straw arsenic under open burning

On-site open burning is a common practice for handling rice straw, but its negative impacts, e.g., biomass loss and air pollution, are largely debated worldwide. To address the negative effects of open burning, many efforts have been made to 'ignite' worldwide bans. However, these bans are likely based on a singular view in which some positive aspects of open burning are overlooked. In this study, we aimed to determine the thermal-induced changes of straw and straw arsenic (As) under open burning and heat-treatments (in the temperature range from 300 to 900 °C). It was found that silica phase in rice straw (so-called phytolith) can encapsulate As in its structure. Open burning or heat-treatment of straw resulted in a tighter association of As and phytolith, thereby reducing dissolution of As. We proposed an opinion that open burning causes air pollution, but it can increase the activity of phytolith in sequestrating As, enabling delayed As cycle in rice ecosystems. The combat of on-site open burning of rice straw to reduce air pollution will alter straw handling routines, thereby changing the cycle of straw phytolith and the route of straw As.

Contrasting effects of rice husk pyrolysis temperature on silicon dissolution and retention of cadmium (Cd) and dimethylarsinic acid (DMA)

Arsenic (As) and cadmium (Cd) are two toxins that affect rice, and their ability to do so may be lessened by soil incorporation of rice husk residues. Rice husks are typically removed from fields and used as a fuel source at rice mills but contain silicon (Si) and other nutrients. It has previously been shown that soil incorporation of rice husk or charred husk can release Si to soil solution to decrease As uptake and promote As methylation, and studies suggest char can additionally decrease Cd availability through several potential mechanisms including adsorption, precipitation, liming, and growth dilution. Charring conditions will impact husk Si dissolution rate and potential to immobilize Cd and possibly methylated As. Here, we compared uncharred husk to husk biochars pyrolyzed at 450, 600, 750, and 900 °C for differences in Si dissolution rate and adsorption of Cd and dimethylarsinic acid (DMA)-the dominant methylated As species present in paddy soils and grain. We hypothesized that Si dissolution rate and Cd adsorption would decrease, and DMA adsorption would increase with pyrolysis temperature. Si release decreased with pyrolysis temperature in the general order: uncharred husk > 450 °C > 600 °C = 750 °C = 950 °C but those differences were not due to SiO₂ crystallization with increasing temperature. Additionally, short (< 5 d) lab-based extractions underestimate Si release from uncharred husk while overestimating release from biochars. Controlling for pH changes/liming effect, adsorption isotherms showed very weak DMA adsorption, while Cd adsorption was favored on higher temperature (950 °C) biochar and was not predicted well by cation exchange capacity (CEC). When applied in a soil incubation study using non-contaminated soil, the biochar had no impact on Cd porewater concentrations while low temperature (450 °C) rice husk biochar led to the highest Si:As ratio. Biochar did not strongly influence Cd and DMA solubility at 1% w/w amendment.

Efficacy of agricultural waste derived biochar for arsenic removal: Tackling water quality in the Indo-Gangetic plain

Arsenic (As), a geogenic and extremely toxic metalloid can jeopardize terrestrial and aquatic ecosystems through environmental partitioning in natural soil-water compartment, geothermal and marine environments. Although, many researchers have investigated the decontamination potential of different mesoporous engineered bio sorbents for a suite of contaminants, still the removal efficiency of various pyrolyzed agricultural residues needs special attention. In the present study, rice straw derived biochar (RSBC) produced from slow pyrolysis process at 600 °C was used to remove As (V) from aqueous medium. Batch experiments were conducted at room temperature (25 ± 2 °C) under different initial concentrations (10, 30, 50, 100 μg L^-1), adsorbent dosages (0.5-5 μg L^-1), pH (4.0-10.0) and contact times (0-180 min). The adsorption equilibrium was established in 120 min. Adsorption process mainly followed pseudo-second order kinetics (R² ≥ 0.96) and Langmuir isotherm models (R² ≥ 0.99), and the monolayer sorption capacity of 25.6 μg g^-1 for As (V) on RSBC was achieved. Among the different adsorbent dosages and initial concentrations used in the present study, 0.2 g L^-1 (14.8 μg g^-1) and 100 μg L^-1 (13.1 μg g^-1) were selected as an optimum parameters. A comparative analysis of RSBC with other pyrolyzed waste materials revealed that RSBC had comparable adsorption ability (per unit area). These acidic groups are responsible for the electron exchange (electrostatic attraction, ion-exchange, π-π/n-πinteractions) with the anionic arsenate, which facilitates optimum removal (>60%) at 7 < pH < pH_PZC. The future areas of research will focus on decontamination of real wastewater samples containing mixtures of different emerging contaminants and installation of biofilter beds that contains different spent adsorbents/organic substrates (including biochar) for biopurification study in real case scenario.

Silicon Cycling in Soils Revisited

Silicon (Si) speciation and availability in soils is highly important for ecosystem functioning, because Si is a beneficial element for plant growth. Si chemistry is highly complex compared to other elements in soils, because Si reaction rates are relatively slow and dependent on Si species. Consequently, we review the occurrence of different Si species in soil solution and their changes by polymerization, depolymerization, and condensation in relation to important soil processes. We show that an argumentation based on thermodynamic endmembers of Si dependent processes, as currently done, is often difficult, because some reactions such as mineral crystallization require months to years (sometimes even centuries or millennia). Furthermore, we give an overview of Si reactions in soil solution and the predominance of certain solid compounds, which is a neglected but important parameter controlling the availability, reactivity, and function of Si in soils. We further discuss the drivers of soil Si cycling and how humans interfere with these processes. The soil Si cycle is of major importance for ecosystem functioning; therefore, a deeper understanding of drivers of Si cycling (e.g., predominant speciation), human disturbances and the implication for important soil properties (water storage, nutrient availability, and micro aggregate stability) is of fundamental relevance.

Arsenic dynamics in paddy soil under traditional manuring practices in Bangladesh

Fertilization with organic matter (farm yard manure and/or rice straw) is thought to enhance arsenic (As) mobilization into soil porewaters, with subsequent As assimilation by rice roots leading to enhanced translocation to the grain. Here, interlinked experiments (field manuring and soil batch culture) were conducted to find the effect of organic matter at a field application rate practiced in Bangladesh (5 t/ha) on As mobilization in soil for paddies impacted by As contaminated groundwater irrigation, a widespread phenomenon in Bangladesh where the experiments were conducted. Total As concentration in a paddy soil (Sonargaon) ranged from 21.9 to 8.1 mg/kg down the soil profile and strongly correlated with TOC content. Arsenic, Fe, Mn, and DOC release into soil solution, and As speciation, are intimately linked to OM amendment, soil depth and temporal variation. Organic matter amendments lead to increased mobilization of As into both soil porewaters and standing surface waters. The As speciation in the porewater was dominated by inorganic As (As_i) (arsenite and arsenate), with traces amounts of methylated species (DMA^V and MMA^V) only being found with OM amendment. It was noted in field trials that OM fertilization greatly enhanced As mobility to surface waters, which may have major implications for the fate of As in paddy agronomic ecosystems.

Redox Dependence of Thioarsenate Occurrence in Paddy Soils and the Rice Rhizosphere

In flooded paddy soils, inorganic and methylated thioarsenates contribute substantially to arsenic speciation besides the much-better-investigated oxyarsenic species, and thioarsenate uptake into rice plants has recently been shown. To better understand their fate when soil redox conditions change, that is, from flooding to drainage to reflooding, batch incubations and unplanted microcosm experiments were conducted with two paddy soils covering redox potentials from E_H -260 to +200 mV. Further, occurrence of thioarsenates in the oxygenated rice rhizosphere was investigated using planted rhizobox experiments. Soil flooding resulted in rapid formation of inorganic thioarsenates with a dominance of trithioarsenate. Maximum thiolation of inorganic oxyarsenic species was 57% at E_H -130 mV and oxidation caused nearly complete dethiolation. Only monothioarsenate formed again upon reflooding and was the major inorganic thioarsenate detected in the rhizosphere. Maximum thiolation of mono- and dimethylated oxyarsenates was about 70% and 100%, respectively, below E_H 0 mV. Dithiolated species dominated over monothiolated species below E_H -100 mV. Among all thioarsenates, dimethylated monothioarsenate showed the least transformation upon prolonged oxidation. It also was the major thiolated arsenic species in the rhizosphere with concentrations comparable to its precursor dimethylated oxyarsenate, which is especially critical since dimethylated monothioarsenate is highly carcinogenic.

Masking Phosphate with Rare-Earth Elements Enables Selective Detection of Arsenate by Dipycolylamine-ZnII Chemosensor

Functional reassessment of the phosphate-specific chemosensors revealed their potential as arsenate detectors. A series of dipicolylamine (Dpa)-ZnII chemosensors were screened, among which acridine Dpa-ZnII chemosensor showed the highest capability in sensing arsenate. The presence of excess ZnII improved sensitivity and strengthened the binding between acridine Dpa-ZnII complex to arsenate as well as phosphate. However, due to their response to phosphate, these sensors are not suited for arsenate detection when phosphate is also present. This study demonstrated for the first time that rare-earth elements could effectively mask phosphate, allowing the specific fluorescence detection of arsenate in phosphate-arsenate coexisting systems. In addition, detection of arsenate contamination in the real river water samples and soil samples was performed to prove its practical use. This sensor was further employed for the visualization of arsenate and phosphate uptake in vegetables and flowering plants for the first time, as well as in the evaluation of a potent inhibitor of arsenate/phosphate uptake.

Distribution and Speciation of Copper and Arsenic in Rice Plants ( Oryza sativa japonica 'Koshihikari') Treated with Copper Oxide Nanoparticles and Arsenic during a Life Cycle

A 6 × 2 factorial study was conducted to investigate the effects of copper oxide nanoparticles (nCuO, 0-100 mg/L), arsenic (As, 0-10 mg/kg), and their interaction on uptake, distribution, and speciation of Cu and As in rice plants ( Oryza sativa japonica 'Koshihikari'). Arsenic (in As-addition treatments) and Cu in seedling roots (SRs) were 1.45 and 1.58 times those in soil, respectively. Arsenic and Cu concentrations further increased in mature plant roots (MRs), which were 2.06 and 2.35 times those in soil, respectively. Arsenic and Cu concentrations in seedling shoots (SSs) were 79% and 54% lower than those in SRs, respectively. The mature stems, however, contained only 3% and 44% of As and Cu in SSs. Copper in flag leaves did not vary much compared to that in stems, whereas As was 14.5 times that in stems. Species transformations of Cu and As were observed in rice including reductions of Cu(II) to Cu(I) and As(V) to As(III). Arsenic in dehusked grains was negatively correlated with Cu and was lowered by nCuO below the WHO (World Health Organization) maximum safe concentration for white rice (200 ng/g). This may alleviate As adverse effects on humans from rice consumption.