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Jarosite, a prominent mineral in oxidised acid sulfate soil, is known to sorb and incorporate a variety of elements, including Al. However, to understand the role that jarosite plays in regulating element cycles, it is crucial to understand the stability and transformation pathways of jarosite in an acid sulfate soil under dynamic biogeochemical conditions. In this study, we observed the transformation of unsubstituted jarosite and Al-substituted jarosite, collectively referred to as 'jarosite', in an acid-sulfate rice paddy topsoil and subsoil from Thailand. Jarosite was incubated in flooded paddy topsoils and subsoils for up to sixteen weeks, both in laboratory mesocosms and in the field, using bags made of polyethylene terephthalate mesh. One set of mesh bags contained jarosite that was not mixed with any other minerals, and referred to as 'pure mineral' incubations. Mineral transformations occurred under the influence of the soil porewater only and were primarily followed by X-ray diffraction analysis. A parallel set of mesh bags contained soil with 57Fe-labelled jarosite enrichment (Fe enriched in soil by a factor of 1.3 but 57Fe enrichment factor of 12.5–13.6), which allowed jarosite transformation to occur in close association with the soil matrix. Mineral transformations in 57Fe-jarosite-enriched soil were followed using 57Fe Mössbauer spectroscopy. In laboratory mesocosms and in the field, jarosite transformed most quickly in topsoil, whereas jarosite underwent limited transformation in subsoils, especially in laboratory mesocosms where Fe reduction was slow. The chemical environment around the jarosite affected the outcome of the transformation processes. Crystalline Fe oxyhydroxides, such as goethite, dominated the products in mesh bags where jarosite was not mixed with soil, whereas short-range-ordered or non-mineral Fe phases, such as Fe(II) sorbed to mineral surfaces or complexed with organic ligands, were dominant transformation products of jarosite when mixed in soil. Furthermore, Al substitution in jarosite caused contrasting effects in mesh bags containing pure minerals or mineral-enriched soil. Aluminium substitution slowed the transformation of jarosite in pure mineral mesh bags, but Al-substituted and unsubstituted jarosite showed similar transformation rates and pathways when incubated as a mixture with soil. The contrasting rate of transformation in topsoil and subsoil, the contrasting products of transformation in pure mineral mesh bags and 57Fe-jarosite-enriched soil, and the contrasting effect of Al substitution in pure mesh bags or jarosite-enriched soil, all demonstrate that the rate and products of jarosite transformation are defined by a balance between competitive transformation pathways that affect the transforming minerals. [ABSTRACT FROM AUTHOR]

Aluminum (Al) is a copious element in the earth's crust, typically causing high acidity in soil-plant systems. Much research has primarily investigated adverse impacts of Al on plants, little is known about its beneficial role in enhancing mineral nutrient availability and acquisition in cassava, which is a vital economic crop relevant to human health. Herein, we examined the effect of Al levels on proton and organic acid release from the roots of two cultivars under an acid-washed sand microcosm. Consequential effects of the Al were examined on the extractability of Al and selected nutrients (iron: Fe and phosphorus: P) in the rhizosphere and bulk sands and the nutrient uptake in the plant. The results demonstrated that the highest Al level significantly demoted fresh root weight (8.53 g) but promoted the proton release from roots (2.03 μmol h−1 g−1 fresh weight), compared to the control treatment (11.92 g and 0.40 μmol h−1 g−1 fresh weight). Water-extractable Al and Fe concentrations in the rhizosphere sand were higher by 188–276 % and 201–291 %, respectively, than bulk sand in the highest Al level. The moderate Al levels (<50 μmol Al L−1) also increased Fe accumulation in the plant, elaborating on the beneficial role of Al in enhancing Fe acquisition. The main organic anions (oxalate and tartrate) released from the roots were cultivar-dependent. Our study highlighted that moderate Al levels showed the benefits of Al in promoting proton release from roots, enhancing Fe availability in the rhizosphere zone, and Fe acquisition in the cassava plant. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxides of silicon (Si), manganese (Mn), and zinc (Zn) have been used as soil amendments to reduce As mobility and uptake in paddy soil systems. However, these amendments are hypothesized to be affected differently depending on the soil pH and their effect on As speciation in rice paddy systems is not fully understood. Herein, we used a microcosm experiment to investigate the effects of natural Si-rich fly ash and synthetic Mn and Zn oxides on the temporal development of porewater chemistry, including aqueous As speciation (As(III), As(V), MMA, DMA, and DMMTA) and solid-phase As solubility, in a naturally calcareous soil with or without soil acidification (with sulfuric acid) during 28 days of flooding and subsequent 14 days of drainage. We found that soil acidification to pH 4.5 considerably increased the solubility of Si, Fe, Mn, and Zn compared to the non-acidified soil. Additions of Mn and Zn oxides decreased the concentrations of dissolved arsenite and arsenate in the non-acidified soil whereas additions of Zn oxide and combined Si–Zn oxides increased them in the acidified soil. The Si-rich fly ash did not increase dissolved Si and As in the acidified and non-acidified soils. Dimethylated monothioarsenate (DMMTA) was mainly observed in the acidified soil during the later stage of soil flooding. The initial 28 days of soil flooding decreased the levels of soluble and exchangeable As and increased As associated with Mn oxides, whereas the subsequent 14 days of soil drainage reversed the trend. This study highlighted that soil acidification considerably controlled the solubilization of Ca and Fe, thus influencing the soil pH-Eh buffering capacity, the solubility of Si, Mn, and Zn oxides, and the mobility of different As species in carbonate-rich and acidic soils under redox fluctuations. [Display omitted] • Solubilities of Si, Mn, and Zn oxides differed in acidified and non-acidified soils. • Soil acidification greatly impacted As, Si, Mn, and Zn solubility in paddy soils. • Mn and Zn oxides decreased arsenite and arsenate in non-acidified soil. • Zn oxide increased dissolved arsenite and arsenate in acidified soil. • Soil acidification and reaeration synergistically increased H 4 SiO 4 -to-H 3 AsO 3 ratio. [ABSTRACT FROM AUTHOR]

Exposure to arsenic (As) through rice consumption potentially threatens millions of people worldwide. Understanding is still lacking the recycling impacts of rice residues on As phytoavailability in paddy soils and is of indisputable importance in providing a sustainable and effective measure to decrease As accumulation in rice grain. Herein, we examined the effects of rice husk biochar (RHB) and rice husk ash (RHA) on As grain speciation, and As dynamics in the soil porewater and solid-phase fractions. The results corroborated that both the RHB and RHA (0.64% w/w) treatments significantly (p < 0.05) decreased inorganic As accumulation in rice grain to 0.27–0.29 mg kg−1, which was below the maximum inorganic As level in husked rice (0.35 mg kg−1) established by the Codex. The residual phase (F6 = 90% of total soil As) as quantified by the sequential extraction was the dominant As pool; the fractions were subsequently transformed into several As pools associated with soluble and exchangeable (F1), organically bound (F2), Mn oxides (F3), poorly crystalline (F4) and crystalline (F5) Fe oxides during the rice growing periods. The Si-rich amendments enhanced the residual phase formation upon soil flooding, which decreased the As availability to rice plant. The inorganic grain-As concentrations were well explained by the soil-extractable As concentrations in the F2, F3, F5, and F6 fractions. The pore-water analysis indicated that Mn oxides were important sources and sinks for As released to the soil solution. Our findings shed light on the beneficial role of RHB and RHA in alleviating inorganic As uptake in paddy rice. Unlabelled Image • Rice husk biochar and ash significantly decreased As(III) accumulation in grain. • Rice husk ash significantly decreased As(V) uptake in grain. • Biochar and ash promoted residual As phase formation and demoted rice grain As. • Oxalate-extractable As of flooded soil could explain inorganic grain As contents. • Mn oxides were the major sources and sinks for As released to soil solution. [ABSTRACT FROM AUTHOR]