Trace Elements in Thai Oxisols on Limestone in Relation to Rainfall

AbstractTrace element concentrations of 7 profiles of Oxisols derived from calcareous sedimentary rocks under tropical monsoonal and the tropical savanna environments have been investigated using XRD, SXRD, XRF, and ICP–OES analytical techniques so as to provide baseline values for estimating environmental contamination of soils with heavy metals. Kaolinite was the dominant mineral of the clay fraction with moderate amounts of goethite and hematite. Gibbsite was only present in Ak1 profile under more humid conditions contributing to high anion exchange capacity (AEC). Trace element concentrations in these soils conform to those for worldwide normal soils. However, the extraordinary high arsenic (As) concentration in Ak1 soil represents a hazardous concentration (313mg kg-1), which may have been inherited from the parent rock. Arsenic in this soil is probably not toxic because it is immobile in an oxidized environment, and is sorbed onto soil constituents particularly sesquioxides. Factor analysis showed that for soils under various rainfalls were deficient. For the whole soil samples, three main groups of similar geochemical behavior existed. Different concentrations of elements of the Ca group (Ca, Ni, Cr, Mg, Ba, Be, Gd, Rb, K, Co, Mn, Zn, La, Cu, V, Ce, P, Cd, Sc, Pb, clay, SSA and CEC) and the Si group (Si and sand), resulted in soils developed under the tropical monsoonal and the tropical savanna climates being clearly different. The Fe group (Fe, Ti, Al, Mo, Ge, Se, Sr, Ga, Th, Hf, U, As and Sb) separated the soils developed of under a tropical monsoonal into two groups based on the very high concentrations of Fe, Al and As in Ak1 profile

Biogeochemical Cycling of Carbon and Nitrogen in Rainfed Rice Production Under Conventional and Organic Rice Farming

Dwindling carbon (C) and nitrogen (N) levels in paddy soils decreases rice production and threaten human food security globally. The efficient maintenance of C and N fluxes in soil-rice systems is a crucial prerequisite for agricultural and environmental sustainability. Herein, we examined the C and N fluxes from 63 rainfed rice paddy fields under conventional farming (CF) and organic farming (OF) systems in Thailand. The C and N fluxes were measured based on a detailed analysis of relevant influxes (fertilizer, manure, and biomass addition) and effluxes (biomass harvest and greenhouse gas emission). The result demonstrated that the harvested grain and straw contributed to the most abundant C and N effluxes for both farming systems. The CH4 effluxes were moderate, whereas the N2O effluxes were meager relative to their total effluxes. Stubble incorporation and animal manure addition to soil were the most extensive C influxes. However, the primary N influxes were stubble incorporation and animal manure addition for the OF system, and chemical-N fertilizers for the CF system. Net C depletions were observed in both the CF and OF systems. However, net N was depleted and accumulated in the CF and OF systems, respectively. Straw incorporation to soils could restore the net C accumulations for the CF and OF systems and elevate the net N accumulation for both systems. This study highlighted that complete straw removal has exacerbated the C and N stock in soil-rice systems, inducing insecurity for the environment and the agricultural systems. Effective straw management is a simple approach for sustaining paddy rice production

The Distribution of Trace Metals in Roadside Agricultural Soils, Thailand

Vehicle emissions have been known to cause trace metal contamination in soils. The extent of such contaminations in soils, and of the effects of traffic density and distance from highways on the concentration of trace metals in roadside agricultural soils is largely unknown. This study examined the total concentrations of common trace metals (Cd, Co, Cr, Cu, Ni, Pb, V, and Zn) in roadside agricultural soils from Thailand with diverse traffic densities (approximately 30⁻200 million vehicles/kilometer/year), roadside distances (0, 10, 20, 50, and 100 m from the road edge), and crops (rice, maize, and sugarcane). Cadmium, Cu, Pb, and Zn concentrations significantly decreased with increasing distance away from the roads (p < 0.05). However, the concentrations of these metals were not correlated with traffic density, probably due to extensive road maintenance and expansion. The contamination factor demonstrated that the road edge soils were moderately- to highly-polluted with Cd, Cu, Pb, and Zn. The safest distance to minimize metal pollution for agricultural production is proposed to be greater than 10 m away from the road edge

Solid Phase Speciation and Solubility of Vanadium in Highly Weathered Soils

Vanadium (V) is increasingly recognized both as a medical trace element with essential biological functions and as a potentially toxic environmental pollutant, yet the current knowledge on V speciation in soils is limited. Here, we investigated the chemical speciation and extractability of V in highly weathered tropical soils, which are often rich in V compared to soils of temperate climatic regions. Vanadium <i>K-</i>edge X-ray absorption near edge structure (XANES) spectra of soil samples, along with a range of reference compounds differing in V-oxidation state and coordination chemistry, revealed the predominance of V^4+/5+ in a primarily octahedral or tetrahedral coordination. The soil spectra were best fitted with linear combinations of reference spectra of V⁴⁺ in the structure of kaolinite, V⁵⁺ adsorbed to kaolinite, and other V⁵⁺-sorbed solids. Vanadate adsorbed to goethite, ferrihydrite, gibbsite, and/or Fe­(III)–natural organic matter complexes and V⁴⁺ in the structure of goethite may be present, but cannot unequivocally be distinguished from each other by XANES spectroscopy. Sequential and single chemical extractions provided complementary information on the solubility of V under various conditions. The most labile V fractions, interpreted as weakly and strongly adsorbed V⁵⁺, are the most relevant to V mobility and bioavailability in the environment, and accounted for only ∼2% of total soil V. Our results demonstrate that kaolinite and Fe oxides can effectively sequester V in highly weathered soils by mechanisms of adsorption and structural incorporation and are relevant to other Fe-oxide-rich environments under acidic and oxic conditions

Solid-Phase Speciation and Solubility of Phosphorus in an Acid Sulfate Paddy Soil during Soil Reduction and Reoxidation as Affected by Oil Palm Ash and Biochar

Understanding phosphorus (P) speciation and how redox conditions control P solubility in acid sulfate paddy soils with limited P availability is crucial for improving soil P availability. We examined P speciation and extractability in an acid sulfate paddy soil incorporated with oil palm ash (OPA) and biochar (OPB) during soil reduction and subsequent oxidation. Phosphorus <i>K</i>-edge X-ray absorption near edge structure (XANES) spectra of the soil samples revealed that P in the soil mainly occurred as P adsorbed to ferrihydrite and P adsorbed to gibbsite. During soil reduction, gibbsite-bound P was transformed into variscite, which was back-transformed to gibbsite-bound P during soil reoxidation. Sequential extraction results confirmed the dominance of Fe/Al (hydr)­oxides-bound P (average 72%) in the soils. The OPA incorporation increased the exchangeable P pool concurring with the decrease in gibbsite-bound P. The OPB incorporation enhanced the dissolved P from the residual pool presumably due to electron shuttling of biochar with Fe­(III) minerals during soil reduction. Our results highlight P dynamics in paddy soils, which are of immense importance for effective P-management strategies in rice cultivation

Temporal development of arsenic speciation and extractability in acidified and non-acidified paddy soil amended with silicon-rich fly ash and manganese- or zinc-oxides under flooded and drainage conditions

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.ISSN:0045-6535ISSN:1879-129