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Landfills have the potential to contribute to mercury (Hg) pollution, due to the burial of waste containing mercury. Mercury from domestic waste can enter the soil surrounding landfills through surface runoff and leachate. In this study, we assessed the levels of Hg in the paddy soil around the Nam Son landfill, the largest landfill in the North of Vietnam, during both rainy (September 2021) and dry (January 2022) seasons. The concentration of Hg was in the range of 20.5 to 79.7 μg/kg dry w.t. in Bac Son and Nam Son, and 16.6 μg/kg dry w.t. at a higher reference site. In most of the samples, the rainy season showed higher Hg concentrations than the dry season. Soil samples taken closer to the landfill exhibited higher levels of Hg contamination compared to those in more distant paddy areas, suggesting a decreasing trend of Hg concentration as one moves away from the pollution source. Additionally, Hg concentration was found to decrease vertically from the surface, with the higher value observed in the surface layer (0–5 cm), and the lower in the bottom layer (20–25 cm). The geo-accumulation index showed that all the sampling points were moderately to heavily polluted, indicating that Hg was lost from the waste source in the landfill. This study provides valuable insights into the spatial and vertical distribution of Hg pollution in the topsoil and highlights the importance of managing and assessing the risks of Hg-containing waste. [ABSTRACT FROM AUTHOR]

In paddy rice farming systems, the degradation of insecticides from the soil reduces their occurrence as well as their levels in surface and groundwater. Degradation of insecticide fenobucarb in the paddy soil-water system was carried out by surface soil collected from paddy rice in the Hue province of Vietnam. Experiments were conducted using a batch technique, the fenobucarb-spiked paddy soils were shaken end-over-end with solutions of pH 7, pH 9, 25 mg L−1 DOC, and 1 critical micelle (cmc) sodium dodecyl sulfate (SDS). After each period (24, 48, 72, ... 144 hours), the soil solutions were separated into the aliquot and the solid phase to analyze fenobucarb and intermediate metabolic products 2-sec-butyl phenol. The decrease of fenobucarb and the appearance of (2-sec-butyl-phenol) were the result of hydrolysis and biodegradation by fenobucarb-acclimatized microorganisms that were capable of degrading fenobucarb in the paddy soil. The degradation of fenobucarb in both the soil and water phases in the paddy soil-water system was enhanced in the experiment with pH 7, followed by those with pH 9, 25 mg L−1 DOC, and 1 cmc SDS. With pH 9, in addition to decomposition by bacteria present in the soil, fenobucarb was hydrolyzed under basic conditions. The presence of 25 mg L−1 DOC and 1 cmc SDS in the aqueous solution increased desorption and microbial activity in the soil and increased the biodegradation efficiency of fenobucarb in the paddy soil-water system. However, the degradation of fenobucarb in the 1 cmc SDS solution was inhibited because SDS solution may be preferentially utilized by microorganisms, and fenobucarb would have a competitive effect with SDS. Irrigation water for paddy fields often receives untreated domestic and industrial wastewater that may contain multiple compounds such as DOC and SDS. Therefore, it contributes to the elimination of fenobucarb insecticide in the paddy soil-water system. [ABSTRACT FROM AUTHOR]

Intensive rice cultivation for a long time resulted in increasing soil degradation and less yield. This study aimed to evaluate effects of the combining reducing nitrogen fertilizer (N) with biochar amendment on soil chemical properties, rice growth parameters, and grain yield in the rice cultivation system in the Mekong Delta region, Vietnam (VMD). Field experiment was designed in the split-plot design with two factors, including N fertilizer (main plot) and biochar (sub-plot). Two N fertilizer rates were: (N50)—50 kg N ha–1 and (N100)—100 kg N ha–1, which is the farmer's practice. Biochar was amended with three rates: no applied biochar (B0), 5 t ha–1 (B5), and 10 t ha–1 (B10). The results indicated that reducing N fertilizer by 50% combined 5–10 t biochar ha–1 resulted in maintaining soil pH, soil electrical conductivity, soil organic carbon, cation exchange capacity, and rice biomass. Applying biochar at a rate of 5–10 t ha–1 significantly increased the available N, available P, and rice height compared to the treatment with no applied biochar (B0). Rice yield in the treatments applied with 5–10 t ha–1 was significantly higher than the treatment without the use of biochar by 11.6–14.7%. The findings of this study confirmed that reducing 50% N fertilizer combined with 5 t ha–1 or 10 t ha–1 of biochar could improve soil available N, available P, rice growth, and grain yield in intensive rice cultivation systems in the VMD region. [ABSTRACT FROM AUTHOR]

Advocating proper phosphorus (P) fertilisation is necessary to save this limited natural resource and to save the investment in rice cultivation. This study aimed to evaluate changes in phosphorus availability, total phosphorus in soil, phosphorus buffering capacity, and phosphorus saturation in the long-term phosphorus fertilisation in the paddy rice system. Soil samples were collected in the harvest stage after seven consecutive crops over three years at Can Tho city, Vietnam. The applied phosphorus fertiliser rates were: no phosphorus fertilisation (P0), 17.4 kg P/ha (P17.4), and 26.2 kg P/ha as farmer's practice (P26.2). The results showed that the soil phosphorus buffering capacity in P0, P17.4 and P26.2 treatments was 9.49, 9.08 and 9.04 mg/kg, respectively. The degree of phosphorus saturation of P17.4 and P26.2 treatments ranged from 17.7% to 25.5%, showing the medium to high risk of phosphorus leaching. This study indicated that the application of phosphorus rate higher than 17.4 kg P/ha might result in the reduced soil phosphorus buffering capacity in the intensive rice cropping system in the Vietnamese Mekong Delta region. Our results implied that the application of a rate lower than 17.4 kg P/ha/crop could be extended to the other rice-growing (double/triple rice) areas in the Vietnamese Mekong Delta region or other paddy rice on alluvial soils in Asia. [ABSTRACT FROM AUTHOR]

• Phytoliths dissolved three times faster in paddy soils than in non-paddy soils. • Phytolith morphology and surface chemistry were not affected by paddy management. • Laboratory dissolution rate decreased with time in soil and with surface Al. • Field dissolution was unrelated to changes in surface chemistry. • Field dissolution is overwhelmingly governed by water regime. Phytoliths are an important component in the cycling of silicon (Si) in rice cultivation, yet little is known about their medium to long-term stability. While it is commonly accepted that phytolith solubility in soil decreases with time, the mechanisms that cause this decrease remain unclear. Most studies on phytolith aging to date have been conducted under laboratory conditions and field studies are rare. Here, we present a comprehensive field study of phytolith aging in paddy and non-paddy fields in the Philippines and Vietnam. Phytoliths extracted from rice straw were placed in mesh bags and exposed to ambient soil conditions for up to 550 days. Following exposure, total mass loss and changes in physical (particle size, specific surface area, ζ potential) and chemical (surface chemical composition by acid digestion and X-ray photoelectron spectroscopy) properties as well as dissolution kinetics (56 days in 0.1 M CaCl 2 at pH 6.5) were determined. Phytolith dissolution in the field was rapid (up to 30 % mass loss) and three times faster in paddy than in non-paddy soils. Differences in phytolith properties were unexpectedly little between paddy and non-paddy soils. Laboratory and field-derived dissolution rates decreased with exposure time but were one order of magnitude lower in the field. While laboratory dissolution rates were negatively related to surface Al contents this was not observed for field-exposed phytoliths. Our findings suggest that under natural field conditions, phytolith dissolution is overwhelmingly governed by water regime, while the influence of protective surface coatings is only minor. We conclude that reliable estimates of phytolith dissolution can be best obtained in field experiments. [ABSTRACT FROM AUTHOR]

The 2030 Agenda was promulgated for the Sustainable Develop Goals (SDGs), which consist of 17 goals created to eradicate poverty and realize a more sustainable world. Among the goals, soil science can contribute significantly toward achieving zero hunger (#2), implementing climate action (#13), bettering life on land (#15), among others. Paddy fields are important sustainable agro-ecosystems for rice (Oryza sativa) production, and rice is the staple food for more than 2 billion people worldwide. Paddy rice is produced mainly in East, South, and Southeast Asia, in region with a monsoon climate. Paddy soils are unique, as they are mostly managed under waterlogged conditions during the rice-growing season. Nitrogen (N) is one of the most important nutrients for many crops, including rice. Some potential sustainable soil management methods are suggested to meet the SDGs, especially in paddy fields and wetlands in East, South, and Southeast Asia, based on current achievements, including N mineralization analysis, soil microbial analysis, and greenhouse gas measurements, to predict soil fertility and effective environmental management practices in the future. One such example of sustainable soil management practice is the amendment of silicate fertilizer produced from steel-making sludge that reduced methane emission from paddy fields (3%–50%) and increased the rice yield in Vietnam. [ABSTRACT FROM AUTHOR]

This study aimed to investigate effects of nitrogen (N) fertilizer rates and inoculation of rice seeds with N-fixing bacterium Gluconacetobacter diazotrophicus on the growth parameters and yield of OM5451 and OM6976 rice varieties in the Vietnamese Mekong Delta region. Nitrogen fertilizer rates of 50 kg N.ha-1 and 100 kg N.ha-1 were used, with latter reflecting farmer practice. Three rice seed inoculation methods were also employed: Seeds soaked in water for 24 hours and allowed to stand for 30 hours (control) (B0); Seeds soaked in water for 24 hours and inoculated with G. diazotrophicus for 30 hours (B1); Seeds soaked with G. diazotrophicus in water for 24 hours and allowed to stand for 30 hours. Applying 50 kg N.ha-1 without combining with NFB bacterium in this experiment generally resulted in less tillers, shorter plants, a lower SPAD index, and lower grain yield. Combining G. diazotrophicus bacterium with reduced N fertilizer of 50 kg N.ha-1 demonstrated rice growth and yield may be maintained in both varieties compared to 100 kg N.ha-1. These results providing a firm foundation for future research of adding NFB to paddy soils to decrease the N fertilizer requirement. [ABSTRACT FROM AUTHOR]

Arsenic (As) contamination has been identified as a tangible risk related to soil and land degradation, and identifying pristine regions is, therefore, important for the long‐term safety considerations of humans. The Red River Basin is among the largest cultivation regions in Southeast Asia that provides rice for local consumers as well as for exports. While the lowland part of the Red River Delta (RRD) has been recently identified as an area suffering from As risk, little is known about the upland terrace part. In this study, terraced paddy was found to be an ecosystem with a lower accumulation of As. The median As value, x~, in the terraced paddy soil was 2.22 mg kg−1, while almost no As was detected in the rice grain. The content of soil As of the RRD (x~= 6.12 mg kg−1) was equal to the global average and led to a significant accumulation of As in rice grain (x~ = 221 μg kg−1). Among the soil properties, soil water‐soluble silicon (SiH2O) appeared to be the most effective factor for reducing As translocations to rice. As the diminution of available Si can narrow the arable land that produces non or low‐As rice, this is likely an emerging factor that affects land degradation. With its low‐As rice status, the terraced paddy can be highlighted as a pristine part for rice growing. The fact that As accumulates in rice at different levels between terraced and lowland paddy regions can be considered in developing As mitigation strategies or rice repricing. [ABSTRACT FROM AUTHOR]

Rice production in floodplain deltas is currently vulnerable to climate change and contamination from anthropogenic activities. The relocation of rice production to upland regions could be an option for increasing the sustainability of rice production. Our study evaluated the spatial patterning of heavy metals, i.e., copper (Cu) and cadmium (Cd), in rice along a topogradient from terrace to lowland areas in the Red River basin. The dataset obtained from the analysis of 61 farm sites throughout the whole basin indicated a large discrepancy in the Cu and Cd contents in rice grains from terrace and lowland paddies. While Cu and Cd were not found in most of the rice grain samples from the terrace paddies, the median Cu and Cd contents of the lowland paddy rice were 1.895 and 0.033 mg kg−1, respectively. Assessing the relationship of Cu and Cd in the soil–rice system to soil properties revealed possible correlations between soil available silicon (Si) and the Cu and Cd contents in rice grain. The enrichment of Si in rice plants likely reduces the translocation of Cu and Cd from soil to grain. Therefore, management of the Si supply, particularly in lowland paddies, should be highlighted as a way to reduce dietary intake of Cu and Cd. [ABSTRACT FROM AUTHOR]

Understanding of the fate of phytolith in soils is important because of its role as an agronomical nutrient resource and for carbon sequestration. Accumulation of phytolith derived silica occurs in paddy soils when phytolith input, here through straw incorporation, is higher than silica removal through crop offtake, dissolution and leaching. Phytolith dissolution is thought to be the main reason for diminishing of phytolith derived silica in soil, with this dissolution being highly dependent on soil chemical properties. In this study, phytolith content from 78 paddy soils sampled in the Red River (RRD) and Mekong River (MRD) Deltas, Vietnam, were quantified and related to pH, electrical conductivity (EC), oxalate-extractable Al and Fe (Al ox and Fe ox ), organic carbon (OC) and clay content. Average phytolith contents within the topsoil (0–20 cm) were 4.8 ± 1.98 and 7.74 ± 2.97 mg g −1 for the RRD and MRD, respectively. Positive correlation coefficients obtained for Al ox , OC, clay content and EC, implying that these parameters might enhance phytolith resistance to dissolution. Soil pH had a negative correlation coefficient with phytolith content. Lower pH and higher Al ox content explained the predominance of the phytolith in the MRD compared to the RRD. Soil pH adjustment can be proposed as an active management strategy to maintain phytolith added Si, source for crops and for carbon sequestration. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the spatial and temporal variations in toposequential methane (CH4) emission is essential for assessing and mitigating CH4 emission from rice cascades in mountainous watersheds. To assess the toposequential variation in CH4 emission among different field positions, two cascades of double-cropping paddy rice fields were investigated in Yen Chau district, Northwest Vietnam. The cascades were divided into fertilized and non-fertilized parts and CH4 measurements at 10days intervals were conducted at top, middle and bottom fields of each part. The results showed that the rate and cumulative amount of CH4 emissions in non-fertilized part were higher than that of fertilized one in both spring and summer rice seasons due to the stimulation of CH4 oxidation by urea and sulfate containing fertilizers. The spatial variation in CH4 emissions among the field positions was high in both cropping seasons with the highest emissions in the bottom fields and the lowest emissions were found in the top fields (i.e. bottom field CH4 emissions 1.8–3.0 times higher than the top field). The differences among field positions were influenced by clay content, total nitrogen and total carbon content which showed toposequential differences. The average CH4 fluxes ranged from 1.0 to 5.1mgCH4 m−2 h−1 being largest at later growth stages for spring rice and during early growth stages for summer rice. Cumulative CH4 emissions for spring rice ranged from 3.1 to 13.7gCH4 m−2 and that for summer rice from 4.3 to 23.5gCH4 m−2. 61.7% was emitted during summer rice season and 38.1% from spring rice season. The higher values for summer crops were due to higher availability of fresh organic substrates under higher soil temperature during the early growing period. The average total CH4 emissions from double-cropping paddy rice fields were 14.8gCH4 m−2 for cascade 1 and 27.3gCH4 m−2 for cascade 2. The higher emission for cascade 2 might be due to the lower soil Eh and higher clay content especially in the lower lying fields. The results highlight that large toposequence differences in CH4 emissions require different site specific management practices for each toposequence position in order to mitigate CH4 emission in paddies in mountainous watersheds. [Copyright &y& Elsevier]

The present study observed bacterial consortia in iron-deposited colonies formed on the surface of paddy soils. Plow layer soils under reduced condition, which were collected from a paddy field in Japan and paddy fields located at two sites in Mekong Delta, Vietnam, were incubated at 25 and 30°C in test tubes under a nitrogen (N2) atmosphere with sleeve stoppers made from a synthetic rubber. The sleeve stopper enabled the establishment of microaerobic conditions because of gradual diffusion of oxygen (O2) into the tube (29.2 nmol mL−1day−1). Many colonies were observed on the soil surface in the test tubes after 2–3 weeks of incubation from both Japanese and Vietnam paddy soils. Bright orange and blood red colonies, which had 0.5–1-mm diameters and circular shapes, were observed. Micro X-ray fluorescence analysis of the colonies showed deposition of iron (Fe) in the colonies, indicating bacteria were enriched in the Fe oxidation. Microscopic observation showed acicular, floc and concentric crystal structures of Fe oxides in the colonies. Long- and short-rod bacterial cells mainly existed inside (or interspace) and outside surface of the colonies. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and clone library analysis of bacterial 16S rRNA genes showed that most of the obtained sequences belonged to Betaproteobacteria, in which bacteria related to “Sideroxydans lithotrophicus” and “S. paludicola” in Gallionellales, known as microaerobic, neutrophilic Fe-oxidizers, were predominant. The sequences related toThiobacillusspp. andDechloromonasspp., some strains of which are known to be anaerobic, nitrate-dependent neutrophilic Fe-oxidizers, were also obtained. The present study showed proliferation of bacteria related to neutrophilic Fe-oxidizers in paddy soils under microaerobic condition. [ABSTRACT FROM AUTHOR]