Flooded Soil Research Articles

Impact of Fe(III) (Oxyhydr)oxides Mineralogy on Iron Solubilization and Associated Microbial Communities.

Iron-reducing bacteria (IRB) are strongly involved in Fe cycling in surface environments. Transformation of Fe and associated trace elements is strongly linked to the reactivity of various iron minerals. Mechanisms of Fe (oxyhydr)oxides bio-reduction have been mostly elucidated with pure bacterial strains belonging to Geobacter or Shewanella genera, whereas studies involving mixed IRB populations remain scarce. The present study aimed to evaluate the iron reducing rates of IRB enriched consortia originating from complex environmental samples, when grown in presence of Fe (oxyhydr)oxides of different mineralogy. The abundances of Geobacter and Shewanella were assessed in order to acquire knowledge about the abundance of these two genera in relation to the effects of mixed IRB populations on kinetic control of mineralogical Fe (oxyhydr)oxides reductive dissolution. Laboratory experiments were carried out with two freshly synthetized Fe (oxyhydr)oxides presenting contrasting specific surfaces, and two defined Fe-oxides, i.e., goethite and hematite. Three IRB consortia were enriched from environmental samples from a riverbank subjected to cyclic redox oscillations related to flooding periods (Decize, France): an unsaturated surface soil, a flooded surface soil and an aquatic sediment, with a mixture of organic compounds provided as electron donors. The consortia could all reduce iron-nitrilotriacetate acid (Fe(III)-NTA) in 1–2 days. When grown on Fe (oxyhydr)oxides, Fe solubilization rates decreased as follows: fresh Fe (oxyhydr)oxides > goethite > hematite. Based on a bacterial rrs gene fingerprinting approach (CE-SSCP), bacterial community structure in presence of Fe(III)-minerals was similar to those of the site sample communities from which they originated but differed from that of the Fe(III)-NTA enrichments. Shewanella was more abundant than Geobacter in all cultures. Its abundance was higher in presence of the most efficiently reduced Fe (oxyhydr)oxide than with other Fe(III)-minerals. Geobacter as a proportion of the total community was highest in the presence of the least easily solubilized Fe (oxyhydr)oxides. This study highlights the influence of Fe mineralogy on the abundance of Geobacter and Shewanella in relation to Fe bio-reduction kinetics in presence of a complex mixture of electron donors.

Burr marigold (Bidens tripartita L.) roots directly and immediately scavenge rhizosphere methane with highly exuded hydrogen peroxide via a rhizosphere Fenton reaction

The major factors controlling the soil methane (CH4) concentration and CH4 emissions of various plant (mainly wetland) species were identified. Five plant species (Oryza sativa, Zizania latifolia, Phragmites australis, Sesbania cannabina, and Bidens tripartita) were separately cultivated under the flooded soil conditions. The direct CH4 scavenging potential of B. tripartita roots was also measured in conjunction with in vitro CH4 scavenging experiments using H2O2 and several transition metal ions. CH4 emissions from the soil-cultivated plants did not depend on the maximum CH4 emission potential for each plant species but on the soil CH4 concentrations, which were positively correlated with the CH4 production potential of the soil and negatively correlated with soil Eh values. Bidens tripartita roots possessed the highest increasing soil Eh potential and a direct, immediate, and continuous CH4 scavenging potential via the Fenton reaction using a considerably high concentration of root apoplastic H2O2 and rhizosphere Fe2+. Bidens tripartita presented the highest soil Eh ascending potential. The in vitro experiments suggested the involvement of・OH/FeIVO2+ via the newly termed rhizosphere Fenton reaction as a strong destructive power for CH4. To our knowledge, this is the first report on direct CH4 scavenging by high H2O2-exuding plant roots.

A Novel Multicriteria Decision Making Model for Sustainable Stormwater Management

Precipitation not absorbed by the soil or local vegetation and remain on the surface leading to stormwater can cause soil erosion, flooding, property damage, and overflow to wastewater treatment facilities. This paper introduces a novel multicriteria decision-making model to choose among various sustainable solutions that can help in managing stormwater. This model is intended to help decision-makers in handling stormwater through proper utilization of precipitation while ensuring public safety and adhering to runoff regulations. The model also aims to present sustainable technologies that can help in reducing harmful stormwater overflows. As a way of constructing and validating the model, precipitation and other relevant data from the North-Eastern region of the United States were used. The model can be altered though to suit other regions in the world. The model was further validated by seeking the opinion of a group of experts on its constructs. Statistical analysis identified high item-to-total correlations for model constructs and a model Cronbach’s alpha value of 0.84 leading to conclude that the model is valid. Yet, green solutions presented in this study and the developed model should be considered as a first step in determining sustainable stormwater solutions and further research in this area is needed.

Cadmium transfer in contaminated soil-rice systems: Insights from solid-state speciation analysis and stable isotope fractionation.

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (δ114/110Cd=-0.37 to-0.39%) compared to the plants that grew on non-flooded soils (δ114/110Cd=-0.45 to-0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl2 root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Δ114/110Cdshoot-root 0.16-0.19‰). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Δ114/110Cdshoot-root-0.02 to 0.08‰). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd-S complexes in the vacuole. Cd-S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems.

Arsenic dynamics and flux assessment under drying-wetting irrigation and enhanced microbial diversity in paddy soils: A four year study in Bengal delta plain

Arsenic (As) assessment in agricultural soils and corresponding crops is necessary from the global health safety perspective. To the best of our knowledge, we are reporting for the first time, As flux determining parametric equations for paddy field with seasonal rice cultivation under conventional flooding and dry-wet irrigation approaches. Rigorous field experiments and measuring quantitative parameters, flushed out or percolated into the deeper soil As flux was assessed. A wintery (boro)-monsoonal (aman) study from 2016 to 2019 has been conducted showing the efficiency of dry-wet irrigation on reduction of soil As bioavailability. The reduction in boro was 52.4% in 2016 to 64.8% in 2019 while in aman, it was 61% in 2016 to 74.9% in 2019. Low bioavailability was correlated to plant’s internal vascular structure that was found more rigid and firm in dry-wet field grown plants. Observed soil physico-chemical parameters clearly influenced As bioavailability as well as soil microbial community. Assessment of microbial diversity using metagenomics under altered water regime was done by population analysis, relative abundance, species richness, Krona chart comparison. Dry-wet field was found to be more diverse and enriched in microbial community than that of the flooded soil indicating an affective reduction of As bioavailability under biotic-abiotic factors.

Influence of soil characteristics on physiological and growth responses of Cytharexyllum myrianthum Cham. (Verbenaceae) to flooding

The success of watershed riparian forest restoration programs can be affected by the selection of plant species tolerant to flooding and soil types that occur along water courses. We evaluated physiological and growth responses of Cytharexyllum myrianthum seedlings to flooding in three different soil types representative of the Almada River Watershed (ARW), southern Bahia, Brazil. The soils selected, based on the relative abundance and importance in the ARW, were: Luvisol, Argisol, and Spodosol. After 35 days of flooding, the Argisol had the lowest and the Spodosol had the highest reduction–oxidation (redox) potential (Eh). After 35 days of flooding, the Luvisol had higher pH and electrical conductivity (Ec) than the other soils. Stomatal conductance (gs) and net photosynthesis (A) significantly declined in all soil types after 7 days of flooding. After that period, morphological changes characteristic of flood-tolerant plants, such as lenticel hypertrophy and adventitious root formation, were observed in all flooded plants. Following the morphological changes, gs and A in flooded plants increased to values close to those of the non-flooded plants. The highest relative growth rates based on mass (RGRm) and net assimilation rates (NAR) for the non-flooded plants were observed in the Argisol. After 35 days of flooding, no significant differences in RGRm or NAR were observed between non-flooded and flooded plants in the Luvisol, but large significant decreases in RGRm and NAR were observed for the flooded plants in the Spodosol. Our results demonstrated that the ability of seedlings of the same species to acclimate to flooded soil conditions differs among soil types. Therefore, the characteristics of soils present in a watershed should be considered when selecting tree species for the reforestation of riparian forests.

MAPPING SPATIAL DISTRIBUTION OF MAIN SEASON RICE FIELDS IN EASTERN NEPAL USING MULTI-TEMPORAL LANDSAT 8 IMAGES

Mapping rice area can be beneficial for change detection, irrigation management, climate change impact and vegetation protection and restoration programs. Remote sensing has provided a vantage means of mapping rice area. The unique physical characteristics of rice plants is that it is grown in flooded soil, which significantly affect the spectral reflectance from the rice fields. After a period of two months, the dense rice canopy cover replaces the flooded soil. This dynamic of the rice field is captured with the help of vegetation indices and are used to identify rice fields. Multi-spectral and multi-temporal data Landsat 8 data is used in the study. An algorithm that uses Normalized difference vegetation index (NDVI) and Land-surface Water Index (LSWI) derived from Landsat 16-days 30-meter data was used to differentiate paddy field from other areas. It works on the basis of sensitivity of LSWI for surface moisture and NDVI for vegetation content. This algorithm was used to detect rice fields in twelve local levels in Sunsari and Morang districts of Nepal. The results were validated using 0.44m resolution digital globe satellite imagery with 79 well-distributed control points. The overall accuracy of the method was found to be 79.746%.

Interspecies variation in survival of soil fauna in flooded soil

While many studies have examined the effects of flooding on earthworm population distributions, few studies have investigated physiological and behavioural responses of earthworms to the low oxygen conditions caused by flooding. An earthworm's skin is its oxygen exchange organ, allowing earthworms to survive in flooded environments provided that the water contains sufficient dissolved oxygen. Individuals of three species of earthworm, the anecic Lumbricus terrestris (Linneaus, 1758), the green morph of the endogeic Allolobophora chlorotica (Savigny, 1826) and the epigeic Lumbricus castaneus (Savigny, 1826) were placed in reconstituted groundwater that was either kept aerated or kept in a sealed container so that dissolved oxygen was gradually consumed as the earthworm respired. Oxygen saturation of the water was measured over time in sacrificial triplicate replicates from each treatment at discrete time points; earthworm mortality and mass were recorded. Before treatments, oxygen levels in all treatment tubes were 9.53 (±0.64) mg O2 L−1. L. terrestris, a large species which emerges at night to forage at the soil surface died when oxygen levels reached 0.82 (±0.46) mg O2 L−1 after approximately 36 h. L. castaneus, a smaller species which lives on the soil surface, died when oxygen levels reached 3.60 (±2.01) mg O2 L−1 after approximately 168 h. A. chlorotica, which is similar in size to L. castaneus, lives in the upper 20 cm of soil and is known to aestivate during the summer, did not die, even when oxygen levels reached 1.49 (±0.40) mg O2 L−1 after 280 h. The results suggest that earthworm respiration is closely linked to both body size and to behavioural ecotype. These findings suggest that if flooding increases in frequency resulting in episodic reductions in soil oxygen levels, the species composition of earthworm communities may change, with an increased presence of endogeic earthworms which show a responsive plasticity to flooding events.

Effects of AG1 and AG2 QTLs on Nonstructural Carbohydrate and Seed Management Options for Rice Seedling Growth and Establishment under Flooding Stress

Rice (Oryza sativa) plants acquired excess photosynthates in the form of nonstructural carbohydrates (NSCs) in their stems and grain. Despite keen interest in rice NSC, the dynamics of NSC accumulation, translocation and re-accumulation have not yet been well investigated. AG1 and AG2 QTLs associated with flooding tolerance through catalyzing starch into soluble sugar in germinating seeds. Here we conducted three experiments, greenhouse and field to lay the groundwork for large-scale diversity studies on grain NSC and some agronomic traits under direct-seeded rice (DSR) system, using elite lines incorporating AG1, AG2 and AG1-AG2 QTLs into the popular varieties PSB Rc82 and Ciherang-Sub1 along with the donors Kho Hlan On (AG1) and Ma-Zhan Red (AG2). In germinating seedlings, soluble sugars increased, while starch concentration decreased gradually especially in the tolerant checks and AG1-AG2 introgression lines under flooded soil. Soluble sugar accumulation in stem started to increase from the vegetative stage and peaked at the panicle initiation stage then gradually decreased towards the maturity stage. But Sub1-AG lines had higher sugar and starch concentrations at different growth stages than other genotypes in wet season 2016 and dry season 2017. Plant survival rate was positively correlated with the stem NSC at the early vegetative stage (21 days after sowing), and stem NSC was positively associated with plant height at different growth stages. Among the tested seeding rate, the most suitable seeding rate, 4g/m2 with shallow burial depth (0.5cm), resulted in better seedling establishment, relatively higher seedling vigor index and higher leaf area index under flooding in DSR system. Introgression of AG1-AG2 QTLs had no any negative impact on nonstructural carbohydrate, germination rate, and growth and biomass production.

Simultaneous measurement of aqueous redox-sensitive elements and their species across the soil-water interface

The redox-sensitive elements, such as iron, manganese, sulfur, phosphorus, and arsenic, shift their speciation every millimeter (mm) across the soil-water interface in the flooded soil environments. Monitoring of element speciation at this high-resolution (HR) within the SWI is still difficult. The key challenge lies in obtaining sufficient porewater samples at specific locations along the soil gradient for downstream analysis. Here with an optimized inductively coupled plasma mass spectrometry (ICP-MS) method and a HR porewater sampler, we demonstrate mm-scale element profiles mapping across the SWI in paddy soils. High-concentrations of iron and manganese (> 10 mg/L) were measured by ICP-MS in an extended dynamic range mode to avoid signal overflow. The iron profile along the SWI generated by the ICP-MS method showed no significant difference (p < 0.05) compared to that measured independently using a colorimetric method. Furthermore, four arsenic (arsenite, arsenate, monomethylarsonic and dimethylarsinic acid), two phosphorus (phosphite and phosphate) and two sulfur (sulfide and sulfate) species were separated in 10 min by ion chromatography -ICP-MS with the NH4HCO3 mobile phase. We verified the technique using paddy soils collected from the field, and present the mm-scale profiles of iron, manganese, and arsenic, phosphorus, sulfur species (relative standard deviation < 8%). The technique developed in this study will significantly promote the measurement throughput in limited samples (e.g. 100 μL) collected by HR samplers, which would greatly facilitate redox-sensitive elements biogeochemical cycling in saturated soils.

Assessing the habitat suitability of paddy fields for avian indicators based on hydropedological parameters of the wet agricultural soil along the Cauvery delta basin, India

Paddy fields provide a broad range of habitats to wintering waterbirds as natural wetlands have declined worldwide. However, the intensification of farming practices has altered the microclimates of the wet agricultural soil, the composition of soil invertebrates, thus indirectly affecting the avifauna. This study evaluated the importance of paddy fields as wintering grounds for two avian indicators Asian openbill stork (Anastomus oscitans), and the Black-headed ibis (Threskiornis melanocephalus) along the Cauvery delta basin, widely known as the granary of Tamil Nadu, South India. Several stretches of this agricultural belt have recorded a drastic reduction in crop cover thereby threatening the species that forage these mosaics. This entire delta region is under threat and is increasingly shrinking due to indiscriminate sand mining activities causing irreparable damage to soil, fauna, and agriculture. The erratic monsoons have forced wading species to initiate long migrations in response to climatic adaptations, thus impairing the existing population. The present study focused on the abundance patterns of two wading birds and their association with the hydropedological parameters of the flooded soil along the deltaic dynamic mosaic. The relative abundance of the avifauna was documented for 24 months using unlimited radius point counts. The hydropedological parameters recorded were pH, electrical conductivity, total alkalinity, calcium carbonate, organic matter, total organic carbon, nitrogen, phosphorus, potassium, chloride, sodium, magnesium, calcium, sulfur, moisture, the particle size distribution of sand, silt and clay, water holding capacity, and bulk density. The results indicate that paddy fields are excellent habitat for A. oscitans and T. melanocephalus and are strongly influenced by landscape characteristics, availability of feeding grounds, and hydropedological parameters of the wet soil. Among the hydropedological parameters, organic matter and nitrogen exhibited a negative correlation and the percent silt content showed a positive correlation to A. oscitans. Similarly, T. melanocephalus showed a negative correlation to chloride and sodium, and a positive correlation to nitrogen. The contrasting effects of hydropedological parameters could be explained by differential thermal tolerance levels and metabolic requirements of the species or may be mediated by changes in the macrofaunal composition. The wet agricultural soil exhibited fewer differences across salinity classes, particle size distribution especially silt and clay, organic matter, total organic carbon, pH and bulk density, and greater differences among calcium content, water holding capacity, the particle size distribution of sand, and electrical conductivity across delta zones. Our study highlights the need to consider hydropedological factors of the wet paddy soil when selecting conservation sites on agro-landscapes for migratory wetland birds. Agricultural policies should aim at considering these cultural landscapes along the delta region as high nature value avian farmlands under the protected agricultural Cauvery zone network to sustain biodiversity and vital ecosystem services of birds in the paddy fields.

Improved grain yield and lowered arsenic accumulation in rice plants by inoculation with arsenite-oxidizing Achromobacter xylosoxidans GD03

Arsenite is the predominant arsenic species in flooded paddy soil, and arsenite bioaccumulation in rice grains has been identified as a major problem in many Asian countries. Lowering arsenite level in rice plants and grain via accelerating arsenite oxidation is a potential strategy to help populations, who depended on rice consumption, to reduce the internal exposure level of arsenic. We herein isolated a strain, Achromobacter xylosoxidans GD03, with the high arsenite-oxidizing ability and plant growth-promoting traits. We observed that arsenite exposure could promote A. xylosoxidans GD03 to excrete indole-3-acetic acid and thus promoted rice growth. The pot culture experiments of Indica rice cultivar Guang You Ming 118 (GYM118) demonstrated that A. xylosoxidans GD03 inoculation of paddy soil (4.5–180 × 108 CFU GD03/kg soil) significantly accelerated arsenite oxidation in flooded soil. The daily arsenic oxidation rate with GD03 inoculation was 1.5–3.3 times as that without strain GD03 inoculation within the whole growth period of Indica GYM118 in the presence of the native microflora. It thus led to a 34–69%, 43–74%, 24–76% and 35–57% decrease in arsenite concentration of the stems, leaves, bran and grain of Indica GYM118 respectively and a 59–96% increase in rice grain yield. The paddy soil inoculated with 40.0 mL/kg of A. xylosoxidans GD03 resulted in a lowest As(III) concentrations in all rice organs of Indica GYM118, which equivalent to only 24–50% of the As(III) concentrations in the group without GD03 inoculation. The results highlight that a highly arsenite-oxidizing bacterium could accelerate arsenite oxidation of paddy soil when facing competition with the native microflora, thus decrease arsenic toxicity and bioavailable soil arsenic.

Physico chemical and biological properties of post flood soils in the south central laterites of southern Kerala

The south central laterites of Kottayam district was one of the worst hit areas during 2018 flood. The post-flood soils of the region required a site specific and detailed analysis of soil fertility parameters to understand the extent of damage. Surface soil samples were analysed for soil texture, particle density, WHC, pH, EC, organic carbon, available primary, secondary nutrients and acid phosphatase. The results showed that sandy clay loam was the predominant soil textural class in the area. Bulk density varied from 1.0 -1.6 Mg m-3 and maximum water holding capacity was less than 30 per cent for most of the area. Soils with moderately acidic pH increased compared to pre flood study. Organic carbon and available N were in high and medium range, respectively. The available P was reduced compared to pre flood soils while K was medium. Available Ca and S levels were sufficient. Mg deficiency was observed in 64.71 percentage of the samples in post flood area. A medium range of acid phosphatase activity was noticed in the region.

Salinity-independent dissipation of antibiotics from flooded tropical soil: a microcosm study

River deltas are frequently facing salinity intrusion, thus challenging agricultural production in these areas. One adaption strategy to increasing salinity is shrimp production, which however, heavily relies on antibiotic usage. This study was performed to evaluate the effect of increasing salinity on the dissipation rates of antibiotics in tropical flooded soil systems. For this purpose, paddy top soil from a coastal Vietnamese delta was spiked with selected frequently used antibiotics (sulfadiazine, sulfamethazine, sulfamethoxazole, trimethoprim) and incubated with flood water of different salt concentrations (0, 10, 20 g L−1). Antibiotic concentrations were monitored in water and soil phases over a period of 112 days using liquid chromatography and tandem mass spectrometry. We found that sulfamethazine was the most persistent antibiotic in the flooded soil system (DT50 = 77 days), followed by sulfadiazine (DT50 = 53 days), trimethoprim (DT50 = 3 days) and sulfamethoxazole (DT50 = 1 days). With the exception of sulfamethoxazole, the apparent distribution coefficient increased significantly (p < 0.05) for all antibiotics in course of the incubation, which indicates an accumulation of antibiotics in soil. On a whole system basis, including soil and water into the assessment, there was no overall salinity effect on the dissipation rates of antibiotics, suggesting that common e-fate models remain valid under varying salinity.

How do phytogenic iron oxide nanoparticles drive redox reactions to reduce cadmium availability in a flooded paddy soil?

While soil redox reactions are known to determine heavy metal soil availability, specific information on how iron (Fe) nanomaterials reduce heavy metal availability in bulk soil and in the rice rhizosphere is limited. Here a pot experiment was performed to examine the effect of phytogenic iron oxide nanoparticles (PION) on the availability of cadmium (Cd) in flooded soil. PION significantly reduced soil Cd availability, with Cd in rice shoot being 2.72, 1.21 and 0.40 mg kg−1 for the control, 1 and 5% PION treatments, respectively. In addition, following PION application, Illumina MiSeq sequencing indicated that the abundance of Lentimicrobium and Anaeromyxobacter increased, while the abundance of Geobacter and Thiobacillus decreased. Structural equation model analysis revealed that redox reactions, driven by carbon, nitrogen, iron and sulfur cycling related functional groups, played an important role in the immobilization of Cd in flooded soil. Co-occurrence network analysis showed that the rhizosphere soil was far more complex than the bulk soil. Overall, PION addition enhanced the inherent soil microbe’s activity and the involved in reducing Cd availability to rice by converting mobile Cd into stabler forms. This initial result paves the way for establishing a practical low-cost remediation strategy for Cd contaminated paddy soils.

Flood duration differentially affects seed germination, growth and ecophysiology of the swamp tree Citronella gongonha from the southern Neotropics

Citronella gongonha is an abundant tree species in flood-prone areas of southern South America. In swamp forests, young and adult individuals of the species are mainly distributed on hummocks. In this study, we address some aspects of the ecology and physiology of C. gongonha that contribute to the knowledge of its distribution in swamp forests. We evaluated seed germination and growth parameters, as well as chlorophyll, malondialdehyde, hydrogen peroxide, nitrate and carbohydrate levels in young C. gongonha plants during different periods of flooding. Germination did not occur under flooding conditions, and seeds persisted only up to 30 days of flooding, losing germination potential as submersion time increased. However, young plants survived a long period of flooding (until 120 days) by reducing growth, developing morphological adaptations and using carbohydrate pools. These results help to explain the distribution pattern of C. gongonha in swamp forests, predominantly on hummocks where soil flooding is attenuated.

Factors destabilizing the control of Monochoria vaginalis by rice bran: its conflicting powers influence both suppression and promotion of germination in paddy soil

ABSTRACT Organic paddy rice (Oryza sativa L.) cultivation can be negatively impacted because of the weeds growth like Monochoria vaginalis. Although the rice bran application is useful in combating weed, its performance is not consistent. In this study, laboratory experiments were conducted to elucidate the effects of rice bran on the suppression and promotion of germination of M. vaginalis. Suppression experiments involved incubating mixtures of flooded soil and rice bran. This was followed by a germination experiment in the collected soil solutions. A negative and significant correlation was observed between germination and the electric conductivity (EC) of the solution. None of the seeds germinated when the EC of the Gray Lowland soil was over 130 mS m−1. Conversely, the seeds in the Andosols’ solution germinated even when EC was over 130 mS m−1. Thus, the solutions of the Andosols had different mechanisms of suppression than the Gray Lowland soils. To understand the promotion of germination, air-dried soil was prepared after the flooded incubation of rice bran and soil mixture, simulating a long-term application of rice bran. Germination was analyzed in a filtrate of the mixture of air-dried soil and distilled water. The addition of rice bran significantly promoted germination. There was a negative and significant correlation between germination and dissolved oxygen (DO) suggesting that hardly decomposable components like antioxidants in rice bran decreased O2 and stimulated germination because the seeds of M. vaginalis require very little O2 for germination.

Ammonium aggravates salt stress in plants by entrapping them in a chloride over-accumulation state in an NRT1.1-dependent manner

Global climate change has exacerbated flooding in coastal areas affected by soil salinization. Ammonium (NH4+) is the predominant form of nitrogen in flooded soils, but the role played by NH4+ in the plant response to salt stress has not been fully clarified. We investigated the responses of Arabidopsis thaliana, Oryza sativa, and Nicotiana benthamiana plants fed with NH4+. All species were hypersensitive to NaCl stress and accumulated more Cl− and less Na+ than those fed with NO3−. Further investigation of A. thaliana indicated that salt hypersensitivity induced by the presence of NH4+ was abolished by removing the Cl− but was not affected by the removal of Na+, suggesting that excess accumulation of Cl− rather than Na+ is involved in NH4+-conferred salt hypersensitivity. The expression of nitrate transporter NRT1.1 protein was also up-regulated by NH4+ treatment, which increased root Cl− uptake due to the Cl− uptake activity of NRT1.1 and the absence of uptake competition from NO3−. Knockout of NRT1.1 in plants decreased their root Cl− uptake and retracted the NH4+-conferred salt hypersensitivity. Our findings revealed that NH4+-aggravated salt stress in plants is associated with Cl− over-accumulation through the up-regulation of NRT1.1-mediated Cl− uptake. These findings suggest the significant impact of Cl− toxicity in flooded coastal areas, an issue of ecological significance.

Dynamic release and transformation of metallic copper colloids in flooded paddy soil: Role of soil reducible sulfate and temperature

Mobile metal Cu colloids can be formed in periodically flooded paddy soils, potentially aggravating the risks to rice cultivated in these soils. Here, we investigated the formation and fate of Cu colloids in flooded soil as influenced by soil reducible sulfate and temperature. In microcosms with different initial sulfate availability (1.30, 5.34, or 7.38 mmol/kg), we found the treatments with higher sulfate concentrations showed the greater and faster release of metal colloids. Sulfate reduction resulted in the transformation of copper in the colloids from Cu(0) to CuxS, and the percentage of CuxS in the colloid phase increased with increasing sulfate content according to the Cu K-edge EXAFS spectra. The batch experiments incubated at 5, 25 or 35 °C proved that high temperature enhanced the microbial activity and released more Cu colloids during flooding. The colloid formation was delayed at low temperature but persisted longer in the soil, which led to greater particle average size because of slow growth and uniform agglomeration. Low temperature appeared to only influence the formation and growth but not the speciation of Cu colloids. Our results highlight the importance of soil reducible sulfate and temperature in mediating the dynamics of colloidal metals in flooded soil.

Wetting-drying cycles during a rice-wheat crop rotation rapidly (im)mobilize recalcitrant soil phosphorus

The residual phosphorus (P) in Hedley’s sequential fractionation procedure is considered to be a relatively stable soil P pool and unavailable for plant uptake. In the present study, we investigated the effect of wetting-drying events on the dynamics of the residual soil P fraction in a flooded rice and aerobic wheat rotation. Soils were taken from a long-term field trial after flooded (rice), and aerobic (wheat) crops were harvested. Hedley’s sequential fractionation method was used to assess the distribution of various P fractions. These changes in these P fractions were then related to Fe cycling in the soil. The residual P (H2SO4-H2O2 digested) was the dominant P fraction (37–51% of total P) in the aerobic soil under wheat, while it was decreased by 18–27% in flooded soil under rice cultivation. In contrast, the sparingly soluble Ca-bound P (HCl-Pi) increased from 25–31% under wheat cultivation to 41–50% under flooded rice (paddy) cultivation where reducing conditions are expected to prevail under submerged paddy soil conditions. The crop rotation not only altered the sparingly available P fraction but also influenced soil labile P, especially the organic P form. Compared with the rice soil, a 4-fold increase in the labile P fraction (NaHCO3-Po) was observed in wheat soil. The moderately labile P fraction (NaOH-extractable) showed a similar trend to that of labile P pool, but the increased NaOH-Po in wheat soil was relatively small. The relatively rapid change in the residual P fraction was attributed to oxidation-reduction cycles of Fe oxides between flooded (rice) and aerobic (wheat) soil conditions. Wetting and drying cycles associated with a rice-wheat crop rotation promoted the transformation of the sparingly soluble soil P fraction between crops, which was attributed to changes in soil redox conditions, particularly Fe cycling. This indicated that the rice-wheat crop rotation can draw upon the sparingly soluble P fraction for crop production, thus relying less on fertilizer-applied P.