Soil-water Interface Research Articles

Carbon-nutrient stoichiometry drives phosphorus immobilization in phototrophic biofilms at the soil-water interface in paddy fields

Phototrophic biofilms are distributed widely at the sediment/soil-water interfaces (SWI) in paddy fields, where they immobilize phosphorus, thereby reducing its runoff loss. However, how soil carbon, nutrient availability and nutrient ratios drive the phototrophic biofilm community and its contribution to phosphorus cycling is largely unknown. A large scale field investigation in Chinese paddy fields reported here shows that soil organic carbon (SOC) and soil total nitrogen (STN) contents rather than soil total phosphorus (STP) triggered phosphorus immobilization of paddy biofilms, as they changed algal diversity and EPS production. High C: P and N: P ratios favored phosphorus immobilization in biofilm biomass via increasing the abundance of green algae. The C: N ratio on the other hand had only a weak effect on phosphorus immobilization, being counteracted by SOC or STN. Results from this study reveal how the in-situ interception of phosphorus in paddy fields is driven by soil carbon, nutrient availability and nutrient ratios and provide practical information on how to reduce runoff losses of phosphorus by regulating SOC and STN contents.

Mathematical Modeling for the Absorption Capacity of Heavy Metals from the Soil in the Case of Phragmites Australis Plant Species

This paper proposes a tridimensional mathematical model of the absorption capacity of heavy metals (cadmium and nickel) from the soil in the case of Phragmites Australis plant species (the soil and plant samples was taken from six locations/areas along the Bistri�a and Crac�u Rivers, belonging to the Siret hydrographic basin). The variable measures taken into consideration when carrying out the experiments and realizing the mathematical model are the distance from the water-soil interface from which the plant samples were taken and the concentration of the heavy metals in the soil. The mathematical model was elaborated and tested by means of the TableCurve 3D program used for generating linear and non-linear equations. A very high absorption capacity of cadmium from the soil was recorded A very high absorption capacity of cadmium from the soil was recorded, in the case of Phragmites Australis plant species (190�234 % higher than in the soil). The correlation coefficient of the mathematic model was between 0.90 and 0.98.

Interpreting redox potential (Eh) and diffusive fluxes of phosphorus (P) and nitrate (NO3−) from commercial rice grown on histosols

In South Florida, approximately 11,000 ha of rice is grown every summer on highly organic histosols. With no added fertilizer inputs of phosphorus (P) or nitrogen (N) as part of the cultivation program, growing flooded rice has the potential to reduce nutrient fluxes from soils to surface water and limit its losses to the water column. Changes in soil redox potential (Eh) will play an important role in the availability of P and nitrate (NO3−) concentration within the groundwater, which ultimately drives the direction of diffusive fluxes across the soil–water interface. The objective of this study was to measure vertical Eh and groundwater concentrations of P and NO3− from commercial flooded rice fields and estimate diffusive fluxes across the soil–water interface. Results indicate that Eh was depth dependent, where deeper soils (30 cm deep) were slightly more anaerobic (mean − 162 mV) than shallow soils (15 cm deep) (mean − 144 mV). Groundwater P concentrations ranged between 0.38 and 1.09 mg L−1 in deep soils (up to 60 cm) and 0.018–0.608 mg L−1 in shallow soils (up to 40 cm). Groundwater NO3− concentrations ranged between 0.065 and 0.64 mg L−1 in deep soils (up to 60 cm) and 0.055–0.499 mg L−1 in shallow soils (up to 40 cm). Diffusive fluxes were mostly positive, which meant they flowed from shallow groundwater to the water column. Phosphorus fluxes were as high as 0.134 mg m−2 d−1 observed in 60-cm deep soils, whereas NO3− fluxes were as high as 0.12 mg m−2 d−1 observed in 40-cm shallow soils. Diffusive fluxes of P and NO3− from flooded rice fields were relatively lower compared to other hydrologic systems.

Effects of dissolved organic carbon on desorption of aged phenanthrene from contaminated soils: A mechanistic study

Dissolved organic carbon (DOC) has a major influence upon sorption/desorption and transport of hydrophobic organic contaminants (HOCs) in soil environments. However, the molecular mechanisms of DOC sorption and its effects on aged HOC desorption in contaminated soils still remain largely unclear. Here, effects of three different DOC (one from commercial peat and two from biochars produced at 300 °C and 500 °C pyrolysis temperatures, respectively) and oxalate (as a reference) on abiotic desorption behavior of aged phenanthrene from three agricultural soils were investigated. Results showed that desorption of aged phenanthrene from soils was predominantly dependent on soil organic carbon content. The presence of DOC and oxalate resulted in higher desorption of phenanthrene compared to water alone, and the effects were positively related to soil organic carbon content and DOC/oxalate concentration. The facilitating effects of DOC were further increased during the second consecutive desorption, whereas oxalate had no such effect. Ultra-high-resolution Fourier transform-ion cyclotron resonance-mass spectrometry confirmed the molecular fractionation of DOC at the soil-water interface during DOC sorption. Specifically, the DOC molecules with O-rich moieties were preferentially adsorbed, whereas the molecules with phenolic and aromatic structures were selectively retained in the soil solutions through competitive displacement and co-sorption reactions during sorption. The enriched phenyl structures in the retained DOC facilitated its association with phenanthrene in the solutions and thus the release of phenanthrene from the soils. In contrast, oxalate replaced some organic carbon from the soils and thus released the associated phenanthrene into the solutions. Our findings highlight the importance of the molecular composition and structure of DOC for the desorption of phenanthrene in soil-water environments, which may help improve our understanding of the release and transport of organic compounds in the environments.

Methane emission and soil microbial communities in early rice paddy as influenced by urea-N fertilization

Urea is widely used in rice farming systems to improve the nitrogen (N) availability and crop yield, yet its influence on CH4 emissions remains unclear. The present study aimed to identify the potential microbial mechanisms resulting in the N fertilizer-induced effects on CH4 emissions from paddies. A field experiment was conducted with (N1, 100 kg N ha−1) and without (N0) N fertilization. CH4 emission rate was measured every 3–7 day using the static chamber-gas chromatography method. Quantitative PCR (qPCR) and Illumina MiSeq sequencing approaches were employed to investigate the community composition and structure of methanogenic archaea and methanotrophic bacteria in rice rhizosphere soil at three time points (tillering stage 17d, heading stage 47d and mature stage 67 d). Urea application significantly stimulated the CH4 emissions from paddies, partially due to the decrease of redox potential (Eh) around the root region and the reduction of dissolved oxygen (DO) concentration at the soil-water interface. qPCR indicated a sharp increase in methanogenic archaeal 16S rRNA and methanotrophic bacterial pmoA gene copies with urea fertilization. Next-generation sequencing of these amplicons revealed that the diversity of both methanogenic and methanotrophic communities was decreased by urea input. Additionally, urea fertilization stimulated the genera Methanoregula (methanogen) and Methylococcus (type I methanotrophs) and inhibited the genus Methylocystis (type II methanotrophs). Urea fertilization changed oxygen state of the paddy soil and water and thus changed the community structure of methanogens and methanotrophs, which resulted in increased CH4 emissions.

Tracing the Dynamic Changes of Element Profiles by Novel Soil Porewater Samplers with Ultralow Disturbance to Soil-Water Interface.

In flooded soils, soil–water interface (SWI) is the key zone controlling biogeochemical dynamics. Chemical species and concentrations vary greatly at micro- to cm-scales. Techniques able to track these changing element profiles both in space and over time with appropriate resolution are rare. Here, we report a patent-pending technique, the Integrated Porewater Injection (IPI) sampler, which is designed for soil porewater sampling with minimum disturbance to saturated soil environment. IPI sampler employs a single hollow fiber membrane tube to passively sample porewater surrounding the tube. When working, it can be integrated into the sample introduction system, thus the sample preparation procedure is dramatically simplified. In this study, IPI samplers were coupled to ICP-MS at data-only mode. The limits of detection of IPI-ICP-MS for Ni, As, Cd, Sb, and Pb were 0.12, 0.67, 0.027, 0.029, and 0.074 μg·L–1, respectively. Furthermore, 25 IPI samplers were assembled into an SWI profiler using 3D printing in a one-dimensional array. The SWI profiler is able to analyze element profiles at high spatial resolution (∼2 mm) every ≥24 h. When deployed in arsenic-contaminated paddy soils, it depicted the distributions and dynamics of multiple elements at anoxic–oxic transition. The results show that the SWI profiler is a powerful and robust technique in monitoring dynamics of element profile in soil porewater at high spatial resolution. The method will greatly facilitate studies of elements behaviors in sediments of wetland, rivers, lakes, and oceans.

Iron plays an important role in molecular fractionation of dissolved organic matter at soil-water interface

Adsorption of dissolved organic matter (DOM) onto soils plays an important role in the mobility and stabilization of organic carbon in soils; however, little attention has been paid to changes in the molecular components of soil DOM during adsorption on soils. In the present study, molecular fractionation of DOM induced by adsorption on a red soil was investigated using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results indicated that compounds high in unsaturation or polarity or rich in oxygen had a high affinity to soil surfaces, while aliphatic compounds with few oxygenated groups and low polarity compounds were preferentially retained in solution. Among soil fractions with different particle sizes, the fine clay fraction with high iron content and surface area was the main contributor to the adsorptive fractionation of DOM. Comparison of the molecular fractionation of DOM derived from adsorption on soil with iron removed and on soil minerals with various iron contents and surface areas further indicated that iron containing minerals in the soil provided the major adsorptive sites and determined the molecular fractionation of DOM at the soil-water interface. The results provide molecular information for further understanding mechanisms underlying the persistence and mobility of organic carbon in soils.

Effects of Citrate on the Rates and Mechanisms of Phosphate Adsorption and Desorption on a Calcareous Soil

Core Ideas Citrate and Ca influence the P sorption in calcareous soil. Labile Ca influences the citrate‐enhanced P desorption. The soil P reactions obey the pseudo‐second‐order kinetics. P sorption in calcareous soil is Ca‐concentration dependent. Adsorption at the soil–water interface is a fundamental chemical process that often controls the retention of compounds in soils. The rates of adsorption therefore have a vital role in the fate and transport of phosphate in the soil environment. In this study, the effects of a range of organic acid (citrate) levels and soil Ca levels on phosphate (PO43−) adsorption and desorption were investigated in detail through a combination of laboratory (miscible displacement kinetics) and synchrotron‐based X‐ray absorption spectroscopy. A pseudo‐second‐order kinetics model was applied to the data that provided good fits for the entire datasets (both adsorption and desorption) with a single set of equations. In general, it was observed that citrate directly competed for adsorption sites in soils with reduced levels of Ca but in soils high in labile Ca, citrate was not competitive in desorbing P. The most likely explanation for this is that citrate instead chelates with Ca to form soluble complexes when an excess of Ca is present.

Molecular understanding of dissolved black carbon sorption in soil-water environment

Dissolved black carbon (DBC) involves in many biogeochemical processes in both terrestrial and aquatic environments. About 26.5 Tg of charcoal- or black carbon-derived DBC was released into aquatic environments annually, accounting to ∼10% of the global riverine flux of dissolved organic carbon (DOC). Yet the sorption behaviors of DBC and their effects on water quality in soil-water environment are poorly understood. Here we examined the molecular composition variations of DOC induced by the sorption of two biochar-derived DBCs (pyrolyzed at 300 °C and 500 °C) on three contrasting soils. The DBCs were adsorbed mainly through competitive displacement of soil surface functional groups and co-sorption with soil indigenous DOC, which varied with soil properties and the aromaticity of the DBCs. Ultrahigh resolution mass spectrometry analysis indicated that compounds with rich oxygen content or unsaturated structures such as tannins and unsaturated aromatics from both DBC and soil DOC, were preferentially adsorbed on the soils in the presence of DBC. In contrast, compounds with high aromatic structures including condensed aromatics and lignins were concentrated in the aquatic phase. Molecular fractionation also occurred to the heteroatomic compounds during the sorption, and the heteroatomic dissolved organic sulphur in the DBCs was easier to be adsorbed relative to dissolved organic nitrogen. Our results suggest that DBC sorption in soil-water environment could have important implications for water quality by altering DOC molecular composition and decreasing DOC molecular diversity at the soil-water interface. This study provides essential information for understanding the behavior of DBC in the environments.

Long-term outdoor lysimeter study with cerium dioxide nanomaterial

Fate and impact of cerium dioxide nanomaterials (CeO2 NM) in soil remains uncertain. Most of the recent environmental studies used high doses of CeO2 NM in short-term laboratory experiments and stated soil as large sink for NM. Recent studies covering the life cycle of plants found evidence for particle uptake in crop plants, and triggered concern about NM entering the food chain. Here, we present a 25 months outdoor lysimeter study that investigates the translocation, biological impact, and transformation of CeO2 NM in soils cultivated with different crops (wheat (Triticum aestivum L.), canola (Brassica napus L.), and barley (Hordeum vulgare L.)). Low doses of CeO2 NM (NM-212, 10 and 50 mg CeO2 NM kg−1 soil) were applied to a Cambisol by sewage sludge or artificial rainwater. To simulate ploughing, the CeO2 NM was mixed into the first 20 cm of the lysimeter soil. Three weeks after the NM application by sewage sludge and five days after the latest NM application by artificial rainwater wheat seeds were sown.We found no vertical Ce translocation to deeper soil layers as well as negligible release of Ce into percolating water, confirming that soil is a sink for CeO2 NM. Furthermore, no inhibition of the NM sensitive ammonium oxidizing bacteria (AOB) was observed which implies a low bioavailability of CeO2 NM in soil. The detected Ce root uptake for canola (Ce < 35.7 mg kg−1) and barley (Ce < 61.8 mg kg−1) indicates that physico-chemical conditions in the rhizosphere or the aging of the NM in soil can remobilize the applied CeO2 NM to some extent. One can speculate that the obtained reduction of Ce4+ to Ce3+ (≈ 50% after 25 months) was coupled to different conditions (ionic strength, pH, water content, soil organic matter content) in the rhizosphere and necessary for the root accumulation of Ce.Due to negligible mobility in the soil-water interface, as well as non-toxicity to AOB, the environmental risk of the tested CeO2 NM appears to be low, compared to other highly mobile emergent pollutants. Nonetheless, further long-term investigations need to focus on the speciation and localization of NM in plants to clarify uptake and impact of NM to crops.

The characteristics of sediment pollution and the identification of oxidation layer under long-term operation of large-scale surface flow constructed wetlands

研究湿地底泥污染特性及水-土界面边界层性质是明确大型人工湿地长期运行下底泥内源污染问题的关键.本文以典型人工湿地为例，通过综合布点垂向分层监测湿地底泥理化性质，明确底泥污染特性，分析水-土界面边界氧化层现象、性质及形成机理，探讨其性质成因.结果表明：与湖泊类似，长期运行人工湿地底泥也会产生氧化层，垂向由上到下分为氧化层、污染层、过渡层和健康层；运行6年的湿地底泥可形成厚约1 cm、无明显臭味、棕黄色可塑状的氧化层；与污染层相比，氧化层色味变淡、流动性减弱，含水率、d（0.9）、亚铁、总有机碳和总氮含量分别降低7.20%、54.04%、54.59%、17.89%和7.00%，氧化还原电位和总磷含量分别升高150.41%和18.17%；氧化层是由氧化、沉降协同作用下形成的底泥表层氧化态微环境，其中上覆水溶解氧水平越高越有利于提高氧化层致密程度，降低氧化层有机质、总氮含量，上覆水悬浮物浓度越高、水深越深越有利于增加氧化层厚度.;The study on the characteristics of sediment pollution and the boundary layer of water-soil interface is the key to identify the problem of sediment pollution under the long-term operation of large constructed wetlands. This paper takes a typical surface flow constructed wetland as an example, through comprehensive vertical layered monitoring of the physicochemical properties of the wetland sediment, to clarify the pollution characteristics of the sediment and analyzed the phenomenon, nature and formation mechanism of the boundary oxidation layer of the water-soil interface. The results show that, similar to lakes, the sediment of constructed wetland in long-term operation also generates oxidation layer, which is divided into oxidation layer, pollution layer, transition layer and health layer from top to bottom. After 6 years of operation, the wetland sediment can form an oxidation layer, which is about 1 cm thick, brown-yellow, malleable, with no obvious odor. Compared with the pollution layer, the color and smell of the oxidation layer are weakened, and its fluidity is weakened. Meanwhile, the water content, d(0.9), diatomic iron, total organic carbon and total nitrogen are decreased by 7.20%, 54.04%, 54.59%, 17.89% and 7.00%. Eh and total phosphorus are increased by 150.41% and 18.17%, respectively. Oxidation layer is a micro-environment of surface oxidation state formed under the synergistic action of oxidation and settlement. The higher the dissolved oxygen level of overlying water is, the more favorable it is to increase the density of the oxidation layer and reduce the organic matter and total nitrogen content of the oxidation layer. In addition, the higher the concentration of overlying water suspended substance and the deeper the water depth, the more favorable it is to increase the thickness of the oxidation layer.

Effects of artificial aeration and iron inputs on the transformation of carbon and phosphorus in a typical wetland soil

Artificial aeration changes the redox conditions at the soil surface. The introduction of iron (Fe) into wetlands can influence carbon (C) and phosphorus (P) cycling under the fluctuating redox conditions. However, artificial Fe introduced into wetlands is uncommon, and there are no Fe dose guidelines. We compared aerobic and anaerobic conditions to test the hypothesis that Fe addition can, although redox-dependent, affect P forms and the coupling of organic C. Twenty-four intact soil cores were collected randomly from a lacustrine wetland of Lake Xiaoxingkai. And representative and homogeneous seedlings of Glyceria spiculosa were collected. The incubation was designed with two treatment factors: Fe/P ratio (5 or 10) and high and low dissolved oxygen (DO) concentrations (> 6 and < 2 mg L−1, respectively). Four groups with three replicates were separated randomly and labeled as aerobic + plant treatment, anaerobic + plant treatment, and aerobic or anaerobic treatment (control). The DO concentrations were stratified, decreasing with soil depth and increasing with time, especially under aerobic conditions. The Eh values generally increased with fluctuations under aerobic conditions. The artificial aeration substantially changed the redox environment at the water–soil interface. Of the total P, 45% was in the reactive Fe-bound P, indicating that Lake Xiaoxingkai had high internal P loading. No significant differences were observed in total Fe, amorphous Fe, and organic C at the soil surface between the two Fe/P ratios; however, a significant difference in free Fe was observed. And soil amorphous Fe was found to be a significant correlation with soil organic C, indicating that iron oxides were related with the soil chemical properties. After short-term incubation, Fe addition can affect the cycling of major elements in wetlands, although this effect is redox dependent. Excessive Fe doses may result in regional environmental risks, such as eutrophication and C sinks of wetland ecosystems. Large-scale controlled experiments are needed to fully understand the behaviors of soil elements in wetlands.

An Improved SWAT for Predicting Manganese Pollution Load at the Soil-Water Interface in a Manganese Mine Area

An Improved SWAT for Predicting Manganese Pollution Load at the Soil-Water Interface in a Manganese Mine Area

Bioavailability of Antibiotics at Soil-Water Interfaces: A Comparison of Measured Activities and Equilibrium Partitioning Estimates.

There are growing concerns that antibiotic pollution impacts environmental microbiota and facilitates the propagation of antibiotic resistance. However, the prediction or analytical determination of bioavailable concentrations of antibiotics in soil is still subject to great uncertainty. Biological assays are increasingly recognized as valuable complementary tools that allow a more direct determination of the residual antibiotic activity. This study assessed the bioavailability of structurally diverse antibiotics at a soil-water interface applying activity-based analyses in conjunction with equilibrium partitioning (EqP) modeling. The activity against Gram-positive and Gram-negative bacteria of nine antibiotics from different classes was determined in the presence and absence of standard soil (LUFA St. 2.2). The addition of soil affected the activity of different antibiotics to highly varying degrees. Moreover, a highly significant correlation ( p < 0.0001) between the experimentally observed and the EqP-derived log EC50 (half-maximal effective concentration) values was observed. The innovative experimental design of this study provided new insights on the bioavailability of antibiotics at soil-water interfaces. EqP appears to be applicable to a broad range of antibiotics for the purpose of screening-level risk assessment. However, EqP estimates cannot replace soil-specific ecotoxicity testing in higher-tier assessments, since their accuracy is still compromised by a number of factors.

Wien effect of Cd/Zn on soil clay fraction and their interaction

BackgroundThe coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. While many studies have been done on the mutual toxicity of Cd2+ and Zn2+, few studies can be found in the literature focused on the interaction of Cd2+ and Zn2+ on soil clay fractions especially in terms of energy relationship.ResultsThe binding energies of Cd2+ on boggy soil (Histosols) particles and Zn2+ on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd2+ and Zn2+ on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd2+ and Zn2+ have a strong capacity to adsorb in the solid phase at the soil–water interface of boggy soil and yellow brown soil, respectively. However, both Cd2+ and Zn2+ adsorbed on paddy soil particles easily release into the solution of the soil suspension. Unlike the binding energy, the higher adsorption energies of ions in boggy and yellow brown soils showed a weak binding force of ions in boggy soil and yellow brown soil. A 1:1 ratio of Cd2+ to Zn2+ promotes the mutual inhibition of their retentions. Cd2+ and Zn2+ have high mobility and bioavailability in paddy soil and yellow drab soil (Ustalfs), whereas they have high potential mobility and bioavailability in boggy soil and yellow brown soil.ConclusionIn the combined system, Zn2+ had preferential adsorption than Cd2+ on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd2+ and Zn2+ while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd2+ and Zn2+ restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+–Zn2+.

The relative contributions of plant quality, simulated rising temperature, and habitat to litter decomposition.

With litter bag methods, we examined mass loss rates and different chemical fractions of litters from two wetland plant species, Zizania caduciflora and Hippuris vulgaris. Those two species examined here varied significantly in their initial litter chemical traits. Experiment was performed under simulated rising temperature (1.5-2.0 ℃), and under three different habitats (air, air-water interface and water-soil interface). The results showed that, during one-year decomposition period, the mass resi-dual rates exhibited distinct seasonal dynamics, and there were strong interactive effects between seasonal dynamics and environmental factors. Different factors contributed differently for the variation of litter decomposition, 28.8% of which being explained by litter quality, 6.3% of which being explained by rising temperature, and 34.9% being explained by habitat. Along with the decomposition, the contents of different chemical fractions (easy or hard to decompose) varied greatly. Among them, nitrogen contents in H. vulgaris decreased by 53.1%, while the lignin contents increased by 45.4%. Overall, habitat was the most important factor driving litter decomposition, the second was litter quality, and rising temperature had minor effect.

A review of shampoo surfactant technology: consumer benefits, raw materials and recent developments.

Surfactants form the core of all shampoo formulations, and contribute to a wide range of different benefits, including cleansing, foaming, rheology control, skin mildness and the deposition of benefit agents to the hair and scalp. The purpose of this review was to assist the design of effective, modern, shampoo surfactant technologies. The mechanisms through which surfactants help deliver their effects are presented, along with the appraisal techniques through which surfactant options can be tested and screened for product development. The steps that should be taken to select the most appropriate blend of surfactants are described, and useful information on the most widely used surfactants is provided. The review concludes with an examination of recent developments in 'greener' surfactants, 'sulphate-free' technologies and structured liquid phases for novel sensory properties and for suspending benefit agents.

Experimental study on impact behavior of submarine landslides on undersea communication cables

Submarine landslides, which are characterized by large scale and long run-out distance, could destroy undersea communication cables, thus resulting in a great number of economic losses. In this work, an experimental apparatus is designed to simulate the relative motion between the submarine landslide and undersea communication cable, and investigate the impact behavior of a submarine landslide. The test results show that submarine landslide with larger thickness and larger particle size can generate larger impact force on the communication cables. With an increase of the sliding velocity, the evolution of the flow behavior and impact force of the soil-water mixture can be divided into three stages: 1) landslide stage, the mixture consists a water layer and a sand layer, the impact force increases with the sliding velocity and reaches a peak value at this stage; 2) transforming stage, sand particles start to be eroded by water, and result into a turbidity current layer above the soil-water interface. At this stage, the impact force decreases to a minimum value; 3) turbidity current stage, all of the sand particles are eroded by water, and the impact force increases again when increasing the velocity continuously.

An Isotopic Dilution Approach for Quantifying Mercury Lability in Soils

The accurate estimation of soil mercury lability is crucial for risk assessment. In comparison to chemical fractionation and speciation, isotopic dilution (ID) offers precise definition of labile mercury fractions while maintaining the natural equilibrium. We developed and applied an ID protocol with 199Hg to estimate the soil mercury (Hg) isotopically exchangeable (labile) pool or HgE using a range of industrially contaminated soils in Switzerland. The measured HgE values were consistent for the same soil against different spike levels (50, 100, and 200% of native 199Hg), indicating that the spiked and soil isotopes achieved required dynamic equilibrium at the soil–water interface. Total soil Hg (THg; mg kg–1) was the best predictor of HgE (mg kg–1) and %HgE and accounted for 96 and 63% of the variance, respectively. Nonetheless, despite the wide range of THg values (0.37–310 mg kg–1) in the studied soils, Hg lability spanned a narrow range (∼12–25% of THg), highlighting the large capacity of soils to se...

Azolla cover significantly decreased CH4 but not N2O emissions from flooding rice paddy to atmosphere

ABSTRACTAzolla (Azolla filiculoides) is a common aquatic fern that has been used successfully as a dual crop with lowland rice. It grows rapidly and has the ability to fix N2 for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. To investigate the effect of azolla cover on methane (CH4) and nitrous oxide (N2O) emissions from rice paddy, a pot experiment with two treatments, control (rice plant only) and azolla cover (rice plus azolla covering on the flooding water), was carried out in Tsuruoka, Yamagata, Japan, in 2016. The results showed that the rice growth parameters, like shoot height, maximum and productive tiller numbers, and plant biomass were not significantly different between the two treatments. Dual cropping of azolla with rice significantly suppressed CH4 emissions, likely due to an increase in dissolved oxygen concentration and redox potential at the soil-water interface between flooding water and soil surface. There were significant (P < 0.05) positive correlations between CH4 flux and night respiration (CO2 emissions) between the two treatments. The cumulated CH4 emissions during the growth period until 106 days after transplanting (DAT) was significantly lower at 36.2 g C m−2 from azolla cover treatment than that from control treatment pot at 55.4 g C m−2. A prolonged nonsignificant N2O emission under the azolla cover treatment after the initial highest peak at 15 DAT was recorded due to denitrification of the nitrate in initial soil. No further N2O emissions were recorded thereafter from both treatments. Azolla cover did not affect N2O emissions from both treatments.