Non-flooded Conditions Research Articles

Not just a methane source: Amazonian floodplain sediments harbour a high diversity of methanotrophs with different metabolic capabilities.

The Amazonian floodplain forests are dynamic ecosystems of great importance for the regional hydrological and biogeochemical cycles and function as a significant CH4 source contributing to the global carbon balance. Unique geochemical factors may drive the microbial community composition and, consequently, affect CH4 emissions across floodplain areas. Here, we report the in situ composition of CH4 cycling microbial communities in Amazonian floodplain sediments. We considered how abiotic factors may affect the microbial community composition and, more specifically, CH4 cycling groups. We collected sediment samples during wet and dry seasons from three different types of floodplain forests, along with upland forest soil samples, from the Eastern Amazon, Brazil. We used high-resolution sequencing of archaeal and bacterial 16S rRNA genes combined with real-time PCR to quantify Archaea and Bacteria, as well as key functional genes indicative of the presence of methanogenic (mcrA) and methanotrophic (pmoA) microorganisms. Methanogens were found to be present in high abundance in floodplain sediments, and they seem to resist the dramatic environmental changes between flooded and nonflooded conditions. Methanotrophs known to use different pathways to oxidise CH4 were detected, including anaerobic archaeal and bacterial taxa, indicating that a wide metabolic diversity may be harboured in this highly variable environment. The floodplain environmental variability, which is affected by the river origin, drives not only the sediment chemistry but also the composition of the microbial communities. These environmental changes seem also to affect the pools of methanotrophs occupying distinct niches. Understanding these shifts in the methanotrophic communities could improve our comprehension of the CH4 emissions in the region.

Sulfur amendments to soil decrease inorganic arsenic accumulation in rice grain under flooded and nonflooded conditions: Insights from temporal dynamics of porewater chemistry and solid-phase arsenic solubility.

Rice cultivation under flooded conditions enhances arsenic (As) solubility and favors As accumulation in rice grain that poses an indisputable threat to human health worldwide. The reduction of sulfur may induce processes that decrease As solubility, but its impact on rice grain As species remains unresolved. Herein, we investigated the influence of sulfur (S)-containing materials, including chicken manure and elemental sulfur powder on As accumulation and speciation in rice grain as well as the dynamics of the porewater chemistry and solid-phase As solubility throughout the entire growth stage under continuous flooding and intermittent flooding conditions in pot experiments. The S amendments (200mgSkg-1) to the soil significantly decreased inorganic As in rice grain under continuous flooding (~65% decrease) as well as under intermittent flooding (~70% decrease). The chicken manure amendment promoted sulfur reduction and enhanced dissolvable Mn, Fe, and As at an earlier growth stage. The sequential extraction results corroborated a decrease in the soluble and exchangeable As (F1) and an increase in residual As (F5) fractions in the S-amended treatments. Solubility data suggested that As adsorption onto Fe oxides was the primary mechanism controlling As solubility rather than the formation of AsFe sulfides. Porewater As, considered to represent the most bioavailable As fraction, failed to explain the grain As accumulation. The time-averaged concentration of oxalate-extractable As explained grain arsenite best, suggesting that poorly crystalline Fe oxides may be the primary dissolvable reactive phases that control As bioavailability in the soil-rice system. Our results suggest that the application of S-containing soil amendments can effectively decrease inorganic As accumulation in rice grains grown under the flooded conditions, which are most widely applied in paddy rice production.

Plastic in global rivers: are floods making it worse?

Riverine plastic pollution is of global concern due to its negative impact on ecosystem health and human livelihood. Recent studies show a strong link between river discharge and plastic transport, but the role of floods is still unresolved. We combined high-resolution mismanaged plastic waste data and river flood extents with increasing return periods to estimate flood-driven plastic mobilisation, from local to global scale. We show that 10 year return period floods already tenfold the global plastic mobilisation potential compared to non-flood conditions. In the worst affected regions, plastic mobilisation increases up to five orders of magnitude. Our results suggest a high inter-annual variability in plastic mobilisation, previously ignored by global plastic transport models. Flood defences reduce plastic mobilisation substantially, but regions vulnerable to flooding often coincide with high plastic mobilisation potential during floods. Consequentially, clean-up and mitigation measures and flood risk management are inherently interdependent and need to be managed holistically.

The effect of Fenton reaction using H2O2 and water control on the distribution and accumulation of As speciation within the soil-rice system

The behavior of arsenic (As) in paddy soil is of great interest because rice is an efficient As accumulator, which can result in potential health risks to humans. In this study, we investigated the distribution and translocation of As speciation within the soil-rice system in the presence of Fenton solution (100 μM H2O2 and 100 μM Fe2+). The results showed that the iron plaque concentrations were enhanced either by the addition of Fenton solution or under flooded condition. In addition, total As, As5+, and As3+ concentrations were lower in rice tissues treated with Fenton solution than those without Fenton solution treatments. Therefore the addition of Fenton solution can reduce As speciation accumulation in rice tissue. This study revealed the function of Fenton solution on the fate of As speciation distribution within soil-rice system and the link between iron plaque and As accumulation in rice tissue under flooded and non-flooded conditions.

生物炭添加对不同水氮条件下芦苇生长和氮素吸收的影响

生物炭能改良土壤从而促进植物生长和氮素吸收，但其作用效果是否受水氮条件的影响尚不清楚。以湿地植物芦苇为研究对象，在3种氮添加水平（无添加，30 kg hm^-2 a^-1和60 kg hm^-2 a^-1）和两种水分（淹水和非淹水）条件下分别进行生物炭添加和不添加处理，结果表明：（1）生物炭添加能促进芦苇根系生长，在非淹水条件下根系生物量增加了40.5%，在淹水条件下根系生物量增加了20.1%。（2）生物炭添加能促进非淹水条件下芦苇的氮素吸收，能提高淹水条件下芦苇的氮素生产力。（3）生物炭添加加剧了土壤氮素损失，且在非淹水高氮条件下作用最强，可能是由于生物炭促进了芦苇的氮素吸收。芦苇氮素吸收速率与土壤氮损失之间存在显著的正相关关系。因此，在添加生物炭时，需要考虑土壤水分状况和氮素富集程度以及植物的氮素吸收偏好。该研究结果可为生物炭在湿地生态系统中的应用提供参考。;Nitrogen is an essential element for plant growth and is closely related to plant life activities. In most terrestrial ecosystems, plant growth is limited by nitrogen availability. Therefore, it is important to improve plant productivity in terrestrial systems by increasing the nitrogen uptake capacity of plants. Biochar is regarded as a kind of soil conditioner produced by pyrolysis of biomass under high temperature and hypoxia conditions. It has attracted more and more attention because of its important role in promoting crop yield, reducing greenhouse gas emissions and environmental pollution control in recent years. Most previous studies have applied biochar in dryland ecosystems, but few in wetland ecosystem. Though biochar has been proved to be beneficial of plant growth and nitrogen uptake, but whether it is influenced by different nitrogen availability and water conditions is still unclear. Therefore, we conducted a greenhouse experiment to investigate the effects of biochar addition on the growth and nitrogen absorption of Phragmites australis with three nitrogen additions (no addition, 30 kg hm^-2 a^-1, 60 kg hm^-2 a^-1) and two water treatments (inundation and no inundation). The results showed that:(1) biochar addition promoted the root growth of P. australis, increased the root biomass by 40.5% in the non-flooded treatment and by 20.1% in the flooded treatment, but have no significant effects on biomass of other parts and total biomass. (2) Biochar addition could improve the nitrogen absorption rate in the non-flooded condition, increased the nitrogen absorption rate by 23% in average. Biochar addition also enhanced the nitrogen productivity of P. australis in the flooded condition by 22% in average. (3) Biochar significantly decreased the soil ammonium-N, nitrate-N and mineral nitrogen concentrations, increased the soil nitrogen loss, and the effects were the strongest in the non-flooded and high nitrogen treatments because biochar promoted nitrogen absorption rate of P. australis. There was a significantly positive correlation between nitrogen absorption rate of P. australis and soil nitrogen loss. Our study shows that biochar has different effects on growth, nitrogen absorption and utilization of P. australis in different soil nitrogen availability and water treatments, and the difference may be related to the absorption preference of plants on different nitrogen chemical forms. Therefore, it is necessary to comprehensive balance soil water status, nitrogen enrichment and nitrogen absorption preference of plants when applying biochar in field. This result can practically provide scientific support for the application of biochar in the wetland ecosystem.

Flood Extent Mapping in the Caprivi Floodplain Using Sentinel-1 Time Series

Deployment of Sentinel-1 (S1) satellite constellation carrying a C-band synthetic aperture radar (SAR) enables regular and timely monitoring of floods from their onset until returning to nonflooded (NF) conditions. The major constraint on using SAR for near-real-time (NRT) flood mapping has been the inability to rapidly process the obtained imagery into reliable flood maps. This study evaluates the efficacy of S1 time series for quantifying and characterizing inundation extents in vegetated environments. A novel algorithm based on statistical time-series modeling of flooded (F) and NF pixels is proposed for NRT flood monitoring. For each new available S1 image, the probability of temporarily F conditions is tested against that of NF conditions by means of likelihood ratio tests. The likelihoods for the two conditions are derived from early acquisitions in the time series. The algorithm calibration consists of adjusting two likelihood ratio thresholds to match the reference F area extent during a single flood season. The proposed algorithm is applied to the Caprivi region, the resulting maps were compared to cloud-free Landsat-8 (LS8) derived maps captured during two flood events. A good spatial agreement (85-87%) between LS8 and S1 flood maps was observed during the flood peak in both 2017 and 2018 seasons. Significant discrepancies were noted during the flood expansion and recession phases, mainly due to different sensitivities of the data sources to the emerging vegetation. Overall, the analysis shows that S1 can stand as an effective standalone or gap-filling alternative to optical imagery during a flood event.

InSAR Multitemporal Data over Persistent Scatterers to Detect Floodwater in Urban Areas: A Case Study in Beletweyne, Somalia

A stack of Sentinel-1 InSAR data in an urban area where flood events recurrently occur, namely Beletweyne town in Somalia, has been analyzed. From this analysis, a novel method to deal with the problem of flood mapping in urban areas has been derived. The approach assumes the availability of a map of persistent scatterers (PSs) inside the urban settlement and is based on the analysis of the temporal trend of the InSAR coherence and the spatial average of the exponential of the InSAR phase in each PS. Both interferometric products are expected to have high and stable values in the PSs; therefore, anomalous decreases may indicate that floodwater is present in an urban area. The stack of Sentinel-1 data has been divided into two subsets. The first one has been used as a calibration set to identify the PSs and determine, for each PS, reference values of the coherence and the spatial average of the exponential of the interferometric phase under standard non-flooded conditions. The other subset has been used for validation purposes. Flood maps produced by UNOSAT, analyzing very-high-resolution optical images of the floods that occurred in Beletweyne in April–May 2018, October–November 2019, and April–May 2020, have been used as reference data. In particular, the map of the April–May 2018 flood has been used for training purposes together with the subset of Sentinel-1 calibration data, whilst the other two maps have been used to validate the products generated by applying the proposed method. The main product is a binary map of flooded PSs that complements the floodwater map of rural/suburban areas produced by applying a well-consolidated algorithm based on intensity data. In addition, a flood severity map that labels the different districts of Beletweyne, as not, partially, or totally flooded has been generated to consolidate the validation. The results have confirmed the effectiveness of the proposed method.

Development of a Distinct Microbial Community Upon First Season Crop Change in Soils of Long-Term Managed Maize and Rice Fields.

The introduction of crop rotation regimes in paddy soils, for example, rice in combination with maize, implements the establishment of new paddy fields to compensate for reduced rice production on existing fields. To study responses of the soil and rhizosphere microbiota upon introduction of a new crop species into continuous cropping agroecosystems, we conducted experiments with soils from adjacent fields where rice and maize were grown successively for more than 30 years. In microcosm experiments, rice and maize plants were cultivated in both soils under the respective plant-required management regime, i.e., rice cultivation under flooded conditions and maize under non-flooded conditions. 16S rRNA gene and fungal ITS region amplicon analysis showed that the soil and rhizosphere microbiota was clearly distinct between soils after long-term rice/maize management. Upon change of the management regime, the bulk soil microbiota became different to both, the former microbial community in the soil and the community being characteristic for the respective type of long-term cropping. Nevertheless, the influence of the soil management history remained clearly visible besides the impact of the new management regime. Similar results were observed for the rhizosphere, though the combined effect of plant species and altered management was even more effective in this compartment compared to the bulk soil. The newly introduced crop plant did not take over characteristic members of the rhizosphere microbiota of the previously cultivated crop; instead, some previously rare taxa became enriched. Thus, the formerly grown crop species did not directly affect the recruitment of microorganisms in the rhizosphere of the following crop species. Further, the results show that the rhizosphere and bulk soil microbiota do not develop straight toward the specific microbiota that is characteristic for a continuous cropping system, but reach a distinct stage upon introduction of a new crop species and new management practices.

Crop residue carbon-to-nitrogen ratio regulates denitrifier N2O production post flooding

The response of nitrifier and denitrifier populations and associated N2O emissions to different carbon-to-nitrogen (C/N = 17 or 45) straw amendments was monitored under flooding-drying and non-flooding conditions. A 10-week laboratory mesocosm study was conducted in two soils, (i) a paddy soil with a long history of managed flooding-drying (CN), and (ii) a wheat cropping soil with no previous history of flooding (UK). We measured N2O fluxes and the abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite reductase (nirK and nirS) genes, and nitrous oxide reductase (nosZI and nosZII) genes during flooding (4 weeks) and post-flooding (6 weeks). Straw addition enhanced N2O emissions, with higher fluxes apparent after incorporation of narrow C/N residues. Moreover, the impact of crop amendment on N2O emission was exacerbated when soil was under flooding-drying conditions. The abundances of nirS and nosZI genes in CN soil and AOA gene in UK soil were increased by straw amendment, with highest in narrow C/N straw amendments. Structural equation modeling showed that the impact of denitrifier gene abundance on the N2O flux was stronger than that of nitrifier gene abundance in the two soils, and significant correlations were observed between N2O fluxes and the consumption of DOC and NO3−, indicating that denitrification was the dominant N2O production pathway during the drying phase. The ratio of (nirS + nirK)/(nosZI + nosZII) in the narrow C/N amendment was greater than in the wide C/N treatment after flooding, suggesting that the straw C/N ratio had an effect on the capacity for N2O production via denitrification. We conclude that crop amendments with an appropriate C/N ratio could minimize N2O fluxes through regulating the denitrification process when soils are subjected to regular flooding and drying and also experiencing greater frequencies of flooding.

Can flooding-induced greenhouse gas emissions be mitigated by trait-based plant species choice?

Intensively managed grasslands are large sources of the potent greenhouse gas nitrous oxide (N2O) and important regulators of methane (CH4) consumption and production. The predicted increase in flooding frequency and severity due to climate change could increase N2O emissions and shift grasslands from a net CH4 sink to a source. Therefore, effective management strategies are critical for mitigating greenhouse gas emissions from flood-prone grasslands. We tested how repeated flooding affected the N2O and CH4 emissions from 11 different plant communities (Festuca arundinacea, Lolium perenne, Poa trivialis, and Trifolium repens in monoculture, 2- and 4-species mixtures), using intact soil cores from an 18-month old grassland field experiment in a 4-month greenhouse experiment. To elucidate potential underlying mechanisms, we related plant functional traits to cumulative N2O and CH4 emissions. We hypothesized that traits related with fast nitrogen uptake and growth would lower N2O and CH4 emissions in ambient (non-flooded) conditions, and that traits related to tissue toughness would lower N2O and CH4 emissions in flooded conditions. We found that flooding increased cumulative N2O emissions by 97 fold and cumulative CH4 emissions by 1.6 fold on average. Plant community composition mediated the flood-induced increase in N2O emissions. In flooded conditions, increasing abundance of the grass F. arundinacea was related with lower N2O emissions; whereas increases in abundance of the legume T. repens resulted in higher N2O emissions. In non-flooded conditions, N2O emissions were not clearly mediated by plant traits related with nitrogen uptake or biomass production. In flooded conditions, plant communities with high root carbon to nitrogen ratio were related with lower cumulative N2O emissions, and a lower global warming potential (CO2 equivalent of N2O and CH4). We conclude that plant functional traits related to slower decomposition and nitrogen mineralization could play a significant role in mitigating N2O emissions in flooded grasslands.

Carbon isotopes and lignin phenols for tracing the floods during the past 70 years in the middle reach of the Changjiang River

The Lake Tian E Zhou (TEZ, an oxbow lake) was formed during the rerouting of the Changjiang River in 1972, with strong influences from the main river channel and flood events. Herein, a sediment core was collected from the Lake TEZ for the measurements of carbon isotopes and biomarkers, including stable carbon isotopes (δ13C), radiocarbon composition (Δ14C), and lignin phenols, as well as lead-210 to reconstruct recent heavy flood events over the past 70 years. At the 24–26 cm interval, the sediment contained the highest OC%, TN%, and lignin phenols content, as well as significantly depleted 13C but enriched 14C, corresponding to the extreme flood event in 1998. In addition, statistics from t-test showed that lignin phenols normalized to OC (Λ8), the concentration of 3, 5-dihydroxy benzoic acid (3, 5-BD), and the ratio of p-hydroxy benzophenone to total hydroxyl phenols (PHB/HP) were all significantly different between the layers containing flood deposits and the layers deposited under normal non-flood conditions (p<0.05). These results indicate that the later three parameters are highly related to flood events and can be used as compelling proxies, along with sediment chronology, for hydrological changes and storm/flood events in the river basin and coastal marine environments.

Buoyancy and Stability Analysis of Ark’a Modulam Amphibious Foundation Model

To solve problems of settlement in floodplain areas, research on anti-flood foundation model is crucial. The anti-flood foundation model should be able to float during flood and to remain on land during non-flood conditions. This research focused on an amphibious foundation model named Ark’a Modulam. The reliability of Ark’a Modulam floating construction was analyzed for buoyancy and stability against vertical and horizontal loads. Two floating constructions were designed based on knock down and rigid systems. Both systems were designed using iron pipes with 2.5” and 4" of diameter. No significant differences were found in the buoyancy forces of the 2 floating construction systems. The load that can be supported by 1 floating construction was a maximum of 0.954 ton and stable at a floating depth limit of 43.2 cm. The stability of the floating construction should include the effect of vertical drive piles. The vertical and horizontal loads acting on the construction positioned inside Kahayan River during an extreme flood was also analyzed. During an extreme flood with 100 years of return period, the construction of one vertical drive piles should able to support vertical forces equal to or larger than 7.50 ton.

Wetland plant litter decomposition occurring during the freeze season under disparate flooded conditions

To investigate the heterogeneity of plant litter decomposition in the freeze and freeze-free seasons and the responses to disparate flooded conditions in seasonally frozen wetlands, in situ simulation experiments of litter decomposition were performed in the Sanjiang Plain, Northeast China. The experiments were conducted using the litter bag method for representative plants, Carex lasiocarpa and Calamagrostis angustifolia, in both non-flooded and flooded areas between November 2011 and November 2013. Heterogeneous effects of the freeze season and its interaction with hydrological regimes on the decomposition of the litter of various species and organs were observed. The litter decomposition occurred during the freeze season and made a significant contribution to the loss throughout the year. The two-year mass-loss of C. lasiocarpa and C. angustifolia and their organs were ordered differently between the freeze season and the freeze-free season. The proportion of litter mass-loss during the freeze season accounting for the whole year in the flooded area were greater than that in the non-flooded area, except for the C. angustifolia root litter. The litter mass-losses of entire C. lasiocarpa and C. angustifolia during the freeze season were greater than those during the freeze-free season in the flooded area, while the pattern was opposite in the non-flooded area. The effect of environmental factors on litter decomposition might override the effects of litter substrate quality. The total N and P of the litter of the entire C. lasiocarpa and entire C. angustifolia increased significantly relative to the initial values after two years and tended to enrich more in the litter under flooded conditions than under non-flooded conditions. The results highlighted the heterogeneous effects of the freeze season and its interaction with hydrological regimes on various species and organs, which would provide management and restoration options for degraded wetlands caused by climate change.

Effects of soil fertility and flooding regime on the growth of Ambrosia trifida

Understanding the effects of abiotic environmental factors on invasive plants species traits is of importance for practical prevention. To examine the effects of soil fertility and flooding regime on the growth of Ambrosia trifida L., a mesocosm experiment was conducted for 18 weeks. Two levels of soil fertility (high and low) and three types of flooding regime (non-flooded, flooded, and periodically flooded) were prepared. Shoot height and dry weight of each plant were measured. We found both individual and interactive effects of soil fertility and flooding regime on the overall growth performance of A. trifida (p < 0.05). The highest shoot height (154.7 ± 4.4 cm) and total dry weight (TDW, 13.0 ± 1.4 g) were obtained under high fertility and non-flooded condition. Height and weight were relatively low under flooding conditions (flooded and periodically flooded). In particular, shoot height (102.3 ± 3.2 cm) and TDW (3.2 ± 0.3 g) were the lowest under low fertility and periodically flooded condition. On the other hand, the ratio of above- to below-ground dry weight was relatively high under flooded conditions, showing the adaptive phenotypic plasticity. Adventitious root formation and more biomass allocation to shoots were a flooding-adaptive mechanism of A. trifida, well developed under high fertility condition. We suggest maintaining appropriate water regime and avoiding eutrophication in wetlands would be necessary to prevent A. trifida from invading. These findings will contribute to the conservation of biodiversity in wetlands by effective management of A. trifida.

Soil properties and not high CO2 affect CH4 production and uptake in periodically waterlogged arable soils

Several studies related to CH4 cycling focus on the effect of elevated atmospheric CO2 levels on soil methanogenesis and methanotrophy. However, periodically waterlogged soils are characterized by much higher CO2 concentrations, while aggregates forming in arable soils due to traditional cultivation may be a natural CH4 source for low-affinity methanotrophs. We present a comprehensive laboratory investigation into both methanogenesis and methanotrophy in three different soils under ambient and high CO2 concentrations. The tested materials were agricultural soils widely distributed in Europe: Eutric Cambisol, Haplic Podzol and Mollic Gleysol with determined properties: Corg, Ntot, clay, silt, sand, pH, NO3−, NH4+. We conducted two independent experiments on methanogenesis (flooded conditions) and methanotrophy (non-flooded conditions). All samples for both studies were divided into three sets which differed in the initial CO2 concentration in headspace: ambient CO2 (0.03% v/v ± 0.005), and two high CO2 levels (5% ± 0.4 and 10% ± 0.5 v/v CO2). Moreover, 1% (v/v) of CH4 was added to all sets prepared for the methanotrophy test since low-affinity methanotrophy in the tested soils had been reported previously. Gas concentrations (CH4, CO2, O2) in headspace were measured using gas chromatography method. We observed that the examined soils differed in their ability to produce CH4 as follows: Haplic Podzol = Eutric Cambisol > Mollic Gleysol, as well as to consume CH4: Mollic Gleysol > Haplic Podzol > Eutric Cambisol. The CH4 emissions started faster in Gleysol, but the final CH4 concentration and CH4 production rate in this soil were significantly lower than in the two other soils. The CH4 uptake rate significantly differed among the tested soils. The MANOVA confirmed the significance of the soil-type factor in determining both process rates, whereas the effect of CO2 level and the interaction between both factors were not significant. The amount of consumed O2 during CH4 oxidation was also characteristic for examined soil, but did not differ significantly between CO2 variants. CH4 production and consumption in three different soils (Eutric Cambisol, Haplic Podzol and Mollic Gleysol) collected from periodically waterlogged arable fields were more strongly affected by soil properties than by high CO2 concentration. In contrast, the high CO2 level significantly decreased CO2 production accompanying tested the CH4-related processes.

Copper stress in flooded soil: Impact on enzyme activities, microbial community composition and diversity in the rhizosphere of Salix integra

Climate change has increased flooding frequency, making the heavy metal polluted areas more vulnerable, and led to increased global land degradation. Information about the alteration of soil microbiota under heavy metal pollution and flooding is still rather limited. Fast-growing trees are candidates for phytoremediation of heavy metal polluted soils. Therefore, the impact of Cu pollution on microbiota in soil used for cultivating Salix integra Thunb. was investigated with and without flooding for 60 d. Bacterial and fungal communities were accessed via partial 16S rRNA (V3–V4) and internal transcribed spacer (ITS) genes. The activity of invertase, urease and cellulase were markedly decreased by 28.5–59%, 55.0–76.7% and 17.3–34.1%, respectively, with increasing Cu levels. Flooding significantly increased the activity of polyphenol oxidase and peroxidase by 56.3% and 41.4% at the highest Cu level compared to its respective non-flooded condition. High Cu concentration significantly decreased the richness and diversity of the bacterial community, and fungi were more sensitive than bacteria under flooding conditions. Redundancy analysis suggests that Cu, Fe and soil organic matter are the key determinants affecting the composition of microbial communities. Our findings provide new insight into the responses of soil microbes to Cu-contamination and contribute to our understanding of metal toxicity in soil-woody plant systems under flooded conditions.

Differences in uptake and accumulation of copper and zinc by Salix clones under flooded versus non-flooded conditions.

The climate-driven flooding poses a challenge for phytoremediation of contaminated soil, and the willow (Salix spp.) is a promising candidate coping with climate change and environmental pollution. In this study, uptake and accumulation of copper (Cu), zinc (Zn) and their bioavailability in the rhizosphere across the Salix clones under flooded versus non-flooded (control) conditions were investigated using a pot experiment. The tested Salix clones grew well without showing any toxic symptoms under non-flooded soil condition; in contrast, the clones showed 100% survival for long-term flooding with the development of hypertrophied lenticels and adventitious roots. There were wide clonal variations in biomass production and accumulation of Cu and Zn under flooded and non-flooded conditions. Flooded treatments dramatically decreased aboveground biomass across the Salix clones to different extents compared to the control. The non-flooded clones exhibited relatively high accumulation capacities of Cu and Zn in aerial parts. However, the flooded clones resulted in more substantial reductions in Cu and Zn accumulation in aerial parts, and most of Cu and Zn were limited in roots. EDTA-extractable Cu and Zn predicted well bioavailability of Cu and Zn to the Salix clones under the current condition. It was concluded that the Salix clones exhibited Cu and Zn phytoextraction traits (non-flooding) or phytostabilization traits (flooding), which provides a valuable insight into phytomanagement of contaminated soils by willows subjected to flooding stress.

The negative impact of cadmium on nitrogen transformation processes in a paddy soil is greater under non-flooding than flooding conditions

Nitrification and denitrification are two important processes in the nitrogen (N) cycle. Under heavy-metal pollution with water management of paddy soils, these two processes are not well understood. This study aimed to examine the effect of cadmium (Cd) on N transformation under flooding and non-flooding conditions. A paddy soil was incubated under two water regimes (flooding and non-flooding) and four Cd levels (0, 2, 5 and 10 mg kg−1). The availability of Cd was higher in the non-flooding than flooding conditions. Cadmium contamination significantly (p ≤ 0.05) decreased the copy number of archaeal and bacterial amoA genes, bacterial nirS, nirK and nosZ genes under both conditions with the decrease being greater under non-flooding. High level of Cd (10 mg kg−1) was more toxic in non-flooding than flooding conditions to the nitrifiers and denitrifiers, which in turn decreased N transformation through microbially-mediated processes. Its contamination decreased N2O emission initially under both water regimes but the effect was greater under the non-flooding condition. However, the non-significant stimulatory effect of Cd on N2O emission was observed during the late phase. The microbial community structure was changed with time and water regimes. Irrespective of water regime, the dominated fungal phyla were Ascomycota and Basidiomycota while the dominated bacteria phyla were Actinobacteria, Proteobacteria, Firmicutes and Acidobacteria. In summary, water regimes and Cd bioavailability changed soil N transformations via microbial mediated processes.

Effect of water-driven changes in rice rhizosphere on Cd lability in three soils with different pH

Pot experiments were conducted to evaluate the effect of water management, namely continuous flooding (CF), intermittent flooding (IF) and non-flooding (NF), on Cd phytoavailaility in three paddy soils that differed in pH and in Cd concentrations. Diffusive gradients in thin films (DGT) technique was employed to monitor soil labile Cd and Fe concentrations simultaneously at three growth stages (tillering, heading and mature stage) of rice. The Cd phytoavailability were generally in the order of NF > IF > CF, and higher rice Cd (over permitted level, 0.2 mg/kg) were only found in neutral and acidic soils under NF conditions. DGT measured soil labile Cd rather than total Cd was the most reliable predictor for Cd accumulation in rice. CF enhanced the formation of root plaques, which related to oxidation of large quantities of available Fe on root surfaces due to the O2 secretion of rice root. The Cd concentration in root plaques shared the same trend with DGT-Cd. Generally, root plaques would inhibit Cd uptake by rice under CF conditions, while under IF and NF conditions, root plaques act as a temporarily store of Cd, and soil labile Cd is the key factor that controls the transfer of Cd from soil to rice. The results of principle component analysis revealed that water management had the greatest effect on soil Cd lability and rice Cd in acidic soil. Thus, it is important to consider the availability of Cd and soil pH when assessing current agricultural practices of contaminated soil in China.

Combined impacts of Si-rich rice residues and flooding extent on grain As and Cd in rice

Increasing plant-availability of Si through soil amendment of Si-rich rice residues can decrease inorganic As without affecting Cd levels in grain under flooded soil conditions. However, the impacts of Si amendments on Cd and As uptake by rice under different flooding extents have not been reported. We investigated the effects of different flooding extent on As and Cd uptake by rice and accumulation in grain in well-weathered soil amended with Si-rich rice husk (Husk) or mixed charred/ashed rice husk (Ash). Our results show that Husk and to a lesser extent Ash amendments decreased grain As under both flooded (~40% and 20% decrease, respectively) and nonflooded (~75% decrease) conditions due to increased Si. Under flooded conditions grain As and yield is higher, and Husk amendment additionally decreased grain inorganic As by ~45%. Under nonflooded conditions grain Cd is higher and yield is lower, and Ash amendment decreased grain, husk, and straw Cd by ~40–50% not due to Si, but due to increased aboveground biomass and an increase in soil pH, which helped to retain Cd in soil. These data illustrate that rice residue addition to paddy soil can lower human health risk under both flooded and nonflooded conditions without affecting grain Zn and Fe.