Non-flooded Conditions Research Articles

Wasted shores: Using drones to monitor the spatio-temporal evolution of debris accumulation hotspots on South Africa's Umgeni River

Rivers are major contributors of plastic waste to the oceans. Running through the northern part of the 1.3 million-inhabitants City of Durban, South Africa, the Umgeni River is estimated to flush in the order of tens to hundreds of tonnes of plastic waste into the Indian Ocean every year. The riverbanks are lined with plastic and other macro-waste accumulation zones formed due to direct littering and occasional deposition of river debris loads. This study presents the use of Unmanned Aerial Vehicles (UAVs) and hydro-meteorological sensors to (1) identify, quantify and monitor such anthropogenic macro-waste hotspots; and (2) investigate the influence of rainfall, river water level and a major flood event on the spatio-temporal evolution of the hotspots, evidencing the debris' availability to leak into the Indian Ocean. The one-year aerial monitoring (2021−2022) of waste hotspots shows that extreme hydrometeorological events have an immediate but short-term effect on the erosion of debris stocks in riverine systems. We observe that reduction in mean index changes of hotspot surface area after flooding were 2–5 times higher than in non-flood conditions. Despite visual evidence of seasonality in debris erosion between the wet and dry season, only the 'natural' type hotspots showed a significant change. Our findings support reported inconsistencies of macro-debris erosion with hydrological factors. Although the data contributes a baseline for macro-debris erosion in the Umgeni River, future ground truth sampling and finer monitoring scales are important to fully understand debris transfer in river systems. The mapping of waste hotspots and understanding their spatio-temporal transfer dynamics supports policymakers in planning and timing to mitigate environmental pollution.

Deep Learning-Based Flood Detection for Bridge Monitoring Using Accelerometer Data

Flooding and consequential scouring are the primary causes of bridge failures, making the detection of such events crucial for structural safety. This study investigates the characteristics of accelerometer data from bridge pier vibrations and proposes a flood detection method with deep learning-based models based on ResNet18 and 1D Convolution architectures. These models were comprehensively evaluated for (1) detecting vehicles passing on bridges and (2) detecting flood events based on axis-specific accelerometer data under various traffic conditions. Continuous Wavelet Transform (CWT) was employed to convert the accelerometer data into richer time-frequency representations, enhancing the detection of passing vehicles. Notably, when vehicles are passing over bridges, the vertical direction exhibits a magnified and more sustained energy distribution across a wider frequency range. Additionally, under flooding conditions, time-frequency representations from the bridge direction reveal a significant increase in energy intensity and continuity compared with non-flooding conditions. For detection of vehicles passing, ResNet18 outperformed the 1D Convolution model, achieving an accuracy of 97.2% compared with 91.4%. For flood detection without vehicles passing, the two models performed similarly well, with accuracies of 97.3% and 98.3%, respectively. However, in scenarios with vehicles passing, the 1D Convolution model excelled, achieving an accuracy of 98.6%, significantly higher than that of ResNet18 (81.6%). This suggests that high-frequency signals, such as vertical vibrations induced by passing vehicles, are better captured by more complex representations (CWT) and models (e.g., ResNet18), while relatively low-frequency signals, such as longitudinal vibrations caused by flooding, can be effectively captured by simpler 1D Convolution over the original signals. Consequentially, the two model types are deployed in a pipeline where the ResNet18 model is used for classifying whether vehicles are passing the bridge, followed by two 1D Convolution models: one trained for detecting flood events under vehicles-passing conditions and the other trained for detecting flood events under no-vehicles-passing conditions. This hierarchical approach provides a robust framework for real-time monitoring of bridge response to vehicle passing and timely warning of flood events, enhancing the potential to reduce bridge collapses and improve public safety.

Methane-cycling microbial communities from Amazon floodplains and upland forests respond differently to simulated climate change scenarios

Seasonal floodplains in the Amazon basin are important sources of methane (CH4), while upland forests are known for their sink capacity. Climate change effects, including shifts in rainfall patterns and rising temperatures, may alter the functionality of soil microbial communities, leading to uncertain changes in CH4 cycling dynamics. To investigate the microbial feedback under climate change scenarios, we performed a microcosm experiment using soils from two floodplains (i.e., Amazonas and Tapajós rivers) and one upland forest. We employed a two-factorial experimental design comprising flooding (with non-flooded control) and temperature (at 27 °C and 30 °C, representing a 3 °C increase) as variables. We assessed prokaryotic community dynamics over 30 days using 16S rRNA gene sequencing and qPCR. These data were integrated with chemical properties, CH4 fluxes, and isotopic values and signatures. In the floodplains, temperature changes did not significantly affect the overall microbial composition and CH4 fluxes. CH4 emissions and uptake in response to flooding and non-flooding conditions, respectively, were observed in the floodplain soils. By contrast, in the upland forest, the higher temperature caused a sink-to-source shift under flooding conditions and reduced CH4 sink capability under dry conditions. The upland soil microbial communities also changed in response to increased temperature, with a higher percentage of specialist microbes observed. Floodplains showed higher total and relative abundances of methanogenic and methanotrophic microbes compared to forest soils. Isotopic data from some flooded samples from the Amazonas river floodplain indicated CH4 oxidation metabolism. This floodplain also showed a high relative abundance of aerobic and anaerobic CH4 oxidizing Bacteria and Archaea. Taken together, our data indicate that CH4 cycle dynamics and microbial communities in Amazonian floodplain and upland forest soils may respond differently to climate change effects. We also highlight the potential role of CH4 oxidation pathways in mitigating CH4 emissions in Amazonian floodplains.

Response mechanism of Cynodon dactylon to flooding stress based on integrating metabonomics and transcriptomics analysis

Cynodon dactylon, a perennial herbaceous of Poaceae, possessing unique abilities in flooding tolerance and clonal propagation. However, evidences on molecular response mechanism from field are missing. Here, the differential responses of above-ground (stolon) and below-ground (root) organs of C. dactylon under flooding and non-flooding (CK) conditions were compared at the transcriptional and metabolomic levels. Under flooding stress, significant enrichment of differentially expressed genes (DEGs) related to dehydration responses, including cell water homeostasis and ion transmembrane transport were observed in both roots and stolons. The stolons were more sensitive to flooding, while enrichment in energy metabolism in roots were higher, such as the glycolytic process and glycogen biosynthetic process. The DEGs of photosynthesis in stolons were significantly down-regulated compared to roots. Under flooding stress, oxidative phosphorylation (OXPHOS) is inactivated, and the terminal oxidase of the mitochondrial electron transport chain (mtETC), cytochrome c oxidase (COX), is inhibited. Consequently, the expression of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) involved in fermentative metabolism is upregulated. The combined analysis indicated that phenylalanine, tyrosine, and tryptophan biosynthetic pathways in roots were significantly enriched in the flooding group. In contrast, there was a significant increase in phenylalanine biosynthesis in stolons, and the metabolite phenol exhibited up-regulated expression. In summary, phenylalanine metabolism and phenylalanine biosynthesis are the major responses to flooding stress, indicating the crucial role of these pathways in the flood tolerance of C. dactylon. These findings provide valuable evidence for understanding plant regulatory mechanisms underlying flooding tolerance.

Non-flooding conditions caused by water table drawdown alter microbial network complexity and decrease multifunctionality in alpine wetland soils

Several soil functions of alpine wetland depend on microbial communities, including carbon storage and nutrient cycling, and soil microbes are highly sensitive to hydrological conditions. Wetland degradation is often accompanied by a decline in water table. With the water table drawdown, the effects of microbial network complexity on various soil functions remain insufficiently understood. In this research, we quantified soil multifunctionality of flooded and non-flooded sites in the Lalu Wetland on the Tibetan Plateau. We employed high-throughput sequencing to investigate the microbial community responses to water table depth changes, as well as the relationships between microbial network properties and soil multifunctionality. Our findings revealed a substantial reduction in soil multifunctionality at both surface and subsurface soil layers (0–20 cm and 20–40 cm) in non-flooded sites compared to flooded sites. The α-diversity of bacteria in the surface soil of non-flooded sites was significantly lower than that in flooded sites. Microbial network properties (including the number of nodes, number of edges, average degree, density, and modularity of co-occurrence networks) exhibited significant correlations with soil multifunctionality. This study underscores the adverse impact of non-flooded conditions resulting from water table drawdown on soil multifunctionality in alpine wetland soils, driven by alterations in microbial community structure. Additionally, we identified soil pH and moisture content as pivotal abiotic factors influencing soil multifunctionality, with microbial network complexity emerging as a valuable predictor of multifunctionality.

Genome-Wide Identification and Analysis of Plasma Membrane H+-ATPases Associated with Waterlogging in Prunus persica (L.) Batsch

Plant plasma membrane H+-ATPase is a transport protein that is generally located on the plasma membrane and generates energy by hydrolyzing adenosine triphosphate (ATP) to pump hydrogen ions (H+) in the cytoplasm out of the cell against a concentration gradient. The plasma membrane H+-ATPases in plants are encoded by a multigene family and potentially play a fundamental role in regulating plant responses to various abiotic stresses, thus contributing to plant adaptation under adverse conditions. To understand the characteristics of the plasma membrane H+-ATPase family in peach (Prunus persica), this study analyzed the plasma membrane H+-ATPase family genes in peach. The results showed that there were 27 members of the plasma membrane H+-ATPase family in peach with amino acid sequences ranging from 943 to 1327. Subcellular localization showed that 23 of the 27 members were located on the cell membrane, and the phylogenetic tree analysis indicated that peach plasma membrane H+-ATPase members were divided into five groups. There were four genes with tandem repeat relationships, and six plasma membrane H+-ATPase genes were differentially expressed after 5 days of flooding and under non-flooding conditions based on the RNA-seq and RT-qPCR analyses. This study also investigated the characteristics and possible functions of the plasma membrane H+-ATPase family members in peach. The results provide theoretical support for further studies on their biological functions in peach.

Exploring metal(loid)s dynamics and bacterial community shifts in contaminated paddy soil: Impact of MgO-laden biochar under different water conditions

In this study, a soil incubation experiment was conducted to explore the influence MgO-treated corn straw biochar (MCB) on the bioavailability and chemical forms of cadmium (Cd), lead (Pb), and arsenic (As), alongside the impact on the bacterial community within paddy soil subjected to both flooded and non-flooded conditions. Raw corn straw biochar (CB) served as the unmodified biochar control, aiding in the understanding of the biochar's role within the composite. The results showed that even at a minimal concentration of 0.5 %, MCB exhibited higher effectiveness in reducing the bioavailability of Pb and Cd compared to 1 % CB. In non-flooded conditions, 0.5 % MCB reduced the bioavailable Pb and Cd by 99.7 % and 87.4 %, respectively, while NaH2PO4-extracted As displayed a 14.5 % increase. With increasing MCB concentrations (from 0.5 % to 1.5 %), soil pH, DOC, EC, available phosphorus, and bioavailable As increased, while bioavailable Pb and Cd exhibited declining tendencies. Flooding did not notably alter MCB's role in reducing Pb and Cd bioavailability, yet it systematically amplified As release. Heavy metal fractions extracted by acetic acid increased in the MCB groups under flooding conditions, especially for As. The inclusion of 0.5 % MCB did not noticeably affect bacterial diversity, whereas higher doses led to reduced diversity and substantial changes in community composition. Specifically, the groups with MCB showed an increase in the Bacteroidetes and Proteobacteria phyla, accompanied by a decrease in Acidobacteria. These alterations were primarily attributed to the increased pH and EC resulting from MgO hydrolysis. Consequently, for Pb/Cd stabilization and soil bacterial diversity, a low dosage of MgO-treated biochar is recommended. However, caution is advised when employing MgO-treated biochar in soils with elevated arsenic levels, particularly under flooded conditions.

GenX uptake by wheat and rice in flooded and non-flooded soils: a greenhouse experiment

GenX (hexafluoropropylene oxide dimer acid) belongs to the group of per- and poly-fluoroalkyl substance (PFAS) compounds introduced to replace perfluorooctanoic acid (PFOA), which has been phased out in industrial and consumer product formulations. While GenX has been investigated in lab animals, there is limited information available regarding its uptake and translocation in wheat and rice. This study reports on a greenhouse experiment in which wheat and rice grown under flooded and non-flooded conditions were exposed to two GenX concentrations in the soil (0.4 mg kg−1 and 2 mg kg−1). GenX was analysed in the soil, porewater and shoots using targeted liquid chromatography-tandem mass spectroscopy (LC-MS/MS) analysis. Extractable organic fluorine (EOF) was determined using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-GFMAS) instrument. Results showed that different species took up different amounts of GenX. The GenX concentration in rice shoots was found to be 2.34 (± 0.45) and 4.11 (± 0.87) μg g−1 under flooded and non-flooded conditions, respectively, at a low exposure level. At high exposure, the GenX concentrations in flooded and non-flooded rice shoots increased threefold to 10.4 (± 0.41) and 13.4 (± 0.72) μg g−1, respectively. Wheat shoots showed similar concentrations and increases between low- and high-level exposure. The translocation factor was significantly higher (P = 0.013) in non-flooded rice compared to flooded rice. The GenX bioaccumulation behaviours under the same culture conditions (e.g. temperature, humidity, light, same GenX concentration in the soil) were significantly different in non-flooded and flooded rice (P < 0.001). Non-flooded rice plants displayed a higher level of GenX bioaccumulation than flooded ones. Following exposure to GenX, flooded rice plants showed a reduction in biomass (25%) compared to the control plants (P < 0.014). Our findings indicate that GenX is a bioaccumulative compound, the presence of which likely inhibits the growth of plants.

Does rice paddy management increase soil organic carbon in the warm temperate and tropical regions?

Enhancing soil organic carbon (SOC) storage in agricultural fields is an effective option for climate change mitigation and food security. In temperate regions, rice farming under paddy is known to increase SOC stock due mainly to the reduced decomposition caused by prolonged submerged conditions compared to the cropping practices under non-flooded conditions. It remains unclear, however, if and to what extent rice paddy management increases SOC storage in warmer climate regions where decomposition is much faster. We thus examined the factors controlling the SOC stock and the net change in SOC normalized to the duration under paddy (ΔSOC) at paddy fields in warm temperate (21 sites) and tropical climatic regions (11 sites) from peer-reviewed literature which was concentrated to Asia. Using collected data (e.g., SOC content, bulk density, mean annual temperature (MAT), precipitation, management duration, soil management, crop rotation), we evaluated predictor variables that account for ΔSOC variability (i.e., mean decrease accuracy) by random forest regression. The mean paddy duration for all data (n = 170) was 22 years. Average SOC stock and ΔSOC at 0–20 cm depth were 38 ± 1.45SE Mg C ha−1 and 0.34 ± 0.04SE Mg C ha−1 yr−1, respectively, indicating that continuous paddy management had a positive effect on SOC storage in both climatic regions. SOC stock was two-fold higher and ΔSOC was four-fold higher in the warm temperate region compared to the tropical region (p < 0.05). The best predictor variables for ΔSOC were MAT followed by soil management and then paddy duration. While organic amendment including both straw and manure increased ΔSOC in both warm temperate and tropical paddies, we did not detect a positive effect of chemical fertilizer application alone or crop rotation type (e.g., rice-upland vs rice-rice). Despite high variability, ΔSOC in the warm temperate region significantly declined over time from 0.82 ± 0.10 Mg C ha−1 yr−1 (< 20 yrs) to 0.22 ± 0.06 Mg C ha−1 yr−1. While SOC stock was 50% lower, the tropical paddies showed no clear decline in ΔSOC up to 41 years (0.04–0.21 Mg C ha−1 yr−1). These results suggest that SOC accretion might have reached equilibrium after 20 years of paddy practice in the warm temperate region and, if so, ΔSOC is likely to diminish to zero in the near future. In tropical paddy fields, on the other hand, slow SOC build-up may continue for a longer term although the ΔSOC value itself is not high. While the current study showed the efficacy of paddy management on SOC build-up under the warmer climate regions, further study on the rate and qualitative changes is necessary to evaluate the trade-offs between soil fertility improvement (e.g., soil N build-up) and methane emission.

Water conditions and arbuscular mycorrhizal symbiosis affect the phytoremediation of petroleum-contaminated soil by Phragmites australis

Phytoremediation of petroleum-contaminated soils using the synergistic functions of plants and rhizosphere microorganisms is a promising technology. However, successfully applying this approach presents challenges under certain conditions (submerged environments). This study analyzed the potential role of Phragmites australis in symbiosis with arbuscular mycorrhizal (AM) fungi during petroleum remediation at two water levels. AM inoculation promoted P. australis aboveground growth under non-flooded conditions, whereas flooding significantly increased P. australis biomass. The highest total petroleum hydrocarbon (TPH) degradation efficiency was observed in non-flooded soils, whereas submergence severely inhibited TPHs dissipation. Plants with AM inoculation treatments substantially enhanced the removal of TPHs under flooded conditions. TPH removal was positively correlated with dehydrogenase activity but negatively correlated with easily extracted glomalin-related soil proteins. Moreover, different petroleum-hydrocarbon-decaying candidates contributed to TPH removal in these two cultured soils. These findings provide valuable information for the remediation of future TPH-contaminated soils, especially applied in intermittently submerged environments.

Evaluating the Performance of Multi-criteria Decision-making Techniques in Flood Susceptibility Mapping

Abstract Performances of multi-criteria decision-making techniques in prediction of flood susceptibility are varied. We evaluated performances of ARAS, CODAS, COPRAS, EDAS, MOORA, TOPSIS, VIKOR, and WASPAS in predicting flood susceptibility of Barpeta district of Assam, India. Elevation, slope, proximity to river, geomorphology, drainage density, rainfall, land use/land cover, lithology, soil, stream power index, topographic wetness index and plan curvature were used as flood conditioning factors. The results show higher flood susceptibility over areas characterized by gentle slopes, low elevation and high proximity to drainage. Performances of the models were evaluated using 216 locations (flood and non-flood conditions) randomly classified into training (70%) and validation (30%) through area under receiver operating characteristic (ROC) curve (AUC). TOPSIS model showed better success (AUC = 0.965) and prediction rate (AUC = 0.962) than other models. Among the best performing models, highest percentage of area under high flood susceptibility was predicted by TOPSIS. Therefore, TOPSIS can be effectively used for flood risk management in areas having similar geographical conditions.

Differential influence of legume and cereal crop residue incorporation on methane production and consumption in a tropical vertisol

Crop residue incorporation to the soil is an essential strategy to improve soil quality and crop productivity in order to attain sustainable development goals. Experiments were conducted to evaluate the differential effect of crop residues on CH4 production and consumption in a tropical vertisol. Soils were incubated with residues of cereals (maize and wheat) and legumes (chickpea and soybean) at 1% w/w, under non-flooded and flooded conditions to estimate CH4 consumption and CH4 production rates, respectively. Rates of CH4 production (ng CH4 produced g/soil/day) varied from 0.068 to 0.107 with lowest in chickpea residue and highest in wheat straw amended soil. CH4 consumption rates (ng CH4 consumed g/soil/day) was highest (0.79) in wheat straw amended soil and lowest (0.53) in chickpea residue amended soil. Organic carbon (%) and available NO3− (mM) contents increased significantly (P &gt; 0.05) in residue amended soils over control under both flooded (methanogenic) and non-flooded (methane consuming) conditions. Abundance of methanogens and methanotrophs was estimated as mcr and pmoA gene copies g−1 soil, indicated that both the microbial groups were stimulated significantly due to the amendment of crop residues. Linear models exhibited significant correlation among CH4 production and consumption with organic carbon, available nitrate and microbial abundance. The study highlights that crop residues incorporation influences both CH4 consumption and production potential of soil and this effect is more pronounced with biomass of cereals than legumes.

Impacts of an extreme Changjiang flood on variations in carbon cycle components in the Changjiang Estuary and adjacent East China sea

River flooding is expected to increase in frequency and severity under climate change. However, the impact of extreme river flooding on the coastal carbon cycle has rarely been studied. A severe Changjiang flood occurred in the summer of 2020, which was the largest Changjiang flood in the last 20 years since 2000. This extreme flood resulted in the export of great amounts of nitrate (6.4 × 108 mol d−1), silicate (7.1 × 108 mol d−1), phosphate (5.1 × 106 mol d−1), dissolved organic carbon (DOC; 13.9 × 109 g C d−1), and inorganic carbon (DIC; 145.5 × 109 g C d−1) from Changjiang to the Changjiang Estuary, which were considerably higher (by ∼8–178%) compared to those observed in previous summers since 2006. The increase in nutrient loads, including a considerable increase in PO43− concentrations due to the flood, has triggered the development of severe algal blooms in the East China Sea. Consequently, the production of DOC and removal of DIC were observed in the offshore region during the flood. Moreover, the increase in NH4+ concentrations likely indicated enhanced organic material remineralization in flood-influenced coastal waters. The decrease in CO2 fluxes across the air-sea interface (FCO2) has been observed in the offshore region, while an increase in FCO2 was found in the nearshore region during the flood compared to the non-flood condition in July 2018. These findings provide valuable references for assessing the impact of the extreme Changjiang flood on the coastal carbon cycle.

Chromium speciation and mobility in contaminated coastal urban soils affected by water salinity and redox conditions

Chromium (Cr) is a redox-sensitive element in contaminated coastal urban soils. Sea level rise (SLR) with subsequent soil inundation may facilitate Cr transformation and mobilization through alterations in local redox conditions and porewater ion composition. We investigated the impact of water salinity and redox conditions on Cr chemistry in these environments. Synchrotron-based X-ray spectroscopy and wet chemical analyses revealed that the soils contained very high levels of Cr (up to 4320 mg kg-1) and that chromite (∼52%) and Fe-Cr hydroxide coprecipitates (∼44%) were the predominant Cr species. The abundance of these two components resulted in low Cr mobility under non-flooded conditions. Chromium(II) was identified in the soils, potentially derived from the waste parent material. Seawater and anoxic conditions resulted in lower Cr release compared to freshwater and aerobic conditions. Up to three to eight times more Cr was released under aerobic conditions versus anaerobic conditions in the freshwater versus saltwater, respectively, with total dissolved Cr values remaining below 0.02 mg L-1. The decrease in Cr release was likely due to Cr reduction by Fe(II) and sulfide. This work provides important information on how salinity and redox fluctuations impact Cr cycling which is likely to occur during SLR.

Effects of Biochar and Organic Additives on CO2 Emissions and the Microbial Community at Two Water Saturations in Saline–Alkaline Soil

The nutrient-limiting conditions in saline–alkali soil as well as the salinity and alkalinity stress are successfully alleviated by water management measures and the addition of organic matter. However, the impacts of these two strategies on the microbe-driven CO2 emissions in saline–alkaline soils are not yet clear. Therefore, a 150-day incubation experiment was conducted in this study to evaluate the short-term effects of water regulation and the addition of organic matter with different characteristics on CO2 emissions and microbial community characteristics in saline–alkali soils under non-flooding conditions. This study was conducted at two water saturations, i.e., 50% WFPS and 80% WFPS. In addition, five organic matter treatments were conducted: CK: control; N: urea; SN: Straw + urea; SNH: Straw + urea + microbial agent; and SNB: Straw + urea + biochar. The results demonstrated that compared with 50% WFPS, 80% WFPS significantly increased cumulative CO2 emission by 27.66%, but significantly decreased salt content and the fungal Chao1 and Shannon indices. The application of the biochar and microbial agent decreased the cumulative CO2 emissions of the SN treatment by 27.39% and 14.92%, respectively. When sufficient carbon supply is available, the decrease in fungal diversity may reduce CO2 emission. The findings demonstrated that SNH and SNB at 80% WFPS might decrease CO2 emissions under straw carbon intake as well as the loss of labile organic carbon (LOC). Additionally, these treatments can alleviate microbial stress caused by salinity, which has a favorable impact on enhancing carbon storage in salinity-affected dryland soils.

Effects of soil habitat changes on antibiotic resistance genes and related microbiomes in paddy fields

The changes of paddy soil habitat profoundly affect the structure and function of soil microorganisms, but how this process drives the growth and spread of manure- derived antibiotic resistance genes (ARGs) after entering the soil is unclear. Herein, this study explored the environmental fate and behavior of various ARGs in the paddy soil during rice growth period. Results showed that most ARG abundances in flooded soil was lower than that in non-flooded soil during rice growth (decreased by 33.4 %). And soil dry-wet alternation altered microbial community structure in paddy field (P < 0.05), showing that Actinobacteria and Firmicutes increased in proportion under non-flooded conditions, and Chloroflexi, Proteobacteria and Acidobacteria evolved into the dominant groups in flooded soil. Meanwhile, the correlation between ARGs and bacterial communities was stronger than that with mobile genetic elements (MGEs) in both flooded and non-flooded paddy soils. Furthermore, soil properties, especially oxidation reduction potential (ORP), were proved to be an essential factor in regulating the variability of ARGs in the whole rice growth stage by structural equation model, with a direct influence (λ = 0.38, P < 0.05), following by similar effects of bacterial communities and MGEs (λ = 0.36, P < 0.05; λ = 0.29, P < 0.05). This study demonstrated that soil dry-wet alternation effectively reduced the proliferation and dissemination of most ARGs in paddy fields, providing a novel agronomic measure for pollution control of antibiotic resistance in farmland ecosystem.

River plastic transport and deposition amplified by extreme flood

Plastic pollution in the world’s rivers and ocean is increasingly threatening ecosystem health and human livelihood. In contrast to what is commonly assumed, most mismanaged plastic waste that enters the environment is not exported into the ocean. Rivers are therefore not only conduits but also reservoirs of plastic pollution. Plastic mobilization, transport and retention dynamics are influenced by hydrological processes and river catchment features (for example, land use, vegetation and river morphology). Increased river discharge has been associated with elevated plastic transport rates, although the exact relation between the two can vary over time and space. However, the precise role of an extreme discharge event on plastic transport is still unknown. Here we show that fluvial floods drive macroplastic (>2.5 cm) transport (items h−1) and accumulation (items m−2) in river systems. We collected unique observational evidence during the July 2021 flood along the whole Dutch part of the Meuse. Plastic transport multiplied by a factor of over 100 compared with non-flood conditions (3.3 × 104 versus 2.3 × 102 items h−1). Over one-third of the modelled annual plastic item transport was estimated to occur within 6 days of extreme discharge. Between Maastricht and Ravenstein (291 km and 131 km from the river mouth), plastic transport during the flood period decreased by 90%, suggesting that the dispersal of plastic mobilized during the flood is limited due to the entrapment on riverbanks, in vegetation and on the floodplains. Plastic transport and accumulation on the riverbanks decreased significantly along the river, corroborating the river’s function as a plastic reservoir. Using new observational evidence, we demonstrate the crucial role of floods as drivers of plastic transport and accumulation in river systems. Floods amplify the mobilization of plastics, but the effects are local, and the river-scale dispersal is limited. We anticipate that our findings will serve as a starting point for improving global estimates of river plastic transport, retention and export into the sea. Moreover, our results provide essential insights for future large-scale and long-term quantitative assessments of river plastic pollution. Reliable observations and a fundamental understanding of plastic transport are key to designing effective prevention and reduction strategies.

Effects of different colors of film mulch on soil temperature and rice growth in a non-flooded condition.

Rice cultivation under film mulching with no flooding is widely used as an effective water-saving technology. Different colors of film mulch have different effects on the soil hydrothermal environment and crop growth because of their different optical properties. However, the effects of different colors of film mulch on soil temperature and rice physiological growth are not clearly understood. Field experiments were conducted in 2019 and 2020 to investigate the effects of different color mulches on soil temperature and rice growth in a non-flooded condition. Transparent film (TM), black film (BM), two-color film (BWM, silver on the front and black on the back), and no film (NM) in a non-flooded condition were designed. Soil temperature variation at different soil depths of 0-0.25 m and rice plant height, stem thickness, dry matter, yield and quality were monitored. The results showed that compared to no mulching, the mulching treatment effectively increased the average soil temperature during the whole rice growth stage with the soil temperature ranked TM > BM > BWM. Compared with NM, the BM and BWM treatments increased rice yield by 12.1-17.7% and 6.4-14.4% in 2019 and 2020, respectively. The BWM had 18.2% and 6.8% greater gel consistency than NM in 2019 and 2020, respectively. Transparent film should be applied with care because of the high soil temperature stress. Black film and two-color film (silver on the front and black on the back) could be better option for rice yield, increasing and quality improving in a non-flooded condition. © 2023 Society of Chemical Industry.

Rice husk and husk biochar soil amendments store soil carbon while water management controls dissolved organic matter chemistry in well-weathered soil

Rice agriculture feeds over half the world's population, and paddy soils impact the carbon cycle through soil organic carbon (SOC) preservation and production of carbon dioxide (CO2) and methane (CH4), which are greenhouse gases (GHG). Rice husk is a nutrient-rich, underutilized byproduct of rice milling that is sometimes pyrolyzed or combusted. It is unresolved how the incorporation of these residues affects C dynamics in paddy soil. In this study, we sought to determine how untreated (Husk), low-temperature pyrolyzed (Biochar), and combusted (CharSil) husk amendments affect SOC levels, GHG emissions, and dissolved organic matter (DOM) chemistry. We amended Ultisol paddy mesocosms and collected SOC and GHG data for three years of rice grown under alternate wetting and drying (AWD) conditions. We also performed a greenhouse pot study that included water management treatments of nonflooded, AWD, and flooded. Husk, Biochar, and CharSil amendments and flooding generally increased SOC storage and CH4 emissions, while nonflooded conditions increased N2O emissions and nonflooded and CharSil treatments increased CO2 emissions. All amendments stored ∼0.15 kg C m−2 y−1 more SOC than CH4 emissions (as CO2 equivalents), but the combustion of husk to produce CharSil resulted in the net release of CO2 which negates any SOC storage. UV–visible absorption/fluorescence spectroscopy from the pot study suggests that nonflooded treatment decreased DOM aromaticity and molecular size. Our data show that flooding and amendment of Husk and Biochar maximized C storage in the highly weathered rice paddy soil under study despite Husk increasing CH4 emissions. Water management affected dissolved organic matter chemistry more strongly than amendments, but this requires further investigation. Return of rice husk that is untreated or pyrolyzed at low temperature shows promise to close nutrient loops and preserve SOC in rice paddy soils.

Evaluation of water management on arsenic methylation and volatilization in arsenic-contaminated soils strengthened by bioaugmentation and biostimulation

Arsenic (As) fate in paddy fields has been one of the most significant current issues due to the strong As accumulation potential of rice plants under flooded conditions. However, no attempt was done to explore As methylation and volatilization under non-flooded conditions. Herein, we investigated the effects of water management on As methylation and volatilization in three arsenic-contaminated soils enhanced by biostimulation with straw-derived organic matter and bioaugmentation with genetic engineered Pseudomonas putida KT2440 (GE P. putida). Under flooded conditions, the application of biochar (BC), rice straw (RS) and their combination (BC+RS) increased total As in porewater. However, these effects were greatly attenuated under non-flooded conditions. Compared with RS amendment alone, the combination of GE P. putida and RS further promoted the As methylation and volatilization, and the promotion percentage under non-flooded conditions were significantly higher than that under flooded conditions. The combined GE P. putida and RS showed the highest efficiency in As methylation (88 µg/L) and volatilization (415.4 µg/(kg·year)) in the non-flooded soil with moderate As contamination. Finally, stepwise multiple linear regression analysis presented that methylated As, DOC and pH in porewater were the most important factors contributing to As volatilization. Overall, our findings suggest that combination of bioaugmentation with GE P. putida and biostimulation with RS/BC+RS is a potential strategy for bioremediation of arsenic-contaminated soils by enhancing As methylation and volatilization under non-flooded conditions.