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Rice is a staple food by an increasing number of people in China. As more issues have arisen in China due to rice contaminated by cadmium (Cd), Cd contamination in arable soils has become a severe problem. In China, many studies have examined Cd contamination in arable soils on a national scale, but little studies have focused on the distribution of Cd in paddy fields. This study explored the spatial pattern of Cd in paddy soils in China, made a preliminary evaluation of the potential risk, and identified the most critically contaminated regions based on the domestic rough rice trade flow. The results showed that Cd concentrations in paddy soils in China ranged from 0.01 to 5.50 mg/kg, with a median value of 0.23 mg/kg. On average, the highest Cd concentrations were in Hunan (0.73 mg/kg), Guangxi (0.70 mg/kg), and Sichuan (0.46 mg/kg) provinces. Cd concentrations in paddy soils in central and western regions were higher than those in eastern regions, especially the southeastern coastal regions. Of the administrative regions, Cd standard exceedance rate was 33.2 %, and the heavy pollution rate was 8.6 %. Regarding to Cd of paddy soil, soil environmental quality was better in Northeast China Plain than in Yangtze River Basin and southeastern coastal region. Mining activities were the main anthropogenic pollution source of Cd in Chinese paddy soil. Based on rice trade, more of the Chinese population would be exposed to Cd through intake of rice produced in Hunan province. Certain regions that output rice, especially Hunan province, should be given priority in the management and control of Cd contamination in paddy soil.

Aims Two field microcosm experiments and ¹⁵N labeling techniques were used to investigate the effects of biochar addition on rice N nutrition and GHG emissions in an Inceptisol and an Ultisol. Methods Biochar N bioavailability and effect of biochar on fertilizer nitrogen-use efficiency (NUE) were studied by ¹⁵N-enriched wheat biochar (7.8803 atom% ¹⁵N) and fertilizer urea (5.0026 atom% ¹⁵N) (Experiment I). Corn biochar and corn stalks were applied at 12 Mgha⁻¹ to study their effects on GHG emissions (Experiment II). Results Biochar had no significant impact on rice production and less than 2 % of the biochar N was available to plants in the first season. Biochar addition increased soil C and N contents and decreased urea NUE. Seasonal cumulative CH₄ emissions with biochar were similar to the controls, but significantly lower than the local practice of straw amendment. N₂O emissions with biochar were similar to the control in the acidic Ultisol, but significantly higher in the slightly alkaline Inceptisol. Carbon-balance calculations found no major losses of biochar-C. Conclusion Low bio-availability of biochar N did not make a significantly impact on rice production or N nutrition during the first year. Replacement of straw amendments with biochar could decrease CH₄ emissions and increase SOC stocks.

Irrigated and rain-fed rice fields are unique agroecosystems and anthropogenic wetlands whose main feature is seasonal flooding. Flooded soils are characterized by spatiotemporal shifts and oscillation of the oxygen status and redox potential, sustaining varieties of microbial metabolisms, where bacteria and methanogenic archaea play principal roles and thus have been the major research targets. In this review, we focus on the diversity and ecology of protists—often overlooked biological entities—in wetland rice field soils. Protists with different ecological functions, i.e., phagotrophs, phototrophs, saprotrophs, and parasites, inhabit a rice field soil with a community- and individual-level adaptation to the wide range of oxygen tensions and redox potential. Other agricultural managements like fertilization and char application also influence the protist community. They link to the material cycling in rice soil and affect the activities and community composition of the microorganisms involved in the biogeochemical cycles. Rice roots are the hot spot for protists, which control the rhizospheric bacterial community and could increase the plant productivity through enhancing nutrient release and altering bacterial activities. This review highlights the essential roles of protists in a wetland rice field soil and needs for further research to fill the gaps in knowledge regarding the diversity and functions of the protists in this unique agroecosystem.

Background Worldwide, there is an increasing interest in using biochar in agriculture to help mitigate global warming and improve crop productivity. Methods The effects of biochar on greenhouse gas (GHG) emissions and rice and wheat yields were assessed using outdoor pot experiments in two different soils (upland soil vs. paddy soil) and an aerobic incubation experiment in the paddy soil. Results Biochar addition to the upland soil increased methane (CH4) emissions by 37 % during the rice season, while it had no effect on CH4 emissions during the wheat season. Biochar amendment decreased nitrous oxide (N2O) emissions up to 54 % and 53 % during the rice and wheat seasons, respectively, but had no effect on the ecosystem respiration in either crop season. In the aerobic incubation experiment, biochar addition significantly decreased N2O emissions and increased carbon dioxide (CO2) emissions from the paddy soil (P<0.01) without urea nitrogen. Biochar addition increased grain yield and biomass if applied with nitrogen fertilizer. Averaged over the two soils, biochar amendments increased the production of rice and wheat by 12 % and 17 %, respectively, and these increases can be partly attributed to the increases in soil nitrate retention. Conclusions Our results demonstrated that although biochar increased the global warming potential at high nitrogen fertilizer application, biochar incorporation significantly decreased N2O emissions while promoting crop production.

Rice is a worldwide staple food and heavy metal contamination is often reported in rice production. Heavy metal can originate from natural sources or be present through anthropogenic contamination. Therefore, this review summarizes the current status of heavy metal contamination in paddy soil and plants, highlighting the mechanism of uptake, bioaccumulation, and health risk assessment. A scoping search employing Google Scholar, Science Direct, Research Gate, Scopus, and Wiley Online was carried out to build up the review using the following keywords: heavy metals, absorption, translocation, accumulation, uptake, biotransformation, rice, and human risk with no restrictions being placed on the year of study. Cadmium (Cd), arsenic (As), and lead (Pb) have been identified as the most prevalent metals in rice cultivation. Mining and irrigation activities are primary sources, but chemical fertilizer and pesticide usage also contribute to heavy metal contamination of paddy soil worldwide. Further to their adverse effect on the paddy ecosystem by reducing the soil fertility and grain yield, heavy metal contamination represents a risk to human health. An in-depth discussion is further offered on health risk assessments by quantitative measurement to identify potential risk towards heavy metal exposure via rice consumption, which consisted of in vitro digestion models through a vital ingestion portion of rice.

Phosphorus (P) cycling in paddy soil is closely related to iron (Fe) redox wheel; its availability to rice has thus generally been ascribed to Fe minerals reductive dissolution. However, the literature aimed to identify the best method for predicting rice available P does not uniformly point to Fe reductants. Rice plants can indeed solubilize and absorb P through many strategies as a function of P supply, modifying the chemical environment. Therefore, this study aims to estimate P availability in paddy soils coupling the redox mechanisms driving P cycling with concurrent plant responses. Soil available P was estimated in three groups of paddy soils with low, medium, or high P content assessing easily desorbable pools (0.01 M calcium chloride, Olsen, Mehlich-III, anion exchanging resins) and Fe-bound P pools (EDTA, citrate-ascorbate, and oxalate). Rice P uptake and responses to P availability were assessed by a mesocosm cultivation trial. Although P released in porewater positively correlated with dissolved Fe(II), it did not with plant P uptake, and readily desorbable P pools were better availability predictors than Fe-bound pools, mainly because of the asynchrony observed between Fe reduction and plant P demand. Moreover, in low P soils, plants showed higher Fe(II) oxidation, enhanced root growth, and up-regulation of P root transporter encoding genes, plant responses being related with changes in P pools. These results indicate the generally assumed direct link between Fe reduction and rice P nutrition in paddy soils as an oversimplification, with rice P nutrition appearing as the result of a complex trade-off between soil redox dynamics, P content, and plant responses.

PurposeFertilization is a vital approach to increase the crop yield by enhancing soil fertility, but some of the fertilizer sources such as pig manure contain non-essential toxic heavy metals, which can produce the environmental and public health risk. Therefore, the purpose of this study was to investigate the soil fertility and heavy metal pollution risk under long-term fertilization in acidic paddy soil.Materials and methodsFertilizer treatments that were arranged in randomized complete block design (RCBD) included CK (no fertilization), NK (inorganic nitrogen and potassium fertilization), NPK (inorganic NK and phosphorus fertilization), NPKM1 (70% of NPK and 30% pig manure application), NPKM2 (50% of NPK and 50% of pig manure application), and NPKM3 (30% of NPK and 70% of pig manure application).Results and discussionThe rice grain yield and soil nutrient contents were highest under NPKM3 treatment. Long-term addition of manure significantly (P ≤ 0.05) increased soil pH and SOC content compared to the NPK fertilization. Soil available and total Cr, Cd, and Hg contents were highest under NPKM3 treatments, while soil total and available Pb content was significantly (P ≤ 0.05) higher under NPK treatment. Highest ecological risk (IR) was (1904) under NPK treatment and highest pollution load index (PLI) was 1.5 under NPKM3 treatment. Cd concentration in rice grain exceeded the maximum permissible limit of 0.1 mg kg−1 under combined application of manure and inorganic fertilization treatments. Grain Cr, Hg, and Pb contents were within safe limits of their concentration in all treatments. Moreover, biological accumulation coefficients of Cr, Cd, Hg, Pb, Zn, and Cu were highest under NPK treatment. Redundancy analysis (RDA) showed that soil pH and nutrient contents showed significant correlation with heavy metal concentrations in soil. Soil pH showed significant (P ≤ 0.05) positive effect on Cd accumulation in rice grain.ConclusionHeavy metals in pig manure should be monitored before application to the field to reduce the risk of heavy metal pollution in soil and plant. Furthermore, combined application at the rate of 70% inorganic fertilization and 30% of manure could be better strategy to produce high crop yield with minimum risk of heavy metal contamination in soil and food crops.

Microbial residues are key components of stable soil organic C (SOC). However, the accumulation patterns of fungal and bacterial residues across climate regions are largely unknown, especially in paddy soils. In this study, the amounts of microbial-derived amino sugars (AS) with their constituents, glucosamine (GlcN), galactosamine (GalN), and muramic acid (MurN, a biomarker of bacterial residues) were quantified in paddy soils, which were collected from mid-temperate, warm-temperate, subtropical, and tropical climate regions across eastern China. The contents of total AS and fungal-derived GlcN (F-GlcN, a biomarker of fungal residues) were lowest in the warm-temperate region, but not significantly different among the other three climate regions. The MurN content and its contribution to SOC accumulation were higher in the warmer and wetter regions (subtropic and tropic) than in the cooler and drier ones (mid-temperate and warm-temperate). Consequently, the ratio of F-GlcN to MurN was lower in the warmer and wetter regions (8.5–15.4) than in the cooler and drier ones (12.8–28.8). These results illustrate that the bacteria participating in SOC transformation and stabilization in paddy soils exerted more prominent activities in the warmer and wetter regions than in the cooler and drier regions. Structure equation models emphasize that the contrasting patterns of fungal and bacterial residues’ contribution to SOC accumulation in paddy ecosystems along the latitudinal gradient were mainly attributed to their different responses to the climate factors of temperature and precipitation.

Background and aims Water and nutrient management influences the allocation and stabilisation of newly assimilated carbon (C) in paddy soils. This study aimed to determine the belowground allocation of C assimilated by rice and the subsequent C stabilisation in soil aggregates and as mineral-organic associates depending on combined alternate wetting and drying (AWD) versus continuous flooding (CF) and P fertilisation. Methods We continuously labelled rice plants in 13 CO 2 atmosphere under AWD versus CF water management, and at two P fertilisation levels (0 or 80 mg P kg −1 soil). The 13 C allocation to soil and its incorporation into the wet-sieved aggregate size classes and density fractions of the rhizosphere and bulk soils were analysed 6, 14, and 22 days after the labelling was started (D6, D14, and D22, respectively). Results Under both water regimes and P fertilisation levels, the proportion of photoassimilates was the highest in the silt- and clay-size aggregate classes and in the mineral-associated fraction. On D6 and D14, P fertilization resulted in smaller 13 C incorporation into soil, independent of water management. In the rhizosphere soil, at D22, P fertilisation increased 13 C incorporation over no P amendment in macroaggregates (>250 μm) by 32% (AWD) and 42% (CF), in microaggregates (250–53 μm) by 97% (CF), and in the silt + clay size class (<53 μm) by 83% (CF). Further, P fertilisation led to larger 13 C incorporation into the rhizosphere soil light fraction (75% at AWD and 90% at CF) and dense fraction (38% and 45%, respectively), and into the bulk soil macroaggregates (71% and 78%, respectively). Conclusions Phosphorus fertilisation increased the contents of recent photoassimilates in soil aggregate classes with longer residence time as well as of the particulate organic matter with the continuation of plant growth. This positive response of the stabilisation of recent plant photosynthates in soil to P fertilisation can increase the potential of paddy soil for C sequestration. This potential is not limited by the introduction of alternate wetting and drying water-saving technique.

Arsenic contamination in paddy soils has aroused global concern due to its threats to food security and human health. Biochar modified with different iron materials was prepared for arsenic (As) immobilization in contaminated soils. Soil incubation experiments were carried to investigate the effects of biochar modified with Fe-oxyhydroxy sulfate (Biochar-FeOS), FeCl3 (Biochar-FeCl3), and zero-valent iron (Biochar-Fe) on the pH, NaHCO3-extractable As concentrations, and the As fractions in soils. The scanning electron microscope and X-ray diffraction analysis demonstrated that iron was successfully loaded onto the surface or embedded into the pores of the biochar. Addition of Biochar-FeOS, Biochar-FeCl3, and Biochar-Fe had no significant effects on the soil pH but significantly decreased the contents of NaHCO3-extractable As in soils by 13.95–30.35%, 10.97–28.39%, and 17.98–35.18%, respectively. Biochar-FeOS, Biochar-FeCl3, and Biochar-Fe treatments decreased the concentrations of non-specifically sorbed and specifically sorbed As fractions in soils, and increased the amorphous and poorly crystalline, hydrated Fe, Al oxide-bound, and residual As fractions. Compared with the other iron-modified biochars, Biochar-FeOS showed the most effective immobilization and has the potential for the remediation of As-contaminated paddy soils.