Inorganic versus organic fertilizers: How do they lead to methylmercury accumulation in rice grains

Mercury supply limits methylmercury production in paddy soils

The neurotoxic methylmercury (MeHg) is a product of inorganic mercury (IHg) after microbial transformation. Yet it remains unclear whether microbial activity or IHg supply dominates Hg methylation in paddies, hotspots of MeHg formation. Here, we quantified the response of MeHg production to changes in microbial activity and Hg supply using 63 paddy soils under the common scenario of straw amendment, a globally prevalent agricultural practice. We demonstrate that the IHg supply is the limiting factor for Hg methylation in paddies. This is because IHg supply is generally low in soils and can largely be facilitated (by 336-747 %) by straw amendment. The generally high activities of sulfate-reducing bacteria (SRB) do not limit Hg methylation, even though SRB have been validated as the predominant microbial Hg methylators in paddies in this study. These findings caution against the mobilization of legacy Hg triggered by human activities and climate change, resulting in increased MeHg production and the subsequent flux of this potent neurotoxin to our dining tables.

Mercury transformations in algae, plants, and animals: The occurrence, mechanisms, and gaps

Mercury (Hg) is a global pollutant showing potent toxicity to living organisms. The transformations of Hg are critical to global Hg cycling and Hg exposure risks, considering Hg mobilities and toxicities vary depending on Hg speciation. Though currently well understood in ambient environments, Hg transformations are inadequately explored in non-microbial organisms. The primary drivers of in vivo Hg transformations are far from clear, and the impacts of these processes on global Hg cycling and Hg associated health risks are not well understood. This hinders a comprehensive understanding of global Hg cycling and the effective mitigation of Hg exposure risks. Here, we focused on Hg transformations in non-microbial organisms, particularly algae, plants, and animals. The process of Hg oxidation/reduction and methylation/demethylation in organisms were reviewed since these processes are the key transformations between the dominant Hg species, i.e., elemental Hg (Hg0), divalent inorganic Hg (IHgII), and methylmercury (MeHg). By summarizing the current knowledge of Hg transformations in organisms, we proposed the potential yet overlooked drivers of these processes, along with potential challenges that hinder a full understanding of in vivo Hg transformations. Knowledge summarized in this review would help achieve a comprehensive understanding of the fate and toxicity of Hg in organisms, providing a basis for predicting Hg cycles and mitigating human exposure.

The Effects of Different Soil Component Couplings on the Methylation and Bioavailability of Mercury in Soil

Soil composition can influence the chemical forms and bioavailability of soil mercury (Hg). However, previous studies have predominantly focused on the influence of individual components on the biogeochemical behavior of soil Hg, while the influence of various component interactions among several individual factors remain unclear. In this study, artificial soil was prepared by precisely regulating its components, and a controlled potted experiment was conducted to investigate the influence of various organic and inorganic constituents, as well as different soil textures resulting from their coupling, on soil Hg methylation and its bioavailability. Our findings show that inorganic components in the soils primarily exhibit adsorption and fixation effects on Hg, thereby reducing the accumulation of total mercury (THg) and methylmercury (MeHg) in plants. It is noteworthy that iron sulfide simultaneously resulted in an increase in soil MeHg concentration (277%). Concentrations of THg and MeHg in soil with peat were lower in rice but greater in spinach. A correlation analysis indicated that the size of soil particles was a crucial factor affecting the accumulation of Hg in plants. Consequently, even though fulvic acid activated soil Hg, it significantly increased the proportion of soil particles smaller than 100.8 μm, thus inhibiting the accumulation of Hg in plants, particularly reducing the concentration of THg (93%) and MeHg (85%) in water spinach. These results demonstrate that the interaction of organic and inorganic components can influence the biogeochemical behavior of soil Hg not only through their chemical properties, but also by altering the soil texture.

The divergent effects of nitrate and ammonium application on mercury methylation, demethylation, and reduction in flooded paddy slurries

The production of methylmercury (MeHg) in flooded paddy fields determines its accumulation in rice grains; this, in turn, results in MeHg exposure risks for not only rice-eating humans but also wildlife. Nitrogen (N) fertilizers have been widely applied in rice cultivation fields to supply essential nutrients. However, the effects of N fertilizer addition on mercury (Hg) transformations are not unclear. This limits our understanding of MeHg formation in rice paddy ecosystems. In this study, we spiked three Hg tracers (200HgII, Me198Hg, and 202Hg0) in paddy slurries fertilized with urea, ammonium, and nitrate. The influences of N fertilization on Hg methylation, demethylation, and reduction and the underlying mechanisms were elucidated. The results revealed that dissimilatory nitrate reduction was the dominant process in the incubated paddy slurries. Nitrate addition inhibited HgII reduction, HgII methylation, and MeHg demethylation. Competition between nitrates and other electron acceptors (e.g., HgII, sulfate, or carbon dioxide) under dark conditions was the mechanism underlying nitrate-regulated Hg transformation. Ammonium and urea additions promoted HgII reduction, and anaerobic ammonium oxidation coupled with HgII reduction (Hgammox) was likely the reason. This work highlighted that nitrate addition not only inhibited HgII methylation but also reduced the demethylation of MeHg and therefore may generate more accumulation of MeHg in the incubated paddy slurries. Findings from this study link the biogeochemical cycling of N and Hg and provide crucial knowledge for assessing Hg risks in intermittently flooded wetland ecosystems.

Are Chokeberry Products Safe for Health? Evaluation of the Content of Contaminants and Health Risk

The health-promoting properties of chokeberry fruit have been confirmed in numerous scientific studies. It has been shown that the consumption of these fruits, due to the high content of bioactive compounds, has beneficial effects in neurodegenerative diseases, in addition to having hypolipemic, hypotensive, hypoglycemic, and anti-inflammatory properties. However, different conditions and methods of fruit cultivation, as well as methods of juice and fiber production, may result in a high content of toxic substances, which reduce the health value of chokeberry products. Many substances are environmental pollutants. In this study, for the first time, we examined the content of toxic elements (As, Hg, Cd, Pb), nitrates, and nitrites in all chokeberry juices (organic, conventional, from concentrate, and not from fruit concentrate) without additives and in all chokeberry fibers available in Poland. In addition, risk indicators of adverse health effects were calculated. The median content of the contaminants tested in juices was 0.461 µg/kg for As, 1.170 µg/kg for Cd, 0.427 µg/kg for Hg, 1.404 µg/kg for Pb, 4.892 mg/kg for NO2-, and 41.788 mg/kg for NO3-. These values did not exceed the permissible standards for the calculated indicators. There were also no statistically significant differences in the content of Cd, Hg, and Pb, as well as nitrates (III) and nitrates (V), in the tested juices depending on the method of cultivation and juice production. However, statistically significant differences in As content were found between juices from conventional and organic cultivation (1.032 µg/kg vs. 0.458 µg/kg) and juices from concentrate and not from concentrate (1.164 µg/kg vs. 0.460 µg/kg). There were no statistically significant differences with respect to impurities in fibers. It is shown that the consumption of chokeberry juice and fiber in the amount normally consumed does not pose a health risk associated with the intake of toxic substances; in the case of long-term fiber consumption, the Pb content should be monitored. In particular, organic juices and those not from fruit concentrate are recommended due to the lower As content.

Nutrient recovery from municipal solid waste leachate in the scope of circular economy: Recent developments and future perspectives

Holistically considering the current situation of the commercial synthetic fertilizer (CSF) market, recent global developments, and future projection studies, dependency on CSFs in agricultural production born significant risks, especially to the food security of foreign-dependent countries. The foreign dependency of countries in terms of CSFs can be reduced by the concepts such as the circular economy and resource recovery. Recently, waste streams are considered as a source in order to produce recovery-based fertilizers (RBF). RBFs produced from different waste streams can be substituted with CSFs as input for agricultural applications. Municipal solid waste leachate (MSWL) is one of the waste streams that have a high potential for RBF production. Distribution of the published papers over the years shows that this potential was noticed by more researchers in the millennium. MSWL contains a remarkable amount of nitrogen and phosphorus which are the main nutrients required for agricultural production. These nutrients can be recovered with many different methods such as microalgae cultivation, chemical precipitation, ammonia stripping, membrane separation, etc. MSWL can be generated within the different phases of municipal solid waste (MSW) management. Although it is mainly composed of landfill leachate (LL), composting plant leachate (CPL), incineration plant leachate (IPL), and transfer station leachate (TSL) should be considered as potential sources to produce RBF. This study compiles studies conducted on MSWL from the perspective of nitrogen and phosphorus recovery. Moreover, recent developments and limitations of the subject were extensively discussed and future perspectives were introduced by considering the entire MSW management. Investigated studies in this review showed that the potential of MSWL to produce RBF is significant. The outcomes of this paper will serve the countries for ensuring their food security by implementing the resource recovery concept to produce RBF. Thus, the risks born with the recent global developments could be overcome in this way besides the positive environmental outcomes of resource recovery.

Kinetic characteristics of and critical stages for mercury accumulation in rice (Oryza sativa L.)

By studying the dynamic characteristics of and key growth stages for mercury (Hg) enrichment in rice, the Hg migration and translocation processes in this species can be better understood. In this study, a pot experiment was conducted, wherein two rice cultivars, Tianyouhuazhan (TYHZ, indica) and Zhendao 18 (ZD18, japonica), were selected and planted for analysing the Hg accumulation kinetic characteristics in rice plants. The plants were sampled at each growth stage, and the biomass and total Hg (THg) and methylmercury (MeHg) concentrations of each tissue were measured. The relative Hg contribution rates (CRs) in whole rice plants and rice grains were calculated, and the growth stage with the highest relative contribution was identified as the key growth stage for Hg accumulation. The results indicated that in rice, the MeHg translocation capability was stronger than the THg translocation capability. Significant differences in the kinetic characteristics of Hg accumulation were found between the two rice cultivars, and the TYHZ rice grains had a stronger Hg accumulation ability than the ZD18 rice grains. The key growth stages for THg accumulation in whole rice plants of both cultivars were the tillering and booting stages, while that for MeHg accumulation was the tillering stage. The key period for Hg accumulation in rice grains was the grain filling stage for both cultivars. The insights from this study could provide scientific guidance for the safe production of rice in Hg-contaminated soil.

A new insight into the role of iron plaque in arsenic and cadmium accumulation in rice (Oryza sativa L.) roots

Iron plaque, naturally iron-manganese (hydr)oxides adhered to the surface of rice roots, controls the sequestration and accumulation of arsenic (As) and cadmium (Cd) in the paddy soil-rice system. However, the effects of the paddy rice growth on the iron plaque formation and As and Cd accumulation of rice roots are often neglected. This study explores the distribution characteristics of iron plaques on rice roots and their effects on As and Cd sequestration and uptake via cutting the rice roots into 5 cm segments. Results indicated that the percentages of rice root biomass of 0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, and 20-25 cm are 57.5 %, 25.2 %, 9.3 %, 4.9 %, and 3.1 %, respectively. Iron (Fe) and manganese (Mn) concentrations in iron plaques on rice roots of various segments are 41.19-81.11 g kg^-1 and 0.94-3.20 g kg^-1, respectively. Increased tendency of Fe and Mn concentrations from the proximal rice roots to the distal rice roots show that iron plaque is more likely to deposit on the distal rice roots than proximal rice roots. The DCB-extractable As and Cd concentrations of rice roots with various segments are 694.63-1517.23 mg kg^-1 and 9.00-37.58 mg kg^-1, displaying a similar trend to the distribution characteristics of Fe and Mn. Furthermore, the average transfer factor (TF) of As (0.68 ± 0.26) from iron plaque to rice roots was significantly lower than that of Cd (1.57 ± 0.19) (P < 0.05). There was a significant positive correlation between the Cd sequestration in iron plaque and the Cd accumulation in rice roots (R = 0.97, P < 0.01). Still, a similar correlation wasn't observed between As sequestration in iron plaque and As accumulation in rice roots (R = -0.04, and P > 0.05). These results indicated that the formed iron plaque might act as a barrier to As uptake by rice roots and a facilitator to Cd uptake. This study provides insight into the role of iron plaque in the sequestration and uptake of As and Cd in paddy soil-rice systems.