The Ins and Outs of Per- and Polyfluoroalkyl Substances in the Great Lakes: The Role of Atmospheric Deposition.

As part of the Integrated Atmospheric Deposition Network, precipitation (n = 207) and air (n = 60) from five sites and water samples (n = 87) from all five Great Lakes were collected in 2021-2023 and analyzed for 41 per- and polyfluoroalkyl substances (PFAS). These measurements were combined with other available data to estimate the mass budget for four representative compounds, PFBA, PFBS, PFOS, and PFOA for the basin. The median Σ41PFAS concentrations in precipitation across the five sites ranged between 2.4 and 4.5 ng/L. The median Σ41PFAS concentration in lake water was highest in Lake Ontario (11 ng/L) and lowest in Lake Superior (1.3 ng/L). The median Σ41PFAS concentration in air samples was highest in Cleveland at 410 pg/m3 and lowest at Sleeping Bear Dunes at 146 pg/m3. The net mass transfer flows were generally negative for Lakes Superior, Michigan, and Huron and positive for Lakes Erie and Ontario, indicating that the three most northern lakes are accumulating PFAS and the other two are eliminating PFAS. Atmospheric deposition is an important source of PFAS, particularly for Lake Superior.

Atmospheric Nitrogen Pollution Control Benefits the Coastal Environment.

Nitrogen is an essential nutrient and a major limiting element for the ocean ecosystem. Since the preindustrial era, substantial amounts of nitrogen from terrestrial sources have entered the ocean via rivers, groundwater, and atmospheric deposition. China serves as a key hub in the global nitrogen cycle, but the pathways, sources, and potential mitigation strategies for land-ocean nitrogen transport are unclear. By combining the CHANS, WRF-Chem, and WNF models, we estimated that 8 million tonnes (Tg) of nitrogen was transferred into the ocean in 2017 in China, with atmospheric deposition contributing 1/3. About half variation of the offshore chlorophyll concentration was explained by atmospheric deposition. The Bohai Sea was the hot spot of nitrogen input, estimated at 214 kg N ha-1, while other areas were around 25-51 kg N ha-1. The largest contributors are agricultural systems (4 Tg, 55%), followed by domestic sewage (2 Tg, 21%). Abatement measures could reduce nitrogen export to the ocean by 43%, and mitigating ammonia and nitrogen oxide emissions accounts for 33% of this reduction, highlighting the importance of addressing air pollution in resolving ocean pollution. The cost-benefit analysis suggests the priority of nitrogen reduction in cropland and transport systems for the ocean environment.

Long-term Trends of Dissolved Organic Carbon Dynamics in a Subtropical Forest Responding to Environmental Changes.

Dissolved organic carbon (DOC) dynamics are critical to carbon cycling in forest ecosystems and sensitive to global change. Our study, spanning from 2001 to 2020 in a headwater catchment in subtropical China, analyzed DOC and water chemistry of throughfall, litter leachate, soil waters at various depths, and streamwater. We focused on DOC transport through hydrological pathways and assessed the long-term trends in DOC dynamics amidst environmental and climatic changes. Our results showed that the annual DOC deposition via throughfall and stream outflow was 14.2 ± 2.2 and 1.87 ± 0.83 g C m-2 year-1, respectively. Notably, there was a long-term declining trend in DOC deposition via throughfall (-0.195 mg C L-1 year-1), attributed to reduced organic carbon emissions from clean air actions. Conversely, DOC concentrations in soil waters and stream waters showed increasing trends, primarily due to mitigated acid deposition. Moreover, elevated temperature and precipitation could partly explain the long-term rise in DOC leaching. These trends in DOC dynamics have significant implications for the stability of carbon sink in terrestrial, aquatic, and even oceanic ecosystems at regional scales.

Substantial accumulation of mercury in the deepest parts of the ocean and implications for the environmental mercury cycle.

Anthropogenic activities have led to widespread contamination with mercury (Hg), a potent neurotoxin that bioaccumulates through food webs. Recent models estimated that, presently, 200 to 600 t of Hg is sequestered annually in deep-sea sediments, approximately doubling since industrialization. However, most studies did not extend to the hadal zone (6,000- to 11,000-m depth), the deepest ocean realm. Here, we report on measurements of Hg and related parameters in sediment cores from four trench regions (1,560 to 10,840 m), showing that the world's deepest ocean realm is accumulating Hg at remarkably high rates (depth-integrated minimum-maximum: 24 to 220 µg ⋅ m-2 ⋅ y-1) greater than the global deep-sea average by a factor of up to 400, with most Hg in these trenches being derived from the surface ocean. Furthermore, vertical profiles of Hg concentrations in trench cores show notable increasing trends from pre-1900 [average 51 ± 14 (1σ) ng ⋅ g-1] to post-1950 (81 ± 32 ng ⋅ g-1). This increase cannot be explained by changes in the delivery rate of organic carbon alone but also need increasing Hg delivery from anthropogenic sources. This evidence, along with recent findings on the high abundance of methylmercury in hadal biota [R. Sun et al, Nat. Commun. 11, 3389 (2020); J. D. Blum et al, Proc. Natl. Acad. Sci. U. S. A. 117, 29292-29298 (2020)], leads us to propose that hadal trenches are a large marine sink for Hg and may play an important role in the regulation of the global biogeochemical cycle of Hg.

Toxic effects of atmospheric deposition in mining areas on wheat seedlings.

Storage and transportation of coal, as well as operation of coal-fired power plants, produce amounts of metallic exhaust that may lead to different atmospheric environment in the overlapped areas of farmland and coal resource (OAFCR) environment. To investigate the effects of different atmospheric environment in the OAFCR region (north of Xuzhou) on wheat seedlings (AK-58), a box experiment was conducted and compared to an area far from the OAFCR (south of Xuzhou). The study revealed that (1) compared to the southern suburb of Xuzhou, the fresh and dry weight, activities of photosynthetic enzymes and POD of wheat seedlings in the OAFCR reduced obviously. (2) Significantly higher levels of Cr, Cd, Pb, Zn, and Cu were found in the shoots and roots of wheat seedlings in the OAFCR, with lower transfer factor for heavy metals (except Cd and As) in comparison to those in the southern suburb. And the bioconcentration factors of heavy metals (except As) in wheat seedlings in the OAFCR were significantly higher. (3) Nearly 90% of heavy metals (Pb, Cu, Cd, Zn, and Cr) absorbed by wheat were stored in cell walls and soluble fractions, with significantly higher contents of Cu and Cr in wheat seedlings' cell walls and higher contents of Pb, Zn, and Cd in soluble components found in the OAFCR. Our results showed that atmospheric deposition in the mining area has a certain toxic effect on wheat seedlings, and this study provides a theoretical basis for OAFCR crop toxicity management.

Accumulation analysis and overall measurement to represent airborne toxic metals with passive tree bark biomonitoring technique in urban areas.

Authorities have long proved the utility of bioindicators in monitoring the state of environmental pollution. Some biological indicators can measure environmental pollutant levels, and many tree species have been tested for suitability for monitoring purposes. The differences in morphological characteristics in the trees have demonstrated the effects of human activities on different materials. Measuring bark and wood biomass from contaminated sites was identified and directly compared with those from a clean site or areas characterized by distinct contamination sources. However, preliminary results demonstrate the approach's potential in the realization of strategies for disease control and promoting health to reduce environmental and health inequalities in at-risk urban areas. Picea orientalis L. and Cedrus atlantica Endl., especially their bark, can be regarded as a more robust storage of Cu (37.95 mg/kg) and Mn (188.25 mg/kg) than Pinus pinaster, Cupressus arizonica, and Pseudotsuga menziesii, which and is therefore a better bioindicator for Cu and Mn pollution. Considering the total concentrations as a result of the study, the pollution is thought to be caused by environmental problems and traffic in the region. The deposition of Cu, Mn, Ni, and Zn elements was found P. menziesii (60, 443, 58, and 258 mg/kg) and P. orientalis (76, 1684, 41, and 378 mg/kg) and seems to reflect atmospheric quite clearly compared to P. pinaster, C. arizonica, and C. atlantica. Ni and Zn concentrations have significantly increased since 1983, and P. menziesii and P. orientalis can be potentially valuable bioindicators for emphasizing polluted fields.

Assessing heavy metal contamination in a Brazilian metropolis: a case study with a focus on (bio)indicators.

The continuous expansion of the global vehicle fleet poses a growing threat to environmental quality through heavy metal contamination. In this scenario, monitoring to safeguard public health in urban areas is necessary. Our study involved the collection of 36 street dust and 29 moss samples from roads of a Brazilian metropolis (Recife) with varying traffic intensities as follows: natural reserve (0 vehicles per day), low (< 15,000 vehicles per day), medium (15,000-30,000 vehicles per day), and high (> 30,000 vehicles per day). ICP-AES analysis was performed to determine the concentrations of nine potentially toxic metals (Ba, Cd, Cr, Cu, Mn, Ni, Pb, V, and Zn) to assess the influence of vehicular flow on urban contamination. In the street dust samples, the mean metal concentrations (mg kg-1) exhibited the following order: Ba (503.7) > Mn (303.0) > Zn (144.4) > Cu (95.3) > Cr (56.1) > Pb (34.2) > V (28.7) > Ni (11.3) > Cd (1.5). Conversely, in the moss samples, the metal concentration order was as follows (mg kg-1): Mn (63.8) > Zn (62.5) > Ba (61.0) > Cu (17.7) > Cr (8.0) > V (7.3) > Pb (7.0) > Ni (2.9) > Cd (0.3). Roads with higher traffic volumes exhibited the highest metal enrichments in moss samples for all metals and in dust samples for Cd, Cr, Mn, Ni, and V. However, dust from low-flow roads had higher enrichments for Ba, Cu, and Zn, indicating the influential role of other traffic-related factors in metal deposition. Our findings highlight traffic flow as the predominant source of pollution in urban centers, with both street dust and moss serving as sensitive indicators of metal input attributable to vehicular traffic. These indicators offer valuable insights for urban quality monitoring and pollution control efforts.

Quantification and characterization of airborne microplastics in the coastal area of Terengganu, Malaysia.

Microplastics (MPs) pose a threat to ecosystems due to their capacity to bind with toxic chemicals. While the occurrence of MPs in aquatic environmental matrices like water, sediments, and biota is well studied, their presence in the atmosphere remains less understood. This study aimed to determine the presence of airborne MPs and their characteristics through ground-based sampling in the coastal city of Kuala Nerus, Terengganu, Malaysia. Airborne MP samples were collected using passive sampling technique in December 2019. MPs were manually counted and identified using a stereomicroscope based on their colour and shape. The average deposition rate of airborne MPs during the sampling period was 5476 ± 3796 particles/m2/day, ranging from 576 to 15,562 particles/m2/day. Various colours such as transparent (38%), blue (25%), black (20%), red (13%), and others (4%) were observed. The predominant shape of airborne MPs was fibres (> 99%). The morphology structure of MPs observed using a scanning electron microscope (SEM) showed a cracked surface on MPs, suggesting weathering and irregular fragmentation. Further elemental analysis using energy dispersive X-ray spectroscopy (EDS) showed the presence of heavy metals such as aluminium (Al) and cadmium (Cd) on the surface of MPs, attributed to the adsorption capacities of MPs. Polymer types of airborne MPs were analysed using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), which revealed particles composed of polyester (PES), polyethylene (PE), and polypropylene (PP). The preliminary findings could provide additional information for further investigations of MPs, especially in the atmosphere, to better understand their sources and potential human exposure.

Atmospheric phosphorus deposition amplifies carbon sinks in simulations of a tropical forest in Central Africa.

Spatial redistribution of nutrients by atmospheric transport and deposition could theoretically act as a continental-scale mechanism which counteracts declines in soil fertility caused by nutrient lock-up in accumulating biomass in tropical forests in Central Africa. However, to what extent it affects carbon sinks in forests remains elusive. Here we use a terrestrial biosphere model to quantify the impact of changes in atmospheric nitrogen and phosphorus deposition on plant nutrition and biomass carbon sink at a typical lowland forest site in Central Africa. We find that the increase in nutrient deposition since the 1980s could have contributed to the carbon sink over the past four decades up to an extent which is similar to that from the combined effects of increasing atmospheric carbon dioxide and climate change. Furthermore, we find that the modelled carbon sink responds to changes in phosphorus deposition, but less so to nitrogen deposition. The pronounced response of ecosystem productivity to changes in nutrient deposition illustrates a potential mechanism that could control carbon sinks in Central Africa. Monitoring the quantity and quality of nutrient deposition is needed in this region, given the changes in nutrient deposition due to human land use.

Future climate change effects on US forest composition may offset benefits of reduced atmospheric deposition of N and S.

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously derived growth and survival responses for 94 tree species, representing >90% of the contiguous US forest basal area, to project how changes in mean annual temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that under the low climate change scenario (RCP 4.5), reductions in aboveground tree biomass from higher temperatures are roughly offset by increases in aboveground tree biomass from reductions in N and S deposition. However, under the higher climate change scenario (RCP 8.5) the decreases from climate change overwhelm increases from reductions in N and S deposition. These broad trends underlie wide variation among species. We found averaged across temperature scenarios the relative abundance of 60 species were projected to decrease more than 5% and 20 species were projected to increase more than 5%; and reductions of N and S deposition led to a decrease for 13 species and an increase for 40 species. This suggests large shifts in the composition of US forests in the future. Negative climate effects were mostly from elevated temperature and were not offset by scenarios with wetter conditions. We found that by 2100 an estimated 1 billion trees under the RCP 4.5 scenario and 20 billion trees under the RCP 8.5 scenario may be pushed outside the temperature record upon which these relationships were derived. These results may not fully capture future changes in forest composition as several other factors were not included. Overall efforts to reduce atmospheric deposition of N and S will likely be insufficient to overcome climate change impacts on forest demography across much of the United States unless we adhere to the low climate change scenario.

Mercury Pollution History in Tropical and Subtropical American Lakes: Multiple Impacts and the Possible Relationship with Climate Change.

Sediment cores obtained from 11 tropical and subtropical American lakes revealed that local human activities significantly increased mercury (Hg) inputs and pollution levels. Remote lakes also have been contaminated by anthropogenic Hg through atmospheric depositions. Long-term sediment-core profiles revealed an approximately 3-fold increase in Hg fluxes to sediments from c. 1850 to 2000. Generalized additive models indicate that c. 3-fold increases in Hg fluxes also occurred since 2000 in the remote sites, while Hg emissions from anthropogenic sources have remained relatively stable. The tropical and subtropical Americas are vulnerable to extreme weather events. Air temperatures in this region have shown a marked increase since the 1990s, and extreme weather events arising from climate change have increased. When comparing Hg fluxes to recent (1950-2016) climatic changes, results show marked increases in Hg fluxes to sediments during dry periods. The Standardized Precipitation-Evapotranspiration Index (SPEI) time series indicate a tendency toward more extreme drier conditions across the study region since the mid-1990s, suggesting that instabilities in catchment surfaces caused by climate change are responsible for the elevated Hg flux rates. Drier conditions since c. 2000 appear to be promoting Hg fluxes from catchments to lakes, a process that will likely be exacerbated under future climate-change scenarios.

Multidimensional Drivers of Mercury Distribution in Global Surface Soils: Insights from a Global Standardized Field Survey.

Soil stores a large amount of mercury (Hg) that has adverse effects on human health and ecosystem safety. Significant uncertainties still exist in revealing environmental drivers of soil Hg accumulation and predicting global Hg distribution owing to the lack of field data from global standardized analyses. Here, we conducted a global standardized field survey and explored a holistic understanding of the multidimensional environmental drivers of Hg accumulation in global surface soils. Hg content in surface soils from our survey ranges from 3.8 to 618.2 µg kg-1 with an average of 74.0 µg kg-1 across the globe. Atmospheric Hg deposition, particularly vegetation-induced elemental Hg0 deposition, is the major source of surface soil Hg. Soil organic carbon serves as the major substrate for sequestering Hg in surface soils and is significantly influenced by agricultural management, litterfall, and elevation. For human activities, changing land-use could be a more important contributor than direct anthropogenic emissions. Our prediction of a new global Hg distribution highlights the hot spots (high Hg content) in East Asia, the Northern Hemispheric temperate/boreal regions, and tropical areas, while the cold spots (low Hg content) are in arid regions. The holistic understanding of multidimensional environmental drivers helps to predict the Hg distribution in global surface soils under a changing global environment.

Anthropogenic Iron Invasion into the Ocean: Results from the East Sea (Japan Sea).

Iron (Fe) is an essential micronutrient for phytoplankton growth, and its availability limits primary production in half of the global ocean. Traditionally, atmospheric input of natural mineral dust has been considered as a main source of Fe in the surface ocean. However, here we show that about 45% of the water-soluble Fe in aerosols collected over the East Sea (Japan Sea) is anthropogenic, which originates mainly from heavy fuel oil combustion, based on the analyses of various chemical tracers (Al, K, V, Ni, Pb, and 210Pb). It is striking that a tiny quantity of oil, less than 1% of the aerosols in mass, can constitute the majority of water-soluble Fe in aerosols due to its high Fe solubility. Furthermore, we show that a quarter of dissolved Fe in the East Sea is anthropogenic using a 210Pb-based scavenging model. Since this sea is almost fully enclosed (200-3000 m) and located at the forefront of the Asian human footprint, our results provide an insight that the marine Fe cycle may be already perturbed by human activities.

Air-Sea Exchange and Atmospheric Deposition of Phthalate Esters in the South China Sea.

Phthalate esters (PAEs) have been investigated in paired air and seawater samples collected onboard the research vessel SONNE in the South China Sea in the summer of 2019. The concentrations of ∑7PAEs ranged from 2.84 to 24.3 ng/m3 with a mean of 9.67 ± 5.86 ng/m3 in air and from 0.96 to 8.35 ng/L with a mean of 3.05 ng/L in seawater. Net air-to-seawater deposition dominated air-sea exchange fluxes of DiBP, DnBP, DMP, and DEP, while strong water-to-air volatilization was estimated for bis(2-ethylhexyl) phthalate (DEHP). The estimated net atmospheric depositions were 3740 t/y for the sum of DMP, DEP, DiBP, and DnBP, but DEHP volatilized from seawater to air with an average of 900 t/y. The seasonally changing monsoon circulation, currents, and cyclones occurring in the Pacific can significantly influence the concentration of PAEs, and alter the direction and magnitude of air-sea exchange and particle deposition fluxes. Consequently, the dynamic air-sea exchange process may drive the transport of PAEs from marginal seas and estuaries toward remote marine environments, which can play an important role in the environmental transport and cycling of PAEs in the global ocean.

Disentangling Source Profiles and Time Trends of Halogenated Flame Retardants in the Great Lakes.

Thirty-five polybrominated diphenyl ethers (PBDEs) and eight other alternative flame retardants were measured in air samples (vapor plus particles) collected at six sites near the North American Great Lakes between 2005 and 2019 as part of the Integrated Atmospheric Deposition Network (IADN). These data were analyzed using a multiple linear regression model to determine spatial and temporal trends. Overall, the levels of flame retardants remain significantly higher in urban sites compared to rural and remote sites except for pentabromoethylbenzene (PBEB), hexabromobenzene (HBB), and total Dechlorane Plus (ΣDP). Here, we report the first findings of decreasing levels of ΣDP at Sturgeon Point, New York. The atmospheric levels of total PBDEs remain unchanged over time near Lakes Michigan and Superior and declined near Lakes Erie and Ontario, with rate constants at the latter two lakes revealing halving times of approximately 7 to 14 years. This work presents results from the first investigation of PBDE source apportionment in the Great Lakes atmosphere. Source apportionment by use of positive matrix factorization (PMF) identified two legacy commercial technical mixtures (i.e., penta-BDE and deca-BDE mixes) and elucidated a factor representing ambient degradation. Our results show that weathered local sources of technical commercial mixtures, and their photolysis contribute most to the total PBDE burden in the Great Lakes atmosphere.

Carbon Stable Isotope Ratio of Dissolved Organic Matter as a Tool To Identify Its Sources and Transformations in a Tropical Montane Forest in Ecuador.

Dissolved organic matter (DOM) contributes to forest C cycling. We assessed temporal variability, sources, and transformations of DOM during four years in a tropical montane forest with the help of stable C isotope ratios (δ13C values). We measured δ13C values of DOM in rainfall (RF), throughfall (TF), stemflow (SF), litter leachate (LL), soil solutions at the 0.15 and 0.30 m depths (SS15, SS30), and streamflow (ST) with TOC-IRMS. The δ13C values of DOM did not vary seasonally. We detected an event with a high δ13C value likely attributable to black carbon from local pasture fires. The mean δ13C values of DOM outside the event decreased in the order, RF (-26.0 ± 1.3‰) > TF (-28.7 ± 0.3‰) > SF (-29.2 ± 0.2‰) > LL (-29.6 ± 0.2‰) because of increasing leaching of C-isotopically light compounds. The higher δ13C values of DOM in SS15 (-27.8 ± 1.0‰), SS30 (-27.6 ± 1.1‰), and ST (-27.9 ± 1.1‰) than in the above-ground solutions suggested that roots and root exudates are major belowground DOM sources. Although in DOM the C/N ratios correlated with the δ13C values when all solutions were considered, this was not the case for SS15, SS30, and ST alone. Thus, the δ13C values of DOM provide an additional tool to assess the sources and turnover of DOM.

Magneto-chemical characterisation of Saharan dust deposited on snow in Poland.

Recent research has convincingly shown the advantages of combining environmental magnetism and geochemical analyses for the proxy estimation of anthropogenic pollution due to their atmospheric deposition in local environments. Few studies have also focused on anthropogenic particles deposited on snow. However, papers reporting on Sahara dust particles deposited on snow in central Europe and which involve magnetic methods are missing. To the best of our knowledge, this is the first study investigating the magnetic features of the SDE recorded in snowfall in this part of Europe (i.e. Poland). Our aim was to provide the magnetic characteristics and chemical elemental compositions of a snow horizon containing Saharan dust deposited near the Polish Jakuszyce meteorological station during a snowfall event that occurred from the 1st to the February 7, 2021. Samples of snow with and without Saharan dust were analysed with respect to iron oxide contents (magnetic susceptibility, hysteresis loop, magnetic remanence acquisition) and compared with chemical compositions. Our results revealed the presence of both ferrimagnetic magnetite and antiferromagnetic hematite in the dust-enriched horizon, and the diamagnetic behaviour of the reference layer consisting of 'pure' snow. The samples recorded the presence of geogenic elements such as Al, Fe, Mn, and Ti, anthropogenic elements such as As, Co, Cr, Cu, Ni, Pb, and Zn, and nutrients including Ca and K. The total concentrations of geogenic elements, nutrients, and anthropogenic elements in the snow samples with deposited Saharan dust were, respectively, >3700, >320, and >110 times greater than in the samples without Saharan dust. These findings may serve as reference data for a variety of environmental magnetic studies.

New insight into the geochemical mechanism and behavior of heavy metals in soil and dust fall of a typical copper smelter.

The desorption mechanism of heavy metals (HMs) in soil around the mining region are complex and affected by multiple pollution sources, including sewage discharge and atmospheric deposition. Meanwhile, pollution sources would change soil physical and chemical properties (mineralogy and organic matter), thus affecting the bioavailability of HMs. This study aimed to investigate the pollution source of HMs (Cd, Co, Cu, Cr, Mn, Ni, Pb, and Zn) in soil near mining, and further evaluate influence mechanism of dust fall on HMs pollution in soil by desorption dynamics processes and pH-dependence leaching test. Result presented that dust fall is the primary pollution source to HMs accumulation in soil. Additionally, the result of mineralogical analysis in dust fall revealed that quartz, kaolinite, calcite, chalcopyrite, and magnetite are the major mineralogical phases by XRD and SEM-EDS. Meanwhile, the abundance of kaolinite and calcite in dust fall is higher than in soil, which is the primary reason of higher acid-base buffer capacity of dust fall. Correspondingly, the weakened or disappeared of hydroxyl after the adding acid extraction (0-0.4 mmol· g-1) demonstrated that hydroxyl is the main participants of HMs absorption in soil and dust fall. These combined findings suggested that atmospheric deposition not only increases the pollution loading of HMs in soil, but also changes the mineral phase composition of soil, which would increase the adsorption capacity and bioavailability of HMs in soil. This is very remarkable that heavy metals in soil influenced by dust fall pollution could be released preferentially when soil pH is changed. The present results of this study would provide efficient and scientific targeted strategies for pollution control of HMs in soil near mining areas.

Influence mechanism of awns on wheat grain Pb absorption: Awns' significant contribution to grain Pb was mainly originated from their direct absorption of atmospheric Pb at the late grain-filling stage.

The spike is the organ that contributes the most to lead (Pb) accumulation in wheat grains. However, as an important photosynthetic and transpiration tissue in spike, the role of awn in wheat grain Pb absorption remains unknown. A field experiment was conducted to investigate the influence mechanism of awn on grain Pb accumulation through two comparative treatments: with and without awns (de-awned treatment). The de-awned treatment decreased wheat yield by 4.1 %; however, it significantly lowered the grain Pb accumulation rate at the late filling stage (15 days after anthesis) and led to a 22.8 % decrease in grain Pb concentration from 0.57 to 0.44 mg·kg-1. Moreover, the relative contribution of awn-to-grain Pb accumulation gradually increased with the filling process, finally reaching 26.6 % at maturity. In addition, Pb isotope source analysis indicated that the Pb in the awn and grain mainly originated from atmospheric deposition, and the de-awned treatment decreased the proportion of grain Pb from atmospheric deposition by 8.9 %. Microstructural observations further confirmed that the contribution of awns to grain Pb accumulation mainly originated from their direct absorption of atmospheric Pb. In conclusion, awns play an important role in wheat grain Pb absorption at the late grain-filling stage; planting awnless or short-awn wheat varieties may be the simplest and effective environmental management measure to reduce the health risks of Pb in wheat in regions with serious atmospheric Pb contamination.

Straw removal or non-removal affects cadmium (Cd) accumulation in soil-rice (Oryza sativa L.) system at different ambient air Cd levels.

Despite the potential importance of the removal of contaminated straw for heavy metal output from agricultural soils, previous studies have primarily focused on the variation in the metal concentration without considering the impact input of heavy metals from atmospheric deposition. Here, rice was grown under field conditions, and, as a reference, in a deposition-free environment, and exposed to different ambient air Cd levels. Two consecutive years of pot experiments were conducted in two study areas (ZZ and LY) to examine the changes in soil physicochemical properties as well as Cd accumulation in the soil-rice (Oryza sativa L.) system in response to straw return or removal. The results showed that rice straw return enhanced the soil pH and organic matter (OM) content, but reduced the soil redox potential (Eh); and the variation in amplitude increased with number of cultivation years. After two years of cultivation, the concentrations of soil total Cd and extractable Cd in the straw-removal treatments reduced by 9.89-29.49% and 4.88-37.74%, respectively, whereas those in the straw-return treatments exhibited a slight decrease, or even an increase. This indicated that straw removal could effectively reduce the concentration and bioavailability of Cd in contaminated farmland, which was further confirmed by the results for accumulation of Cd in rice tissues. In addition, the contribution from atmospheric deposition was confirmed by the greater variation in Cd concentration in soils and rice tissues under deposition-free conditions. A major implication of our findings is that the adoption of reasonable straw-treatment measures and proper control over ambient air heavy metals can promote the remediation efficiency of Cd-contaminated fields.