10 research outputs found
Silicate Mineral Impacts on the Uptake and Storage of Arsenic and Plant Nutrients in Rice (Oryza sativa L.)
Arsenic-contaminated rice grain may threaten human health
globally.
Since H<sub>3</sub>AsO<sub>3</sub><sup>0</sup> is the predominant
As species found in paddy pore-waters, and H<sub>4</sub>SiO<sub>4</sub><sup>0</sup> and H<sub>3</sub>AsO<sub>3</sub><sup>0</sup> share an
uptake pathway, silica amendments have been proposed to decrease As
uptake and consequent As concentrations in grains. Here, we evaluated
the impact of two silicate mineral additions differing in solubility
(+Si<sub>L</sub>, diatomaceous earth, 0.29 mM Si; +Si<sub>H</sub>,
Si-gel, 1.1 mM Si) to soils differing in mineralogy on arsenic concentration
in rice. The +Si<sub>L</sub> addition either did not change or decreased
As concentration in pore-water but did not change or increased grain-As
levels relative to the (+AsāSi) control. The +Si<sub>H</sub> addition increased As in pore-water, but it significantly decreased
grain-As relative to the (+AsāSi) control. Only the +Si<sub>H</sub> addition resulted in significant increases in straw- and
husk-Si. Total grain- and straw-As was negatively correlated with
pore-water Si, and the relationship differed between two soils exhibiting
different mineralogy. These differing results are a consequence of
competition between H<sub>4</sub>SiO<sub>4</sub><sup>0</sup> and H<sub>3</sub>AsO<sub>3</sub><sup>0</sup> for adsorption sites on soil solids
and subsequent plant-uptake, and illustrate the importance of Si mineralogy
on arsenic uptake
Arsenic, Lead, and Cadmium in U.S. Mushrooms and Substrate in Relation to Dietary Exposure
Wild mushrooms can
absorb high quantities of metalĀ(loid)Ās, yet
the concentration, speciation, and localization of As, Pb, and Cd
in cultivated mushrooms, particularly in the United States, are unresolved.
We collected 40 samples of 12 types of raw mushrooms from 2 geographic
locations that produce the majority of marketable U.S. mushrooms and
analyzed the total As, Pb, and Cd content, the speciation and localization
of As in select samples, and assessed the metal sources and substrate-to-fruit
transfer at one representative farm. Cremini mushrooms contained significantly
higher total As concentrations than Shiitake and localized the As
differently; while As in Cremini was distributed throughout the fruiting
body, it was localized to the hymenophore region in Shiitake. Cd was
significantly higher in Royal Trumpet than in White Button, Cremini,
and Portobello, while no difference was observed in Pb levels among
the mushrooms. Concentrations of As, Pb, and Cd were less than 1 Ī¼g
g<sup>ā1</sup> d.w. in all mushroom samples, and the overall
risk of As, Cd, and Pb intake from mushroom consumption is low in
the U.S. However, higher percentages of tolerable intake levels are
observed when calculating risk based on single serving-sizes or when
substrate contains elevated levels of metalĀ(loid)Ās
How Rice (<i>Oryza sativa</i> L.) Responds to Elevated As under Different Si-Rich Soil Amendments
Several
strategies exist to mitigate As impacts on rice and each
has its set of trade-offs with respect to yield, inorganic As content
in grain, and CH<sub>4</sub> emissions. The addition of Si to paddy
soil can decrease As uptake by rice but how rice will respond to elevated
As when soil is amended with Si-rich materials is unresolved. Here,
we evaluated yield impacts and grain As content and speciation in
rice exposed to elevated As in response to different Si-rich soil
amendments including rice husk, rice husk ash, and CaSiO<sub>3</sub> in a pot study. We found that As-induced yield losses were alleviated
by Husk amendment, partially alleviated by Ash amendment, and not
affected by CaSiO<sub>3</sub> amendment. Furthermore, Husk was the
only tested Si-amendment to significantly decrease grain As concentrations.
Husk amendment was likely effective at decreasing grain As and improving
yield because it provided more plant-available Si, particularly during
the reproductive and ripening phases. Both Husk and Ash provided K,
which also played a role in yield improvement. This study demonstrates
that while Si-rich amendments can affect rice uptake of As, the kinetics
of Si dissolution and nutrient availability can also affect As uptake
and toxicity in rice
Arsenic Concentrations in Paddy Soil and Rice and Health Implications for Major Rice-Growing Regions of Cambodia
Despite
the global importance of As in rice, research has primarily
focused on Bangladesh, India, China, and the United States with limited
attention given to other countries. Owing to both indigenous As within
the soil and the possible increases arising from the onset of irrigation
with groundwater, an assessment of As in rice within Cambodia is needed,
which offers a ābase-caseā comparison against sediments
of similar origin that comprise rice paddy soils where As-contaminated
water is used for irrigation (e.g., Bangladesh). Here, we evaluated
the As content of rice from five provinces (Kandal, Prey Veng, Battambang,
Banteay Meanchey, and Kampong Thom) in the rice-growing regions of
Cambodia and coupled that data to soil-chemical factors based on extractions
of paddy soil collected and processed under anoxic conditions. At
total soil As concentrations ranging 0.8 to 18 Ī¼g g<sup>ā1</sup>, total grain As concentrations averaged 0.2 Ī¼g g<sup>ā1</sup> and ranged from 0.1 to 0.37 with Banteay Meanchey rice having significantly
higher values than Prey Veng rice. Overall, soil-extractable concentrations
of As, Fe, P, and Si and total As were poor predictors of grain As
concentrations. While biogeochemical factors leading to reduction
of AsĀ(V)-bearing FeĀ(III) oxides are likely most important for predicting
plant-available As, husk and straw As concentrations were the most
significant predictors of grain-As levels among our measured parameters
Unraveling the Mechanisms of Fe Oxidation and Mn Reduction on Mn Indicators of Reduction in Soil (IRIS) Films
Indicators of reduction in soil (IRIS)
devices are low-cost soil
redox sensors coated with Fe or Mn oxides, which can be reductively
dissolved from the device under suitable redox conditions. Removal
of the metal oxide coating from the surface, leaving behind the white
film, can be quantified and used as an indicator of reducing conditions
in soils. Manganese IRIS, coated with birnessite, can also oxidize
Fe(II), resulting in a color change from brown to orange that complicates
the interpretation of coating removal. Here, we studied field-deployed
Mn IRIS films where Fe oxidation was present to unravel the mechanisms
of Mn oxidation of Fe(II) and the resulting minerals on the IRIS film
surface. We observed reductions in the Mn average oxidation state
when Fe precipitation was evident. Fe precipitation was primarily
ferrihydrite (30ā90%), but lepidocrocite and goethite were
also detected, notably when the Mn average oxidation state decreased.
The decrease in the average oxidation state of Mn was due to the adsorption
of Mn(II) to the oxidized Fe and the precipitation of rhodochrosite
(MnCO3) on the film. The results were variable on small
spatial scales (<1 mm), highlighting the suitability of IRIS in
studying heterogeneous redox reactions in soil. Mn IRIS also provides
a tool to bridge lab and field studies of the interactions between
Mn oxides and reduced constituents
Silicon Decreases Dimethylarsinic Acid Concentration in Rice Grain and Mitigates Straighthead Disorder
While root Si transporters play a
role in the uptake of arsenite
and organic As species dimethylarsinic acid (DMA) and monomethylarsonic
acid (MMA) in rice (<i>Oryza sativa</i> L.), the impact
of Si addition on the accumulation of DMA and MMA in reproductive
tissues has not been directly evaluated, particularly in isolation
from inorganic As species. Furthermore, DMA and MMA are suspected
causal agents of straighthead disorder. We performed a hydroponic
study to disentangle the impact of Si on accumulation of DMA and MMA
in rice grain. At 5 Ī¼M, MMA was toxic to rice, regardless of
Si addition, although Si significantly decreased root MMA concentrations.
Plants dosed with 5 Ī¼M DMA grew well vegetatively but exhibited
straighthead disorder at the lowest Si dose, and this DMA-induced
yield loss reversed with increasing solution Si. Increasing Si also
significantly decreased DMA concentrations in roots, straw, husk,
and grain, particularly in mature plants. Si restricted grain DMA
through competition for root uptake and downregulation of root Si
transporters particularly at later stages of growth when Si uptake
was greatest. Our finding that DMA causes straighthead disorder under
low Si availability but not under high Si availability suggests Si
as a straighthead management strategy
Unraveling the Mechanisms of Fe Oxidation and Mn Reduction on Mn Indicators of Reduction in Soil (IRIS) Films
Indicators of reduction in soil (IRIS)
devices are low-cost soil
redox sensors coated with Fe or Mn oxides, which can be reductively
dissolved from the device under suitable redox conditions. Removal
of the metal oxide coating from the surface, leaving behind the white
film, can be quantified and used as an indicator of reducing conditions
in soils. Manganese IRIS, coated with birnessite, can also oxidize
Fe(II), resulting in a color change from brown to orange that complicates
the interpretation of coating removal. Here, we studied field-deployed
Mn IRIS films where Fe oxidation was present to unravel the mechanisms
of Mn oxidation of Fe(II) and the resulting minerals on the IRIS film
surface. We observed reductions in the Mn average oxidation state
when Fe precipitation was evident. Fe precipitation was primarily
ferrihydrite (30ā90%), but lepidocrocite and goethite were
also detected, notably when the Mn average oxidation state decreased.
The decrease in the average oxidation state of Mn was due to the adsorption
of Mn(II) to the oxidized Fe and the precipitation of rhodochrosite
(MnCO3) on the film. The results were variable on small
spatial scales (<1 mm), highlighting the suitability of IRIS in
studying heterogeneous redox reactions in soil. Mn IRIS also provides
a tool to bridge lab and field studies of the interactions between
Mn oxides and reduced constituents
Unraveling the Mechanisms of Fe Oxidation and Mn Reduction on Mn Indicators of Reduction in Soil (IRIS) Films
Indicators of reduction in soil (IRIS)
devices are low-cost soil
redox sensors coated with Fe or Mn oxides, which can be reductively
dissolved from the device under suitable redox conditions. Removal
of the metal oxide coating from the surface, leaving behind the white
film, can be quantified and used as an indicator of reducing conditions
in soils. Manganese IRIS, coated with birnessite, can also oxidize
Fe(II), resulting in a color change from brown to orange that complicates
the interpretation of coating removal. Here, we studied field-deployed
Mn IRIS films where Fe oxidation was present to unravel the mechanisms
of Mn oxidation of Fe(II) and the resulting minerals on the IRIS film
surface. We observed reductions in the Mn average oxidation state
when Fe precipitation was evident. Fe precipitation was primarily
ferrihydrite (30ā90%), but lepidocrocite and goethite were
also detected, notably when the Mn average oxidation state decreased.
The decrease in the average oxidation state of Mn was due to the adsorption
of Mn(II) to the oxidized Fe and the precipitation of rhodochrosite
(MnCO3) on the film. The results were variable on small
spatial scales (<1 mm), highlighting the suitability of IRIS in
studying heterogeneous redox reactions in soil. Mn IRIS also provides
a tool to bridge lab and field studies of the interactions between
Mn oxides and reduced constituents
Soil Incorporation of Silica-Rich Rice Husk Decreases Inorganic Arsenic in Rice Grain
Arsenic
decreases rice yield, and inorganic grain As threatens
human health; thus, strategies to decrease rice As are critically
needed. Increased plant-available silica (Si) can decrease rice As,
yet the source of Si matters. Rice husk, an underutilized and Si-rich
byproduct of rice production that contains less labile C and an order
of magnitude less As than rice straw, may be an economically viable
Si resource to decrease rice As, yet the impact of rice husk incorporation
on As in the riceāsoil nexus has not been reported. This proof-of-concept
study shows that rice husk incorporation to soil (1% w/w) decreases
inorganic grain As by 25ā50% without negatively affecting grain
Cd, yield, or dissolved CH<sub>4</sub> levels. Rice husk is a critical
yet perhaps overlooked resource to improve soil quality through enhanced
nutrient availability and attenuate human health risks through consumption
of As-laden grain
<b>Dataset: High methane concentrations in tidal salt marsh soils: where does the methane go?</b>
Tidal salt marshes produce and emit CH4. Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 Ī¼mol mol-1 positively correlated with S2- (salinity range: 6.6 to 14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 Ā± 684.4 nmol m-2 s-1). CH4 and CO2 within the soil pore water were produced from young carbon, with most Ī14C-CH4 and Ī14C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.</p