20 research outputs found
Factors controlling spatiotemporal variability of soil carbon accumulation and stock estimates in a tidal salt marsh
Tidal salt marshes are important contributors to soil carbon (C) stocks despite their relatively small land surface area. Although it is well understood that salt marshes have soil C burial rates orders of magnitude greater than those of terrestrial ecosystems, there is a wide range in accrual rates among spatially distributed marshes. In addition, wide ranges in C accrual rates also exist within a single marsh ecosystem. Tidal marshes often contain multiple species of cordgrass due to variations in hydrology and soil biogeochemistry caused by microtopography and distance from tidal creeks, creating distinct subsites. Our overarching objective was to observe how soil C concentration and dissolved organic carbon (DOC) vary across four plant phenophases and across three subsites categorized by unique vegetation and hydrology. We also investigated the dominant biogeochemical controls on the spatiotemporal variability of soil C and DOC concentrations. We hypothesized that subsite biogeochemistry drives spatial heterogeneity in soil C concentration, and this causes variability in total soil C and DOC concentrations at the marsh scale. In addition, we hypothesized that soil C concentration and porewater biogeochemistry vary temporally across the four plant phenophases (i.e., senescence, dormancy, green-up, maturity). To test these interrelated hypotheses, we quantified soil C and DOC concentrations in 12 cm sections of soil cores (0–48 cm depth) across time (i.e., phenophase) and space (i.e., subsite), alongside several other porewater biogeochemical variables. Soil C concentration varied significantly (p < 0.05) among the three subsites and was significantly greater during plant dormancy. Soil S, porewater sulfide, redox potential, and depth predicted 44 % of the variability in soil C concentration. There were also significant spatial differences in the optical characterization properties of DOC across subsites. Our results show that soil C varied spatially across a marsh ecosystem by up to 63 % and across plant phenophase by 26 %, causing variability in soil C accrual rates and stocks depending on where and when samples are taken. This shows that hydrology, biogeochemistry, and plant phenology are major controls on salt marsh C content. It is critical to consider spatiotemporal heterogeneity in soil C concentration and porewater biogeochemistry to account for these sources of uncertainty in C stock estimates. We recommend that multiple locations and sampling time points are sampled when conducting blue C assessments to account for ecosystem-scale variability.</p
Si and Water Management Drives Changes in Fe and Mn Pools that Affect As Cycling and Uptake in Rice
Arsenic availability to rice is tied to biogeochemical cycling of Fe and Mn in rice soils. Two strategies to minimize As uptake by rice—increasing Si and decreasing water—affect soil Fe and Mn pools. We synthesized data from several soil-based experiments with four rice cultivars across pot and field trials with manipulations of Si, water, or both. Increasing Si alters the mineral composition of Fe plaque more than decreasing water, with the former promoting relatively more ferrihydrite and less lepidocrocite. Nonflooded conditions decrease lepidocrocite but slightly increase goethite compared to flooded rice. Plaque As, which was a mixture of arsenite (15–40%) and arsenate (60–85%), was correlated positively with ferrihydrite and negatively with lepidocrocite and goethite. Plaque As was also positively correlated with F1 and F2 soil As, and F2 was correlated positively with porewater As, total grain As, and grain organic As (oAs). Grain inorganic As (iAs) was negatively correlated with oxalate-extractable Fe and Mn. Our data and multiple linear regression models suggest that under flooded conditions iAs is released by poorly crystalline Fe oxides to porewater mainly as iAs(III), which can either be taken up by the plant, adsorbed to Fe plaque, oxidized to iAs(V) or methylated to oAs. Increasing Si can promote more desorption of iAs(III) and promote more poorly-ordered phases in plaque and in bulk soil. The ultimate effectiveness of a Si amendment to decrease As uptake by rice depends upon it being able to increase exogenous Si relative to As in porewater after competitive adsorption/desorption processes. Our data further suggest that poorly crystalline Fe and Mn soil pools can retain inorganic As and decrease plant uptake, but these pools in bulk soil and plaque control grain organic As
Effects of Genotype and Transpiration Rate on the Uptake and Accumulation of Perchlorate (ClO 4
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 (+As–Si) control. The +Si<sub>H</sub> addition increased As in pore-water, but it significantly decreased
grain-As relative to the (+As–Si) 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
Rice husk and charred husk amendments increase porewater and plant Si but water management determines grain As and Cd concentration
Purpose
Rice is a staple crop worldwide and a silicon (Si) hyperaccumulator with Si levels reaching 5–10% of its mass; this can result in desilication and Si-deficiency if plant residues are not managed correctly. Rice is also uniquely subject to arsenic (As) and cadmium (Cd) contamination depending on soil conditions. Our goal is to quantify the effects of rice husk (a Si-rich milling byproduct) amendments and different water management strategies on rice uptake of Si, As, and Cd.
Methods
We employed 4 husk amendment treatments: Control (no husk), Husk (untreated husk), Biochar (husk pyrolyzed at 450 °C), and CharSil (husk combusted at > 1000 °C). Each of these amendments was studied under nonflooded, alternate wetting and drying (AWD), and flooded water management in a pot study. Porewater chemistry and mature plant elemental composition were measured.
Results
Husk and Biochar treatments, along with flooding, increased porewater and plant Si. Vegetative tissue As decreased with increasing porewater Si, but grain As and plant Cd were primarily controlled by water management. Grain As and Cd were inversely correlated and are simultaneously minimized in a redox potential (Eh) range of 225–275 mV in the studied soil. Ferrihydrite in root iron plaque decreased As translocation from porewater to grain, but amendments were not able to increase plaque ferrihydrite content.
Conclusion
We conclude moderate husk amendment rates (i.e., 4 years’ worth) with minimal pretreatment strongly increases rice Si content but may not be sufficient to decrease grain As in low Si and As soil.ISSN:0032-079XISSN:1573-503
Arsenic, Lead, and Cadmium in U.S. Mushrooms and Substrate in Relation to Dietary Exposure
Wild mushrooms can
absorb high quantities of metalÂ(loid)Âs, 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 ÎĽg
g<sup>–1</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 metalÂ(loid)Âs
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 “base-case” 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 μg g<sup>–1</sup>, total grain As concentrations averaged 0.2 μg g<sup>–1</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 AsÂ(V)-bearing FeÂ(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 (30–90%), 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 (30–90%), 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