Anoxic Soil Research Articles

Biochemical Investigations of Five Recombinantly Expressed Tyrosinases Reveal Two Novel Mechanisms Impacting Carbon Storage in Wetland Ecosystems.

Wetlands are globally distributed ecosystems characterized by predominantly anoxic soils, resulting from water-logging. Over the past millennia, low decomposition rates of organic matter led to the accumulation of 20-30% of the world's soil carbon pool in wetlands. Phenolic compounds are critically involved in stabilizing wetland carbon stores as they act as broad-scale inhibitors of hydrolytic enzymes. Tyrosinases are oxidoreductases capable of removing phenolic compounds in the presence of O2 by oxidizing them to the corresponding o-quinones. Herein, kinetic investigations (kcat and Km values) reveal that low-molecular-weight phenolic compounds naturally present within wetland ecosystems (including monophenols, diphenols, triphenols, and flavonoids) are accepted by five recombinantly expressed wetland tyrosinases (TYRs) as substrates. Investigations of the interactions between TYRs and wetland phenolics reveal two novel mechanisms that describe the global impact of TYRs on the wetland carbon cycle. First, it is shown that o-quinones (produced by TYRs from low-molecular-weight phenolic substrates) are capable of directly inactivating hydrolytic enzymes. Second, it is reported that o-quinones can interact with high-molecular-weight phenolic polymers (which inhibit hydrolytic enzymes) and remove them through precipitation. The balance between these two mechanisms will profoundly affect the fate of wetland carbon stocks, particularly in the wake of climate change.

Ephemeral microbial responses to pulses of bioavailable carbon in oxic and anoxic salt marsh soils

Roots of salt marsh grasses contribute to soil building but also affect decomposition by releasing bioavailable carbon exudates and oxygen. Disentangling exudate and oxygen effects on decomposition is difficult in the field but essential for marsh carbon models and predicting the impacts of global change disturbances. We tested how pulsed, simulated exudates affect soil metabolism under oxic and anoxic conditions, and whether carbon and oxygen availability facilitate mineralization of existing organic matter (i.e., priming). We conducted a laboratory experiment in flow-through reactors, adding carbon pulses weekly for 84 days and then following starvation under low carbon conditions. Oxygen consumption and sulfide production were inhibited under anoxic and oxic conditions and slowed by 21 ± 10% and 55 ± 8%, respectively, between 1- and 5- days following exudate pulses. Respiration rates immediately following and between pulses increased over time, suggesting that microbes capitalize on and may acclimate to patchy resources. Starvation caused oxygen consumption and sulfide production to fall 28% and 78% in oxic and anoxic treatments. Smaller decreases in oxygen consumption following pulses could suggest greater access to secondary carbon sources and that sulfate reducers were more reliant on exudates. Soil organic carbon was not the likely secondary source because porewater dissolved inorganic carbon δ13C values did not change during transit through the reactors, despite a ∼26‰ difference between the supplied seawater and marsh soil. Interpretation of oxygen consumption rates is complicated by non-respiratory oxidation of reduced inorganic compounds and possibly significant lithoautotrophy. Exudate pulses elicited rapid and ephemeral respiratory responses, particularly under anoxia, but non-respiratory oxidation of reduced compounds obscured the impact of oxygen availability in our experimental system. Despite this, greater aerobic respiration rates suggest that oxygen availability has more potential to regulate carbon mineralization in coastal wetlands than root exudates.

Methylotrophic methanogens and bacteria synergistically demethylate dimethylarsenate in paddy soil and alleviate rice straighthead disease.

Microorganisms play a key role in arsenic (As) biogeochemistry, transforming As species between inorganic and organic forms and different oxidation states. Microbial As methylation is enhanced in anoxic paddy soil, producing primarily dimethylarsenic (DMAs), which can cause rice straighthead disease and large yield losses. DMAs can also be demethylated in paddy soil, but the microorganisms driving this process remain unclear. In this study, we showed that the enrichment culture of methylotrophic methanogens from paddy soil demethylated pentavalent DMAs(V) efficiently. DMAs(V) was reduced to DMAs(III) before demethylation. 16S rRNA gene diversity and metagenomic analysis showed that Methanomassiliicoccus dominated in the enrichment culture, with Methanosarcina and Methanoculleus also being present. We isolated Methanomassiliicoccus luminyensis CZDD1 and Methanosarcina mazei CZ1 from the enrichment culture; the former could partially demethylate trivalent DMAs(III) but not DMAs(V) and the latter could demethylate neither. Addition of strain CZDD1 to the enrichment culture greatly accelerated DMAs(V) demethylation. Demethylation of DMAs(V) in the enrichment culture was suppressed by ampicillin, suggesting the involvement of bacteria. We isolated three anaerobic bacterial strains including Clostridium from the enrichment culture, which could produce hydrogen and reduce DMAs(V) to DMAs(III). Furthermore, augmentation of the Methanomassiliicoccus-Clostridium coculture to a paddy soil decreased DMAs accumulation by rice and alleviated straighthead disease. The results reveal a synergistic relationship whereby anaerobic bacteria reduce DMAs(V) to DMAs(III) for demethylation by Methanomassiliicoccus and also produce hydrogen to promote the growth of Methanomassiliicoccus; enhancing their populations in paddy soil can help alleviate rice straighthead disease.

Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in an Anoxic Soil.

It is known that the association of soil organic matter (SOM) with iron minerals limits carbon mobilization and degradation in aerobic soils and sediments. However, the efficacy of iron mineral protection mechanisms under reducing soil conditions, where Fe(III)-bearing minerals may be used as terminal electron acceptors, is poorly understood. Here, we quantified the extent to which iron mineral protection inhibits mineralization of organic carbon in reduced soils by adding dissolved 13C-glucuronic acid, a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate, or pure 57Fe-ferrihydrite to anoxic soil slurries. In tracking the re-partitioning and transformation of 13C-glucuronic acid and native SOM, we find that coprecipitation suppresses mineralization of 13C-glucuronic acid by 56% after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks, owing to ongoing reductive dissolution of the coprecipitated 57Fe-ferrihydrite. Addition of both dissolved and coprecipitated 13C-glucuronic acid resulted in increased native SOM mineralization, but the reduced bioavailability of the coprecipitated versus dissolved 13C-glucuronic acid decreased the priming effect by 35%. In contrast, the addition of pure 57Fe-ferrihydrite resulted in negligible changes in native SOM mineralization. Our results show that iron mineral protection mechanisms are relevant for understanding the mobilization and degradation of SOM under reducing soil conditions.

Coupled anaerobic methane oxidation and metal reduction in soil under elevated CO2.

Continued current emissions of carbon dioxide (CO2 ) and methane (CH4 ) by human activities will increase global atmospheric CO2 and CH4 concentrations and surface temperature significantly. Fields of paddy rice, the most important form of anthropogenic wetlands, account for about 9% of anthropogenic sources of CH4 . Elevated atmospheric CO2 may enhance CH4 production in rice paddies, potentially reinforcing the increase in atmospheric CH4 . However, what is not known is whether and how elevated CO2 influences CH4 consumption under anoxic soil conditions in rice paddies, as the net emission of CH4 is a balance of methanogenesis and methanotrophy. In this study, we used a long-term free-air CO2 enrichment experiment to examine the impact of elevated CO2 on the transformation of CH4 in a paddy rice agroecosystem. We demonstrate that elevated CO2 substantially increased anaerobic oxidation of methane (AOM) coupled to manganese and/or iron oxides reduction in the calcareous paddy soil. We further show that elevated CO2 may stimulate the growth and metabolism of Candidatus Methanoperedens nitroreducens, which is actively involved in catalyzing AOM when coupled to metal reduction, mainly through enhancing the availability of soil CH4 . These findings suggest that a thorough evaluation of climate-carbon cycle feedbacks may need to consider the coupling of methane and metal cycles in natural and agricultural wetlands under future climate change scenarios.

Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates.

Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.

ROS production upon groundwater oxygenation: Implications of oxidative capacity during groundwater abstraction and discharging

Production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and hydroxyl radical (•OH), has been increasingly discovered upon oxygenation of reduced species in anoxic soil and sediment pore waters. We therefore hypothesize that H2O2 and •OH can be produced during groundwater abstraction and discharging due to oxygenation. To evaluate the capacities of H2O2 and •OH accumulation during groundwater oxygenation, here we measured H2O2 and •OH accumulations during dark oxygenation of groundwater abstracted from different depths of 11 wells adjacent to the Han River. This abstraction could simulate the scenarios of groundwater abstraction by riverside pumping and also of groundwater discharging in hyporheic zones. Results showed that H2O2 and •OH formed during oxygenation of the groundwater with low Eh values (< 63 mV), and only H2O2 (< 3.37 μM) formed for the groundwater with high Eh values (63–206.5 mV). Statistical analysis indicated that dissolved Fe(II) was mainly accountable for the production of measurable •OH in the low Eh locations. The aquifer conditions (i.e., aquifer lithology) and external environmental factors (i.e. pumping disturbance) within short-term abstraction process had negligible influence on ROS production. The relationship between ROS production and groundwater chemistry was further explored by multiple linear regression, which deduced the quantitative models for estimating ROS production upon oxygenation of groundwater from different sources with different chemistry. Using the abstracted groundwater in western Bengal basin as an example, the estimated results indicated that ROS production had a great potential to oxidize the typical contaminant of As(III). As ROS, particularly •OH, represents strong oxidants, our findings implicate an overlooked oxidative capacity during groundwater abstraction and discharging in hyporheic zones or other areas, which could lead to the oxidative transformation of substances and the oxidative stress of microbial metabolism and associated biogeochemical processes in the environments suffering from groundwater oxygenation. This oxidative capacity is particularly important when the groundwater with low Eh values, i.e., under strongly reductive conditions, were abstracted and oxygenated in the surface.

A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

A rationale for higher ratios of <scp>CH₄</scp> to <scp>CO₂</scp> production in warmer anoxic freshwater sediments and soils

The barrier to radial oxygen loss protects roots against hydrogen sulphide intrusion and its toxic effect.

The root barrier to radial O2 loss (ROL) is a key root trait preventing O2 loss from roots to anoxic soils thereby enabling root growth into anoxic, flooded soils. We hypothesized that the ROL barrier can also prevent intrusion of hydrogen sulphide (H2 S), a potent phytotoxin in flooded soils. Using H2 S and O2 sensitive microsensors, we measured the apparent permeance to H2 S of rice roots, tested whether restricted H2 S intrusion reduced its adverse effects on root respiration, and if H2 S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H2 S by almost 99%, greatly restricting H2 S intrusion. The ROL barrier acted as a shield towards H2 S; O2 consumption in roots with a ROL barrier remained unaffected at high H2 S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H2 S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H2 S intrusion, and H2 S can act as an environmental signalling molecule for the induction of the barrier. The study demonstrates the multiple functions of the suberized/lignified outer part of the rice root beyond that of restricting ROL.

Reconnaissance of Oxygenic Denitrifiers in Agriculturally Impacted Soils.

Row crop production in the agricultural Midwest pollutes waterways with nitrate, and exacerbates climate change through increased emissions of nitrous oxide and methane. Oxygenic denitrification processes in agricultural soils mitigate nitrate and nitrous oxide pollution by short-circuiting the canonical pathway to avoid nitrous oxide formation. Furthermore, many oxygenic denitrifiers employ a nitric oxide dismutase (nod) to create molecular oxygen that is used by methane monooxygenase to oxidize methane in otherwise anoxic soils. The direct investigation of nod genes that could facilitate oxygenic denitrification processes in agricultural sites is limited, with no prior studies investigating nod genes at tile drainage sites. Thus, we performed a reconnaissance of nod genes at variably saturated surface sites, and within a variably to fully saturated soil core in Iowa to expand the known distribution of oxygenic denitrifiers. We identified new nod gene sequences from agricultural soil and freshwater sediments in addition to identifying nitric oxide reductase (qNor) related sequences. Surface and variably saturated core samples displayed a nod to 16S rRNA gene relative abundance of 0.004% to 0.1% and fully saturated core samples had relative nod gene abundance of 1.2%. The relative abundance of the phylum Methylomirabilota increased from 0.6% and 1% in the variably saturated core samples to 3.8% and 5.3% in the fully saturated core samples. The more than 10-fold increase in relative nod abundance and almost 9-fold increase in relative Methylomirabilota abundance in fully saturated soils suggests that potential oxygenic denitrifiers play a greater nitrogen cycling role under these conditions. IMPORTANCE The direct investigation of nod genes in agricultural sites is limited, with no prior studies investigating nod genes at tile drains. An improved understanding of nod gene diversity and distribution is significant to the field of bioremediation and ecosystem services. The expansion of the nod gene database will advance oxygenic denitrification as a potential strategy for sustainable nitrate and nitrous oxide mitigation, specifically for agricultural sites.

Properties and pelletization of Camellia oleifera shell after anoxic storage

The anoxic storage of the Camellia oleifera shell (COS) was carried out with respect to dry matter losses, color dimension, moisture content, pellet properties, and combustion analysis, in order to investigate the effects of anoxic storage on feedstock's properties and pelletization. After 34-days anoxic storage, the dry matter losses of COS were varied between 12.48% and 18.95%. The hemicellulose content was decreased, and the relative content of lignin was gradually increased after anoxic storage. The higher heating value (HHV), single pellet density, and bulk density of the stored COS were decreased, while the fixed carbon and ash content were increased. The microbial fungi richness and diversity were increased after anoxic storage. After 34-days anoxic storage, the anoxic stored COS was processed into pellets. The compaction and extrusion energy consumption were increased with the increase in storage temperature and by mixing with fresh soil in COS field which contains amounts of microorganisms. The combustion of stored COS became more stable, and the stored COS with soil was more difficult to be pyrolyzed. The results showed that anoxic storage could be against to enhance the properties of COS pellets and pelletization, which should be reduced the anoxic storage process and soil stain in storage period as far as possible.

Source data of manuscript: A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

Source data of manuscript: A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

Anaeromicropila herbilytica gen. nov., sp. nov., a plant polysaccharide-decomposing anaerobic bacterium isolated from anoxic soil subjected to reductive soil disinfestation, and reclassification of Clostridium populeti as Anaeromicropila populeti comb. nov.

An obligately anaerobic bacterial strain (TB5T) was isolated from a soil sample subjected to reductive or biological soil disinfestation. Cells of the strain were Gram-stain-positive, spore-forming and motile rods. The strain grew at 15–40 °C (optimum, 37 °C) and pH 5.4–7.5 (optimum, pH 7.3). Strain TB5Tutilized a wide variety of carbohydrates including polysaccharides (cellulose, xylan, starch, inulin, glucomannan and laminarin) and organic acids. Acetate, ethanol, H2 and CO2 were products from the substrates utilized. The major components of the cellular fatty acids were C16 : 1 ω7c DMA, C16 : 0 DMA and C18 : 1 ω7c DMA. The diagnostic amino acid of the cell-wall peptidoglycan was meso-diaminopimelic acid. The closest related species to strain TB5T based on 16S rRNA gene sequences was Clostridium populeti 743AT (95.4 % sequence similarity). The genome size of strain TB5T was 5.09 Mb and the genomic DNA G+C content was 32.7 mol%. Strain TB5T had genes encoding polysaccharide-decomposing enzymes such as cellulase, xylanase, β-glucosidase and β-mannosidase in the genome. Based on the phylogenetic, genomic and phenotypic data, a novel species of a novel genus in the family Lachnospiraceae , Anaeromicropila herbilytica gen. nov., sp. nov., is proposed to accommodate the strain. The type species is Anaeromicropila herbilytica with strain TB5T (=NBRC 112093T=DSM 110037T) as the type strain. For the closest related species C. populeti , Anaeromicropila populeti comb. nov. is proposed with an emended description of the species.

Reactive iron, not fungal community, drives organic carbon oxidation potential in floodplain soils

Wetlands host ∼20% of terrestrial organic carbon and serve as a major sink for atmospheric carbon. Anoxic soils and sediments accrue soil organic carbon (SOC) partly by hampering the activity of extracellular oxidative enzymes that break down phenolic polymers. Upon aeration, fungal-driven oxidative enzymatic depolymerization and microbial respiration of released monomers ensue. Redox-active metals can simultaneously catalyze abiotic nonspecific oxidation of SOC, notable examples including Mn(III) or Fe(II) through Fenton-like, hydrogen peroxide-catalyzed oxidative radical production. However, the extent of reactive metal contributions to biotic and abiotic SOC degradation is not understood in the context of natural environments with diverse redox chemistry. We tested the relative contributions of fungi, Mn(III) and Fe(II) to phenolic substrate (L-DOPA) oxidation in floodplain soils representing a range of transient redox conditions driven by permanent vs. periodic flooding. Phenol oxidative potential was highest in permanently flooded soils with fewer fungal taxa known for observed (per)oxidase activity and instead correlated with HCl-extractable Fe(II), Fe(total) and Fe(II)/Fe(total), suggesting a specific role of Fe(II). Fe(II) additions enhanced phenol oxidative potential in sterilized and non-sterilized soils in the presence of hydrogen peroxide, indicating abiotic Fe-mediated radical chemistry could significantly enhance wetland SOC oxidative depolymerization throughout redox-active floodplain soils. Fungal community composition did not correlate to phenol oxidative potential overall and only more oxic soils adjacent to the river with diverse fungal communities showed declining oxidative potential after sterilization. Mn(III) addition did not significantly enhance phenol oxidative potential across all soils, although it appeared to drive fungal-mediated oxidative potential in the most aerated floodplain soils. Understanding how metals mediate SOC depolymerization as abiotic oxidants or microbially-harnessed enzyme cofactors and substrates in soils under variable hydrologic controls will improve our ability to represent depolymerization in terrestrial carbon models in wetland and other frequently saturated soils.

Aerobic and denitrifying methanotrophs: Dual wheels driving soil methane emission reduction

The greenhouse gas methane in soils has been considered to be consumed mainly by aerobic methane-oxidizing bacteria for a long time. In the last decades, the discovery of anaerobic methanotrophs greatly complemented the methane cycle, but their contribution rates and ecological significance in soils remain undescribed. In this work, the soil samples from forest, grassland and cropland in four different climatic regions were collected to investigate these conventional and novel methanotrophs. A dual-core microbial methane sink, responsible for over 80 % of soil methane emission reduction, was unveiled. The aerobic core was performed by aerobic methanotrophic bacteria in topsoil, who played important roles in stabilizing bacterial communities. The anaerobic core was denitrifying methanotrophs in anoxic soils, including denitrifying methanotrophic bacteria from NC10 phylum and denitrifying methanotrophic archaea from ANME-2d clade. They were ubiquitous in terrestrial soils and potentially led to around 50 % of the total methane removal. Human activities such as livestock farming and rice cultivation further promoted the contribution rates of these denitrifying methanotrophs. This work elucidated the emission reduction contribution of different methanotrophs in the continental setting, which would help to reduce uncertainties in the estimations of the soil methane emission.

Environmental predictors of electroactive bacterioplankton in small boreal lakes.

Extracellular electron transfer (EET) by electroactive bacteria in anoxic soils and sediments is an intensively researched subject, but EET's function in planktonic ecology has been less considered. Following the discovery of an unexpectedly high prevalence of EET genes in a bog lake's bacterioplankton, we hypothesized that the redox capacities of dissolved organic matter (DOM) enrich for electroactive bacteria by mediating redox chemistry. We developed the bioinformatics pipeline FEET (Find EET) to identify and summarize predicted EET protein-encoding genes from metagenomics data. We then applied FEET to 36 bog and thermokarst lakes and correlated gene occurrence with environmental data to test our predictions. Our results provide indirect evidence that DOM may participate in bacterioplankton EET. We found a similarly high prevalence of genes encoding putative EET proteins in most of these lakes, where oxidative EET strongly correlated with DOM. Numerous novel clusters of multiheme cytochromes that may enable EET were identified. Taxa previously not considered EET-capable were found to carry EET genes. We propose that EET and DOM interactions are of ecologically important to bacterioplankton in small boreal lakes, and that EET, particularly by methylotrophs and anoxygenic phototrophs, should be further studied and incorporated into methane emission models of melting permafrost.

PH dependence of arsenic speciation in paddy soils: The role of distinct methanotrophs

Arsenic (As) is a priority environmental pollutant in paddy field. The coupling of arsenate (As(V)) reduction with anaerobic methane (CH4) oxidation was recently demonstrated in paddy soils and has been suggested to serve as a critical driver for As transformation and mobilization. However, whether As(V)-dependent CH4 oxidation is driven by distinct methanotrophs under different pH conditions remains unclear. Here, we investigated the response of As(V)-dependent CH4 oxidation to pH shifts (pH 5.5–8.0) by employing isotopically labelled CH4. Furthermore, the underlying mechanisms were also investigated in well-controlled anoxic soil suspension incubations. Our results showed that As(V)-dependent CH4 oxidation is highly sensitive to pH changes (1.6–6.8 times variation of arsenite formation). A short-term (0–10 d) pH shift from near-neutral pH to acidic conditions (i.e., pH 5.5, −85% arsenite formation) had an inhibitory effect on As(V)-dependent CH4 oxidation. However, prolonged acidic conditions (i.e., >15 d) had no significant influence on As(V)-dependent CH4 oxidation. The microbial analyses indicated that As reduction in paddies can be driven by anaerobic CH4 oxidation archaea (ANME) and methanotrophs. And, methanotrophs may serve as a critical driver for As(V)-dependent CH4 oxidation. Moreover, type I methanotrophs Methylobacter were more active in oxidizing CH4 than type II methanotrophs Methylocystis when the pH ≥ 6.5. However, Methylocystis had a higher tolerance to soil acidification than Methylobacter. This study illustrates that As(V)-dependent CH4 oxidation could be dominated by distinct methanotrophs along with pH shifts, which eventually enhances As release in paddy soils.

Varied response of carbon dioxide emissions to warming in oxic, anoxic and transitional soil layers in a drained peatland

AbstractPeatlands are an important natural store of carbon. Here, we investigate how carbon dynamics in soils from three distinct layers from a drained peatland in the Qinghai-Tibet Plateau might respond to global warming. We incubated Zoige Plateau soil samples that represent oxic surface soil, permanently waterlogged anoxic deep soil, and a transitional interlayer, which varies between oxic and anoxic conditions with water table fluctuations, in the laboratory at 8 °C and 18 °C for 154 days. We find that carbon quality and microbial activity were highest in surface soils and lowest in interlayer soils. Carbon dioxide emissions were lowest in interlayer soils, where they were less sensitive to warming and more sensitive to nitrogen content than in the other layers. Conversely, carbon quality and microbial activity were the primary predictors of carbon dioxide emissions from the surface and deep soil, respectively. Our findings suggest a low vulnerability of soil carbon in interlayer soils of drained peatlands to warming.

Reduction of Dimethylarsenate to Highly Toxic Dimethylarsenite in Paddy Soil and Rice Plants.

Dimethylarsenate [DMAs(V)] is a common methylated As species in soils and plants and can cause the physiological disorder straighthead disease in rice. Because DMAs(V) is relatively noncytotoxic, we hypothesize that phytotoxicity of DMAs(V) may arise from trivalent dimethylarsenite [DMAs(III)]. DMAs(III) has been detected in human urine samples but not in environmental samples, likely due to its instability under oxic conditions. We first established methods for preservation and detections of DMAs(III) in soil and plant samples. We showed that DMAs(III) was a major As species in soil solution from an anoxic paddy soil. Enrichment cultures for fermentative, sulfate-reducing, and denitrifying bacteria from the paddy soil could reduce DMAs(V) to DMAs(III). Twenty-two strains of anaerobic bacteria isolated from the soil showed some ability to reduce DMAs(V). Rice plants grown in hydroponic culture with DMAs(V) also showed the ability to reduce DMAs(V) to DMAs(III). Rice plants and grains grown in a flooded paddy soil contained both DMAs(V) and DMAs(III); their concentrations were higher in the spikelets with straighthead disease than those without. DMAs(III) was much more toxic to the protoplasts isolated from rice plants than DMAs(V). Taken together, the ability to reduce DMAs(V) to highly toxic DMAs(III) is common to soil anaerobes and rice plants.

Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils.

Mountain floodplain soils often show spatiotemporal variations in redox conditions that arise due to changing hydrology and resulting biogeochemistry. Under oxygen-depleted conditions, solid phase terminal electron acceptors (TEAs) can be used in anaerobic respiration. However, it remains unclear to what degree the redox properties of solid phases limit respiration rates and hence organic matter degradation. Here, we assess such limitations in soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified CO2 production and microbial Fe(III) reduction, the main microbial metabolic pathway, as well as the reactivity of whole-soil solid phase TEAs toward mediated electrochemical reduction. Fe(III) reduction occurred together with CO2 production in native oxic soils, while neither Fe(II) nor CO2 production was observed in native anoxic soils. Initial CO2 production rates increased with increasing TEA redox reactivity toward mediated electrochemical reduction across all soil depths. Low TEA redox reactivity appears to be caused by elevated Fe(II) concentrations rather than crystallinity of Fe(III) phases. Our findings illustrate that the buildup of Fe(II) in systems with long residence times limits the thermodynamic viability of dissimilatory Fe(III) reduction and thereby limits the mineralization of organic carbon.