Labile Dissolved Organic Matter Research Articles

Karenia brevis and Pyrodinium bahamense Utilization of Dissolved Organic Matter in Urban Stormwater Runoff and Rainfall Entering Tampa Bay, Florida

This study investigated how nitrogen and dissolved organic matter (DOM) from stormwater runoff and rainfall support the growth of Karenia brevis and Pyrodinium bahamense. Excitation–emission matrix spectroscopy coupled with parallel factor analysis tracked changes in the optical properties of DOM in each bioassay, revealing greater reactivity of terrestrial humic-like DOM. Significant increases in cell yield and specific growth rates were observed upon additions of runoff for both species, with significant increases in specific growth rates upon the addition of a 2 in simulated rain event for P. bahamense only. By hour 48, 100% of the dissolved organic nitrogen (DON) in each treatment was utilized by P. bahamense, and by hour 72, over 50% of the DON was utilized by K. brevis. The percentage of bioavailable dissolved organic carbon (DOC) was greater for P. bahamense compared to K. brevis, suggesting a greater affinity for DOC compounds by P. bahamense. However, the bioavailability of DOM for each species could be owed to distinct chemical characteristics of labile DOM conveyed from each site. This study demonstrates that stormwater runoff and rainfall are both sources of labile DOM and DON for K. brevis and P. bahamense, which has implications for blooms of these species in Tampa Bay waters.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L−1, and 20.3 ± 6.6 nmol L−1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004–2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

Probing the optical and molecular properties of sedimentary dissolved organic matter in the laminated diatom mats from the southern Mariana Trench

Laminated diatom mats (LDMs) deposition, formed by the aggregation of giant diatoms (e.g., Ethmodiscus rex), are recognized as a crucial driver of the global carbon and silicon cycles due to their substantial contribution to the flux of organic carbon and biogenic silica. The biogenic silica-rich sediment is one of the typical sediment types in the deep-sea setting of the tropical Pacific Ocean. Here, we analyzed the concentrations of pore-water dissolved organic carbon (DOC) and phosphate (PO43−), the optical properties, and molecular compositions of dissolved organic matter (DOM) in the two LDMs cores and one non-LDMs core retrieved from the southern Mariana Trench. Our results showed that the LDMs sediments with higher total organic carbon (TOC) contents have lower concentrations of DOC and chromophoric dissolved organic matter (CDOM), compared to the non-LDMs core. Meanwhile, the LDMs samples exhibited lower relative intensity of fluorescent proteins-like, molecular lability boundary (MLBL%) and higher values for humification index (HIX) and aromaticity modification (AImod) compared to the non-LDM core, indicating a higher level of stability and recalcitrance of DOM likely resulting from microbial degradation of relatively labile DOM compounds. Furthermore, the LDMs samples were characterized by a high abundance of CHO-containing DOM components and a low abundance of nitrogen- and sulfur-containing components, potentially linked to microbial degradation of hydrocarbon compounds that were enriched in the LDMs sediments. Additionally, the unique formulas in the LDMs samples demonstrated higher degree of recalcitrance suggested by higher aromaticity and fraction of CRAM, which may be attributed to microbial transformation of DOM. These findings add to our limited knowledge of the optical properties and molecular composition of DOM in the deep-sea opal-rich sediments, and have implications for understanding DOM cycling in the deep oceans.

Spatial distribution characteristics and interaction effects of DOM and microbial communities in kelp cultivation areas

The influence of macroalgae cultivation on aquaculture carbon sinks is significant, with microbial carbon (C) pumps contributing to a stable inert dissolved carbon pool in this context. Concurrently, dissolved organic matter (DOM) exchange at the marine sediment-water interface profoundly affects global ecosystem element cycling. However, the interactions between DOM and bacterial communities at the sediment-water interface in kelp cultivation areas, especially regarding microbial function prediction, have not been fully explored. This study analyzed the DOM characteristics, environmental factors, and bacterial community structure in the Tahewan kelp——Saccharina japonica cultivated area and compared them with those in non-cultivated areas. The results indicated significantly higher dissolved organic carbon (DOC) concentrations in the kelp culture area, particularly in surface seawater and overlying water. The dominant bacterial phyla in both regions included Pseudomonadota, Actinomycetota, and Bacteroidota in both regions, while Desulfobacterota was more prevalent in the sediment environment of the cultivated region. Parallel factor analysis (EEM-PARAFAC) was used to identify DOM components, among which component C2 (a microbial humic-like substance DOM) was highly resistant to microbial degradation. We infer that C2 has similar properties to recalcitrant dissolved organic matter (RDOM). Analysis of the predicted functional genes based on 16S rRNA gene data showed that methanol oxidation, methylotrophy, and methanotrophy were significant in the bottom seawater of the cultivation area. The carbon (C), nitrogen (N), and sulfur (S) cycle functional genes in the sediment environment of the kelp cultivation area were more active than those in other areas, especially in which sulfate reduction and denitrification were the two main processes. Furthermore, a DOM priming effect was identified in the cultivated sediment environment, where kelp-released labile dissolved organic matter (LDOM) stimulates rapid degradation of the original RDOM, potentially enhancing C sequestration.

Suboxic DOM is bioavailable to surface prokaryotes in a simulated overturn of an oxygen minimum zone, Devil's Hole, Bermuda.

Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand through respiration, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil's Hole, Bermuda is a model ecosystem to study OMZ microbial biogeochemistry because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop, with organic matter and nutrients accumulating at depth. In this study, the bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of suboxic waters was assessed using a simulated overturn experiment. The surface inoculated prokaryotic community responded to the deep (formerly suboxic) 0.2 μm filtrate with cell densities increasing 2.5-fold over 6 days while removing 5 μmol L-1 of DOC. After 12 days, the surface community began to shift, and DOC quality became less diagenetically altered along with an increase in SAR202, a Chloroflexi that can degrade recalcitrant dissolved organic matter (DOM). Labile DOC production after 12 days coincided with an increase of Nitrosopumilales, a chemoautotrophic ammonia oxidizing archaea (AOA) that converts ammonia to nitrite based on the ammonia monooxygenase (amoA) gene copy number and nutrient data. In comparison, the inoculation of the deep anaerobic prokaryotic community into surface 0.2 μm filtrate demonstrated a die-off of 25.5% of the initial inoculum community followed by a 1.5-fold increase in cell densities over 6 days. Within 2 days, the prokaryotic community shifted from a Chlorobiales dominated assemblage to a surface-like heterotrophic community devoid of Chlorobiales. The DOM quality changed to less diagenetically altered material and coincided with an increase in the ribulose-1,5-bisphosphate carboxylase/oxygenase form I (cbbL) gene number followed by an influx of labile DOM. Upon reoxygenation, the deep DOM that accumulated under suboxic conditions is bioavailable to surface prokaryotes that utilize the accumulated DOC initially before switching to a community that can both produce labile DOM via chemoautotrophy and degrade the more recalcitrant DOM.

Processes in the Surface Ocean Regulate Dissolved Organic Matter Distributions in the Deep

AbstractMarine dissolved organic matter (DOM) is a major global carbon pool, consisting of thousands of compounds with distinct lifetimes. While marine DOM persists for millennia, its molecular and isotopic composition imply that it is dynamic on shorter timescales. To determine the extent to which DOM deviates from conservative water mass mixing, we conducted a two‐endmember mixing analysis on dissolved organic carbon (DOC) concentration and DOM molecular composition in the Atlantic and Pacific. Endmembers were the deep water masses near their formation sites. For DOM composition, we considered 6118 molecular formulae (MF) identified via Fourier‐transform ion cyclotron resonance mass spectrometry in solid‐phase extracts (SPE) of 837 samples. Bulk DOC and SPE‐DOC concentrations behaved conservatively in both basins and ≥70% of the MF (14–20 μM SPE‐DOC) mixed conservatively. However, a small fraction (10%–20%) of the MF (<3 μM SPE‐DOC) were added or removed during mixing. These MF were more reduced and oxidized, respectively, than the conservative fraction. There were also MF absent from the endmembers; these accounted for ≤1 μM of SPE‐DOC and positively correlated with DOM lability. Based on their distribution across the two basins, we conclude that the conserved MF are formed in the surface subtropical ocean and modified in overturning areas. In the deep ocean, however, these MF are solely controlled by mixing. This finding contrasts with the current paradigm of slow, continuous degradation of recalcitrant DOM in the deep ocean. Our analysis illustrates the importance of the surface ocean in controlling DOM cycling in the deep.

Organic amendment application affects the release behaviour, bioavailability, and speciation of heavy metals in zinc smelting slag: Insight into dissolved organic matter

Organic amendments are commonly used in assisted phytostabilization of mine wastes by improving their physicochemical and biological properties. These amendments are susceptible to leaching and degradation, resulting in the generation of dissolved organic matter (DOM), which significantly influences the geochemical behaviour of heavy metals (HMs). However, the geochemical behaviour of HMs in metal smelting slag driven by organic amendment-derived DOM remains unclear. In this study, we investigated the impact of cow manure-derived DOM on the release behaviour, bioavailability, and speciation of HMs (Cu, Pb, Zn, and Cd) in zinc smelting slag using a multidisciplinary approach. The results showed that DOM enhanced the weathering of the slag, with a minimal impact on the slag’s mineral phases, except for causing gypsum dissolution. The DOM addition resulted in a slight increase in HM release from the slag during the initial inoculation period, followed by a reduction in HM release during the later period. Furthermore, the DOM addition increased the diversity and relative abundance of the bacterial community. This, in turn, led to a decrease in the dissolved organic carbon (DOC) content and enhanced the transformation of labile DOM compounds into recalcitrant compounds. The variation in HM release during various inoculation periods can be attributed to the bacterial decomposition and transformation of DOM, which further enhanced the transformation of HM fractions. Specifically, during the later period, DOM promoted the conversion of a portion of the reducible and oxidizable fractions of Cu, Pb, and Zn into the acid-soluble and residual fractions. Moreover, it partially transformed the reducible, oxidizable, and residual fractions of Cd into the acid-soluble fraction. Overall, this study provides new insights into the geochemical behaviour of HMs in slag governed by the coupling effect of DOM and the bacterial community. These findings have implications for the use of organic amendments in assisted phytostabilization of metal smelting slag. Environmental implicationMetal smelting slag is hazardous due to its high levels of HMs, and its improper disposal has serious consequences for the ecosystem. Organic amendments are employed in assisted phytostabilization of the slag site by improving its microecological properties. However, the impact of organic amendment-derived DOM on HM migration and transformation in slag remains unclear. This study indicated that the coupling effects of DOM and microbes governed the geochemical behaviour of HMs in slag. These findings provide new insights into how organic amendments impact the geochemical behaviour of HMs in slag, contributing to the development of phytostabilization technology.

Bacterial community regulation of soil organic matter molecular structure in heavy metal-rich mangrove sediments

Heavy metals (HMs) profoundly impact soil carbon storage potential primarily through soil carbon structure. The association between HM content and soil carbon structure in mangrove sediments remains unclear, likely due to the involvement of microorganisms. In this study, surface sediments in the Futian National Mangrove Nature Reserve were sampled to investigate the chemical structure of soil organic carbon (SOC), the molecular composition of dissolved organic matter (DOM), and potential interactions with microorganisms. HMs, except for Ni, were positively correlated with soil carbon. HMs significantly reduced the alkyl C/O-alkyl C ratio, aromaticity index, and aromatic C values, but increased the labile carboxy/amide C and carbonyl C ratio in SOC. HMs also increased DOM stability, as reflected by the reduced abundance of labile DOM (lipids and proteins) and increased proportion of stable DOM (tannins and condensed aromatics). Bacteria increased the decomposition of labile DOM components (unsaturated hydrocarbons) and the accumulation of stable DOM components (lignins) under HM enrichment. In addition, the association between the bacterial groups and DOM molecules was more robust than that with fungal groups, indicating bacteria had a more significant impact on DOM molecular composition. These findings help in understanding the molecular mechanisms of soil carbon storage in HM-rich mangroves.

Dissolved organic matter and nutrient processing in organic-rich subterranean estuaries: Implications for future land use and climate scenarios

The subterranean estuary (STE) is a recognized reactive biogeochemical zone at the groundwater–seawater interface in coastal regions. However, few studies have explored dissolved organic matter (DOM) and nutrient processes in buried organic-rich STEs (with up to 20 % of organic matter content), despite their potential contributions to coastal water hypoxia and greenhouse gas emissions. This study conducted controlled laboratory experiments to investigate the physicochemical controls on DOM and nutrient processing in organic-rich STEs, considering variations in groundwater nitrate (NO3–) concentrations, salt concentration, seawater ions, and incubation times. Results showed elevated levels of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in pore water, primarily from particulate organic matter (POM) degradation that originated from decayed vascular plants. POM-to-DOM degradations further resulted in the accumulation of ammonium (NH4+) and highly unsaturated compounds in all treatments. The addition of high-ionic-strength treatments (e.g., seawater and sodium chloride [NaCl] solution) reduced DOC solubility and selectively removed dissolved aromatic compounds in porewater. The cation-rich seawater shifted the porewater NH4+ generation pathway from predominantly biological (re)mineralization in freshwater to abiotic (desorption via ion exchange) pathways in the saline water setup. Orthophosphate (PO43−) dynamics were likely to be influenced by the iron-rich sediment surface in both freshwater and saline water scenarios. Our experiments further indicated that the long-term incubation enriched DOM concentration with a high degradation state. This work demonstrated that organic-rich STEs are significant sources of less labile DOM and NH4+ to coastal waters. In the context of global climate change, our results illustrate the sensitivity of DOM and nutrient biogeochemical cycling in STEs to salinization and altered residence time, consequently affecting the quantity and quality of DOM and nutrients transported to coastal waters.

Evidence for a Temporary Positive Priming Effect in Aquatic Systems With Certain Substrates and Isotopic Discrimination of DOM Sources

AbstractThe “priming effect” (PE) is well documented in soils, but contradictory results have been reported in aquatic ecosystems. To investigate whether the presence of easily degradable labile dissolved organic matter (LDOM) changes the biodegradation rate of nonlabile DOM (NLDOM), incubations were performed with various sources of LDOM and NLDOM in microcosms amended with nutrients and a microbial inoculum. Stable carbon isotope analysis of DOM was used for the first time to estimate the PE. A significant positive PE (15%–34% of initial NLDOM) was measured, but only with mixed (glucose + amino acids) or complex (disaccharide) LDOM. Regardless of the LDOM, no PE was measured with more recalcitrant NLDOM (half‐life of 128 days here), when the LDOM/NLDOM ratio was low (0.3 versus 1), and when the NLDOM and microbial inoculum were from the same water (no mixing). The presence of sediments likely enhances microbial diversity and NLDOM degradation rate, but it did not increase the PE. Microbial use of LDOM produced new microbial NLDOM that should be discriminated (here by isotopes) from initial NLDOM for accurate PE measurements. The PE was temporary and lasted about 2 weeks after one LDOM addition. In areas with frequent additions of LDOM, a long‐lasting PE is expected and may significantly increase CO2 production. Exposing refractory DOM to new conditions and microbial community, such as during natural water mixing, seems to exert a strong control on its dynamics. Here, the addition of a new microbial inoculum had a stronger effect than adding LDOM or the PE.

Advancing chemical lability assessments of organic matter using a synthesis of FT-ICR MS data across diverse environments and experiments

Patterns in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) data provide perspective on how organic matter (OM) character is shaped by natural processes. Here, we reevaluate and update the molecular lability boundary (MLB) approach to assess OM lability using a synthesis of FT-ICR MS data. Now included are MLBL (biolability) indices (i.e., the OM proportion of hydrogen-to-carbon ratios ≥ 1.5) of freshwaters, plant litter and biochar leachates, soils, and oils across the US, trends in OM and dissolved OM (DOM) lability from laboratory experiments examining isolated microbial, photochemical, and thermal degradation, and biogeochemical interpretations of biolability patterns across ecosystems influenced by human activity (e.g., fertilizer addition and agricultural management). Photo-processing increased the hydrogen saturation of OM, describing products of higher biolability (>MLBL values). DOM MLBL values decreased during microbial and thermal degradation processes. Laboratory experiments with isolated treatments showed larger changes in MLBL than exhibited by field samples from systems managed by humans. We interpret changes in N- and/or S-containing DOM above the MLB as the production or use of heterogeneously biolabile material. We interpret heterogeneously labile DOM composition to indicate autochthonous production, biomass growth, nutrient accumulation, or biological degradation in stream and soil samples. The most recent data in our synthesis suggest that the MLB approach should be reframed to provide lability indices for biotic, thermal, and photochemical processes. A broader perspective on lability provides a useful tool to decompose large, complex, and process-driven DOM data into more simple and informative indicators of trends in ecosystem form and function.

Soil microbial community, dissolved organic matter and nutrient cycling interactions change along an elevation gradient in subtropical China

To identify possible dominating processes involved in soil microbial community assembly, dissolved organic matter (DOM) and multi-nutrient cycling (MNC) interactions and contribute to understanding of climate change effects on these important cycles, we investigated the interaction of soil chemistry, DOM components and microbial communities in five vegetation zones - ranging from evergreen broad-leaved forest to alpine meadow - along an elevation gradient of 290–1960 m in the Wuyi Mountains, Fujian Province, China. Soil DOM composition and microbial community assembly were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Illumina MiSeq high-throughput sequencing, respectively. Sloan's neutral model and the modified stochasticity ratio were used to infer community assembly processes. Key microbial drivers of the soil MNC index were identified from partial least squares path models. Our results showed that soil DOM composition is closely related to the vegetation types along an elevation gradient, the structure and composition of the microbial community, and soil nutrient status. Overall, values of the double bond equivalent (DBE), modified aromaticity index (AImod) increased, and H/C ratio and molecular lability boundary (MLBL) percentage decreased with elevation. Lignins/CRAM-like structures compounds dominated soil DOM in each vegetation type and its relative abundance decreased with elevation. Aliphatic/protein and lipids components also decreased, but the relative abundance of aromatic structures and tannin increased with elevation. The alpha diversity index of soil bacteria gradually decreased with elevation, with deterministic processes dominating the microbial community assembly in the highest elevation zone. Bacterial communities were conducive to the decomposition of labile degradable DOM compounds (H/C ≥ 1.5) at low elevation. In the cooler and wetter conditions at higher-elevation sites the relative abundance of potentially resistant soil DOM components (H/C < 1.5) gradually increased. Microbial community diversity and composition were important predictors of potential soil nutrient cycling. Although higher elevation sites have higher nutrient cycling potential, soil DOM was assessed to be a more stable carbon store, with apparent lower lability and bioavailability than at lower elevation sites. Overall, this study increases understanding of the potential linkage between soil microbial community, multiple nutrient cycling and DOM fate in subtropical mountain ecosystems that can help predict the effect of climate change on soil carbon sequestration and thus inform ecosystem management.

Molecular characterization of dissolved organic matter in urban stormwater pond and municipal wastewater discharges transformed by the Florida red tide dinoflagellate Karenia brevis

Karenia brevis blooms occur almost annually in southwest Florida, imposing significant ecological and human health impacts. Currently, 13 nutrient sources have been identified supporting blooms, including nearshore anthropogenic inputs such as stormwater and wastewater outflows. A 21-day bioassay was performed, where K. brevis cultures were inoculated with water sourced from three stormwater ponds along an age gradient (14, 18, and 34 yrs.) and one municipal wastewater effluent sample, with the aim of identifying biomolecular classes and transformations of dissolved organic matter (DOM) compounds used by K. brevis. All sample types supported K. brevis growth and showed compositional changes in their respective DOM pools. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) catalogued the molecular composition of DOM and identified specific compound classes that were biodegraded. Results showed that K. brevis utilized species across a wide range of compositions that correspond to amino sugars, humic, and lignin-like biomolecular classes. The municipal wastewater and the youngest stormwater pond (SWP 14) effluent contained the largest pools of labile DOM compounds which were bioavailable to K. brevis, which indicates younger stormwater pond effluents may be as ecologically important as wastewater effluents to blooms. Conversely, generation of DOM compounds of greater complexity and a wide range of aromaticity was observed with the older (SWP 18 and SWP 34) stormwater pond treatments. These data confirm the potential for stormwater ponds and/or wastewater to contribute nutrients which can potentially support K. brevis blooms, revealing the need for improved nutrient retention strategies to protect coastal waters from the potential ill effects of urban effluent.

Transformation of dissolved organic matter and related arsenic mobility at a surface water-groundwater interface in the Hetao Basin, China

Porewater arsenic mobility above the groundwater table has been recognized as a potential cause of arsenic-rich groundwater, but the processing pathways of dissolved organic matter (DOM) in that hyporheic zone and their effect on porewater arsenic release remain poorly understood. To address these issues, two porewater profiles were sampled in a surface water-groundwater interaction zone from the Hetao Basin, China, to monitor the porewater geochemistry and DOM molecular characteristics. The results show that the porewater arsenic, Fe(II), and DOC concentrations were all significantly higher than those of the intruding pond water, and were located above the conservative mixing model lines. This indicates a net release of these solutes from the sediment. By comparing the porewater with pond water DOM, we found that the carboxyl-rich alicyclic molecules (CRAM) were selectively preserved, carbohydrates and aliphatics/proteins were preferentially consumed, and low O/C-ratio compounds with high bioproduction index (I_bioprod) and terrestrial index (I_terr) were produced. The transformation of CHO to CHOS compounds also represented a pathway of recalcitrant DOM production. The produced recalcitrant organic compounds mostly contributed to the elevated porewater DOC concentrations, but their contribution decreased along the filtration path. The consumption of labile DOM compounds would be responsible for Fe(III) hydroxide reduction and arsenic release. The generated recalcitrant DOM may also be a driver of porewater arsenic mobility by acting as electron shuttles. This study highlights the importance of the hyporheic zone in shaping shallow groundwater DOM composition and the potential contribution to arsenic enrichment.

Co-occurrence of arsenic and iodine in the middle-deep groundwater of the Datong Basin: From the perspective of optical properties and isotopic characteristics

Redox processes can induce arsenic (As) and iodine (I) transformation and thus change As and I co-occurrence, yet there is no evidence that Fe–C–S coupled redox processes have such an impact on the co-occurrence of As and I. To fill this gap, middle-deep groundwater from the Datong Basin were samples for the purpose of exploring how dissolved organic matter (DOM) reactivity affects As and I enrichment and how iron reduction and sulfate reduction processes influence As and I co-occurrence. We identified three DOM components: reduced and oxidized quinone compounds (C1 and C3) and a labile DOM from terrestrial inputs (C2). Two pathways of DOM processing take place in the aquifer, including the degradation of labile DOM to HCO3− and the transformation of oxidized quinone compounds to reduced quinone compounds. Electrons transfer drives the reduction of the terminal electron acceptors. The supply of electrons promotes the reduction of iron and sulfate by microbes, enhancing As and I co-enrichment in groundwater. Thus, the reduction processes of iron and sulfate triggered by the dual roles of DOM affect dissolved As and I co-enrichment. As and I biogeochemical cycling interacts with C, Fe, and S cycling. These results provide isotopic and fluorescence evidence that explains the co-occurrence of arsenic and iodine in middle-deep aquifers.

Distinct seasonal variations of dissolved organic matter across two large freshwater lakes in China: Lability profiles and predictive modeling

Biological lability of dissolved organic matter (DOM) is a crucial indicator of carbon cycle and contaminant attenuation in freshwater lakes. In this study, we employed a multi-stage plug-flow bioreactor and spectrofluorometric indices to characterize the seasonal variations in DOM composition and lability across Poyang Lake (PY) and Lake Taihu (TH), two large freshwater lakes in China with distinct hydrological seasonality. Our findings showed that the export of floodplain-derived organics and river-lake interaction led to a remarkable increase in terrestrial aromatic and humic-like DOM with high molecular weights and long turnover times in PY. Consequently, the labile fraction was extremely low (average LDOC% of 3%) during the rising-to-flood season (spring and summer). Conversely, autochthonous production in TH considerably enriched semi-labile (average SDOC% of 26%) and biodegradable DOM (average BDOC% of 34%) during the phytoplankton bloom to post-bloom season (summer and autumn). This was reflected by the accumulation of low-light-absorbing and protein-like components with high biological and fluorescence indices. In the dry and non-bloom season (winter), the better preservation of humic substances maintained the high molecular weight and humic degree of DOM in PY, while the decay of aquatic plants strengthened autochthonous production, resulting in a similar BDOC% of PY samples (23%–34%) to TH samples (18%–33%). We further applied partial least squares regression using DOM optical indices as predictive proxies, which generated a greater prediction strength for BDOC% (R2 = 0.80) compared to SDOC% (R2 = 0.57) and LDOC% (R2 = 0.28). The regression model identified aromaticity (SUVA254) as the most effective and negative predictor and low molecular weight (A250/A365) as the highly and positively influential factor. Our study provides new evidence that the seasonality of DOM lability profiles is regulated by the trade-off between flow-related variation and phytoplankton production, and presents an approach to describe and predict DOM lability across freshwater lakes.

Relevance of tributary inflows for driving molecular composition of dissolved organic matter (DOM) in a regulated river system

River regulation by dams can alter flow regimes and organic matter dynamics, but less is known about how unregulated tributaries regulate organic matter composition and processing in the regulated river below the confluence. This study reports on water chemistry, especially dissolved organic matter (DOM) concentration and composition (dissolved organic carbon (DOC), organic nitrogen (DON), organic phosphorus (DOP) and combined amino acids (DCAA)) along the regulated Tumut and unregulated Goobarragandra (tributary) rivers under different flow conditions (base flow vs storm event) in south-east Australia. The tributary was significantly different from regulated and downstream sites during base flow conditions with higher temperature, pH, buffering capacity, DOC and nutrient concentrations (DON, DOP, DCAA). DOM characterisation by spectrometry and size exclusion chromatography revealed that the tributary contained a higher proportion of terrestrially derived humic-like and fulvic-like DOM. In contrast, regulated and downstream sites contained higher proportion of microbially derived DOM such as low molecular weight neutrals and protein-like components. Storm pulses of tributary flows into the regulated system, influenced both concentration and composition of DOM at the downstream site, which more strongly resembled the tributary site than the regulated site during the storm event. Additionally, we found that the tributary supplied fresh DOM, including small organic molecules to the regulated system during storm events. The presence of these different types of labile DOM can increase primary productivity and ecological functioning within regulated river reaches downstream of tributary junctions. This has important implications for the protection of unregulated tributary inflows within regulated river basins.

Loss of Selenium from Mollisol Paddy Wetlands of Cold Regions: Insights from Flow-through Reactor Experiments and Process-Based Modeling

Mollisols are critical agricultural resources for securing global food supply. Due to its health importance, selenium (Se) fate in the Mollisols attracts growing concerns. Land use change from conventional drylands to paddy wetlands impacts Se bioavailability in the vulnerable Mollisol agroecosystems. The underlying processes and mechanisms however remain elusive. Here, results of flow-through reactor experiments with paddy Mollisols from northern cold-region sites indicate that continuous flooding with surface water for 48 d induced redox zonation that facilitated the loss of Mollisol Se by up to 51%. Further process-based biogeochemical modeling suggests largest degradation rates of dissolved organic matter (DOM) in 30 cm deep Mollisols that contained the highest-level labile DOM and organic-bound Se. Electron shunting from degradation of Se-bearing DOM coupled to reductive dissolution of Se-adsorbed Fe oxides accounts mainly for Se(IV) release into the pore water. Consequent changes in DOM molecular composition make the reservoir of organic-bound Se vulnerable to flooding-induced redox zonation and likely enhance Se loss through destruction of thiolated Se and emission of gaseous Se from the Mollisol layer. This study highlights a neglected scenario where the speciation-driven loss of bioavailable Se from the paddy wetlands can be a significant consequence in the cold-region Mollisol agroecosystems.

Dissolved organic matter defines microbial communities during initial soil formation after deglaciation

Ecosystem succession and pedogenesis reshuffle the composition and turnover of dissolved organic matter (DOM) and its interactions with soil microbiome. The changes of these connections are especially intensive during initial pedogenesis, e.g. in young post-glacial areas. The temporal succession and vertical development of DOM effects on microbial community structure remains elusive. Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), high-throughput sequencing, and molecular ecological networks, we characterized the molecular diversity of water-extractable DOM and identified its links to microbial communities in soil profiles along deglaciation chronosequence (12, 30, 40, 52, 80, and 120 years) in the southeastern Tibetan Plateau. Low-molecular-weight compound content decreased, whereas the mid- and high-molecular-weight compounds increased with succession age and soil depth. This was confirmed by the increase in double bond equivalents and averaged oxygen-to‑carbon ratios (O/C), and decrease in hydrogen-to‑carbon ratios (H/C), which reflect DOM accumulation and stabilization. Microbial community succession shifted towards the dominance of oligotrophic Acidobacteria and saprophytic Mortierellomycota, reflecting the increase of stable DOM components (H/C < 1.5 and wider O/C). Less DOM-bacterial positive networks during the succession reduced specialization of labile DOM production (such as lipid- and protein-like compounds), whereas more DOM-fungal negative networks increased specialization of stable DOM decomposition (such as tannin- and condensed aromatic-like compounds). Consequently, DOM stability is not intrinsic during initial pedogenesis: stable DOM compounds remain after fast bacterial utilization of labile DOM compounds, whereas fungi decompose slowly the remaining DOM pools.

Phototrophic Biofilms Transform Soil-Dissolved Organic Matter Similarly Despite Compositional and Environmental Differences.

Dissolved organic matter (DOM) is the most reactive pool of organic carbon in soil and one of the most important components of the global carbon cycle. Phototrophic biofilms growing at the soil-water interface in periodically flooding-drying soils like paddy fields consume and produce DOM during their growth and decomposition. However, the effects of phototrophic biofilms on DOM remain poorly understood in these settings. Here, we found that phototrophic biofilms transformed DOM similarly despite differences in soil types and initial DOM compositions, with stronger effects on DOM molecular composition than soil organic carbon and nutrient contents. Specifically, growth of phototrophic biofilms, especially those genera belonging to Proteobacteria and Cyanobacteria, increased the abundance of labile DOM compounds and richness of molecular formulae, while biofilm decomposition decreased the relative abundance of labile components. After a growth and decomposition cycle, phototrophic biofilms universally drove the accumulation of persistent DOM compounds in soil. Our results revealed how phototrophic biofilms shape the richness and changes in soil DOM at the molecular level and provide a reference for using phototrophic biofilms to increase DOM bioactivity and soil fertility in agricultural settings.