Microbial regulation on refractory dissolved organic matter in inland waters.

The production of refractory dissolved organic matter (RDOM) is complex and closely related to microbial consortia in aquatic ecosystems; however, it is still unclear how microorganisms regulate the production of RDOM and its molecular composition in inland waters. Therefore, we conducted a large-scale survey of inland waters and analyzed the optical and mass spectrometric characteristics of DOM, the microbial community and functional genes, as well as related environmental parameters, to understand the abovementioned issues. Here, the RDOM production was found mainly regulated by microbial (e.g., phylogeny and community assembly) rather than other environmental factors in inland waters. Biostatistical analyses and carbon isotopic evidence indicated that the successive microbial processing from labile DOM to RDOM (i.e., carboxyl-rich alicyclic molecules, CRAMs) was widely present in inland waters, involving the microbially mediated carbon skeleton turnover and heteroatom conversion. There was a significant empirical relationship between CRAMs and the ratio of Proteobacteria to Actinobacteria, highlighting the intraspecific interaction of bacteria more important than other microbial groups (i.e., archaea, eukaryotes, and fungi) for the RDOM production. This study demonstrated a fundamental role of microbial regulation in RDOM production within the inland waters, thereby facilitating future estimation of carbon sequestration potential in inland aquatic ecosystems.

Comprehensive source identification of heavy metals in atmospheric particulate matter in a megacity: A case study of Hangzhou.

Megacities face significant pollution challenges, particularly the elevated levels of heavy metals (HMs) in particulate matter (PM). Despite the advent of interdisciplinary and advanced methods for HM source analysis, integrating and applying these approaches to identify HM sources in PM remains a hurdle. This study employs a year-long daily sampling dataset for PM1 and PM1-10 to examine the patterns of HM concentrations under hazy, clean, and rainy conditions in Hangzhou City, aiming to pinpoint the primary sources of HMs in PM. Contrary to other HMs that remained within acceptable limits, the annual average concentrations of Cd and Ni were found to be 20.6 ± 13.6 and 46.9 ± 34.8 ng/m³, respectively, surpassing the World Health Organization's limits by 4.1 and 1.9 times. Remarkably, Cd levels decreased on hazy days, whereas Ni levels were observed to rise on rainy days. Using principal component analysis (PCA), enrichment factor (EF), and backward trajectory analysis, Fe, Mn, Cu, and Zn were determined to be primarily derived from traffic emissions, and there was an interaction between remote migration and local emissions in haze weather. Isotope analysis reveals that Pb concentrations in the Hangzhou region were primarily influenced by emissions from unleaded gasoline, coal combustion, and municipal solid waste incineration, with additional impact from long-range transport; it also highlights nuanced differences between PM1 and PM1-10. Pb isotope and PCA analyses indicate that Ni primarily stemmed from waste incineration emissions. This explanation accounts for the observed higher Ni concentrations on rainy days. Backward trajectory cluster analysis revealed that southern airflows were the primary source of high Cd concentrations on clean days in Hangzhou City. This study employs a multifaceted approach and cross-validation to successfully delineate the sources of HMs in Hangzhou's PM. It offers a methodology for the precise and reliable analysis of complex HM sources in megacity PM.

Global distribution and drivers of relative contributions among soil nitrogen sources to terrestrial plants.

Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (fNO3-), ammonium (fNH4+), and organic N (fEON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of fNO3-, fNH4+, and fEON. The fNO3- increases with MAT, reaching 46% at 28.5 °C. The fNH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the fEON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems.

Metagenomics and Stable Isotopes Uncover the Augmented Sulfide-Driven Autotrophic Denitrification in a Seasonally Hypoxic, Sulfate-Abundant Reservoir.

The mechanism governing sulfur cycling in nitrate reduction within sulfate-rich reservoirs during seasonal hypoxic conditions remains poorly understood. This study employs nitrogen and oxygen isotope fractionation in nitrate, along with metagenomic sequencing to elucidate the intricacies of the coupled sulfur oxidation and nitrate reduction process in the water column. In the Aha reservoir, a typical seasonally stratified water body, we observed the coexistence of denitrification, bacterial sulfide oxidation, and bacterial sulfate reduction in hypoxic conditions. This is substantiated by the presence of abundant N/S-related genes (nosZ and aprAB/dsrAB) and fluctuations in N/S species. The lower 15εNO3/18εNO3 ratio (0.60) observed in this study, compared to heterotrophic denitrification, strongly supports the occurrence of sulfur-driven denitrification. Furthermore, we found a robust positive correlation between the metabolic potential of bacterial sulfide oxidation and denitrification (p < 0.05), emphasizing the role of sulfide produced via sulfate reduction in enhancing denitrification. Sulfide-driven denitrification relied on ∑S2- as the primary electron donor preferentially oxidized by denitrification. The pivotal genus, Sulfuritalea, emerged as a central player in both denitrification and sulfide oxidation processes in hypoxic water bodies. Our study provides compelling evidence that sulfides assume a critical role in regulating denitrification in hypoxic water within an ecosystem where their contribution to the overall nitrogen cycle was previously underestimated.

Tracing phosphorus sources in the river-lake system using the oxygen isotope of phosphate.

The biogeochemical cycling of phosphorus (P) in river-lake systems presents significant challenges in tracing P sources, highlighting the importance of effective traceability approaches for formulating targeted management measures to mitigate lake eutrophication. In this study, we used the oxygen isotope of phosphate (δ18Op) as a tracer in the river-lake systems, establishing a tracing pathway from potential end-members, through inflow rivers, and eventually to the lake. Taking Dianshan Lake and its main inflow rivers as the study area, we measured δ18Op values of potential end-members, including domestic sewage treatment plant effluents, industrial effluents from phosphorus-related enterprises (printing and dyeing, electroplating, plastics, etc.), and farmland soils. Notably, the industrial effluent signatures ranged from 13.1 ‰ to 21.0 ‰ with an average of 16.8 ‰ ± 3.2 ‰, enriching the δ18Op threshold database. Using the MixSIAR model, it was found that phosphorus in the Jishuigang River primarily originated from agricultural non-point sources and domestic sewage in the dry season, while the Qiandengpu River, with a higher proportion of urban area, had a greater influence from domestic sewage and industrial effluents. Moreover, significant differences were observed between δ18Op values at the lake entrances of the inflow rivers (13.7 ‰ ± 1.0 ‰) and in acid-soluble phosphate of the lake sediments (9.9 ‰ ± 1.0 ‰). Isotopic tracing revealed that phosphorus in the lake originated from both external inputs (80.6 %) and internal release (19.4 %) in the dry season. Alongside pollutant flux calculations based on the hydrological conditions and water quality of the inflow rivers, our findings indicated that phosphorus in Dianshan Lake was mainly attributed to agricultural non-point sources, domestic sewage and sediment release in the dry season. This study provided novel insights into the identification of pollution sources in the river-lake systems, with broad implications for pollution control and environmental protection.

Organic matter composition and stability in estuarine wetlands depending on soil salinity.

Coastal wetlands are key players in mitigating global climate change by sequestering soil organic matter. Soil organic matter consists of less stable particulate organic matter (POM) and more stable mineral-associated organic matter (MAOM). The distribution and drivers of MAOM and POM in coastal wetlands have received little attention, despite the processes and mechanisms differ from that in the upland soils. We explored the distribution of POM and MAOM, their contributions to SOM, and the controlling factors along a salinity gradient in an estuarine wetland. In the estuarine wetland, POM C and N were influenced by soil depth and vegetation type, whereas MAOM C and N were influenced only by vegetation type. In the estuarine wetland, SOM was predominantly in the form of MAOM (> 70 %) and increased with salinity (70 %-76 %), leading to long-term C sequestration. Both POM and MAOM increased with SOM, and the increase rate of POM was higher than that of MAOM. Aboveground plant biomass decreased with increasing salinity, resulted in a decrease in POM C (46 %-81 %) and N (52 %-82 %) pools. As the mineral amount and activity, and microbial biomass decreased, the MAOM C (2.5 %-64 %) and N pool (8.6 %-59 %) decreased with salinity. When evaluating POM, the most influential factors were microbial biomass carbon (MBC) and dissolved organic carbon (DOC). Key parameters, including MBC, DOC, soil salinity, soil water content, aboveground plant biomass, mineral content and activity, and bulk density, were identified as influencing factors for both MAOM abundance. Soil water content not only directly controlled MAOM, but together with salinity also indirectly regulated POM and MAOM by controlling microbial biomass and aboveground plant biomass. Our findings have important implications for improving the accumulation and increased stability of soil organic matter in coastal wetlands, considering the global sea level rise and increased frequency of inundation.

Peatland Wildfires Enhance Nitrogen-Containing Organic Compounds in Marine Aerosols over the Western Pacific.

Peatland wildfires contribute significantly to the atmospheric release of light-absorbing organic carbon, often referred to as brown carbon. In this study, we examine the presence of nitrogen-containing organic compounds (NOCs) within marine aerosols across the Western Pacific Ocean, which are influenced by peatland fires from Southeast Asia. Employing ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in electrospray ionization (ESI) positive mode, we discovered that NOCs are predominantly composed of reduced nitrogenous bases, including CHN+ and CHON+ groups. Notably, the count of NOC formulas experiences a marked increase within plumes from peatland wildfires compared to those found in typical marine air masses. These NOCs, often identified as N-heterocyclic alkaloids, serve as potential light-absorbing chromophores. Furthermore, many NOCs demonstrate pyrolytic stability, engage in a variety of substitution reactions, and display enhanced hydrophilic properties, attributed to chemical processes such as methoxylation, hydroxylation, methylation, and hydrogenation that occur during emission and subsequent atmospheric aging. During the daytime atmospheric transport, aging of aromatic N-heterocyclic compounds, particularly in aliphatic amines prone to oxidation and reactions with amine, was observed. The findings underscore the critical role of peatland wildfires in augmenting nitrogen-containing organics in marine aerosols, underscoring the need for in-depth research into their effects on marine ecosystems and regional climatic conditions.

Significant accrual of soil organic carbon through long-term rice cultivation in paddy fields in China.

Paddy fields serve as significant reservoirs of soil organic carbon (SOC) and their potential for terrestrial carbon (C) sequestration is closely associated with changes in SOC pools. However, there has been a dearth of comprehensive studies quantifying changes in SOC pools following extended periods of rice cultivation across a broad geographical scale. Using 104 rice paddy sampling sites that have been in continuous cultivation since the 1980s across China, we studied the changes in topsoil (0-20 cm) labile organic C (LOC I), semi-labile organic C (LOC II), recalcitrant organic C (ROC), and total SOC. We found a substantial increase in both the content (48%) and density (39%) of total SOC within China's paddy fields between the 1980s to the 2010s. Intriguingly, the rate of increase in content and density of ROC exceeded that of LOC (I and II). Using a structural equation model, we revealed that changes in the content and density of total SOC were mainly driven by corresponding shifts in ROC, which are influenced both directly and indirectly by climatic and soil physicochemical factors; in particular temperature, precipitation, phosphorous (P) and clay content. We also showed that the δ13 CLOC were greater than δ13 CROC , independent of the rice cropping region, and that there was a significant positive correlation between δ13 CSOC and δ13 Cstraw . The δ13 CLOC and δ13 CSOC showed significantly negative correlation with soil total Si, suggesting that soil Si plays a part in the allocation of C into different SOC pools, and its turnover or stabilization. Our study underscores that the global C sequestration of the paddy fields mainly stems from the substantial increase in ROC pool.

Microbial contribution estimated by clumped isotopologues (13CH3D and 12CH2D2) characteristics in a CO2 enhanced coal bed methane reservoir.

Carbon capture and storage (CCS) of CO2 is a key technology for substantially mitigating global greenhouse gas emissions. Determining the biogeochemical processes in host rocks after CO2 injection informs the viability of carbon storage as a long-term sink for CO2, the complexity of reservoir CH4 cycling, as well as the direct and indirect environmental impacts of this strategy. The doubly substituted ('clumped') isotopologues of methane (13CH3D and 12CH2D2) provide novel insights into methane origins and post-generation processing. Here, we report the chemical compositions of hydrocarbons (C1/C2+ molecular ratios), and methane bulk and clumped isotopes (δ13C, δD, Δ13CH3D and Δ12CH2D2) of a CO2 enhanced coal bed methane recovery (CO2-ECBM) area in Qinshui basin, China and is an analogue for carbon capture and storage. The clumped isotopologue compositions observed in the study area are generally consistent with a range of temperatures spanning 73 to 193 °C. The range in apparent temperature and correlations among clumped and bulk isotopic indices are best explained by mixing between a high maturity thermogenic methane (high in δ13C and δD, with a clumped isotope composition equilibrated near ∼249 °C) and biogenic methane formed or processed in the reservoir (low in δ13C and δD, with a clumped isotope composition equilibrated near 16-27 °C). We hypothesize that the biogenic endmember may result from slow methanogenesis and/or anaerobic oxidation of methane (AOM). This study demonstrates that the potential of methane clumped isotope approach to identify in situ microbial metabolic processes and their association with carbon cycling in CO2-ECBM area, improving our understanding of biogeochemical mechanisms in analogous geological reservoirs.

Synergistic binding mechanisms of co-contaminants in soil profiles: Influence of iron-bearing minerals and microbial communities.

In contaminated soil sites, the coexistence of inorganic and organic contaminants poses a significant threat to both the surrounding ecosystem and public health. However, the migration characteristics of these co-contaminants within the soil and their interactions with key components, including Fe-bearing minerals, organic matter, and microorganisms, remain unclear. This study involved the collection of a 4.3-m-depth co-contaminated soil profile to investigate the vertical distribution patterns of co-contaminants (namely, arsenic, cadmium, and polychlorinated biphenyls (PCBs)) and their binding mechanisms with environmental factors. The results indicated a notable downward accumulation of inorganic contaminants with increasing soil depth, whereas PCBs were predominantly concentrated in the uppermost layer. Chemical extraction and synchrotron radiation analysis highlighted a positive correlation between the abundance of reactive iron (FeCBD) and both co-contaminants and microbial communities in the contaminated site. Furthermore, Mantel tests and structural equation modeling (SEM) demonstrated the direct impacts of FeCBD and microbial communities on co-contaminants within the soil profile. Overall, these results provided valuable insights into the migration and transformation characteristics of co-contaminants and their binding mechanisms mediated by minerals, organic matter, and microorganisms.

Dominant contribution of combustion-related ammonium during haze pollution in Beijing.

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Silicon fractionations in coastal wetland sediments: Implications for biogeochemical silicon cycling.

Coastal wetland sediment is important reservoir for silicon (Si), and plays an essential role in controlling its biogeochemical cycling. However, little is known about Si fractionations and the associated factors driving their transformations in coastal wetland sediments. In this study, we applied an optimized sequential Si extraction method to separate six sub-fractions of non-crystalline Si (Sinoncry) in sediments from two coastal wetlands, including Si in dissolved silicate (Sidis), Si in the adsorbed silicate (Siad), Si bound to organic matter (Siorg), Si occluded in pedogenic oxides and hydroxides (Siocc), Si in biogenic amorphous silica (Siba), and Si in pedogenic amorphous silica (Sipa). The results showed that the highest proportion of Si in the Sinoncry fraction was Siba (up to 6.6 % of total Si (Sitot)), followed by the Sipa (up to 1.8 % of Sitot). The smallest proportion of Si was found in the Sidis and Siad fractions with the sum of both being <0.1 % of the Sitot. We found a lower Siocc content (188 ± 96.1 mg kg-1) when compared to terrestrial soils. The Sidis was at the center of the inter-transformation among Si fractions, regulating the biogeochemical Si cycling of coastal wetland sediments. Redundancy analysis (RDA) combined with Pearson's correlations further showed that the basic biogenic elements (total organic carbon and total nitrogen), pH, and sediment salinity collectively controlled the Si fractionations in coastal wetland sediments. Our research optimizes sediment Si fractionation procedure and provides insights into the role of sedimentary Si fractions in controlling Si dynamics and knowledge for unraveling the biogeochemical Si cycling in coastal ecosystems.

Microbial Necromass, Lignin, and Glycoproteins for Determining and Optimizing Blue Carbon Formation.

Coastal wetlands contribute to the mitigation of climate change through the sequestration of "blue carbon". Microbial necromass, lignin, and glycoproteins (i.e., glomalin-related soil proteins (GRSP)), as important components of soil organic carbon (SOC), are sensitive to environmental change. However, their contributions to blue carbon formation and the underlying factors remain largely unresolved. To address this paucity of knowledge, we investigated their contributions to blue carbon formation along a salinity gradient in coastal marshes. Our results revealed decreasing contributions of microbial necromass and lignin to blue carbon as the salinity increased, while GRSP showed an opposite trend. Using random forest models, we showed that their contributions to SOC were dependent on microbial biomass and resource stoichiometry. In N-limited saline soils, contributions of microbial necromass to SOC decreased due to increased N-acquisition enzyme activity. Decreases in lignin contributions were linked to reduced mineral protection offered by short-range-ordered Fe (FeSRO). Partial least-squares path modeling (PLS-PM) further indicated that GRSP could increase microbial necromass and lignin formation by enhancing mineral protection. Our findings have implications for improving the accumulation of refractory and mineral-bound organic matter in coastal wetlands, considering the current scenario of heightened nutrient discharge and sea-level rise.

Stepwise degradation of organic matters driven by microbial interactions in China΄s coastal wetlands: Evidence from carbon isotope analysis.

The microbial "unseen majority" as drivers of carbon cycle represent a significant source of uncertain climate change. To comprehend the resilience of life forms on Earth to climate change, it is crucial to incorporate knowledge of intricate microbial interactions and their impact to carbon transformation. Combined with carbon stable isotope analysis and high-throughput sequencing technology, the underlying mechanism of microbial interactions for organic carbon degradation has been elucidated. Niche differentiation enabled archaea to coexist with bacteria mainly in a cooperative manner. Bacteria composed of specialists preferred to degrade lighter carbon, while archaea were capable of utilizing heavier carbon. Microbial resource-dependent interactions drove stepwise degradation of organic matter. Bacterial cooperation directly facilitated the degradation of algae-dominated particulate organic carbon, while competitive feeding of archaea caused by resource scarcity significantly promoted the mineralization of heavier particulate organic carbon and then the release of dissolved inorganic carbon. Meanwhile, archaea functioned as a primary decomposer and collaborated with bacteria in the gradual degradation of dissolved organic carbon. This study emphasized microbial interactions driving carbon cycle and provided new perspectives for incorporating microorganisms into carbon biogeochemical models.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution.

The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO2 oxidation at different levels of air pollution. The Δ33S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ33S signatures (0‰ < Δ33S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Δ33S signatures (-0.236‰ < Δ33S < ∼0‰) observed during winter haze events (PM2.5 > 200 µg/m3) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO2 can produce significant negative Δ33S values in sulfate aerosols.

Agricultural ditches are hotspots of greenhouse gas emissions controlled by nutrient input.

Agricultural ditches are pervasive in agricultural areas and are potential greenhouse gas (GHG) hotspots, since they directly receive abundant nutrients from neighboring farmlands. However, few studies measure GHG concentrations or fluxes in this particular water course, likely resulting in underestimations of GHG emissions from agricultural regions. Here we conducted a one-year field study to investigate the GHG concentrations and fluxes from typical agricultural ditch systems, which included four different types of ditches in an irrigation district located in the North China Plain. The results showed that almost all the ditches were large GHG sources. The mean fluxes were 333 µmol m-2 h-1 for CH4, 7.1 mmol m-2 h-1 for CO2, and 2.4 µmol m-2 h-1 for N2O, which were approximately 12, 5, and 2 times higher, respectively, than that in the river connecting to the ditch systems. Nutrient input was the primary driver stimulating GHG production and emissions, resulting in GHG concentrations and fluxes increasing from the river to ditches adjacent to farmlands, which potentially received more nutrients. Nevertheless, the ditches directly connected to farmlands showed lower GHG concentrations and fluxes compared to the ditches adjacent to farmlands, possibly due to seasonal dryness and occasional drainage. All the ditches covered approximately 3.3% of the 312 km2 farmland area in the study district, and the total GHG emission from the ditches in this area was estimated to be 26.6 Gg CO2-eq yr-1, with 17.5 Gg CO2, 0.27 Gg CH4, and 0.006 Gg N2O emitted annually. Overall, this study demonstrated that agricultural ditches were hotspots of GHG emissions, and future GHG estimations should incorporate this ubiquitous but underrepresented water course.

Machine Learning Predicts the Methane Clumped Isotopologue (12CH2D2) Distributions Constrain Biogeochemical Processes and Estimates the Potential Budget.

Methane (CH4) is a matter of environmental concern; however, global methane isotopologue data remain inadequate. This is due to the challenges posed by high-resolution testing technology and the need for larger sample volumes. Here, worldwide methane clumped isotope databases (n = 465) were compiled. We compared machine-learning (ML) models and used random forest (RF) to predict new Δ12CH2D2 distributions, which cover valuable and hard-to-replicate methane clumped isotope experimental data. Our RF model yields a reliable and continuous database including ruminants, acetoclastic methane, multiple pyrolysis, and controlled experiments. We showed the effectiveness of utilizing a new data set to quantify isotopologue fractionations in biogeochemical methane processes, as well as predicting the steady-state atmospheric methane clumped isotope composition (Δ13CH3D of +2.26 ± 0.71‰ and Δ12CH2D2 of +62.06 ± 4.42‰) with notable biological contributions. Our measured summer and winter water emitted gases (n = 6) demonstrated temperature-driven seasonal microbial community evolution determined by atmospheric clumped isotope temporal variations (Δ 13CH3D ∼ -0.91 ± 0.25 ‰ and Δ12CH2D2 ∼ +3.86 ± 0.84 ‰), which in turn is relevant for future models quantifying the contribution of methane sources and sinks. Predicting clumped isotopologues translates our methane geochemical understanding into quantifiable variables for modeling that can continue to improve predictions and potentially inform global greenhouse gas emissions and mitigation policy.

Organic carbon preservation in wetlands: Iron oxide protection vs. thermodynamic limitation.

The sequestration of organic carbon (OC) in wetland sediments is influenced by the presence of oxygen or lack thereof. The mechanisms of OC sequestration under redox fluctuations, particularly by the co-mediation of reactive iron (Fe) protection and thermodynamic limitation by the energetics of the OC itself, remain unclear. Over the past 26 years, a combination of field surveys and remote sensing images had revealed a strong decline in both natural and constructed wetland areas in Tianjin. This decline could be attributed to anthropogenic landfill practices and agricultural reclamation efforts, which may have significant impacts on the oxidation-reduction conditions for sedimentary OC. The Fe-bound OC (CBD extraction) decreased by 2 to 10-fold (from 8.3 to 10% to 0.7-4.5%) with increasing sediment depth at three sites with varying water depths (WD). The high-resolution spectro-microscopy analysis demonstrated that Fe (oxyhydr)oxides were colocalized with sedimentary OC. Corresponding to lower redox potential, the nominal oxidation state of C (NOSC), which corresponds to the energy content in OC, became more negative (energy content increased) with increasing sediment depth. Taken together, the preservation of sedimentary OC is contingent on the prevailing redox conditions: In environments where oxygen availability is high, reactive Fe provides protection for OC, while in anoxic environments, thermodynamic constraints (i.e., energetic constraints) limit the oxidation of OC.

Anthropogenic Disturbance Stimulates the Export of Dissolved Organic Carbon to Rivers on the Tibetan Plateau.

The impacts of human activities on the riverine carbon (C) cycle have only recently been recognized, and even fewer studies have been reported on anthropogenic impacts on C cycling in rivers draining the vulnerable alpine areas. Here, we examined carbon isotopes (δ13CDOC and Δ14CDOC), fluorescence, and molecular compositions of riverine dissolved organic matters (DOM) in the Bailong River catchment, the eastern edge of the Tibetan Plateau to identify anthropogenic impacts on the C cycle. Human activities show limited impact on dissolved organic carbon (DOC) concentration, but significantly increased the age of DOC (from modern to ∼1600 yr B.P.) and changed the molecular compositions through agriculture and urbanization despite in the catchment with low population density. Agricultural activities indirectly increased the leaching of N-containing aged organic matter from deep soil to rivers. Urbanization released S-containing aged C from fossil products into rivers directly through wastewater. The aged DOC from agricultural activity and wastewater discharge was partly biolabile and/or photolabile. This study highlights that riverine C is sensitive to anthropogenic disturbance. Additionally, the study also emphasizes that human activities reintroduce aged DOC into the modern C cycle, which would accelerate the geological C cycle.

Spatial and molecular variations in forest topsoil dissolved organic matter as revealed by FT-ICR mass spectrometry.

Forest soils cover about 30 % of the Earth's land surface and play a fundamental role in the global cycle of organic matter. Dissolved organic matter (DOM), the largest active pool of terrestrial carbon, is essential for soil development, microbial metabolism and nutrient cycling. However, forest soil DOM is a highly complex mixture of tens of thousands of individual compounds, which is largely composed of organic matter from primary producers, residues from microbial process and the corresponding chemical reactions. Therefore, we need a detailed picture of molecular composition in forest soil, especially the pattern of large-scale spatial distribution, which can help us understand the role of DOM in the carbon cycle. To explore the spatial and molecular variations of DOM in forest soil, we choose six major forest reserves located in different latitudes ranging in China, which were investigated by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Results show that aromatic-like molecules are preferentially enriched in DOM at high latitude forest soils, while aliphatic/peptide-like, carbohydrate-like, and unsaturated hydrocarbon molecules are preferentially enriched in DOM at low latitude forest soils, besides, lignin-like compounds account for the highest proportion in all forest soil DOM. High latitude forest soils have higher aromatic equivalents and aromatic indices than low latitude forest soils, which suggest that organic matter at higher latitude forest soils preferentially contain plant-derived ingredients and are refractory to degradation while microbially derived carbon is dominant in organic matter at low latitudes. Besides, we found that CHO and CHON compounds make up the majority in all forest soil samples. Finally, we visualized the complexity and diversity of soil organic matter molecules through network analysis. Our study provides a molecular-level understanding of forest soil organic matter at large scales, which may contribute to the conservation and utilization of forest resources.