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During wildfires and fossil fuel combustion, biomass is converted to black carbon (BC) via incomplete combustion. BC enters the ocean by rivers and atmospheric deposition contributing to the marine dissolved organic carbon (DOC) pool. The fate of BC is considered to reside in the marine DOC pool, where the oldest BC 14C ages have been measured (>20,000 14C y), implying long-term storage. DOC is the largest exchangeable pool of organic carbon in the oceans, yet most DOC (>80%) remains molecularly uncharacterized. Here, we report 14C measurements on size-fractionated dissolved BC (DBC) obtained using benzene polycarboxylic acids as molecular tracers to constrain the sources and cycling of DBC and its contributions to refractory DOC (RDOC) in a site in the North Pacific Ocean. Our results reveal that the cycling of DBC is more dynamic and heterogeneous than previously believed though it does not comprise a single, uniformly old 14C age. Instead, both semilabile and refractory DBC components are distributed among size fractions of DOC. We report that DBC cycles within DOC as a component of RDOC, exhibiting turnover in the ocean on millennia timescales. DBC within the low-molecular-weight DOC pool is large, environmentally persistent and constitutes the size fraction that is responsible for long-term DBC storage. We speculate that sea surface processes, including bacterial remineralization (via the coupling of photooxidation of surface DBC and bacterial co-metabolism), sorption onto sinking particles and surface photochemical oxidation, modify DBC composition and turnover, ultimately controlling the fate of DBC and RDOC in the ocean.

Drier and hotter conditions linked with anthropogenic climate change can increase wildfire frequency and severity, influencing terrestrial and aquatic carbon cycles at broad spatial and temporal scales. The impacts of wildfire are complex and dependent on several factors that may increase terrestrial deposition and the influx of dissolved organic matter (DOM) from plants into nearby aquatic systems, resulting in the darkening of water color. We tested the effects of plant biomass quantity and its interaction with fire (burned vs. unburned plant biomass) on dissolved organic carbon (DOC) concentration and degradation (biological vs. photochemical) and DOM composition in 400 L freshwater ponds using a gradient experimental design. DOC concentration increased nonlinearly with plant biomass loading in both treatments, with overall higher concentrations (>56 mg/L) in the unburned treatment shortly after plant addition. We also observed nonlinear trends in fluorescence and UV-visible absorbance spectroscopic indices as a function of fire treatment and plant biomass, such as greater humification and specific UV absorbance at 254 nm (a proxy for aromatic DOM) over time. DOM humification occurred gradually over time with less humification in the burned treatment compared to the unburned treatment. Both burned and unburned biomass released noncolored, low molecular weight carbon compounds that were rapidly consumed by microbes. DOC decomposition exhibited a unimodal relationship with plant biomass, with microbes contributing more to DOC loss than photodegradation at intermediate biomass levels (100-300 g). Our findings demonstrate that the quantity of plant biomass leads to nonlinear responses in the dynamics and composition of DOM in experimental ponds that are altered by fire, indicating how disturbances interactively affect DOM processing and its role in aquatic environments.

Marine dissolved organic matter (DOM) is one of the largest reservoirs of organic carbon and nitrogen in the world. Yet, despite its global importance, most DOM remains molecularly uncharacterized. Solid-state nuclear magnetic resonance (NMR) spectroscopy of isolated DOM fractions represents one of the most powerful techniques to understand overall structural composition. However, it is well known that standard cross polarization magic angle spinning (CP/MAS) NMR, the technique used for almost all past solid-state NMR studies of DOM, is at best "semi-quantitative," and underestimates fully substituted NMR active nuclei. Additionally, almost all past solid-state NMR work analyzed high molecular weight (HMW) material isolated by ultrafiltration, which is now understood to represent mostly 14C-young, "semi-labile" compounds. In contrast, there is far less information regarding the composition of older, low molecular weight (LMW) DOM, which represents the vast majority of the ocean's accumulated refractory DOM pool. Here, we applied 13C and 15N solid-state multiCP/MAS NMR, improved NMR methods optimized to more quantitatively resolve fully substituted NMR nuclei, to both HMW and LMW DOM isolated from the surface and deep North Pacific Subtropical Gyre. These methods confirm past work indicating most nitrogen containing HMW DOM as amide compounds, but also demonstrate a modest heterocyclic N component not previously identified. In contrast, we found that LMW DON is almost entirely aromatic heterocyclic N, consistent with the hypothesis that heterocyclic N structures may be largely responsible for the accumulation of the ocean's refractory DON pool. Surprisingly, however, we find DOC aromatic functionalities still represent only a very minor portion of either the HMW or the LMW refractory carbon pools, in marked contrast to refractory DON composition. Together, these more quantitative solid-state NMR techniques likely represent the most accurate picture of DON and DOC functional and compound-class makeup to date, and so have broad implications for our understanding of marine DOM structure and cycling. Specifically, our new data suggests that while chemical composition likely acts as a key control on DOM lability, the most refractory components of DOC and DON have very different compositions, sources, and cycling, supporting the idea that DOC and DON cycling in the ocean may be largely decoupled. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is a significant component of the global carbon cycle both in terrestrial and aquatic systems. Dissolved black carbon (DBC) is a significant portion of the total dissolved organic carbon (DOC) pool and represents a major flux of recalcitrant carbon to the coastal and deep oceans. Dissolved black carbon can originate from multiple sources related to its relative biogeochemical reactivity with the dynamics of highly recalcitrant DBC integral to long-term sequestration. Thus, understanding how the more recalcitrant fractions of DBC varies in diverse catchments is critical and currently underexplored. We used hydrogen pyrolysis to isolate the fraction of DBC with aromatic clusters above 7 rings, representing the more stable components. Here we report the dynamics of DBCHyPy over a hydrological year in a temperate catchment, with a long history of coal mining extraction. Quarterly measurements of DBC were undertaken from two main channel and four tributary sites. Hydrogen pyrolysis derived DBC comprised a significant percentage of the total DOC flux (3.2% to 28.3%) and included significant spatial variability. Unlike other studies examining more reactive DBC fractions, bulk DOC concentrations and DBCHyPy were poorly correlated when considered over an annual scale. Rather, DBCHyPy was correlated with indicators of groundwater such as dissolved inorganic carbon and conductivity. Data suggest a consistent source of DBCHyPy not subject to the same mobilisation drivers as DOC, which shows substantial seasonality. Rather, our data shows a potentially consistent supply of stable DBC originating from the coal mining-influenced groundwater. Petrogenic sources of DBC have been poorly constrained to date, the data presented here suggests in some catchments it may be significant and yield catchment scale DOC-DBC decoupling. The dynamics of DBC have implications for carbon fluxes, pollution transport and water quality/treatment requirements. These preliminary findings suggest potentially complex drivers in spatially heterogeneous catchments, contrasting with previous work finding tight DOC-DBC mobilisation dynamics. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) released by macroalgae supports coastal ocean carbon cycling and contributes to the total oceanic DOC pool. Salinity fluctuates substantially in coastal marine environments due to natural and anthropogenic factors, yet there is limited research on how salinity affects DOC release by ecologically important macroalgae. Here we determined the effect of short‐term salinity changes on rates of DOC release by the habitat‐forming fucalean seaweed Sargassum fallax (Ochrophyta). Lateral branches (~4 g) cut at the axes of mature individuals were incubated across a salinity gradient (4–46) for 24 h under a 12:12 light:dark cycle, and seawater was sampled for DOC at 0, 12, and 24 h. Physiological assays (tissue water content, net photosynthesis, respiration, tissue carbon, and nitrogen content) were undertaken at the end of the 24‐h experiment. Dissolved organic carbon release increased with decreasing salinity while net photosynthesis decreased. Dissolved organic carbon release rates at the lowest salinity tested (4) were ~3.3 times greater in the light than in the dark, indicating two potential DOC release mechanisms: light‐mediated active exudation and passive release linked to osmotic stress. Tissue water content decreased with increasing salinity. These results demonstrate that hyposalinity stress alters the osmotic status of S. fallax, reducing photosynthesis and increasing DOC release. This has important implications for understanding how salinity conditions encountered by macroalgae may affect their contribution to the coastal ocean carbon cycle. [ABSTRACT FROM AUTHOR]

The fluxes of dissolved organic carbon (DOC) through tidal marsh‐influenced estuaries remain poorly quantified and have been identified as a missing component in carbon‐cycle models. The extreme variability inherent to these ecosystems of the land‐ocean interface challenge our ability to capture DOC‐concentration dynamics and to calculate accurate DOC fluxes. In situ discrete and continuous measurements provide high‐quality estimates of DOC concentration, but these strategies are constrained spatially and temporally and can be costly to operate. Here, field measurements and high‐spatial‐resolution remote sensing were used to train and validate a predictive model of DOC‐concentration distributions in the Plum Island Estuary (PIE), a mesotidal saltmarsh‐influenced estuary in Massachusetts. A large set of field measurements collected between 2017 and 2023 was used to develop and validate an empirical algorithm to retrieve DOC concentration with a ±15% uncertainty from Sentinel‐2 imagery. Implementation on 141 useable images produced a 6‐year time series (2017–2023) of DOC distributions along the thalweg. Analysis of the time series helped identify river discharge, tidal water level (WL), and a marsh enhanced vegetation index 2 as predictors of DOC distribution in the estuary, and facilitated the training and validation of a simple model estimating the distribution. This simple model was able to predict DOC along the PIE thalweg within ±16% of the in situ measurements. Implementation for three years (2020–2022) illustrated how this type of remote‐sensing‐informed models can be coupled with the outputs hydrodynamic models to calculate DOC fluxes in tidal marsh‐influenced estuaries and estimate DOC export to the coastal ocean. Plain Language Summary: The lateral transfer of DOC from tidal saltmarsh estuaries to the coastal ocean is not well represented in carbon‐cycle models. These estuaries are influenced by terrestrial processes, riverine and marsh inputs, and tidal fluctuations that combine to create complex dynamics of DOC concentration and complicate the quantification of DOC fluxes used to compute carbon budgets. Here, we used a combination of field measurements and high‐spatial‐resolution satellite imagery to develop and test a model predicting DOC concentrations along a tidal saltmarsh estuary in Massachusetts from three environmental variables (i.e., tidal WL, river discharge, and a satellite‐derived index of saltmarsh‐biomass). A test using an independent data set revealed this simple model can estimate DOC concentrations within ±16% of the values measured in the laboratory. A simulation conducted for a continuous period of 3 years (15‐min time step) showed that the model produced realistic DOC distributions and captured DOC dynamics adequately in the estuary. The model outputs can easily be coupled with hydrodynamic models to quantify the lateral transfer of DOC from tidal saltmarsh estuaries to the coastal ocean and inform carbon‐cycle models. Key Points: In situ data and satellite remote sensing used to develop and validate a predictive model of dissolved organic carbon (DOC) concentration in tidal saltmarsh estuaryDOC dynamics in tidal estuary were predicted accurately (within 16% of in situ DOC) from river discharge, water level, and marsh enhanced vegetation index 2Modeled DOC dynamics can be combined with hydrodynamic models to calculate DOC fluxes in tidal estuaries and inform carbon cycle models [ABSTRACT FROM AUTHOR]

Forests are critical source regions of high-quality drinking water but forest disturbances such as harvesting can alter stream dissolved organic carbon (DOC) concentrations and influence source water treatability. Most stream DOC-centric forest harvesting impact studies report on effects <10 years post-harvest; less is known about the legacy effects of forest harvesting on stream DOC. Here, inter- and intra-catchment variability in stream DOC concentration and export were evaluated in two northern hardwood-dominated headwater catchments (unharvested reference and 24 years post-clearcut). The relationship between stream DOC and the concentration, spatial distribution, and hydrologic connectivity of hillslope solute pool DOC was investigated. Stream DOC concentrations in the legacy clearcut catchment exceeded those in the reference catchment for all flow conditions. Inter-catchment differences in DOC export were inconsistent. Hillslope solute pool DOC concentrations decreased with soil depth but were not significantly different between catchments. Concentration–discharge regression analysis indicated that DOC was primarily transport-limited (flushing) in both catchments. Aqueous potassium silica molar ratio data indicate the influence of groundwater on stream chemistry and streamflow was similar in both catchments. Results suggest that while clearcut harvesting can have detectable decadal-scale effects on stream DOC concentrations in northern hardwood-dominated headwater catchments, the effects are limited and likely do not pose a reasonable threat to downstream drinking water treatment operations. [ABSTRACT FROM AUTHOR]

Urban beaver and stormwater ponds provide hydrologic retention in the landscape while collecting dissolved organic matter (DOM)-rich runoff that can promote primary productivity. Our objective was to determine how the quantity, source, and bioavailability of DOM changed across urban stormwater and beaver pond systems, then compare the two pond types to each other. We measured dissolved organic carbon (DOC) and specific ultraviolet absorbance at 254 nm (SUVA254) from upstream, within, and downstream of seven ponds in Atlanta, GA, USA, biweekly from March to December 2021. Additionally, we completed 28-day laboratory microcosm incubations of pond in- and out-flow during summer and autumn of 2021. We found higher concentrations of DOC in the pond and outflows of stormwater ponds, whereas beaver ponds did not cause any change. Effects of pond type (beaver vs. stormwater) were greater than other controls on concentration, including flow and season. In contrast, SUVA254 showed a shift toward more aromatic carbon below both systems without a clear difference between pond types. Beaver and stormwater pond outflows had similar ranges of DOM bioavailability in summer, but during autumn bioavailability at both sites declined to near zero. Overall, we found that stormwater ponds and beaver ponds had similar impacts on aromaticity and bioavailability, however stormwater ponds increased the quantity of DOC while beaver ponds did not. This suggests that in addition to increasing hydrologic residence times in urbanized systems, urban beaver ponds may limit the export of bioavailable carbon and reduce microbial processing downstream. [ABSTRACT FROM AUTHOR]

Forested headwater catchments ensure good water quality for downstream ecosystems and human consumption. Climate change and the exacerbating likelihood of extreme events elevate the risk of severe forest dieback. However, the effects of forest dieback on the quantity and quality of stream water are not fully understood. Here, we analyse high‐frequency observations of discharge, electrical conductivity (EC), dissolved organic carbon (DOC) and nitrate (NO3N) obtained before, during and after a drought‐induced forest dieback in a headwater stream in the German Harz Mountains. We focus on the characteristics of concentration‐discharge (C‐Q) relationships at the scale of runoff events to assess the effects of forest dieback on the sources, mobilisation and pathways of EC, DOC and NO3N. When comparing pre‐ and post‐dieback conditions, we found a significant increase in runoff efficiency and a doubling of DOC loads exported from the catchment, while DOC concentrations increased only moderately and their C‐Q patterns did not change. EC exhibit no changes in concentrations but a steepening of C‐Q dilution patterns. We explain these findings with a dieback‐induced decrease in evapotranspiration, which leads to more intensive drainage of the upper organic soil layers in the riparian zone. In contrast, we observed a strong increase in NO3N concentrations and fluxes by a factor of ~5, while C‐Q patterns at the event scale changed from enrichment to dilution. We argue that the dieback led to an excess of NO3N on the hillslopes that connect to the stream via surficial flowpaths. In this way, NO3N bypasses the riparian zone, reducing the catchment's efficiency in attenuating this nutrient. Our study emphasises the pivotal role of riparian zones in mediating water quality in headwater streams. Different configurations of the riparian zone and its connection to the hillslopes and the stream network may be a missing piece in explaining differences in water quality responses of catchments to forest dieback. [ABSTRACT FROM AUTHOR]

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The nitrate--nitrogen (NO3-N) concentration is a key variable affecting the ecosystem services supported by headwater streams. The availability of such data monitored continuously at a high frequency (in parallel to hydrometric and other water quality data) potentially permits a greater insight into the dynamics of this key variable. This study demonstrates how single-input single-output (SISO) system identification tools can make better use of these high-frequency data to identify a reduced number of numerical characteristics that support new explanatory hypotheses of rain-driven NO3-N dynamics. A second-order watershed managed for commercial forestry in upland Wales (United Kingdom) provided the illustrative data. Fifteen-minute rainfall time series were used to simulate NO3-N concentration dynamics and the potentially associated dynamics in dissolved organic carbon (DOC) and runoff, monitored at the same high resolution for two 30-day periods with a differing temperature regime. The approach identified robust, high-efficiency models needing few parameters. Comparison of only three derived dynamic response characteristics (DRCs) of δ, TC, and SSG for the three variables for the two different periods led to new hypotheses of rain-driven NO3-N dynamics for further exploratory field investigation. [ABSTRACT FROM AUTHOR]

Forests cover one-third of the global land and are important components of carbon and nitrogen cycling. Anthropogenic disturbances, such as forest road systems or skid trails for timber harvesting, can dramatically change the nutrient cycling in these ecosystems. Skid trails increase soil erosion and thus the displacement of soil organic carbon (SOC) and total nitrogen (Nt). Additionally, runoff transports high amounts of dissolved organic carbon (DOC), which can have a negative impact on aquatic ecosystems. One of the most important countermeasures against soil erosion is the quick recolonization of vegetation. To date, the extent to which natural vegetation succession influences the relocation of SOC and Nt, and in particular the role of mosses in this context, has not been well investigated. This study investigates the influence of natural vegetation succession and in particular of mosses on the displacement process of SOC and Nt as well as DOC caused by soil erosion. To this end, we combine the results of a field study using in-situ rainfall simulations with small-scale runoff plots in skid trails of the Schönbuch Nature Park in southwestern Germany with the results of ex-situ rainfall simulation experiments with infiltration boxes containing the substrate from the respective skid trails. The eroded sediments of skid trails were on average enriched in SOC by 16% and in Nt by 35% compared to the original soil, which lead to a decrease of the C/N ratio in sediments. As vegetation succession progressed, the displacement of SOC and Nt was reduced, confirmed by a negative correlation between the enrichment ratios of SOC (ERSOC), Nt and total vegetation cover. However, mosses tended to reduce ERSOC more than vascular plants. Additionally, mosses significantly decreased DOC concentration in surface runoff compared to bare soils, while no difference in DOC concentration in percolated water could be observed. Future research should explore the role of mosses in the storage of SOC and Nt in the soil and their impact on soil stability. Thus, utilizing mosses could potentially minimize environmental impacts from soil disturbances in forests. [ABSTRACT FROM AUTHOR]

We examined the interactions between stream water and subsurface sediment to quantify how these interactions influenced organic C respiration and dissolved inorganic C (DIC) production in the hyporheic zone of a high-gradient headwater mountain stream draining a forested catchment at the H. J. Andrews Experimental Forest, Oregon, USA. We compared measurements from a well network with those from six 2-m-long hyporheic mesocosms. The patterns in hyporheic metabolism were similar in wells and mesocosms: O2 declined and DIC increased with travel time. However, the dissolved organic C (DOC) showed little net change in concentration. The mesocosms showed that net losses of DOC could account for 7% of O2 consumed during summer and autumn and 24% of O2 consumed in the winter and spring. Previous research at our study site suggested that large volumes of hyporheic exchange are likely to result in continual processing of streamwater DOC through the hyporheic zone, which would limit the accumulation of bioavailable DOC. Consequently, hyporheic respiration in this forested headwater stream appears to rely primarily on organic C ultimately derived from sediment-bound or buried particulate organic matter (POC). We modeled the consumption of O2 (k O2) and the production of DIC (k DIC) as zero-order kinetic reactions. In the mesocosms, k O2 and k DIC were correlated to temperature and were 2× higher in the summer and autumn than in the winter and spring. In the well network, however, neither k O2 nor k DIC showed seasonal differences. The k DIC was also correlated to initial DOC concentrations in both the mesocosms and the wells. Further, the k DIC was correlated to the time since the mesocosms were packed, which suggests that sediment disturbance or incorporation of relatively fresh organic matter from packing the mesocosms increased the bioavailability of the POC. Further, changes in concentrations of O2, DOC, and DIC were approximately linear when plotted against travel time for both the mesocosms and the well field. These data suggest that the processes that release bioavailable DOC from POC are relatively constant along hyporheic flow paths and govern the rate of hyporheic metabolism in the hyporheic zone of this forested headwater stream. [ABSTRACT FROM AUTHOR]

The ability of fish to sustain environmental perturbations during fragile early‐life stages can be of vital importance for populations and species facing rapid global environmental change. One of the potential challenges for aquatic organisms, including teleost fish, is rising concentration of dissolved organic carbon (DOC) in freshwater environments, known as brownification, which causes a cascade of changes including a decreased visibility and pH. Conversely, humic substances are increasingly recognised for their potential antimicrobial and immunostimulant properties. The questions remain whether exposure to humic waters high in DOC during the development causes significant perturbation, and to what extent genetic and parental factors may maintain optimal development in such a possibly challenging environment.To address these questions, we generated pure and hybrid (inter‐population) crosses of Eurasian perch (Perca fluviatilis) originating from clear‐water and humic lakes. Fertilised eggs from each cross were exposed to natural humic and clear‐water lake water. Intra‐chorionic space diameter (embryo diameter) and hatching success were recorded.Perch embryos successfully developed even in a small volume of water (5 mL) and majority hatched (70%) by 13 day post‐fertilisation. Embryo diameter was influenced by the interaction between cross type and treatment where embryos with maternal clear‐water origin were smaller than embryos of humic water origin in both clear‐water and humic water treatments. The probability of hatching was not related to female size, but rather to embryo diameter at the eyed stage and the interaction between cross type and treatment, resulting in pure clear‐water crosses hatching earlier and at higher numbers.Our study shows that effects of humic water are not uniformally negative in perch and discloses population‐dependent responses to humic water exposure during development. Our experimental design also illustrates the utility of multi‐well plates for conducting complex high‐throughput experiments beyond traditional model species and opens new possibilities for testing perch genotype–environment associations in early‐life traits.Taken together, results of this study add to the growing evidence that humic substances may have complex effects on fish during early life, motivating further investigations on their role in shaping natural phenotypes and potential use in the applied sector. [ABSTRACT FROM AUTHOR]

Returning straw to the field is a crucial practice for enhancing soil quality and increasing efficient use of secondary crop products. However, maize straw has a higher carbon-to-nitrogen ratio compared to other crops. This can result in crop nitrogen loss when the straw is returned to the field. Therefore, it is crucial to explore how different methods of straw return affect maize (Zea mays L.) farmland. In this study, a field experiment was performed with three treatments (I, no straw returned, CK; II, direct straw return, SR; and III, straw returned in deep furrows, ISR) to explore the effects of the different straw return modes on soil carbon and nitrogen content and greenhouse gas emissions. The results indicated that the SR and ISR treatments increased the dissolved organic carbon (DOC) content in the topsoil (0–15 cm). Additionally, the ISR treatment boosted the contents of total nitrogen (TN), nitrate nitrogen (NO3−-N), ammonium nitrogen (NH4+-N), dissolved organic nitrogen (DON), and DOC in the subsurface soil (15–30 cm) compared with CK. When it comes to greenhouse gas emissions, the ISR treatment led to an increase in CO2 emissions. However, SR and ISR reduced N2O emissions, with ISR showing a more pronounced reduction. The ISR treatment significantly increased leaf and grain biomass compared to CK and SR. The correlation analyses showed that the yield was positively correlated with soil DOC, and soil greenhouse gas emission was correlated with soil NO3−-N. The ISR technology has great potential in sequestering soil organic matter, improving soil fertility, and realizing sustainable agricultural development. [ABSTRACT FROM AUTHOR]

Through emission processes, palladium (Pd) particulates from industrial sources are introduced into a range of ecosystems including freshwater environments. Despite this, research on Pd‐induced bioaccumulation, uptake, and toxicity is limited for freshwater fishes. Unlike other metals, there are currently no regulations or protective guidelines to limit Pd release into aquatic systems, indicating a global absence of measures addressing its environmental impact. To assess the olfactory toxicity potential of Pd, the present study aimed to explore Pd accumulation in olfactory tissues, olfactory disruption, and oxidative stress in rainbow trout (Oncorhynchus mykiss) following waterborne Pd exposure. Olfactory sensitivity, measured by electro‐olfactography, demonstrated that Pd inhibits multiple pathways of the olfactory system following 96 h of Pd exposure. In this study, the concentrations of Pd for inhibition of olfactory function by 20% (2.5 μg/L; IC20) and 50% (19 μg/L; IC50) were established. Rainbow trout were then exposed to IC20 and IC50 Pd concentrations in combination with varying exposure conditions, as changes in water quality alter the toxicity of metals. Independent to Pd, increased water hardness resulted in decreased olfactory perception owing to ion competition at the olfactory epithelium. No other environmental parameter in this study significantly influenced Pd‐induced olfactory toxicity. Membrane‐associated Pd was measured at the olfactory rosette and gill following exposure; however, this accumulation did not translate to oxidative stress as measured by the production of malondialdehyde. Our data suggest that Pd is toxic to rainbow trout via waterborne contamination near field‐measured levels. This study further demonstrated Pd bioavailability and uptake at water‐adjacent tissues, adding to our collective understanding of the toxicological profile of Pd. Taken together, our results provide novel insights into the olfactory toxicity in fish following Pd exposure. Integr Environ Assess Manag 2024;20:1407–1419. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC). Key Points: Palladium (Pd) inhibits multiple fish olfaction pathways. Shifts in water quality did not alter Pd olfactory inhibition. [ABSTRACT FROM AUTHOR]

This study aimed to determine the quantity and quality of dissolved organic carbon (DOC) released from sewage sludge-derived biochar and digestion-derived biochar under different extraction conditions (deionised water, hot water, 0.1 mol·L-1 NaOH) using TOC analyser, UV-vis spectroscopy. Biochars were produced through the pyrolysis process at temperatures of 400, 500, 600, and 800 ℃. The objective of this article was to examine the influence of diverse extraction solutions on the amount of dissolved organic carbon (BDOC) released from biochars and to delineate alterations in the composition and characteristics of DOC contingent on the extraction parameters. The findings demonstrated that elevated pyrolysis temperatures resulted in a notable reduction in DOC concentration, with fractions extracted using NaOH exhibiting the highest DOC concentrations. SUVA254 analysis and the E2/E3 ratio indicated that biochars produced at higher temperatures contained a greater proportion of aromatic and hydrophobic substances. These results indicate that pyrolysis temperature, feedstock type and extraction conditions are of significant importance for the properties of DOC in biochar. This has important implications for their potential applications in soil management and carbon sequestration strategies. [ABSTRACT FROM AUTHOR]

Climate change leads to an increase in both the frequency and intensity of extreme precipitation. Surface runoff generated by extreme precipitation has a significant impact on water. However, the impact of persistent precipitation on surface water quality is easy to neglect, due to its prolonged duration and lower-intensity rainfall. This study established eight sampling points within selected surface waters to observe the variation of microbial characteristics in a typical persistence precipitation event. The primary difference between Furong Lake (FL) and Chengqian Reservoir (CR) was: the concentrations of dissolved organic carbon (DOC) were 21.3 ± 0.7 and 8.3 ± 1.5 mg/L in FL and CR, respectively. The concentrations of R2A culturable bacteria and coliforms were 104.57 and 101.58 colony-forming units (CFU)/mL in FL, and were 105.46 and 102.64 CFU/mL in CR, respectively. During precipitation, the maximum increase concentrations of R2A, NA culturable bacteria, and coliforms were 100.75, 101.30, and 102.27 CFU/mL in FL, respectively. Furthermore, microbial concentration and rainfall did not increase simultaneously, and a delay phenomenon was observed in the increasing microbial concentrations. Through analyzing the concentration change trends and correlation of various water quality indicators during persistent precipitation, the significant correlation between the DOC concentration and the changes in the dominant species of microbial community structure was found in this study (p < 0.05). For example, as the DOC concentration declined, the abundance of hgcl_clade and CL500-29_marine_group increased. Consequently, although persistent precipitation might not obviously alter the water quality visibly, it could still pose potential microbial risks. [ABSTRACT FROM AUTHOR]

ImportanceThe ability to detect extracellular environmental conditions is a fundamental property of all life forms. Because microbial two-component sensor systems convert information about extracellular conditions into biochemical information that controls their behaviors, we evaluated how two-component sensor systems evolved within the deep Earth across multiple sites where abiotic and biotic properties vary. We show that these sensor systems remain abundant in microbial consortia at all subterranean sampling sites and observe correlations between sensor system abundances and abiotic (dissolved organic carbon variation) and biotic (consortia diversity) properties. These results suggest that multiple environmental properties may drive sensor protein evolution and highlight the need for further studies of metagenomic and geochemical data in parallel to understand the drivers of microbial sensor evolution.

The Altamaha River estuary off the southeastern U.S. is an important source of dissolved organic carbon (DOC) to the coastal ocean. The estuary is formed by three interconnected sounds, and it is characterized by a complex network of narrow channels and creeks with high spatial heterogeneity, making it difficult to study with in situ observations alone. Here, use used satellite data from Landsat available in high resolution (30 m) to investigate DOC distribution and variability in the system. Our analyses show that DOC variability in the estuary is characterized by two seasonal peaks, one in spring and one in fall, while over the shelf maximum DOC content is observed during summer. A multiple regression analysis was used to quantify physical mechanisms controlling DOC variability in the estuary. In addition to seasonal variations, anomalies in river discharge are the dominant factor controlling DOC variability throughout much of the estuarine system. Tides play a key role near the mouth of each sound and in some upstream regions, likely associated with inputs from salt marshes. The influence of winds is smaller and is restricted to the area near the mouth of the Altamaha Sound. Landsat data also captured an input of ~ 2 Gg of DOC to the estuary associated with the passage of Hurricane Irma in 2017. Our results demonstrate that Landsat can provide useful information about scales of variability in narrow estuaries, including capturing the occurrence of sharp fronts that would be difficult to observe with traditional in situ measurements alone. [ABSTRACT FROM AUTHOR]

Many flatfish species are partially euryhaline, such as the Pacific sanddab which spawn and feed in highly dynamic estuaries ranging from seawater to near freshwater. With the rapid increase in saltwater invasion of freshwater habitats, it is very likely that in these estuaries, flatfish will be exposed to increasing levels of dissolved organic carbon (DOC) of freshwater origin at a range of salinities. As salinity fluctuations often coincide with changes in DOC concentration, two natural freshwater DOCs [Luther Marsh (LM, allochthonous) and Lake Ontario (LO, autochthonous) were investigated at salinities of 30 and 7.5 ppt. Optical characterization of the two natural DOC sources indicate salinity-dependent differences in their physicochemistry. LO and LM DOCs, as well as three model compounds [tannic acid (TA), sodium dodecyl sulfate (SDS) and bovine serum albumin (BSA)] representing key chemical moieties of DOC, were used to evaluate physiological effects on sanddabs. In the absence of added DOC, an acute decrease in salinity resulted in an increase in diffusive water flux (a proxy for transcellular water permeability), ammonia excretion and a change in TEP from positive (inside) to negative (inside). The effects of DOC (10 mg C L-1) were salinity and source-dependent, with generally more pronounced effects at 30 than 7.5 ppt, and greater potency of LM relative to LO. Both LM DOC and SDS increased diffusive water flux at 30 ppt but only SDS had an effect at 7.5 ppt. TA decreased ammonia excretion at 7.5 ppt. LO DOC decreased urea-N excretion at both salinities whereas the stimulatory effect of BSA occurred only at 30 ppt. Likewise, the effects of LM DOC and BSA to reduce TEP were present at 30 ppt but not 7.5 ppt. None of the treatments affected oxygen consumption rates. Our results demonstrate that DOCs and salinity interact to alter key physiological processes in marine flatfish, reflecting changes in both gill function and the physicochemistry of DOCs between 30 and 7.5 ppt. [ABSTRACT FROM AUTHOR]

The ocean is the largest carbon pool on earth and plays an important role in the global carbon cycle. Marine organisms contribute over half of the global carbon fixed by photosynthesis every year. Therefore, much attention has been paid to the role of the ocean in the carbon cycle and carbon sequestration. Macroalgae are an important component of coastal ecosystems, with strong and highly efficient carbon fixation capacity. The net primary productivity (NPP) of macroalgae was higher than that of phytoplankton and other primary producers in coastal waters. Parts of the photosynthates are released into the sea in the form of dissolved organic carbon (DOC) and particulate organic carbon (POC) during algae growth. Dissolved organic carbon and POC have attracted widespread attention because they can play important roles in carbon cycling and carbon sequestration in coastal ecosystems. However, the organic carbon release rate of macroalgae, its relationship with primary productivity, and the regulatory factors require further study. The growth of algae and the organic carbon release are influenced by temperature, light intensity, photoperiod, and other factors. However, organic carbon release rates among algae species might be quite different. Therefore, studying the regulatory effects of different factors on organic carbon release by algae and organic carbon release rate distribution has important implications for quantitatively evaluating the organic carbon release ability of algae and its contribution to the coastal ecosystem carbon cycle.In recent years, Sargassum horneri is the most common wild macroalgae in the Yellow Sea. Its ecological role in the coastal ecosystem has attracted widespread attention. Few studies have focused on organic carbon release by S. horneri. This study conducted an orthogonal experiment to measure DOC and POC release rates of S. horneri at different temperatures (5, 15, and 25 ℃), light intensity [86, 172, and 258 μmol/(m2·s)] and photoperiod (L: D=6 h: 18 h, L: D=12 h: 12 h, L: D=24 h: 0 h, L means light time, D means dark time) to study the organic carbon release rate of S. horneri, its relationship with NPP, and the main regulatory factors. The release rates of DOC were 0.653–4.785 mg/(g·h) and the release rates of POC were 0.066–0.322 mg/(g·h). There is an order of magnitude difference between DOC and POC. This indicated that DOC was the main form of organic carbon released by S. horneri. The highest DOC release rate [4.785 mg/(g·h)] occurred at 25 ℃, 172 μmol/(m2·s) light intensity, and L: D = 6 h: 18 h. The highest POC release rate [0.322 mg/(g·h)] occurred at 25 ℃, 258 μmol/(m2·s) light intensity, and L: D = 24 h: 0 h. The organic carbon release ability of S. horneri is consistent with previous studies of other macroalgae.This study found that changes in temperature and light conditions significantly affect the ability of S. horneri to release organic carbon. Temperature was the main factor affecting the release rate of DOC; the DOC release rate increases with increasing temperature. The DOC release rate initially increases, then decreases with increasing light intensity; it increases with a decrease in the photoperiod. Light intensity was the main factor influencing the POC release rate; increasing light intensity and photoperiod accelerated the POC release rate. However, the POC release rate slightly decreased when the light intensity increased from 172 to 258 μmol/(m2·s). There was a greater POC release rate under extreme temperature stress (5 and 25 ℃).This study showed that a proportion of DOC and POC in NPP were unstable in macroalgae. The range of DOC/NPP was 4%–130%. POC/NPP was relatively concentrated to approximately 0.4%–5.9%. The NPP was significantly lower in the experimental group with a high ratio of organic carbon compared with other experimental groups. The DOC release rate negatively correlated with NPP, but the negative correlation was poor in the low temperature groups. There was no significant correlation between the POC release rate and NPP. POC/NPP was relatively stable [approximately (1±0.6)% when NPP>9 mg C/(g·h)]. The growth pressure of S. horneri was high when NPP

Granular activated carbon (GAC) filtration is a commonly used method for advanced wastewater treatment. Filters can be operated continuously or discontinuously, with continuous operation not requiring feed flow interruption for backwashing and circulation (B/C). This study investigated the influence of B/C on the effluent quality of continuous filters. Two continuous GAC filters were operated for 1.5 years, with analysis of dissolved substances and particulate matter in the influent and effluent. The results indicated that various B/C modes had no impact on the removal of dissolved organic carbon and organic micropollutants (OMP), achieving an OMP removal of over 70% after 5,600 treated bed volumes (m3 treated wastewater per m3 GAC). However, it was evident that continuous B/C over 2–4 h resulted in increased turbidity, total suspended solids over 30 mg/L and total phosphorus concentrations of 1.3 mg/L in the filter effluent. Additionally, the study demonstrated that longer and more intensive B/C processes resulted in GAC size degradation with AC concentrations of up to 6.9 mg/L in the filter effluent, along with a change in GAC particle size. Furthermore, the importance of pre-filtration in reducing particulate matter in the filter influent and decreasing hydraulic head loss could be demonstrated. HIGHLIGHTS Operation of two continuous granular activated carbon (GAC) filters.; Different backwashing and circulation (B/C) modes were investigated.; Dissolved substances removal (DOC and micropollutants) showed no dependence on B/C mode.; Intensive and long-lasting B/C resulted in GAC abrasion and increased particulate matter and activated carbon concentration in filter effluent.; Pre-filtration was necessary for filter operation.;

Chironomids are keystone primary benthic consumers with semi‐aquatic life cycles. They support aquatic and terrestrial consumers at higher trophic levels by conveying dietary nutrients, such as fatty acids. In this study, we combined field sampling and laboratory experiments to examine the effects of environmental parameters, including diet, on fatty acid composition and metabolism in chironomid larvae and imagines. Results from 53 lakes showed that lake size, depth, dissolved organic carbon (DOC) concentrations, and trophic state had only marginal effects on the content of long‐chain polyunsaturated fatty acids (LC‐PUFA) in chironomids. Compound‐specific stable hydrogen isotope analyses confirmed that chironomids actively bioconvert dietary fatty acid precursors to LC‐PUFA in all lake types, independent of nutrient or DOC concentrations. Moreover, fatty acid‐specific stable carbon isotope data indicated that the diet of chironomids was subsidized, particularly in oligotrophic lakes in spring, by terrestrial C18 fatty acid precursors that were converted to LC‐PUFA. Data from feeding experiments further confirmed that decreased dietary availability of LC‐PUFA enhanced the conversion of dietary short‐chain precursors to LC‐PUFA. These results suggest that chironomids are PUFA regulators that can sustain LC‐PUFA levels under varying environmental conditions. Furthermore, our results indicate that they bioconvert terrestrial low‐quality material to high‐quality resources, which, via chironomid emergence, support terrestrial food webs. Chironomids are abundant and widespread, and thus, the trophic transfer of LC‐PUFA can have significant implications for the fitness and production of upper trophic level consumers in both aquatic and terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Sugarcane/soybean intercropping and reduced nitrogen (N) application as an important sustainable agricultural pattern can increase crop primary productivity and improve soil ecological functions, thereby affecting soil organic carbon (SOC) input and turnover. To explore the potential mechanism of sugarcane/soybean intercropping affecting SOC sequestration, a two-factor long-term field experiment was carried out, which included planting pattern (sugarcane monocropping (MS), sugarcane/soybean 1:1 intercropping (SB1), and sugarcane/soybean 1:2 intercropping (SB2)) and nitrogen addition levels (reduced N application (N1: 300 kg·hm−2) and conventional N application (N2: 525 kg·hm−2)). The results showed that the shoot and root C fixation in the sugarcane/soybean intercropping system were significantly higher than those in the sugarcane monocropping system during the whole growth period of sugarcane, and the N application level had no significant effect on the C fixation of plants in the intercropping system. Sugarcane/soybean intercropping also increased the contents of total organic C (TOC), labile organic C fraction [microbial biomass C (MBC) and dissolved organic C (DOC)] in the soil during the growth period of sugarcane, and this effect was more obvious at the N1 level. We further analyzed the relationship between plant C sequestration and SOC fraction content using regression equations and found that both plant shoot and root C sequestration were significantly correlated with TOC, MBC, and DOC content. This suggests that sugarcane/soybean intercropping increases the amount of C input to the soil by improving crop shoot and root C sequestration, which then promotes the content of each SOC fraction. The results of this study indicate that sugarcane/soybean intercropping and reduced N application patterns can synergistically improve plant and soil C fixation, which is of great significance for improving crop yields, increasing soil fertility, and reducing greenhouse gas emissions from agricultural fields. [ABSTRACT FROM AUTHOR]

Over a period of 30 years (1993–2022), headwater catchments in the Slavkov Forest (Czech Republic) exhibited a robust increase in stream water DOC (dissolved organic carbon) concentrations following a significant reduction in acidic atmospheric deposition. Sulfur deposition decreased from 34 kg ha−1 yr−1 in 1993 to 2.6 kg ha−1 yr−1 in 2022. Three Norway-spruce-dominated research sites—Černý Potok (CEP), a 15.2 ha peatbog catchment, Lysina (LYS), a 27.3 ha granitic catchment, and Pluhův Bor (PLB), a 21.6 ha serpentinite catchment, were investigated. The three–year average DOC concentration increased from 48.2 mg L−1 (1993–1995) to 68.3 mg L−1 (2020–2022) at CEP (0.69 mg L−1 yr−1). LYS showed an increase from 16.9 mg L−1 to 25.4 mg L−1 (0.30 mg L−1 yr−1 annually). The largest increase was recorded at PLB, with an increase from 15.7 mg L−1 to 36.7 mg L−1 (0.89 mg L−1 yr−1). A decline in ionic strength was identified as the main driver of the DOC increase. The annual runoff declined significantly at CEP and LYS from 465 mm to 331 mm as a result of rising air temperatures and reduced precipitation between 2014 and 2022. PLB (average of 266 mm) did not show a statistically significant decline. Recently, PLB experienced significant deforestation that likely lowered transpiration and thus increased catchment runoff. As a result, DOC fluxes did not change significantly at CEP (average 210 kg ha−1 yr−1) and LYS (90 kg ha −1 yr−1). However, PLB's DOC flux more than doubled, increasing from 44 to 106 kg ha−1 yr−1. Drivers connected with global change, such as increasing temperatures, or potential chemical drivers, such as reductions in Al concentrations and pH changes, were not able to explain the observed changes in DOC concentra tions and fluxes. [ABSTRACT FROM AUTHOR]

Macroalgae influence local and global biogeochemical cycles through their production of dissolved organic carbon (DOC). Yet, data remain scarce and annualized estimates are typically based on high growth periods without considering seasonal variability. Although the mechanisms of active exudation and passive leakage need clarifying, ecophysiological stress is known to enhance DOC release. Therefore, DOC leakage from seasonally senescent macroalgae may be overlooked. This study focuses on the annual kelp Saccharina japonica var. religiosa (class Phaeophyceae) from Oshoro Bay, Hokkaido, Japan. Three years (2020–2022) of seasonal data were collected and analyzed, with least squares mean DOC release rates established for kelp (n = 88) across 16 incubation experiments (t ≥ 4 d, DOC samples ≥1 · d−1) under different photosynthetically active radiation (PAR) treatments (200, 400, 1200, or 1500 μmol photons · m−2 · s−1). Differences in PAR, dry weight biomass (g DW), sea surface temperature, or salinity could not explain DOC release‐rate variability, which was high between individual kelp. Instead, there were significant intra‐annual differences, with mean DOC release rates (mg C · g−1 DW · d−1 ± standard error between n kelp) higher during the autumn "late decay" period (0.71 ± 0.10, n = 27) compared to the winter "early growth" period (0.14 ± 0.025, n = 10) and summer "early decay" period (0.25 ± 0.050, n = 24). This relationship between seasonal senescence and macroalgal DOC release is further evidence that long‐term, place‐based studies of DOC dynamics are essential and that global extrapolations are premature. [ABSTRACT FROM AUTHOR]

Wheat is the most consumed crop worldwide. Zeolite application combined with good tillage practices are good combinations that provide better soil conditions for wheat crops. Zeolite also provides a good layer for carbon to be absorbed into the soil and can retain carbon for hundreds of years. The current study aimed to investigate the effect of tillage practices and zeolite treatments on soil carbon retention and wheat crop productivity. Arranging the treatments implemented according to a factorial randomized block design which includes three replications. Tillage treatments include three levels vis: T1= 6 tillage practices with the help of cultivator (farmer practice/control), T2 (minimum tillage), and T3 (2 cultivation with cultivator + Mold-board plough). The zeolite applications consist of four levels: Z1 = 0, Z2 = 5, Z3 = 10 and Z4 = 15 t ha−1. The effect of the interaction between zeolite treatments and tillage practices on various factors related to soil and crops such as emission of carbon dioxide (CO2), dissolved organic carbon, soil organic carbon, and the productivity and components of wheat productivity. Zeolite applied at 10 t ha−1 in combination with minimum tillage gave significant differences in terms of CO2 emission, dissolved organic carbon, and on soil organic carbon. The experimental results showed that minimum CO2 emission (25.43 and 31.12 (kg CO2-C ha−1 h−1), dissolved organic carbon (4.80 and 4.90 g C kg−1), soil organic carbon (7.88 and 7.97 g C kg−1), plant height (92.14 and 92.97 cm), spike length (11.88 ad 12.11 cm), number of spikelets (20.11 and 20.98), number of tillers (278.65 and 283.93) per unit area, 1000 grain weight (50.74 and 51.54 g), biological yield (8134.87 and 8187.38 kg ha−1) and grain yield (2984.28 and 3028.96 kg ha−1) and harvest index (36.69 and 37.04%) of wheat was observed in zeolite applied at 10 t ha−1 with minimum tillage practice (T2 × Z3) compared to control and other treatments for both the years, respectively. It is therefore concluded that minimum tillage should be practiced in wheat crops with the application of zeolite at 10 t ha−1 to obtain better yield and soil carbon retention under rain-fed conditions. [ABSTRACT FROM AUTHOR]

To keep groundwater levels stable, Jinan's government has implemented several water management measures. However, considerable volumes of dissolved organic carbon (DOC) can enter groundwater via water exchange, impacting groundwater stability. In this study, a SWAT-MODFLOW-RT3D model designed specifically for the Yufu River Basin is developed, and part of the code of the RT3D module is modified to simulate changes in DOC concentrations in groundwater under different artificial recharge scenarios. The ultimate objective is to offer valuable insights into the effective management of water resources in the designated study region. The modified SWAT-MODFLOW-RT3D model simulates the variations of DOC concentration in groundwater under three artificial recharge scenarios, which are (a) recharged by Yellow River water; (b) recharged by Yangtze River water; and (c) recharged by Yangtze River and Yellow River water. The study shows that the main source of groundwater DOC in the basin is exogenous water. The distribution of DOC concentration in groundwater in the basin shows obvious spatial variations due to the influence of infiltration of surface water. The area near the upstream riverbank is the earliest to be affected. With the prolongation of the artificial recharge period, the DOC concentration in groundwater gradually rises from upstream to downstream, and from both sides of the riverbank to the surrounding area. By 2030, the maximum level of DOC in the basin will exceed 6.20 mg/l. The Yellow River water recharge scenario provides more groundwater recharge and less DOC input than the other two scenarios. The findings of this study indicate that particularly when recharge water supplies are enhanced with organic carbon, DOC concentrations in groundwater may alter dramatically during artificial recharge. This coupled modeling analysis is critical for assessing the impact of recharge water on groundwater quality to guide subsequent recharge programs. [ABSTRACT FROM AUTHOR]

The factors limiting micropollutant biodegradation in the environment and how to stimulate this process have often been investigated. However, little information is available on the capacity of microbial communities to retain micropollutant biodegradation capacity in the absence of micropollutants or to reactivate micropollutant biodegradation in systems with fluctuating micropollutant concentrations. This study investigated how a period of 2 months without the addition of micropollutants and other organic carbon affected micropollutant biodegradation by a micropollutant-degrading microbial community. Stimulation of micropollutant biodegradation was performed by adding different types of dissolved organic carbon (DOC)—extracted from natural sources and acetate—increasing 10 × the micropollutant concentration, and inoculating with activated sludge. The results show that the capacity to biodegrade 3 micropollutants was permanently lost. However, the biodegradation activity of 2,4-D, antipyrine, chloridazon, and its metabolites restarted when these micropollutants were re-added to the community. Threshold concentrations similar to those obtained before the period of no substrate addition were achieved, but biodegradation rates were lower for some compounds. Through the addition of high acetate concentrations (108 mg-C/L), gabapentin biodegradation activity was regained, but 2,4-D biodegradation capacity was lost. An increase of bentazon concentration from 50 to 500 µg/L was necessary for biodegradation to be reactivated. These results provide initial insights into the longevity of micropollutant biodegradation capacity in the absence of the substance and strategies for reactivating micropollutant biodegrading communities. [ABSTRACT FROM AUTHOR]

In order to adjust the structure of inefficient Pinus massoniana(P)forest and explore the dynamic changes for dissolved organic carbon(DOC)during the mixed decomposition process of P. massoniana and native broad-leaved tree species. Our research focuses on the leaf litter of P. massoniana, Cinnamomum camphora(C)and Toona sinensis(T), and combine them into the 15 treatments(3 single tree treatments + 12 mixed treatments)according to different tree species combinations and mass proportions, and then conducted field decomposition experiments to further explore the optimal tree species combination and the proportion of mixed planting for DOC release. The results were as follows:(1)DOC content of Pinus massoniana and most of mixed treatments(except for PT64)significantly increased during the early stages of decomposition(0-6 months), leading to carbon enrichment. The DOC content of leaf litter decreased with the extension of decomposition time, and also exhibited carbon enrichment in small degree during the middle to late stages of decomposition(12-18 months)or the late stages of decomposition(18-24 months). Mixed leaf litter showed that the higher the proportion of broad-leaved, the lower the DOC content in the later stage. (2)The antagonistic effect of DOC release rate in the early stage of decomposition(0-6 months)was strong(58. 33%), and only 8. 33%(1/12)of mixed treatments showed the synergistic effect. Its synergistic effect gradually strengthened(91. 67%)after 6-18 months, but the synergistic effect of leaf litter(66. 67%)during the late stage of decomposition weakened(18-24 months). Among all mixed treatments, PT64 exhibited synergistic effects throughout the entire decomposition stage, followed by PT73, PCT622 and PCT613(3/4). (3)Partial least-squares(PLS)regression showed that N content, P content, lignin content, condensed tannin content, C/N, C/P, lignin/N, and lignin/P were important quality factors affecting the release of DOC from litter in this study area. Overall, the DOC release from mixed Pinus massoniana and broad-leaved leaf litter is affected by tree species, mixed proportion and decomposition time. Compared to other mixed treatments, the proportion of broad-leaved greater is than or equal to 30. 00% and the mixed leaf litter treatments including Toona sinensis(PT64, PT73, PCT622 and PCT613)show strong synergistic effects for DOC release. [ABSTRACT FROM AUTHOR]

Water‐logged peatlands store tremendous amounts of soil carbon (C) globally, accumulating C over millennia. As peatlands become disturbed by human activity, these long‐term C stores are getting destabilized and ultimately released as greenhouse gases that may exacerbate climate change. Oxidation of the dissolved organic carbon (DOC) mobilized from disturbed soils to streams and canals may be one avenue for the transfer of previously stored, millennia‐aged C to the atmosphere. However, it remains unknown whether aged peat‐derived DOC undergoes oxidation to carbon dioxide (CO2) following disturbance. Here, we use a new approach to measure the radiocarbon content of CO2 produced from the oxidation of DOC in canals overlying peatland soils that have undergone widespread disturbance in Indonesia. This work shows for the first time that aged DOC mobilized from drained and burned peatland soils is susceptible to oxidation by both microbial respiration and photomineralization over aquatic travel times for DOC. The bulk radiocarbon age of CO2 produced during canal oxidation ranged from modern to ~1300 years before present. These ages for CO2 were most strongly influenced by canal water depth, which was proportional to the water table level where DOC is mobilized from disturbed soils to canals. Canal microbes preferentially respired older or younger organic C pools to CO2, and this may have been facilitated by the use of a small particulate organic C pool over the dissolved pool. Given that high densities of canals are generally associated with lower water tables and higher fire risk, our findings suggest that peatland areas with high canal density may be a hotspot for the loss of aged C on the landscape. Taken together, the results of this study show how and why aquatic processing of organic C on the landscape can enhance the transfer of long‐term peat C stores to the atmosphere following disturbance. [ABSTRACT FROM AUTHOR]

This study aimed to detect non-conservative processes that affect the distribution of fluorescent dissolved organic matter (FDOM) in the Baltic Sea. An extensive data set comprised of 408 FDOM data, optical and physical profiles, and the development of a water masses balance model allowed us to ascertain the sources of mixing anomalies. These were seen as second-order deviations in the FDOM distribution as a function of salinity in three layers: surface water, Baltic Sea Winter Water, and deep water. The difference between modeled and measured FDOM values at three different excitation/emission wavelengths allowed to show the strength of non-conservative processes, such as photochemical and microbial decomposition (negative residual values) or extracellular release of dissolved organic matter from phytoplankton, heterotrophic uptake and release from anoxic sediments (positive residual values). Humic-like FDOMfractions displayed positive residuals in all seasons for intermediate and deep layers and negative residuals in surface waters. Largest accumulation rates of humic-like fractions were reached in the Gulf of Gdańsk during summer in intermediate and deep layers, while the greatest removal in surface waters was observed during spring in the Bornholm and Gotland Basins and during summer in the Gulf of Gdańsk, probably due to photodegradation. Positive residuals of the protein-like fraction were observed at the surface in summer and autumn in the Gulf of Gdańsk, probably linked to the abundance of phytoplankton and also due to the low molecular weight by-products of photodegradation of humic-like components. Spatial transects revealed an increase in humic-like residuals with depth and a strong correlation with apparent oxygen utilization, increasing with higher fluorescence and exhibiting an asymptotic trend. A relationship was found between the protein-like fractions and phytoplankton biomass proxies. A generalized concept for FDOM cycling in the Baltic Sea was proposed, highlighting photobleaching as the dominant non-linear process determining the efficiency of humic-like FDOM removal. The protein-like component was found to be more efficiently taken up by aerobic prokaryotes at the surface. Microbial utilization and reworking of organic matter, release from sediments, and a decade-long stagnation of bottom water masses, all contribute to the observed accumulation of FDOM in mesohaline deep waters below the permanent pycnocline in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Coastal ecosystems are rapidly shifting due to changes in hydrologic presses (e.g., sea‐level rise) and pulses (e.g., seasonal hydrology, disturbances, and restoration of degraded wetlands). Changing water levels and sources are master variables in coastal wetlands that can alter carbon concentrations, sources, processing, and export. Yet, how long‐term increases in water levels from marine and freshwater sources influence dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition is uncertain. We quantified how long‐term changes in water levels are affecting DOC concentration (2001–2021) and DOM composition (2011–2021) differently across the Florida Everglades. DOC concentrations decreased with high water depths in peat marshes and increased with high water levels in marl marshes and across mangroves, and these relationships were reproduced in freshwater peat marshes and shrub mangroves. In the highly productive riverine mangroves, cross‐wavelet analysis highlighted variable relationships between DOC and water level were largely modulated by hurricane disturbances. By comparing relationships between water level and DOC concentrations with carbon sources from DOM fluorescence indices, we found that changing water sources between the dry and wet season shift DOM from algal to detrital sources in freshwater marshes, from detrital marsh to detrital mangrove sources in the brackish water ecotone, and from detrital mangrove to algal marine sources in downstream mangroves. As climate change and anthropogenic drivers continue to alter water levels in coastal wetlands, integrating spatial and temporal measurements of DOC concentrations and DOM compositions is essential to better constrain the transformation and export of carbon across these coastal ecosystems. Plain Language Summary: Water inputs to coastal ecosystems are changing in response to both sea‐level rise and water management. Changes in the amount and source of water can have different effects on organic carbon cycling across coastal ecosystems. We analyzed 20 years of data from the Florida Everglades to understand how changing water levels control carbon cycling. Our results show that higher water levels dilute carbon concentrations, and slow the release of carbon from peat soils, while also contributing to the transport of organic carbon downstream. We additionally found evidence of seasonal shifts in the composition of organic carbon, with higher algal contributions in the dry season for freshwater sites and higher marine contributions during the dry season for sites with marine influence. Our findings highlight that inputs, of both fresh and marine water, are transporting carbon within and between coastal wetlands, but only freshwater inputs prevent the release of carbon from peat stores. This research provides valuable insights into how inputs of water from different sources control the production, release, and movement of carbon throughout coastal wetland ecosystems. These insights are needed to preserve the value of organic carbon stored in coastal ecosystems, as the water within them changes. Key Points: Dissolved organic carbon concentrations decrease with water level in peat marshes but increase with level in mangroves and marl marshesDetrital carbon is mobilized by both fresh and marine hydrologic pulses, but only stored during freshwater hydrologic pulsesFreshwater restoration is increasing humic, colored dissolved organic matter entering the Everglades [ABSTRACT FROM AUTHOR]

We use observations from the Southern Ocean (SO) biogeochemical profiling float array to quantify the meridional pattern of particle export efficiency (PEeff) during the austral productive season. Float estimates reveal a pronounced latitudinal gradient of PEeff, which is quantitatively supported by a compilation of existing ship‐based measurements. Relying on complementary float‐based estimates of distinct carbon pools produced through biological activity, we find that PEeff peaks near the region of maximum particulate inorganic carbon sinking flux in the polar antarctic zone, where net primary production (NPP) is the lowest. Regions characterized by intermediate NPP and low PEeff, primarily in the subtropical and seasonal ice zones, are generally associated with a higher fraction of dissolved organic carbon production. Our study reveals the critical role of distinct biogenic carbon pool production in driving the latitudinal pattern of PEeff in the SO. Plain Language Summary: Microbial organisms in seawater transform carbon dioxide into different types of carbon through photosynthesis and food web cycling. These carbon types include particulate and dissolved phases, with particles being more efficiently transferred out of the sunlit ocean via gravitational sinking. The ratio of sinking particulate organic carbon to total organic carbon production, commonly referred to as the particle export efficiency, is a metric used to describe how efficiently carbon moves from the surface to the deep ocean. Using observations from a large array of robots in the Southern Ocean, we find that the different types of biogenic carbon produced control the latitudinal gradient in particle export efficiency, which is highest in regions where particulate inorganic carbon export is greatest, even when photosynthetically fixed carbon is minimal. In other areas where phytoplankton carbon production is moderate but largely comprised of dissolved organic carbon, the particle export efficiency is lower. Key Points: Meridional pattern of particle export efficiency (PEeff) estimated from BGC‐Argo aligns with ship‐based observations in the Southern OceanLow PEeff in subtropical and ice‐covered regions and high PEeff in subpolar regions is linked to the biogenic carbon pools producedMost global models struggle to reproduce the meridional pattern of PEeff in the Southern Ocean [ABSTRACT FROM AUTHOR]

Dissolved Organic Carbon (DOC) is the largest organic carbon pool in the ocean. Considering the biotic and abiotic factors controlling DOC processes, indirect satellite methods for open ocean DOC estimation can be developed, using conceptual, empirical or statistical models, driven by multiple satellite products. In this study, we infer a time series of global DOC from data of the European Space Agency's (ESA) Ocean Colour Climate Change Initiative (OC-CCI) in combination with a global database of in situ DOC observations. We tested empirical machine learning modelling approaches in which the available in situ data are used to train the models and to find empirical relationships between DOC and variables available from remote sensing. Of the tested methods, a random forest regression showed the best results, and the details of this model are further reported here. We present a time series of global open ocean DOC concentrations between 2010-2018 that is made freely available through the archive of the UK Centre for Environmental Data Analysis (CEDA). [ABSTRACT FROM AUTHOR]

Thicker snow cover in permafrost areas causes deeper active layers and thaw subsidence, which alter local hydrology and may amplify the loss of soil carbon. However, the potential for changes in snow cover and surface runoff to mobilize permafrost carbon remains poorly quantified. In this study, we show that a snow fence experiment on High‐Arctic Svalbard inadvertently led to surface subsidence through warming, and extensive downstream erosion due to increased surface runoff. Within a decade of artificially raised snow depths, several ice wedges collapsed, forming a 50 m long and 1.5 m deep thermo‐erosion gully in the landscape. We estimate that 1.1–3.3 tons C may have eroded, and that the gully is a hotspot for processing of mobilized aquatic carbon. Our results show that interactions among snow, runoff and permafrost thaw form an important driver of soil carbon loss, highlighting the need for improved model representation. Plain Language Summary: Snow cover is steadily disappearing as a result of climate change, but in areas that remain below 0°C we can still expect an increase in snow depth in the middle of winter. Since snow acts akin to a blanket, this warms the soil and accelerates the thaw of permafrost—thereby potentially contributing to carbon release from these frozen soils. Ice wedges, which are typical for permafrost landscapes, are particularly vulnerable to thaw because they hold a large amount of ice. When this ice melts, the surface sinks down, and soil carbon may be lost. In this study, we show how experimentally raised snow cover triggered the collapse of several ice wedges, not only through a warming effect of the snow but also due to an increase in the flow of water through the ice wedge network. As a result, we estimate that 1.1–3.3 tons of carbon were removed from this location, of which a portion could have entered the atmosphere as CO2. We emphasize the importance of studying the interactions among snow, runoff, and permafrost thaw to better understand how this may affect the release of greenhouse gases to the atmosphere. Key Points: A decade of raised snow depths led to strong local thaw subsidence, while increased runoff triggered the downstream collapse of ice wedgesThe abrupt thaw process mobilized about 1.1–3.3 tons of soil organic carbon in total, and dissolved organic carbon was degraded in the thermo‐erosion gullyFeedbacks linking changes in snow cover, surface drainage, and permafrost thaw are important drivers of thermokarst and soil carbon loss [ABSTRACT FROM AUTHOR]

The classic model of the carbon cycle suggests that the extensive burial of 12C-enriched organic carbon leads to a positive carbon isotope (δ13C) excursion (CIE), while massive oxidation of organic carbon results in a negative CIE. However, global events such as the BAsal Cambrian Carbon isotope Excursion (BACE) and the Steptoean Positive Carbon Isotope Excursion (SPICE) are global negative and positive δ13C excursions, respectively, and they also exhibit significant organic carbon burial anomalies, displaying decoupling between carbon isotope anomalies and organic carbon burial. Based on the analyses of the Cambrian carbon cycle and paleoceanographic evolution records from well Tadong2 in the Tarim Basin, we propose a novel model of the carbon cycle in the Cambrian ocean that incorporates oceanic dissolved organic carbon (DOC). Our findings are as follows. (1) The Cambrian ocean maintained substantial DOC reservoirs, which were regulated by ocean currents and paleo-redox conditions and exerted significant influence on the oceanic carbon cycle. (2) The oxidation of the oceanic DOC reservoirs during the early Cambrian led to the BACE and the Asian Phenomenon of the Cambrian petroleum systems, while the expansion of the oceanic DOC reservoirs during the SPICE resulted in a global positive δ13C excursion and the absence of significant organic carbon burial. (3) The deep-basin sedimentary environment in the eastern depression of the Tarim Basin may have fostered the development of organic-rich black shales during the Furongian Series, corresponding to organic carbon burial during the SPICE and representing potential prospects for ultra-deep oil and gas exploration. Future research should focus on the formation mechanism, reserve scale, and influencing factors of the oceanic DOC reservoirs, as well as their resource and environmental effects. It is expected that new breakthroughs will be made in the fields of Earth system science and oil and gas exploration. [ABSTRACT FROM AUTHOR]

Carbon cycles in coastal waters are highly sensitive to human activities and play important roles in global carbon budgets. CO2 sink–source behavior is regulated by spatiotemporal variations in net biological productivity, but the contribution of macrophyte habitats including macroalgae aquaculture to atmospheric CO2 removal has not been well quantified. We investigated the variations in the carbonate system and dissolved organic carbon (DOC) in human-impacted macrophyte habitats and analyzed the biogeochemical drivers for the variations of these processes. Cultivated macroalgal metabolism (photosynthesis, respiration, calcification, and DOC release) was quantified by in situ field-bag experiments. Cultivated macroalgae took up dissolved inorganic carbon (DIC) (16.2–439 mmol-C m−2 day−1) and released DOC (1.2–146 mmol-C m−2 day−1). We estimated that seagrass beds and macroalgae farming contributed 0.8 and 0.4 mmol-C m−2 day−1 of the in situ total CO2 removal (5.7 and 6.7 mmol-C m−2 day−1, respectively) during their growing period in a semi-enclosed embayment but efficient water exchange (i.e., short residence time) in an open coastal area precluded detection of the contribution of macrophyte habitats to the CO2 removal. Although hydrological processes, biological metabolism, and organic carbon storage processes would contribute to the net CO2 sink–source behavior, our analyses distinguished the contribution of macrophytes from other factors. Our findings imply that macroalgae farming, in addition to restoring and creating macrophyte habitats, has potential for atmospheric CO2 removal. [ABSTRACT FROM AUTHOR]

This study evaluated the impact of different land use types on groundwater dissolved organic carbon (DOC) concentrations and annual DOC efflux from drained peatlands to catchment runoff, providing insights into the mechanisms of carbon stock changes in peatland soils. We measured groundwater chemical properties and various environmental variables, and calculated daily runoff and evapotranspiration for 2021 to estimate monthly and annual DOC efflux and analyzed main affecting factors in different peatland land use types. The highest DOC concentrations in groundwater were found in Scots pine forests and active peat extraction sites, with values of 113.7 mg L−1 and 109.7 mg L−1, respectively, and the lowest in silver birch forests and croplands, at 51.9 mg L−1 and 18.6 mg L−1, respectively. There were statistically significant correlations, including a strong negative correlation between DOC concentrations and several groundwater chemical properties, such as pH, electrical conductivity (EC), Ca, Mg, and K concentrations. The concentrations of DOC in the groundwater of drained peatland showed significant variation between different land use types. The highest annual DOC efflux was observed in active peat extraction sites, at 513.1 kg ha−1 y−1, while the lowest was in grasslands, at 61.9 kg ha−1 y−1, where Ca and Mg concentrations, as well as EC, were the highest. Continuous monitoring of these concentration patterns is essential. [ABSTRACT FROM AUTHOR]

Environmental context: Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. Rationale: Dissolved organic carbon (DOC) is both ubiquitous and heterogeneous in freshwater. Freshwaters are browning universally and pH values are decreasing with climate change. DOCs influence water pH, whereas changes in water pH potentially alter the conformation and function of DOCs. The physicochemical properties of DOCs can be characterised by optical and chemical indices, but the inter-relationships among them, and the effects of low pH, are not well understood. Methodology: We characterised five naturally sourced DOCs, spanning large differences in origin and composition, at pH 7 and 4, using multiple indices: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol–water partition coefficient, molecular charge, proton binding index, size-fractionation, and percentage humic-acid-like, percentage fulvic-acid-like and percentage protein-like components by fluorescence-based parallel factor analysis. Results: Many of the indices changed between pH 7 and 4 as reflected in the corresponding absorbance and fluorescence profiles. Generally, apparent aromaticity, apparent molecular weight and molecular charge all decreased with low pH, while lipophilicity increased. Key positive correlations occurred between aromaticity and apparent molecular weight, chemical reactivity and apparent molecular weight, and aromaticity and chemical reactivity, and a negative correlation between lipophilicity and molecular charge. These relationships were pH dependent. Discussion: Our results highlight that physicochemical indices used to characterise DOCs from distinct sources should consider pH and be interpreted carefully. The pH-dependent changes in many of the indices are likely alterations in the conformation, reflected in the optical signatures, rather than changes in the composition of DOCs. In contrast, increased lipophilicity and reduced charge at lower pH are due to actual changes in DOC molecules, resulting from proton binding. The ecological functions of DOCs are dependent on source and will likely change with natural acidification events such as increasing atmospheric CO2. Environmental context. Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. (Image credit: Carolyn Morris.) [ABSTRACT FROM AUTHOR]

Pyrogenic carbon (PyC) is a significant component of the global soil carbon pool due to its longer environmental persistence than other soil organic matter components. Despite PyC's persistence in soil, recent work has indicated that it is susceptible to loss processes such as mineralization and leaching, with the significance and magnitude of these largely unknown at the hillslope and watershed scales. We present a review of the work concerning dissolved PyC transport in soil and freshwater. Our analysis found that the primary environmental controls on dissolved PyC (dPyC) transport are the formation conditions and quality of the PyC itself, with longer and higher temperature charring conditions leading to less transport of dPyC. While correlations between dPyC and dissolved organic carbon in rivers and other pools are frequently reported, the slope of these correlations was pool‐dependent (i.e., soil‐water, precipitation, lakes, streams, rivers), suggesting site‐specific environmental controls. However, the lack of consistency in analytical techniques and sample preparation remains a major challenge to quantifying environmental controls on dPyC fluxes. We propose that future research should focus on the following: (a) consistency in methodological approaches, (b) more quantitative measures of dPyC in pools and fluxes from soils to streams, (c) turnover times of dPyC in soils and aquatic systems, and (d) improved understanding of how mechanisms controlling the fate of dPyC in dynamic post‐fire landscapes interact. With more refined quantitative information about the controls on dPyC transport at the hillslope and landscape scale, we can increase the accuracy and utility of global carbon models. Key Points: Burn intensity of pyrogenic carbon (PyC) controls its dissolution and aqueous transportFrom headwater streams to oceans dissolved PyC increases in radiocarbon age, implying environmental persistenceEnvironmental controls, including climate, topography, biota, parent material, and time, impact the pool size and flux of dissolved PyC [ABSTRACT FROM AUTHOR]

Jones JR, Graham JL, Obrecht DV, Harlan JD, Knowlton MF, Pollard C, Parris J, Thorpe AP. 2024. Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams. Lake Reserv Manage. 40:177–195. In Missouri, distinct geophysical gradients influence statewide patterns in water quality. Here, we quantify the spatiotemporal variability of dissolved organic carbon (DOC) in reservoirs and streams and the edaphic, hydrologic, and land cover variables that account for cross-system variation. Datasets included statewide inventories collected over decades and studies with greater temporal resolution (n = >6350 DOC measurements). Among reservoirs, the smallest DOC concentration was measured in a spring-fed system within a forested watershed, and the largest was where agricultural biosolids were applied to the land (range 1.0–15.9 mg/L, overall mean 5.8 mg/L). Reservoir values increased from the southern forested Highlands (mean 4.7 mg/L) to the northern agricultural Plains (mean 7.0 mg/L). Stream DOC was similar to reservoir values (overall mean in streams 6.3 mg/L; Highlands mean 4.0 mg/L; Plains mean 6.6 mg/L), despite differences in study design and collection period. Reservoir DOC increased in spring, indicative of allochthonous loading, with small autochthonous additions during a broad summer peak. Temporal variability in DOC was low relative to macronutrients and chlorophyll in both reservoirs and streams, indicating DOC may be a sensitive and readily detected indicator of temporal change in these systems. In regression analyses, watershed features accounted for more than 60% of overall cross-system variability in DOC in both reservoirs and streams. Driver-response relations, however, differed between regions. This analysis extends our understanding of environmental influences on surface water chemistry in Missouri and indicates DOC is nearly as predictable as macronutrients using landscape-level features. [ABSTRACT FROM AUTHOR]

Study region: Agricultural Irrigation and Drainage system - Ulansuhai Lake Continuum, located in the Hetao Irrigation District, Inner Mongolia, China. Study focus: In this study, carbon isotope tracing technology was applied to analyze the sources and migration-transformation characteristics of Dissolved Organic Carbon (DOC) and Dissolved Inorganic Carbon (DIC) in the continuous system of the agricultural irrigation and drainage system - Ulansuhai Lake in the Hetao Irrigation Area of Inner Mongolia during typical irrigation periods. New hydrological insights for the region: During typical irrigation periods, DOC and DIC increase simultaneously with the increase in salinity, and dissolved carbon dioxide in the irrigation and drainage channels escapes. The main sources of DOC in farmland drainage water are terrestrial C3 plants (94.00 %) and plankton (6.00 %). The primary sources of DOC in the lake are farmland drainage (98.19 %) and macroaquatic plants (1.81 %). The main sources of DIC in farmland drainage water are irrigation water from the Yellow River (64.73 %) and the equilibrium dissolution process between carbonates and soil CO₂ (35.27 %); while the primary sources of DIC in the lake are farmland drainage (93.10 %) and air-water exchange (6.90 %). Crop types in the irrigated area, soil salinization, rock weathering, metabolic processes, and the hydrological connectivity induced by irrigation from the Yellow River jointly control the sources, migration, and transformation of carbon within the continuum.

Apportioning pollution sources under compound pollution conditions is challenging in river pollution source analysis. The positive matrix factorization (PMF) model is widely used to analyze river pollution sources. However, the identification of pollutants in this model relies primarily on the subjective experience of the researchers, leading to ineffective identification of different contaminants from similar sources. In this study, we propose a comprehensive deviation index (CDI) to quantitatively identify pollution source types based on the PMF and observed source profiles. Taking the subtropical Xizhijiang River Basin as a case study, we quantitatively identified the pollution sources and their contributions to dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) using observed water quality and pollution sources data. The results showed that the eight major pollutants in the study region exhibited significant positive correlations, indicating the similarity of pollutant sources in the watershed. The PMF model identified three primary pollution sources with coefficients of determination for observed versus predicted concentrations ranging from 0.60 to 0.98. The CDI unveiled that the watershed's three pollution sources were farmland, rural, and wastewater treatment plants (WTPs). Farmland emerges as the predominant contributor to DOC (68.04 %), TC (63.29 %), and TDP (44.51 %). Rural notably contributes to NH3-N, PO43−, TDP, and TN, with percentages of 86.37 %, 57.65 %, 41.40 %, and 30.45 %, respectively. WTPs significantly contribute to NO2−, NO3−, and TN, accounting for 71.81 %, 57.39 %, and 37.26 %, respectively. Incorporating source fingerprints into the PMF model, the CDI can accurately identify pollution sources, improve the interpretability of source identification, and mitigate uncertainty in the multiple-source unknown receptor model. These findings have immediate and practical implications for river ecosystem management and pollution control, providing a more effective method for identifying and addressing pollution sources.

The nitrate–nitrogen (NO3-N) concentration is a key variable affecting the ecosystem services supported by headwater streams. The availability of such data monitored continuously at a high frequency (in parallel to hydrometric and other water quality data) potentially permits a greater insight into the dynamics of this key variable. This study demonstrates how single-input single-output (SISO) system identification tools can make better use of these high-frequency data to identify a reduced number of numerical characteristics that support new explanatory hypotheses of rain-driven NO3-N dynamics. A second-order watershed managed for commercial forestry in upland Wales (United Kingdom) provided the illustrative data. Fifteen-minute rainfall time series were used to simulate NO3-N concentration dynamics and the potentially associated dynamics in dissolved organic carbon (DOC) and runoff, monitored at the same high resolution for two 30-day periods with a differing temperature regime. The approach identified robust, high-efficiency models needing few parameters. Comparison of only three derived dynamic response characteristics (DRCs) of δ, TC, and SSG for the three variables for the two different periods led to new hypotheses of rain-driven NO3-N dynamics for further exploratory field investigation.

Forests cover one-third of the global land and are important components of carbon and nitrogen cycling. Anthropogenic disturbances, such as forest road systems or skid trails for timber harvesting, can dramatically change the nutrient cycling in these ecosystems. Skid trails increase soil erosion and thus the displacement of soil organic carbon (SOC) and total nitrogen (Nt). Additionally, runoff transports high amounts of dissolved organic carbon (DOC), which can have a negative impact on aquatic ecosystems. One of the most important countermeasures against soil erosion is the quick recolonization of vegetation. To date, the extent to which natural vegetation succession influences the relocation of SOC and Nt, and in particular the role of mosses in this context, has not been well investigated. This study investigates the influence of natural vegetation succession and in particular of mosses on the displacement process of SOC and Nt as well as DOC caused by soil erosion. To this end, we combine the results of a field study using in-situ rainfall simulations with small-scale runoff plots in skid trails of the Schönbuch Nature Park in southwestern Germany with the results of ex-situ rainfall simulation experiments with infiltration boxes containing the substrate from the respective skid trails. The eroded sediments of skid trails were on average enriched in SOC by 16% and in Nt by 35% compared to the original soil, which lead to a decrease of the C/N ratio in sediments. As vegetation succession progressed, the displacement of SOC and Nt was reduced, confirmed by a negative correlation between the enrichment ratios of SOC (ERSOC), Nt and total vegetation cover. However, mosses tended to reduce ERSOC more than vascular plants. Additionally, mosses significantly decreased DOC concentration in surface runoff compared to bare soils, while no difference in DOC concentration in percolated water could be observed. Future research should explore the role of mosses in the storage of SOC and Nt in the soil and their impact on soil stability. Thus, utilizing mosses could potentially minimize environmental impacts from soil disturbances in forests.

A wet chemical oxidation (WCO) method has been widely used to obtain the dissolved organic carbon (DOC) content and carbon isotope (δ13CDOC) ratios. However, it is sometimes difficult to get high precision results because not enough CO2 was oxidized from the natural water samples with low DOC concentrations. This improvement primarily aims to increase the water sample volume, improve the removal rate of dissolved inorganic carbon (DIC), and minimize the blank DOC from the standard solution. Following the improved procedure, the δ13C ratios of standardized DOC solutions were consistent with their actual values, and their differences were less than 0.2‰. The improved method demonstrated good accuracy and stability when applied to natural water samples with DOC concentrations ≥0.5 mg L−1, with the precisions of DOC concentrations and δ13C ratios were better than 0.07 mg L−1 and 0.1‰, respectively. More importantly, this method saved much pre-treatment time and realized batch processing of water samples to obtain their DOC contents and isotope ratios. HIGHLIGHTS The CO2 peak values of the water samples were within the optimal testing range of the instrument.; Low-temperature nitrogen purging can more thoroughly remove DIC from water samples.; The precision of the DOC concentration in the water samples was better than 0.07 mg L−1.; The precision of the δ13CDOC values in the water samples was better than 0.1‰.;

The drinking water parameter - water hardness - is defined as the sum of calcium and magnesium ion concentrations. In this study, their precipitation during boiling was investigated, using water of different hardnesses, calcium to magnesium ratios, natural tap and bottled waters. During boiling, pH value, conductivity, and calcium and magnesium concentrations were measured. Further, the morphology and composition of precipitates were analyzed. Boiling tests with durations of 30 min showed only a decrease of calcium concentration by precipitation. Longer boiling of water with a high proportion of magnesium showed decreasing magnesium concentrations once the calcium precipitation was completed. Calcium carbonate precipitated as calcite and aragonite crystals with aragonite needles occurring in the presence of high magnesium concentrations. Magnesium hydroxide formed during precipitation was identified as hydromagnesite crystals with different crystallization morphologies. Unexpectedly, for a water with low and equimolar concentrations of calcium and magnesium only magnesium precipitated at the relatively low pH 8.7. In summary, calcium ions in drinking water are evaluated as being of considerably higher relevance for the hardness properties than the magnesium ions. However, the precipitation of both minerals depends on their concentrations and ratio as well as on the pH value and the water matrix.