Variability Of Dissolved Organic Carbon Research Articles

Spatiotemporal variation in methylmercury and related water quality variables in a temperate river under highly dynamic hydrologic conditions

The understanding of the essential environmental factors affecting the spatiotemporal variation in methylmercury (MeHg) in river water is limited to date, despite its importance for predicting the effect of ongoing climate change on MeHg accumulation in freshwater ecosystems. This study aimed to explore the variation in MeHg concentration and related environmental factors in the downstream zone of the Yeongsan River under highly dynamic hydrologic conditions by using water quality and hydrologic data collected from 1997 to 2022, and Hg and MeHg data collected from 2017 to 2022. The mean concentration of unfiltered MeHg was 35.7 ± 13.7 pg L−1 (n = 24) in summer and 26.7 ± 7.43 pg L−1 (n = 24) in fall. Dissolved oxygen (DO), conductivity, nitrate, and dissolved organic carbon (DOC) were determined to be the most influential variables in terms of MeHg variation based on the partial least squares regression model, and their effects on the MeHg concentration were negative, except for DOC. Heatmaps representing the similarity distances between temporal trends of hydrologic and water quality variables were constructed to determine fundamental factors related to the time-based variations in DO, conductivity, nitrate, and DOC using a dynamic time warping algorithm. The heatmap cluster analysis showed that the temporal trends of these variables were closely related to rainfall variation rather than irradiance or water temperature. Overall, biogeochemical factors directly related to in situ methylation rate of Hg(II)—rather than transport of Hg(II) and MeHg from external sources—mainly control the spatiotemporal variation of MeHg in the downstream zone of the Yeongsan River.

Contribution of marine macrophytes to pCO2 and DOC variations in human-impacted coastal waters

AbstractCarbon 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.

Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams

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. XX:XXX–XXX. 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.

New insight into and characterization of DOC, DON and CDOM for urban waters in the lower reaches of the Yellow River, China

Dissolved organic matter (DOM), usually measured as concentrations of dissolved organic carbon (DOC) and nitrogen (DON) and chromophoric dissolved organic matter (CDOM), plays a crucial role in the global carbon and nitrogen cycles in inland waters. The aim of this study is to characterize spatial variations of DOC, DON, and C:N molar ratios and CDOM using absorption spectroscopy and further determine their relationships using pearson correlations (r) for urban waters along trophic gradients in the lower reaches of the Yellow River (YR) of China. A dataset of 122 urban water samples were collected from four typical cities during 2021–2022. Principal component analysis (PCA) was performed to assess the relative distribution of these urban water samples. These urban waters can be grouped into two categories: mesotrophic waters (n = 57) and eutrophic waters (n = 65) based on trophic state index (TSI) threshold value of 50. Results revealed that there were significantly higher DOC, CDOM absorption and SUVA254 along with lower S275–295 and M (t-test, p < 0.01) in eutrophic waters compared with mesotrophic waters. DOC was significantly higher than DON (p < 0.01). DOC was highly related to DON (R2 = 0.649) and COD (R2 = 0.840) suggesting they were derived from a common source. CDOM absorption a(254) was strongly correlated with DOC (R2 = 0.770) and COD (R2 = 0.800). DON can be calculated based on the strong correlations between DOC and DON and then C:N molar ratios can be further obtained for these urban waters. These results implied that CDOM optical absorption a(254) can be viewed as the best indicators of DOC and associated with water quality parameters. These findings can help us to better understand DOC, DON and CDOM of DOM coupled cycling processes for urban waters with similar climatic and hydrologic conditions on basin scale in China.

Spatial-temporal distribution and source apportionment of riverine dissolved organic carbon in the Miyun section of the Chaobai River in Beijing, China

Dissolved organic carbon (DOC) is indicative of water quality in drinking source water and has an impact on drinking water treatment. Exploring occurrence of DOC in drinking source water is of great significance for the drinking water safety in Beijing. This study elucidated spatial-temporal DOC dynamics in the Miyun section of Chaobai River in Beijing of China and identified sources of DOC based on DOC fluorescent spectral characteristics. Results showed that the average riverine DOC concentration was 30.60 mg/L. DOC concentrations in the Miyun section of the Chaobai River were 30.40, 30.24, and 31.09 mg/L in spring, summer and autumn, respectively. DOC concentrations at the three segments of the Miyun section of the Chaobai River (the Chaohe River, the Baihe River and the Chaobai River) were 29.93, 30.29 and 32.57 mg/L, respectively. Land use contributed to DOC variations rather than season and river segment. Significant increases in DOC concentration were observed in river water flowing through farmland and urban areas, and DOC concentration presented the highest after flowing through the farmland area. The results of FI, HIX and BIX showed that DOC mainly came from endogenous sources such as aquatic biological activities, and was less affected by terrestrial inputs. Special attentions ought to be paid on prevention and control of endogenous DOC inputs.

Dissolved Organic Radiocarbon in the West Indian Ocean

AbstractWe report marine dissolved organic carbon (DOC) concentrations, and DOC Δ14C and δ13C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb 14C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆14C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. Variability of DOC ∆14C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ13C are present at two of the five stations studied, suggesting a source of low δ13C DOC, or additional microbial utilization of deep DOC.

Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. OC in the cryosphere rivers plays vital roles in the carbon cycle and river ecosystem health. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC across the Northern Hemisphere cryosphere. Here, we compiled OC concentration, radiocarbon (14C), and the specific ultraviolet absorbance (SUVA) of dissolved OC (DOC) at 254 nm data from 1007 unique sites, extracted from 138 published literature between 1972 and 2022. Overall, the average DOC and particulate OC (POC) concentrations are 6.34 and 2.61 mg C L−1, respectively, with the average age of DOC and POC being ~1100 and ~4300 years BP, respectively, indicating the release of aged carbon pools. Seasonal variations in DOC and POC concentrations, Δ14C-DOC and SUVA254 were observed, with distinct spatial variations closely linked to specific watershed characteristics. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Meanwhile, in boreal forest watersheds, DOC is the most concentrated and youngest in varied ecoregions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic. We estimated that specific OC fluxes in glacier rivers are higher than that in permafrost rivers (4.77 and 1.86 g C m−2 yr−1, respectively). Our results highlight the complex and variable spatiotemporal patterns of riverine OC in the northern cryosphere, which are essential for assessing the impact of OC on the global carbon cycle and climate warming.

Eutrophication and salinization elevate the dissolved organic matter content in arid lakes

Dissolved organic matter (DOM) plays an essential role in the global lake carbon cycle. Understanding DOM composition and monitoring its spatiotemporal dynamics are of great significance for understanding the lake carbon cycle, controlling water pollution, and protecting water resources. However, previous studies have focused mainly on eutrophic freshwater lakes, with limited attention given to saline lakes. Based on in situ data collected in ten lakes in northwestern China, this study reported the changes in DOM components in different lake types. Parallel factor analysis (PARAFAC) was used to analyze the three-dimensional excitation emission matrix (EEMs) to obtain the DOM fluorescence components. The contributions of different environmental factors to the changes in DOM components were quantified by the generalized linear model (GLM). The results showed that the eutrophication index was significantly positively related to dissolved organic carbon (DOC) (R2 = 0.95, p < 0.01) and colored DOM (CDOM) (R2 = 0.96, p < 0.01) concentrations. Terrestrial humic-like and tryptophan-like components, which are highly correlated with human activities, explained 62% and 64% of the variations in DOC and CDOM, respectively. In sum, the contributions of human activities to the DOC and CDOM variations were 61% and 57%, respectively. Salinity also showed significant positive correlations with both DOC (R2 = 0.88, p < 0.01) and CDOM (R2 = 0.87, p < 0.01). Lake salinization led to increases in DOM concentration, and salinity contributed 20% and 16% to the DOC and CDOM variations, respectively. Therefore, human activities and salinity codetermined the DOM concentration and its composition in the western arid lakes.Based on these findings, this study proposed a feasible flowchart for remotely estimating DOM in saline lakes using satellite data. This study is significant for the long-term monitoring of the carbon cycle and the effective protection of lake water resources in saline lakes.

Dissolved organic carbon load variability in the rainfall season dominated its interannual variability in the snowmelt driven basin: Evidence from 41 years data of headwater streams

AbstractNumerous reports have observed variations of dissolved organic carbon (DOC) under the changing environment. From the DOC load variability perspective, however, how much of the variability in DOC load occurred among different seasons, and which season was most synchronized with the interannual variability remains unknown. The weekly DOC concentration and daily discharge records from four headwater streams with long‐term (1978–2018) records in the snowmelt driven basin of Harp Lake, south‐central Ontario, Canada, were used to answer these issues. We also examined the contributions of variabilities in discharge and DOC concentration to the variability in DOC load. We found that the variability of DOC load in the rainfall season (autumn) instead of snowmelt season (spring) was most closely synchronized with its interannual variability, suggesting that if the DOC variability in the rainfall season stabilized, the interannual variability would be more stable. The interannual variabilities for discharge, DOC concentration and DOC load showed an insignificant decreasing trend in the monitored 41 years, and the respective contributions of the variability trend of discharge and DOC concentration to DOC load in rainfall season increased from 28.82% to 144.58% and decreased from 71.18% to −44.58% because the change trend of discharge variability was greater than that in DOC concentration. This study provides a case information in determining which factors contribute most to the variability of DOC load in the watershed, and which season may dominate its variability throughout the year.

Dissolved Organic Carbon Dynamics Variability from Ponds Draining Different Landscapes in a Typical Agricultural Watershed

Dissolved organic carbon (DOC) in inland waters (rivers, reservoirs, lakes, and small ponds) plays a significant role in the global carbon cycle and affects global climate change. In addition, DOC is also a vital indicator of the water environment due to its multiple physical, chemical, and ecological roles. Lakes and ponds of small sizes are abundant on a global and regional scale, and a large increase in ponds is expected with global agricultural land expansion. However, the DOC characteristics of ponds in agricultural watersheds are still unclear, posing a challenge to better understanding the carbon cycle of inland waters. In this study, we explored the DOC variability and their influencing factors in ponds draining different landscapes in a typical agricultural watershed to address the issue. The field measurements over a year showed the DOC concentration varied among ponds draining different landscapes. Specifically, the mean DOC concentrations in the natural pond, sewage pond, aquaculture pond, and irrigation pond were (6.17 ± 1.49) mg/L, (12.08 ± 2.92) mg/L, (9.36 ± 2.92) mg/L, and (8.91 ± 2.71) mg/L, respectively. Meanwhile, monthly measurements found the DOC varied across sampling dates. The DOC variability was positively correlated with nutrients, primary production, and precipitation, suggesting anthropogenic loadings, an internal production rate, and hydrological regime that regulated the substantial variability of DOC in these ponds at the watershed scale. Further, large pollutant discharge and high primary production led to peak DOC occurring in the sewage pond. Our results implied that more attention should be paid to ponds in agricultural watersheds to better understand the roles of inland waters in the global carbon cycle.

Fluxes of dissolved organic matter and nitrate and their contribution to soil acidification across changing permafrost landscapes in northwestern Canada

Climate warming is predicted to change the fluxes of dissolved organic matter and nitrate by increasing active layer thickness, plant productivity, and organic matter decomposition. These changes are hypothesized to increase mineral weathering and soil acidification rates. We investigated whether acidification rates and solute leaching fluxes are variable between permafrost-affected soils with different active layer thicknesses. We compared the fluxes of dissolved organic carbon (DOC) and total dissolved N and the ion fluxes associated with solute leaching and plant uptake to calculate proton budgets in the soils that differed in slope positions (upper slope and lower slope) and soil texture (clayey and sandy soils) in NW Canada. We found the wide variation in DOC and nitrate-N fluxes, depending on slope positions and soil texture. The nitrate-N fluxes were higher at the lower slope position of the sandy soil, compared to the upper slope position. Compared to sandy soils, the DOC fluxes from the organic horizons were higher in the clayey soils on shallower permafrost table. Organic acids were major proton sources in the organic horizons at all sites, but acidity was also contributed by nitrate (sandy and clayey soils at lower slope position) and carbonic acid (clayey soil at upper slope position). The weathering by carbonic acid lead to accumulation of short-range-order minerals in the clayey soils, while incipient podzolization of the sandy soils included the weathering of illite to vermiculite and the dissolution of short-range-order minerals. The shallower active layer thickness at the lower slope position resulted in lower plant productivity and acidification rates. In the permafrost-affected soils, the active layer thickness, the DOC and nitrate-N fluxes, and their contribution to acidification are dependent on the local variation in slope position and soil texture, as well as climate change.

Constraints on the spatial variations of soil carbon fractions in a mangrove forest in Southeast China

Mangroves, carbon-rich ecosystems, store significant amounts of carbon in the soil to mitigate climate change. For better mangrove carbon management, the spatial distributions and driving factors of distinct soil carbon fractions, notably soil inorganic carbon (IC) and labile carbon, require clarification. Here, we quantified variations in the soil carbon fractions (total organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), permanganate-oxidizable carbon (POC), and IC) with landward distance and soil depth, and identified the means by which edaphic and biotic factors affect soil carbon fractions in a mangrove forest in Southeast China. Average TOC, IC, POC, DOC, and MBC densities were 15.22 kg/m3, 1.39 kg/m3, 4.23 kg/m3, 43.49 g/m3, and 63.25 g/m3, respectively. Soil total nitrogen was the most important factor affecting TOC, POC, and MBC. Soil IC was found to be controlled by pH, and plant biomass was the main factor explaining soil DOC variation. Except for MBC, landward distance was negatively correlated with organic carbon fractions and positively correlated with IC via soil nutrients, pH, bulk density, and plant biomass. Soil depth directly or indirectly altered soil carbon fractions via soil nutrients, pH, clay, and salinity, resulting in negative relationships with TOC, POC, and MBC and a positive relationship with IC. Collectively, our results indicate that future changes in soil total nitrogen content and pH are likely to impact carbon storage in mangroves, and that the conservation of landward mangroves is important with respect to their high labile carbon contents.

Estimation of dissolved organic carbon from inland waters at a large scale using satellite data and machine learning methods

Dissolved Organic Carbon (DOC) in inland waters plays an essential role in the global carbon cycle and has significant public health effects. Machine learning (ML) together with remote sensing has emerged as a powerful and promising combination to quantify water quality parameters from space. However, inland water sample data for DOC is limited. Hence, little is known about the potential to quantify DOC content in inland waters, especially over large-scale areas. This study presents the first attempt to estimate DOC in inland waters over a large-scale area using satellite data and ML methods with the newly published open-source dataset AquaSat. Four ML approaches, namely Random Forest Regression (RFR), Support Vector Regression (SVR), Gaussian Process Regression (GPR), and a Multilayer Backpropagation Neural Network (MBPNN) were trained using more than 16 thousand samples across the continental United States matched with satellite data from Landsat 5, 7 and 8 missions. Satellite data from the Landsat missions were further extended with environmental data from the ERA5-Land product and used as input to train the ML algorithms. Our results show that including environmental data as inputs considerably improved the prediction of DOC for all ML algorithms, with GPR showing the most promising performance results with moderate estimation errors (RMSE: 4.08 mg/L). Permutation feature importance analysis showed that the wavelength range in the visible Green band (from Landsat) and the monthly average air temperature (from ERA5-Land) were the most important variables for the ML approaches. The results demonstrate the predictive strength of GPR and its useful feature to derive per pixel standard deviations for detailed analysis. Our results further highlight the important role of considering environmental processes to explain DOC variations over large scales. The application and performance of the GPR in mapping spatiotemporal variations of DOC in an entire water body were discussed by taking Lake Okeechobee (the 8th largest freshwater lake in the U.S.) as an illustrative example. While performance evaluation showed that DOC concentrations can be retrieved with adequate accuracy, algorithm development was challenged by the heterogenous nature of large-scale open source in situ data, issues related to atmospheric correction, and the low spatial and temporal resolution of the environmental predictors. This research demonstrates how open source, large-scale datasets like AquaSat in combination with ML and satellite remote sensing can make research toward large-scale estimation of inland water DOC more realistic while highlighting its remaining limitations and challenges.

Challenges in quantifying and characterizing dissolved organic carbon: Sampling, isolation, storage, and analysis.

Despite the advancements in analytical techniques, there are still great challenges and difficulties in accurately and effectively quantifying and characterizing dissolved organic carbon (DOC) in environmental samples. The objectives of this review paper are (a) to understand the roles and variability of DOC along the water continuum; (b) to identify the constraints, inconsistences, limitations, and artifacts in DOC characterization; and (c) to provide recommendations and remarks to improve the analytical accuracy. For the first objective, we summarize the four ecological and engineering roles of DOC along the water continuum from source water to municipal utility, including nutrients and energy sources, controlling the fates of micropollutants, buffering capacity, and treatability and precursors of disinfection byproducts. We also discuss three major challenges in DOC analysis, including spatial and temporal variations, degradability and stability, and unknown structures and formulas. For the second objective, we review the procedures and steps in DOC analysis, including sampling in diverse environmental matrices, isolation of DOC fraction, storage and preservation techniques, and analyses on bulk chemical characteristics. We list and discuss the available options and evaluate the advantages and disadvantages of each choice. Last, we provide recommendations and remarks for each stage: sampling, isolation, storage, and analysis.

Distribution and microbial degradation of dissolved organic carbon in the northern South China Sea

Dissolved organic carbon (DOC) is the largest reduced carbon pool in the ocean, and it plays significant roles not only in the ocean carbon cycle but also in the control of many biogeochemical processes in the ocean. We present the concentrations and distribution of DOC in the northern South China Sea (SCS) and western North Pacific (NP) in the spring and summer seasons of 2015-2016 and 2019. Laboratory incubation bioassay experiments were also conducted to determine the microbiological respiration of DOC. In the SCS, the concentrations of DOC varied within a range of 38-95 μM, and the large spatial variations in DOC in the upper 100 m depth were influenced by a combination of factors, including primary production, terrestrial inputs from the Pearl River and the intrusion of the Kuroshio Current. The mesopelagic DOC distribution in the northern SCS basin was largely influenced by the physical mixing of upwelled deep water; however, biological processes were estimated to account for 6-20% of the modulation in DOC concentrations. Compared with the deep DOC levels in open ocean areas, a slightly excessive DOC concentration (~ 3-4 μM) was observed in the deep water of the SCS basin. Approximately 10-20% of the DOC was consumed by mesopelagic and/or deep water bacteria in the incubation bioassay experiments, and labile DOC was preferentially respired, resulting in decreased δ13C and Δ14C values of DOC.

Physiographic Controls and Wildfire Effects on Aquatic Biogeochemistry in Tundra of the Yukon‐Kuskokwim Delta, Alaska

AbstractNorthern high‐latitude deltas are hotspots of biogeochemical processing, terrestrial‐aquatic connectivity, and, in Alaska's Yukon‐Kuskokwim Delta (YKD), tundra wildfire. Yet, wildfire effects on aquatic biogeochemistry remain understudied in northern delta regions, thus limiting a more comprehensive understanding of high latitude biogeochemical cycles. In this study, we assess wildfire impacts on summertime aquatic biogeochemistry in YKD tundra using a multi‐year (2015–2019) data set of water chemistry measurements (n = 406) from five aquatic environments: peat plateau ponds, fen ponds, fen channels, lakes, and streams. We aimed to (a) characterize variation in hydrochemistry among aquatic environments; (b) determine wildfire effects on hydrochemistry; and (c) assess post‐fire multi‐year patterns in hydrochemistry in lakes (lower terrestrial‐freshwater connectivity) and fen ponds (higher connectivity). Variation in hydrochemistry among environments was more strongly associated with watershed characteristics (e.g., terrestrial‐aquatic connectivity) than wildfire. However, certain hydrochemical constituents showed consistent wildfire effects. Decreases in dissolved organic carbon (DOC) and CO2, and increases in pH, specific conductance, NH4+, and NO3− indicate that, by combusting soil organic matter, wildfire reduces organics available for hydrologic transport and microbial respiration, and mobilizes nitrogen into freshwaters. Multi‐year post‐fire variation in specific conductance, DOC, and CO2 in lakes and fen ponds suggest that watershed characteristics underlie ecosystem response and recovery to wildfire in the YKD. Together, these results indicate that increasing tundra wildfire occurrence at northern high latitudes could drive multi‐year shifts toward stronger aquatic inorganic nutrient cycling, and that variation in terrain characteristics is likely to underlie wildfire effects on aquatic ecosystems across broader scales.

Long‐term fertilization affects chemical composition of dissolved organic carbon by changing soil properties

AbstractSoil dissolved organic carbon (DOC) plays a key role in fundamental biogeochemical processes; however, the influences of different types of fertilization on the chemical composition and properties of DOC molecules in soils are poorly understood. In this study, DOC samples were extracted from gray desert soils treated with five fertilization treatments: (a) chemical nitrogen, phosphorus and potassium fertilizers; (b) pig and cattle manure; (c) 50% nitrogen from manure and 50% nitrogen from chemical fertilizer; (d) total chemical fertilizers and total manure; and (e) no fertilization (control). Results showed that manure application increased the aromatic carbon by 135.7% and decreased the aliphatic carbon levels by 29.6%, implying manure application promotes the labile carbon consumption and the stable carbon enrichment. The available nutrients and available forms of magnesium were positively associated with the labile carbon groups, and the available forms of calcium were positively associated with the stable carbon groups, demonstrating that the availability of soil nutrients and minerals are influential factors on composition of carbon‐containing groups in DOC. Moreover, the fertilization method had negative direct effects and positive indirect effects on composition of carbon‐containing groups in DOC via soil physicochemical factors, and various forms of mineral ions were the strongest explanatory factors (explaining nearly 51%) of composition of carbon‐containing groups variation in DOC. These findings indicated that soil physicochemical factors play an important role as mediators in the influence of fertilization practices on the composition of carbon‐containing groups in DOC.

Groundwater travel times predict DOC in streams and riparian soils across a heterogeneous boreal landscape

Dissolved organic carbon (DOC) in surface waters is an important component of the boreal landscape carbon budget and a critical variable in water quality. A dominant terrestrial DOC source in the boreal landscape is the riparian zone. These near stream areas play a key role in regulating DOC transport between land and aquatic ecosystems. The groundwater dynamics at this interface have been considered a major controlling variable for DOC export to streams. This study focuses on the regulating role of groundwater levels and mean travel times (MTT) on riparian DOC concentrations and, subsequently, stream DOC. This is done by comparing them as explanatory variables to capture the spatial and intra-annual variability of the stream and riparian groundwater DOC. We used a physically based 3D hydrological model, Mike SHE, to simulate DOC concentrations of the riparian zones for 14 sub-catchments within the Krycklan catchment (Sweden). The model concept assumes that DOC concentrations will be higher in groundwater moving through shallow flow paths. In the model, this can be linked to the position of the groundwater table at a point of observation or the travel time, which will generally be shorter for water that has travelled through shallow and more conductive soil layers. We compared the results with both observed stream and groundwater concentrations. The analysis revealed that the correlation between modelled and observed annual averages of stream DOC increased from r = 0.08 to r = 0.87 by using MTT instead of groundwater level. MTT also better captured the observed spatial variability in riparian DOC concentrations and more successfully represented seasonal variability of stream DOC. We, therefore, suggest that MTT is a better predictor than groundwater level for riparian DOC concentration because it can capture a greater variety of catchment heterogeneities, such as variation in soil properties, catchment size, and input from deep groundwater sources.

A distinctive mode of dissolved organic carbon biodegradation in karst lakes and reservoirs: Evidence from trophic controls and compositional transformations

Lakes and artificial reservoirs present as funnels for global and regional carbon cycling. However, limited information is available about dissolved organic carbon (DOC) bioavailability, especially for its trophic controls and compositional transformations in widely distributed karst waters. Here we examined DOC biodegradation (%BDOC), temperature sensitivity (Q10 value), nutrients and their stoichiometric ratios, as well as DOC composition over a 28-day laboratory incubation experiment in karst lakes and reservoirs. Fluorescence, Fourier transform infrared (FTIR) and UV–Visible spectroscopy were employed to trace DOC variability. We found that %BDOC ranged between 1.17% and 66.6% (median: 30.1%) at 15 °C and 5.95%–68.5% (median: 31.0%) at 30 °C, respectively. The Q10 values were mainly distributed between 0.75 and 1.50, suggesting a large amount of degraded substrate with low activation energy. Aquatic DOC and total dissolved nitrogen (TDN) were tightly linked to the %BDOC, whereas significant phosphorus (P) limitation (TDN:TDP > 25) did not induce discernible correlations. Carbohydrates as a dominant contributor to biodegradable DOC (BDOC) largely drove DOC remineralization. The incomplete degradation of humic-like fractions increased the relative abundance of fulvic acid, tyrosine-like and tryptophan-like fractions. A distinctive mode as biological utilization of extreme active and inactive components could be summarized in the karst lakes and reservoirs. These findings hope to supplement special mechanisms of DOC transformation and fate in the context of climate change mitigation for the UN Sustainable Development Goals.

Dissolved Organic Carbon (DOC) in Ground Ice on Northeastern Tibetan Plateau

Ground ice in permafrost stores substantial amounts of dissolved organic carbon (DOC) upon thaw, which may perpetuate a carbon feedback in permafrost regions, yet little is known to date about the dynamics of DOC and source variability of ground ice on the Tibetan Plateau. Here, the high-resolution data of DOC in ground ice (4.8 m in depth) from two permafrost profiles on the Northeastern Tibetan Plateau (NETP) were firstly presented. We quantified the DOC concentrations (mean: 9.7–21.5 mg/L) of ground ice and revealed sizeable—by a factor of 7.0–36.0—enrichment of the ground ice relative to the other water elements on the TP. Results indicated remarkable depth differences in the DOC of ground ice, suggestive of diverse sources of DOC and different sequestration processes of DOC into ice during permafrost evolution. Combined with DOC and carbon isotopes (δ13CDOC), we clarified that decomposition of soil organic matter and leaching of DOC from organic layers and surrounding permafrost sediments are the important carbon sources of ground ice. The DOC sequestration of ground ice in the upper layers was related to the active layer hydrology and freeze–thaw cycle. However, the permafrost evolution controlled the decomposition of organic carbon and sequestration of DOC in the deep layers. A conceptual model clearly illustrated the dynamics of DOC in ground ice and suggested a significant impact on the carbon cycle on the NETP. The first attempt to explore the DOC in ground ice on the NETP is important and effective for further understanding of carbon cycle under permafrost degradation on the Tibetan Plateau.