Dissolved Organic Carbon Quantity Research Articles

Insights into dissolved organic carbon biodegradation process and influencing factors in shallow lakes in a metropolitan, China

Lakes perform a pivotal role in both regional and global carbon cycles. However, seasonal variations and drivers of dissolved organic carbon (DOC) biodegradability are poorly understood in shallow urban lakes. To address this knowledge gap, a 28-day incubation experiment in urban shallow lakes was carried out to quantify proportion of biodegradable DOC (%BDOC) and temperature sensitivity (Q10 value). %BDOC and Q10 value were significantly higher in the wet season (%BDOC20: 14.48% ± 7.92%, %BDOC30: 17.61% ± 8.98%, Q10:1.32 ± 0.62) over the dry season (%BDOC20: 10.35% ± 6.98%, %BDOC30: 11.59% ± 7.36%, Q10: 1.20 ± 0.65). The absorbance at 350nm (a350) followed a decreasing-increasing-decreasing trend, while a250 / a365 (E2:E3) had a contrasting trend to a350 during the incubation. Three (dry season) and five (wet season) fluorescent components were captured by parallel factor analysis (PARAFAC). The components C2 (terrestrial humic-like) and C3 (tryptophan-like) were largely biodegraded on day 28 in the wet season. The %BDOC was positively related to total dissolved nitrogen (TDN), total dissolved phosphorus (TDP) and fluorescence index (FI), but negatively related to humification index (HIX) during the dry season. However, in the wet season, the %BDOC was positively related to TDN, TDP and DOC. The results show that the controls of DOC biodegradation shift from the combined DOC quality and nutrients (dry season) to DOC quantity (wet season). The study sheds light on the drivers of BDOC seasonality, and underscores the significance of both nutrients and DOC in understanding carbon-geochemistry within lake ecosystems.

Review article: Terrestrial dissolved organic carbon in northern permafrost

Abstract. As the permafrost region warms and permafrost soils thaw, vast stores of soil organic carbon (C) become vulnerable to enhanced microbial decomposition and lateral transport into aquatic ecosystems as dissolved organic carbon (DOC). The mobilization of permafrost soil C can drastically alter the net northern permafrost C budget. DOC entering aquatic ecosystems becomes biologically available for degradation as well as other types of aquatic processing. However, it currently remains unclear which landscape characteristics are most relevant to consider in terms of predicting DOC concentrations entering aquatic systems from permafrost regions. Here, we conducted a systematic review of 111 studies relating to, or including, concentrations of DOC in terrestrial permafrost ecosystems in the northern circumpolar region published between 2000 and 2022. We present a new permafrost DOC dataset consisting of 2845 DOC concentrations, collected from the top 3 m in permafrost soils across the northern circumpolar region. Concentrations of DOC ranged from 0.1 to 500 mg L−1 (median = 41 mg L−1) across all permafrost zones, ecoregions, soil types, and thermal horizons. Across the permafrost zones, the highest median DOC concentrations were in the sporadic permafrost zone (101 mg L−1), while lower concentrations were found in the discontinuous (60 mg L−1) and continuous (59 mg L−1) permafrost zones. However, median DOC concentrations varied in these zones across ecosystem type, with the highest median DOC concentrations in each ecosystem type of 66 and 63 mg L−1 found in coastal tundra and permafrost bog ecosystems, respectively. Coastal tundra (130 mg L−1), permafrost bogs (78 mg L−1), and permafrost wetlands (57 mg L−1) had the highest median DOC concentrations in the permafrost lens, representing a potentially long-term store of DOC. Other than in Yedoma ecosystems, DOC concentrations were found to increase following permafrost thaw and were highly constrained by total dissolved nitrogen concentrations. This systematic review highlights how DOC concentrations differ between organic- or mineral-rich deposits across the circumpolar permafrost region and identifies coastal tundra regions as areas of potentially important DOC mobilization. The quantity of permafrost-derived DOC exported laterally to aquatic ecosystems is an important step for predicting its vulnerability to decomposition.

Hydrological connectivity controls dissolved organic carbon exports in a peatland-dominated boreal catchment stream

Abstract. The magnitudes of dissolved organic carbon (DOC) exports from boreal peatlands to streams through lateral subsurface flow vary during the ice-free season. Peatland water table depth and the alternation of low and high flow in peat-draining streams are thought to drive this DOC export variability. However, calculation of the specific DOC exports from a peatland can be challenging considering the multiple potential DOC sources within the catchment. A calculation approach based on the hydrological connectivity between the peat and the stream could help to solve this issue, which is the approach used in the present research. This study took place from June 2018 to October 2019 in a boreal catchment in northeastern Canada, with 76.7 % of the catchment being covered by ombrotrophic peatland. The objectives were to (1) establish relationships between DOC exports from a headwater stream and the peatland hydrology; (2) quantify, at the catchment scale, the amount of DOC laterally exported to the draining stream; and (3) define the patterns of DOC mobilization during high-river-flow events. At the peatland headwater stream outlet, the DOC concentrations were monitored at a high frequency (hourly) using a fluorescent dissolved organic matter (fDOM) sensor, a proxy for DOC concentration. Hydrological variables, such as stream outlet discharge and peatland water table depth (WTD), were continuously monitored at hourly intervals for 2 years. Our results highlight the direct and delayed control of subsurface flow from peat to the stream and associated DOC exports. Rain events raised the peatland WTD, which increased hydrological connectivity between the peatland and the stream. This led to increased stream discharge (Q) and a delayed DOC concentration increase, typical of lateral subsurface flow. The magnitude of the WTD increase played a crucial role in influencing the quantity of DOC exported. Based on the observations that the peatland is the most important contributor to DOC exports at the catchment scale and that other DOC sources were negligible during high-flow periods, we propose a new approach to estimate the specific DOC exports attributable to the peatland by distinguishing between the surfaces used for calculation during high-flow and low-flow periods. In 2018–2019, 92.6 % of DOC was exported during flood events despite the fact that these flood events accounted for 59.1 % of the period. In 2019–2020, 93.8 % of DOC was exported during flood events, which represented 44.1 % of the period. Our analysis of individual flood events revealed three types of events and DOC mobilization patterns. The first type is characterized by high rainfall, leading to an important WTD increase that favours the connection between the peatland and the stream and leading to high DOC exports. The second is characterized by a large WTD increase succeeding a previous event that had depleted DOC available to be transferred to the stream, leading to low DOC exports. The third type corresponds to low rainfall events with an insufficient WTD increase to reconnect the peatland and the stream, leading to low DOC exports. Our results suggest that DOC exports are sensitive to hydroclimatic conditions; moreover, flood events, changes in rainfall regime, ice-free season duration, and porewater temperature may affect the exported DOC and, consequently, partially offset the net carbon sequestration potential of peatlands.

Hydrologic and Organic Carbon Quality Controls on Nitrous Oxide Dynamics Across a Variably Confined Karst Aquifer

AbstractAtmospheric concentrations of the potent greenhouse gas nitrous oxide (N2O) have increased approximately 20% since the pre‐industrial era. Riverine systems represent an important atmospheric source of N2O however, the influence of varying hydrogeological conditions and dissolved organic carbon (DOC) quantity and quality on N2O dynamics remains poorly understood. We investigate effects of these processes on N2O dynamics in the Santa Fe River watershed in north‐central Florida, USA where a gradient in DOC quantity and quality is caused by varying surface water‐groundwater exchange. The watershed is underlain by the karstic Floridan aquifer, which is confined in the headwaters and contains terrigenous recalcitrant DOC, while the unconfined lower reaches contain protein‐like labile DOC substrates. Where extensive surface water‐groundwater exchange occurs at the boundary between the confined and unconfined aquifer, incomplete denitrification raises N2O concentration to 6.1 μg N‐N2O L−1, approximately 20 times greater than the concentration expected from equilibration with atmospheric N2O. These elevated concentrations create average local fluxes reaching ∼950 μg N‐N2O m−2 hr−1 within the mixing zone. Summation of spatially explicit emission estimates for upstream, mixing zone, and downstream river sections yield a total average emission rate of ∼3,758 kg N‐N2O yr−1. Despite having the smallest surface area across the river transect (∼8%), emissions from the mixing zone account for 26% of the total river emission rate. These findings highlight the potential contributions from karst landscapes to N2O cycling caused by varying redox conditions during surface water‐groundwater exchange, which creates hotspots of N2O production and emissions.

Responses of dissolved organic carbon to freeze–thaw cycles associated with the changes in microbial activity and soil structure

Abstract. Arctic warming accelerates snowmelt, exposing soil surfaces with shallow or no snow cover to freeze–thaw cycles (FTCs) more frequently in early spring and late autumn. FTCs influence Arctic soil C dynamics by increasing or decreasing the amount of dissolved organic carbon (DOC); however, mechanism-based explanations of DOC changes that consider other soil biogeochemical properties are limited. To understand the effects of FTCs on Arctic soil responses, we designed microcosms with surface organic soils from Alaska and investigated several soil biogeochemical changes for seven successive temperature fluctuations of freezing at −9.0 ± 0.3 ∘C and thawing at 6.2 ± 0.3 ∘C for 12 h each. FTCs significantly changed the following soil variables: soil CO2 production (CO2), DOC and total dissolved nitrogen (TDN) contents, two DOC quality indices (SUVA254 and A365 / A254), microaggregate (53–250 µm) distribution, and small-sized mesopore (0.2–10 µm) proportion. Multivariate statistical analyses indicated that the FTCs improved soil structure at the scale of microaggregates and small-sized mesopores, facilitating DOC decomposition by soil microbes and changes in DOC quantity and quality by FTCs. This study showed that FTCs increased soil CO2 production, indicating that FTCs affected DOC characteristics without negatively impacting microbial activity. Soil microaggregation enhanced by FTCs and the subsequent increase in microbial activity and small-sized pore proportion could promote DOC decomposition, decreasing the DOC quantity. This study provides a mechanism-based interpretation of how FTCs alter DOC characteristics of the organic soil in the active layer by incorporating structural changes and microbial responses, improving our understanding of Arctic soil C dynamics.

Depth Moderates DOC Impact on Cold‐Water Refugia in Small, Northern Temperate Lakes

AbstractWidespread changes in dissolved organic carbon (DOC) concentrations in lakes of the Northern Hemisphere are well documented, and there is a need to understand the ecological implications for changes in cold‐water refugia. DOC controls thermal structure in small, wind‐sheltered lakes, thus influencing the volume and persistence of the hypolimnion and cold‐water habitat. We explored the relationship between the quantity and quality of DOC and the availability of cold‐water habitat over the course of summer stratification in small lakes of varying depths. The ratio of the hypolimnetic volume to total lake volume was calculated to assess the relative changes in cold‐water habitat availability in each lake over time. DOC concentration, maximum lake depth, and the interaction between DOC concentration and maximum lake depth explained 87% of the variability in percent change in hypolimnion volume ratio across studied lakes. Hypolimnion compression was greater in shallow, low DOC lakes, whereas deeper, intermediate DOC lakes had more persistent hypolimnion volumes over the course of summer stratification. These results demonstrate that depth moderates the effect of DOC on the volume and persistence of cold‐water refugia availability in north temperate lakes, advancing our understanding of differing lake sensitivity in a rapidly changing physical and chemical environment.

Effects of cornstalk mulching and rainfall intensity on the quantity and quality of dissolved organic carbon in runoff: A field rainfall simulation at the plot scale

AbstractUnderscoring how crop mulching changes the dissolved organic carbon (DOC) in runoff is essential to understanding the sustainable application of cornstalk mulching. This study examined the runoff DOC quantity and quality in plots with and without cornstalk mulching under various rainfall intensities in a representative steep fallow land of southwestern China. Field rainfall simulations within a 60 min duration were conducted on the triplet plots (1.5 m × 1 m for a single plot) with bare soils under 30‐ and 90‐mm h−1 intensities and with cornstalk mulching (air‐dried whole plants with 60% coverage and weight 1.65 kg per plot) under 90 mm h−1 intensity, respectively. The concentration and loss of runoff DOC and its quality variables, including SUVA254 (aromaticity of DOC), C:C (proportion of colored humic substances in DOC), and E4:E6 (fulvic acid to humic acid in DOC), are determined. Results showed that runoff DOC concentration and loss were both significantly higher at 90 mm h−1 intensity than at 30 mm h−1 intensity (p < 0.05), simultaneously, they were 52.6% and 53.2% higher in mulched plots than in bare plots (p < 0.05), respectively. Runoff SUVA254 and C:C in bare plots were 90% and 88.9% higher in 30 mm h−1 intensity than in 90 mm h−1 intensity (p < 0.05), respectively. Whereas, E4:E6 of runoff was 12.3% higher in mulched plots than that in bare plots (p < 0.05). Our findings imply that cornstalk mulching enhances the export of runoff DOC with a decline in aromaticity and the molecular weight of humic substances.

Response of dissolved organic carbon dynamics to salinity in a Constructed Fen Peatland in the Athabasca Oil Sands region

AbstractIn northern Alberta, oil sands mining disturbs the boreal landscape, and reclamation to an ‘equivalent land capability’ is required. Industry is testing peatland construction as part of landscape reclamation. To determine if constructed peatlands can be self‐sustaining, an understanding of the cycling of solutes in pore water and their interactions with dissolved organic carbon (DOC) is needed since DOC can represent an important carbon loss from peatlands. DOC is of interest due to its biotic origin and use by the microbial community and impact on carbon budgets. Additionally, salinity as a control on DOC quantity and quality may be important in oil sands reclaimed systems due to the likelihood of elevated sodium (Na+) from saline groundwater input derived from tailings used to construct catchments, and natural sources. For this research, DOC concentration and quality, and Na+ concentration were measured in the rooting zone (10 and 30 cm depth) of Nikanotee Fen to evaluate the role of Na+ in DOC dynamics. DOC concentration and quality suggested that DOC in the fen was largely sourced from vegetation inputs, with quality also suggesting increases in vegetation inputs between years. Elevated Na+ at 30 cm below ground surface corresponded with high concentrations of labile DOC. At 10 cm below ground surface, sampling location and temperature were the best predictors of DOC concentration and quality. With expected increases in Na+, increased production of mobile and microbially active DOC may lead to higher rates of carbon export.

Soil warming decreased dissolved organic carbon quantity and quality in subtropical forests.

Soil dissolved organic carbon (DOC) is the most active part in forest soil carbon pool, the responses of which to climate warming has profound effects on forest carbon cycling. Based on a manipulative soil warming experiment in subtropical evergreen broad-leaved forests, we collected soil solutions in situ and used ultraviolet-visible, infrared and three-dimensional fluorescence spectroscopy analyses to explore the effects of soil warming (+4 ℃, 1 year) on soil DOC quantity and quality along the soil profile. The results showed that soil DOC flux remained constant along the soil profile. Soil DOC mainly included two humic-like fractions and one microbial metabolite. Warming significantly decreased soil DOC flux and the abundance of aromatic and hydrophobic components, and increased the amount of low molecular weight carbohydrates. Furthermore, soil warming increased the relative proportion of humic-like fractions in the surface soil layer (0-10 cm) and microbial metabolite in the deep soil layer (30-40 cm), indicating that warming might accelerate microbial turnover in the deep layer. Overall, soil warming not only decreased soil DOC content, but also simplified the composition of soil DOC in subtropical evergreen broad-leaved forests.

Dynamics of dissolved organic carbon in runoff discharge under different rainfall patterns in a representative agricultural catchment

Rainfall-induced runoff transported plentiful terrestrial dissolved organic carbon (DOC) to downstream waters, significantly regulating the biogeochemical cycling of carbon in downstream waters. However, the dynamics of DOC exporting from agricultural catchments in response to different rainfall patterns remain largely unknown. This study aimed to understand the effects of rainfall patterns on the runoff DOC quantity and quality in a representative agricultural catchment in Southwest China. Six rainfall events producing runoff, including three moderate rainfalls, two heavy rainfalls, and one rainstorm, were recorded to investigate the shifts in runoff DOC concentration and its quality indicators, i.e., SUVA254 (indicating the aromaticity content), C:C ratio (denoting the proportion of colored substances to uncolored substances), and E4:E6 ratio (denoting the fulvic acid to humic acid of DOC). The results showed that DOC concentration in runoff discharge was highest in the moderate and heavy rainfalls occurring at the early rainy season, while was much lower in the rainstorm. Exported DOC flux reached 1.82 kg hm-2 h−1 in the rainstorm, which was 3.95–60.67 times that in the other rainfall events. SUVA254 and C:C ratio fluctuated softly during the moderate rainfalls and rainstorm, and changed strongly during the heavy rainfall with longest antecedent dry day. The moderate and heavy rainfalls with longer antecedent dry day and the rainstorm delivered DOC with higher SUVA254 and C:C ratio than the other rainfall events did. Discharge was positively correlated with DOC concentration during rainfall across all rainfall patterns, while was negatively correlated with SUVA254 and C:C ratio. Forward stepwise regression analysis showed that DOC flux per hour was only positively linearly correlated with rainfall intensity. Our results were valuable for understanding the biogeochemical cycling of DOC in analogous catchments worldwide.

Origin, accumulation and fate of dissolved organic matter in an extreme hypersaline shallow lake

Hypersaline endorheic aquatic systems (H-SEAS) are lakes/shallow playas in arid and semiarid regions that undergo extreme oscillations in salinity and severe drought episodes. Although their geochemical uniqueness and microbiome have been deeply studied, very little is known about the availability and quality of dissolved organic matter (DOM) in the water column.. A H-SEAS from the Monegros Desert (Zaragoza, NE Spain) was studied during a hydrological wetting-drying-rewetting cycle. DOM analysis included: (i) a dissolved organic carbon (DOC) mass balance; (ii) spectroscopy (absorbance and fluorescence) and (iii) a molecular characterization with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The studied system stored a large amount of DOC and under the highest salinity conditions, salt-saturated waters (i.e., brines with salinity > 30%) accumulated a disproportionate quantity of DOC, indicating a significant in-situ net DOM production. Simultaneously, during the hydrological transition from wet to dry, the DOM pool showed strong alterations of it molecular composition. Spectroscopic methods indicated that aromatic and degraded DOM was rapidly replaced by fresher, relatively small, microbial-derived moieties with a large C/N ratio. FT-ICR-MS highlighted the accumulation of small, saturated and oxidized molecules (molecular O/C > 0.5), with a remarkable increase in the relative contribution of highly oxygenated (molecular O/C>0.9) compounds and a decrease of aliphatic and carboxyl-rich alicyclic moleculesThese results indicated that H-SEAS are extremely active in accumulating and processing DOM, with the notable release of organic solutes probably originated from decaying microplankton under large osmotic stress at extremely high salinities.

Integrating remote sensing of hydrological processes and dissolved organic carbon fluxes in long-term Lake Studies

Dissolved organic carbon (DOC) is an important fraction of organic carbon (OC) that together with colored dissolved organic matter (CDOM) plays a key role in the carbon cycling of lakes. Lake Võrtsjärv is a large and shallow eutrophic lake where water quality measurements have been carried out for a long time. The monthly characteristics of DOC and CDOM in Võrtsjärv and its input/output rivers were traced in 5-years observations. We attempted to link the hydro-morphological indices as lake area (A), water level (WL) and volume variations (Qc) with lake DOC fluxes and CDOM-DOC relationships to improve understanding of carbon fluxes in a very shallow, large and highly eutrophic lake, as well as of the possibility to estimate DOC fluxes by CDOM via remote sensing. The research questions were: how the properties and quantity of DOC, its fluxes and CDOM-DOC relationship in a very shallow, large and highly eutrophic lake are responding to 1) variable proportions of allochthonous and autochthonous processes and 2) changing hydrological conditions. The increased lake volume variation in spring caused the higher level of CDOM, signifying the terrestrial (allochthonous) DOC (DOCAlloch) domination in Lake Võrtsjärv. The proportion of DOCAlloch increased together with increasing WL, A and CDOM concentration. Although the generally high proportion of DOCAlloch in Võrtsjärv, the autochthonous DOC determines the water color (CDOM/DOC) in this highly productive ecosystem with short residence time (280–365 days). Our study on DOC flux revealed that the internal carbon sources, driven by hydrological factors, contribute significantly to the role of large and shallow lakes in global carbon cycling. The further development of water color remote sensing could provide novel directions for understanding the feedbacks of lake DOC flux and changing hydrology.

The contribution of deadwood to soil carbon dynamics in contrasting temperate forest ecosystems

Deadwood forms a significant carbon pool in forest systems and is a potential source of dissolved organic carbon (DOC) input to soil, yet little is known about how deadwood effects forest soil carbon cycling. Deadwood DOC inputs to soil may be retained through sorption or may prime microbial decomposition of existing organic matter to produce additional DOC. To determine impacts of deadwood on soil C cycling, we analysed surface soil from beneath deadwood or leaf litter only, along chronosequences of stands of lowland oak and upland Sitka spruce. The concentration and quality (by optical indices) of water-extracted soil DOC (water-extractable organic carbon; WEOC), in situ decomposition ‘tea bag index’ (TBI) parameters and enzymatic potential assays (β-D-cellubiosidase, β-glucosidase, β-xylosidase, leucine aminopeptidase, phosphatase, phenol oxidase) were determined. Presence of deadwood significantly (p < 0.05) increased WEOC concentration (~ 1.5 to ~ 1.75 times) in the mineral oak soil but had no effect on WEOC in spruce soils, potentially because spruce deadwood DOC inputs were masked by a high background of WEOC (1168 mg kg−1 soil) and/or were not retained through mineral sorption in the highly organic (~ 90% SOM) soil. TBI and enzyme evidence suggested that deadwood-derived DOC did not impact existing forest carbon pools via microbial priming, possibly due to the more humified/aromatic quality of DOC produced (humification index of 0.75 and 0.65 for deadwood and leaf litter WEOC, respectively). Forest carbon budgets, particularly those for mineral soils, may underestimate the quantity of DOC if derived from soil monitoring that does not include a deadwood component.

Seasonal dynamics of chromophoric dissolved organic matter in Poyang Lake, the largest freshwater lake in China

Chromophoric dissolved organic matter (CDOM) plays a vital role in the biogeochemical cycles of elements in aquatic ecosystems. Seasonal dynamics of CDOM of the lake are sensitive indicators of the biogeochemical processes and water quality, which are critical to the water security of the lake. Thus, a field investigation of the CDOM and dissolved organic carbon (DOC) properties was conducted during four hydrological seasons in 2016 to track the temporal variability of CDOM and DOC properties and to explore the influence of rivers and wetlands on the quantity and quality of CDOM in Poyang Lake. The results showed that the alternations of flood and dry periods have a different influence on the quantity and quality of CDOM and DOC in the main lake and the river mouths districts. DOC and aromaticity in the main lake district were more influenced by seasonal variations, while CDOM concentrations and molecular weight in the river mouths were more affected. CDOM fluorescent components in both these districts varied obviously across the hydrological seasons. More terrestrial humic-like substances with higher aromaticity and lower DOC in the rising and flood season than in the dry and retreating season. The contributing rate of CDOM and its terrestrial humic-like substances from input rivers to the lake in the rising season can account for more than 50%, which was more than that in the flood season. Flooding causes wetlands to contribute a higher abundance of CDOM and DOC with lower aromaticity to the lake, contrary to the CDOM features in the flood season, highlighting the important role of wetlands in the organic carbon pool. The DOC can be modeled using CDOM fluorescent and optical parameters by the Support Vector Regression (SVR) method, which provides a tool for DOC dynamic monitoring and water quality management.

Microbial community composition controls carbon flux across litter types in early phase of litter decomposition.

SummaryLeaf litter decomposition is a major carbon input to soil, making it a target for increasing soil carbon storage through microbiome engineering. We expand upon previous findings to show with multiple leaf litter types that microbial composition can drive variation in carbon flow from litter decomposition and specific microbial community features are associated with synonymous patterns of carbon flow among litter types. Although plant litter type selects for different decomposer communities, within a litter type, microbial composition drives variation in the quantity of dissolved organic carbon (DOC) measured at the end of the decomposition period. Bacterial richness was negatively correlated with DOC quantity, supporting our hypothesis that across multiple litter types there are common microbial traits linked to carbon flow patterns. Variation in DOC abundance (i.e. high versus low DOC) driven by microbial composition is tentatively due to differences in bacterial metabolism of labile compounds, rather than catabolism of non‐labile substrates such as lignin. The temporal asynchrony of metabolic processes across litter types may be a substantial impediment to discovering more microbial features common to synonymous patterns of carbon flow among litters. Overall, our findings support the concept that carbon flow may be programmed by manipulating microbial community composition.

Molecular links between whitesand ecosystems and blackwater formation in the Rio Negro watershed

Tropical rivers such as the Rio Negro constitute a major portion of the global aquatic flux of dissolved organic carbon (DOC) entering the ocean, but the exact amount, source contributions and fate of terrestrial DOC remain unknown. We investigated the role of valley and upland whitesand ecosystems (WSEs) and terra firme plateaus in forming blackwater tributaries in the Rio Negro basin to develop novel constraints for the terrestrial export of carbon. 5709 molecular markers from ground- and surface waters of two contrasting valley and upland sites feeding Rio Negro tributaries were identified by ultrahigh resolution mass spectrometry (FT-MS), analyzed by multivariate statistics and compared to known Rio Negro markers. In a Principal Coordinates Analysis, valley and upland DOC molecular composition differed by 78% from plateau DOC, which was characterized by reworked, aliphatic and unsaturated N- and S-containing molecules, while valley and upland DOC contained mainly condensed aromatics, aromatics and oxidized unsaturated structures. Valley and upland samples differed by 10% in molecular DOC composition and by their isotopic content (14C of SPE-DOC, 18O and 2H of water) which indicated differences in hydrology and C turnover. Against expectation, markers of widespread whitesand valleys did not emerge as a major source of Rio Negro markers, but specific upland markers did. Pubchem suggested chromene and benzofuran structures as promising candidates for further study. Our findings indicate that the export of molecular markers diverges from expected transport-limited DOC behavior, and thereby opens new avenues for source annotation beyond DOC quantity. Terrestrial DOC from upland whitesand areas is a major source of specific blackwater molecules missing in the regional ecosystem C balance, whereas C export from the whitesand valleys and especially from terra firme plateaus represents mainly recycled and transformed carbon not directly affecting the ecosystem C balance and possibly, the watersheds downstream molecular signature. Our study highlights the potential of high-resolution techniques to constrain carbon balances of ecosystems and landscapes by novel molecular markers. A comparison with other terrestrial DOM datasets indicated molecular similarities with temperate acidic soils and tropical rivers that warrant further analysis of common DOM markers. Implications, limitations, and future challenges are discussed in the light of potential applications of diagnostic molecular links for DOC source annotation and estimation of terrestrial DOM export in the land-to-ocean continuum.

Summertime Amino Acid and Carbohydrate Patterns in Particulate and Dissolved Organic Carbon Across Fram Strait

Amino acids (AA) and carbohydrates (CHO) are important components of the marine organic carbon cycle. Produced mainly by phytoplankton as part of the particulate organic carbon (POC) fraction, these compounds can be released into the outer medium where they become part of the dissolved organic carbon (DOC) pool and are rapidly taken up by heterotrophs (e.g., bacteria). We investigated the quantity and quality of POC and DOC, AA and CHO composition in both pools in three different water masses in the Fram Strait (Arctic Ocean) in summer 2017. Polar Waters and Atlantic Waters showed similar concentrations of particulate and dissolved AA and CHO, despite Polar Waters showing the highest DOC concentrations. In Mixed Waters, where the two water masses mix with each other and with melting sea ice, the concentrations of particulate and dissolved AA and CHO were highest. AA and CHO composition differed substantially between the particulate and dissolved fractions. The particulate fraction (&amp;gt;0.7 μm) was enriched in essential AA and the CHO galactose, xylose/mannose, and muramic acid. In the dissolved fraction non-essential AA, several neutral CHO, and acidic and amino CHO were enriched. We further investigated different size fractions of the particulate matter using a separate size fractionation approach (0.2–0.7 μm, 0.7–10 μm and &amp;gt;10 μm). The chemical composition of the 0.2–0.7 μm size-fraction had a higher contribution of non-essential AA and acidic and amino sugars, setting them apart from the 0.7–10 μm and &amp;gt;10 μm fractions, which showed the same composition. We suggest that the relative differences observed between different size fractions and DOC with regards to AA and CHO composition can be used to evaluate the state of organic matter processing and evaluate the contribution of autotrophic phytoplankton or more heterotrophic biomass. In the future, changing conditions in the Central Arctic Ocean (Atlantification, warming, decreasing ice concentrations) may increase primary production and consequently degradation. The AA and CHO signatures left behind after production and/or degradation processes occurred, could be used as tracers after the fact to infer changes in microbial loop processes and food web interactions.

Dissolved Organic Matter Dynamics in Reference and Calcium Silicate‐Treated Watersheds at Hubbard Brook Experimental Forest, NH, USA

AbstractDissolved organic matter (DOM) can serve as an indicator of watershed carbon cycling, and links terrestrial to aquatic ecosystems. The extent DOM is microbially processed before it enters aquatic ecosystems is expected to change with soil depth and landscape position, and as some research suggests, in response to recovery from chronic acidification. Here we examined DOM within freely flowing soil pore water and stream water collected from reference and calcium silicate‐treated watersheds at the Hubbard Brook Experimental Forest, New Hampshire, USA. Dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as, absorption and fluorescence properties were analyzed to assess patterns in DOM quantity and quality. Parallel factor analysis was used to identify three predominant fluorescing DOM components (C1‐3). All three components were terrestrial in origin, however C3 exhibited spectral properties previously linked to the microbial processing of humic material. Distinct horizonal patterns emerged between genetic soil horizons. DOC quantity decreased with soil depth, so too did, the plant derived lignin‐like fraction (C2), while the C3 fraction increased. High‐elevation coniferous zones with shallow bedrock contributed to higher DOC concentrations in soil and stream water, while high and low‐elevation mineral soils exhibited greater C3 fractions and fluorescence indicators of microbially processed DOM. This pattern suggests these mineral soils are important organic carbon sinks, due to greater rates of microbial DOM processing. DOC in forest floor solutions, C3 fractions, and indicators of smaller molecular size (E2:E3) were all higher in the calcium silicate‐treated watershed, suggesting greater microbial processing of DOM.

Amount and biodegradation of dissolved organic matter leached from tree branches and roots in subtropical plantations of China

Dissolved organic matter (DOM) plays a crucial role in biogeochemical cycles and ecosystem functions in forests. However, our understanding of wood litter-derived DOM amounts and biodegradation in forest ecosystems remains limited. In this study, we collected branches and roots with varying diameters (2.0–5.0 mm and 5.0–12.0 mm in diameter) from four common tree species (Liquidambar formosana, Schima superba, Pinus massoniana, and Pinus elliottii) in subtropical plantations of southern China. We measured the amounts of dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP), and DOM aromaticity in the litter leachates from a short-term leaching experiment. Here, the specific ultraviolet absorbances at 254 nm and 350 nm were used to indicate DOM aromaticity. We subsequently performed a 28-day incubation experiment to investigate wood litter-derived DOM biodegradation. Our results showed that, irrespective of tree species, branches often leached lower amounts of DOC with lower aromatic content than roots at each diameter class. For both branches and roots, DOC quantity generally increased with decreasing diameter class, but aromatic content showed an opposite trend. Moreover, DOM biodegradation increased with increasing diameter class, and branches had greater DOM biodegradation than did roots. In addition, DOM biodegradation exhibited no significant relationship with DOC:DTN and DOC:DTP ratios, but correlated negatively with DOM aromaticity. These observations highlight the crucial roles of organ type and diameter in regulating wood litter-derived DOM amount and biodegradation, and suggest that DOM chemistry rather than stoichiometry drives the variation of wood-derived DOM biodegradation in subtropical plantations of China.

The dynamics and stoichiometry of dissolved organic carbon release by kelp.

Canopy-forming kelps are foundational species in coastal ecosystems, fixing tremendous amounts of carbon, yet we know little about the ecological and physiological determinants of dissolved organic carbon (DOC) release by kelps. We examined DOC release by the bull kelp, Nereocystis luetkeana, in relation to carbon fixation, nutrient uptake, tissue nitrogen content, and light availability. DOC release was approximately 3.5 times greater during the day than at night. During the day, N. luetkeana blades released an average of 16.2% of fixed carbon as DOC. Carbon fixation increased with light availability but DOC release did not, leading to a lower proportion of fixed carbon released as DOC at high light levels. We found no relationship between carbon fixation and DOC release rates measured concurrently. Rather, DOC release by N. luetkeana blades declined with marginal significance as blade tissue nitrogen content increased and with experimental nitrate addition, supporting the role of stoichiometric relationships in DOC release. Using a stable isotope (13 C) tracer method, we demonstrated that inorganic carbon is rapidly fixed and released by N. luetkeana blades as 13 DOC, within hours. However, recently fixed carbon (13 DOC) comprised less than 20% of the total DOC released, indicating that isotope studies that rely on tracer production alone may underestimate total DOC release, as it is decoupled from recent kelp productivity. Comparing carbon and nitrogen assimilation dynamics of the annual kelp N. luetkeana with the perennial kelp Macrocystis pyrifera revealed that N. luetkeana had significantly higher carbon fixation, DOC production and nitrogen uptake rates per unit dry mass. Both kelp species were able to perform light-independent carbon fixation at night. Carbon fixation by the annual kelp N. luetkeana is as high as 2.35kg C·m-2 ·yr-1 , but an average of 16% of this carbon (376g C·m-2 ·yr-1 ) is released as DOC. As kelp forests are increasingly viewed as vehicles for carbon sequestration, it is important to consider the fate of this substantial quantity of DOC released by canopy-forming kelps.