Mobilization Of Dissolved Organic Carbon Research Articles

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.

Seasonal and interannual dissolved organic carbon transport process dynamics in a subarctic headwater catchment revealed by high-resolution measurements

Abstract. Dissolved organic carbon (DOC) dynamics are evolving in the rapidly changing Arctic and a comprehensive understanding of the controlling processes is urgently required. For example, the transport processes governing DOC dynamics are prone to climate-driven alteration given their strong seasonal nature. Hence, high-resolution and long-term studies are required to assess potential seasonal and interannual changes in DOC transport processes. In this study, we monitored DOC at a 30 min resolution from September 2018 to December 2022 in a headwater peatland-influenced stream in northern Finland (Pallas catchment, 68° N). Temporal variability in transport processes was assessed using multiple methods: concentration–discharge (C–Q) slope for seasonal analysis, a modified hysteresis index for event analysis, yield analysis, and random forest regression models to determine the hydroclimatic controls on transport. The findings revealed the following distinct patterns: (a) the slope of the C–Q relationship displayed a strong seasonal trend, indicating increasing transport limitation each month after snowmelt began; (b) the hysteresis index decreased post-snowmelt, signifying the influence of distal sources and DOC mobilization through slower pathways; and (c) interannual variations in these metrics were generally low, often smaller than month-to-month fluctuations. These results highlight the importance of long-term and detailed monitoring to enable separation of inter- and intra-annual variability to better understand the complexities of DOC transport. This study contributes to a broader comprehension of DOC transport dynamics in the Arctic, specifically quantifying seasonal variability and associated mechanistic drivers, which is vital for predicting how the carbon cycle is likely to change in Arctic ecosystems.

Response times as explanatory variable for export of dissolved organic carbon (DOC) from small forested catchments

Rising trends in the concentrations of dissolved organic carbon (DOC) are observed in many inland waters, including the headwater catchment of the Große Ohe river in the Bavarian Forest National Park (Germany). During flood events, DOC is mobilized via different hydrological pathways, affecting the hydrochemistry of aquatic ecosystems and the viability of drinking water supply. In our field experiments we observed different phases of DOC mobilization during six intensively studied rainfall-runoff events with contrasting antecedent wetness conditions. We propose response time diagrams to link the different phases of DOC mobilization to different response times along a hillslope-riparian-zone-transect. Depending on the antecedent wetness conditions, the hillslope and riparian zone participated differently to phases of the runoff event and shaped the flow hydrograph and DOC export at the catchment outlet. The hillslope always responded with little time delay to precipitation events regardless of the antecedent wetness condition. In contrast, response times in the riparian zone varied. For wet antecedent conditions we observed little delay between the hillslope- and riparian zone peak response, which caused high peak discharge, fast DOC mobilization and high DOC export from the catchment. In contrast, for dry antecedent conditions, the riparian zone response was significantly lower and much delayed. This led not only to attenuated peak discharge but also to larger time lags between the flow hydrograph- and the DOC concentration peak. The combination of low runoff rates and delayed DOC concentration peaks resulted into lower DOC export from the catchment. Thus, the antecedent wetness condition and response times of the hillslope and riparian zone are important indicators for DOC export in the catchment.

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.

Seasonal and Storm Event‐Based Dynamics of Dissolved Organic Carbon (DOC) Concentration in a Mediterranean Headwater Catchment

AbstractThis study investigates the spatial and temporal dynamics of Dissolved Organic carbon (DOC) concentration in a Mediterranean headwater catchment (Turbolo River catchment, southern Italy) equipped with two multi‐parameter sondes providing more than two‐year (May 2019–November 2021) continuous high‐frequency measurements of several DOC‐related parameters. The sondes were installed in two nested sections, a quasi‐pristine upstream sub‐catchment and a downstream outlet with anthropogenic water quality disturbances. DOC estimates were achieved by correcting the fluorescent dissolved organic matter—fDOM—values through an original procedure not requiring extensive laboratory measurements. Then, DOC dynamics at the seasonal and storm event scales were analyzed. At the seasonal scale, results confirmed the climate control on DOC production, with increasing background concentrations in hot and dry summer months. The hydrological regulation proved crucial for DOC mobilization and export, with the top 10th percentile of discharge associated with up to 79% of the total DOC yield. The analysis at the storm scale using flushing and hysteresis indices highlighted substantial differences between the two catchments. In the steeper upstream catchment, the limited capability of preserving hydraulic connection over time with DOC sources determined the prevalence of transport as the limiting factor to DOC export. In the downstream catchment, transport‐ and source‐limited processes were observed almost equally. The correlation between the hysteretic behavior and antecedent precipitation was not linear since the process reverted to transport‐limited for high accumulated rainfall values. Exploiting high‐resolution measurements, the study provided insights into DOC export dynamics in nested headwater catchments at multiple time scales.

Organic Carbon in the Bottom Sediments of Lake Baikal: Geochemical Processes of Burial and Balance Values

This is the first study of dissolved organic matter (DOM) at the Lake Baikal water-bottom interface. High-resolution profiles of dissolved organic carbon (DOC) were obtained simultaneously with dissolved inorganic carbon (DIC), total dissolved carbon, cations (Na+, K+, Ca2+, Mg2+, Fe2+, and Mn2+), and anions (HCO3−, Cl−, NO3−, and SO42−) in the pore water of Lake Baikal deepwater oxidized sediments. We evaluated the DOC fluxes quantitatively and qualitatively. They changed their direction twice under different redox conditions in the sediments (at the redox interfaces). The study revealed that the mobilization of DOC in anoxic sediments was closely related to the reductive dissolution of Fe(III) minerals, and the oxidized surface lake sediments represented an effective DOC trap binding DOC to ferric minerals. Redox conditions appeared to be the main regulator of the DOC exchange. Oxygen conditions led to the uptake of DOC by sediments (31–78 mmol C m−2 yr−1), i.e., the Lake Baikal sediments are a sink of DOC. The DOC flux was approximately 25–35% of the carbon flux at the sediment–water interface. The results of this study allow for a better understanding of the nature and properties of DOC in freshwater ecosystems and compensate for the underestimation of DOC in the internal carbon cycle of the lake.

Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production

AbstractThe marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.

Oxygen Dependent Temperature Regulation of Benthic Fluxes in Reservoirs

AbstractTemperature and dissolved oxygen concentration are critical factors affecting the exchange of solutes between sediment and water; both factors will be affected by warming of lakes and thereby influence water quality. Temperature and oxygen responses of single solute fluxes are well known; however, not much is known about the interaction of temperature and oxygen in regulating the balance of different fluxes in the benthic environment. We analyzed benthic flux (mobilization and immobilization) data of various solutes (dissolved organic carbon (DOC), CH4, NO3−‐N, NH4+‐N, SRP, SO4−, Fe, Mn, and O2) collected from laboratory incubations of 142 sediment cores from 5 different reservoirs incubated under varying in situ temperature and oxygen conditions. Oxygen was the primary driver of benthic fluxes, while temperature and total organic content were secondary. Temperature effects on benthic fluxes were stronger under anoxic conditions which imply that warming will substantially increase the benthic fluxes if the sediment surface becomes anoxic. The varying temperature response of processes underlying the studied fluxes will result in a shift of their relative importance in the benthic environment, especially in shallow lakes that are more vulnerable to warming. For example, more anoxic conditions will shift the equilibrium between net sulfate reduction and methane release toward the latter. We also predict that physical effects of warming leading to hypolimnetic oxygen depletion, that is, stronger stratification and longer hypolimnetic confinement will increase the benthic mobilization of phosphorus, DOC, and methane into water and immobilization of sulfate by the sediments even in deep lakes.

Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment

AbstractThe transport of dissolved organic carbon (DOC) from the soils to inland waters plays an important role in the global carbon cycle. Widespread increases in DOC concentrations have been observed in surface waters over the last few decades, affecting carbon balances, ecosystem functioning and drinking water treatment. However, the primary hydrological controls on DOC mobilization are still uncertain. The aim of this study was to investigate the role of microtopography in the riparian zone for DOC export and DOM quality. DOC concentration and DOM quality in the shallow groundwater of a riparian zone and in streamflow in a forested headwater catchment was investigated using fluorescence and absorbance characteristics. We found higher DOC concentrations with a higher aromaticity in the microtopographical depressions, which were influenced by highly dynamic shallow groundwater levels, than in the flat forest soil. As a result of the frequent wet‐dry cycles in the upper soil layers, aromatic DOC accumulated in the shallow groundwater within and below the microtopographical depressions. Rising groundwater levels during precipitation events led to the connection of the microtopographical depressions to the stream, resulting in a change toward more aromatic DOC in the stream. Increasing stream DOC concentrations were accompanied by increasing concentrations of iron and aluminum, suggesting the coupled release of these metals with DOC from the riparian zone. Our results highlight the importance of the interplay between microtopography and groundwater level dynamics in the riparian zone for DOC export from headwater catchments.

Seasonal and spatial variations in riverine DOC exports in permafrost-dominated Arctic river basins

• Seasonality of riverine DOC exports is strongly associated with hydrological regimes. • The extent and degradation of permafrost regulate riverine DOC mobilization. • Water temperature is an important factor influencing riverine DOC exports. Climate warming is accelerating the release of voluminous organic carbon from thawing permafrost into the Arctic Ocean via riverine transport. However, the seasonal variations in riverine dissolved organic carbon ( DOC ) exports in Arctic river basins with different areal extents of permafrost and how changes in water temperature ( T w ) impact seasonal DOC exports are not fully understood. In this study, the concentrations, ages and seasonality of riverine DOC in the estuaries of six major Arctic rivers (Ob, Yenisei, Lena, Kolyma, Yukon and Mackenzie) were analysed using Arctic Great Rivers Observatory (ArcticGRO) datasets from 2003 to 2019. The results showed that DOC concentrations generally increased with the increases in the streamflow, but always dropped to the minimum with the oldest Δ 14 C-DOC ages (as old as 1650 years BP) in the freezing period (November–April), when the streamflow originates predominantly from groundwater. During the flood pulse period (May or June), a rapid increase in riverine DOC concentration with younger organic carbon (Δ 14 C values from −61 to 152‰) was observed, likely associated with snowmelt-dominated runoff regimes (lower δ 18 O-H 2 O of approximately −20.4 ± 1.6‰). During the ice-free period (June–September), DOC concentrations decreased due to the enhanced dilution of streamflow from precipitation. In the Lena and Kolyma River basins with large areal extents of continuous permafrost, over 70% of DOC flux exported during the ice-free period originated from DOC sources from ∼410 years and ∼230 years BP to the present, respectively; this suggests that greater permafrost extents restrict the release of older DOC into rivers. However, riverine DOC exports likely respond positively to changes in T w during the ice-free period. In addition, such a positive response is likely to be enhanced in basins with larger percentages of sporadic permafrost, thicker active layers, more precipitation and less soil organic carbon. Ultimately, under a warming climate, riverine DOC exports are expected to rise with increasing river water temperatures.

Low hydrological connectivity after summer drought inhibits DOC export in a forested headwater catchment

Abstract. Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in southeastern Germany. Discharge and stream DOC concentrations were measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration–discharge relationships exhibited pronounced but almost exclusively counterclockwise hysteresis loops which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a DOC load up to 7.4 kg h−1. The DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the low hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent wetness conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of subcatchments, depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different subcatchments to DOC export may change in the future when hydrological connectivity will be reduced more often.

Fire reduces riverine DOC concentration draining a watershed and alters post-fire DOC recovery patterns

The loading of dissolved organic carbon (DOC) from soils to inland waters and ultimate transport to the ocean is a critical flux pathway in the terrestrial biosphere carbon cycle. Fires can significantly affect this flux through biogeochemical processes related to oxidation and mobilization of DOC in the soil. Therefore, in order to accurately estimate and model terrestrial carbon storage and export to the marine environment, we need to better understand the effects of fire on DOC flux. In this analysis, we compiled available observational data sets from seven watersheds across the conterminous United States generally spanning the years 1999–2019. We used these data sets to examine the effects of fire on riverine DOC concentration draining a watershed to study both the immediate impacts and the post-fire recovery patterns. Our results suggest that these fires result in an immediate decrease in riverine DOC concentration draining the watershed by 26 ± 15%, and the time required for DOC concentrations to recover to pre-fire levels was estimated to be, on average, approximately 9 months. During recovery, DOC concentration was 24 ± 11% lower than the long-term average for the watershed. In addition, the larger the proportion of the watershed that burned, the greater the concentration decrease and the longer the time period for post-fire recovery.

Soothsaying DOM: A Current Perspective on the Future of Oceanic Dissolved Organic Carbon

The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.

High-frequency measurements explain quantity and quality of dissolved organic carbon mobilization in a headwater catchment

Abstract. Increasing dissolved organic carbon (DOC) concentrations and exports from headwater catchments impact the quality of downstream waters and pose challenges to water supply. The importance of riparian zones for DOC export from catchments in humid, temperate climates has generally been acknowledged, but the hydrological controls and biogeochemical factors that govern mobilization of DOC from riparian zones remain elusive. A high-frequency dataset (15 min resolution for over 1 year) from a headwater catchment in the Harz Mountains (Germany) was analyzed for dominant patterns in DOC concentration (CDOC) and optical DOC quality parameters SUVA254 and S275−295 (spectral slope between 275 and 295 nm) on event and seasonal scales. Quality parameters and CDOC systematically changed with increasing fractions of high-frequency quick flow (Qhf) and antecedent hydroclimatic conditions, defined by the following metrics: aridity index (AI60) of the preceding 60 d and the quotient of mean temperature (T30) and mean discharge (Q30) of the preceding 30 d, which we refer to as discharge-normalized temperature (DNT30). Selected statistical multiple linear regression models for the complete time series (R2=0.72, 0.64 and 0.65 for CDOC, SUVA254 and S275−295, resp.) captured DOC dynamics based on event (Qhf and baseflow) and seasonal-scale predictors (AI60, DNT30). The relative importance of seasonal-scale predictors allowed for the separation of three hydroclimatic states (warm and dry, cold and wet, and intermediate). The specific DOC quality for each state indicates a shift in the activated source zones and highlights the importance of antecedent conditions and their impact on DOC accumulation and mobilization in the riparian zone. The warm and dry state results in high DOC concentrations during events and low concentrations between events and thus can be seen as mobilization limited, whereas the cold and wet state results in low concentration between and during events due to limited DOC accumulation in the riparian zone. The study demonstrates the considerable value of continuous high-frequency measurements of DOC quality and quantity and its (hydroclimatic) key controlling variables in quantitatively unraveling DOC mobilization in the riparian zone. These variables can be linked to DOC source activation by discharge events and the more seasonal control of DOC production in riparian soils.

Mobilisation and transport of dissolved organic carbon and iron in peat catchments—Insights from the Lehstenbach stream in Germany using generalised additive models

AbstractDuring the last decades, increasing exports of both dissolved organic carbon (DOC) and iron were observed from peat catchments in North America and Europe with potential consequences for water quality of streamwater and carbon storages of soils. As mobilisation and transport processes of DOC and iron in peat catchments are only partly understood, the purpose of this study was to elucidate these processes in an intensively monitored and studied system. Specifically, it was hypothesised that dissimilatory iron reduction in riparian peatland soils mobilises DOC initially adsorbed to iron minerals. During stormflow conditions, both DOC and iron will be transported into the stream network. Ferrous iron may be reoxidised at redox interfaces on its way to the stream, and subsequently, ferric iron could be transported together with DOC as complexes. To test these hypotheses, generalised additive models (GAMs) were applied to 14 years of weekly time series of discharge and concentrations of selected solutes measured in a German headwater stream called Lehstenbach. This stream drains a 4.19‐km2 forested mountain catchment; one third of which is covered by riparian peatland soils. We interpreted results of different types of GAM in the way that (a) iron reduction drove the mobilisation of DOC from peatland soils and that (b) both iron and DOC were transported as complexes after their joint mobilisation to and within the steam. It was speculated that low nitrate availability in the uppermost wetland soil layer, particularly during the growing season, promoted iron reduction and thus the mobilisation of DOC. However, the influence of nitrate on the DOC mobilisation remains relatively uncertain. This influence could be further investigated using methods similar to the GAM analysis conducted here for other catchments with long‐term data as well as detailed measurements of the relevant species in riparian wetland soils and the adjacent stream network.

Association between extracted copper and dissolved organic matter in dairy-manure amended soils

Dairy manure often has elevated concentrations of copper (Cu) that when applied to soil may create toxicity risks to seedlings and soil microbes. Manure application also increases dissolved organic matter (DOM) in soil solution. We hypothesize that high rates of dairy manure amendment over several years will cause increased DOM in the soil that complexes Cu, increasing its mobility. To test this hypothesis, this study investigated water soluble Cu concentrations and dissolved organic carbon (DOC) in soil samples from 3 years of manure-amended soils. Samples were collected at two depths over the first 3 years of a long-term manure-amendment field trial. DOC, Cu, Fe, and P concentrations were measured in water extracts from the samples. Ultraviolet/visible (UV/Vis) spectra were used to assess the DOC characteristics. After 3 years of manure application, extractable Cu concentration was approximately four times greater in the surface and two times greater in subsurface samples of manure-amended soils as compared to non-amended control soils and traditional mineral fertilizer-amended soils. The extractable Cu concentration was greatest in plots that had the highest manure amendment rates (35 t ha−1 and 52 t ha−1, dry weight). The UV/Vis parameters SUVA254 and E2/E3 correlated with Cu concentration in the extracts (p < 0.05), suggesting that DOC characteristics are important in Cu-binding. The molecular characteristics of the DOC in the subsurface after 3 years of manure amendment were distinct from the DOC in the control plot, suggesting that manure amendment creates mobile DOC that may facilitate Cu mobilization through soil. The 10-fold increase in extractable Cu concentration after only 3 years of manure application indicates that repeated applications of the dairy manure sources used in this study at rates of 35 t/ha or greater may create risks for Cu toxicity and leaching of Cu into ground and surface waters.

Biologically driven DOC release from peatlands during recovery from acidification

Peatlands store 1/3 of global soil carbon, destabilisation of which contributes much to the recent increase in DOC (dissolved organic carbon) in freshwater ecosystems. One suggested mechanism for the enhanced decomposition of peat and the releases of DOC is recovery from acidification. However, no biological role in the process has yet been identified. Here we report extracellular enzyme activities and microbial composition in peatlands of Korea, the UK, Japan and Indonesia, and find higher pH to promote phenol oxidase activities, greater abundances in Actinobacteria and fungi, and enhanced pore-water DOC concentrations. Our pH manipulation experiments also showed that increase in pH enhanced phenol oxidase activity and DOC production with greater Actinobacterial and fungal abundances. Finally, knockout or addition of phenol oxidase dramatically changed DOC and phenolic production, indicating the central role of phenol oxidase in DOC mobilisation. Our findings provide evidence to support a previously unrecognized biological mechanism through which pH increases activate phenol oxidase, accelerating the release of DOC and phenolics.

Spatio-temporal controls of dissolved organic carbon stream water concentrations

Elevated concentration and high variance of dissolved organic carbon (DOC) in surface waters are a challenge for the production of drinking water. Past studies have indicated a dominant role of wetlands in DOC mobilization, but have mainly been focused on boreal and oceanic catchments. Here we analyze the observational DOC time series from 89 temperate humid catchments which drain into German drinking water reservoirs. We characterize the DOC concentration median and variability and utilize partial least squares regression in order to quantify the relation to catchment characteristics such as land use, climate, and topography. We found that the long-term median DOC concentration in the catchment is well predicted by the 90th percentile of the distribution of the topographic wetness index (0.9P TWI) over the entire catchment area. The 0.9P TWI can be directly connected to the abundance of riparian wetlands in the catchments. DOC concentration variability (represented as the ratio of the interquartile range and the median concentrations) was also found to be well predictable. Concentration variability was highest in cold and wet catchments with a high 0.9P TWI. Here we also found stronger correlations between DOC concentrations and discharge, with positive concentration-discharge patterns. Catchments with elevated DOC-concentration variance also exhibited the most severe long-term increases in concentrations. Our results thus indicate that, in temperate climates, riparian wetlands can be the dominant source zones of DOC and control the hydrological mobilization and potentially also the spatial difference in long-term concentration trends observed in surface waters. We conclude that the dominance of topography and climatic conditions in controlling spatio-temporal patterns in DOC concentrations leads to very limited management options.

Factors Affecting the Leaching of Dissolved Organic Carbon after Tree Dieback in an Unmanaged European Mountain Forest.

Forest disturbances affect ecosystem biogeochemistry, water quality, and carbon cycling. We analyzed water chemistry before, during, and after a dieback event at a headwater catchment in the Bohemian Forest (central Europe) together with an un-impacted reference catchment, focusing on drivers and responses of dissolved organic carbon (DOC) leaching. We analyzed data regarding carbon input to the forest floor via litter and throughfall, changes in soil moisture and composition, streamwater chemistry, discharge, and temperature. We observed three key points. (i) In the first 3 years following dieback, DOC production from dead biomass led to increased concentrations in soil, but DOC leaching did not increase due to chemical suppression of its solubility by elevated concentrations of protons and polyvalent cations and elevated microbial demand for DOC associated with high ammonium (NH4+) concentrations. (ii) DOC leaching remained low during the next 2 years because its availability in soils declined, which also left more NH4+ available for nitrifiers, increasing NO3- and proton production that further increased the chemical suppression of DOC mobility. (iii) After 5 years, DOC leaching started to increase as concentrations of NO3-, protons, and polyvalent cations started to decrease in soil water. Our data suggest that disturbance-induced changes in N cycling strongly influence DOC leaching via both chemical and biological mechanisms and that the magnitude of DOC leaching may vary over periods following disturbance. Our study adds insights as to why the impacts of forest disturbances are sometime observed at the local soil scale but not simultaneously on the larger catchment scale.

Retrogressive thaw slumps temper dissolved organic carbon delivery to streams of the Peel Plateau, NWT, Canada

Abstract. In Siberia and Alaska, permafrost thaw has been associated with significant increases in the delivery of dissolved organic carbon (DOC) to recipient stream ecosystems. Here, we examine the effect of retrogressive thaw slumps (RTSs) on DOC concentration and transport, using data from eight RTS features on the Peel Plateau, NWT, Canada. Like extensive regions of northwestern Canada, the Peel Plateau is comprised of thick, ice-rich tills that were deposited at the margins of the Laurentide Ice Sheet. RTS features are now widespread in this region, with headwall exposures up to 30 m high and total disturbed areas often exceeding 20 ha. We find that intensive slumping on the Peel Plateau is universally associated with decreasing DOC concentrations downstream of slumps, even though the composition of slump-derived dissolved organic matter (DOM; assessed using specific UV absorbance and slope ratios) is similar to permafrost-derived DOM from other regions. Comparisons of upstream and downstream DOC flux relative to fluxes of total suspended solids suggest that the substantial fine-grained sediments released by RTS features may sequester DOC. Runoff obtained directly from slump rill water, above entry into recipient streams, indicates that the deepest RTS features, which thaw the greatest extent of buried, Pleistocene-aged glacial tills, release low-concentration DOC when compared to paired upstream, undisturbed locations, while shallower features, with exposures that are more limited to a relict Holocene active layer, have within-slump DOC concentrations more similar to upstream sites. Finally, fine-scale work at a single RTS site indicates that temperature and precipitation serve as primary environmental controls on above-slump and below-slump DOC flux, but it also shows that the relationship between climatic parameters and DOC flux is complex for these dynamic thermokarst features. These results demonstrate that we should expect clear variation in thermokarst-associated DOC mobilization across Arctic regions. However, they also show that within-region variation in thermokarst intensity and landscape composition is critical for determining the biogeochemical response. Geological and climate legacy shape the physical and chemical composition of permafrost and thermokarst potential. As such, these factors must be considered in predictions of land-to-water carbon mobilization in a warming Arctic.