Dissolved Organic Carbon Dynamics Research Articles

DOC and POC in the water column of the southern Baltic. Part I. Evaluation of factors influencing sources, distribution and concentration dynamics of organic matter

Organic substances are important components of the marine environment as they determine the properties of seawater and the key biogeochemical processes taking place in it. Organic carbon (OC) is a measure of organic matter. For practical purposes, OC is divided into dissolved organic carbon (DOC) and particulate organic carbon (POC). Both DOC and POC play a major role in the carbon cycle, especially in shelf seas like the Baltic, where their concentrations are substantial. In a three-year study (2009–2011) seawater samples for DOC and POC measurements were collected from stations located in the Gdańsk Deep, the Gotland Deep and the Bornholm Deep. The accuracy and precision of analysis were satisfactory; the recovery was better than 95%, and the relative standard deviation was 4% (n =5). Concentrations of chlorophyll a, phaeopigment a, salinity, pH and temperature were also measured in the same samples. These parameters were selected as proxies of processes contributing to DOC and POC abundance.The aim of the study was to address questions regarding the vertical, horizontal and seasonal dynamics of both DOC and POC in the Baltic Sea and the factors influencing carbon concentrations. In general, the highest concentrations of both DOC and POC were recorded in the surface water layer (DOC~4.7mgdm−3, POC~0.6mgdm−3) as a consequence of intensive phytoplankton activity, and in the halocline layer (DOC~5.1mgdm−3, POC~0.4mgdm−3). The lowest DOC and POC concentrations were measured in the sub-halocline water layer, where the values did not exceed 3.5mgdm−3 (DOC) and 0.1mgdm−3 (POC). Seasonally, the highest DOC and POC concentrations were measured during the growing season: surface DOC~5.0mgdm−3; sub-halocline DOC~4.1mgdm−3 and surface POC~∼0.9mgdm−3, sub-halocline POC~∼0.2mgdm−3. The ANOVA Kruskal-Wallis test results indicate statistically significant differences among the three study sites regarding average concentrations, and concentrations in particular water layers and seasons. It shows that concentrations of DOC and POC differ in sub-basins of the Baltic Sea. The differences were attributed to the varying distances from river mouths to study sites or the different starting times and/or durations of the spring algal blooms. Statistically significant dependences were found between both DOC and POC concentrations and Chl a (phytoplankton biomass), pH (phytoplankton photosynthetic rate), pheo (zooplankton sloppy feeding), salinity (river run-off and North Sea water inflows) and water temperature (season). This was taken as proof that these factors influence DOC and POC in the study areas.

Modeling dissolved organic carbon in temperate forest soils: TRIPLEX-DOC model development and validation

Abstract. Even though dissolved organic carbon (DOC) is the most active carbon (C) cycling in soil organic carbon (SOC) pools, it receives little attention from the global C budget. DOC fluxes are critical to aquatic ecosystem inputs and contribute to the C balance of terrestrial ecosystems, but few ecosystem models have attempted to integrate DOC dynamics into terrestrial C cycling. This study introduces a new process-based model, TRIPLEX-DOC, that is capable of estimating DOC dynamics in forest soils by incorporating both ecological drivers and biogeochemical processes. TRIPLEX-DOC was developed from Forest-DNDC, a biogeochemical model simulating C and nitrogen (N) dynamics, coupled with a new DOC process module that predicts metabolic transformations, sorption/desorption, and DOC leaching in forest soils. The model was validated against field observations of DOC concentrations and fluxes at white pine forest stands located in southern Ontario, Canada. The model was able to simulate seasonal dynamics of DOC concentrations and the magnitudes observed within different soil layers, as well as DOC leaching in the age sequence of these forests. Additionally, TRIPLEX-DOC estimated the effect of forest harvesting on DOC leaching, with a significant increase following harvesting, illustrating that land use change is of critical importance in regulating DOC leaching in temperate forests as an important source of C input to aquatic ecosystems.

Integrating parsimonious models of hydrological connectivity and soil biogeochemistry to simulate stream DOC dynamics

AbstractTo improve understanding and prediction of dissolved organic carbon (DOC) sources and fluxes in northern peat‐dominated catchments, we present the development and application of a parsimonious tracer‐aided rainfall‐runoff model coupled with a biogeochemistry subroutine able to concurrently simulate streamflow and DOC dynamics. The modeling approach which included quantitative assessment of associated uncertainties was conditioned by geochemical tracers which discriminate dominant water sources. Integration of DOC was predicated on statistical time series models which identified air temperature and streamflow as the key proxies that capture DOC supply and transport processes in two upland catchments in Scotland, UK. Conceptualizing the nonlinear partitioning of quick near‐surface and slower groundwater runoff sources in combination with a DOC mass balance resulted in a coupled, low‐parameter mechanistic model. Model tests showed mostly sensitive parameters and reasonable simulation results with seasonally controlled DOC supply and event‐based DOC transport. Transport is facilitated even for smaller events by overland flow from saturated histosols connected to the stream network. However, during prolonged dry periods, near‐surface runoff “switches off” and stream DOC is dominated by low concentration groundwaters. Furthermore, the model was able to explain subtle differences in DOC dynamics between the two catchments mainly reflecting the distribution of saturated soils and available storage. We conclude that tracers and statistical time series models can successfully guide the development of parsimonious yet structurally consistent water quality models. Parsimonious models provide tools for estimating DOC dynamics and loads with reduced uncertainty and potentially greater transferability.

Effect of litter type on soil microbial parameters and dissolved organic carbon in a laboratory microcosm experiment

A laboratory microcosm experiment was conducted to evaluate the effects of the four single-species (Pinus tabulaeformis (Pt), Pinus radiata (Pr), Cercidiphyllum japonicum (Cj), and Ostryopsis davidiana (Od) litters from southwestern China and mixed pine-broadleaf (Pt + Cj, Pr + Cj, Pt + Od, Pr + Od) litters on soil microbial activities and dissolved organic carbon (DOC). Microcosms with the local typical soil and litterbags containing the eight litter types were incubated with 60% water field capacity for 84 days at 20°C. The results showed that the dynamics of soil microbial parameters and DOC were influenced by the litter types with different initial chemical quality. Due to their initial poor nutrient contents, the Pt and Pr litter treatments always showed lower soil microbial activities and DOC at each sampling compared with the Cj and Od litter treatments. However, compared with the single-species pine litter treatments, the inclusion of broadleaf Cj or Od litter into pine litter significantly increased soil microbial activities, and the concentrations of soil DOC during the whole incubation process. The current work thus provided a good implication for plantation management that it should be appropriate to consider Cj as an ameliorative species or retain Od in the pine plantations to improve soil conditions.

Cross-scale ensemble projections of dissolved organic carbon dynamics in boreal forest streams

Climate is an important driver of dissolved organic carbon (DOC) dynamics in boreal catchments characterized by networks of streams within forest-wetland landscape mosaics. In this paper, we assess how climate change may affect stream DOC concentrations ([DOC]) and export from boreal forest streams with a multi-model ensemble approach. First, we apply an ensemble of regional climate models (RCMs) to project soil temperatures and stream-flows. These data are then used to drive two biogeochemical models of surface water DOC: (1) The Integrated Catchment model for Carbon (INCA-C), a detailed process-based model of DOC operating at the catchment scale, and (2) The Riparian Integration Model (RIM), a simple dynamic hillslope scale model of stream [DOC]. All RCMs project a consistent increase in temperature and precipitation as well as a shift in spring runoff peaks from May to April. However, they present a considerable range of possible future runoff conditions with an ensemble median increase of 31 % between current and future (2061–2090) conditions. Both biogeochemical models perform well in describing the dynamics of present-day stream [DOC] and fluxes, but disagree in their future projections. Here, we assess possible futures in three boreal catchments representative of forest, mire and mixed landscape elements. INCA-C projects a wider range of stream [DOC] due to its temperature sensitivity, whereas RIM gives consistently larger inter-annual variation and a wider range of exports due to its sensitivity to hydrological variations. The uncertainties associated with modeling complex processes that control future DOC dynamics in boreal and temperate catchments are still the main limitation to our understanding of DOC mechanisms under changing climate conditions. Novel, currently overlooked or unknown drivers may appear that will present new challenges to modelling DOC in the future.

The effect of long-term water table manipulations on dissolved organic carbon dynamics in a poor fen peatland

Dissolved organic carbon (DOC) production, consumption, and quality displayed differences after long-term (~55 years) hydrological alterations in a poor fen peatland in northern Michigan. The construction of an earthen levee resulted in areas of a raised and lowered water table (WT) relative to an unaltered intermediate WT site. The lowered WT site had greater peat aeration and larger seasonal vertical WT fluctuations that likely elevated peat decomposition and subsidence with subsequent increases in bulk density, vertical hydraulic gradient, decreased hydraulic conductivity (Ksat), and a greater pore water residence time relative to the unaltered site. The raised WT site displayed a decreased Ksat combined with seasonal upwelling events that contributed to a longer residence time in comparison to the unaltered site. These differences are potentially contributing to elevated DOC concentrations at the lowered and raised WT site relative to the unaltered site. Additionally, spectrophotometric indices and chemical constituent assays indicated that the lowered site DOC was more aromatic and contained elevated concentrations of phenolics compared to the intermediate site. The raised site DOC was less aromatic, less humified, and also had a greater phenolic content than the intermediate site. Furthermore, an incubation experiment showed that DOC in the raised site contained the greatest labile carbon source. Based on our results, long-term WT alterations will likely impose significant effects on DOC dynamics in these peatlands; however, WT position alone was not a good predictor of DOC concentrations, though impoundment appears to produce a more labile DOC whereas drainage increases DOC aromaticity.

Intra‐annual variability of organic carbon concentrations in running waters: Drivers along a climatic gradient

AbstractTrends in surface water dissolved organic carbon (DOC) concentrations have received considerable scientific interest during recent decades. However, intra‐annual DOC variability is often orders of magnitude larger than long‐term trends. Unraveling the controls on intra‐annual DOC dynamics holds the key to a better understanding of long‐term changes and their ecological significance. We quantified and characterized intra‐annual DOC variability and compared it with long‐term DOC trends in 136 streams and rivers, varying in size and geographical characteristics, across a 1400 km latitudinal gradient during 2000–2010. Discharge, temperature, and month of the year were the most significant predictors of intra‐annual DOC variability in a majority of the running waters. Relationships between DOC, discharge, and temperature were, however, different along a mean annual temperature (MAT) gradient. Running waters with low MAT generally displayed positive DOC‐discharge correlations whereas the relationships in sites with higher MAT were more variable. This reflected contrasting relationships between temperature and discharge with discharge positively correlated with temperature in cold areas, while it was negatively correlated with temperature in catchments with higher MAT. Sites where flow, temperature, and month were poorly related to intra‐annual DOC dynamics were large catchments or sites with extensive upstream lake cover. DOC trends were generally much smaller than intra‐annual DOC variability and did not show any north‐south gradient. Our findings suggest that DOC in running waters could respond to a changing climate in ways not predictable, or even discernible, from extrapolation of recent interannual trends.

Environmental dynamics of dissolved black carbon in wetlands

Wetlands are ecosystems commonly characterized by elevated levels of dissolved organic carbon (DOC), and although they cover a surface area less than 2 % worldwide, they are an important carbon source representing an estimated 15 % of global annual DOC flux to the oceans. Because of their unique hydrological characteristics, fire can be an important ecological driver in pulsed wetland systems. Consequently, wetlands may be important sources not only of DOC but also of products derived from biomass burning, such as dissolved black carbon (DBC). However, the biogeochemistry of DBC in wetlands has not been studied in detail. The objective of this study is to determine the environmental dynamics of DBC in different fire-impacted wetlands. An intensive, 2-year spatial and temporal dynamics study of DBC in a coastal wetland, the Everglades (Florida) system, as well as one-time sampling surveys for the other two inland wetlands, Okavango Delta (Botswana) and the Pantanal (Brazil), were reported. Our data reveal that DBC dynamics are strongly coupled with the DOC dynamics regardless of location, season or recent fire history. The statistically significant linear regression between DOC and DBC was applied to estimate DBC fluxes to the coastal zone through two main riverine DOC export routes in the Everglades ecosystem. The presence of significant amounts of DBC in these three fire-impacted ecosystems suggests that sub-tropical wetlands could represent an important continental-ocean carrier of combustion products from biomass burning. The discrimination of DBC molecular structure (i.e. aromaticity) between coastal and terrestrial samples, and between samples collected in wet and dry season, suggests that spatially-significant variation in DBC source strength and/or degree of degradation may also influence DBC dynamics.

Lysimeter trials to assess the impact of different flood–dry-cycles on the dynamics of pore water concentrations of As, Cr, Mo and V in a contaminated floodplain soil

We hypothesize that the dynamics of water soluble arsenic (As), chromium (Cr), molybdenum (Mo), and vanadium (V) in soils might be controlled by the period of flooding due to changes of redox potential (EH), pH, and carriers of metals such as dissolved organic carbon (DOC), iron (Fe), manganese (Mn) and sulfate (SO42−). Therefore, we aimed to assess the impact of different flood–dry-cycles on the temporal dynamics of pore water concentrations of As, Cr, Mo and V as affected by changes of soil EH/pH and dynamics of DOC, Fe, Mn and SO42− in a contaminated floodplain soil collected at the Elbe River (Germany). For this purpose a specific groundwater lysimeter technique with two separate lysimeters which served as replicates was used. The groundwater level inside the lysimeters was controlled to simulate different flood–dry-cycles sequentially as follows: the long term (LT) includes 94days of flooding followed by similar drying term. The short term (ST) comprises 21days flooding followed by a similar drying term and was repeated six times. The entire experimental period (LT_ST) was about 450days. The presented data are mean values of both lysimeters.Flooding of the soil caused a significant decrease of EH and pH. Concentrations of soluble As, Cr, Fe, Mn, Mo, and DOC were higher under reducing conditions than under oxidizing conditions in LT. However, As and Cr tended to be mobilized under oxidizing conditions during ST, which might be due to slow kinetics of the redox reaction of As and Cr. Dynamics of Mo were more affected by changes of EH/pH as compared to As, Cr, and V and governed mainly by Fe–Mn chemistry. Concentrations of V in ST were higher than in LT and were controlled particularly by pH and chemistry of Fe. The interactions between the elements and carriers studied were stronger during long flood–dry-cycles than during short cycles, which confirmed our hypothesis. We conclude that the dynamics of As, Cr, Mo, and V are determined by the length of time soils are exposed to flooding, because drivers of element mobility need a certain time to provoke reactions in soils under changing conditions.

Monsoonal-type climate or land-use management: Understanding their role in the mobilization of nitrate and DOC in a mountainous catchment

Summary The linkage between hydrologic dynamics and the delivery of nitrate and DOC (dissolved organic carbon) to streams was studied in the Haean catchment, a mixed land-use mountainous catchment in South Korea. Three monsoonal precipitation events were analyzed, which varied in total rainfall amount (39–70 mm) and intensities (mean: 1.6–5.6 mm h −1 ), by high-resolution (2–4 h interval) stream water-quality sampling along the topographic elevation gradient of the catchment, from an upland deciduous forest stream, over areas intensively used for agriculture (dryland farming and rice paddies) down to the catchment outlet. The dynamics of river-aquifer exchange were investigated at two piezometer transects at mid and lower elevations. DOC and nitrate sources and their transport pathways to the receiving surface waters differed between the forested and the agricultural stream site. In the forest stream, elevated DOC concentrations (max: 3.5 mgC l −1 ) during precipitation events were due to hydrologic flushing of soluble organic matter in upper soil horizons, with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the forested stream occurred along shallow subsurface transport pathways. At the agricultural sites stream DOC concentrations were considerably higher (max: 23.5 mgC l −1 ) supplied from adjacent rice paddies. The highest in-stream nitrate concentrations (max: 4.1 mgN l −1 ) occurred at river reaches located in the lower agricultural part of the catchment, affected by groundwater inputs. Groundwater nitrate concentrations were high (max: 7.4 mgN l −1 ) owing to chemical fertilizer leaching from dryland fields forced by monsoonal rainfalls. Overall, this study demonstrates that the hydrologic dynamics resulting from the monsoonal climate drive the in-stream DOC dynamics in the forested 1st-order catchment whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the prevailing land-use type and irrigation management. Nitrate dynamics in higher order agricultural streams and their connected aquifers reflect combined effects of land-use type and monsoonal hydrology.

The role of irrigation runoff and winter rainfall on dissolved organic carbon loads in an agricultural watershed

We investigated the role of land use/land cover and agriculture practices on stream dissolved organic carbon (DOC) dynamics in the Willow Slough watershed (WSW) from 2006 to 2008. The 415km2 watershed in the northern Central Valley, California is covered by 31% of native vegetation and the remaining 69% of agricultural fields (primarily alfalfa, tomatoes, and rice). Stream discharge and weekly DOC concentrations were measured at eight nested subwatersheds to estimate the DOC loads and yields (loads/area) using the USGS developed stream load estimation model, LOADEST. Stream DOC concentrations peaked at 18.9mgL−1 during summer irrigation in the subwatershed with the highest percentage of agricultural land use, demonstrating the strong influence of agricultural activities on summer DOC dynamics. These high concentrations contributed to DOC yields increasing up to 1.29gm−2 during the 6 month period of intensive agricultural activity. The high DOC yields from the most agricultural subwatershed during the summer irrigation period was similar throughout the study, suggesting that summer DOC loads from irrigation runoff would not change significantly in the absence of major changes in crops or irrigation practices. In contrast, annual DOC yields varied from 0.89 to 1.68gm−2yr−1 for the most agricultural watershed due to differences in winter precipitation. This suggests that variability in the annual DOC yields will be largely determined by the winter precipitation, which can vary significantly from year to year. Changes in precipitation patterns and intensities as well as agricultural practices have potential to considerably alter the DOC dynamics.

Dynamics, chemical properties and bioavailability of DOC in an early successional catchment

Abstract. The dynamics of dissolved organic carbon (DOC) have been intensively studied in mature ecosystems, but little is known about DOC dynamics and the significance of DOC as a substrate for microbial activity in early-successional catchments. We determined the concentration, chemical composition, source, radiocarbon age, and bioavailability of DOC along the hydrological flow path from soil solution to a downstream pond in a recently constructed catchment (Chicken Creek Catchment, Germany). Soil solution, upwelling ground water, stream water, subsurface water in an alluvial fan, and pond water all had high DOC concentrations (averages: 6.0–11.6 mg DOC L–1), despite small carbon stocks in both vegetation and soil of the catchment. Solid-state CPMAS 13C NMR of DOC in upwelling ground water revealed a higher proportion of aromatic compounds (32%) and a lower proportion of carbohydrates (33%) than in pond water (18% and 45%, respectively). The average 14C age of DOC in upwelling ground water was 2600 to 2900 yr, while organic matter of the Quaternary substrate of the catchment had a 14C age of 3000 to 16 000 yr. Both the 14C age data and 13C NMR spectra suggest that DOC partly derived from organic matter of the Quaternary substrate (about 40 to 90% of the C in the DOC), indicating that both recent and old C of the DOC can support microbial activity during early ecosystem succession. However, in a 70 day incubation experiment, only about 11% of the total DOC was found to be bioavailable. This proportion was irrespective of the water type. Origin of the microbial communities within the catchment (enriched from soil, stream sediment or pond water) also had only a marginal effect on overall DOC utilization.

Differentiating the degradation dynamics of algal and terrestrial carbon within complex natural dissolved organic carbon in temperate lakes

It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO2 in 2 week lake water incubations and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and 10 times as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four times higher than labile algal DOC, accounting for almost the entire long‐term residual C metabolism, but also contributing to short‐term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO2 production even in more productive lakes.

Long-term nitrogen addition decreases carbon leaching in a nitrogen-rich forest ecosystem

Abstract. Dissolved organic carbon (DOC) plays a critical role in the carbon (C) cycle of forest soils, and has been recently connected with global increases in nitrogen (N) deposition. Most studies on effects of elevated N deposition on DOC have been carried out in N-limited temperate regions, with far fewer data available from N-rich ecosystems, especially in the context of chronically elevated N deposition. Furthermore, mechanisms for excess N-induced changes of DOC dynamics have been suggested to be different between the two kinds of ecosystems, because of the different ecosystem N status. The purpose of this study was to experimentally examine how long-term N addition affects DOC dynamics below the primary rooting zones (the upper 20 cm soils) in typically N-rich lowland tropical forests. We have a primary assumption that long-term continuous N addition minimally affects DOC concentrations and effluxes in N-rich tropical forests. Experimental N addition was administered at the following levels: 0, 50, 100 and 150 kg N ha−1 yr−1, respectively. Results showed that seven years of N addition significantly decreased DOC concentrations in soil solution, and chemo-physical controls (solution acidity change and soil sorption) rather than biological controls may mainly account for the decreases, in contrast to other forests. We further found that N addition greatly decreased annual DOC effluxes from the primary rooting zone and increased water-extractable DOC in soils. Our results suggest that long-term N deposition could increase soil C sequestration in the upper soils by decreasing DOC efflux from that layer in N-rich ecosystems, a novel mechanism for continued accumulation of soil C in old-growth forests.

Influence of stratification on marine dissolved organic carbon (DOC) dynamics: The Mediterranean Sea case

Vertical distributions of dissolved organic carbon (DOC) were determined in different seasons in the southern Adriatic and Tyrrhenian Seas to study the role of stratification in DOC patterns. These two seas are located at similar latitude but differ in extents of vertical stratification. Stratification affects DOC dynamics in both basins, but with interesting differences. In the Tyrrhenian Sea, where the upper water column was stratified during all cruises, DOC showed high surface layer values without substantial seasonality. By contrast, in the southern Adriatic Sea, the seasonal cycle of stratification forced opposite trends in the 0–50m and 50–800m stocks, with DOC removal from the upper 50m associated with DOC increase below 50m. Regarding DOC export via deep water formation in the southern Adriatic Sea, we estimate that 0.19TgCyr−1 was exported to the 50–800m layer by convective overturn, while 0.85–1.19TgCyr−1 was sequestered below 1000m due to a continental shelf pump mechanism. We hypothesize that enhanced stratification associated with a warmer ocean could further increase DOC concentrations in the mixed layer, changing the role of DOC in the oceanic carbon cycle.

Dissolved Organic Carbon Dynamics and Controls of Planted Slash Pine Forest Soil in Subtropical Region in Southern China

Soil dissolved organic carbon (DOC) is an active fraction of the soil organic carbon (SOC) pool and links terrestrial and aquatic systems. The degradation of DOC can affect carbon cycling, nutrient dynamics and energy supply to microorganism, and consequently change biogeochemical processes. This study investigated the vertical and seasonal variability of soil DOC concentrations and its controls in a 23-year-old planted slash pine (Pinus elliottii) forest at Qianyanzhou Forest Experimental Station (QFES) in Southern China. Soil solutions were collected at bimonthly intervals at depths of 10, 20 and 30 cm by a mechanical-vacuum extractor from November 2007 to March 2009, and at monthly intervals at depths of 10, 30 and 50cm from April 2009 to October 2010. The DOC concentrations were determined with a total organic carbon (TOC) analyzer. Mean (±standard deviation) DOC concentrations at depths of 10cm, 20cm, 30cm and 50cm were 12.4±4.4, 10.6±6.3, 8.7±2.6, and 8.0±5.9 mg L-1, respectively. Both seasonal and spring means of DOC concentration showed a decreasing trend with increasing depth, while there was no clear trend for the summer, autumn, or winter seasons. DOC concentrations during spring, summer, autumn and winter ranged from 4.8 to 21.5, 4.9 to 26.2, 5.4 to 17.1, 4.9 to 14.6 mg L-1, respectively, their mean DOC concentrations were 10.2, 10.5, 10.8 and 8.3 mg L-1, respectively. No consistent pattern of seasonal variability of DOC concentrations at different depths was observed. No obvious relationship between organic carbon content of forest litter and DOC concentration was found. There was a positive linear relationship between SOC and DOC concentration (R2=0.19, p<0.01), which showed that SOC was one of the main controls of DOC. A positive exponential relationship existed between soil temperature at 5 cm and DOC concentrations at 10 cm depth in slash pine, masson pine (Pinus massoniana) and Chinese fir (Cunninghamia lanceolate) planted forests (R2=0.12, p<0.01). DOC concentrations showed a negative linear relationship with soil moisture at all depths in slash pine forest (R2=0.15, p<0.001), and DOC concentrations at depth of 10 cm demonstrated a negative exponential relationship with soil moisture at 5 cm depth in three planted forests (R2=0.13, p<0.001). Precipitation in sampling months and mean seasonal DOC concentration were not correlated. However, a more detailed analysis of precipitation events at different times before sampling and seasonal DOC concentration showed that the timing of precipitation events prior to sampling had different effects on seasonal DOC concentrations at different depths. Our study highlights the importance of DOC dynamics for the carbon cycle in planted slash pine forest and it provides evidence for evaluating the effects of ecological restoration in subtropical red soil region.

Controls on dissolved organic carbon quantity and chemical character in temperate rivers of North America

Understanding the processes controlling the transfer and chemical composition of dissolved organic carbon (DOC) in freshwater systems is crucial to understanding the carbon cycle and the effects of DOC on water quality. Previous studies have identified watershed‐scale controls on bulk DOC flux and concentration among small basins but fewer studies have explored controls among large basins or simultaneously considered the chemical composition of DOC. Because the chemical character of DOC drives riverine biogeochemical processes such as metabolism and photodegradation, accounting for chemical character in watershed‐scale studies will improve the way bulk DOC variability in rivers is interpreted. We analyzed DOC quantity and chemical character near the mouths of 17 large North American rivers, primarily between 2008 and 2010, and identified watershed characteristics that controlled variability. We quantified DOC chemical character using both specific ultraviolet absorbance at 254 nm (SUVA254) and XAD‐resin fractionation. Mean DOC concentration ranged from 2.1 to 47 mg C L−1 and mean SUVA254 ranged from 1.3 to 4.7 L mg C−1 m−1. We found a significant positive correlation between basin wetland cover and both bulk DOC concentration (R2 = 0.78; p &lt; 0.0001) and SUVA254 (R2 = 0.91; p &lt; 0.0001), while other land use characteristics were not correlated. The strong wetland relationship with bulk DOC concentration is similar to that found by others in small headwater catchments. However, two watersheds with extremely long surface water residence times, the Colorado and St. Lawrence, diverged from this wetland relationship. These results suggest that the role of riverine processes in altering the terrestrial DOC signal at the annual scale was minimal except in river systems with long surface water residence times. However, synoptic DOC sampling of both quantity and character throughout river networks will be needed to more rigorously test this finding. The inclusion of DOC chemical character will be vital to achieving a more complete understanding of bulk DOC dynamics in large river systems.

Dissolved organic matter uptake in a temperate seagrass ecosystem

We assessed the utilization of inorganic and organic nitrogen compounds of different complexity by primary producers and bacteria in a seagrass ecosystem. Using double-labeled ( 13 C and 15 N) substrates, the net transfers from the dissolved nitrogen and carbon pools to phytoplank- ton, planktonic bacteria, epiphytes, seagrasses (Zostera noltii and Cymodocea nodosa), and a macroalga (Caulerpa prolifera) were quantified in field incubations. Phytoplankton was the largest nitrogen sink, followed by the epiphytic community. In contrast, carbon fixation was dom- inated by the macrophytes. Although compartment-specific variations existed, NH4 + was gener- ally preferred over NO3 � and urea. Specific uptake rates of individual amino acids were inversely related to their C:N ratio and their structural complexity (glycine > L-leucine > L-phenylalanine). In addition, biomarker-specific measurements (polar lipid-derived fatty acids and D-alanine) indi- cated increasing bacterial contributions to carbon uptake with increasing amino acid structural complexity. All primary producers acquired nitrogen from a complex pool of algae-derived dis- solved organic matter (DOM), but algae-derived dissolved organic carbon (DOC) was almost exclusively used by the planktonic compartment. In contrast, a similar complex pool of bacteria- derived DOM was not taken up in significant quantities by any of the primary producers. Our results illustrate that dissolved organic nitrogen (DON) can provide bioavailable nitrogen to pri- mary producers and bacteria in seagrass ecosystems on very short time-scales, and DON and DOC dynamics are largely uncoupled and must be investigated as 2 separate interacting pools.

Dissolved organic carbon characteristics in surface ponds from contrasting wetland ecosystems: a case study in the Sanjiang Plain, Northeast China

Abstract. Dissolved organic carbon (DOC) is a significant component of carbon and nutrient cycling in fluvial ecosystems. Natural wetlands, as important DOC sources for river and ocean ecosystems, have experienced extensive natural and anthropogenic disturbances such as climate change, hydrological variations and land use change in recent years. The DOC characteristics in surface ponds from contrasting wetlands in the Sanjiang Plain, Northeastern China were investigated. Surface ponds at seven sites (two natural phialiform wetlands, three natural riparian wetlands, one degraded wetland and one artificial wetland, i.e., rice paddy) were monitored during the growing seasons of 2009 and 2010. The results show that the surface ponds at the five natural wetland sites exhibited a wide range of DOC concentrations (10.06–48.73 mg L−1) during the study period. The DOC concentrations showed no annual differences (P &gt; 0.05) at all the wetland sites, except one of the phialiform wetland sites. The two phialiform wetlands exhibited higher DOC concentrations than the three riparian wetlands (P &lt; 0.05). The DOC concentration in the surface pond at the artificial wetland site was relatively low (P &lt; 0.05) compared to that at the degraded wetland site. The C/C ratios (the color per carbon unit ratio, Abs400/DOC concentration) showed inconsistent variations among these seven wetland sites, while the E4/E6 ratio (Abs465/Abs665, fulvic acid/humic acid) from the surface pond in the rice paddy land exerted 42.07–55.36% reductions (P &lt; 0.05), compared to those at the five natural wetland sites. Furthermore, the E4/E6 ratio in the surface pond at the rice paddy site was significantly lower compared to that at the degraded wetland site (P &lt; 0.05), which indicated that disturbance to wetland DOC in surface ponds might be stronger when natural wetlands were converted to rice paddies in comparison with wetland degradation. This study could not only provide insightful points for understanding the aquatic DOC dynamics from different wetland ecosystems, but also support data information for incorporating the aquatic DOC into the model for regional carbon budgets in the future.

Simulation of monthly dissolved organic carbon concentrations in small forested watersheds

Dissolved organic carbon (DOC) plays an important role in both terrestrial and aquatic ecosystems. In-stream DOC concentration is an important indicator of water quality, because high in-stream DOC concentrations naturally lead to reductions in dissolved oxygen. Recent studies indicate that DOC is responsible in transferring toxic metals (e.g., mercury, Hg) from terrestrial sources to aquatic ecosystems. However, assessing DOC or Hg dynamics is fairly challenging as a result of extreme spatiotemporal variation. Typically, process-based models with short-to-medium term temporal resolutions are required to model DOC and Hg dynamics. The objective of this research is to develop a watershed-based monthly DOC production and export model that integrates the treatment of (i) forest-litter decomposition (DOC production), (ii) wetland-to-watershed area ratio (DOC storage), and (iii) relevant hydrological processes (DOC export) in the simulation of in-stream concentrations. Model results are compared against DOC concentrations collected at the stream outlets of two separate forested watersheds in Kejimkujik National Park, Atlantic Canada. Comparisons for the two watersheds show that predicted monthly DOC concentrations are generally in good agreement with field-based concentrations, giving R2-values of 0.61 and 0.63 for Pine Marten Brook watershed and 0.72 and 0.75 for Moose Pit Brook watershed for model calibration and validation, respectively.