Annual Dissolved Organic Carbon Concentrations Research Articles

Dissolved organic carbon response to hydrological drought characteristics: Based on long-term measurements of headwater streams

Influence of extreme hydrological events on water quality has been widely concerned. For instance, droughts can inhibit dissolved organic carbon (DOC) exports or imports. However, the response relationship of DOC to hydrological drought characteristics (i.e., duration and severity) requires more in-depth research. We propose an integrated framework for constructing, validating, and applying the response relationship model, and investigate the capability of response model to simulate DOC based on hydrological drought characteristics. Three headwater basins (HP3a, HP4, and HP6), with different drainage areas (9.28–122.80 ha) and long-term (>40 year) observed DOC concentration and hydrometeorological data, in Harp Lake catchment, south-central Ontario, southeastern Canada, are used to demonstrate the proposed framework. Run theory and variable drought thresholds (VDTs) are used to identify hydrological drought characteristics, and DOC during hydrological drought is extracted. Based on the extracted hydrological drought characteristics and DOC for one basin (i.e., HP3a), the response relationship model is constructed and validated, and then applied to other two basins (i.e., HP4 and HP6). Three evaluation indicators: coefficient of determination (R2), root-mean-square-error (RMSE), and mean absolute percentage error (MAPE), are served to test the goodness-of-fit performance of the response relationship model. The results show that (i) annual DOC concentration showed a significant (a = 0.01) increasing trend during 1978–2018 in the study basin. (ii) During the hydrological drought, the variation of temperature affected DOC variation indirectly through direct influence on SO4 variation. (iii) The response sensitivity of DOC to hydrologic process with different timescales is varying within a year, namely, there is a larger response sensitivity from March to May than in other months. (iv) DOC during the hydrological drought has a close and regular linear relationship with hydrological drought characteristics, i.e., with the increase of drought duration and severity, DOC concentration also increases. The relationship with drought duration is better than that of severity (R2 = 0.92 vs 0.35). (v) The response relationship model (autoregressive integrated moving average) can simulate DOC in hydrological drought (R2 ≥ 0.87, RMSE ≤ 0.86, MAPE ≤ 13.69%) at HP3a, and also has good applications at HP4 and HP6 basins. These results provide an improved understanding of DOC-drought relationship, and may support policy makers that look for increased resilience of aquatic ecological security to droughts.

Increased Dissolved Organic Carbon Concentrations in Peat‐Fed UK Water Supplies Under Future Climate and Sulfate Deposition Scenarios

AbstractPeatlands are globally‐important terrestrial carbon stores as well as regional sources of potable water supply. Water draining from peatlands is rich in dissolved organic carbon (DOC), which can be problematic for water treatment. However, it is unclear how future climate and sulfate deposition changes may impact DOC in peatland‐derived potable water. The United Kingdom (UK) is a global hotspot that consumes 79% of all potable water derived directly from peatlands. Here, a physically‐based hydrological model and a biogeochemical organic carbon model were used to predict discharge and DOC concentration in nine hotspots of peatland‐derived potable water use in the UK under a range of 21st century climate and sulfate deposition scenarios. These nine catchments supply 72% of all peatland‐derived water consumed in the UK and 57% of the global total, equivalent to the total domestic consumption of over 14 million people. Our simulations indicate that annual discharges will decrease and that mean annual DOC concentrations will increase under all future scenarios (by as much as 53.4% annually for the highest emissions scenario) in all catchments. Large increases (by as much as a factor of 1.6) in DOC concentration in the 2090s over the baseline period are projected for autumn and winter, seasons when DOC concentrations are already high in the baseline datasets such that water treatment works often reach their capacity to cope. The total DOC flux is largely insensitive to future climate change because the projected increase in DOC concentration is mostly counterbalanced by the projected decrease in discharge.

The Spatial and Temporal Distribution of Dissolved Organic Carbon Exported from Three Chinese Rivers to the China Sea.

The lateral transport of dissolved organic carbon (DOC) plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. Neglecting the lateral flow of dissolved organic carbon can lead to an underestimation of the organic carbon budget of terrestrial ecosystems. It is thus necessary to integrate DOC concentrations and flux into carbon cycle models, particularly with regard to the development of models that are intended to directly link terrestrial and ocean carbon cycles. However, to achieve this goal, more accurate information is needed to better understand and predict DOC dynamics. In this study, we compiled an inclusive database of available data collected from the Yangtze River, Yellow River and Pearl River in China. The database is collected based on online literature survey and analysed by statistic method. Overall, our results revealed a positive correlation between DOC flux and discharge in all three rivers, whereas the DOC concentration was more strongly correlated with the regional net primary productivity (NPP). We estimated the total DOC flux exported by the three rivers into the China Sea to be approximately 2.73 Tg yr-1. Specifically, the annual flux of DOC from the Yangtze River, Yellow River and Pearl River was estimated to be 1.85 Tg yr-1, 0.06 Tg yr-1 and 0.82 Tg yr-1, respectively, and the average annual DOC concentrations were estimated to be 2.24 ± 0.53 mg L-1, 2.70 ± 0.38 mg L-1 and 1.51 ± 0.09 mg L-1, respectively. Seasonal variations in DOC concentrations are greatly influenced by the interaction between temperature and precipitation. NPP is significantly and positively related to the DOC concentration in the Yangtze River and the Pearl River. In addition, differences in climate and the productivity of the vegetation may influence both the flux and concentrations of DOC transported by the rivers and thus potentially affect estuarine geochemistry.

Dry‐season length and runoff control annual variability in stream DOC dynamics in a small, shallow groundwater‐dominated agricultural watershed

AbstractAs a phenomenon integrating climate conditions and hydrological control of the connection between streams and terrestrial dissolved organic carbon (DOC) sources, groundwater dynamics control patterns of stream DOC characteristics (concentrations and fluxes). Influence of intra‐annual variations in groundwater level, discharge and climatic factors on DOC concentrations and fluxes were assessed over 13 years at the headwater watershed of Kervidy‐Naizin (5 km2) in western France. Four seasonal periods were delineated within each year according to groundwater fluctuations (A: rewetting, B: high flow, C: recession, and D: drought). Annual and seasonal base flow versus stormflow DOC concentrations were defined based on daily hydrograph readings. High interannual variability of annual DOC fluxes (5.4–39.5 kg ha−1 yr−1) indicates that several years of data are required to encompass variations in water flux to evaluate the actual DOC export capacity of a watershed. Interannual variability of mean annual DOC concentrations was much lower (4.9–7.5 mg C L−1), with concentrations decreasing within each year from ca. 9.2 mg C L−1 in A to ca. 3.0 mg C L−1 in C. This indicates an intra‐annual pattern of stream DOC concentrations controlled by DOC source characteristics and groundwater dynamics very similar across years. Partial least squares regressions combined with multiple linear regressions showed that the dry season characteristics (length and drawdown) determine the mean annual DOC concentration while annual runoff determines the annual flux. Antagonistic mechanisms of production‐accumulation and dilution‐depletion combined with an unlimited DOC supply from riparian wetland soils can mitigate the response of stream concentrations to global changes and climatic variations.

Effects of climate and land-surface processes on terrestrial dissolved organic carbon export to major U.S. coastal rivers

This study aims to understand the influences of climate change and land surface processes on the variation of in-stream DOC concentrations in coastal rivers crossing different climate zones. Monthly observations spanning multiple years in seven major rivers in four different climate zones within the U.S. were analyzed for correlations between dissolved organic carbon (DOC) concentration and surface air temperature, precipitation, land cover and discharge. The major watersheds were the Altamaha River (GA), the Apilachicola River (FL), the Columbia River (OR), the Delaware River (NJ), the Sacramento and San Joaquin Rivers (CA) and the Susquehanna River (MD). One minor watershed, the Neponset River (MA) was also analyzed. Results indicate that temperature is the most important variable for DOC export when the variation of annual mean temperatures is large (e.g., >5°C) with sufficient precipitation levels. Land-surface characteristics and discharges are better correlated to DOC concentrations when the variations of annual mean temperatures are small (e.g. <2°C). However, results from the small watershed (Neponset) showed that land surface processes can vary annual DOC concentrations about ±1.65mg/L from mean value. This study is the first to examine DOC relationships in watersheds in multiple climate zones, and it was determined that weak correlations between temperature and DOC found in previous studies may be attributable to the fact that those studies examined small watersheds contained within a single climate zone. DOC flux per square meter was calculated based on incremental temperature increases. The results indicate that an increase of 1°C would result in a 0.476mg/L increase of in-stream DOC in large watersheds. Climate warming would have a greater impact on riverine DOC yields in cooler climate zones (up to 26% per °C) than on those in warmer climate zones (up to 6% per °C).

Dissolved organic carbon and nitrogen in urban and rural watersheds of south-central Texas: land use and land management influences

Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were quantified in urban and rural watersheds located in central Texas, USA between 2007 and 2008. The proportion of urban land use ranged from 6 to 100% in our 12 study watersheds which included nine watersheds without waste water treatment plants (WWTP) and three watersheds sampled downstream of a WWTP. Annual mean DOC concentrations ranged 20.4–52.5 mg L−1. Annual mean DON concentrations ranged 0.6–1.9 mg L−1. Only the rural watersheds without a WWTP had significantly lower DOC concentrations compared to those watersheds with a WWTP but all the streams except two had significantly reduced DON compared to those with a WWTP. Analysis of the nine watersheds without a WWTP indicated that 68% of the variability in mean annual DOC concentration was explained by urban open areas such as golf courses, sports fields and neighborhood parks under turf grass. There was no relationship between annual mean DON concentration and any land use. Urban open area also explained a significant amount of the variance in stream sodium and stream sodium adsorption ratio (SAR). Ninety-four percent of the variance in annual mean DOC concentration was explained by SAR. Irrigation of urban turf grass with domestic tap water high in sodium (>181 mg Na+ L−1) may be inducing sodic soil conditions in watershed soils in this region resulting in elevated mean annual DOC concentrations in our streams.

Spatial and temporal variation of dissolved organic carbon export from gauged and ungauged watersheds of Dee Valley, Scotland: Effect of land cover and C:N

Dissolved organic carbon (DOC) concentration and export were quantified in 56 streams within the Dee Valley, Scotland. Percentage land cover was calculated for each of the watersheds. For a subset of 21 watersheds, organic and mineral soil layers were collected for carbon and nitrogen analysis. Peat cover derived from land cover data was a good predictor of annual DOC export for the selected 21 upland watersheds but not for the set of 56 watersheds. Using DOC values and mean watershed C:N ratio, we produced an empirical predictive model for estimation of annual DOC export for the 21 watersheds as well as a model of weekly fluxes in two of the larger watersheds to meet the needs of watershed managers. Annual exports for the 56 streams in the Dee Valley ranged from 1700 to 10,500 kg km−2 yr−1. Land cover was not related to annual DOC export for the 56 streams, but by season, arable land and grassland were both correlated to spring DOC export (R = −0.39 arable land and R = −0.33 grassland p&lt;0.01). Summertime DOC export was correlated to heather, montane, and conifer land cover (R = −0.029 heather, R = −0.39 montane, and R = 0.33 conifer, p&lt;0.05). Autumn and winter DOC export was best described by peat (R = 0.29 and 0.39, respectively, p&lt;0.05). In the subset of 21 watersheds with soil information, watershed soil organic matter C:N ratio and peat had a higher correlation with annual DOC concentrations and export than any other land cover class. Source of peat distribution data may impact the relationship between DOC and peat cover, and this issue is examined. Modeled weekly DOC load compared well with observed DOC load in a time series evaluation.

13C composition of dissolved organic carbon in upland forested catchments of the Morvan Mountains (France): Influence of coniferous and deciduous vegetation

One of the main environmental changes caused by human activities is that of land use. These changes influence the quantity and quality of the dissolved organic matter (DOM) fluxes through the vegetation–soil–stream system. The aim of this work is to evaluate the influence of the substitution of native deciduous forests by well managed coniferous forests on dissolved organic carbon (DOC) fluxes and their associated carbon isotopic composition (δ13CDOC). DOC fluxes and δ13CDOC were monitored for 2 years in the streams of four similar upland forested catchments in the Morvan Mountains (France). Mean annual DOC concentrations and fluxes were 2–4 times lower in streams in catchments with predominant coniferous vegetation than in those with deciduous vegetation. δ13CDOC values were lower by 0.5–1.0‰ in the stream under deciduous vegetation (−28.3‰ to −30‰) than in the other streams (−27.1‰ to −29.5‰). Furthermore, stream DOC was 13C depleted compared to the isotopic composition of the soil organic matter (SOM). This 13C depletion was higher under deciduous cover where SOM was enriched in 13C with increasing depth, producing 13C depleted solute fraction. Under coniferous cover, no significant isotopic differentiation was observed between the solute and solid phase in the soil. Our results show that the δ13CDOC in stream is marked by the vegetation cover because of differences in the degradation pathway and dynamics of SOM. Besides, it means that the substitution of deciduous forests by coniferous plantations may significantly modify the chemical composition of DOC in streams.

Budgets and chemistry of dissolved organic carbon in forest soils: effects of anthropogenic soil acidification

Budgets and chemistry of dissolved organic carbon (DOC) are compared at three spruce stands receiving different rates of acid deposition. Generally, an increasing input of mineral acids is associated with increased soil acidification and an increased degree of forest decline phenomena. In order to assess the effects of anthropogenic soil acidification on DOC control, we combined measurements of DOC concentrations and fluxes with an analytical approach in terms of DOC fractionation, wet-chemical degradation, and pyrolysis-field ionization mass spectrometry. Mean annual DOC concentration in the mineral soil input varies around 27.0 mg 1 −1 at the less damaged stands, and reaches 59.4 mg 1 −1 at the severely damaged stand. We suggest a biotic control of DOC release in the forest floor, which is in turn influenced by environmental conditions. Enhanced anthropogenic N deposition and crown thinning increase the DOC release in the forest floor due to a stimulation of microbial organic matter mineralization. Furthermore, N hampers the ligninase production which results in the accumulation of partly water-soluble soft-rot lignin-degredation products. Sorption of DOC by sesquioxides in the B horizons reduces the mean annual DOC concentration to values between 2.63 mg 1 −1 in the least acidified soil and 31.15 mg 1 −1 in the most acidified soil. In this soil DOC retention is impeded by soil indigenous factors such as covering of sesquioxides with organic carbon. Moreover, anthropogenic soil acidification results in an increasing DOC output of the mineral soil due to (i) elevated DOC release in the forest floor, (ii) competition of DOC with sulfate for sorption sites, and (iii) loss of Al and Fe oxides/hydroxides due to buffer processes. The less acidified mineral soils are characterized by a pronounced preferential retention of polymeric hydrophobic acids. In contrast, the soil receiving the highest rates of acid deposition shows only a minor qualitative alteration of DOC in the profile.

Dissolved organic carbon control in acid forest soils of the Fichtelgebirge (Germany) as revealed by distribution patterns and structural composition analyses

Dissolved organic carbon (DOC) was investigated in different compartments of three acid forest soils of the Fichtelgebirge (northeastern Bavaria, Germany) developed under Norway spruce stands. Soil acidification increases from Wülfersreuth (Wü) over Oberwarmensteinach (Ow) to Hohe Matzen (HM). In order to assess factors of DOC control, DOC concentrations and fluxes in the compartments were measured. The structural composition of DOC was evaluated by using wet-chemical methods, CPMAS 13C NMR spectroscopy, pyrolysis-field ionization mass spectrometry, and analytical DOC fractionation. DOC is mainly mobilized in the forest floor; it exhibits high spatial and temporal variation in concentrations and fluxes. Mean annual DOC concentrations of the mineral soil input vary between 2.25 mmol C l −1 in the less acid soils Wü and Ow and 4.53 mmol C l −1 in the highly acidified HM soil. Chemical investigations suggest a biotic control of DOC release, which is in turn influenced by environmental factors. Mobilized DOC consists of highly degraded plant material and microbially synthesized products, but still occurs mainly as macromolecules. Enhanced anthropogenic N inputs and crown thinning seem to accelerate the DOC release in the HM forest floor due to stimulated microbial activity. Furthermore, solubility of organic carbon is enhanced by the decline of divalent and polyvalent cation bridging of organic compounds due to exchange processes with protons. Contact of the soil solution with the mineral B horizons drastically reduce the DOC concentrations to 0.14–0.84 mmol C l −1 mainly by abiotic mechanisms (precipitation, sorption by Fe- and Al-oxides). Between 14.1 and 32.5 g C m −2 yr −1 are translocated into the B horizons. DOC in the mineral soil output corresponds well with DOC in the headwater stream. While in the less acidified soils Wü and Ow most of the DOC is retained in the Bs horizon, the highly podzolized HM soil shows a lower DOC retention capacity over the whole solum and still exhibits an unusually high DOC output of 6.6 g C m −2 yr −1. It is suggested that the elevated content of organic carbon surrounding the oxides, and competition of sulfate with components of DOC for sorption sites is partly responsible for the reduced DOC retention in this soil.

Retention of Dissolved Organic Carbon and Sulfate in Aggregated Acid Forest Soils

AbstractAdsorption of dissolved organic carbon (DOC) and sulfate was examined in mineral A and B horizons of aggregated acid forest soils from declining (Oberwarmensteinach) and healthy (Wülfersreuth) Norway spruce [Picae abies (L.) Karst.] stands in northeastern Bavaria, Germany. Adsorption studies were conducted in the laboratory on undisturbed, aggregated soil samples under nonequilibrium conditions using a miscible displacement method. Initial mass relationships suggest a net release of DOC in the A horizons of both stands at all DOC concentration levels added. In contrast, the B horizons retained DOC from solutions containing more than 4 to 5 g C m−3 in the Oberwarmensteinach B horizon, and 2 to 3 g m−3 in the Wülfersreuth B horizon, respectively. These concentrations correspond to mean annual DOC concentrations in the B horizon solutions of both sites. Addition of sulfate (1.60 molc SO4 m−3) significantly reduced DOC retention, suggesting competition between SO4 and parts of DOC for adsorption sites. Retention of sulfate from solution occurred concentrations with more than 0.38 to 0.42 molc SO4 m−3 in Oberwarmensteinach, and 0.25 to 0.31 molc SO4 m−3 in Wülfersreuth, respectively. Compared with batch experiments, the miscible displacement method resulted in lower affinities of the soils for DOC. Reduction of DOC and sulfate retention by the use of the flow‐generated technique is due to the consideration of sorption kinetics, which are influenced by soil structure and texture. Although this method underestimates the affinity of a soil for DOC to a certain extent, we suggest that it reflects the situation in the field quite well. Flow‐generated experiments showed that the decline phenomena of spruce in Oberwarmensteinach correspond with a reduced potential for DOC retention in the upper B horizon, as well as with lower sulfate retention throughout the profile.