Stable Dissolved Organic Carbon Research Articles

In-reservoir transformation of dissolved organic matter as a function of hydrological flow

Reservoirs are vital to meet the ever-increasing demands for freshwater in a warming climate. Dissolved organic matter (DOM) represents an important pool of carbon and can be a major concern in drinking water sources. However, insights into DOM dynamics in temperate, semi-arid reservoirs remain limited. Therefore, we investigated the variations in DOM properties in Lake Diefenbaker, a large reservoir on the Canadian Prairies, by analyzing eight years of DOM concentrations and composition through linear mixed effect modeling. Contrary to expectations, reservoir dissolved organic carbon (DOC) concentration showed no correlation with inflow from the South Saskatchewan River (p = 0.12), while dissolved organic nitrogen (DON) increased with decreasing inflow (p = 0.002). DOM optical indices (SUVA254 and E4:E6 ratio) and DOC:DON ratio revealed a pronounced influence of inflow on reservoir DOM composition (p < 0.001), i.e., allochthonous characteristics increased with increasing flow, and autochthonous characteristics increased with declining flow. Travel time corrected comparison of approximately the same water parcel along the reservoir length revealed that increasing water residence time in downstream regions led to a significant transformation in DOM composition, favoring autochthonous characteristics (mean SUVA254 reduced by 0.52 L mg-C−1 m−1, and the E4:E6 and spectral slope ratio increased by 1.6 and 0.06, respectively). Autochthonous DOC inputs likely offset the allochthonous DOC losses, which resulted in a relatively stable DOC concentration throughout the reservoir (mean 3.7 mg L−1). Additionally, the effect of a large aquaculture operation on reservoir DOM properties was investigated, but no effect was detected. The results have significant implications for managing large river-reservoirs. Autochthonous DOM poses challenges to water processing, necessitating monitoring of DOM composition for reservoir drinking water quality. Insights on climate-induced changes in DOM properties will also assist with understanding changes to habitat conditions and contaminant transport.

Effects of different regeneration patterns on soil dissolved organic matter degradation in Castanopsis carlesii forests of subtropical China

Biodegradability of dissolved organic matter (DOM) affects stabilization and mineralization of soil organic matter, which is of great significance to soil nutrient cycling. In order to explore the effects of forest regeneration on soil DOM degradation, soil DOM solution was sampled in a natural Castanopsis carlesii forest (NF), a secondary forest of C. carlesii (SF), and an artificial-assisted regeneration forest of C. carlesii (AR) in a sub-tropical area and conducted 42-day laboratory incubation. The results showed that: 1) both the degradation rate of soil dissolved organic carbon (DOC) and the ratio of labile DOC were as follows: SF>AR>NF; dissolved organic nitrogen (DON) and microbial biomass carbon (MBC) are the factors significantly affecting the ratio of labile DOC; 2) stable DOC accounted for the majority of soil DOC in all the three forest types (72.3%-94.6%), which had long turnover time and contributed to the formation of stable soil organic matter (SOC); 3) the initial humification index in emission mode (HIXem) of soil DOM would affect the turnover time of labile DOC. The spectral structure of DOM changed dynamically during the degradation process, indicating that microorganism would turn to degrade aromatic and hydrophobic fractions for carbon source after the depletion of labile DOM. Overall, the transformation from NF of C. carlesii into SF and AR could increase the proportion of the easily degradable DOC, and enhance the biodegradability of soil DOM, which were not conducive to the accumulation of SOC.

Dissolved Organic Carbon Turnover in Permafrost-Influenced Watersheds of Interior Alaska: Molecular Insights and the Priming Effect

Increased permafrost thaw due to climate change in northern high-latitudes has prompted concern over impacts on soil and stream biogeochemistry that affect the fate of dissolved organic carbon (DOC). Few studies to-date have examined the link between molecular composition and biolability of dissolved organic matter (DOM) mobilized from different soil horizons despite its importance in understanding carbon turnover in aquatic systems. Additionally, the effect of mixed DOM sources on microbial metabolism (e.g., priming) is not well understood. No studies to-date have addressed potential priming effects in northern high-latitude or permafrost-influenced aquatic ecosystems, yet these ecosystems may be hot spots of priming where biolabile, ancient permafrost DOC mixes with relatively stable, modern stream DOC. To assess biodegradability and priming of DOC in permafrost-influenced streams, we conducted 28 day bioincubation experiments utilizing a suite of stream samples and leachates of fresh vegetation and different soil horizons, including permafrost, from Interior Alaska. The molecular composition of unamended DOM samples at initial and final time points was determined by ultrahigh resolution mass spectrometry. Initial molecular composition was correlated to DOC biodegradability, particularly the contribution of energy-rich aliphatic compounds, and stream microbial communities utilized 50-56% of aliphatics in permafrost-derived DOM within 28 days. Biodegradability of DOC followed a continuum from relatively stable stream DOC to relatively biolabile DOC derived from permafrost, active layer organic soil, and vegetation leachates. Microbial utilization of DOC was ~3-11% for stream bioincubations and ranged from 9% (active layer mineral soil-derived) to 66% (vegetation-derived) for leachate bioincubations. To investigate the presence or absence of a priming effect, bioincubation experiments included treatments amended with 1% relative carbon concentrations of simple, biolabile organic carbon substrates (i.e., primers). The amount of DOC consumed in primed treatments was not significantly different from the control in any of the bioincubation experiments after 28 days, making it apparent that the addition of biolabile permafrost-derived DOC to aquatic ecosystems will likely not enhance the biodegradation of relatively modern, stable DOC sources. Thus, future projections of carbon turnover in northern high-latitude region streams may not have to account for a priming effect.

Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Abstract. Oceanic dissolved organic carbon (DOC) represents one of the largest carbon reservoirs on Earth, and its distribution and biogeochemical cycles play important roles in carbon cycling and other biogeochemical processes in the ocean. We report the distribution and concentrations of DOC for water samples collected from the shelf-edge and slope regions in the East China Sea (ECS) and the Kuroshio Extension (KE) in the northwestern North Pacific during two cruises in 2014–2015. The DOC concentrations were 45–88 µM in the ECS and 35–65 µM in the KE. In addition to biological processes that are estimated to account for 7 % and 8 %–20 % in shaping the DOC distribution in the ECS and KE regions, respectively, the DOC distribution is largely controlled by hydrodynamic mixing of different water masses. By comparing the DOC results with dissolved inorganic carbon (DIC) and dissolved inorganic radiocarbon (Δ14C-DIC) measured from the same water samples, we further demonstrate that the intrusion of the Kuroshio Current could dilute the DOC concentrations at stations in the outer shelf and slope regions of the ECS. The concentrations of DOC in the KE were significantly lower in surface waters than in the ECS, and a relatively low and stable DOC level (∼40 µM) was found in deep water (below 1500 m) at all stations. Based on the previously reported DIC and Δ14C-DIC values for the stations, the observed spatial variations of DOC in the upper 700 m among the stations in the KE were mainly influenced by mixing of the two water masses carried by the Kuroshio and Oyashio, the two dominant western boundary currents in the region. The hydrodynamic processes are thus important factors in the distribution and dynamics of DOC in the KE region.

An Assessment of Dissolved Organic Carbon Biodegradability and Priming in Blackwater Systems

AbstractRivers deliver approximately 260 Tg of dissolved organic carbon (DOC) to the ocean annually, yet there is little evidence of terrigenous DOC (tDOC) in the ocean. While tDOC was historically believed to be stable and resistant to microbial degradation, it has recently been shown that freshwater systems mineralize more tDOC than originally thought. The priming effect is a possible mechanism by which inputs of biolabile DOC enhance the bioavailability of stable DOC components in aquatic systems, resulting in higher rates of microbial remineralization. Here we investigate tDOC biodegradability by conducting bioincubation experiments and utilizing ultrahigh resolution mass spectrometry to characterize the chemical composition of blackwater and leachate dissolved organic matter (DOM) samples. The role of priming in blackwater ecosystems was assessed through the inclusion of bioincubation treatments amended with a variety of simple biolabile OC substrates. Blackwaters lost 6.10 ± 3.85% DOC within 1 month, while leachates lost 38.10 ± 16.74% DOC. There were no significant differences between DOC remineralization in control and primed treatments, indicating that priming is not an important factor in the biodegradation of DOC in blackwater ecosystems. However, the proportion of biodegradable DOC and DOM composition were significantly correlated, mostly driven by the contribution of aliphatic compounds (H/C ≥ 1.5, O/C &lt; 0.9) that were abundant (9.3 ± 5.2%) in leachate DOM. The molecular signature of biodegraded leachate DOM resembled that of stable blackwater DOM, indicating that bioavailable DOM components leached from plant litter are rapidly utilized and stable DOM is exported downstream.

A comparison of extraction procedures for water‐extractable organic matter in soils

SummaryThe characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.

Modeling priming effects on microbial consumption of dissolved organic carbon in rivers

AbstractRivers receive and process large quantities of terrestrial dissolved organic carbon (DOC). Biologically available (unstable) DOC leached from primary producers may stimulate (i.e., prime) the consumption of more stable terrestrially derived DOC by heterotrophic microbes. We measured microbial DOC consumption (i.e., decay rates) from contrasting C sources in 10 rivers in the western and Midwestern United States using short‐term bioassays of river water, soil and algal leachates, glucose, and commercial humate. We added inorganic nutrients (ammonium and phosphorus) to a subset of bioassays. We also amended a subset of river, soil, and commercial humate bioassays with glucose or algal leachates to test the hypothesis that unstable DOC primes consumption of more stable DOC. We used prior measurements of source‐specific DOC bioavailability, linked with a Bayesian process model, to estimate means and posterior probability distributions for source‐specific DOC decay rates in multisource bioassays. Modeled priming effects ranged from a −130 to +370% change in more stable DOC decay when incubated with unstable DOC. Glucose increased modeled river DOC decay by an average of 87% among all rivers. Glucose and algal leachates increased soil leachate and commercial humate decay by an average of 25% above background rates. Inorganic nutrient additions did not have consistent effects on DOC decay, likely because most of the study rivers had high ambient background nutrients. Our results demonstrate that the priming effect can augment DOC decay in rivers. In addition, Bayesian models can be used to estimate mechanisms driving aquatic ecosystem processes that are difficult to measure directly.

Developing an understanding of dissolved organic matter dynamics in the giant Lake Baikal by ultrahigh resolution mass spectrometry

To elucidate the molecular characteristics of dissolved organic matter (DOM) in Lake Baikal, 3D excitation-emission matrix spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were employed. From the linear relationship between the humic-like peak intensities (excitation/emission = 305 nm/430 nm) and dissolved organic carbon (DOC) concentrations in the water samples extending from the Selenga River mouth to offshore (central lake in the south basin), allochthonous DOM appeared to be a main contributor to the DOC concentrations. However, DOM with fewer fluorophores dominated in the South Basin of the lake at stable DOC concentrations of ca. 0.84 mg C l−1. Meanwhile, FT-ICR MS analysis and subsequent principal component analysis across the transect revealed a transition of compounds with high H-deficiency (aromatic) to compounds with low H-deficiency (aliphatic) that dominate pelagic open-lake water. We believe that this molecular change is induced by photo-degradation, which mainly alters aromatic compounds.

Pyrolysis temperature-dependent release of dissolved organic carbon from plant, manure, and biorefinery wastes

Limited information is available to understand the chemical structure of biochar's labile dissolved organic carbon (DOC) fraction that will alter soil organic carbon (SOC) composition. This study utilized the high sensitivity of fluorescence excitation–emission (EEM) spectrophotometry to understand the structural changes in biochar-derived DOC as a function of (1) chemical property of feedstock and (2) pyrolysis temperature (350–800°C). Regardless of feedstock (almond shell, broiler litter, lignin, cottonseed hull, and pecan shell), low temperature (350–400°C) pyrolysis shifted EEM of hot water (80°C for 16h) extracts toward longer emission wavelengths, higher aromaticity regions. Five component parallel factor (PARAFAC) modeling of EEM afforded: (C1) (poly)phenolic pyrolysis products and other water-soluble aromatic structures similar to fulvic-like SOC; (C2–C3) aromatic structures similar to humic-like SOC having low water solubility that decomposed above 350°C; (C4) carboxyl and other transient thermochemical conversion intermediates that decomposed above 500°C; and (C5) thermally stable DOC fraction of lignin-rich biomass (pecan shell and lignin). Relative contribution of fulvic-like component exceeded that of humic-like components, and increased as a function of pyrolysis temperature. The C4 showed disproportionately high contributions to 500°C broiler litter and cottonseed hull biochars. The carboxyl-enriched DOC of biochars pyrolyzed at 350–500°C can have diverse environmental consequences, including the mobilization of heavy metals.

Effect of benthic-pelagic coupling on dissolved organic carbon concentrations in permeable sediments and water column in the northeastern Gulf of Mexico

Large areas of the continental shelf are covered by permeable sand beds that filter substantial volumes of coastal water. This study investigated the temporal changes in and coupling between dissolved organic carbon (DOC) concentrations in the water column and pore water of nearshore permeable sediments in the northeastern Gulf of Mexico. Time series samples, collected in the nearshore environments at an exposed and a nearby protected site, showed very similar patterns of sedimentary DOC concentrations at both sites indicating that large-scale, seasonal, and weather-related processes controlled these distributions. A summer situation, with the sediment surface layer depleted in DOC and upward diffusion of recalcitrant DOC from deeper layers, was separated by a fall transition phase, where increasing winds and waves caused a mixing of the sedimentary DOC, resulting in relatively even concentrations over the investigated sediment depth (0–12cm). The subsequent winter situation was characterized by increased DOC in the surface layer caused by filtration of degradable organic material into the sands and subsurface removal of degradable DOC. A second transition phase in early spring marked the end of the winter situation, with strong winds and waves and thorough pore water mixing, leading again to even DOC distribution in the investigated sediment layer. This transition phase initiated the next summer situation with depletion of DOC in the sediment surface layers. Our results indicate that DOC in the upper layer (≤12cm) of the shallow sands is controlled by benthic-pelagic coupling facilitated by relatively rapid solute and particle exchange in the highly permeable sediments at our study sites. The prompt responses of the surface layer DOC concentrations to the changes in the overlying water underline the dynamic character of substrate supply in the permeable sediments, setting them apart from fine-grained cohesive beds characterized by relatively stable DOC distributions.

Biodegradation of soil-derived dissolved organic matter as related to its properties

Abstract Quantifying the contribution of dissolved organic matter (DOM) to C sequestration in soils requires knowledge about extent and rate of its biodegradation. Since degradation experiments are time consuming, estimating the biodegradability of DOM by more easily measurable properties seems valuable. Our goal was therefore to investigate the biodegradation of DOM of different origin and to relate its extent and rate to properties such as UV absorbance, synchronous and emission scan fluorescence, XAD-8 sorption chromatography and 1 H-NMR spectroscopy. We extracted DOM from 13 different samples (maize straw, forest floors, peats, agricultural soils) and carried out a 90-day liquid incubation experiment. DOM biodegradation was quantified by CO 2 evolution. Rapidly and slowly mineralizable portions of dissolved organic carbon (DOC) as a measure of labile and stable DOC and the respective mineralization rate constants and half-lives were calculated by a double exponential model. The extent and rate of DOM biodegradation from less humified organic material (straw, litter and fermentation layers of forest floors) were high resulting in 61–93% of DOC being mineralized. The labile fraction comprised 59–88% of total DOC. DOM extracted from agricultural soils was of an intermediate biodegradability with a CO 2 evolution comprising 17–32% of total DOC. Labile DOC represented 14–25% of total DOC. DOM extracted from peats and Oa forest floor layers was relatively stable (mineralization of 4–9% of total DOC, labile DOC: 3–6%). The half-life of the labile DOC pool was short (2–5 days), whereas that of the stable DOC pool ranged from 0.2 years (DOM from less humified material) to 8.6 years (DOM from the Oa layer under spruce). Extent and rate of DOM biodegradation were closely but nonlinearly related to DOM properties. Solutions exceeding a threshold value of UV absorbance, aromaticity, XAD-8 adsorbable C or of humification indices derived from fluorescence spectra or having carbohydrate contents below a certain level were stable against biodegradation. Relatively simple methods like UV spectroscopy, XAD-8 fractionation and fluorescence emission spectroscopy were suitable to estimate the biodegradation of DOM.