Dynamics of Respired Dissolved Organic Carbon in a Stream

Soil labile organic carbon indicating seasonal dynamics of soil organic carbon in northeast peatland

A strong mitigation scenario maintains climate neutrality of northern peatlands

Research in stream metabolism, gas exchange, and sediment dynamics indicates that rivers are an active component of the global carbon cycle and that river form and process can influence partitioning of terrestrially derived carbon among the atmosphere, geosphere, and ocean. Here we develop a conceptual model of carbon dynamics (inputs, outputs, and storage of organic carbon) within a river corridor, which includes the active channel and the riparian zone. The exchange of carbon from the channel to the riparian zone represents potential for storage of transported carbon not included in the “active pipe” model of organic carbon (OC) dynamics in freshwater systems. The active pipe model recognizes that river processes influence carbon dynamics, but focuses on CO2 emissions from the channel and eventual delivery to the ocean. We also review how human activities directly and indirectly alter carbon dynamics within river corridors. We propose that dams create the most significant alteration of carbon dynamics within a channel, but that alteration of riparian zones, including the reduction of lateral connectivity between the channel and riparian zone, constitutes the most substantial change of carbon dynamics in river corridors. We argue that the morphology and processes of a river corridor regulate the ability to store, transform, and transport OC, and that people are pervasive modifiers of river morphology and processes. The net effect of most human activities, with the notable exception of reservoir construction, appears to be that of reducing the ability of river corridors to store OC within biota and sediment, which effectively converts river corridors to OC sources rather than OC sinks. We conclude by summarizing knowledge gaps in OC dynamics and the implications of our findings for managing OC dynamics within river corridors.

Carbon dynamics in peatlands are regulated by biogeochemical processes, including heterotrophic respiration, where microorganisms utilize organic carbon (OC) as an electron donor and respire CO2. In the absence of oxygen, ferric iron (FeIII) is an important electron acceptor. However, the presence of FeIII minerals can also modify carbon dynamics by adsorbing OC or occluding OC in microaggregates, thus limiting the mineralization of OC. Along with its speciation and heterogeneity across soils, the overall impact of iron (Fe) on OC mineralization in mineral-rich peatlands remains unclear. To investigate this complex role of Fe and its impact on OC mobilization, we designed model anoxic soil incubations, where multiple Fe species were added to mimic soil heterogeneity. To this end, reactive Fe species (ferrihydrite; Fh, a ferrihydrite-silicate coprecipitate; FhSi in which Si:Fe = 0.05 mol/mol, FeIII-peat complex; FePeat) and more stable Fe species (goethite; Gt) either in pure forms (Fh, FhSi, and Gt) or mixtures of the two (95/5% Gt/Fh; GtFh, 95/5% Gt/FePeat; GtFePeat) were added to an ombrotrophic peat, increasing the Fe content of the soil from 0.1% to 6% (w/w). The incubations were prepared anoxically in crimp-sealed vials and lasted for 70 days. Two incubation series were established to allow for (1) measurements of the headspace CO2 concentrations (over 60 days) and (2) sampling of the soil slurry after 4, 17, 35, and 70 days. The latter was used to follow trends in pH, Eh, dissolved organic carbon (DOC) and Fe speciation in the aqueous-phase, and amounts of OC and Fe mobilized from the solid-phase in sequential chemical extractions: 0.5 M HCl (sorbed Fe), hydroxylamine-HCl (short-ranged-ordered Fe oxyhydroxides), and 6 M HCl (crystalline Fe hydroxides).The results show that the addition of Fe species changes the carbon dynamics. The addition of reactive Fe species (Fh, FhSi) promoted CO2 production and resulted in higher concentrations of aqueous Fe, suggesting reductive dissolution of the minerals as they served as extra electron acceptors for microbial respiration. In contrast, in the Gt treatment, the goethite addition alone did not affect CO2 production until the 20th day, after which CO2 production was first inhibited and then promoted (after 42 days) compared to a control treatment which received no Fe additions. However, when small fractions (5%) of reactive Fe species were added alongside goethite (GtFh and GtFePeat), CO2 production was up to 1.5-2.2 times higher than in the Gt treatment. Yet, the lowest DOC concentrations were measured in Fh and FhSi, suggesting that the ferrihydrites re-adsorbed the released OC. Furthermore, while fractions of extractable Fe in Fh and FhSi did not change significantly over the incubation, a strong increase in 6 M HCl extractable Fe in all goethite-containing treatments suggests that, although less reductive dissolution occurred, mineral recrystallization may have occurred.These results highlight the complex impacts of exogenous Fe species on carbon dynamics and shed light on the vulnerability of peatlands as carbon sinks in the context of climate change, where changes in groundwater geochemistry, including Fe content and watertable fluctuation, may be expected.

<p>The main driver of the Carpathian landscape is the process of natural forest succession, which causes the overgrowing of the unique semi-natural meadows. Land-use changes influence the balance of organic carbon in the soil, simultaneously may cause carbon sequestration or CO<sub>2</sub> emission. Whereas, there is still a lack of knowledge covering the impact of natural forest succession on organic carbon cycling. The purpose of this study was to investigate the dynamics of organic carbon in the different land-use soils. The selected properties showing the rate of mineralization process as well as soil biological activity were taken into account.</p><p>This study was located in three selected Carpathians’ national parks. Soil samples were taken from 0-10 cm and 10-20 cm soil layers of ten transects each consisting three different land use: semi-natural meadow, succession (30-75 aged trees), and old-growth forest (more than 150 years). Measurements of microbial biomass carbon (MBC), dissolved organic carbon (DOC), dehydrogenase (DHA) and invertase (INW) activity and microbial respiration were made on fresh soil samples. Based on the first-order kinetic model of microbial respiration the cumulative respiration was calculated. Additionally, the metabolic quotient (qCO<sub>2</sub>), the microbial quotient (qMIC), and the mineralization quotient (qM) were calculated.</p><p>The mean C<sub>org</sub> content ranged from 17.6 g kg<sup>-1</sup> in the 10-20 cm layer of succession to 41.5 g kg<sup>-1</sup> in the 0-10 cm layer of forest. Considering the individual land use variants in the 0-10 cm layer meadow characterised the highest MBC, DHA and qM, and the lowest qCO<sub>2</sub> values. In the succession, the highest cumulative respiration and qCO<sub>2</sub> and the lowest MBC and INW were noted. Whereas the forest characterised the highest INW and the lowest cumulative respiration, DHA, qMIC and qM. Similarity, in the 10-20 cm layer meadow the highest MBC and DHA as well as qMIC were found. The succession characterised the highest cumulative respiration, qCO<sub>2</sub> and qM and the lowest qMIC. However, in the forest the highest INW and the lowest qCO<sub>2</sub>, qMIC and qM were noticed.</p><p>Overall, for all investigated soils the positive correlations between C<sub>org</sub> and MBC, DHA and negative correlations C<sub>org</sub> with qMIC, qCO<sub>2</sub> and DOC were shown. Whereas, when we take into consideration the individuals land use variants and depths can be stated that the content of organic carbon was shaped by different properties. In the 0-10 cm content of C<sub>org</sub> in meadow and forest positive correlated with cumulative respiration and DHA, and negative with qM. Additionally, in forest negative correlations C<sub>org</sub> with DOC, INW and qCO<sub>2</sub> were found. While in succession the positive correlations C<sub>org</sub> with MBC and INW and negative correlations C<sub>org</sub> with DHA, qMIC and DOC were noted. In the 10-20 cm layers of meadow and succession C<sub>org</sub> positive correlated with MBC, INW, qCO<sub>2</sub> and negative with qM and DOC. Additionally, the qMIC positive correlation with C<sub>org</sub> in meadow and negative correlation in succession was found. Whereas, in forest C<sub>org </sub>positive correlated with qM and MBC, while negative correlations between C<sub>org</sub> and qMIC, DOC and qCO<sub>2</sub> were noticed.</p>

Carbon dynamics determined by natural 13C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Carbon dynamics in the freshwater part of the Elbe estuary, Germany: Implications of improving water quality

Recent warming at high-latitudes has accelerated permafrost thaw in northern peatlands, and thaw can have profound effects on local hydrology and ecosystem carbon balance. To assess the impact of permafrost thaw on soil organic carbon (OC) dynamics, we measured soil hydrologic and thermal dynamics and soil OC stocks across a collapse-scar bog chronosequence in interior Alaska. We observed dramatic changes in the distribution of soil water associated with thawing of ice-rich frozen peat. The impoundment of warm water in collapse-scar bogs initiated talik formation and the lateral expansion of bogs over time. On average, Permafrost Plateaus stored 137 ± 37 kg C m -2 , whereas OC storage in Young Bogs and Old Bogs averaged 84 ± 13 kg C m -2 . Based on our reconstructions, the accumulation of OC in near-surface bog peat continued for nearly 1,000 years following permafrost thaw, at which point accumulation rates slowed. Rapid decomposition of thawed forest peat reduced deep OC stocks by nearly half during the first 100 years following thaw. Using a simple mass-balance model, we show that accumulation rates at the bog surface were not sufficient to balance deep OC losses, resulting in a net loss of OC from the entire peat column. An uncertainty analysis also revealed that the magnitude and timing of soil OC loss from thawed forest peat depends substantially on variation in OC input rates to bog peat and variation in decay constants for shallow and deep OC stocks. These findings suggest that permafrost thaw and the subsequent release of OC from thawed peat will likely reduce the strength of northern permafrost-affected peatlands as a carbon dioxide sink, and consequently, will likely accelerate rates of atmospheric warming.

林冠氮添加和林下植被去除对杉木林土壤有机碳组分的影响

We investigated the variation patterns of organic carbon in soil and soil solution of four selected Robiniapseudoacacia forests aged 10a, 25a, 31a, and 35a, as well as a contrastive tillage site in a similar topography condition in Loess Plateau, China. The purpose was to explore the dynamics of soil organic carbon (SOC) and dissolved organic carbon (DOC) in R. pseudoacacia forests. On average depths of 20, 40, and 60 cm, SOC, active organic carbon (AOC), and DOC gradually increase with increased forest age. After forest restoration, the AOC/SOC ratio and resistant organic carbon/SOC ratio increase, whereas the slow organic carbon/SOC ratio decreases. The soil solutions in the subsoil layer have low DOC:DON ratio and high UV absorption at 280 nm. At 40 and 60 cm, the depth distribution is indicated as special low values for DOC concentration in the C99 site (10a site), as well as for soil water content, SOC, and AOC in the 25a forest site. Our results provide evidence that during forest restoration, SOC does not consistently increase linearly. The change points of different SOC proportions and DOC concentrations at various depths are not same, i.e., asynchronous changes exist.

Excretion of organic matter and nutrients from invasive quagga mussels and potential impact on carbon dynamics in Lake Michigan

Stable carbon isotopes are a powerful tool to assess the origin and dynamics of carbon in soils. However, direct analysis of the (13)C/(12)C ratio in the dissolved organic carbon (DOC) pool has proved to be difficult. Recently, several systems have been developed to measure isotope ratios in DOC by coupling a total organic carbon (TOC) analyzer with an isotope ratio mass spectrometer. However these systems were designed for the analysis of fresh and marine water and no results for soil solutions or (13)C-enriched samples have been reported. Because we mainly deal with soil solutions in which the difficult to oxidize humic and fulvic acids are the predominant carbon-containing components, we preferred to use thermal catalytic oxidation to convert DOC into CO(2). We therefore coupled a high-temperature combustion TOC analyzer with an isotope ratio mass spectrometer, by trapping and focusing the CO(2) cryogenically between the instruments. The analytical performance was tested by measuring solutions of compounds varying in the ease with which they can be oxidized. Samples with DOC concentrations between 1 and 100 mg C/L could be analyzed with good precision (standard deviation (SD) < or = 0.6 per thousand), acceptable accuracy, good linearity (overall SD = 1 per thousand) and without significant memory effects. In a (13)C-tracer experiment, we observed that mixing plant residues with soil caused a release of plant-derived DOC, which was degraded or sorbed during incubation. Based on these results, we are confident that this approach can become a relatively simple alternative method for the measurement of the (13)C/(12)C ratio of DOC in soil solutions.

Plant detritus plays a crucial role in regulating belowground biogeochemical processes in forest ecosystems, particularly influencing labile carbon (C) dynamics and overall soil C storage. However, the specific mechanisms by which litter and roots affect soil organic carbon (SOC) and its components in plantations remain insufficiently understood. To investigate this, we conducted a detritus input and removal treatment (DIRT) experiment in a Larix principis-rupprechtii Mayr plantation in the Taiyue Mountains, China, in July 2014. The experiment comprised three treatments: root and litter retention (CK), litter removal (LR), and root and litter removal (RLR). Soil samples were collected from depths of 0–10 cm and 10–20 cm during June, August, and October 2015 to evaluate changes in soil pH, water content (SW), SOC, dissolved organic carbon (DOC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC). The removal of litter and roots significantly increased soil pH (p &lt; 0.05), with pH values being 8.84% and 8.55% higher in the LR and RLR treatments, respectively, compared to CK treatment. SOC levels were significantly reduced by 26.10% and 12.47% in the LR and RLR treatments, respectively (p &lt; 0.05). Similarly, DOC and MBC concentrations decreased following litter and root removal, with DOC content in August being 2.5 times lower than in June. Across all treatments and sampling seasons, SOC content was consistently higher in the 0–10 cm depth, exhibiting increases of 35.15% to 39.44% compared to the 10–20 cm depth (p &lt; 0.001). Significant negative correlations were observed between SOC and the ratios of ROC/SOC, pH, DOC/SOC, and MBC/SOC (R = −0.54 to −0.37; p &lt; 0.05). Path analysis indicated that soil pH had a significant direct negative effect on SOC (p &lt; 0.05), with a standardized path coefficient (β) of −0.36, while ROC had a significant direct positive effect on SOC (β = 0.66, p &lt; 0.05). Additionally, pH indirectly affected SOC by significantly influencing ROC (β = −0.69), thereby impacting SOC indirectly. Random forest analysis also confirmed that the ROC/SOC ratio plays a critical role in SOC regulation. This study reveals the complex interactions between litter and root removal and soil C dynamics in larch plantations, identifying soil pH and ROC as crucial regulator of SOC content. However, the short-term duration and focus on shallow soil depths limit our understanding of long-term impacts and deeper soil C storage. Future research should explore these aspects and consider varying climate conditions to enhance the applicability of our findings. These insights provide a scientific foundation for developing effective forest management strategies and forecasting changes in soil C storage in the context of climate change.

Abstract. Current land surface models (LSMs) typically represent soils in a very simplistic way, assuming soil organic carbon (SOC) as a bulk, and thus impeding a correct representation of deep soil carbon dynamics. Moreover, LSMs generally neglect the production and export of dissolved organic carbon (DOC) from soils to rivers, leading to overestimations of the potential carbon sequestration on land. This common oversimplified processing of SOC in LSMs is partly responsible for the large uncertainty in the predictions of the soil carbon response to climate change. In this study, we present a new soil carbon module called ORCHIDEE-SOM, embedded within the land surface model ORCHIDEE, which is able to reproduce the DOC and SOC dynamics in a vertically discretized soil to 2 m. The model includes processes of biological production and consumption of SOC and DOC, DOC adsorption on and desorption from soil minerals, diffusion of SOC and DOC, and DOC transport with water through and out of the soils to rivers. We evaluated ORCHIDEE-SOM against observations of DOC concentrations and SOC stocks from four European sites with different vegetation covers: a coniferous forest, a deciduous forest, a grassland, and a cropland. The model was able to reproduce the SOC stocks along their vertical profiles at the four sites and the DOC concentrations within the range of measurements, with the exception of the DOC concentrations in the upper soil horizon at the coniferous forest. However, the model was not able to fully capture the temporal dynamics of DOC concentrations. Further model improvements should focus on a plant- and depth-dependent parameterization of the new input model parameters, such as the turnover times of DOC and the microbial carbon use efficiency. We suggest that this new soil module, when parameterized for global simulations, will improve the representation of the global carbon cycle in LSMs, thus helping to constrain the predictions of the future SOC response to global warming.

Soil carbon dynamics were studied at four different forest stands developed on bedrocks with contrasting geology in Slovenia: one plot on magmatic granodiorite bedrock (IG), two plots on carbonate bedrock in the karstic-dinaric area (CC and CD), and one situated on Pleistocene coalluvial terraces (FGS). Throughfall (TF) and soil water were collected monthly at each location from June to November during 2005–2007. In soil water, the following parameters were determined: T, pH, total alkalinity, concentrations of Ca2+ and Mg2+, dissolved organic carbon (DOC), and Cl− as well as δ13CDIC. On the other hand, in TF, only the Cl− content was measured. Soil and plant samples were also collected at forest stands, and stable isotope measurements were performed in soil and plant organic carbon and total nitrogen and in carbonate rocks. The obtained data were used to calculate the dissolved inorganic carbon (DIC) and DOC fluxes. Statistic analyses were carried out to compare sites of different lithologies, at different spatial and temporal scales. Decomposition of soil organic matter (SOM) controlled by the climate can explain the 13C and 15 N enrichment in SOM at CC, CD, and FGS, while the soil microbial biomass makes an important contribution to the SOM at IG. The loss of DOC at a soil depth of 5 cm was estimated at 1 mol m−2 year−1 and shows no significant differences among the study sites. The DOC fluxes were mainly controlled by physical factors, most notably sorption dynamics, and microbial–DOC relationships. The pH and pCO2 of the soil solution controlled the DIC fluxes according to carbonate equilibrium reactions. An increased exchange between DIC and atmospheric air was observed for samples from non-carbonate subsoils (IG and FGS). In addition, higher δ13CDIC values up to −19.4 ‰ in the shallow soil water were recorded during the summer as a consequence of isotopic fractionation induced by molecular diffusion of soil CO2. The δ13CDIC values also suggest that half of the DIC derives from soil CO2 indicating that 2 to 5 mol m−2 year−1 of carbon is lost in the form of dissolved inorganic carbon at CC and CD after carbonate dissolution. Major difference in soil carbon dynamics between the four forest ecosystems is a result of the combined influence of bedrock geology, soil texture, and the sources of SOM. Water flux was a critical parameter in quantifying carbon depletion rates in dissolved organic and inorganic carbon forms.