Dissolved Organic Carbon Fluxes Research Articles

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

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

Lateral carbon export has low impact on the net ecosystem carbon balance of a polygonal tundra catchment

Abstract. Permafrost-affected soils contain large quantities of soil organic carbon (SOC). Changes in the SOC pool of a particular ecosystem can be related to its net ecosystem carbon balance (NECB) in which the balance of carbon (C) influxes and effluxes is expressed. For polygonal tundra landscapes, accounts of ecosystem carbon balances in the literature are often solely based on estimates of vertical carbon fluxes. To fill this gap, we present data regarding the lateral export rates of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from a polygonal tundra site in the north Siberian Lena River delta, Russia. We use water discharge observations in combination with concentration measurements of waterborne carbon to derive the lateral carbon fluxes from one growing season (2 June–8 September 2014 for DOC, 8 June–8 September 2014 for DIC). To put the lateral C fluxes into context, we furthermore present the surface–atmosphere eddy covariance fluxes of carbon dioxide (CO2) and methane (CH4) from this study site. The results show cumulative lateral DIC and DOC fluxes of 0.31–0.38 and 0.06–0.08 g m−2, respectively, during the 93 d observation period (8 June–8 September 2014). Vertical turbulent fluxes of CO2-C and CH4-C accumulated to −19.0 ± 1.2 and 1.0 ± 0.02 g m−2 in the same period. Thus, the lateral C export represented about 2 % of the net ecosystem exchange of (NEE) CO2. However, the relationship between lateral and surface–atmosphere fluxes changed over the observation period. At the beginning of the growing season (early June), the lateral C flux outpaced the surface-directed net vertical turbulent CO2 flux, causing the polygonal tundra landscape to be a net carbon source during this time of the year. Later in the growing season, the vertical turbulent CO2 flux dominated the NECB.

Coupled dynamics of aqueous biogeochemistry in contrasting floodplain environments: Implications for Critical Zone carbon sequestration along redox gradients

This study contributes to our evolving understanding of how coupled biogeochemical dynamics of Fe and Mn along the redox gradients affect C cycling and sequestration at Critical Zones. A 12-month monitoring of the in-situ soil pore-water biogeochemistry and redox gradients across the floodplain at the White Clay Creek Watershed in the Christina River Basin - Critical Zone Observatory was investigated. Topographically, eastern floodplain was narrow with a steep hillslope connected to the agricultural field while western floodplain was flat and wide. Soil profile consisted of post-colonial oxic deposits, pre-colonial anoxic buried wetland soils underlain by suboxic valley-bottom gravel. Contrasting soil pore-water chemistry including DOM quality, DOC, Fe and Mn, δ18O and δD profiles, and redox potentials were observed between the eastern and western floodplains due to seasonal fluctuations, divergent topographic and hydrologic settings. We observed diverse types of C pools across the floodplain from short (labile) to long term (recalcitrant) C forms: microbial origin DOM at eastern versus terrestrial or plant-derived DOM at western buried wetland soil pore-waters. Gravel was linked to the soluble microbial byproduct-like fractions indicating bioavailable DOM and microbial activity. Gravel beds may act as a natural filter (or C sink) of terrestrial DOM via adsorption and precipitation with Mn- and Fe (hydr)oxides, stimulating the long-term sequestration of sedimentary OC while retaining the microbially-derived DOM in the pore-water. In contrast, reductive dissolution of Mn- and Fe (hydr)oxides may result in re-mobilization of aromatic DOM and increase of DOC flux (C source) to the stream. Future studies are needed to improve our understanding of how in-situ redox gradients and/or microbial activity transform OC-mineral complexes and the coupled C dynamics at Critical Zones.

Biogeochemical properties and fate of dissolved organic matter in wet deposition: Insights from a mariculture area in North Yellow Sea

To address the potential roles of atmospheric wet deposition in carbon cycling in coastal waters, a comprehensive study of the biogeochemical properties of dissolved organic matter (DOM) in precipitation and the resulting implication in a mariculture area in North Yellow Sea was conducted. The annual mean concentrations of dissolved organic carbon (DOC), chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) were 1.52 ± 1.52 mg C L−1, 0.36 ± 0.66 m−1 and 0.38 ± 0.35 QSU, respectively. The concentrations of most DOM proxies exhibited significant negative correlations with the corresponding precipitation amount (R2 = 0.15–0.40, P < 0.01), but the dilution effects became less significant when the precipitation amount exceeded 10.2, 10.7, 10.2 and 2.4 mm for DOC, CDOM, highly‑oxygenated and hypoxic structured humic-like substances, respectively. Seasonally, the dominant precipitation type in winter was snowfall, in which the DOM contained more high-molecular-weight compounds with higher aromaticity and humification degree, while the characteristics of DOM in intensive rainfall in summer were contrary to those in winter. The wet deposition flux of DOC to this region was estimated to be 6.31 × 108 g C a−1, which was 3.3 and 1.4 times that of the dry deposition and local riverine input, thereby contributing to 4.0 % of the DOC storage in the study area. In summer, the intensive input of DOC through wet deposition (0.43 g C m−2) to surface seawater could enrich its bioavailable DOC by 10.7 μmol L−1, the complete aerobic decomposition of which would cause an obvious dissolved oxygen depletion in the surface seawater by 21.4 μmol L−1, demonstrating the influence of wet deposition on summer deoxygenation in coastal waters.

Simultaneous Measurements of Dissolved Organic Carbon and Soil Respiration Reveal Reduced Soil Carbon Loss Under Nitrogen Addition in a Montane Forest

AbstractA major uncertainty in the estimation of soils acting as net carbon (C) sinks or sources stem from the effects of anthropogenic nitrogen (N) input on the balance between plant C input and soil C loss. In contrast to the generally observed increasing pattern of plant C input, the response of soil C loss to increased N deposition remains elusive, largely due to its large temporal variation. Here simultaneous measurements of two major soil C loss pathways, including dissolved organic carbon (DOC) leaching and soil respiration, were conducted for 5 and 3 yr, respectively, to assess the effects of N addition on soil C loss in an N‐limited montane forest. The effects were seasonal, depth and N level dependent and the two pathways responded asynchronously to N addition. Significant decreases in DOC concentrations and fluxes in leachates from the organic layer were observed during autumn/winter under a high N addition rate (40 kg N/ha/yr). No significant impact of N addition on DOC concentrations or fluxes was observed for leachates from the mineral soil horizon. Biodegradability was low for DOC from both soil layers and was not consistently influenced by N addition. Soil respiration was significantly decreased under high N addition. Annual soil C loss (estimated by summing DOC leaching from the mineral horizon and soil respiration) showed that N addition reduced soil C loss consistently over years, implying that the forest soil is likely a C sink under excess N deposition, which should be confirmed with longer term monitoring.

Eutrophication reduced the release of dissolved organic carbon from tropical seagrass roots through exudation and decomposition

Seagrass bed ecosystem is one of the most effective carbon capture and storage systems on earth. Seagrass roots are the key link of carbon flow between leaf-root-sediment, and the release of dissolved organic carbon (DOC) from seagrass roots through exudation and decomposition are vital sources to the sediment organic carbon (SOC) in the seagrass beds. Unfortunately, human-induced eutrophication may change the release process of DOC from seagrass roots, thereby affecting the sediment carbon storage capacity. However, little is known about the effect of nutrient enrichment on the release of DOC from seagrass roots, hindering the development of seagrass underground ecology. Therefore, we selected Thalassia hemprichii, the tropical dominant seagrass species, as the research object, and made a comparison of the release of DOC from roots through exudation and decomposition under different nitrate treatments. We found that under control, 10 μmol L−1, 20 μmol L−1 and 40 μmol L−1 nitrate treatments, soluble sugar of T. hemprichii roots were 71.37 ± 3.43 mg g−1, 67.03 ± 5.33 mg g−1, 49.14 ± 3.48 mg g−1, and 18.51 ± 2.09 mg g−1, respectively, while the corresponding root DOC exudation rates were 7.00 ± 0.97 mg g DW root−1 h−1, 5.11 ± 0.42 mg g DW root−1 h−1, 4.08 ± 0.23 mg g DW root−1 h−1, and 3.78 ± 0.74 mg g DW root−1 h−1, respectively. There was a significant positive correlation between root soluble sugar and DOC exudation rate. DOC concentration of sediment porewater and SOC content also decreased under nitrate enrichment (though not significantly), which were both significantly positively correlated with the rate of root exuded DOC. Meanwhile, nitrate enrichment also reduced the release rate of DOC from seagrass roots during initial decomposition, and the release flux of DOC from decomposition. Therefore, nutrient enrichment could decrease nonstructural carbohydrates of seagrass roots, reducing the rate of root exuded DOC, thereby lowered SOC, as well as the DOC release from seagrass root decomposition. In order to increase the release of DOC from seagrass roots and improve the carbon sequestration capacity of seagrass beds, effective measures should be taken to control the coastal nutrients input into seagrass beds.

Enrichment of 13C with depth in soil organic horizons is not explained by CO2 or DOC losses during decomposition

Stable isotope ratios of soil organic carbon (SOC) are a potentially powerful, integrative tool for analyzing the soil C cycle. However, limited understanding of the mechanisms for C isotope fractionation in soil prevents their widespread application. Soil organic carbon (SOC) is progressively enriched in 13C with age and depth in the soil profile even though CO2 produced during soil respiration is typically enriched in 13C compared to SOC. This results in an apparent mass balance paradox. To resolve this paradox, we hypothesized that the loss of 13C-depleted DOC during soil diagenesis, the combination of microbial decomposition and abiotic processes such as leaching, can account for the increase in SOC δ13C with depth. We combined three independent approaches (field measurements, a laboratory incubation experiment, and seasonal sampling of DOC across a climate transect) to systematically evaluate the relationship between the δ13C of soil respiration, DOC, and the SOC it is derived from. However, DOC was not significantly depleted in 13C compared to SOC in any of the three approaches, and mass balance calculations indicated that the DOC flux cannot account for the full extent of 13C enrichment of SOC with depth in the soil profile. We suggest that vertical C transport by plant roots or fungi, rather than diagenesis, may be largely responsible for the observed C isotope profile. Future studies aimed at understanding these vertical transport processes should enable increased application of soil δ13C, enhancing soil biogeochemical studies.

Carbon metabolism and bioavailability of dissolved organic carbon (DOC) fluxes in seagrass communities are altered under the presence of the tropical invasive alga Halimeda incrassata

Seagrass beds act as blue carbon sinks globally as they enhance the trapping of recalcitrant (i.e., low biodegradability) organic carbon in their sediments. Recent studies also show that the recalcitrant fraction of the dissolved organic carbon (DOC) pool in seawater has an important role as long-term carbon sequestration in oceans. Although seagrasses are known for the large amount of DOC they export, little attention has been given to its biodegradability, which ultimately determinates its fate in the coastal carbon cycle. In turn, invasive algae are a major global concern in seagrass ecosystems since they can deeply modify their structure and functions, which may affect carbon metabolism and DOC release. This work assesses how the presence of Halimeda incrassata, an invasive tropical calcareous macroalga, modifies carbon metabolism and DOC fluxes in invaded areas dominated by the seagrass Cymodocea nodosa. Our results show that stands with the presence of this seagrass (i.e., both monospecific and mixed meadow) had the highest production values, acting as high DOC producers in both winter (mainly of labile DOC; DOCL) and summer (mainly as recalcitrant DOC; DOCR). In contrast, monospecific H. incrassata beds exhibited low production values, and the presence of this macroalga (either as monospecific beds or mixed with C. nodosa) triggered the shift from a net DOC-producing-system in summer (mainly DOCL) to a net DOC-consuming-system in winter. This work thus suggests that C. nodosa meadows have the potential to export a significant fraction of both labile and recalcitrant DOC, and that the spread of this invasive alga might decrease the C export capacity of seagrass meadows. Such shift would imply the reduction of a quick and efficient transfer of carbon and energy to higher trophic levels, and might reduce the blue carbon potential of seagrasses as dissolved form in the water column.

Rising dissolved organic carbon concentrations in coastal waters of northwestern Borneo related to tropical peatland conversion.

Southeast Asia’s peatlands are considered a globally important source of terrigenous dissolved organic carbon (DOC) to the ocean. Human disturbance has probably increased peatland DOC fluxes, but the lack of monitoring has precluded a robust demonstration of such a regional-scale impact. Here, we use a time series of satellite ocean color data from northwestern Borneo to show that DOC concentrations in coastal waters have increased between 2002 and 2021 by 0.31 μmol liter−1 year−1 (95% confidence interval, 0.18 to 0.44 μmol liter−1 year−1). We show that this was caused by a ≥30% increase in the concentration of terrigenous DOC and coincided with the conversion of 69% of regional peatland area to nonforest land cover, suggesting that peatland conversion has substantially increased DOC fluxes to the sea. This rise in DOC concentration has also increased the underwater light absorption by dissolved organic matter, which may affect marine productivity by altering underwater light availability.

Carbon isotopic constraints on the degradation and sequestration of organic matter in river‐influenced marginal sea sediments

AbstractOrganic matter degradation and sequestration in marine sediments are important processes involved in carbon cycling in the ocean. Here, we present the results of carbon isotope (14C and 13C) and concentration measurements of sedimentary organic carbon (SOC), pore‐water dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in sediments collected from the East China Sea, Yellow/Bohai Sea, and South China Sea. Our results indicated that selective degradation and preservation of organic matter occurred in these sediments, and marine‐derived young organic carbon degraded preferentially and rapidly, resulting in high concentrations of pore‐water DOC and DIC with distinct carbon isotopic signatures. The average 14C age of pore‐water DOC was thousands of years younger than that of SOC in the sediment, suggesting that DOC was newly produced and cycled much faster than SOC. Aged SOC was refractory and preserved in sediments. Using a dual‐isotope three‐end‐member model, the contributions of potential sources to SOC, DOC, and DIC were estimated. Marine‐derived biomass organic carbon contributed the most to DOC, and dissolution of biogenic carbonate contributed the most to DIC. Riverine inputs of pre‐aged soil and fossil organic carbon dominated the SOC pool. Our study demonstrated that marginal sea sediments are important sites of young DOC and DIC fluxes into the water column, thus acting as a major pathway for carbon and nutrient cycling in the ocean.

Peatland Dissolved Organic Carbon Export to Surface Waters: Global Significance and Effects of Anthropogenic Disturbance

AbstractPeatlands are key components of the global carbon cycle. We compiled peatland dissolved organic carbon (DOC) export from 62 sites to provide a global estimate of the contribution of these ecosystems to fluvial organic carbon fluxes. Despite their modest coverage on land (less than 3%), peatlands contribute at least 91 ± 54 Tg DOC to surface waters, and could represent from 12% to 20% of the DOC ultimately transferred to oceans. Boreal peatlands are the main contributors, with 58% of peatland inputs. Tropical peatlands contribute 37% of global peatland fluxes although representing only 13% of global peatland areas. Temperature is the leading factor explaining the variability in DOC fluxes for natural sites, when it is discharge for sites impacted by direct anthropogenic activities. This suggests different trajectories under climate change pressure for non–disturbed and disturbed sites and reinforces the need for hydrological restoration of disturbed peatland sites worldwide.

Contribution of dissolved organic carbon from a tropical river system to the Arabian Sea, southwestern India

The Payaswini-Chandragiri river system comprises two small tropical rivers originating in the Western Ghats (Patti Ghats reserve forest) of Peninsular India and contributes to an annual discharge of 4.40 km3 of water into the Arabian sea. This work mainly focuses on the fluxes of total dissolved organic carbon (DOC) from these rivers into the Arabian Sea. Quantifying the amount of terrestrial carbon fluxes into the ocean is important in better understanding long-term climate change because a significant portion of atmospheric carbon dioxide is getting locked up in the ocean sediments. However, DOC fluxes from the tropical southwest flowing rivers of India are sparse, especially in the backdrop of increased human activities since the onset of the Anthropocene. This study is an attempt to fill this gap. The DOC fluxes were measured between April 2016 and Dec 2017 on a seasonal basis. The flux of DOC in the post-monsoon season was 1.3 and 1.2 times higher than the monsoon and the pre-monsoon seasons, respectively. This riverine system contributes ∼0.4% of the total DOC flux received by the Arabian sea.

Rapid decline in atmospheric organic carbon deposition in rural Beijing, North China between 2016 and 2020

Atmospheric deposition of dissolved organic carbon (DOC) is equivalent to the magnitude of riverine export of organic carbon (OC) to the oceans. Interpreting the fluctuations of DOC in precipitation is therefore essential in understanding the global carbon balance. This study measured DOC in 137 rain events collected from November 2016 to November 2020 at a rural site in agricultural areas of Beijing on the North China Plain. Concentrations of DOC ranged from 0.09 to 19.69 mg C L−1 with a volume weighted mean value of 2.76 ± 0.27 mg C L−1, which is comparable with those measured in North China a decade ago (3.0 ± 0.4 mg C L−1) and the global mean level (2.64 ± 2.04 mg C L−1). While differences in these values were small, their overall mean values were statistically different (p < 0.05). When classified via air mass back-trajectories, precipitation events with trajectories from south demonstrated higher DOC concentrations than those from north, matching with the spatial distribution of OC emission in the region. The annual precipitation DOC fluxes varied from 6.60 to 15.85 kg C ha−1 yr−1, with an average of 11.05 ± 3.83 kg C ha−1 yr−1 in this study. Both the annual mean concentrations and fluxes of precipitation DOC decreased by approximately 60% between 2017 and 2020, coinciding with the declining trend in OC emissions and aerosol OC concentrations due to the clean air actions. These unique data will allow for a better understanding of current status of rainwater DOC deposition on the global scale and will be critical for evaluating the impacts of future changes, e.g., replacing coal with natural gas, on atmospheric composition.

Spatiotemporal patterns and drivers of terrestrial dissolved organic carbon (DOC) leaching into the European river network

Abstract. Leaching of dissolved organic carbon (DOC) from soils into the river network is an important component of the land carbon (C) budget. At regional to global scales, its significance has been estimated through simple mass budgets, often using multi-year averages of observed fluvial DOC fluxes as a proxy of DOC leaching due to the limited availability of observations of the leaching flux itself. This procedure leads to a systematic underestimation of the leaching flux because of the decay of DOC during fluvial transport. Moreover, this procedure does not allow for revealing spatiotemporal variability in DOC leaching from soils, which is vital to better understand the drivers of DOC leaching and its impact on the local soil C budget. In this study, we use the land surface model (LSM) ORCHILEAK to simulate the terrestrial C budget, including leaching of DOC from the soil and its subsequent reactive transport through the river network of Europe. The model performance is evaluated not only against the sparse observations of the soil DOC leaching rate, but also against the more abundant observations of fluxes and reactivity of DOC in rivers, providing further evidence that our simulated DOC fluxes are realistic. The model is then used to simulate the spatiotemporal patterns of DOC leaching across Europe over the period 1972–2012, quantifying both the environmental drivers of these patterns and the impact of DOC leaching on the land C budget. Over the simulation period, we find that, on average, 14.3 Tg C yr−1 of DOC is leached from land into European rivers, which is about 0.6 % of the terrestrial net primary production (NPP), a fraction significantly lower than that reported for tropical river networks. On average, 12.3 Tg C yr−1 of the leached DOC is finally exported to the coast via the river network, and the rest is respired during transit. DOC leaching presents a large seasonal variability, with the maximum occurring in winter and the minimum in summer, except for most parts of northern Europe, where the maximum occurs in spring due to snowmelt. The DOC leaching rate is generally low in warm and dry regions, and high in the cold and wet regions of Europe. Furthermore, runoff and the ratio between runoff from shallower flow paths on one hand and deep drainage and groundwater flow on the other hand are the main drivers of the spatiotemporal variation of DOC leaching. Temperature, as a major control of DOC production and decomposition rates in the soils, plays only a secondary role.

Export of Dissolved Organic Carbon from the Source Region of Yangtze River in the Tibetan Plateau

The carbon release and transport in rivers are expected to increase in a warming climate with enhanced melting. We present a continuous dataset of DOC in the river, precipitation, and groundwater, including air temperature, discharge, and precipitation in the source region of the Yangtze River (SRYR). Our study shows that the average concentrations of DOC in the three end-members are characterized as the sequence of groundwater &gt; precipitation &gt; river, which is related to the water volume, cycle period, and river flow speed. The seasonality of DOC in the river is observed as the obvious bimodal structure at Tuotuohe (TTH) and Zhimenda (ZMD) gauging stations. The highest concentration appears in July (2.4 mg L−1 at TTH and 2.1 mg L−1 at ZMD) and the secondary high value (2.2 mg L−1 at TTH 1.9 mg L−1 at ZMD) emerges from August to September. It is estimated that 459 and 6751 tons of DOC are transported by the river at TTH and ZMD, respectively. Although the wet deposition flux of DOC is nearly ten times higher than the river flux, riverine DOC still primarily originates from soil erosion of the basin rather than precipitation settlement. Riverine DOC fluxes are positively correlated with discharge, suggesting DOC fluxes are likely to increase in the future. Our findings highlight that permafrost degradation and glacier retreat have a great effect on DOC concentration in rivers and may become increasingly important for regional biogeochemical cycles.

Large fluxes of continental-shelf-borne dissolved organic carbon in the East China Sea and the Yellow Sea

Fluxes of dissolved organic carbon (DOC) from continental shelves to the ocean may play a critical role in marine carbon cycling and budget. However, these fluxes have been poorly constrained because complicated biogeochemical reactions of riverine, atmospheric, and marine organic carbon occur in continental shelf-waters. We used multiple tracers of DOC such as stable- and radiocarbon isotope ratios of DOC, fluorescent properties of dissolved organic matter (FDOM), and 228Ra as a water age tracer to investigate the sources and fluxes of DOC in the northwest Pacific continental margin of the East China Sea and the Yellow Sea. Here, we show that there are significant additional (excess) supplies of DOC in the central Yellow Sea relative to the Changjiang (Yangtze River) source, based on these tracers. The marine δ13C signature (−21.1 ± 1.1‰) and the radiocarbon age (2000 ± 400 yr) of DOC suggest that the additional DOC (Δ14DOC = −44‰) is supplied from a combination of newly produced DOC and the degradation of particulate as well as sedimentary organic matter. The flux of this additional DOC produced in the continental shelf of the East China Sea to the open ocean is estimated to be ~1.9 ± 0.8 Tg C yr−1, which is comparable to that from the Changjiang discharge. Our study implies that the fluxes of continental shelf-borne DOC may be important globally and should be considered in estimating global DOC budgets in the marine environment.

High dissolved organic carbon deposition is buffered by surface soil in a headwater catchment of a subtropical plantation

High atmospheric dry and wet deposition of dissolved organic carbon (DOC) may contribute to net ecosystem carbon balance (NECB), but the input and output fluxes of DOC and their transport processes in a catchment remain unclear. Here, to elucidate the importance of DOC in NECB from the view of Earth’s critical zone, we measured the water fluxes of inputs (precipitation, throughfall, and stemflow) and outputs (soil leachate, surface flow, interflow, streamflow and groundwater) and their DOC concentrations in a subtropical plantation headwater catchment. Our results showed that dry DOC deposition from plant-derived dust and exudates (22,317 kg C km−2 yr−1) was about 4.5 times higher than the wet DOC deposition from precipitation (4966 kg C km−2 yr−1), and accounted for 3.88 and 0.86% of the net ecosystem C productivity (NEP) (5.75×105 kg C km−2 yr−1), respectively. In comparison, DOC fluxes in stream and groundwater were 5912 and 3905 kg C km−2 yr−1, indicating that soil-buffered DOC flux was 17,466 kg C km−2 yr−1 and accounted for 3.04% of NEP. A positive correlation between DOC and pH and total dissolved solids in soil leachate indicated that the biophysical and chemical absorption bridged the interaction between soil minerals and organic carbon. Our results underline the importance of including plant-derived DOC deposition in carbon sink estimations in subtropical plantation headwater catchments.

Joint control by soil moisture, functional genes and substrates on response of N2O flux to climate extremes in a semiarid grassland

Nitrous oxide (N2O), the third most important greenhouse gas, contributes to the increasing frequency and severity of climate extremes. Disentangling feedbacks of climate extremes on terrestrial N2O emission is important for forecasting future climate changes. Here, we experimentally imposed extreme drought and heat wave events during three years in a semiarid grassland to investigate the responses of N2O flux. We identified that N2O flux suppression during droughts was mediated by soil water content (SWC), microbial biomass carbon (MBC), soil inorganic nitrogen (SIN) and dissolved organic carbon (DOC) contents, and the abundance of archaeal amoA, nirK, and narG. However, bacterial amoA, nirS, and nosZ remained stable. Upon rewetting following droughts, the SWC, SIN, DOC, archaeal amoA, nirK, narG, and resultant N2O fluxes recovered to the magnitude of the ambient control. In contrast, heat waves alone or in combination with drought did not impact N2O fluxes or the underlying physical, chemical and microbial states. Stepwise multiple linear regression suggested that SWC, DOC, and MBC were the key factors regulating immediate responses of N2O flux to climate extremes while the major factors regulating seasonal mean N2O flux in response to climate extremes were archaeal amoA abundance, nirK abundance, and MBC. Our results suggest that N2O fluxes were sensitive to droughts but insensitive to heat waves. Soil moisture induced changes in substrate availability, and the community size of total and functional microorganisms in soil jointly regulated N2O responses to climate extremes. The relative importance of regulating factors shifted at different timescales.

Reservoirs change pCO2 and water quality of downstream rivers: Evidence from three reservoirs in the Seine Basin

The global increase in the construction of reservoirs has drawn attention given its documented hydrological and biogeochemical impacts on downstream rivers; however, the impact of reservoirs on downstream pCO2 (partial pressure of carbon dioxide) is still poorly understood. To evaluate these impacts, the interactions between reservoirs and their corresponding upstream and downstream rivers were analyzed for three reservoirs in the Seine Basin based on monthly measurement during two hydrological years. The seasonal variations of water quality in the reservoirs were mainly driven by the entering water and the biogeochemical processes occurring in the reservoirs. Our results unravel the crucial role of reservoir in downstream water quality, which significantly increased DOC (dissolved organic carbon) and BDOC (biodegradable DOC) concentrations, while lowered DSi (dissolved silica) concentrations during emptying period (p < 0.01). Furthermore, the impacts of reservoirs on the annual fluxes of DOC, BDOC, and DSi were quantified and suggested that the three reservoirs respectively increased 20% and 23% of annual fluxes of DOC and BDOC, while decreased 33% of annual DSi fluxes in their downstream rivers. Additionally, the reservoirs significantly decreased downstream riverine pCO2 (p < 0.01), and enhanced the gas transfer coefficient of CO2 in downstream rivers by 1.3 times during the emptying period, which highlights the necessity to consider the potential impact of reservoirs on riverine CO2 emissions. Overall, our results highlight the importance of combining biogeochemical and hydrological characteristics to understand the impacts of reservoirs on downstream rivers, and emphasize the need of similar studies under the current context of increasing reservoir constructions.

The role of terrestrial productivity and hydrology in regulating aquatic dissolved organic carbon concentrations in boreal catchments.

The past decades have witnessed an increase in dissolved organic carbon (DOC) concentrations in the catchments of the Northern Hemisphere. Increasing terrestrial productivity and changing hydrology may be reasons for the increases in DOC concentration. The aim of this study is to investigate the impacts of increased terrestrial productivity and changed hydrology following climate change on DOC concentrations. We tested and quantified the effects of gross primary production (GPP), ecosystem respiration (RE) and discharge on DOC concentrations in boreal catchments over 3 years. As catchment characteristics can regulate the extent of rising DOC concentrations caused by the regional or global environmental changes, we selected four catchments with different sizes (small, medium and large) and landscapes (forest, mire and forest‐mire mixed). We applied multiple models: Wavelet coherence analysis detected the delay‐effects of terrestrial productivity and discharge on aquatic DOC variations of boreal catchments; thereafter, the distributed‐lag linear models quantified the contributions of each factor on DOC variations. Our results showed that the combined impacts of terrestrial productivity and discharge explained 62% of aquatic DOC variations on average across all sites, whereas discharge, gross primary production (GPP) and RE accounted for 26%, 22% and 3%, respectively. The impact of GPP and discharge on DOC changes was directly related to catchment size: GPP dominated DOC fluctuations in small catchments (<1 km2), whereas discharge controlled DOC variations in big catchments (>1 km2). The direction of the relation between GPP and discharge on DOC varied. Increasing RE always made a positive contribution to DOC concentration. This study reveals that climate change‐induced terrestrial greening and shifting hydrology change the DOC export from terrestrial to aquatic ecosystems. The work improves our mechanistic understanding of surface water DOC regulation in boreal catchments and confirms the importance of DOC fluxes in regulating ecosystem C budgets.