Transport Of Dissolved Carbon Research Articles

Spatiotemporal variability of dissolved carbon and sources of dissolved inorganic carbon influenced by freeze–thaw and subsurface flow in an alpine headwater catchment of the Qinghai-Tibetan Plateau

The temporal freeze–thaw (FT) and the spatial rock distribution have great impacts on riverine dissolved carbon transport and dissolved inorganic carbon (DIC) sources. However, the effects of the subsurface flow (SSF) process during the FT process and spatial riverine transport on dissolved carbon transport and DIC sources are still unclear due to the lack of available spatiotemporal measurements. Herein, we designed and implemented a complete scheme of water sampling including stream water (from upper headwater to the middle and downward) and SSF (in the soil, saprolite, and weathered bedrock layers) in an alpine headwater catchment. The characteristics of the dissolved organic and inorganic carbon (DOC and DIC), and δ13C-DIC were analyzed, and the spatiotemporal variability of the DIC source was explored. Our findings reveal that the impact of the FT process on the dissolved carbon dynamics and DIC sources is revealed in the dissolved carbon transport process controlled by the SSF process. Specifically, the deepening of the active layer during the FT process enhances the movement of SSF into the deeper layer, thereby activating DIC in this deep layer. Concurrently, DOC from the shallow layer rapidly infiltrates deeper layers, leading to minimal differences in DOC concentrations across layers. This results in an enriched behavior of DOC (b = 0.21) and a depleted behavior of DIC (b = −0.27) during the FT process. In terms of influence on DIC sources, the deepening active layer promotes silicate weathering in the silicate-rich deep layers and strengthens organic carbon mineralization. On a spatial scale, the rock distribution, particularly the Si/Ca ratio, predominantly controls the chemical weathering process and, consequently, determines the DIC sources. In contrast, the influence of organic carbon mineralization on DIC sources becomes less significant. In this study, the impact mode of SSF on dissolved carbon transport and transformation is elucidated, and the spatial influence of rock distribution on riverine DIC sources is emphasized.

Connectivity of floodplain influences riverine carbon outgassing and dissolved carbon transport

Rivers not only function as a conduit for the delivery of terrestrial constituents to oceans, but they also serve as an essential medium for biogeochemical processing of the constituents. While extensive research has been conducted on carbon transport in many rivers, little is known about carbon transformation in engineered rivers reconnected with their floodplain network. Being the largest distributary of the levee-confined Mississippi River (MR), the Atchafalaya River (AR) carries 25 % of the MR water, flowing through North America's largest freshwater swamp basin and emptying into the Gulf of Mexico. Previous studies reported that this 200-km long, 5–30-km wide river basin can remove a substantial amount of riverine nutrients and organic carbon. This study aimed to test the hypothesis that the AR emits significantly higher CO2 into the atmosphere as it flows through its extensive floodplain network than the levee-confined MR does. From January 2019 to December 2021, we conducted biweekly – monthly in-situ measurements in the lower AR at Morgan City and in the lower Mississippi River at Baton Rouge. Field measurements included partial pressure of dissolved CO2 (pCO2), water temperature, chlorophyll a, colored dissolved organic matter, dissolved oxygen, pH, and turbidity. During each field sampling, water samples were collected and analyzed for concentrations of dissolved organic and inorganic carbon (DOC and DIC). Mass transport of DOC and DIC and outgassing of CO2 were quantified for the two rivers. We found that pCO2 levels were significantly higher in the AR (mean: 3563 μatm; min-max: 1130–8650 μatm) than those in the MR (1931 μatm, 836–3501 μatm), resulting in a doubled CO2 outgassing rate in the AR (486 mmol m2 d−1) than in the MR (241 mmol m2 d−1). The AR had higher DOC (8.5 mg L−1) but lower chlorophyll a (153.9 AFU) when compared with the MR (7.5 mg L−1 and 164.0 AFU). Water temperature was constantly higher in the AR than in the MR, especially during the wintertime. Since the Mississippi-Atchafalaya River system is among the world's largest and most engineered river systems, our assessment offers a field case study to inform on the potential implications of reconnecting rivers with their floodplains networks.

Dissolved Carbon Transport and Processing in North America’s Largest Swamp River Entering the Northern Gulf of Mexico

Transport and transformation of riverine dissolved carbon is an important component of global carbon cycling. The Atchafalaya River (AR) flows 189 kilometers through the largest bottomland swamp in North America and discharges ~25% of the flow of the Mississippi River into the Gulf of Mexico annually, providing a unique opportunity to study the floodplain/wetland impacts on dissolved carbon. The aim of this study is to determine how dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the AR change spatially and seasonally, and to elucidate which processes control the carbon cycling in this intricate swamp-river system. From May 2015 to May 2016, we conducted monthly river sampling from the river’s inflow to its outflow, analyzing samples for concentrations and δ13C stable isotope composition of DOC and DIC. We found that DIC concentrations in the AR were three times higher than the DOC concentrations on average, and showed more pronounced downstream changes than the DOC. During the study period, the river discharged a total of 5.35 Tg DIC and a total of 2.34 Tg DOC into the Gulf of Mexico. Based on the mass inflow–outflow balance, approximately 0.53 Tg (~10%) of the total DIC exported was produced within the floodplain/wetland system, while 0.24 Tg (~10%) of the DOC entering the basin was removed. The AR’s water was consistently oversaturated with CO2 partial pressure (pCO2) above the atmospheric pCO2 (with pCO2 varying from 551 µatm to 6922 µatm), indicating a large source of DIC from river waters to the atmosphere as well as to the coastal margins. Largest changes in carbon constituents occurred during periods of greatest inundation of the swamp-river basin and corresponded with shifts in isotopic composition. This effect was particularly pronounced during the initial flood stages, supporting the hypothesis that subtropical floodplains can act as effective enhancers of the biogeochemical cycling of dissolved carbon.

Dissolved carbon fluxes in a vegetation restoration area of an eroding landscape

Dissolved carbon (DC) is a critical component of the global carbon (C) cycle. DC transport occurs through water-driven erosion and infiltration during rain storms. To explore the specific role of DC flux in topsoil C pool dynamics during rainfall events and predict the trend of ratios of lateral versus vertical DC efflux from topsoil in a vegetation restoration area, we measured the major DC fluxes at four runoff plots, during rainfall events in an eroding soil landscape on the Chinese Loess Plateau. The results show that topsoil vertical DC efflux into deep soil layers accounted for approximately 98.7 (±1.0) % of the total dissolved carbon efflux in plots with vegetation versus 95.3% in a plot without vegetation. The carbon sequestration capacity of the top soil would be underestimated by up to 38 (±5) % if the vertical DC efflux was omitted. The ratios of lateral versus vertical DC efflux tended to increase with rainfall intensity. The results of this study improve understanding of the carbon cycle processes during rainfall events in general and estimation of carbon sequestration rates in vegetation restoration regions such as the Chinese Loess Plateau in particular.

Dissolved carbon transport in a river-lake continuum: A case study in a subtropical watershed, USA

Rivers and lakes have been traditionally studied as separate entities for carbon transport. However, there is a gap in our knowledge of the connectivity of dissolved carbon in a river-lake continuum. In this study, we analyzed dissolved carbon along the Little River-Catahoula Lake in subtropical Louisiana, United States to assess carbon biogeochemistry in such a river-lake continuum. Monthly in-situ measurements and water sample collections were made at four locations during April 2015 to February 2016 to determine riverine carbon transport into and out of the lake. Results show that much of the dissolved inorganic carbon (DIC) in the river-lake continuum originated from 13C depleted sources with an average δ13CDIC of −18.5‰. Significant decreases in DIC were found after the river passed through the lake (from 482 to 399 μmol L−1), which was most prevalent when the lake was not affected by backwater flow from the downstream river. CO2 outgassing could be mainly responsible for the sink behavior of the lake for DIC. Dissolved organic carbon (DOC) in the studied watershed were mostly terrigenous with low δ13CDOC averaged at −29.2‰. Significant, consistent decreases in DOC concentrations were found from the river to the lake inflow and then to the lake outflow. During the majority of the year, the lake reduced DOC concentrations from the river inflow water, but switched to functioning as a source of DOC during warmer, dryer conditions in September and October due to increased water residence time. Therefore, the lake functioned both as a sink and as a source for DOC.