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

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

Radiocarbon signatures of carbon phases exported by Swiss rivers in the Anthropocene

Lateral carbon transport through the land-to-ocean-aquatic-continuum (LOAC) represents a key component of the global carbon cycle. This LOAC involves complex processes, many of which are prone to anthropogenic perturbation, yet the influence of natural and human-induced drivers remains poorly constrained. This study examines the radiocarbon (14C) signatures of particulate and dissolved organic carbon (POC, DOC) and dissolved inorganic carbon (DIC) transported by Swiss rivers to assess controls on sources and cycling of carbon within their watersheds. Twenty-one rivers were selected and sampled during high-flow conditions in summer 2021, a year of exceptionally high rainfall. Δ14C values of POC range from -446‰ to -158‰, while corresponding ranges of Δ14C values for DOC and DIC are -377‰ to -43‰ and -301‰ to -40‰, respectively, indicating the prevalence of pre-aged carbon. Region-specific agricultural practices seem to have an influential effect on all three carbon phases in rivers draining the Swiss Plateau. Based on Multivariate Regression Analysis, mean basin elevation correlated negatively with Δ14C values of all three carbon phases. These contrasts between alpine terrain and the lowlands reflect the importance of overriding ecoregional controls on riverine carbon dynamics within Switzerland, despite high spatial variability in catchment properties. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Biodegradable Dissolved Organic Carbon (BDOC) Removal from Micro-Polluted Water Source Using Ultrafiltration: Comparison with Conventional Processes, Operation Conditions and Membrane Fouling Control

The biodegradable dissolved organic carbon (BDOC) in micro-polluted water sources affects the drinking water quality and safety in the urban water supply. The conventional technology of “coagulation-sedimentation-filtration” in a water plant located in the lower reaches of the Yangtze River removed dissolved organic carbon (DOC) with a molecular weight (MW) > 30 kDa effectively, but the BDOC elimination only ranged 27.4−58.1%, due to their predominant smaller MW (<1 kDa), leading to a high residual BDOC of 0.22−0.33 mg/L. To ensure the biological stability of drinking water, i.e., the inability to support microbial growth (BDOC < 0.2 mg/L), a pilot-scale ultrafiltration process (UF, made of aromatic polyamide with MW cut-off of 1 kDa) was operated to remove BDOC as an advanced treatment after sand-filtration. Results showed the membrane flux decreased with the increase in the influent BDOC concentration and decrease in operating pressure. With an operating pressure of 0.25 MPa, the BDOC removal by UF reached 80.7%, leading to a biologically stable BDOC concentration of 0.08 mg/L. The fouling of the membrane was mainly caused by organic pollution. The H2O2−HCl immersion washing method effectively cleaned the membrane surface fouling, with a recovery of membrane flux of 98%.

Dynamics of Soil Carbon Fractions and Carbon Stability in Relation to Grassland Degradation in Xinjiang, Northwest China

Grassland degradation usually results in significant shifts in vegetation species composition and plant biomass, thus altering the soil organic carbon (SOC) content and stability. Dynamics of labile carbon fractions after grassland degradation were well addressed; however, the changes in stable carbon fractions were poorly quantified. Soil samples at 0–10 cm and 10–20 cm depth were collected from a native grassland (NA), a lightly degraded grassland (LD), a moderately degraded grassland (MD), and a severely degraded grassland (SD) in northwest China to assess the influence of grassland degradation on the total SOC content, four SOC fractions (very labile carbon, CF1; labile carbon, CF2; less labile carbon, CF3; non-labile carbon, CF4), and SOC stability. Compared with the NA, the contents under LD, MD, and SD at 0–20 cm depth reduced by 20.58%, 29.22%, and 64.58% for total SOC, 21.38%, 23.00%, and 63.66% for CF1, 13.81%, 20.58%, and 62.26% for CF2, 24.30%, 35.05%, and 68.63% for CF3, and 22.17%, 38.80%, and 63.82% for CF4, respectively. The linear relationships between the total SOC and the four fractions of CF1, CF2, CF3, and CF4 were significant in this study. The lability index of SOC under the NA, LD, MD, and SD was 1.57, 1.59, 1.67, and 1.57, respectively, and no significant difference was found among the four grasslands. To conclude, grassland degradation changes the contents of total SOC and its labile and stable fractions but did not change the SOC stability in northwest China.