Hulun Lake, located in a remote, semi-arid area in the northeast part of Inner Mongolia, China, shares a transboundary basin with Mongolia and supports a unique wetland ecosystem that includes many endangered species. Decadal scale decreases in the lake stage and increased salinity make an understanding of the lake’s water and salt sources critical for appropriate design of strategies to protect and manage the lake. Multiple tracers (chloride, and δ¹⁸O and δ²H in water) in samples collected from lake water, rivers, and nearby water wells were used in conjunction with an annual water balance based on historic data to better understand the lake’s major water and salt sources. The average annual water balance was conducted for two time periods: 1981–2000 and 2001–2013. The contribution of river discharge to the annual lake input decreased by half (from 64% to 31%) between the two time periods, while the volumetric contribution of groundwater discharge increased four-fold (from about 11% to about 50% of the total lake input). Significant evaporation was apparent in the stable isotope composition of the present-day lake water, however, evaporation alone could not account for the high lake water chloride concentrations. Limited domestic well water sampling, a regional salinity survey, and saline soils suggest that high chloride groundwater concentrations exist in the region south of the lake. The chloride mass balance suggested that groundwater currently contributes more than 90% of the annual chloride loading to the lake, which is likely four times greater than the earlier period (1981–2000) with lower groundwater input. The use of water and chloride mass balances combined with water isotope analyses could be applied to other watersheds where hydrologic information is scarce.

The organic matter of lake sediment plays an important role in paleolimnological reconstruction. Here, we report a detailed study of organic matter components (C_org%, N%, δ¹³C, δ¹⁵N) in a dated sediment core of Hulun Lake in northern China. Multiple mixing models based on the stoichiometric ratios and stable isotopic compositions were applied to quantify the contributions of organic matter sources in lake sediment. The results show that the organic matter in the sediments from Hulun Lake mainly comes from terrestrial organic matter: the proportion of terrestrial organic matter is more than 80%. The results of the SIAR mixing model further reveal that the proportions of terrestrial C₃ plants-derived organic matter, soil organic matter, and lake plankton-derived organic matter were 76.0%, 13.9%, and 10.1%, respectively. The organic matter content of lake sediment from terrestrial sources began to increase significantly from 1980 onward, which is consistent with the growth in overgrazing in the Hulun Lake basin. The content of organic matter from endogenous lake-derived sources began to increase significantly after 2000 due to the nutrients gradually becoming concentrated in lake water, indicating that the reduction in rivers’ discharge and the downgrade of the lake water level were the immediate causes of the lake’s environmental deterioration during this period.

The distribution of sediment grain size can record past environmental conditions and human activity. In this study, radioisotope of ²¹⁰Pb and ¹³⁷Cs and a grain size of a 41 cm core in Hulun Lake were applied to reconstruct the high-resolution sedimentation history. The profiles of the grain size of the lake sediments show that silt (4–63 µm) was the largest contribution with an average content of 84.05%, and the second largest contribution was sand (>63 µm) with an average content of 15.68%. The median grain size and the mean grain size in the whole sediment core was 22.39 μm and 36.85 μm, respectively. Correlations of the sedimentological variables with instrumental measurements were also analyzed. The peak–trough value of the mean grain size of the sediments in Hulun Lake can reflect the magnitude of rainfall intensity and river discharge. The clay and silt contents at a depth of approximately 32–38 cm was different from other depths throughout the core, which showed continuous maxima with an average content of 0.35% and 94.08%. These changes in grain size correspond to the period of dam construction in 1963–1970. Therefore, the sediment grain size of Hulun Lake effectively recorded the dam-building activity.

This study successfully synthesized a BC/TiO ₂ /g-C ₃ N ₄ composite photocatalyst using the sol–gel method, conducted an in-depth analysis of the influence mechanism of BC on charge transfer performance in heterojunction materials, and revealed its key role in regulating the charge transport process. The introduction of biochar significantly increased the specific surface area of BC/TiO ₂ /g-C ₃ N ₄ compared to the TiO ₂ /g-C ₃ N ₄ catalyst, broadening the visible light absorption range. Under simulated sunlight irradiation with a wavelength greater than 420 nm, the three-component material MBC-500, calcined at 500 °C, exhibits the best catalytic performance, with an adsorption photocatalytic degradation rate of 98.13% for sulfadiazine (SDZ) within 60 min, which is 3.46, 3.40, and 2.36 times that of TiO ₂ , g-C ₃ N ₄ , and TiO ₂ /g-C ₃ N ₄ , respectively. Characterization analysis and density functional theory (DFT) calculations revealed the energy band structure and electron transfer pathways of the composite photocatalyst, indicating the significant role of biochar in electron transfer and storage. Additionally, the calculated adsorption energies demonstrated the good adsorption performance of MBC-500 for O ₂ and sulfadiazine. This composite photocatalyst exhibited good stability and reusability even after five cycles of use. During the degradation process of SDZ, ·O ₂ ⁻ , h ⁺ , ·OH play a major role. Graphical Abstract

Microbial communities play a pivotal role in material cycling, energy flow, and pollutant degradation within river ecosystems. Thus, gaining a clear understanding of how wastewater discharge affects microbial community structure and function is essential for the protection and management of the surface water environment. In this study, a total of 9 samples were collected from the Sha River in March 2024. Subsequently, 16S rRNA sequencing technology combined with investigation of physicochemical properties of water was used to investigate the compositional diversity, spatial distribution, and explore the environmental effects of wastewater discharged on microorganisms. The sequencing results of species at the phylum level revealed that the dominant microbial phyla in the Sha River were primarily Proteobacteria (55.4%), Actinobacteriota (24.0%), Bacteroidota (14.3%), and Verrucomicrobiota (2.6%). The most dominant phylum, Proteobacteria, exhibited varying abundances across different sampling sites in the Sha River basin, with the highest abundances observed at Sites S2, S4, S5, and S6. This is mainly due to the fact that the upstream areas of Sites S2, S4, S5, and S6 are characterized by high concentrations of COD and NH₃-N, which are caused by wastewater discharge. Quantitative analysis was also conducted using the Source Tracker model; the results showed that S2 (36.7%) and S4 (31.3%) in the upper reaches of the Sha River are the primary contributors to the microbial community in the downstream catchment area (S6). The study found that the impact of wastewater discharge on the microbial community in the downstream water body exhibits a “longitudinal persistence of microbial signatures” even though the physicochemical pollution indicators of the water body have decreased. These findings of this study represent the application in microbial source tracking in the upstream and downstream sections of rivers, providing strong support for formulating more effective environmental protection strategies in the Sha River basin.