Time-series monitoring of river hydrochemistry and multiple isotope signals in the Yarlung Tsangpo River reveals a hydrological domination of fluvial nitrate fluxes in the Tibetan Plateau

Chemodiversity of dissolved organic matter exports from subtropical humid catchment driven by hydrological connectivity

The quantity and quality of dissolved organic matter (DOM) exported from source areas are closely related to hydrological linkage between source areas and streams, that is hydrological connectivity. However, understanding of how hydrological connectivity regulates the export of catchment DOM components remains inadequate. In this study, high-frequency monitoring of groundwater and runoff from subtropical humid catchment was conducted for 20 months, and hydrological connectivity was quantitatively characterized by considering both surface and subsurface hydrological processes. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was utilized to investigate the DOM molecular composition. Results showed that over half of the areas in the catchment could not persistently establish hydrological connectivity with the stream during the rainfall. The average proportion of lignin was the highest in DOM components, followed by tannin and proteins. Additionally, both modified aromaticity index and double bond equivalence reached maximums at peak discharge, reflecting terrestrial materials could increase DOM aromaticity and unsaturated degree. Partial least square-structural equation modeling revealed significantly direct effects of rainfall, antecedent conditions, and hydrological connectivity on dissolved organic carbon (DOC) export. Furthermore, nonlinear relationships were observed between hydrological connectivity and DOC, tannin, and condensed aromatics. Specifically, the instantaneous DOC flux increased dramatically when the hydrological connectivity strength exceeded 0.14; tannin and condensed aromatics exhibited a rapid increase with rising connectivity strength, but remained stable at connectivity strength above 0.25. However, hydrological connectivity showed no significant correlation with unstable components (such as lipids, protein, amino sugars, and carbohydrates). These results provide new insights into hydrological controls on the quantity and quality of DOM export and contribute to developing appropriate catchment management strategies for carbon storage.

Nitrate transformation and source tracking of Yarlung Tsangpo River using a multi-tracer approach combined with Bayesian stable isotope mixing model

Excessive levels of nitrate nitrogen (NO3--N) could lead to ecological issues, particularly in the Yarlung Tsangpo River (YTR) region located on the Qinghai Tibet Plateau. Therefore, it is crucial to understand the fate and sources of nitrogen to facilitate pollution mitigation efforts. Herein, multiple isotopes and source resolution models were applied to analyze key transformation processes and quantify the sources of NO3-. The δ15N-NO3- and δ18O-NO3- isotopic compositions in the YTR varied between 1.23‰ and 13.64‰ and -7.88‰-11.19‰, respectively. The NO3--N concentrations varied from 0.08 to 0.86 mg/L in the dry season and 0.20-1.19 mg/L during the wet season. Nitrification remained the primary process for nitrogen transformation in both seasons. However, the wet season had a widespread effect on increasing nitrate levels, while denitrification had a limited ability to reduce nitrate. The elevated nitrate concentrations during the flood season were caused by increased release of NO3- from manure & sewage (M&S) and chemical fertilizers (CF). Future endeavors should prioritize enhancing management strategies to improve the utilization efficiency of CF and hinder the direct entry of untreated sewage into the water system.

Soil organic nitrogen variation shaped by diverse agroecosystems in a typical karst area: evidence from isotopic geochemistry

Background

Soil organic nitrogen (SON) levels can respond effectively to crop metabolism and are directly related to soil productivity. However, simultaneous comparisons of SON dynamics using isotopic tracing in diverse agroecosystems are lacking, especially in karst areas with fragile ecology.

Methods

To better understand the response of SON dynamics to environmental changes under the coupling of natural and anthropogenic disturbances, SON contents and their stable N isotope (δ15NSON) compositions were determined in abandoned cropland (AC, n = 16), grazing shrubland (GS, n = 11), and secondary forest land (SF, n = 20) from a typical karst area in southwest China.

Results

The SON contents in the SF (mean: 0.09%) and AC (mean: 0.10%) profiles were obviously lower than those in the GS profile (mean: 0.31%). The δ15NSON values ranged from 4.35‰-7.59‰, 3.79‰-7.23‰, and 1.87‰-7.08‰ for the SF, AC, and GS profiles, respectively. Decomposition of organic matter controlled the SON variations in the secondary forest land by the covered vegetation, and that in the grazing shrubland by goat excreta. δ15NSON ranges were controlled by the covered vegetation, and the δ15NSON fractionations during SON transformation were influenced by microorganisms in all surface soil.

Conclusions

The excreta of goats that contained 15N-enriched SON induced a heavier δ15NSON composition in the grazed shrubland. Long-term cultivation consumes SON, whereas moderate grazing increases SON content to reduce the risk of soil degradation. This study suggests that optimized crop-livestock production may benefit the sustainable development of agroecosystems in karst regions.

Multiple isotopes reveal the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of Yellow River

The continuous increase of nitrate (NO3-) level in rivers is a hot issue in the world. However, the driving mechanism of high NO3- level in large rivers is still lacking, which has limited the use of river water and increased the cost of water treatment. In this study, multiple isotopes and source resolution models are applied to identify the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of the Yellow River (LRYR). The major sources of NO3- were sewage and manure (SAM) in the low-flow season and soil nitrogen (SN) and chemical fertilizer (CF) in the high-flow season. Nitrification was the most key process of nitrogen cycling in the LRYR. However, in the biological removal processes, denitrification may not occur significantly. The temporal variation of contributions of NO3- sources were estimated by a source resolution model in the LRYR. The proportional contributions of SAM and CF to NO3- in the low-flow and high-flow season were 32.5%-52.3%, 44.2%-46.2% and 36.0%-40.8%, 54.9%-56.9%, respectively. The driving mechanisms of high NO3- level were unreasonable sewage discharge, intensity rainfall runoff, nitrification and lack of nitrate removal capacity. To control the NO3- concentration, targeted measures should be implemented to improve the capacity of sewage and wastewater treatment, increase the utilization efficiency of nitrogen fertilizer and construct ecological engineering. This study deepens the understanding of the driving mechanism of high nitrate level and provides a vital reference for nitrogen pollution control in rivers to other area of the world.

Subsurface hydrological connectivity controls nitrate export flux in a hilly catchment

Subsurface runoff represents the main pathway of nitrate transport in hilly catchments. The magnitude of nitrate export from a source area is closely related to subsurface hydrological connectivity, which refers to the linkage of separate regions of a catchment via subsurface runoff. However, understanding of how subsurface hydrological connectivity regulates catchment nitrate export remains insufficient. This study conducted high-frequency monitoring of shallow groundwater in a hilly catchment over 17 months. Subsurface hydrological connectivity of the catchment over 38 rainfall events was analyzed by combining topography-based upscaling of shallow groundwater and graph theory. Moreover, cross-correlation analysis was used to evaluate the time-series similarity between subsurface hydrological connectivity and nitrate flux during rainfall events. The results showed that the maximum subsurface hydrological connectivity during 32 out of 38 rainfall events was below 0.5. Although subsurface flow paths (i.e., the pathways of lateral subsurface runoff) exhibited clear dynamic extension and contraction during rainfall events, most areas in the catchment did not establish subsurface hydrological connectivity with the stream. The primary pattern of nitrate export was flushing (44.7%), followed by dilution (34.2%), and chemostatic behavior (21.1%). A threshold relationship between subsurface hydrological connectivity and nitrate flux was identified, with nitrate flux rapidly increasing after the subsurface connectivity strength exceeded 0.121. Moreover, the median value of cross-correlation coefficients reached 0.67, which indicated subsurface hydrological connectivity exerts a strong control on nitrate flux. However, this control effect is not constant and it increases with rainfall amount and intensity as a power function. The results of this study provide comprehensive insights into the subsurface hydrological control of catchment nitrate export.

Nitrate dynamics in the streamwater-groundwater interaction system: Sources, fate, and controls

Nitrate (NO3-) pollution has attracted widespread attention as a threat to human health and aquatic ecosystems; however, elucidating the controlling factors behind nitrate dynamics under the context of changeable hydrological processes, particularly the interactions between streamwater and groundwater (SW-GW), presents significant challenges. A multi-tracer approach, integrating physicochemical and isotopic tracers (Cl-, δ2H-H2O, δ18O-H2O, δ15N-NO3- and δ18O-NO3-), was employed to identify the response of nitrate dynamics to SW-GW interaction in the Fen River Basin. The streamwater and groundwater NO3- concentrations varied greatly with space and time. Sewage and manure (28 %-73 %), fertilizer (14 %-36 %) and soil organic nitrogen (12 %-28 %) were the main NO3- sources in water bodies. Despite the control of land use type on streamwater nitrate dynamics in losing sections, SW-GW interactions drove NO3- dynamics in both streamwater and groundwater under most circumstances. In gaining streams, streamwater nitrate dynamics were influenced either by groundwater dilution or microbial nitrification, depending on whether groundwater discharge ratios exceeded or fell below 25 %, respectively. In losing streams, groundwater nitrate content increased with streamwater infiltration time, but the influence was mainly limited within 3 km from the river channel. This study provides a scientific basis for the effective management of water nitrate pollution at the watershed scale.

Riverine sulfate sources and behaviors in arid environment, Northwest China: Constraints from sulfur and oxygen isotopes

The fate of riverine sulfate ion (SO42-) and its environmental effects in arid environment are difficult to evaluate due to its complicated sources and strongly coupled behaviors with water cycle which is significantly modified by humans. To understand the sulfur cycle in aquatic systems in arid environment, the chemical and sulfur and oxygen isotopic compositions (δ34SSO4 and δ18OSO4) of major rivers around the Badain Jaran Desert, northwestern China, were investigated. These rivers had averaged SO42- content at 1336 µmol/L, over 10 times higher than the global average. The δ34SSO4 and δ18OSO4 values ranged from -5.3‰ to +11.8‰ and +1.6‰ to +12.8‰, respectively. The end-member analysis and the inverse model showed that riverine sulfate was mainly derived from evaporites dissolution (0-87%), sulfide oxidation (13%-100%) and precipitation (0-33%), indicating heterogeneity in sulfur sources and behaviors along the river drainage with the lithology variations and climate gradients. Multiple isotopic tools combining with hydro-chemistry compositions could be applied to reveal sulfur cycle in arid environment. Based on the calculation, sulfide oxidation plays the primary role in the headwater and upstream in the Qilian-Mountains area, where sulfide is widely exposed. While the proportion of evaporites dissolution contributing to riverine sulfate is much higher in downstream in a drier environment. Besides, less precipitation and higher temperature can lead to more intensive evaporation, affecting the process of sulfide oxidation and enhancing the rates of evaporites dissolution and sulfate precipitation in the basin.

Data-driven assessment of soil total nitrogen on the Qinghai-Tibet Plateau

The investigation of soil total nitrogen (STN) holds significant importance in the preservation and sustainability of Earth's ecosystems. The Qinghai-Tibet Plateau (QTP), renowned as the world's most expansive plateau and characterized by its exceptionally delicate ecosystem, demands an in-depth exploration of its STN content. In this study, we use a machine learning approach to extrapolate point-scale measured STN stocks to the entire QTP and calculated STN storage from 0 to 2 m. Our results show that the XGB algorithm performs well in modeling STN despite variations in simulation accuracy for specific depth ranges. The spatial distribution of STN across the QTP exhibits pronounced heterogeneity, especially for the 0-50 cm soil layer, with relatively higher STN stocks in the southeast and lower stocks in the northwest of QTP. The vertical distribution reveals a gradual decrease in STN storage with increasing depth. The 0-50 cm soil layer holds the highest STN stocks, averaging around 0.78 kg/m2, which is almost the sum of STN stocks in the 50-100 cm and 100-200 cm soil layers. Meanwhile, the STN stocks are smaller in permafrost zone than that in non-permafrost zone. We also investigate the impact factors that control the spatiotemporal distribution of STN. It indicates that vegetation, precipitation, temperature, and elevation are the major factors for STN distribution, while physical properties of the soil have a relatively smaller impact. These findings are crucial for understanding the distribution and evolution of STN on the QTP.

Nitrate budget of a terrestrial-to-marine continuum in South China: Insights from isotopes and a Markov chain Monte Carlo model

Anthropogenic nitrate (NO3-) production has been increasing and is exported to the ocean via river networks, causing eutrophication and ecological damage. While studies have focused on river NO3- pollution, what has been lacking is the quantification of the sources of NO3- in coastal rivers. This study applied the dual isotopes (δ15N/δ18O-NO3-) to quantify the sources and their fluxes of NO3- in two inflow rivers of the Qinzhou Bay. By adding our results to the NO3- source apportionment in Qinzhou Bay, we, for the first time, established the NO3- budgets of the terrestrial-to-marine continuum in both high- and low-flow seasons. We quantitatively showed the direct and indirect roles (e.g., the stimulation of nitrification by sewage ammonium-NH4+) of terrestrial sources in driving the high NO3- loading in the estuary. The results highlighted the necessity to consider coastal rivers and estuary as a whole, which could shed light on the effective reduction of NO3- pollution in coastal environments.

Hydrogeochemical characteristics and recharge sources identification based on isotopic tracing of alpine rivers in the Tibetan Plateau

Alpine rivers originating from the Tibetan Plateau (TP) contain large amounts of water resources with high environmental sensitivity and eco-fragility. To clarify the variability and controlling factors of hydrochemistry on the headwater of the Yarlung Tsangpo River (YTR), the large river basin with the highest altitude in the world, water samples from the Chaiqu watershed were collected in 2018, and major ions, δ²H and δ¹⁸O of river water were analyzed. The values of δ²H (mean: -141.4‰) and δ¹⁸O (mean: -18.6‰) were lower than those in most Tibetan rivers, which followed the relationship: δ²H = 4.79*δ¹⁸O-52.2. Most river deuterium excess (d-excess) values were lower than 10‰ and positively correlated with altitude controlled by regional evaporation. The SO₄^2- in the upstream, the HCO₃^- in the downstream, and the Ca²⁺ and Mg²⁺ were the controlling ions (accounting for >50% of the total anions/cations) in the Chaiqu watershed. Stoichiometry and principal component analysis (PCA) results revealed that sulfuric acid stimulated the weathering of carbonates and silicates to produce riverine solutes. This study promotes understanding water source dynamics to inform water quality and environmental management in alpine regions.

Unexpectedly high nitrate levels in a pristine forest river on the Southeastern Qinghai-Tibet Plateau

River nitrate (NO₃^-) pollution is a global environmental issue. Recently, high NO₃^- levels in some pristine or minimally-disturbed rivers were reported, but their drivers remain unclear. This study integrated river isotopes (δ¹⁸O/δ¹⁵N-NO₃^- and δD/¹⁸O-H₂O), ¹⁵N pairing experiments, and qPCR to reveal the processes driving the high NO₃^- levels in a nearly pristine forest river on the Qinghai-Tibet Plateau. The river isotopes suggested that, at the catchment scale, NO₃^- removal was prevalent in summer, but weak in winter. The pristine forest soils contributed more than 90 % of the riverine NO₃^-, indicating the high NO₃^- backgrounds. The release of soil NO₃^- to the river was "transport-limited" in both seasons, i.e., the NO₃^- production/stock in the soils exceeded the capacity of hydrological NO₃^- leaching. In summer, this regime and the NO₃^--plentiful conditions in the soils associated with the strong NO₃^- nitrification led to the high riverine NO₃^- levels. While the in-soil nitrification was weak in winter, the leaching of legacy NO₃^- resulted in the consistently high NO₃^- levels. This study provides insights into the reasons for high NO₃^- levels in pristine or minimally-disturbed rivers worldwide and highlights the necessity to consider NO₃^- backgrounds when evaluating anthropogenic NO₃^- pollution in rivers.

Nitrogen sources and conversion processes in shallow groundwater around a plain lake (Northwest China): Evidenced by multiple isotopes and water chemistry

The groundwater quality is severely impacted by Nitrate (NO₃^--N) pollution worldwide. Effective lake quality management depends on understanding the origin and fate of nitrogen (N) in the groundwater around lakes. This study combined data for multiple stable isotopes (δ²H-H₂O and δ¹⁸O-H₂O, δ¹⁵N-NO₃ and δ¹⁸O-NO₃) and hydrochemistry with the hydrodynamic monitoring profile and a Bayesian isotope mixing (MixSIAR) model to clarify the sources and transformation of N within shallow groundwater around Shahu Lake in the arid area plain of Northwest China. In May 2022, multiple water samples were collected from aquifers (n = 33), drainage water (n = 1), channel water (n = 1), and lake water (n = 7). The results showed that 57% of groundwater samples had high NO₃^--N concentrations exceeding the World Health Organisation threshold for drinking water (10 mg/L). The high variation in δ¹⁵N-NO₃ (from -9.21‰ to +27.57‰) and δ¹⁸O-NO₃ (from -8.32‰ to +19.04‰) revealed multiple N sources and conversion processes. According to nitrate isotopes and the MixSIAR model, N fertilizer, soil organic N and manure, and sewage are the main sources of nitrogen in groundwater and lake water, which account for 40.61%, 35.86%, and 21.55% of groundwater NO₃^--N, respectively, and 35.07%, 34.43%, and 27.49% of lake water NO₃^--N. Hydrodynamic monitoring combined with water isotopes showed that upper groundwater (5-10 m) within 1.22 km of the adjacent lake shore strongly interacted with the lake. In groundwater, nitrification predominated, while local denitrification remained a possibility. In conclusion, this research offers a comprehensive approach to determining the sources and conversion of N in contaminated groundwater.

From Soil to River: Revealing the Mechanisms Underlying the High Riverine Nitrate Levels in a Forest Dominated Catchment

Elevated riverine nitrate (NO₃^-) levels have led to increased eutrophication and other ecological implications. While high riverine NO₃^- levels were generally ascribed to anthropogenic activities, high NO₃^- levels in some pristine or minimally disturbed rivers were reported. The drivers of these unexpectedly high NO₃^- levels remain unclear. This study combined natural abundance isotopes, ¹⁵N-labeling techniques, and molecular techniques to reveal the processes driving the high NO₃^- levels in a sparsely populated forest river. The natural abundance isotopes revealed that the NO₃^- was mainly from soil sources and that NO₃^- removal processes were insignificant. The ¹⁵N-labeling experiments also quantitatively showed that the biological NO₃^- removal processes, i.e., denitrification, dissimilatory NO₃^- reduction to ammonium (DNRA), and anaerobic ammonia oxidation (anammox), in the soils and sediments were weak relative to nitrification in summer. While nitrification was minor in winter, the NO₃^- removal was insignificant relative to the large NO₃^- stock in the catchment. Stepwise multiple regression analyses and structural equation models revealed that in summer, nitrification in the soils was regulated by the amoA-AOB gene abundances and NH₄⁺-N contents. Low temperature constrained nitrification in winter. Denitrification was largely controlled by moisture content in both seasons, and anammox and DNRA could be explained by the competition with nitrification and denitrification on their substrate (nitrite-NO₂^-). We also revealed the strong hydrological control on the transport of soil NO₃^- to the river. This study effectively revealed the mechanisms underlying the high NO₃^- levels in a nearly pristine river, which has implications for the understanding of riverine NO₃^- levels worldwide.

Riverine nitrate source and transformation as affected by land use and land cover

A mixed land use/land cover (LULC) catchment increases the complexity of sources and transformations of nitrate in rivers. Spatial paucity of sampling particularly low-resolution sampling in tributaries can result in a bias for identifying nitrate sources and transformations. In this study, high spatial resolution sampling campaigns covering mainstream and tributaries in combination with hydro-chemical parameters and dual isotopes of nitrate were performed to reveal spatio-temporal variations of nitrate sources and transformations in a river draining a mixed LULC catchment. This study suggested that point sources dominated the nitrate in the summer and winter, while non-point sources dominated the nitrate in the spring and autumn. A positive correlation was observed between proportions from sewage and land use index (LUI). However, negative correlations between soil nitrogen/nitrogen fertilizer and LUI were observed. With an increase of urban areas, the increased contribution from domestic sewage resulted in an increase of NO₃^- concentrations in rivers. Both urban and agricultural inputs should be considered in nitrate pollution management in a mixed LULC catchment. We concluded that the seasonal variations of nitrate sources were mainly affected by flow velocity conditions and agricultural activities, while spatial variations were mainly affected by LULC. In addition, we found a novel underestimation of dominated sources from Bayesian model because of mixing effect of isotope values from the tributaries to mainstream, however, high spatial resolution sampling can make up for this shortcoming. δ¹⁵N and δ¹⁸O values of nitrate indicated that nitrate originated from nitrification in soils. The nitrate concentrations and correlation between δ¹⁵N and 1/[NO₃^-] suggested little contribution of nitrate removal by denitrification. Thus, the nitrate reduction in the Yuehe River basin needs to be strengthened. The study provides new implications for estimation of nitrate sources and transformations and basis for nitrate reduction in the river with mixed LULC catchment.

Spatial patterns in water quality and source apportionment in a typical cascade development river southwestern China using PMF modeling and multivariate statistical techniques

River cascade development is one of the human activities that have the most significant impact on the water environment. However, the mechanism of cascade development affecting river hydrochemical components still needs to be further studied. In this study, water quality index(WQI), positive matrix factorization(PMF) model and multivariate statistical techniques were used to identify the mechanism of cascade development affecting river hydrochemical components in an typical cascade development Rivers, Lancang River, China. The results showed that the water quality of Lancang River is relatively good due to less affected by human activity. The spatial variation of river hydrochemistry is affected by the development of cascade reservoirs, and shows three patterns: irregular variation (pH and DO), fluctuating decreasing (Na+, Cl-, SO42- and HCO3-) and multi-peak variation (TN, TDN, NO3--N and NH4+-N). It's worth noting that the concentration of the most hydrochemical parameters is higher in the upper reaches (less human activities) than that in the middle and lower reaches of river due to the retention effect of the reservoir on the chemical composition. The PMF model outputs revealed that the rock weathering and internal source, sewage and soil nitrogen, and chemical fertilizer were primary material sources of Lancang River. Compared with the natural channel zone (41.0%), the interaction of water-rock has more influence on chemical component in the reservoir area (56.3%), while the contribution of fertilizer (11.2%) to the river hydrochemistry is less. The sites of downstream of the reservoir dam were affected by the retention of the reservoir and the disturbance of the bottom drainage, which leads to the weakening of the influence of the sewage (44.7%) on the river material and the increase of the contribution of fertilizer (25.0%). These results could provide valuable information in controlling the eutrophication of cascade reservoirs and the scientific construction of river cascade reservoirs.