Si-ang Li

Tracing nitrate sources with dual isotopes and long term monitoring of nitrogen species in the Yellow River, China

A heavy load of nitrogenous compounds reflects nutrient loss and influences water quality in large rivers. Nitrogenous concentrations and dual isotopes of nitrate were measured to ascertain the spatial and temporal distributions of nitrate transformation in the Yellow River, the second-longest river in China. Assessment of the long-term record indicates that [NO3−–N] has increased by two-fold over the past three decades. Weekly observation of ammonium over a twelve-year period revealed high concentrations and suggests impairment of water quality, particularly since 2011. The estimated total dissolved nitrogen flux was 7.2 times higher in middle reaches than that at head waters. Anthropogenic nitrogen sources become more important in lower section of the upper reaches and middle reaches because of intensive agricultural activities and urban input. Nitrate in the lower reaches was mainly derived from transportation of upstream nitrate and point sources from cities. The spatial variation of ammonium and nitrate isotopes show that nitrification is a key process governing nitrogen transformation. Riverine biological processes could potentially be responsible for the shift of nitrate isotope signature. The first step to reducing nitrogen load and improving water quality will be containment and careful management of sources from urban input, sewage waste and irrigation runoff.

Response of methane production via propionate oxidation to carboxylated multiwalled carbon nanotubes in paddy soil enrichments

Carboxylated multiwalled carbon nanotubes (MWCNTs-COOH) have become a growing concern in terms of their fate and toxicity in aqueous environments. Methane (CH4) is a major product of organic matter degradation in waterlogged environments. In this study, we determined the effect of MWCNTs-COOH on the production of CH4 from propionate oxidation in paddy soil enrichments. The results showed that the methanogenesis from propionate degradation was accelerated in the presence of MWCNTs-COOH. In addition, the rates of CH4 production and propionate degradation increased with increasing concentrations of MWCNTs-COOH. Scanning electron microscopy (SEM) observations showed that the cells were intact and maintained their structure in the presence of MWCNTs-COOH. In addition, SEM and fluorescence in situ hybridization (FISH) images revealed that the cells were in direct contact with the MWCNTs and formed cell-MWCNTs aggregates that contained both bacteria and archaea. On the other hand, nontoxic magnetite nanoparticles (Fe3O4) had similar effects on the CH4 production and cell integrity as the MWCNTs-COOH. Compared with no nanomaterial addition, the relative abundances of Geobacter and Methanosarcina species increased in the presence of MWCNTs-COOH. This study suggests that MWCNTs-COOH exerted positive rather than cytotoxic effects on the syntrophic oxidation of propionate in paddy soil enrichments and affected the bacterial and archaeal community structure at the test concentrations. These findings provide novel insight into the consequences of nanomaterial release into anoxic natural environments.

Sources and key processes controlling particulate organic nitrogen in impounded river–reservoir systems on the Maotiao River, southwest China

ABSTRACT The δ15N of nitrate (NO3−) and particulate organic nitrogen (PON) was used to study the sources and fate of nitrogen in the impounded Maotiao River, southwest China. During months when the reservoirs thermally stratified, denitrification was the key process in the hypolimnion in Hongfeng and Baihua reservoirs. Based on the δ15N of PON and NO3−, PON in the epilimnion of reservoirs was dominated by NO3− assimilation during stratification. Most δ15N of PON was higher than that of NO3− in the reservoirs in October, indicating that PON in reservoirs was mainly derived from exogenous nitrogen input or denitrification rather than assimilation of NO3− in the epilimnion. Relationships between the molar carbon:nitrogen (C:N) ratio and δ15N-PON indicated the main sources of PON in reservoirs were from phytoplankton, produced in inflowing rivers and the reservoir under the joint influence of soil organic matter, denitrification, and exogenous nitrogen inputs. The δ15N in PON and NO3− increased gradually from upstream to downstream, by 8.21 ± 0.65‰ and 1.74 ± 3.66‰, respectively, suggesting an accumulative effect caused by the downstream waters. Overall, this study provides insight into the nitrogen biogeochemical cycling of river–reservoirs systems using stable nitrogen isotopes and C:N to specify the effects of river impoundment on nitrogen dynamics.

Spatial variation of nitrogen cycling in a subtropical stratified impoundment in southwest China, elucidated by nitrous oxide isotopomer and nitrate isotopes

ABSTRACT Estimates of biogeochemical processes and the proportion of N2O production in the aquatic system of impoundments are important to quantify nitrogen cycling, particularly during stratification periods. In this study, we used the dual isotopes of nitrate (NO3−) and nitrous oxide (N2O) to estimate the nitrogen dynamics and contributions to N2O production and reduction at varying zones in Lake Baihua, located in southwest China. The lake was strongly stratified during the sampling period, with the oxic zone from the surface to 12  m and the anoxic zone from 12 to 21  m. The assimilation shifted δ15N and δ18O of NO3− significantly in the epilimnion (0−4  m), and denitrification contributed to the shift in the low dissolved oxygen zone of the hypolimnion. The semiquantitative analysis showed that nitrification accounted for >67% of the N2O production between 0 and 4  m while higher nitrification contributions were also found between 6 and 12  m. The contribution of denitrification between 15 and 21  m was >43%. The mechanism responsible for the vertical variations should be considered in the estimation of nitrogen cycling and N2O production in subtropical stratified impoundments.

Temporal transport of major and trace elements in the upper reaches of the Xijiang River, SW China

Abstract This study investigated the temporal variations of major and trace element contents and controlling factors in the upper reaches of the Xijiang River, SW China. The results showed the major ions principally derived from the weathering of carbonate and silicate rocks, with a seasonal variation impacted by the monsoonal climate. The contents of dissolved trace elements also varied seasonally in the two main tributaries: the Nanpan and Beipan rivers. Most dissolved trace elements such as Mn, Zn and Pb being affected by anthropogenic contributions displayed a large seasonal variability, from onefold to hundred-fold, while V, Rb and U show a relative stable trend uncorrelated with the different seasons, suggesting a natural origin by rock weathering. Suspended particle matter in the Nanpan and Beipan rivers was depleted in Ca, Mg, Na, K and Sr, but enriched in Al, Fe and Sb compared to the upper crust (UC) values. A binary mixture of silicate-rich and carbonate-rich sources dominates the behavior of elements such as Ca and Sr in riverine sediments, while a third end-member with an anthropogenic signature influenced the content of trace elements like Mn, Sb, Pb, Zn, Cd, Cr and Cu. The enrichments of Sb relative to the UC would be related to the mining activity in the studied area. The study highlights the importance of investigating temporal variations of major and trace element contents of rivers controlled by the monsoon climate and impacted by human activity.

Sulfate sources constrained by sulfur and oxygen isotopic compositions in the upper reaches of the Xijiang River, China

While it is critical to accurately understand the sources and transformation of sulfate based on time-series analysis, there are limited studies on temporal variation of sulfate in rivers and on rock weathering by sulfuric acids. We conducted a monthly sampling campaign in the Beipan, Nanpan, and Hongshui Rivers over the course of one hydrological year. This study examined seasonal variations in riverine sulfate impacted by the monsoon climate in the upper reaches of the Xijiang River basin. In general, the SO42− contents in these rivers dropped from relatively high levels to low values during the high-flow season, in response to increasing discharge. The sulfate was generally enriched in heavy isotopes during the low-flow season compared to the high-flow season. The calculated results indicate that the riverine sulfate was mainly derived from sulfide oxidation, but that evaporite dissolution could be an important source during the low-flow season, based on isotopic evidence. Mine drainage is likely an important source of sulfate to these rivers during the high-flow season due to contributions from fast surface flow, which responds to frequent heavy rain in monsoonal climate regions. A relatively high proportion of HCO3− was found to be derived from rock weathering by sulfuric acid during the high-flow season when compared to that observed during the low-flow season. The results suggest that approximately one quarter of the HCO3− in the Hongshui River originated from carbonate weathering by sulfuric acid derived from the oxidation of sulfide. Such information on the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal variability of sulfate concentrations and weathering processes in rivers.

The influence of urbanization on karst rivers based on nutrient concentration and nitrate dual isotopes: an example from southwestern China

China is experiencing rapid urbanization that has changed the water quality of rivers, especially nutrient loads. In this study, a typical urban river located in a karst area, Chengguan River, was chosen to explore the influence of urbanization on river ecosystems based on nutrient concentration and nitrate isotopes. The results show monthly variability of water chemistry and nutrient concentration. Nutrient concentration in two tributaries and the mainstem showed significant spatial variability, with heavy N and P pollution in one tributary near a suburban area, indicating a response to different levels of urbanization. Measurements of nitrate dual isotopes suggest that volatilization, assimilation, nitrification, and denitrification all occur in the polluted river. Water chemistry and nitrate isotopes show that major nitrogen sources included domestic waste and agricultural input, such as chemical fertilizer and manure. The results suggest that urbanization increases nutrient concentrations and accelerates the riverine nitrogen dynamic, and point to the need to manage point sources of sewage effluents to improve the water quality of urban rivers in southwestern China.

Identification of sources and transformations of nitrate in the Xijiang River using nitrate isotopes and Bayesian model

Coupled nitrogen and oxygen isotopes of nitrate have proven useful in identifying nitrate sources and transformation in rivers. However, isotopic fractionation and low-resolution monitoring limit the accurate estimation of nitrate dynamics. In the present study, the spatio-temporal variations of nitrate isotopes (15N and 18O) and hydrochemical compositions (NO3- and Cl-) of river water were examined to understand nitrate sources in the Xijiang River, China. High-frequency sampling campaigns and isotopic analysis were performed at the mouth of the Xijiang River to capture temporal nitrate variabilities. The overall values of δ15N-NO3- and δ18O-NO3- ranged from +4.4‰ to +14.1‰ and from -0.3‰ to +6.8‰, respectively. The results of nitrate isotopes indicated that NO3- mainly originated from soil organic nitrogen (SON), chemical fertilizer (CF), and manure and sewage wastes (M&S). The negative correlation of nitrate isotopic values with NO3-/Cl- ratios suggested the importance of denitrification in NO3- loss. The results of Bayesian model with incorporation of isotopic fractionation during the denitrification showed that SON and CF contributed to the most (72-73%) nitrate in the wet season; whereas approximately 58% of nitrate was derived from anthropogenic inputs (M&S and CF) in the dry season. The nitrate flux was 2.08 × 105 tons N yr-1 during one hydrologic year between 2013 and 2014, with 86% occurring in the wet season. Long-term fluctuations in nitrate flux indicated that nitrate export increased significantly over the past 35 years, and was significantly correlated with nitrate concentrations. The seasonal pattern of nitrate dynamics indicated the mixing of nitrified NO3- and denitrified NO3- between surface flow and groundwater flow under different hydrological conditions. Overall, the present study quantitatively evaluates the spatio-temporal variations in nitrate sources in a subtropical watershed, and the high-frequency monitoring gives a better estimate of nitrate exports and proportional contributions of nitrate sources.

Contamination of heavy metals and isotopic tracing of Pb in surface and profile soils in a polluted farmland from a typical karst area in southern China

Farmland top soils and soil profiles situated in the karst area of Guilin, Guangxi Zhuang Autonomous Region, southern China, reveal different degrees of heavy metal pollution, both in respect to the lateral as well as the vertical dimension. Pb isotope ratios clearly identify that heavy metal contributions to the soil represent the legacy of former Pb-Zn mining and smelting in the area. Depending upon soil properties, differences in the intensity of the vertical penetration of heavy metal pollution are discernible. Top soil coverage by local farmers provides little remediation. Consequently, hazardous conditions for the regional ecology, for agricultural usage and ultimately for human health remain in place. Based on chemical and isotopic results obtained, more effective remediation strategies need to be developed.

Mechanisms controlling dissolved CO2 over-saturation in the Three Gorges Reservoir area

ABSTRACT The emission of CO2 to the atmosphere from inland waters is an important part of the global carbon cycle. In this study, we made spatial and temporal measurements of CO2 partial pressure (pCO2) along the Three Gorges Dam system. The pCO2 ranged from 619 to 2383 μatm and was supersaturated relative to atmospheric CO2. Further, pCO2 showed obvious spatial and temporal variations: pCO2 at the high-flow season was much lower than that at the low-flow season near the upstream part of the reservoir, whereas pCO2 in the reservoir water and after the dam showed an opposite seasonal trend. Organic matter mineralization produced more CO2 in the surface water of the reservoir area at the high-flow season and should be responsible for the δ13C-depleted dissolved inorganic carbon (δ13CDIC). In addition, organic carbon mineralization is sensitive to temperature variability, which is expected to be an important driver of the dissolved CO2 over-saturation in the reservoir. This study suggested that the construction of Three Gorges Reservoir increased the water transit time and accelerated the organic carbon mineralization in the Changjiang River. The results indicate that carbon cycling changes markedly in large impounded rivers.