ABSTRACT
The nitrogen (N) pollution of water is a common global problem. To understand the key production process of N2 O and identify the dominant N sources, Tuojia River, a typical agricultural watershed in a subtropical area, was investigated. To analyze the characteristics of dual nitrate isotopes (δ15N-NO3-,δ18O-NO3-) in water, and N isotope (δ15Norg) and carbon-nitrogen ratio (C/N) in sediment organic matter from four reaches(S1-S4), the stable isotopes method was used. The results showed that the sources of nitrate varied significantly among river segments and were affected by agricultural production and human habitation on the land surface. The average δ15N-NO3- in reaches S1, S2, S3, and S4 were 1.72, 2.62, 4.10, and -1.28, respectively, while the average δ18O-NO3- were 2.60,-0.06, 0.85, and -0.62. The N in terrestrial soil made a large contribution to nitrate sources in reach S1, while soil N, ammonium N fertilizer, and manure played a main role in reaches S2 and S3. Most of the nitrate came from ammonium N fertilizer in reach S4. We also found that δ15Norg in sediment organic matter ranged from -0.69 to 11.21, and C/N was between 7.30 and 12.02. The mean δ15Norg in reaches S1-S4 were 1.91, 2.96, 4.72, and 3.23, respectively, and the mean C/N values were 10.62, 8.63, 9.05, and 9.22, respectively. Although there were some differences in δ15Norg among reaches S2-S4, the dominant N source was sewage in those reaches. However, soil organic matter was the main N source in the sediments of reach S1. The mean δ18O-NO3- in reaches S1-S4 were -7.01,-0.17,-0.28, and -0.60, respectively, indicating that nitrification was the key N2 O production process in these reaches. The ratios of δ15N-NO3- and δ18O-NO3- were 0.66,-41.01,-30.23, and 9.39 in reaches S1-S4, respectively. Finally, we found that there was a positive correlation between NO3--N and δ15N-NO3-. To summarize, the N transformation and N2 O production could be dominated by the nitrification process in Tuojia River.
ABSTRACT
This study aimed at exploring the key pathway of methane production and clarifying the composition and distribution of carbon (C) isotopes in the Tuojia River waterbody in Hunan Pro-vince. We estimated CH4 concentrations and fluxes of four reaches (S1, S2, S3 and S4) by a two-layer diffusion model and gas chromatography. The spatial and temporal distribution of CH4 flux and its relationship with environmental factors were examined. The key pathway of CH4 production was investigated by stable C isotope method to analyze the distribution characteristics of 13C isotope (δ13C) of water dissolved CH4 and seston/benthic organic matter. There was significant seasonal variability in water pH, with mean value of (7.27±0.03). The concentration of dissolved oxygen (DO) showed strong seasonal and spatial variations, with the range of 0.43-13.99 mg·L-1. The maximum value of DO occurred in S1 and differed significantly in summer and autumin. In addition, DO differed significantly in winter and other seasons in S2, S3 and S4. The concentration of dissolved organic carbon (DOC) showed a gradual increasing trend from source to estuary. The highest concentration of DOC (8.32 mg·L-1) was found in S2, while the lowest was observed in S1 (0.34 mg·L-1). The electrical conductivity (EC) and oxidation-reduction potential (ORP) of water ranged from 17 to 436 µS·cm-1 and from -52.30 to 674.10 mV, respectively, which were significantly different among the four reaches (Pï¼0.05). Water ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) concentrations were in the ranges of 0.30-1.35 (averaged 0.90±0.10) mg·L-1 and 0.82-2.45 (averaged 1.62±0.16) mg·L-1, respectively. The dissolved concentration and diffusion flux of CH4 ranged from 0 to 5.28 µmol·L-1 and from -0.34 to 619.72 µg C·m-2·h-1, respectively, with significant temporal and spatial variations. They showed a similar trend among reaches. Their values were highest in spring, followed by in winter and lowest in summer and autumn. Spatially, the CH4 concentration and flux followed the order of S2>S3>S4>S1. The correlation analysis showed that CH4 flux was positively correlated with NH4+-N and DOC. The pathway of CH4 production of all reaches was dominated by acetic acid fermentation, while there were obvious differences among the four reaches. The contribution of CH4 from acetic acid fermentation was greatest (87%) in S1, followed by S4(81%), S2(78%) and S3(76%). The mean value of the δ13C for dissolved CH4, seston organic matter and benthic organic matter was -41.64±1.91, -14.07±1.06 and -26.20±1.02, respectively. There was a positive correlation between the δ13C of dissolved CH4 and benthic organic matter, whereas the δ13C value of dissolved CH4 was negatively correlated with CH4 flux.
Subject(s)
Carbon Isotopes , Methane , Rivers/chemistry , Carbon , Environmental Monitoring , NitrogenABSTRACT
The characteristics of hydrogen and oxygen stable isotopes in river is important for regional hydrologic cycle research. To uncover water supply sources in subtropical agricultural basin from a perspective of stable isotopes, field measurements were conducted in four reaches (S1, S2, S3 and S4) of Tuojia River from April to August 2017. We analyzed the spatial and temporal variations in hydrogen and oxygen isotopes and deuterium excess parameters and their relationship with precipitation, altitude and water quality. Results showed that hydrogen and oxygen isotopes and deuterium excess values ranged from -43.17 to -26.43 (-35.50±5.44), -7.94 to -5.70 (-6.86±0.74), and 16.77 to 23.49 (19.39±1.95), respectively. Under the influence of monsoon circulation, hydrogen and oxygen isotopes showed substantial seasonal variation, with spring (δD: -29.88±3.31; δ18O: -6.18±0.57) > summer (δD: -39.25±2.65; δ18O: -7.32±0.42). The spatial distribution of hydrogen and oxygen isotopes values increased fluctuantly with the position from the sampling site to the river's source, with δD: S1ï¼S4ï¼S3ï¼S2, and δ18O: S1ï¼S3ï¼S4ï¼S2. The deuterium excess values had no significant temporal variation, while it spatially increased gradually with the river levels. The slope and intercept of water line in this river were smaller than that of the local meteoric water line, suggesting that precipitation was the primary water source for this river. At the seasonal scale, both δD and δ18O were significantly negatively correlated with water temperature (δD: r=-0.92; δ18O: r=-0.88) and δ18O was negatively correlated with altitude (r=-0.96). At spatial scale, δ18O had a significantly positive correlation with water temperature. The δD and δ18O had negative correlation with precipitation, but being not statistically significant.
