The technique of using nitrogen and oxygen isotopic compositions to identify
nitrogen sources and assess the nitrogen cycle process has been extensively applied in
fresh water research in the last few decades. Nitrate pollution in fresh water has
caused concern worldwide: if nitrate sources could be identified, this major form of
pollution could be combated. Thus, this technique provides much-needed scientific
help with the remediation of fresh water. As the method of determining nitrogen and
oxygen isotopic ratios has become more precise, many deficiencies in earlier methods
of determination have been overcome. Moreover, each kind of nitrate source and
some isotopic fractionation processes have also been well studied; all these factors
play an important role in research into the use of isotopic tools in tracing nitrate
sources. The joint use dual isotopes with water chemistry are indeed providing very
detailed and useful information relating to the identification of nitrate sources and the
tracking of nitrate transformation in freshwater systems. Surface water samples were
collected from Taihu Lake and Chaohu Lake in three different periods. The current
study can be divided into three parts to discuss, such as Taihu Lake (collected in
December, 2007); Taihu Lake (collected in January and July of 2009, respectively);
Chaohu Lake (collected in January and July of 2009, respectively). The δ15N and δ18O
- NO3
- values, as well as major ions tracers, from collected water samples in Taihu
Lake and Chaohu Lake were investigated to identify the primary nitrate sources and
assess nitrogen biogeochemical process in the present study.
The results of Taihu Lake (collected in December, 2007) showed that the nitrate
concentration in North Taihu Lake (WTL) was generally higher than those in East
Taihu Lake (ETL) and its upstream inflow rivers. The NO3
-/Cl- value combined with
mapping of δ15N-NO3
- and NO3
- concentration [There was a positive correlations
(R2=0.83, 0.63 respectively) between Cl- and NO3
- levels can be observed in WTL]
suggested that mixing process should play a major effect in WTL, and denitrification
was the dominant nitrogen transformation process in WTL. A linear relationship of
close to ~ 1: 2 was observed between δ15N-NO3
- and δ18O-NO3
- values in NTL,
confirming the occurrence of denitrification in NTL. The nitrogen isotopic
composition of local nitrate sources were comparable with previously published
values. The δ15N-NO3
- in ETL ranged from 3.32‰ to 6.17‰ (n=5, mean =5.1‰), and
the δ15N-NO3
- in WTL and rivers ranged from 5.69‰ to 10.35‰ (n= 7, mean =8.79‰)
and from 7.33‰ to 10.11‰ (n= 5, mean =9.35‰), respectively. The δ15N-NO3
- data
implied that sewage and manure were the principal nitrate sources in WTL and its
feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and
atmospheric deposition. The range of determined δ18O-NO3
- value in the current study
varied from 4.0‰ to 12.4‰ (n=17, mean =7.9‰). The δ18O-NO3
- data indicated most
of nitrate from microbial nitrification of organic nitrogen matter could possibly make
a significant contribution to the lake.
In January and July of 2009, the Cl- concentrations in Taihu Lake basin were
found to range from 58.33 to 85.72 mg/L in winter and from 30.52 to 69.22 mg/L in
summer, with average values of 66.68 and 56.21 mg/L, respectively. This seasonal
variation was also observed in SO4
2- and NO3
- concentrations, in which the mean
values of SO4
2- and NO3
- in summer (73.54 and 3.87 mg/L, respectively) were lower
than those in winter (84.49 and 5.53 mg/L, respectively), which clearly suggested a
dilution effect by rain water in summer months. The relationship between NO3
- and
Cl- suggested that a mixing effect dominated the nitrate transformation process in
Taihu Lake in winter. The δ15N-NO3
- values in watersheds varied from +5.3‰ to
+22.2‰ (n=6, mean value = +15.1‰) in winter, and from +0.5‰ to +20.6‰ (n=8,
mean value = +9.2‰) in summer. The δ15N-NO3
- values in NTL varied from +4.0‰
to +17.8‰ (n=7, mean value = +8.7‰) in winter, and from +5.2‰ to +11.1‰ (n= 7,
mean value = +9.7‰) in summer. Moreover, the δ15N-NO3
- values in ETL varied
from +1.8‰ to +5.9‰ (n=5, mean value = +4.4‰) in winter, and from +8.1‰ to
+10.2‰ (n= 7, mean value = +9.4‰) in summer. The δ15N-NO3
- values suggested
that: In winter, sewage and manure were the primary nitrate sources in major inflow
rivers and NTL, while nitrate sources in ETL probably derived from soil organic
nitrogen. In summer, atmospheric deposition and swage/manure inputs appeared to
play an important role in controlling the distribution of nitrates in the whole Lake.
The δ18O-NO3
- values in Taihu Lake basin ranged from +0.4‰ to +17.9‰ in winter
and from +0.4‰ to +17.9‰ in summer, with average value of +6.3‰ and +6.6‰,
respectively, suggesting that the nitrate produced from microbial nitrification was
another major nitrate sources during both winter and summer months. The variations
in isotopic values in nitrate suggested that denitrification enriched the heavier isotopes
of nitrate in NTL in winter and in ETL in summer.
About the results of Chaohu Lake (collected in January and July of 2009,
respectively). The nitrate concentrations and their nitrogen and oxygen isotopic
compositions in the water samples were analyzed. The nitrate concentration ranged
from 0.30 to 13.94 mg/L in winter and from 0.08 to 5.14 mg/L in summer, with
average values for 6.72 and 2.10 mg/L, respectively, which exhibited a large seasonal
variation, suggesting the dilution effect of rainwater. There was a positive correlation
(y = 0.2x + 4.0, R2=0.74) between NO3
- and Cl- in WCL in summer, indicating the
occurrence of mixing process in WCL in summer. In the current study, the δ15N-NO3
-
values in the water samples ranged from +5.8 to +16.1‰ in winter with a mean value
of +10.9‰ (n=8), and from +5.2 to +9.9‰ in summer with a mean value of +6.7‰
(n=15), and the δ15N-NO3
- values for the rivers in summer (mean value = 13.7‰, n=5)
were evidently lower than those in winter (mean value = 6.9‰, n=5). The data
concerning δ15N-NO3
- values in winter suggested that the nitrate sources of inflow
rivers derived mainly from sewage and manure inputs, while the nitrate sources in the
southern part of Chaohu Lake were possibly influenced by chemical fertilizer or plant
derived nitrogen; in summer, seasonal fertilizer application in the lake watersheds and
rainwater probably contributed more nitrates into the rivers and the lake. The
δ18O-NO3
-
values did not exhibit distinct seasonal variations in rivers and Chaohu
Lake, and the δ18O-NO3
-
values of collected water samples in both winter and summer
scatted from +0.7 to +13.5‰, with a mean value +7.1‰ (n=23).The δ18O-NO3
- values
suggested that nitrate produced from microbial nitrification could also be a significant
nitrate source in the Chaohu Lake basin. The slope of the linear regression between
δ15N and δ18O-NO3
- values supported the hypothesis that the denitrification process
may control the isotopic composition of nitrates in East Chaohu Lake in summer.
| Date of Award | 3 Oct 2012 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Hon Wah Michael LAM (Supervisor) |
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