Application of Nitrate Stable Isotopes, δN-NO3 and δO-NO3, in Bukit Merah Reservoir, Malaysia

Identifying nitrate sources in Bukit Merah Reservoir (BMR), Perak, Malaysia is important in order to understand the productivity of the ecosystem. For the first time, the dual stable isotopes of nitrate, δN-NO3 and δO-NO3, were applied to assess its potential use to trace the sources of nitrate in surface water of BMR. Water samples were collected in the months of August 2016, December 2016, March 2017 and October 2017 from 10 sites within Kurau River Basin covering the upper stream, tributaries and reservoir. Water samples were analysed using the bacterial denitrification method. The variation of nitrate isotopes ranged from +0.4‰ to +14.91‰ for δN-NO3 and –0.01‰ to +39.4‰ for δO-NO3, respectively. Overall, based on the crossplots between δN-NO3 and δO-NO3, majority of the δO-NO3 in the samples reflects atmospheric deposition Application of Nitrate Stable Isotopes 2 whereas δN-NO3 indicates sources from both ammonium fertiliser and soil nitrogen, and a minor contribution of sewage and manure in BMR water catchment.

To cite this article: Yaccob, N. S. et al. (2018). Application of nitrate stable isotopes, δ 15 N-NO 3 - Since nitrate can be derived from multiple possible sources, it is a challenge for freshwater system to trace its sources which includes atmospheric deposition, nitrogenous fertiliser, animal manure, discharge of domestic sewage and soil organic nitrogen. 4 Traditional method of measuring nitrate concentration and evaluating the potential nitrate source faced difficulties due to multiple potential sources of nitrate including the non-point sources and complex biological processes. In contrast, stable isotopes give a specific isotopic signature which can specify potential source by using δ 15 N-NO 3and δ 18 O-NO 3 -. 5 Realising the risks of the current condition in BMR, the dual stable isotopes of δ 15 N-NO 3and δ 18 O-NO 3were used to see its potential application in identifying the nitrate source in BMR for better understanding of the system.

EXPERIMENTAL
BMR was built in 1902 and is known as the oldest man-made reservoir in Malaysia. It was built to provide irrigation water for double cropping of 24,000 ha of paddy land in the district of Kerian, Perak and to provide domestic and industrial water supply of the populace in the Kerian and Larut Matang districts. 6 It is also used as a tool for flood and drought control. In 2015, 40.87% of land use around BMR was categorised as reserved forest by state and 28.88% as permanent reserved forest area. The land use for agricultural activities was dominated by 18.55% of palm oil plantation, 9.37% of rubber plantation and the other 2.33% was used for farming area.
Ten sampling points were selected from the upper stream, tributary and the reservoir respectively (Figure 1)

RESULTS AND DISCUSSION
In our study, δ 15 N-NO 3and δ 18 O-NO 3varied between +0.4‰ to +14.91‰ and -0.01‰ to +39.4‰, respectively for the four selected sampling months ( Figure 2). As indicated in Figure 2, nitrate from BMR was derived from four different sources: atmospheric deposition, soil nitrate, ammonium fertiliser, sewage or manure. As reported by Kendall et al., nitrate from atmospheric deposition is heavily enriched in 18 O up to 75‰ while lower value of 18 O are usually found in ammonium fertiliser, nitrate fertiliser, soil nitrogen or sewage and manure. 8 From the figure, enriched 15 N-NO 3showed potential sources from sewage and manure while the lighter 15 N-NO 3showed soil nitrogen and ammonium fertiliser. Thus, from the result, the δ 15 N-NO 3of our water samples indicated sources from both soil nitrogen and ammonium fertiliser. Sand mining activities near Sungai Kurau is one of the main sources that have contributed to the re-suspended sediment at the surrounding area suggesting soil nitrogen as the primary source for the upper stream and tributaries. 9 As supported by previous researchers, 93% of the sediment input and nutrients in the catchment of BMR were claimed to originate from Sungai Kurau. 6  Kendall and Wassenaar. 8,11,12. In this analysis, the δ 18 O-NO 3values varied from as low as -0.01‰ in October 2017 to as high as +39.4‰ in December 2016. The nitrate derived from atmospheric deposition showed δ 15 N values from +0.4‰ to +11.45‰ with relatively high δ 18 O(>+14‰). 10,11 The result of δ 18 O-NO 3from all the four months indicated that there is an atmospheric deposition influence as the major nitrate source in the catchment. Hence, based on the dual isotopic pattern, the nitrates content in the water catchment are mainly derived from both soil nitrogen and ammonium fertiliser together with atmospheric deposition.

CONCLUSION
Major portion of nitrate in BMR reflects atmospheric deposition and an overlap between ammonium fertilisers and soil organic nitrogen. Only a minor portion points toward sewage and manure. The results presented the potential application of nitrate stable isotopes as the tool for tracing the source of nitrate in water catchment. Further studies should incorporate analysing the potential sources of nitrate within BMR to confirm the δ 15 N-NO 3and δ 18 O-NO 3ranges.