Relationship Between Nitrate Sources and Tropical Land Use in the Langat River Basin, Malaysia

Nitrogen loading due to urbanization, industrialization, and agricultural expansion is increasing in tropical regions, and releases nitrogen into water as nitrate ion (NO₃^―), causing negative effects on the environment. However, there are limited studies on the nitrogen loadings and NO₃^― sources in the watershed with mixed tropical land use. To address this gap, this study aims to determine the NO₃^― sources and tropical land use effect on the nitrogen loadings in the Langat River basin, Malaysia, which includes tropical rainforest upstream, urban areas midstream, and oil palm plantation areas downstream. The stream and irrigation water in oil palm plantations and the river water were collected during the wet and dry seasons. The samples were analyzed for major dissolved ion concentrations, water isotope ratios (δ²H and δ¹⁸O of H₂O), and nitrate nitrogen and oxygen isotope ratios (δ¹⁵N and δ¹⁸O of NO₃^―). The NO₃^―concentration ranged from 1.41 mg/L to 19.09 mg/L in river water and from 0.13 mg/L to 2.44 mg/L in the stream and irrigation water in the oil palm plantation, much lower than in river water. The NO₃^― concentration was higher in the dry season than in the wet season, likely due to flushing during the wet season and water retention during the dry season. The δ¹⁵N-NO₃^― and δ¹⁸O-NO₃^― ratios ranged from -2.52‰ to 19.12‰ and from -3.62‰ to 23.90‰, respectively. The stream water in oil palm plantations showed a low value of δ¹⁵N-NO₃^―, while the irrigation water isotope values were high. This could be due to denitrification and absorption by oil palm trees during the discharge from the oil palm plantation to the outside as irrigation drainage. The NO₃^― concentration decreased with an increase in the proportion of forest area in the catchment, assuming each water sampling point to be the outlet. In contrast, NO₃^― concentration increased with the proportion of built-up areas. The δ¹⁵N-NO₃^― decreased as the proportion of oil palm plantations increased. These findings indicated that NO₃^― discharge from tropical rainforests could not contribute substantially to river water. The main sources of nitrate in river water could be ammonia fertilizer from plantation areas and sewage water from urban areas. Overall, as in previous studies, fertilizers and sewage were identified as the main sources of nitrate ions in tropical urban and agricultural areas. In addition, this study in the case of oil palm plantations revealed that although denitrification and oil palm trees absorb the NO₃^―, the δ¹⁵N-NO₃^― in the river water showed fertilizer-derived runoff, indicating that the impact of the plantations on water quality cannot be overlooked.