Nitrate sources and transformations along a mountain-to-plain gradient in the Taizi River basin in Northeast China

Fifty-seven riverine samples in three typical regions, namely, upper mountainous (zone 1), middle hilly (zone 2), and lower plain (zone 3) regions, were collected in May (low flow) and August (high flow) of 2016, and chemical parameters and isotopes were analyzed to enrich the knowledge of riverine nitrate sources and transformations in the Taizi River basin. Results showed that NO₃^- concentrations in zone 3 were the highest, followed by zones 2 and 1. NO₃^-/Cl^- molar ratios and nitrate dual isotopes indicated that NO₃^- was mainly from chemical fertilizer (CF) in zones 1 (57.0%) and 2 (43.1%) according to a Bayesian mixing model (SIAR) and mixed sources of CF, nitrification of soil organic nitrogen (SON), and manure and sewage (M&S) in zone 3 (92.8%), during the high-flow season. NO₃^- was mainly from CF and SON in zones 1 (76.7%) and 2 (74.0%), during the low-flow season. NO₃^-sources were different in the three rivers of zone 3 mainly due to various urban inputs. Contributions of CF, SON, and M&S increased by 13%, 8.3%, and 7.5% in zones 1, 2, and 3, respectively, from the low-flow to the high-flow season. NO₃^- in the Taizi River was mainly influenced by nitrification in soils, while no significant denitrification was found in the three zones. Measures for reducing NO₃^- inputs to rivers should be considered by improving effectively utilizing rate of chemical fertilizer and inhibit nitrification.