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The focus of urban water environment renovation has shifted to high nitrate (NO 3 −) load. Nitrate input and nitrogen conversion are responsible for the continuous increase in nitrate levels in urban rivers. This study utilized nitrate stable isotopes (δ15N–NO 3 − and δ18O–NO 3 −) to investigate NO 3 − sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO 3 − was the most common form of dissolved inorganic nitrogen (DIN), accounting for 66 ± 14% of total DIN with a mean value of 1.86 ± 0.85 mg L−1. The δ15N–NO 3 − and δ18O–NO 3 − values ranged from 5.72 to 12.42‰ (mean value: 8.38 ± 1.54‰) and −5.01 to 10.39‰ (mean value: 0.58 ± 1.76‰), respectively. Based on isotopic evidence, the river received a significant amount of nitrate through direct exogenous input and sewage ammonium nitrification, while nitrate removal (denitrification) was insignificant, resulting in nitrate accumulation. Analysis using the MixSIAR model revealed that treated wastewater (68.3 ± 9.7%), soil nitrogen (15.7 ± 4.8%) and nitrogen fertilizer (15.5 ± 4.9%) were the main sources of NO 3 − in rivers. Despite the fact that Shanghai's urban domestic sewage recovery rate has reached 92%, reducing nitrate concentrations in treated wastewater is crucial for addressing nitrogen pollution in urban rivers. Additional efforts are needed to upgrade urban sewage treatment during low flow periods and/or in the main stream, and to control non-point sources of nitrate, such as soil nitrogen and nitrogen fertilizer, during high flow periods and/or tributaries. This research provides insights into NO 3 − sources and transformations, and serves as a scientific basis for controlling NO 3 − in urban rivers. • Urban rivers have received significant amounts of nitrate through direct exogenous input and ammonium nitrification from sewage. • A weak permanent nitrate removal mechanism results in the accumulation of nitrate. • The main source of nitrate pollution in urban rivers is mainly due to the substantial input of treated wastewater. [ABSTRACT FROM AUTHOR]

Plant litter input is an important driver of soil/sediment organic carbon (SOC) turnover. A large number of studies have targeted litter-derived C input tracing at a global level. However, little is known about how litter carbon (C) input via various plant tissues affects SOC accumulation and mineralization. Here, we conducted laboratory incubation to investigate the effects of leaf litter and stem litter input on SOC dynamics using the natural 13C isotope technique. A 122-day laboratory incubation period showed that litter input facilitated SOC accumulation. Leaf and stem litter inputs increased soil total organic carbon content by 37.6% and 15.5%, respectively. Leaf litter input had a higher contribution to SOC accumulation than stem litter input. Throughout the incubation period, the δ13C values of stem litter and leaf litter increased by 1.5‰ and 3.3‰, respectively, while δ13CO 2 derived from stem litter and δ13CO 2 derived from leaf litter decreased by 4.2‰ and 6.1‰, respectively, suggesting that the magnitude of δ13C in litter and δ13CO 2 shifts varied, depending on litter tissues. The cumulative CO 2 –C emissions of leaf litter input treatments were 27.56%–42.47% higher than those of the stem litter input treatments, and thus leaf litter input promoted SOC mineralization more than stem litter input. Moreover, the proportion of increased CO 2 –C emissions to cumulative CO 2 –C emissions (57.18%–92.12%) was greater than the proportion of litter C input to total C (18.7%–36.8%), indicating that litter input could stimulate native SOC mineralization, which offsets litter-derived C in the soil. Overall, litter input caused a net increase in SOC accumulation, but it also accelerated the loss of native SOC. These findings provide a reliable basis for assessing SOC stability and net C sink capacity in wetlands. [Display omitted] • The effects of litter input from various plant tissues on SOC dynamics were studied. • Leaf litter input had a higher contribution to SOC storage than stem litter input. • The magnitude of δ13C in litter and δ13CO 2 shifts varied depending on litter tissues. • Leaf litter input promoted SOC mineralization more than stem litter input. • Litter input caused net increases in SOC storage, but it accelerated native SOC loss. [ABSTRACT FROM AUTHOR]