Responses of Net Anthropogenic N Inputs and Export Fluxes in the Megacity of Chengdu, China

Abstract

Anthropogenic N inputs have become progressively more problematic and have profoundly affected the water quality in megacities throughout China. Thus, to design and implement appropriate megalopolis watershed management, it is important to understand the relationship between N inputs and exports and to identify the N pollution sources. To that end, in this work, the net anthropogenic N inputs (NANI) in Chengdu City were estimated based on statistical data collected between 1970 and 2019. N input fluxes and pollution sources were estimated through sample collection and field measurements that were performed between 2017 and 2019, while nitrate (NO3−) was identified using stable isotope and Bayesian model (SIAR) analysis. The NANI was found to be affected primarily by livestock and poultry consumption of N rich feed. Moreover, the N export fluxes and runoff showed a high degree of correlation. Notably, NO3− fluxes exhibited a significant increase over the course of the study period, such that, by 2019, the total N fluxes (18,883.85 N kg/km2) exceeded the NANI (17,093.87 N kg/km2). The results indicate that although livestock and poultry farming were the original primary sources of NANI, their contributions declined on an annual basis. Moreover, with the emphasis placed on point source management in Chengdu City, domestic sewage discharge has been significantly reduced. Therefore, N retention in groundwater is thought to be the factor driving the N flux increase. These findings are pivotal to solving the N pollution problem in megacities like Chengdu (China).

Isotopic source identification of nitrogen pollution in the Pi River in Chengdu

“…The assessment of the different forms of N is essential to a correct interpretation of N cycling (Ding et al, 2021; Fadhullah et al, 2020; X.‐L. Liu et al, 2021).…”

“…However, the DO concentration (mean 7.58 ± 0.93 mg/L) measured in the main stream during the dry season was not ideal for denitrification (<1 mg O 2 /L; Soto et al, 2019). It is suggested that these results reflect groundwater input with significant denitrification characteristics (Ding et al, 2021). Therefore, it is reasonable to ignore denitrification, and assume that mixing with atmospheric NO 3 − was the dominant factor for the high 18 O-NO 3 − obtained during the rainy season (Ding et al, 2014).…”

“…Dissolved oxygen (DO), electrical conductivity (EC), pH, and temperature (T) were measured in situ by using a portable meter (models HQ11D and HQ40D, HACH Instruments). Then, water samples were collected for nitrate ($${\text{NO}}_{3}^{&#x02010;}$$), ammonia ($${\text{NH}}_{4}^{&#x0002B;}$$), nitrite ($${\text{NO}}_{2}^{&#x02010;}$$), and total dissolved nitrogen (TDN) analysis, and subsequently filtered through 47 mm diameter and 0.7 μm‐pore sized glass filters (GF/F, Whatman) in the laboratory (Ding et al, 2021). The $${\text{NO}}_{3}^{&#x02010;}$$ and $${\text{NO}}_{2}^{&#x02010;}$$ concentrations were measured using ion chromatography (883 Basic IC, Metrohm), whereas the $${\text{NH}}_{4}^{&#x0002B;}$$ concentration was measured using spectrophotometry (722 N, Shanghai Jingke).…”

“…The $${\text{NO}}_{3}^{&#x02010;}$$ and $${\text{NO}}_{2}^{&#x02010;}$$ concentrations were measured using ion chromatography (883 Basic IC, Metrohm), whereas the $${\text{NH}}_{4}^{&#x0002B;}$$ concentration was measured using spectrophotometry (722 N, Shanghai Jingke). After digestion of the TDN with alkaline potassium persulfate, $${\text{NO}}_{3}^{&#x02010;}$$‐N was reduced to $${\text{NO}}_{2}^{&#x02010;}$$‐N and then analyzed with spectrophotometry (Ding et al, 2021; Xing & Liu, 2016). The concentration of dissolved organic nitrogen (DON) was calculated by mass balance (DON = TDN−DIN, where DIN refers to dissolved inorganic nitrogen, that is, the sum of $${\text{NO}}_{3}^{&#x02010;}$$‐N, $${\text{NO}}_{2}^{&#x02010;}$$‐N, and $${\text{NH}}_{4}^{&#x0002B;}$$‐N species; Xing & Liu, 2016).…”

“…The assessment of the different forms of N is essential to a correct interpretation of N cycling (Ding et al, 2021;Fadhullah et al, 2020;X.-L. Liu et al, 2021). As shown in Figure 5 N, but the delayed propagation of nitrifying bacteria failed to transform nitrite into nitrate (Ma et al, 2019).…”