Mechanisms of nitrate accumulation in highly urbanized rivers: Evidence from multi-isotopes in the Pearl River Delta, China

Highlights

• Reveal the mechanisms of nitrate accumulation in urbanized rivers by multi-isotopes.

• Anthropogenic input is the predominant contributor to increase of NO₃⁻

• Nitrification of sewage NH₄⁺ contributes to a “new” source of NO₃⁻.

• NO₃⁻ removal via denitrification from urban landscapes is weakened.

• High NH₄⁺ causes a repression of NO₃⁻ removal by assimilation.

Abstract

Nitrate accumulation due to anthropogenic input in highly urbanized rivers is a general recognition. The excess anthropogenic nitrogen (N) can alter N transformations and would further accelerate nitrate accumulation. However, there is lack of direct evidence to reveal the mechanisms of nitrate accumulation in the urbanized river. In this study, multi isotopes of nitrate (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻), ammonium (δ¹⁵N-NH₄⁺) and suspended particulate nitrogen (δ¹⁵N-PN) were obtained to investigate mechanisms of nitrate accumulation in typical highly urbanization rivers in the Pearl River Delta (PRD). Firstly, source apportionment revealed sewage was the dominant source of nitrate and ammonium, evidenced by higher δ¹⁵N values (>+5‰). Secondly, in the dry season, a significant correlation between the δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values, an increasing trend of δ¹⁵N-NO₃⁻ with NO₃⁻-N concentration, and lower δ¹⁸O-NO₃⁻ values provided isotopic evidence of nitrate accumulation that NO₃⁻ was increased by enhanced nitrification of sewage effluent NH₄⁺. Thirdly, in the wet season, the relationships between the δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻, and lower isotope enrichment factor of denitrification indicated that denitrification in soils or riparian zones was weakened due to urbanization, which increased NO₃⁻ fluxes to the river water. Fourthly, the δ¹⁵N-PN was significantly correlated with δ¹⁵N-NH₄⁺ rather than δ¹⁵N-NO₃⁻, revealing the preference of NH₄⁺ uptake by phytoplankton. The NH₄⁺ preferred assimilation repressed NO₃⁻ removal and consequently result in nitrate accumulation. This study provided direct lines of evidence by multi isotopes to understand the mechanisms of nitrate accumulation in the urbanized rivers, which deepens our understanding of roles of isotopes in the study of N biogeochemical processes and has important implication for regional and global N cycling.

Introduction

Nitrate accumulation in highly urbanized rivers is a hot topic due to intensive urbanization and increased human population, which have significantly increased nitrogen (N) loading and altered N transformation in the river ecosystem (Archana et al., 2018, Jani and Toor, 2018, Peierls et al., 1991). In this way, urbanized rivers have become channels for “pollution collection and transport” of reactive N and hotspots of regional pollution (Hale et al., 2014, Zhang et al., 2015). Undoubtedly, exogenous input of nutrients to the rivers is recognized as the most important factor disturbing the natural N cycling. Numerous studies have reported that the excess input of nutrients caused by human activities resulting in elevated nitrate (NO₃⁻; Archana et al., 2018, Bu et al., 2011, Liu et al., 2018a, McIsaac et al., 2001) and ammonium (NH₄⁺; Pennino et al., 2016, Zhang et al., 2007) in the rivers. However, how the nutrient elements with high-bioactivity, with the elevated nitrate resists the transformations and accumulates in the river are still unclear. Therefore, it is essential to investigate the transformations of N in highly urbanized rivers, which will provide further understanding of nitrate accumulation in the urban rivers that has important implication for regional and global N cycling.

Numerous studies simulated the flux and behaviors of nutrients in the rivers by both hydrological and hydrodynamic model equipped with nutrient modules (Beaujouan et al., 2002, Kadlec and Hammer, 1988, Nguyen et al., 2018). These models were established based on assumed processes and validate the model parameters by fitting procedures. However, in the field research, there is a lack of direct evidence to prove whether the N transformation has occurred, and if so, which transformation process can be attributed to nitrate accumulation in the urbanized rivers. The development and progress of environmental multi-isotope tracing technology provide a tool to solve these problems (Archana et al., 2018, Xia et al., 2017, Xuan et al., 2019). In general, N transformations could be classified into source and sink processes of nitrate. Nitrification is a typical nitrate source process, which could be traced by depleted δ¹⁵N-NO₃⁻ values and enriched δ¹⁵N-NH₄⁺ values due to preferential incorporation of the lighter isotopes into the production of NO₃⁻ (Barnes and Raymond, 2010, Mariotti et al., 1981). Assimilation is a temporary sink process of N and could exchange N with sediment by sedimentation and suspension, which results in decease of the δ¹⁵N of the produced organic N and leads to the increase of the δ¹⁵N and δ¹⁸O values of the remaining nitrate closed to 1:1 during NO₃⁻ uptake by phytoplankton (Granger et al., 2004). The denitrification is a permanent sink process of nitrate, which leads to the simultaneous increase of residual δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ (Denk et al., 2017, Nikolenko et al., 2018), and the ratios of the increase of the δ¹⁵N and δ¹⁸O values of the remaining nitrate are assumed to be close to 1.5:1 or even 2:1 (Kendall et al., 2007, Mengis et al., 1999). These three typical source and sink processes coexist and determinate the behaviors of N in the river ecosystems. Thus, in order to identify the mechanisms of nitrate accumulation in an urbanized river, multi-isotopes can be used to investigate the N transformations including nitrification, denitrification and assimilation.

The Pearl River, also known as the Zhujiang River, is the second largest river in China in terms of annual runoff. The Pearl River Delta (PRD) region is a collection of numerous distributaries and a highly dynamic river ecosystem. During the last two decades, the PRD region has been dramatically affected by human activities, such as rapid industrialization, intensive agricultural activities and urbanized developments, which have posed significant impact on the water environment (Chen et al., 2019, Lu et al., 2008, Lu et al., 2009). Due to the large population (over 60 million in 2018, Guangdong Statistical Yearbook, 2018) as well as the massive sewage discharge (over 7000 million tons in 2018, Guangdong Statistical Yearbook, 2018) in this region (Hui et al., 2018), large quantities of biological reactive N are discharged into the rivers, which leads to frequent eutrophication and deteriorative water quality, and results in increase in nutrients, especially nitrate accumulation (Hu and Li, 2009, Lu et al., 2008, Wang et al., 2013). In addition, runoff in the PRD region is highly seasonal due to wet southwesterly monsoon in summer and dry northeasterly monsoon in winter, causing seasonal variation of physical and biogeochemical processes of N (Ye et al., 2016, Ye et al., 2017). Previous studies concerned on sources and transformations of N in the PRD region focused on the riverine fluxes of N (Hu and Li, 2009, Lu et al., 2009), organic matter sources (Yu et al., 2010) and biogeochemical processes of nutrients by physical-biological model (Hu and Li, 2009). However, few studies have been conducted to investigate the origin and transformations of different forms of N in the PRD region. Although numerous studies also have investigated N cycling in other urbanized watersheds, such as N fluxes and retention (Groffman et al., 2004, Zhu et al., 2005), NO₃⁻ sources and NO₃⁻ transformation processes (Archana et al., 2018, Liu et al., 2018a, Liu et al., 2018b) and effects of urban sewage on N concentrations (Daniel et al., 2002), there is still unclear about which mechanisms are likely to facilitate nitrate accumulation in the urbanized river.

Therefore, in this study, we measured dissolved N concentrations, as well as stable isotopes of dissolved inorganic N (δ¹⁸O-NO₃⁻, δ¹⁵N-NO₃⁻, δ¹⁵N-NH₄⁺) and suspended particulate N (δ¹⁵N-PN) in the rivers of the PRD region to investigate seasonal (wet and dry seasons) and spatial variations of N sources and N transformation processes. Working hypothesis of this investigation is that the seasonal change of runoff and different degree of urbanization significantly alter the N dynamics in this highly urbanized river ecosystem, and the nitrate accumulation is associated with this changes of N transformations, including nitrification, denitrification and assimilation.