Introduction

Nitrogen (N) is an essential nutrient for organisms and the excessive inputs of N have led to serious eutrophication and frequent harmful algal blooms in aquatic systems (Diaz and Rosenberg, 2008; Di et al. , 2015). As an important reduced form of N, NH4+ needs the least energy for the uptake of phytoplankton and bacteria, and thus commonly shows low water column concentrations when the primary production is high (Gardneret al. , 2004; Paerl et al. , 2011). Despite within the conditions of low ambient NH4+concentrations and/or net fluxes (i.e., differences between the regeneration and uptake of NH4+), evidence have shown that raid NH4+recycling provides hidden N source for algal species (Bruesewitzet al. , 2015; Jiang et al. , 2019; Gardner et al. , 2017). Therefore, to comprehensively understand the sources and sinks of N and serve eutrophication control, it is crucial to evaluate NH4+ recycling and its contribution to the ecosystem.
Water column NH4+ recycling involves the intensively coupling of NH4+regeneration and uptake. The regeneration of NH4+ is primarily contributed by the bacterial ammonification of labile organic N and metabolic release of zooplankton (Gardner et al. , 2004), and the uptake of NH4+ mainly include planktonic (i.e., algae and heterotrophic bacteria) assimilation and bacterial nitrification (Hampel et al. , 2017). The balance between NH4+ regeneration and uptake can be reflected by calculating community biological NH4+ demand (CBAD), which indicates net microbial community demand for NH4+ (Gardner et al. , 2017).
Recycling rates of NH4+ and the CBAD varied among different water bodies under the influence of various environmental factors and ecosystem characteristics. Nutrient concentrations, organic matter, and chlorophyll a (Chl-a ) are common factors influencing NH4+recycling rates in eutrophic lakes, estuaries, and coasts (Bruesewitzet al. , 2015; Gudasz et al. , 2015; Jiang et al. , 2019). In high turbid rivers, besides the factors mentioned above, suspended solid (SS) is also an important factor influencing NH4+ recycling processes (Xue et al. , 2019). SS supports the metabolism of attached heterotrophic bacteria (i.e., nitrification, denitrification, and nitrous oxide emission) by providing ideal oxic or anoxic interfaces with easily available substrates from SS or its surrounding water (Xia et al. , 2009; Liu et al. , 2013; Yao et al. , 2016; Zhouet al. , 2019; Zheng et al. , 2017). However, the influence of SS on microbial NH4+ recycling remains unclear.
Characterized by low retention time, drastic flushing effects, and dramatic SS changes, river-estuary continuums exhibit typical gradients of nutrient concentrations and dynamics (Mccarthy et al. , 2007b; Bruesewitzet al. , 2015; Liang and Xian, 2018). Previous studies in river-estuary continuums usually focused on N loads and its exports (Liang and Xian, 2018), long-term changes of nutrient concentrations (Liu et al. , 2018), and responses of phytoplankton community to increased nutrients input (Zhou et al. , 2008). Relatively fewer studies are focused on NH4+ recycling in river-estuary continuums and its effect factors. NH4+ recycling in river-estuary continuums may not support severe algal blooms as in eutrophic lakes, while it is still playing an important role in microbial growth and can impact nutrient dynamics in the systems and even lower coasts.
In recent decades, algal blooms increased dramatically in the estuary of Yangtze River in terms of frequency, area, and persistence time (Zhouet al. , 2008; Tang et al. , 2006; Dai et al. , 2011; Jiang et al. , 2014). It has been known that external N inputs play an important role in supporting algal blooms in the estuary of Yangtze River (Zhou et al. , 2008), while the contribution of water column N recycling is unclear. We hypothesized that NH4+ recycling would provide enormous N for microbes in the river-estuary continuum of Yangtze River, and the volume of regenerated NH4+ have the potential to support algal blooms in the estuary. The aims of this study are to: (1) quantify NH4+ uptake and regeneration rates as well as CBAD; (2) examine the effects of SS and other environmental factors on NH4+recycling rates; (3) evaluate the contribution of regenerated NH4+ to N budgets and explore implications. This study is the first to report NH4+ recycling and CBAD in the river-estuary continuum of Yangtze River. The results of our study provide a basis for the comprehensive understanding of N sources in the Yangtze River and the formulation of N management strategies.