Variations of dissimilatory nitrate reduction processes along reclamation chronosequences in Chongming Island, China
Introduction
Over the past few decades, overloaded reactive nitrogen transported into coastal areas has caused various environmental issues, especially aquatic eutrophication (Cui et al., 2013; Hou et al., 2015; Chen et al., 2016). Coastal wetlands are key transitional zones between terrestrial ecosystem and marine ecosystem, which can provide extensive ecosystem services in global biogeochemical cycles such as storm protection (Moeller et al., 2014), carbon storage (Duarte et al., 2005), and habitat for flora and fauna (Roberts et al., 2012). Besides, coastal wetlands can filter contaminants including reactive nitrogen from terrestrial ecosystem to open seas (Canfield et al., 2010), and nitrogen removal in coastal wetland can significantly alleviate N loading to the coastal ocean (Peng et al., 2016). Dissimilatory nitrate reduction processes (DNRP) such as denitrification (DNF), anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) are major pathways of nitrate reduction in coastal wetland, playing an important role in controlling the nitrate dynamics and fate in estuarine and coastal ecosystems (Deng et al., 2015; Gao et al., 2017).
Nowadays, about 50% of the global coastal wetlands have been reclaimed due to the demand for farming and housing (Barbier et al., 2011), and rice planting is a long-term implement in wetland reclamation with great agronomic significance (Zong et al., 2007). Rice paddy is an important way of agricultural cultivation with high amounts of fertilizer N (up to 250 kg N ha−1) and low efficiency of N use (below 40%) (Cassman, 1999; Peng et al., 2006). The conversion of coastal wetland into paddy soil can alter soil physicochemical properties due to fertilization application and other agricultural management (Cui et al., 2012; Ding et al., 2017), and changes of soil physicochemical properties are important factors affecting nitrogen cycling and N2O emission (Li et al., 2015; Liu et al., 2016). For example, long-term application of N fertilization can significantly increase rates of DNF, mineralization and nitrogen fixation in soils (Wu et al., 2017a; Wang et al., 2019; Peng et al., 2016). Therefore, conversion of coastal wetland into paddy soil may significantly increase N loading to coastal regions and affect global warming. Although DNRP in rice paddy soils has been previously studied (Pandey et al., 2019; Shan et al., 2016, 2018), the effects of reclamation chronosequences in coastal wetland on DNRP are poorly understood.
There was extensive reclamation of coastal wetland to paddy soil in the Yangtze River Delta (Zong et al., 2007). As the largest estuarine island in China, approximately half of the present area of Chongming Island were obtained through coastal wetland reclamation (Cui et al., 2012a). Previous studies indicated an increasing trend of nutrients and bacterial diversity and decreasing trend of amorphous Fe oxyhydrates along reclamation chronosequences in the Chongming Island (Cui et al., 2012a, Cui et al., 2012b). However, the knowledge of soil DNRP and related functional gene abundances succession along the rice cultivation chronosequences are still limited. The objectives of this study were to (1) expound the evolution of DNRP, related functional genes abundances, and N2O emission along reclamation chronosequences; (2) identify the main factors regulating DNRP and N2O emission; (3) reveal the fate and environmental implications of N transformation in coastal wetland reclamation. This study can improve the understanding of N removal and transformation in agricultural land and coastal wetland.
Section snippets
Study area and sampling
Chongming Island locates in the Yangtze Estuary with a total area of 1267 km2, which is the largest estuarine island in China (Ma et al., 2015). The average temperature, precipitation and groundwater table of Chongming Island are 15.3 °C, 1003.7 mm and 85.7 cm, respectively. In the reclaimed areas of Chongming Island, nearly 300 kg ha-1 N fertilizers were applied annually, and 15-30 cm high of stubbles were returned to soil after harvesting in the paddy fields (Cui et al., 2012a). The southern
Physicochemical properties
Significant differences were observed in concentrations of NO2—, NH4+, NO3—, Fe2+, TOC, and sulfides between paddy soil and coastal wetland sediment (one-way ANOVA, p < 0.05), while similar concentrations of Fe3+ were detected. Concentrations of NO2—, sulfides and TOC were higher in paddy soil than coastal wetland sediment, and concentrations of NH4+ and Fe2+ were higher in paddy soil than high tidal flats. Contrarily, concentrations of NO3— were lower in paddy soil than coastal wetland
Discussion
Land use change can shift a set of soil abiotic and biotic properties (Aon and Colaneri, 2001; Liu et al., 2018; Singh et al., 2010), and further significantly alter nitrogen cycling processes (Van Lent et al., 2015). Long-term nitrogen (N) fertilization on agricultural soil have caused significant imbalance in biogeochemical N cycling (Galloway et al., 2004; Pandey et al., 2019), nitrous oxide emissions (Liu et al., 2016) and soil microbial communities (Manoharan et al., 2017).
In this study,
Conclusions
This study reported the effects of coastal wetland conversion to paddy soil on DNRP in eastern intertidal flat of Chongming Island, China. Conversion of coastal wetland to paddy soil can significantly increase the rates of DNF, anammox and DNRA, and the nitrate reduction rates generally increased along the reclamation chronosequences. Coastal wetland conversion to paddy soil regulated soil physicochemical characteristics and abundances of related functional genes, further leading to the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by National Natural Science Foundations of China (grant numbers 41730646, 41761144062, 91851111, 41725002, 41671463, 41501524 and 41971105), and Chinese National Key Programs for Fundamental Research and Development (grant numbers 2016YFE0133700, and 2016YFA0600904). Data presented in this paper can be obtained by sending a written request to the corresponding author.
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