Numerous studies have unveiled the importance of nitrogen (N) transformation processes over the past decades, but comprehensive studies of key N-cycling processes are still rare in estuarine and coastal ecosystems. Here, we used isotope pairing, isotope-tracing, and isotope dilution techniques combined with quantitative polymerase chain reaction to investigate microbial N-cycling processes in surface sediments (0–5 cm) of the Pearl River Estuary. The average rates of denitrification, anammox, DNRA, N2 fixation, N mineralization, and NH4+ immobilization were 1.41 ± 0.89, 0.067 ± 0.033, 0.47 ± 0.28, 0.31 ± 0.30, 1.86 ± 1.09, and 1.30 ± 0.83 μg N g−1 dry d−1, respectively. Sediment grain size, organic matter, nutrients, and Fe2+/Fe3+ rather than gene abundances controlled these rates. The abundances of bacterial 16S rRNA, anammox 16S rRNA, nirS, nrfA, nifH, bacteria-amoA, and archaea-amoA genes were significantly correlated with organic matter, nutrients, and sediment grain size. In general, higher rates and gene abundance was occurred in outer than inner estuary. Among these pathways, denitrification contributed 41.83–90.13% of the total nitrate reduction, as compared to 0.94–8.58% for anammox and 8.55–54.56% for DNRA. The sediment N-loss fluxes caused by denitrification and anammox in our study area (1.5 × 1010 m2) was about 6.2 × 107 mol N d−1, accounting for ~42.1% of the riverine dissolved inorganic N fluxes, suggesting that the sediment of the Pearl River Estuary has great significance to the mitigation and controlling of N pollution in this ecosystem. Additionally, the net NH4+ production via sediment microbial pathways (N mineralization, N2 fixation, DNRA, NH4+ immobilization, and anammox) was estimated at ~ 5.5 × 107 mol N d−1, while the net NO3− consumption (denitrification, anammox, and DNRA) was ~8.3 × 107 mol N d−1. Overall, these results highlight the importance of complicated N-cycling processes in controlling the N budget in the Pearl River Estuary and improve the understanding of both the processes and associated controlling factors in estuarine and coastal ecosystems.

Nitrogen cycling processes in sediments of the Pearl River Estuary: Spatial variations, controlling factors, and environmental implications

Distribution determination, risk assessment, and source identification of heavy metals in mangrove wetland sediments from Qi’ao Island, South China

This study focuses on the coastal features, environments, and dynamics to accurately describe and regularly monitor the Qingdao shoreline in eastern China. It collects categorical ETM+ and OLI data from 2000, 2010, and 2019 on the mainland coastline and explores the characteristics and spatiotemporal differences across the past 19 years by using remote sensing and geographic information system (GIS) technologies. The results show that the length of the Qingdao coastline has increased continuously over the last two decades, for a total increase of 18.14 km. There are different natural and artificial coastlines that have undergone major changes. The human-induced deterioration of coastlines has gradually and substantially risen from 53.63% in 2000 to 68.40% in 2019, while the length of the natural coastlines has decreased dramatically. Jiaozhou Bay focuses on areas with significantly changing coastlines, and major changes have occurred in the west and east of the Qingdao coast. The coastline has largely expanded seaward because of the comprehensive impact of natural and anthropogenic factors. The leading factor in coastal evolution is coastal engineering constructions. In addition, the top three other construction activities are the restoration of the aquaculture pond, salt field, and harbor edifices. The driving force that triggered the shift in the coastline reveals significant temporal heterogeneity.

/pdf/a-spatiotemporal-change-detection-analysis-of-coastline-data-16s20q5mgy.pdf

A Spatiotemporal Change Detection Analysis of Coastline Data in Qingdao, East China

A new model for evaluating sustainable utilization of coastline integrating economic output and ecological impact: A case study of coastal areas in Beibu Gulf, China

Shoreline changes are of great importance for evaluating the interaction between humans and ecosystems in coastal areas. They serve as a useful metric for assessing the ecological costs of socioeconomic developmental activities along the coast. In this paper, we present an assessment of shoreline changes along the eastern coast of mainland China from ~1990 to 2019 by applying a novel method recently developed by us. This method which we call the Nearest Distance Method (NDM) is used to make a detailed assessment of shorelines delineated from Landsat Thematic Mapper (TM) and Operational Land Imager (OLI) images. The results indicate a dramatic decline in natural shorelines that correspond to the rapid increase in the construction of artificial shorelines, driven by China’s economic growth. Of the entire coast of mainland China, the biggest change occurred along the Bohai Sea, where artificial shorelines expanded from 42.4% in ~1990 to 81.5% in 2019. Over this period, this study indicates that China lost > 60% of its biological shorelines, a trend that is especially worrisome because these include areas that were once biologically diverse and extremely rich. As anticipated, shoreline losses were greatest where regions of low economic value had been transformed to areas of higher economic value. Overall, this influence of human activities on shorelines in China is unprecedented. The repercussions of these changes on ecosystems, and the susceptibility of new shoreline developments to population growth and sea-level rise, need to be assessed urgently before additional changes are effected.

https://www.mdpi.com/2220-9964/9/10/572/pdf

Rui Zhang

Papers

Dynamic process and artificial mechanism of coastline change in the Pearl River Estuary