Sandy seepage faces as bioactive nitrate reactors: Biogeochemistry, microbial ecology and metagenomics

Subterranean estuaries are highly dynamic in processing dissolved inorganic nitrogen (DIN). Here we investigate DIN turnover in surface sediments (0-20 cm depth) at the higher, medium and lower inter-tidal of a seepage face, i.e., the outer mouth of the subterranean estuary, during four consecutive sea-sons in Sanggou Bay, China. Throughout the studied period, ammonium (NH4+) and nitrite (NO2 -) concentrations in the sampled porewaters did not vary significantly with depth or season. In contrast, peaks in porewater nitrate (NO3-) concentration and decreases in .515N-NO3-and .518O-NO3-were observed in the 15-20 cm depth (bottom) sediment, particularly during summer and autumn. Coupled with NO3- production, the sediment total nitrogen was also markedly peaking in the bottom layer of the studied seepage face. Together with abundant heterotrophic microbes in the sediment, this NO3- accumulation was linked to a reaction chain including organic matter decomposition, ammonification and nitrification. During winter, porewater enrichment in total nitrogen occurred closer to the surface of the seepage face but triggered also active NO3- production. This pattern reinforced the importance of pelagic organic matter supply on NO3- production. In the shallower depths of the seepage face (<12 cm), active net NO3- removal occurred except in winter. The isotopic fractionation (.515N-NO3-and .518O-NO3-) and metagenomic results revealed denitrification as the main pathway for NO3- reduction. Biological assimilation from benthic primary producers may also consume a fraction of NO3- at the sediment water interface. Both NO3- production and removal significantly varied in magnitude with season (-13.6 to 6.2 nmol cm-3 h-1). Substrate supply was the key driver for nitrate cycling, as evidenced by the high NO3- production rate in spring by comparison to autumn. The highest NO3- turnover rates were found in summer, suggesting the combined influence of advection rates and sediment microbiota composition. In spite of active removal (peak NO3- removal capability: 61%), a significant amount of NO3- was still trans-ported from the seepage face into the bay waters. The magnitude of NO3- fluxes ranged from 312 to 476 kg N d-1, accounting for approximately 15% of the total exogenous NO3- loading into the bay. NO3- isotopic fingerprint revealed chemical fertilizer as the main source of terrestrial NO3- in SGD, highlighting the importance of land use to coastal system nitrogen budgets.(c) 2022 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Automated summary

This study investigates the turnover of dissolved inorganic nitrogen in subterranean estuaries in Sanggou Bay, China. The results show that ammonium and nitrite concentrations did not vary significantly with depth or season, while nitrate concentrations peaked in the bottom sediment layer and were linked to nitrogen production through organic matter decomposition, ammonification, and nitrification.