ABSTRACT
Mangrove aquatic ecosystems receive substantial nitrogen (N) inputs from both land and sea, playing critical roles in modulating coastal N fluxes. The microbially-mediated competition between denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in mangrove sediments significantly impacts the N fate and transformation processes. Despite their recognized role in N loss or retention in surface sediments, how these two processes vary with sediment depths and their influential factors remain elusive. Here, we employed a comprehensive approach combining 15N isotope tracer, quantitative PCR (qPCR) and metagenomics to verify the vertical dynamics of denitrification and DNRA across five 100-cm mangrove sediment cores. Our results revealed a clear vertical partitioning, with denitrification dominated in 0-30 cm sediments, while DNRA played a greater role with increasing depths. Quantification of denitrification and DNRA functional genes further explained this phenomenon. Taxonomic analysis identified Pseudomonadota as the primary denitrification group, while Planctomycetota and Pseudomonadota exhibited high proportion in DNRA group. Furthermore, genome-resolved metagenomics revealed multiple salt-tolerance strategies and aromatic compound utilization potential in denitrification assemblages. This allowed denitrification to dominate in oxygen-fluctuating and higher-salinity surface sediments. However, the elevated C/N in anaerobic deep sediments favored DNRA, tending to generate biologically available NH4+. Together, our results uncover the depth-related variations in the microbially-mediated competition between denitrification and DNRA, regulating N dynamics in mangrove ecosystems.
ABSTRACT
Dissolved inorganic nitrogen (DIN) is very high in the Pearl River Estuary (PRE) and nitrate (NOx-) removal processes such as denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) are important for determining export of DIN to coastal waters. However, fluxes of NOx- removal and influencing factors in the PRE are still unclear. We conducted 4 cruises at 11 sites in the PRE to investigate potential NOx- removal rates, their contributions, and corresponding gene abundances, and controlling factors in surface sediments (0-5 cm). The results showed that the potential rates of denitrification, anammox, and DNRA as well as their contributions varied spatially and seasonally. Denitrification (1.98 ± 1.7 µg N g-1 d-1) was the major NOx- removal processes (68.43 ± 14.61%) while DNRA (0.45 ± 0.28 µg N g-1 d-1) contributed 22.61 ± 14.89% in NOx- removal. The NOx- removal processes and corresponding gene abundances were correlated with the chlorophyll concentrations in both overlying water and sediment, indicating that marine-produced organic matter was the major driver for benthic NOx- removal processes. In addition, water column turbidity had important effects on primary production, which affects benthic N processes. Our study provides evidences for that the turbidity-regulated primary production in overlying water is the primary driver for benthic NOx- removal processes. The contribution of sediment NOx- removal fluxes to water column NOx- concentration was low in the upper estuary and increased in the lower estuary where marine produced chlorophyll a was higher. However, daily fluxes of NOx- removal were estimated to account for only 0.18-7.22% (mean 1.85 ± 1.62%) of NOx- in the whole overlying water column. This suggests that most riverine NOx- was exported out into the adjacent coastal waters.
