Search

Background and aims: Nitrogen (N) mineralization and immobilization are two important N biogeochemical processes in estuarine and coastal sediments. However, the changes of those two processes in sediment following Spartina alterniflora (S. alterniflora) invasions into mangrove wetlands under aerobic and anaerobic conditions remain limited. Methods: We used a 15N stable isotope dilution technique to investigate sediment gross N mineralization (GNM) and NH4+ immobilization (GAI) under aerobic and anaerobic conditions in mangrove communities, mangrove communities mixed with S. alterniflora, and pure S. alterniflora communities in the Zhangjiang Estuary, China. Results: GNM and GAI rates varied spatially and seasonally. Summer N mineralization and immobilization rates under aerobic and anaerobic conditions were both significantly higher than those in winter, and the high rates were observed mainly in mangrove sediments and decreased with S. alterniflora invasion. GNM rates in aerobic conditions were significantly higher than those in anaerobic conditions, while GAI rates in aerobic conditions were less than those in anaerobic conditions. GNM and GAI rates were related closely to temperature, water content and organic matter. S. alterniflora invasion can weaken GNM and GAI rates by reducing organic matter accumulation, and S. alterniflora invasion could have reduced sediment N mineralization (2.10 × 103 t N yr−1) and immobilization (1.46 × 103 t N yr−1) in mangrove wetlands of China from 1980 to 2020. Conclusions: Our results highlight the importance of S. alterniflora invasion and hydrological conditions in controlling N mineralization and immobilization processes in mangrove sediments and are essential for the evaluation of N-balance in this N-limited ecosystem. [ABSTRACT FROM AUTHOR]

Mangroves are important ecosystems on the coasts of southeastern and southern China, which have a significant role in altering microbial communities and nutrient cycles. However, the response of sediment nitrogen (N) mineralization (GNM) and ammonium immobilization (GAI) on the different coastal types (muddy and sandy) to mangrove growth remains poorly understood. Here, surface sediment (0–5 cm) GNM and GAI were examined simultaneously in bare flat and mangrove habitats on both muddy and sandy coasts along a coastal transect of South China, using 15N isotope dilution techniques. The results showed that GNM and GAI rates in sediment ranged from 0.31 to 33.94 μg N g−1 d−1 and 0.24 to 41.02 μg N g−1 d−1, respectively, and mangroves significantly increased the two process rates on both muddy and sandy coasts. The magnitude of increase in GNM and GAI was more pronounced for sandy coasts than for muddy coasts. Mangroves also promoted the ratio of GAI to GNM (RAI), but the ratio of GNM to total N (PAM%) generally did not differ between bare flat and mangrove habitats. Sediment total organic carbon, total N, ammonium, ferrous iron, and microbial abundances were crucial factors in regulating GNM and GAI on both two contrasting coasts, but the process rates on sandy coasts were also related to sediment grain size composition and specific surface area. Our results suggested that mangrove growth is more effective in promoting GNM and GAI on sandy coasts than on muddy coasts, which may be important to improving the nutrient cycle and energy flow on sandy coasts. Overall, these findings provide valuable insights into the N transformations and influencing mechanisms on different coastal types influenced by the growth of mangroves and imply that it is significant to advance mangrove restoration on sandy coasts, yet widely overlooked. [ABSTRACT FROM AUTHOR]

Nitrogen mineralization is a key biogeochemical process transforming organic nitrogen to inorganic nitrogen in estuarine and coastal sediments. Although sedimentary nitrogen mineralization is an important internal driver for aquatic eutrophication, few studies have investigated sedimentary nitrogen mineralization in these environments. Sediment-slurry incubation experiments combined with 15N isotope dilution technique were conducted to quantify the potential rates of nitrogen mineralization in surface sediments of the Yangtze Estuary. The gross nitrogen mineralization (GNM) rates ranged from 0.02 to 5.13 mg N kg-1 d-1 in surface sediments of the study area. The GNM rates were generally higher in summer than in winter, and the relative high rates were detected mainly at sites near the north branch and frontal edge of this estuary. The spatial and temporal distributions of GNM rates were observed to depend largely on temperature, salinity, sedimentary organic carbon and nitrogen contents, and extracellular enzyme (urease and L-glutaminase) activities. The total mineralized nitrogen in the sediments of the Yangtze Estuary was estimated to be about 6.17 × 105 t N yr-1, and approximately 37% of it was retained in the estuary. Assuming the retained mineralized nitrogen is totally released from the sediments into the water column, which contributed 12–15% of total dissolved inorganic nitrogen (DIN) sources in this study area. This result indicated that the mineralization process is a significant internal nitrogen source for the overlying water of the Yangtze Estuary, and thus may contribute to the estuarine and coastal eutrophication. [ABSTRACT FROM AUTHOR]

Estuarine and tidal wetlands with high primary productivity and biological activity play a crucial role in coastal nutrient dynamics. Here, to better reveal the effects of extracellular enzymes and microbial community on carbon (C) and nitrogen (N) mineralization, the incubation experiments with different C and N addition patterns to the tidal sediments of the Yangtze Estuary (China) were conducted. The results suggested a significant increase in cumulative CO effluxes in the C and CN treatment experiments, while no significant difference in cumulative CO effluxes between the N treatment and control (CK) experiments was observed. In addition, the nutrient addition patterns had a great influence on dissolve organic C and N levels, but a small effect on microbial biomass C and N. Microbial community composition and microbial activity were found to be positively correlated with organic C (OC) and the molar ratio of C to N (C/N). Partial correlation analysis, controlling for C/N, supported direct effects of OC on the activity of carbon-cycling extracellular enzymes (cellulase and polyphenol oxidase), while C/N exhibited negatively correlations with urease and Gram-positive bacteria to Gram-negative bacteria (G+/G−). Strong relationships were found between CO efflux and mineral nitrogen with the activity of specific enzymes (sucrase, cellulase, and polyphenol oxidase) and abundances of Gram-negative bacteria, arbuscular mycorrhizal fungi, and fungi, suggesting the significant influences of microbial community and enzyme activity on C and N mineralization in the estuarine and tidal wetlands. Furthermore, this study could highlight the need to explore effects of nutrient supply on microbial communities and enzyme activity changes associated with the C and N mineralization in these wetlands induced by the climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • The settling phytoplankton was major driver for benthic N mineralization. • NNM was higher in lower estuary mainly due to higher chlorophyll. • NNM was higher in low temperature season mainly due to the inhibition of GAI. • NNM is gradually important for supporting primary production along the estuary. Nitrogen (N) mineralization and immobilization are two important N transformation processes in estuarine sediments. However, the biotic and abiotic mechanisms regulating those two processes in sediments of a eutrophic estuary remains poorly understood. Here, we used 15N stable isotope dilution technique to investigate the spatial and temporal variations of gross N mineralization (GNM) and gross NH 4 + immobilization (GAI) in surface sediment (0–5 cm) along the Pearl River Estuary (PRE) in 4 cruises. Results showed that GNM rates varied spatially and increased with salinity, ranging from 0.15 to 1.99 μg N g−1 d−1, while GAI varied more temporally and increased with temperature, ranging from 0 to 1.96 μg N g−1 d−1. The net N mineralization (NNM) rates ranged from −0.24 to 1.84 μg N g−1 d−1, NNM was higher in lower estuary mainly due to higher chlorophyll-induced higher GNM, while NNM was higher in low temperature season mainly due to the inhibition of GAI. As high turbidity in upper estuary reduced light penetration and hence primary production, the decrease of settling labile organic matter from the water column weaken benthic bacterial abundances, GNM and NNM rates, whereas inorganic N (especially NH 4 +) had greater effect on GAI than organic matter. As an NH 4 + source, the contribution of sediment daily NNM to NH 4 + and dissolved inorganic N (DIN) in the overlying water column increased from upstream to downstream (−5.81 to 110.74% and −0.09 to 1.27%, respectively), suggesting an increased importance of NNM for supporting primary production in the lower estuary. Our study reveals the dynamics of net NH 4 + production by distinguishing N mineralization and immobilization processes, and enhances the understanding of benthic-pelagic coupling influence on N-cycling in a eutrophic estuary. [ABSTRACT FROM AUTHOR]