Effects of marine produced organic matter on the potential estuarine capacity of NOx- removal

Labile organic matter favors a low N2O yield during nitrogen removal in estuarine sediments

Estuary harbors the active sediment denitrification and nitrous oxide (N2O) emission, while the knowledge of environmental controls on the denitrification-derived N2O yield remains underexplored. Here, we quantitatively assess the potential and in situ rates of N2O production during sediment denitrification in the Pearl River Estuary (PRE), China. Organic matter determines the product stoichiometry and capacity of nitrogen removal. In particular, labile organic matter (LOM) reduces N2O yield via enhancing the complete coupled nitrification-denitrification. Our results reveal that the chain processes, primary production-LOM settling-sedimentary respiration-coupled nitrification-denitrification, control the sediment denitrification and N2O production, linking the carbon and nitrogen biogeochemical cycles in the atmosphere-water column-sediment continuum. The PRE sediments serve as nitrogen removal hotspots but with low efficiency (~25 % of riverine input) and strong N2O release (~66 % of daily sea-air N2O efflux). These findings contribute to policy makers to develop knowledge-based management actions for achieving sustainable coastal environments and mitigating N2O emission.

Effects of salinity on nitrogen reduction pathways in estuarine wetland sediments

Denitrification, anammox, and DNRA are three important nitrogen (N) reduction pathways in estuarine sediments. Although salinity is an important variables controlling microbial growth and activities, knowledge about the effects of changing salinity on those three processes in estuarine and coastal wetland sediments are not well understood. Herein, we performed a 60-d microcosms experiment with different salinities (0, 5, 15, 25 and 35 ‰) to explore the vital role of salinity in controlling N-loss and N retention in estuarine wetland sediments. The results showed that sediment organic matter, sulfide, and nitrate (NO3-) were profoundly decreased with increasing salinity, while sediment ammonium (NH4+) and ferrous (Fe2+) varied in reverse patterns. Meanwhile, N-loss and N retention rates and associated gene abundances were differentially inhibited with increasing salinity, while the contributions of denitrification, anammox, and DNRA to total nitrate reduction were apparently unaffected. Moreover, denitrification rate was the most sensitive to salinity, and then followed by DNRA, while anammox was the weakest among these three processes. In other words, anammox bacteria showed a wide range of salinity tolerance, while both denitrification and DNRA reflected a relatively limited dynamic range of it. Our findings could provide insights into temporal interactive effects of salinity on sediment physico-chemical properties, N reduction rates and associated gene abundances. Our findings can improve understanding of the effects of saltwater incursion on the N fate and N balance in estuarine and coastal sediments.

Nitrogen migration and transformation during re-suspension and photo-induction in landscape water replenished by reclaimed water

Sediment re-suspension plays a crucial role in releasing endogenous nitrogen and greenhouse gases in shallow urban waters. However, the impacts of repeated re-suspension and photo-induced processes on migration and transformation from endogenous nitrogen, as well as the emission of greenhouse gases, remain unclear. This study simulated three conditions: re-suspension (Rs), re-suspension combined with ultravioletirradiation (Rs + UV), and ultraviolet irradiation (UV). The findings revealed that both repeated sediment re-suspension and exposure to UV light altered the characteristics of surface sediments. Decrease of convertible nitrogen in sediments, leading to the release of ion-exchangeable nitrogen (IEF-N) into NH4+-N and NO3--N, influenced greenhouse gas production differently under various conditions. The study observed the highest concentration of dissolved N2O in under UV irradiation, positively correlated with NO2--N and NO3--N. Re-suspension increased the turbidity of the overlying water and accelerated nitrification, resulting in the highest NO3--N concentration and the lowest dissolved N2O concentration. Additionally, in the Rs + UV dissolved N2O maintained the higher concentrations than in Rs, with greatest amount of N conversion in surface sediments, and a 59.45% reduction in IEF-N. The production of N2O during re-suspension was mainly positively correlated with NH4+-N in the overlying water. Therefore, this study suggest that repeated re-suspension and light exposure significantly influence nitrogen migration and transformation processes in sediment, providing a theoretical explanation for the eutrophication of water and greenhouse gas emissions.

Denitrification dominates dissimilatory nitrate reduction across global natural ecosystems

Denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are three competing processes of microbial nitrate reduction that determine the degree of ecosystem nitrogen (N) loss versus recycling. However, the global patterns and drivers of relative contributions of these N cycling processes to soil or sediment nitrate reduction remain unknown, limiting our understanding of the global N balance and management. Here, we compiled a global dataset of 1570 observations from a wide range of terrestrial and aquatic ecosystems. We found that denitrification contributed up to 66.1% of total nitrate reduction globally, being significantly greater in estuarine and coastal ecosystems. Anammox and DNRA could account for 12.7% and 21.2% of total nitrate reduction, respectively. The contribution of denitrification to nitrate reduction increased with longitude, while the contribution of anammox and DNRA decreased. The local environmental factors controlling the relative contributions of the three N cycling processes to nitrate reduction included the concentrations of soil organic carbon, ammonium, nitrate, and ferrous iron. Our results underline the dominant role of denitrification over anammox and DNRA in ecosystem nitrate transformation, which is crucial to improving the current global soil N cycle model and achieving sustainable N management.

Sediment nitrates reduction processes affected by non-native Sonneratia apetala plantation in South China

Numerous studies have highlighted the importance of nitrates (NOx-) reduction processes in estuarine and coastal ecosystems over the past decades. However, the biotic and abiotic factors sediment NOx- reduction processes in mangrove of varying ages are still not fully understood. Here, we investigated the dynamics of sediment NOx- reduction processes and associated gene abundances in mangroves of different ages (including 0-year unvegetated mudflats, 10 and 20-years Sonneratia apetala, as well as >40 years of mature native Kandelia obovate) on the Qi'ao Island using 15N stable-isotope pairing techniques and quantitative PCR. The denitrification (2.64-11.30 nmol g-1 h-1), anammox (0.06-0.83 nmol g-1 h-1), and dissimilatory nitrate reduction to ammonium (DNRA, 0.58-16.34 nmol g-1 h-1) rates varied spatially and seasonally, but their contributions to the total NOx- reduction (DEN%, ANA%, and DNRA%), associated gene abundance (nirS, anammox 16S rRNA, and nrfA), and organic matter only varied spatially. Organic matter and microbial abundances are the dominating factors controlling N loss and retention. Without considering confounding factors, mangroves conservation and restoration significantly increased DNRA rates, NIRI (DNRA/(denitrification + anammox)), organic matter content, and microbial abundances (p < 0.05 for all), but reduced N loss rates. Mangroves conservation and restoration are estimated to have increased sediment N retention (~931.81 t N yr-1) and reduced N loss (~481.32 t N yr-1) in coastal wetlands of China over the past 40 years (1980-2020). Overall, our results indicate that mangrove restoration and conservation can significantly increase sediment N retention due to the rapid biomass accumulation, and it can provide more nutrients for mangrove and microorganism growth, thus creating a virtuous cycle in these N-limited ecosystems.

Dynamics of Benthic Nitrate Reduction Pathways and Associated Microbial Communities Responding to the Development of Seasonal Deoxygenation in a Coastal Mariculture Zone

Intensive mariculture activities result in eutrophication and enhance coastal deoxygenation. Deoxygenation profoundly influences nitrate reduction processes and further the fate of nitrogen (N) in coastal systems. Herein, 15N isotope labeling, real-time PCR, and high-throughput sequencing techniques were jointly used to investigate the participation and seasonal dynamics of sediment nitrate reduction pathways and the succession of functional microbial communities during the development of seasonal deoxygenation in a coastal aquaculture zone. Denitrification dominated benthic nitrate reduction (46.26-80.91%). Both denitrification and dissimilatory nitrate reduction to ammonium were significantly enhanced by summer deoxygenation (dissolved oxygen levels fell to 2.94 ± 0.28 mg L-1), while anammox remained unchanged. The abundance of the nitrous oxide reductase gene nosZ increased during deoxygenation. The community of the nosZ gene was sensitive to deoxygenation, with Azospirillum and Ruegeria accounting for the majority. Pelobacter was overwhelming in the nrfA gene (encoding dissimilatory nitrite reductase) community, which was less affected by deoxygenation. The variations of benthic nitrate reduction processes were driven by bottom water oxygen combined with temperature, chlorophyll a, and microbial gene abundances and community compositions. Our results implicated that seasonal oxygen-deficient zones could be substantial N sinks of coastal ecosystems and important for N balance. Effective management measures need to be developed to avoid further exacerbation of coastal deoxygenation and maintain the sustainable development of mariculture.

Sediment nitrogen mineralization and immobilization affected by non-native Sonneratia apetala plantation in an intertidal wetland of South China

The mineralization and immobilization of nitrogen (N) are critical biogeochemical transformations in estuarine and coastal sediments. However, the biotic and abiotic mechanisms that regulate the two processes in different aged mangrove sediments remain poorly understood. Here, we used ¹⁵N isotope dilution method to investigate the changes in sediment N mineralization (GNM) and NH₄⁺ immobilization (GAI) of different aged mangrove habitats (including 0, 10, and 20 years Sonneratia apetala, as well as >40 years mature native Kandelia obovata) in Qi'ao Island, Guangdong Province, China. Measured GNM and GAI rates ranged from 2.69 to 17.53 μg N g^-1 d^-1 and 2.29-21.38 μg N g^-1 d^-1, respectively, which varied both spatially and seasonally. The ratio of GNM to total N (PAM%, 0.24-0.86%) also varied spatially and seasonally, but the ratio of GAI to GNM (RAI, 0.79-1.54) only varied spatially. Mangrove restoration significantly increased the N mineralization and immobilization rates, but remained lower than those of mature native Kandelia obovata habitat. The sediment bacterial abundance, labile organic matter and temperature are the dominant factors in controlling N mineralization and immobilization. Our findings suggested that exotic mangrove Sonneratia aperale plantation can enhance sediment N mineralization and immobilization rates and improve N stability through accumulated biomass rapidly. Overall, these results provide new insights into sediment N transformation processes and associated influencing mechanisms in such intertidal wetlands profoundly influenced by human activities.

Effect of Aquaculture Reclamation on Sediment Nitrates Reduction Processes in Mangrove Wetland

Sediment denitrification, anaerobic ammonium oxidation (anammox), and nitrate dissimilation to ammonium (DNRA) play an important role in controlling the dynamics of nitrates (NOx−) and their fate in estuarine and coastal ecosystems. However, the effects of land-use change on NOx− reduction processes in mangrove sediments are still unclear. Here, we used a mud experiment method combined with a 15N stable isotope tracer method to study the mechanism and ecological environment of the change of land use pattern on the sediment NOx− reduction processes in mangrove wetlands. Our study showed that most physicochemical parameters, NOx− reduction rates, and their gene abundances varied considerably. The denitrification, anammox, and DNRA rates in mangrove sediment cores were in a range of 1.04–4.24 nmol g−1 h−1, 0.14–0.36 nmol g−1 h−1, and 0–2.72 nmol g−1 h−1, respectively. The denitrification, anammox, and DNRA rates in aquaculture sediment cores were in a range of 1.06–10.96 nmol g−1 h−1, 0.13–0.37 nmol g−1 h−1, and 0–1.96 nmol g−1 h−1, respectively. The highest values of denitrification, anammox, DNRA, the contribution of denitrification and DNRA to total NOx− reduction (DEN% and DNRA%), gene abundances (nirS, Amx 16S rRNA, and nrfA), total organic carbon (TOC), total nitrogen (TN), and TOC/TN in sediments were generally found in the top layer (0–5 cm) and then decreased with depth, while the contribution of anammox to total NOx− reduction (ANA%), Fe2+, and Fe2+/Fe3+ were generally increased with sediment depth in both mangrove and aquaculture ecosystems. When mangrove wetlands are transformed into pools, some properties (including TOC, TN, and Fe3+), DNRA rates, DRNA%, and nrfA gene abundances were decreased, while some properties (including NH4+, TOC/TN, Fe2+, and Fe2+/Fe3+), denitrification rates, DEN%, nirS, and ANAMMOX 16S gene abundances were increased. Sediment organic matter (TOC and TN) content and Fe2+ both affected NO3− reduction rates, with organic matter the most prominent factor. Thus, aquaculture reclamation enhances N loss while reducing N retention in sediments of mangrove wetlands, which plays an important role in regulating the source and fate of reactive N in mangrove ecosystems.

A multi-stable isotopic constraint on water column oxygen sinks in the Pearl River Estuary, South China

Bottom water oxygen depletion is a central concern in estuaries and coastal oceans worldwide. However, a mechanistic understanding and quantitative diagnosis of different oxygen-consuming processes are less clear. In this study, a multi-stable isotope approach is developed to delineate the role of oxygen respiration and nitrification contributing to total oxygen consumption in the Pearl River Estuary (PRE), a large eutrophic estuary in south China. The approach highly couples with analysis of the carbon isotope composition of dissolved inorganic carbon (δ¹³C-DIC) and with stable nitrogen isotope analysis in ammonium (δ¹⁵N-NH₄⁺) and nitrate (δ¹⁵N-NO₃^-). In all seasons, relatively low DO concentrations were observed in the upper reach and, to some extent, in the outer estuary during summer, while high concentrations of DO were found in the transition zone between the inner and outer estuary. On the basis of isotopic differentiation, our data reveal that much more depleted δ¹³C-DIC is coincident with DIC additions and low oxygen in the upper reach and inner estuary during most seasons. This is most likely a consequence of organic carbon (OC) degradation via aerobic respiration. Based on the carbon isotopic mass balance of DIC and the stoichiometry ratio of -ΔDO/ΔDIC, we found that the OC degradation dominates the total oxygen consumption in the upper reach, as well as in the inner estuary during summer (48.3%-93.5%). In addition, nitrification is another key process in contributing to total oxygen loss in the upper reach, as supported by the well-coupled variations of δ¹⁵N of NH₄⁺ and NO₃^- and apparent oxygen utilization (AOU). Using the formerly determined N isotopic fractionation and observed δ¹⁵N variation, we estimated that nitrification could account for 35.3%-44.1% and 28.5%-31.6% of the total oxygen consumption in the upper reach during winter and summer, respectively, while its contribution to total oxygen loss is minor in the inner and outer estuary. Overall, this study demonstrates the potential of the multi-stable isotopic approach to assess oxygen sink partitioning in large human-perturbed estuaries.

Sediment Nitrate Dissimilatory Reduction Processes along a Salinity Gradient in an Estuarine and Coastal Wetland, China

Nitrate (NO3−) dissimilatory reduction processes (denitrification, anammox and dissimilatory NO3− reduction to ammonium (DNRA)) in estuarine and coastal ecosystems play a crucial role in regulating reactive nitrogen loadings. However, nitrate reduction process rates and relative proportions along the estuarine salinity gradient remain poorly understood. Here, denitrification, anammox and DNRA were explored simultaneously along a salinity gradient in Yangtze Estuary based on nitrogen isotope-tracing experiments. Measured denitrification, anammox and DNRA process rates were in the range of 2.33–28.21 nmol g−1 h−1, 0.43–1.87 nmol g−1 h−1 and 0.28–0.74 nmol g−1 h−1, respectively, with a large spatio-temporal variation. The changes in these nitrate reduction process rates were mainly affected by the TOC, TN, NH4+ and NOx− concentrations, rather than salinity and related functional gene abundance. Denitrification dominated the total NO3− reduction process (67.52 to 93.85%), while anammox (3.67 to 25.01%) and DNRA (2.48 to 11.21%) also played a substantially important role in nitrate reduction. The proportions of denitrification to gross nitrate reduction in high-salinity areas were generally lower than those in freshwater, but the opposite was true for DNRA. Overall, our study reported the simultaneous observation of nitrate dissimilatory reduction processes along the salinity gradient of the estuary and highlighted that changes in sediment environmental variables affected by human activities can alter the distribution patterns of NO3− reduction processes.

Spartina alterniflora Invasion Enhances Dissimilatory Nitrate Reduction to Ammonium (DNRA) Rates in the Yangtze River Estuary, China

Dissimilatory nitrate reduction to ammonium (DNRA) can save N by converting nitrate into ammonium and avoiding nitrate leaching and runoff in saltmarshes. However, little is known about the effects of invasive plants on DNRA in the upper and deeper soil layers in salt marshes. Here, we investigated DNRA rates in the soils of six different depth layers (0–5, 5–10, 10–20, 20–30, 30–50, and 50–100 cm) from the invasive Spartina alterniflora marshland, two native plants Scirpus mariqueter and Phragmites australis marshlands, and bare mudflat on Chongming Island, located in the Yangtze River Estuary, China. Our results show that S. alterniflora significantly increased DNRA rates in both the upper 50 cm soil and deeper 50–100 cm soil layers. With respect to the entire soil profile, the NO3− reduction content calculated from DNRA in S. alterniflora marshland was 502.84 g N m−2 yr−1, increased by 47.10%, 49.42%, and 38.57% compared to bare mudflat, S. mariquete, and P. australis, respectively. Moreover, NO3− reduction content from the 50–100 cm soil layers was almost identical to that in the upper 50 cm of the soil. In the month of May, DNRA is primarily regulated by SO42− and pH in the upper and deeper soil layers, respectively, whereas, in the month of October, soil pH accounted for the most variables of DNRA in both the upper and deeper soil layers. Altogether, these results from a new perspective confirm that S. alterniflora invasion increases soil N pool and may further push its invasion in salt marshes, and the importance of deeper soil in nitrogen cycling cannot be ignored.