Dissimilatory nitrate reduction to ammonium (DNRA) potentially facilitates the accumulation of phosphorus in lake water from sediment

Characteristics of nitrogen and phosphorus migration at sediment-water interface in seasonal frozen lakes and the mechanism of microbial driven cycling: a case study of Lake Daihai

Nitrogen and phosphorus play pivotal roles in determining the eutrophic conditions and nutrient provision in lakes. However, the mechanisms and processes of nutrient release at the sediment-water interface of shallow lakes in cold regions remain unclear, especially under the complex environmental conditions of freezing and open-water periods. Therefore, Diffusive Gradients in Thin-films (DGT) and High-resolution Peeper technologies (HR-Peeper) were used to investigate the nitrogen and phosphorus characteristics of the sediment water interface, and the process of bacteria affecting the nitrogen and phosphorus cycle was clarified by the high-throughput sequencing technology. The results indicated that sediment phosphorus (PO43-) flux ranged from -1.39 to 3.6 mg/m2·d, with the interstitial water-Soluble Reactive PO43- presenting notable fluidity and potential bioavailability. The ammonia nitrogen (NH4+-N) flux varied from -4.71 to 3.65 mg/m2·d. The nitrate nitrogen (NO3--N) flux varied from -11.64 to 1.18 mg/m2·d, exhibiting an opposite trend to NH4+-N, which was released into water bodies during the freezing period and migrated to the sediments in the open water period. Common metabolic pathways and functional genes for nitrogen and phosphorus were identified in Methylomicrobium, Marinobacter, and Psychrobacter. The dissimilatory nitrate reduction to ammonium (DNRA) facilitated the transformation of polyphosphates and the release of phosphorus. Water temperature indirectly regulated the fluxes of nitrogen and phosphorus at the sediment-water interface (SWI) by modulating the microbial abundance and dissolved oxygen (DO) content.

Nitrogen Recovery through Dissimilatory Nitrate Reduction to Ammonium: Impact of Environmental Factors

The application of the bacterial dissimilatory nitrate reduction to ammonium (DNRA) process for treating nitrate-rich wastewater offers an environmentally friendly and resource-efficient strategy with significant potential for ammonium nitrogen recovery. This study investigates the impact of carbon sources, C/N ratios, pH, and temperature on the DNRA efficiency of Pseudomonas sp. strain LZ-1 (strain LZ-1). The results revealed that sodium citrate is the most favorable carbon source among sodium formate, sodium acetate, sodium propionate, and sodium citrate for enhancing DNRA in strain LZ-1. Ammonia production by strain LZ-1 peaks at a C/N of 8 within the range of 3 to 20, increasing before and decreasing thereafter. Furthermore, neutral to alkaline conditions (pH 7-10) are favorable for the DNRA process, with an optimal initial pH of 9. Temperature studies indicate a similar trend of initial increase followed by a decline in DNRA efficiency as temperatures rise from 20 to 35 °C, with peak ammonia production at 30 °C. The presence of sulfur ions inhibits the DNRA process in the strain LZ-1. However, this inhibitory effect diminished as the S/N ratio increased from 1/4 to 1. These insights contribute to a deeper understanding of the impact of environmental factors on DNRA and serve as a valuable reference for the utilization of strain LZ-1 in nitrogen recovery from nitrate-rich wastewaters.

Sediment grain size regulates the biogeochemical processes of nitrate in the riparian zone by influencing nutrient concentrations and microbial abundance

Riparian zones play a crucial role in reducing nitrate pollution in both terrestrial and aquatic environments. Complex deposition action and dynamic hydrological processes will change the grain size distribution of riparian sediments, affect the residence time of substances, and have a cascade effect on the biogeochemical process of nitrate nitrogen (NO3--N). However, simultaneous studies on NO3--N transformation and the potential drivers in riparian zones are still lacking, especially neglecting the effect of sediment grain size (SGS). To fill this knowledge gap, we first systematically identified and quantified NO3--N biogeochemical processes in the riparian zone by integrating molecular biotechnology, 15N stable isotope tracing, and microcosmic incubation experiments. We then evaluated the combined effects of environmental variables (including pH, dissolved organic carbon (DOC), oxidation reduction potential, SGS, etc.) on NO3--N transformation through Random Forest and Structural Equation Models. The results demonstrated that NO3--N underwent five microbial-mediated processes, with denitrification, dissimilatory nitrate reduction to ammonium (DNRA) dominated the NO3--N attenuation (69.4 % and 20.1 %, respectively), followed by anaerobic ammonia oxidation (anammox) and nitrate-dependent ferric oxidation (NDFO) (8.4 % and 2.1 %, respectively), while nitrification dominated the NO3--N production. SGS emerged as the most critical factor influencing NO3--N transformation (24.96 %, p < 0.01), followed by functional genes (nirS, nrfA) abundance, DOC, and ammonia concentrations (14.12 %, 16.40 %, 13.08 %, p < 0.01). SGS influenced NO3--N transformation by regulating microbial abundance and nutrient concentrations. RF predicted that a 5 % increase in the proportion of fine grains (diameter < 50 μm) may increase the NO3--N transformation rate by 3.8 %. This work highlights the significance of integrating machine learning and geochemical analysis for a comprehensive understanding of nitrate biogeochemical processes in riparian zones, contributing valuable references for future nitrogen management strategies.

Chronic increasing nitrogen and endogenous phosphorus release from sediment threaten to the water quality in a semi-humid region reservoir

The water quality in the drinking water reservoir directly affects people's quality of life and health. When external pollution input is effectively controlled, endogenous release is considered the main cause of water quality deterioration. As the major nitrogen (N) and phosphorus (P) sources in reservoirs, sediment plays a vital role in affecting the water quality. To understand the spatial and temporal variation of N and P in the sediment, this study analyzed the current characteristics and cumulative effects of a semi-humid reservoir, Yuqiao Reservoir, in North China. The N and P concentrations in the reservoir sediment were decreased along the flow direction, while the minimum values were recorded at the central sediment profile. External input and algal deposition were the main factors leading to higher sediment concentrations in the east (Re-E) and west (Re-W) areas of reservoir sediment profiles. According to the long-term datasets, the peaks of both sediment total nitrogen content and deposition rate were observed in the 2010s, which has increased about three times and six times than in the1990s, respectively. Therefore, the increase in phosphorus concentration may be the main reason for eutrophication in water in recent years. The mineralization of organic matter has a significant promoting effect on releasing N and P from sediments, which will intensify eutrophication in water dominated by P and bring huge challenges to water environment management. This study highlights that the current imbalance in N and P inputs into reservoirs and the endogenous P release from sediment will have a significant impact on water quality.

Nitrogen metabolism pathways and functional microorganisms in typical karst wetlands

Aha Lake artificial reservoir wetland, Niangniang Mountain karst mountain wetland, and Caohai plateau lake wetland are typical karst wetlands in Guizhou Province with unique topography and geomorphic features. They were selected as research objects in this study to explore microorganisms and functional genes in nitrogen metabolism adopting macro-genome sequencing technology. It was found that Proteobacteria, Actinobacteria, and Acidobacteria were the dominant phyla in nitrogen metabolism in these three wetlands, similar to previous studies. However, at the genus level, there was a significant difference, with the dominant bacteria being Bradyrhizobium, Methylocystis, and Anaeromyxobacter. Six nitrogen metabolism pathways, including nitrogen fixation, nitrification, denitrification, dissimilatory nitrate reduction, assimilatory nitrate reduction, and complete nitrification, comammox, were revealed, but anaerobic ammonia oxidation genes were not detected. Nitrogen metabolism microorganisms and pathways were more affected by SOM, pH, NO3-, and EC in karst wetlands. This study further discussed microorganisms and functions of nitrogen metabolism in karst wetlands, which was of great significance to nitrogen cycles of karst wetland ecosystems.

Seasonal variation regulate the endogenous phosphorus release in sediments of Shijiuhu Lake via water-level fluctuation

Freshwater lakes undergo substantial alterations of the phosphorus (P) cycle in the water-sediment ecosystem due to thermal change. The impact process of seasonal fluctuation on P cycling in sediments has been scarcely investigated. P forms in sediments from a freshwater lake in China were analyzed using sequential extraction technique. The vertical distribution of soluble reactive P (SRP), Fe2+, and S2- in the interstitial water was measured using diffusion gradient technique (DGT). Fick's Law and DIFS model were used to obtain the diffusion fluxes of SRP and the kinetic parameters in the water-sediment system. The results showed that total P (TP) concentrations in the solid sediments varied from 207.5, 266.6 and 130.3 mg/kg to 614.7, 1053.1, and 687.6 mg/kg in winter, spring, and summer, respectively. The concentrations of individual P forms in spring were higher than those in other seasons, with Fe-bound P (Fe-P) concentration being the highest across all seasons. Notably, significant variations of SRP concentrations were found in the interstitial water between sedimentary depths of approximately 2 cm and 6 cm, particularly in the summer. Furthermore, higher diffusion fluxes of SRP through the interface were found in summer. A stable anaerobic environment failed to develop in spring with high water level, preventing the desorption of solid Fe-P and diffusion of Fe2+ into the water due to the afflux and deposition of P-containing particulate into deeper sediment layers along with organic material. Under extreme high-temperature in summer, decreased rainfall and rising temperatures boosted the activity of aquatic organisms in the water, thereby reducing P fixation by sediments and leading to P release. This process increased the risk of P excess and potential eutrophication in the water. Generally, clarifying the resupplying processes of endogenous P in sediment systems experiencing seasonal variations is critical for eutrophication management of lakes.

Nitrogen removal in freshwater sediments of riparian zone: N-loss pathways and environmental controls

The riparian zone is an important location of nitrogen removal in the terrestrial and aquatic ecosystems. Many studies have focused on the nitrogen removal efficiency and one or two nitrogen removal processes in the riparian zone, and less attention has been paid to the interaction of different nitrogen transformation processes and the impact of in situ environmental conditions. The molecular biotechnology, microcosm culture experiments and 15N stable isotope tracing techniques were used in this research at the riparian zone in Weinan section of the Wei River, to reveal the nitrogen removal mechanism of riparian zone with multi-layer lithologic structure. The results showed that the nitrogen removal rate in the riparian zone was 4.14-35.19 μmol·N·kg-1·h-1. Denitrification, dissimilatory reduction to ammonium (DNRA) and anaerobic ammonium oxidation (anammox) jointly achieved the natural attenuation process of nitrogen in the riparian zone, and denitrification was the dominant process (accounting for 59.6%). High dissolved organic nitrogen and nitrate ratio (DOC:NO3-) would promote denitrification, but when the NO3- content was less than 0.06 mg/kg, DNRA would occur in preference to denitrification. Furthermore, the abundances of functional genes (norB, nirS, nrfA) and anammox bacterial 16S rRNA gene showed similar distribution patterns with the corresponding nitrogen transformation rates. Sedimentary NOX-, Fe(II), dissolved organic carbon (DOC) and the nitrogen transformation functional microbial abundance were the main factors affecting nitrogen removal in the riparian zone. Fe (II) promoted NO3- attenuation through nitrate dependent ferrous oxidation process under microbial mediation, and DOC promotes NO3- attenuation through enhancing DNRA effect. The results of this study can be used for the management of the riparian zone and the prevention and control of global nitrogen pollution.

Terrestrial sources regulate the endogenous phosphorus load in Taihu Lake, China after exogenous controls: Evidence from a representative lake watershed

Identifying phosphorus (P) sources and contributions from terrestrial sources is important for clean water and eutrophication management in lake watersheds. However, this remains challenging owing to the high complexity of P transport processes. The concentrations of different P fractions in the soils and sediments from Taihu Lake, a representative freshwater lake watershed, were obtained using sequential extraction procedure. The dissolved phosphate (PO₄-P) and alkaline phosphatase activity (APA) in the lake's water were also surveyed. The results showed that different P pools in the soil and sediments displayed different ranges. Higher concentrations of P fractions were measured in the solid soils and sediments from the northern and western regions of the lake watershed, indicating a larger input of P from exogenous sources, including agriculture runoff and industrial effluent from the river. Generally, higher Fe-P and Ca-P concentrations of up to 399.5 and 481.4 mg/kg were detected in soils and lake sediments, respectively. Similarly, the lake's water had higher concentrations of PO₄-P and APA in the northern region. A significant positive correlation was found between Fe-P in the soil and PO₄-P concentrations in the water. Statistical analysis indicated that appropriately 68.75% P was retained in the sediment from terrigenous sources, and 31.25% P experienced dissolution and shifted to the solution phase in the water-sediment ecosystems. The dissolution and release in Fe-P in the soils were responsible for the increase of Ca-P in the sediment after the afflux of soils into the lake. These findings suggest that soil runoff predominantly controls P occurrence in lake sediments as an exogenous source. Generally, the strategy of reducing terrestrial inputs from agricultural soil discharge is still an important step in P management at the catchment scale of lakes.

In situ remediation mechanism of internal nitrogen and phosphorus regeneration and release in shallow eutrophic lakes by combining multiple remediation techniques

Large anthropogenic inputs of N and P alter the nutrient cycle and exacerbate global eutrophication problems in aquatic ecosystems. This study in Lake Datong, China, investigates the remediation mechanism of multiple remediation technique combinations (dredging, adsorbent amendment, and planting aquatic vegetation) on sediment N and P loads based on two high-resolution sampling techniques (HR-Peeper and DGT) and P sequential extraction procedures. The results showed that high temperature and low dissolved oxygen considerably enhanced pore water dissolved reactive P (DRP) and NH₄⁺ concentrations attributable to abundant Fe-P and organic matter content in the sediment. Fe reduction is critical for regulating pore water DRP release and promoting N removal. Overall, for Lake Datong, combining multiple remediation techniques is more effective in controlling sediment P loads (pore water DRP, P fluxes, forms of P, and labile P), from a long-term perspective, than a single remediation. Lanthanum-modified bentonite (LMB) inactivation treatment can transfer mobile P in the surface sediment into more refractory forms over time, thereby reducing the risk of sediment labile P release. However, it is difficult to effectively remediate internal P loads owing to inappropriate dredging depths and low biomass of aquatic vegetation. Future lake restoration practices should optimize the selection of different remediation technique combinations based on internal N and P pollution characteristics, while reducing external wastewater input. These results are important for understanding the remediation mechanisms of internal N and P and provide suggestions for sediment management of shallow eutrophic lakes.