Nitrogen Fixation Rates Research Articles

Nitrogen fixation of municipal sewage sludge by co-pyrolysis with biomass and KH2PO4-modified biochar addition

In order to reduce nitrogen emissions during municipal sewage sludge (MSS) pyrolysis, the effects of co-pyrolysis of MSS with biomass and the second adsorption of KH2PO4-modified biochar were investigated. It was found that co-pyrolysis with cellulose inhibited the conversion of nitrogen into pyrolytic gas, and increased the N content in pyrolytic biochar. And the nitrogen fixation rate increased with cellulose addition increasing. Besides, compared with different biomass, MSS co-pyrolysis with 30% sugarcane bagasse had the optimal effect on nitrogen fixation (39%). Moreover, KH2PO4-modified biochar addition increased the second adsorption of nitrogenous volatiles and fixed nitrogen into oxides-N. The total nitrogen fixation rate reached maximum (47.65%) at 5% KH2PO4 impregnation. Thus, it is an effective nitrogen fixation method that MSS co-pyrolysis of 30% sugarcane bagasse with the second absorption of 5% KH2PO4-modified biochar. This study provided an effective solution to improve the treatment of municipal sewage sludge by pyrolysis technology.

Geographic distribution of micronektonic fish communities in the subtropical North Pacific: The effect of primary productivity and nitrogen fixation

We investigated the geographic variations of micronektonic fish communities along the transects in the subtropical North Pacific (23° N, 138° E–120° W) and California Current (30–45° N, 123–125° W) during August–October 2017 in relation to environmental factors, including primary productivity and nitrogen fixation. Throughout the cruise, 102 species across 31 families were collected at a depth of 0–500 m at night. Clustering analysis indicated that the micronektonic fish were categorized into three communities along the east–west subtropical transect, in addition to the subarctic-transitional water community. Species in the subtropical communities were composed of indicator species, which were characteristically found in their respective community, in addition to the common species (Vinciguerria nimbaria, Notolychnus valdiviae and Bolinichthys longipes), which were regularly found in all communities. Redundancy analysis indicated that the distribution of indicator species in western (Benthosema suborbitale and Lampanyctus alatus) and central (Cyclothone alba and Hygophum proximum) subtropical communities were positively associated with chlorophyll a concentration and nitrogen fixation rate, respectively. Regarding the eastern community indicator species (Ceratoscopelus townsendi, Cyclothone acclinidens, and Diogenichthys laternatus), various factors such as chlorophyll a concentration, dissolved oxygen, temperature and salinity were negatively related to their specific occurrences. In addition, specifically abundant occurrence of V. nimbaria was observed at a station where the thermocline became shallower and the chlorophyll a concentration was highest. Our results suggest that the productivity of the lower trophic level affects the geographic distribution of micronektonic fish in the subtropical North Pacific in addition to physicochemical parameters.

Presence and activity of nitrogen-fixing bacteria in Scots pine needles in a boreal forest: a nitrogen-addition experiment.

Endophytic nitrogen-fixing bacteria have been detected and isolated from the needles of conifer trees growing in North American boreal forests. Because boreal forests are nutrient-limited, these bacteria could provide an important source of nitrogen for tree species. This study aimed to determine their presence and activity in a Scandinavian boreal forest, using immunodetection of nitrogenase enzyme subunits and acetylene-reduction assays of native Scots pine (Pinus sylvestris L.) needles. The presence and rate of nitrogen fixation by endophytic bacteria were compared between control plots and fertilized plots in a nitrogen-addition experiment. In contrast to the expectation that nitrogen-fixation rates would decline in fertilized plots, as seen, for instance, with nitrogen-fixing bacteria associated with bryophytes, there was no difference in the presence or activity of nitrogen-fixing bacteria between the two treatments. The extrapolated calculated rate of nitrogen fixation relevant for the forest stand was 20gNha-1year-1, which is rather low compared with Scots pine annual nitrogen use but could be important for the nitrogen-poor forest in the long term. In addition, of 13 colonies of potential nitrogen-fixing bacteria isolated from the needles on nitrogen-free media, 10 showed in vitro nitrogen fixation. In summary, 16S rRNA sequencing identified the species as belonging to the genera Bacillus, Variovorax, Novosphingobium, Sphingomonas, Microbacterium and Priestia, which was confirmed by Illumina whole-genome sequencing. Our results confirm the presence of endophytic nitrogen-fixing bacteria in Scots pine needles and suggest that they could be important for the long-term nitrogen budget of the Scandinavian boreal forest.

Reduction in the Use of Some Herbicides Favors Nitrogen Fixation Efficiency in Phaseolus vulgaris and Medicago sativa.

In recent decades, the quality of agricultural soils has been seriously affected by the excessive application of pesticides, with herbicides being one of the most abundant. Continuous use of herbicides alters the soil microbial community and beneficial interactions between plants and bacteria such as legume-rhizobia spp. symbiosis, causing a decrease in the biological nitrogen fixation, which is essential for soil fertility. Therefore, the aim of this work was to study the effect of two commonly used herbicides (pendimethalin and clethodim) on the legume-rhizobia spp. symbiosis to improve the effectiveness of this process. Phaseolus vulgaris plants grown in pots with a mixture of soil:perlite (3:1 v/v), showed a 44% inhibition of nitrogen fixation rate with pendimethalin. However, clethodim, specifically used against monocots, did not induce significant differences. Additionally, we analyzed the effect of herbicides on root exudate composition, detecting alterations that might be interfering with the symbiosis establishment. In order to assess the effect of the herbicides at the early nodulation steps, nodulation kinetics in Medicago sativa plants inoculated with Sinorhizobium meliloti were performed. Clethodim caused a 30% reduction in nodulation while pendimethalin totally inhibited nodulation, producing a reduction in bacterial growth and motility as well. In conclusion, pendimethalin and clethodim application reduced the capacity of Phaseolus vulgaris and Medicago sativa to fix nitrogen by inhibiting root growth and modifying root exudate composition as well as bacterial fitness. Thus, a reduction in the use of these herbicides in these crops should be addressed to favor a state of natural fertilization of the soil through greater efficiency of leguminous crops.

Construction of CuS/ZnO Z-scheme heterojunction as highly efficient piezocatalyst for degradation of organic pollutant and promoting N2 fixation properties

Piezocatalytic technology has great potential in addressing water-system pollution and countering energy crises issues. Herein, high-performance CuS/ZnO Z-scheme heterojunction piezocatalyst was prepared by environmentally friendly solid-state chemistry approach and explored piezocatalytic performances toward degradation of organic methylene blue (MB) dye pollutant and nitrogen (N2) fixation activity under ultrasonic vibration. The CuS/ZnO piezocatalyst presents outstanding property in MB degradation process with high efficiency (94.7% in 40 min), high rate constant (0.06804 min−1) and good recyclability stability in comparison with the numerous ZnO-based piezocatalysts. In addition, this catalyst also exhibits superior piezocatalytic activity with a production rate of nitrogen fixation rate of 77.5 μmol L−1 gcat−1 h−1 in the reduction of N2 to ammonia process, which is approximately 4-folds higher than that of pristine ZnO. Such improvement was mainly attributed to the facilitating charge carriers separation via rational construction of Z-scheme heterojunction as well as enhanced redox capacity. A novel piezocatalytic Z-scheme heterojunction mechanism of CuS/ZnO has been proposed and elucidated. This work suggests that designing highly efficient CuS/ZnO piezocatalyst will be a promising candidate material for prospects application in coping with the environmental remediation pollutants and energy crisis problems.

Prediction of char yield and nitrogen fixation rate from pyrolysis of sewage sludge based on machine learning

Based on sewage sludge composition and pyrolysis processing conditions, machine learning was employed as a tool to predict the nitrogen fixation rate and char yield of sewage sludge pyrolysis. Multiple linear regression, decision tree, support vector machine, random forest, and gradient boosting tree methods were used in the research to model the collected data, and the model results were further discussed. The required data set is sorted by the existing articles, and the missing values are filled by k-Nearest Neighbors method. Predicting the char yield and nitrogen fixation rate by considering the ultimate and proximate composition of the sewage sludge can give good results. The results showed the gradient boosting tree method is the most accurate in predicting nitrogen fixation rate and char yield, with the coefficient of determination for char yield and nitrogen fixation rate reaching 0.864 and 0.860, respectively. Fusion of existing models by voting or stacking methods revealed promising further improvement in prediction accuracy, but it was limited. In further analysis, using the mean decrease impurity and SHapely Additive exPlanations methods to more accurately evaluate the feature importance. For example, HTT was the most important feature affecting char yield and nitrogen fixation rate, especially when predicting the nitrogen fixation rate, with a feature importance of 0.625. This study also quantified the effects of different features on char yield and nitrogen fixation rate using partial dependence and individual condition expectation plots to provide a reference for the utilization and research of sewage sludge pyrolysis.

Defect-induced charge redistribution of MoO3-x nanometric wires for photocatalytic ammonia synthesis

Photocatalytic ammonia synthesis technology has become one of the effective methods to replace the Haber method for nitrogen fixation in the future for its low energy consumption and green environment. However, limited by the weak adsorption/activation ability of N2 molecules at the photocatalyst interface, the efficient nitrogen fixation still remains a daunting job. Defect-induced charge redistribution as a catalytic site for N2 molecules is the most prominent strategy to enhance the adsorption/activation of N2 molecules at the interface of catalysts. In this study, MoO3-x nanowires containing asymmetric defects were prepared by a one-step hydrothermal method via using glycine as a defect inducer. It is shown that at the atomic scale, the defect-induced charge reconfiguration can significantly improve the nitrogen adsorption and activation capacity and enhance the nitrogen fixation capacity; at the nanoscale, the charge redistribution induced by asymmetric defects effectively improved the photogenerated charge separation. Given the charge redistribution on the atomic and nanoscale of MoO3-x nanowires, the optimal nitrogen fixation rate of MoO3-x reached 200.35 µmol g−1h−1.

Control factors of soil diazotrophic community assembly and nitrogen fixation rate across eastern China

Biological nitrogen (N) fixation is an essential N input into terrestrial ecosystems and plays a critical role in global N cycling. However, the driving factors of the diazotrophic community composition and soil N fixation rate at the regional scale remain largely unclear, especially in different land-use types. Here, we explored the environmental adaptation of diazotrophic communities and controlling factors of N fixation in maize, rice, and forest soils across eastern China. Results showed that the diversity, composition, and co-occurrence network differed significantly among these habitats, with a relatively higher alpha diversity and network complexity in rice fields. The mean annual precipitation plays a predominant role in shaping diazotrophic community composition. Dispersal limitation, belonging to stochastic processes, contributed most to diazotrophic community assembly in terrestrial ecosystems. Habitat niche breadth analysis indicated that the environmental adaptation of diazotrophic communities was highest in rice fields, followed by forest and maize fields. The overall N fixation rate was higher in rice fields, followed by maize and forest soils. Structural equation modeling revealed that the N fixation rate was mainly affected by soil properties in maize and forest soils. In contrast, the N fixation rate was more influenced by diazotrophic community composition and climatic factors in rice habitats. Our results revealed that the environmental adaptation of the diazotrophic communities and controlling factors of the N fixation function were different among maize, rice, and forest ecosystems. These findings will help us understand and predict the N bioavailability in different ecosystem types under climate changes.

Electrodriven Chemical Looping Ammonia Synthesis Mediated by Lithium Imide

The chemical looping process for ammonia synthesis (CLAS) has advantages in avoiding competitive adsorptions of nitrogen and hydrogen and optimizing the thermodynamics and kinetics of a reaction step by step. Nitrogen carriers with favorable thermodynamic and kinetic properties are critical for such a process, especially under mild conditions. Recent studies have shown that alkali- and alkaline-earth-metal imides are attractive nitrogen carriers; however, the equilibrium concentration of NH3 in the hydrogenation of imides to ammonia and hydrides is low at atmospheric pressure. Moreover, the reaction rate of nitrogen fixation on the hydrides is slow. Here, we show that an electrodriven CLAS (ECLAS) mediated by a lithium imide is feasible in a LiCl–NaCl–KCl eutectic electrolytic cell with a nickel-foam electrode. We demonstrate that the reactions of lithium imide hydrogenation and lithium hydride nitridation are both promoted by the input of electric energy. The average ammonia production rate over such an electrodriven CLAS is ca. 8 times higher than that of a thermal-driven CLAS.

Nitrogen fixation and nifH gene diversity in cyanobacteria living on feather mosses in a subalpine forest of Mt. Fuji

In the boreal forests, feather mosses such as Hylocomium splendens and Pleurozium schreberi are colonized by cyanobacteria, which provide large amounts of nitrogen to forest ecosystems through nitrogen fixation. Although these feather mosses are also ubiquitous in subalpine forests of East Asia, little is known regarding their associated cyanobacteria and their ability to fix nitrogen. In this study, we investigated (1) whether cyanobacteria co-exist and fix nitrogen in the two species of feather mosses that cover the ground surface in a subalpine forest of Mt. Fuji, (2) whether cyanobacteria belonging to a common cluster with boreal forests are found in feather mosses in Mt. Fuji, and (3) whether moss-associated nitrogen fixation rates differed among moss growing substrates, canopy openness, and moss nitrogen concentrations in the same forest area. Our results showed that cyanobacteria colonized feather mosses in the subalpine forests of Mt. Fuji and acetylene reduction rates as an index of nitrogen fixation tended to be higher in H. splendens than in P. schreberi. Based on analysis of the nifH gene, 43 bacterial operational taxonomic units (OTUs) were identified, 28 of which represented cyanobacteria. Among the five clusters of cyanobacteria classified based on their nifH gene and identified in northern Europe, four (Nostoc cluster I, Nostoc cluster II, Stigonema cluster, and nifH2 cluster) were also found at Mt. Fuji. The acetylene reduction rate differed depending on the moss growing substrate and the total nitrogen concentration of moss shoots, and a strong negative correlation was observed with the total nitrogen concentration.

Anthracnose Disease and Physiological Aspects of Soybean in Saturated Soil Culture in Tidal Land

Saturated soil culture in tidal land could be an alternative measure for achieving sufficient soybean production to meet demand and reduce the current 56% dependence on imports. Therefore, this study aims to investigate disease development and the nitrogen fixation rate of soybean in water-saturated cultivation on tidal land. It was conducted in Simpang Village, Berbak District, East Tanjung Jabung, Jambi Province, Indonesia, and subjected to a randomized block design as an environmental design. The treatment consisted of 3 water inundation levels in the trench, namely, 15, 20, and 25 cm from the soil surface. The results showed that the height of standing water in a water-saturated cultivation system affected the soil moisture. Furthermore, high soil moisture results in a high anthracnose disease in soybean caused by the fungus Colletotrichumdematinum var. truncatum. The disease attack rate reached 15.7% with 72% soil moisture at a depth of 15 cm below the soil surface. There was also an attack rate of 16.3%, with soil moisture of 70.5% at a depth of 20 cm from the soil surface. The highest percentage of fallen pods in the treatment plots was up to 15cm and 20 cm in standing water, which was 17.3% and 16.7%, respectively. The root nodules' weight decreased in height by 25 cm from the soil surface. Furthermore, the rate of nitrogen fixation decreased, as indicated by a reduction in the activity of the root nodules' acetylene, which decreased at the height of the waterlogging 25 cm from the soil surface. Soybean crop yield was affected by the height of standing water in the trench. The numbers of filled pods, the weight of dry seeds per plant, and the weight of 100 seeds were lower at the height of water 25 cm from the soil surface. Additionally, disease severity was strongly correlated with root nodules and seed/plant dry weight. Increased disease severity significantly contributed to the decline in dry weight of root nodules, very strongly to dry seed/plant weight and moderately to acetylene-reducing activity.

Cyanobacteria and Algae in Biological Soil Crusts of Frost Boils in the Mountain Tundra of the Urals

Diversity of cyanobacteria and eukaryotic algae was recovered from biological soil crusts formed on bare spots in tundras of the Ural mountains. The research was carried out in two typical tundra types, dominated by shrub-moss-lichen and dwarf birch-lichen-moss communities. In total, 46 species belonging to five divisions were identified: Cyanobacteria (19), Ochrophyta (1), Bacillariophyta (2), Chlorophyta (22), Charophyta (2). Core species of the biological crusts come from Cyanobacteria (Stigonema minutum, Gloeocapsopsis magma, Schizothrix fuscescens, Dasygloea cf. lamyi, Fischerella muscicola, Nostoc commune, Scytonema hofmannii) and Chlorophyta (Sporotetras polydermatica, Coccomyxa simplex, Elliptochloris bilobata, E. subsphaerica, Lobochlamys culleus, Pleurastrum terricola). The low diversity of algae and cyanobacteria results from harsh environmental conditions of their habitat: extreme fluctuation of upper ground temperatures, acidic soils with low base content, and reduced levels of nitrogen, phosphorus and other crucial biogenic elements. The total number of cyanobacterial and algal cells per g of soil was estimated at 0.03 to 34.19 million. Based on the acetylene reduction method, the average nitrogen fixation rates in biological crusts ranged from 0.009 to 0.015 mg С2Н4 m–2h–1. The activity varied between soil crust types from 1.48 to 2.25 mg С2Н4 m–2h–1. In future, regular observations are planned on the studied sites to reconstruct and predict succession processes in cryogenic landscapes under mountain tundra conditions.

Functional significance of microbial diversity in arid soils: biological soil crusts and nitrogen fixation as a model system.

Microbial communities respond to changes in environmental conditions; however, how compositional shifts affect ecosystem processes is still not well-understood and it is often assumed that different microbial communities will function equally under similar environmental conditions. We evaluated this assumption of functional redundancy using biological soil crusts (BSCs) from two arid ecosystems in Mexico with contrasting climate conditions (hot and cold deserts) following an experimental approach both in the field (reciprocal transplants) and in laboratory conditions (common garden), focusing on the community's composition and potential for nitrogen fixation. Potential of nitrogen fixation was assessed through the acetylene reduction assay. Community composition and diversity was determined with T-RFLPs of nifH gene, high throughput sequencing of 16S rRNA gene amplicons and metagenomic libraries. BSCs tended to show higher potential nitrogen fixation rates when experiencing temperatures more similar to their native environment. Moreover, changes in potential nitrogen fixation, taxonomic and functional community composition, and diversity often depended on an interactive effect of origin of the communities and the environment they experienced. We interpret our results as legacy effects that result from ecological specialization of the BSC communities to their native environment. Overall, we present evidence of nonfunctional redundancy of BSCs in terms of nitrogen fixation.

Construction of Co3O4 nanopolyhedra with rich oxygen vacancies from ZIF-67 for efficient photocatalytic nitrogen fixation.

Photocatalytic nitrogen fixation has attracted much attention due to the fact that it is a way of using solar energy to achieve clean and sustainable conversion of nitrogen to ammonia under mild conditions. In this paper, different proportions of Zn-doped Co3O4 nanopolyhedrons were synthesized using bimetallic ZIFs containing Co2+ and Zn2+ as precursors for the construction of photocatalytic nitrogen fixation semiconductor materials for the first time. The synthesized Co3O4 nano-polyhedron still retains the rhombic dodecahedron shape of ZIF-67 and exhibits a large specific surface area. Moreover, Zn doping results in abundant oxygen vacancies on the surface of Co3O4 polyhedron. These oxygen vacancies not only provide active sites for nitrogen adsorption and activation, but also enhance the separation ability of photocarriers, which can significantly improve the efficiency of photocatalytic nitrogen fixation of the material. When Zn-Co3O4-30 is utilized as the catalyst for photocatalytic nitrogen fixation, the nitrogen fixation rate is 96.8μmolg-1h-1, which is much higher than that of pure-Co3O4. In this study, heteroatom-doped Co3O4 polyhedron with rich oxygen vacancy was synthesized by low-temperature oxidation method, which provides a new idea for the design and synthesis of skeleton-based photocatalytic nitrogen fixation materials.

Seasonal Hypoxia Enhances Benthic Nitrogen Fixation and Shapes Specific Diazotrophic Community in the Eutrophic Marine Ranch

Recently, a growing number of studies have confirmed that biological nitrogen fixation is also an important reactive nitrogen source in coastal regions. However, how benthic nitrogen fixation and diazotrophic community in coastal regions respond to seasonal hypoxia remains largely unknown. In this study, we investigated the spatiotemporal pattern of potential nitrogen fixation rate and diazotrophic abundance and community in sediments of a eutrophic marine ranch experiencing summer hypoxia using 15N tracing and high throughput sequencing techniques. The results showed that potential nitrogen fixation rates ranged from 0.013 to 10.199 μmol kg−1 h−1, and were significantly enhanced by summer hypoxia (ANOVA, p < 0.05). However, nifH gene abundance peaked in June. The diazotrophic community was dominated by Geobacteraceae (>60%), followed by Desulfobulbaceae (13.61%). Bottom water oxygen, pH, Chl-a concentration, and sediment NH4+ significantly regulated benthic nitrogen fixation, while the variation of diazotrophic community was explained by sediment TOC, TN, and Fe content (p < 0.05). This study highlighted that hypoxia stimulated benthic nitrogen fixation, which counteracted the nitrogen removal by denitrification and anammox, and could further aggregate eutrophication of the coastal marine ranch. Moreover, the result emphasized the importance of nitrogen fixation in coastal regions for the global N budget.

Comparative assessment of nitrogen fixation rate by 15N2 tracer assays in the South China Sea

Comparative assessment of nitrogen fixation rate by 15N2 tracer assays in the South China Sea

Phosphoenolpyruvate reallocation links nitrogen fixation rates to root nodule energy state.

Legume-rhizobium symbiosis in root nodules fixes nitrogen to satisfy the plant's nitrogen demands. The nodules' demand for energy is thought to determine nitrogen fixation rates. How this energy state is sensed to modulate nitrogen fixation is unknown. Here, we identified two soybean (Glycine max) cystathionine β-synthase domain-containing proteins, nodule AMP sensor 1 (GmNAS1) and NAS1-associated protein 1 (GmNAP1). In the high-nodule energy state, GmNAS1 and GmNAP1 form homodimers that interact with the nuclear factor-Y C (NF-YC) subunit (GmNFYC10a) on mitochondria and reduce its nuclear accumulation. Less nuclear GmNFYC10a leads to lower expression of glycolytic genes involved in pyruvate production, which modulates phosphoenolpyruvate allocation to favor nitrogen fixation. Insight into these pathways may help in the design of leguminous crops that have improved carbon use, nitrogen fixation, and growth.

Nitrogen-fixing sulfate reducing bacteria in shallow coastal sediments under simulated resuspension

Nitrogen (N2) fixation by heterotrophic non-cyanobacterial diazotrophs is common in marine deep-sea sediments. However, in shallow coastal areas, where resuspension of sediments is extensive, the magnitude of sediment-associated N2 fixation during resuspension is unknown. We examined nitrogen fixation in dark slurry incubations with sediments (0–5 cm and 0–10 cm depths) under simulated resuspension from five shallow stations (water depth <1 m) and an anoxic site in the Bay of Gdansk (109 m) in the Baltic Sea. Abiotic variables and the composition of nitrogen fixing organisms (diazotrophs) were measured at the study sites. To estimate the contribution of nitrogen fixing sulfate reducing bacteria to total nitrogen fixation, parallel incubations with sodium molybdate as inhibitor were performed. Our data show low but variable nitrogen fixation rates (n.d. - 23.7 nmol N g−1 d−1), promoted in small-grained sediments associated with increased organic carbon content and high nutrient concentrations in pore waters. Highest nitrogen fixation at the shallow sites was encountered in the upper 0–5 cm of the sediments while rates were negligible below. Sulfate reducing bacteria (e.g. Desulfobacterales and Desulfovibrionales) were responsible for most of the heterotrophic nitrogen fixation and appear as key players for pelagic N2 fixation during resuspension. Our study reveals an important sediment – water coupling, which may be accentuated by the increased storms and resuspension events predicted for the Baltic Sea region.

Facile preparation of perylene imide based supramolecular structure with strong internal electric-field for enhancing photocatalytic nitrogen fixation

Rational regulating supramolecular structure with built-in electric field is a promising tactic to improve the separation of charge carriers and photocatalytic nitrogen fixation efficiency. However, those synthetic procedures are complicated, time-consuming, and low-scale preparation. Here, we reported a simple tactic to prepare perylene diimide (PDI)-triazine-based supramolecular using the solution-dispersed supramolecular self-assembly method. The PDI-triazine-based supramolecular obtains an outstanding nitrogen fixation rate and near IR yield in pure water, which was about 10 times higher than that of pristine PDI. The DFT calculation and experiment reveal that embeds triazine units reasonably regulated the HOMO and LOMO band and the absorption properties, which guaranteed the PDIMA enable to absorb the near IR light and activate the N2 molecule. Moreover, the adjustable face-to-face arrangement of π–π stacking and intrinsic molecular dipole is conducive to the construction of built-in electric field, resulting in high carrier separation efficiency. This study provides a facile strategy to incorporate foreign groups for enhancing photocatalytic efficiency over organic-based supramolecular.

The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

Abstract. The availability of iron (Fe) and phosphorus (P) has been shown to be a key factor regulating rates of nitrogen fixation in the western subtropical Pacific. However, the relative importance of Fe and P at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by (i) carrying out measurements of finer-scale spatial variabilities in N2 fixation rates and diazotroph nifH gene abundances throughout these regions and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the nifH gene abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates and nifH gene abundances were lower in the NSCS than around the Luzon Strait and the western North Pacific. The nutrient addition bioassay experiments demonstrated that N2 fixation rates in the central NSCS were co-limited by Fe and P, whereas at the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in N2 fixation rates, and in six out of eight experiments the largest responses in nifH gene abundances were dominated by either Trichodesmium or UCYN-B (unicellular diazotrophic cyanobacteria group B). In general, nutrient addition had a relatively restricted impact on the composition of the six phylotypes that we surveyed apart from on UCYN-B. This unicellular cyanobacterium group showed increased contribution to the total nifH gene abundance following P addition at sites where N2 fixation rates were P-limited. Our study provides comprehensive evidence of nutrient controls on N2 fixation biogeography in the margin of the western North Pacific. Future research that more accurately constrains nutrient supply rates to this region would be beneficial for resolving what controls diazotroph community structure.