N2 Fixation Rates Research Articles

Heterogeneity of nitrogen fixation in the mesopelagic zone of the South China Sea

Biological dinitrogen (N2) fixation, the energetically expensive conversion of N2 gas to ammonia, plays an essential role in balancing the nitrogen budget in the ocean. Accumulating studies show detectable N2 fixation rates below the euphotic zone in various marine systems, revealing new insights of marine N2 fixation. However, the reported rates are highly variable and frequently fall close to detection limits, raising the question of the ubiquity and significance of N2 fixation in the global dark ocean. Using highly sensitive isotopic labeling incubation including a set of control incubations, we confirm the occurrence of mesopelagic N2 fixation in the South China Sea. Interestingly, we consistently observed that ca. 30% of samples show a significant elevation of 15N in the particulate nitrogen after incubation at most depths (200 - 1000 m). Although this approach does not allow accurate quantification of N2 fixation rates, our data suggest the occurrence of dark N2 fixation yet with highly heterogeneous signals in the mesopelagic zone of the South China Sea. A data compilation of reported N2 fixation in the global dark ocean further reveals that such heterogeneity has also been observed elsewhere, unveiling the ubiquitous heterogeneity in mesopelagic N2 fixation. Thus, we call for more observations to constrain mesopelagic N2 fixation budgets and to understand the underlying mechanism for such heterogeneity.

Nitrogen fixation in the North Atlantic supported by Gulf Stream eddy-borne diazotrophs

Mesoscale oceanic eddies contribute to the redistribution of resources needed for plankton to thrive. However, due to their fluid-trapping capacity, they can also isolate plankton communities, subjecting them to rapidly changing environmental conditions. Diazotrophs, which fix dinitrogen (N2), are key members of the plankton community, providing reactive nitrogen, particularly in large nutrient-depleted regions such as subtropical gyres. However, there is still limited knowledge about how mesoscale structures characterized by specific local environmental conditions can affect the distribution and metabolic response of diazotrophs when compared with the large-scale dynamics of an oceanic region. Here we investigated genetic diazotroph diversity and N2 fixation rates in a transect across the Gulf Stream and two associated eddies, a region with intense mesoscale activity known for its important role in nutrient transport into the North Atlantic Gyre. We show that eddy edges are hotspots for diazotroph activity with potential community connectivity between eddies. Using a long-term mesoscale eddy database, we quantified N2 fixation rates as up to 17 times higher within eddies than in ambient waters, overall providing ~21 µmol N m−2 yr−1 to the region. Our results indicate that mesoscale eddies are hotspots of reactive nitrogen production within the broader marine nitrogen cycle.

Substrate Competition of Diazotrophic Nitrous Oxide Assimilation Over Dinitrogen Fixation

AbstractNitrous oxide (N2O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N2O assimilation to biomass represents a novel biological N2O consumption pathway in addition to canonical denitrification. Thermodynamically, N2O assimilation is more favorable than dinitrogen (N2) fixation in natural environments, especially under higher N2O concentration and cooler conditions. Via isotopic tracing experiments, N2O assimilation was detected on cultured diazotrophs Crocosphaera and Trichodesmium with specific rates from 1.27 ± 0.16 × 10−4 to 2.00 ± 0.25 × 10−4 hr−1 under elevated [N2O]/[N2] conditions (0.0005–0.01) within 24‐hr incubation. The rates of N2O assimilation during the light and dark periods were statistically insignificant compared with N2 fixation activity. In a eutrophic estuary, N2O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N2O assimilation and N2 fixation in varying substrate concentrations. The low [N2O]/[N2] ratio in natural conditions leads to N2O assimilation rate being <0.1% of N2 fixation rate, rendering negligible impact of N2O assimilation. The model was also used to predict the time required for experimental detection of N2O assimilation in isotopic tracing experiments under varying [N2O]/[N2] ratios. This study enhances the mechanistic understanding of N2O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N2O sink and nitrogen source for production.

Investigating drivers of free-living diazotroph activity in paddy soils across China

Microbially mediated N fixation is widespread in rice paddy ecosystems and crucial in maintaining soil fertility. However, our understanding of the factors determining the distribution of free-living diazotrophic microorganisms that perform this process in paddy fields is limited. This study investigated the spatial distribution and factors influencing presence and potential activity of free-living microorganisms capable of N2 fixation in addition to dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonium oxidation (anammox), and denitrification in 50 paddy soils across China. Using 15N isotope tracing in laboratory incubations and microbial community analysis via metagenomics, we demonstrate that paddy soils may represent a previously underappreciated hotspot for N2 fixation with mean potential rates of 24.4 ± 17.8 nmol N g−1 h−1, 10-fold higher than DNRA (2.55 ± 0.4 nmol N g−1 h−1), and could counterbalance a portion of N2 losses through anammox and denitrification (9.24 ± 1.1 nmol N g−1 h−1). Site longitude and organic carbon (C) concentrations, as well as the diazotrophic community composition, were the dominant abiotic and biotic factors accounting for regional variations in potential N2 fixation rates. The N2 metabolic pathways predicted from the metagenome-assembled genomes (MAGs) revealed significant co-occurrence of the diazotroph marker gene nifH with denitrification-associated genes (nirS/K and nosZ) and organic C oxidation-related genes (yiaY and galM). Furthermore, enzymes involved in organic C oxidation, particularly glycoside hydrolases and glycosyltransferases, were not only phenotypically correlated with free-living N2 fixation rates but were also identified in nifH-containing MAGs, indicating the heterotrophic capabilities of diazotrophs in paddy soils. Collectively, our results underscore the substantial contribution of free-living N2 fixation to soil N fertility in paddy fields, and highlight the importance of coupling organic C oxidation with nitrate reduction to enhance N2 fixation.

Hyperspectral Imaging Predicts Differences in Carbon and Nitrogen Status Among Representative Biocrust Functional Groups of the Colorado Plateau

AbstractBiological soil crusts (biocrusts) are widespread soil photosynthetic communities covering about 12% of Earth's land surface, and play crucial roles in terrestrial carbon (C) and nitrogen (N) cycles, yet scalable quantifications of biocrusts and their biogeochemical contributions are notably lacking. While remote sensing has enormous potential to assess, scale, and contextualize biocrusts and their functions, the applicability of hyperspectral data in predicting C‐ and N‐related biocrust traits remains largely unexplored. We address this issue by evaluating the potential of in situ hyperspectral data to predict C and N across a range of biocrust species and different environmental conditions. We found that in situ hyperspectral reflectance measurements can be used to predict biocrust tissue C/N ratios and N concentrations with relatively high accuracy but to a lesser extent for potential biocrust N2 fixation rates. Critical wavelength domains included the visible region of the spectrum from roughly 490–600 nm, which most effectively captured variations in biocrust tissue C, and the shortwave infrared region from 1,150 to 1,350 nm and 1,550–1,650 nm, which most effectively captured biocrust tissue N and N2 fixation potential. Finally, we provide evidence that multi‐ and hyperspectral missions with targeted band placement, such as the proposed 26‐band Landsat Next, could be effective in predicting biocrust traits. This work provides a critical step in understanding how to apply data from new and upcoming satellite missions to the monitoring of biocrusts.

Heteropoly blue-modified ultrathin bismuth oxychloride nanosheets with oxygen vacancies for efficient photocatalytic nitrogen fixation in pure water

Photocatalytic nitrogen reduction is a promising green technology for ammonia synthesis under mild conditions. However, the poor charge transfer efficiency and weak N2 adsorption/activation capability severely hamper the ammonia production efficiency. In this work, heteropoly blue (r-PW12) nanoparticles are loaded on the surface of ultrathin bismuth oxychloride nanosheets with oxygen vacancies (BiOCl-OVs) by electrostatic self-assembly method, and a series of xr-PW12/BiOCl-OVs heterojunction composites have been prepared. Acting as a robust support, ultrathin two-dimensional (2D) structure of BiOCl-OVs inhibits the aggregation of r-PW12 nanoparticles, enhancing the interfacial contact between r-PW12 and BiOCl. More importantly, the existence of oxygen vacancies (OVs) provides abundant active sites for efficient N2 adsorption and activation. In combination of the enhanced light absorption and promoted photogenerated carriers separation of xr-PW12/BiOCl-OVs heterojunction, under simulated solar light, the optimal 7r-PW12/BiOCl-OVs exhibits an excellent photocatalytic N2 fixation rate of 33.53 µmol g-1h−1 in pure water, without the need of sacrificial agents and co-catalysts. The reaction dynamics is also monitored by in situ FT-IR spectroscopy, and an associative distal pathway is identified. Our study demonstrates that construction of heteropoly blues-based heterojunction is a promising strategy for developing high-performance N2 reduction photocatalysts. It is anticipated that combining of different defects with heteropoly blues of different structures might provide more possibilities for designing highly efficient photocatalysis systems.

Nitrogen fixation rate and phosphorus enrichment effects on diazotrophic cyanobacteria in the Gulf of Riga

In eutrophied marine systems such as the Baltic Sea, diazotrophic cyanobacteria have the potential to add additional bioavailable nitrogen (N) to the system through fixation of atmospheric dinitrogen (N2). However, their growth is regarded to be limited by phosphorus availability (P). This study investigates the response of two cyanobacteria species, Aphanizomenon flosaquae and Nodularia spumigena, collected from the Gulf of Riga under different environmental conditions to a short-period dissolved inorganic phosphorus (DIP) enrichment. The samples were collected during the summer cyanobacterial bloom of 2022 in the central region of the Gulf of Riga. Contrary to expectations, neither species demonstrated a significant increase in biomass. The study also established that N2-fixation rates did not correlate directly with the total diazotrophic cyanobacteria biomass, but showed a significant correlation with heterocyst presence in both species addressed during this study. The findings suggest the influence of additional factors beyond DIP availability on the N2-fixing cyanobacteria growth in the Gulf of Riga.

Evolutionary history and root trait coordination predict nutrient strategy in tropical legume trees.

Plants express diverse nutrient use and acquisition traits, but it is unclear how trait combinations at the species level are constrained by phylogeny, trait coordination, or trade-offs in resource investment. One trait - nitrogen (N) fixation - is assumed to correlate with other traits and used to define plant functional groups, despite potential confounding effects of phylogeny. We quantified growth, carbon metabolism, fixation rate, root phosphatase activity (RPA), mycorrhizal colonization, and leaf and root morphology/chemistry across 22 species of fixing and nonfixing tropical Fabaceae trees under common conditions. Belowground trait variation was high even among closely related species, and most traits displayed a phylogenetic signal, including N-fixation rate and nodule biomass. Across species, we observed strong positive correlations between physiological traits such as RPA and root respiration. RPA increased ~ fourfold per unit increase in fixation, supporting the debated hypothesis that N-fixers 'trade' N for phosphatases to enhance phosphorus acquisition. Specific root length and root N differed between functional groups, though for other traits, apparent differences became nonsignificant after accounting for phylogenetic nonindependence. We conclude that evolutionary history, trait coordination, and fixation ability contribute to nutrient trait expression at the species level, and recommend explicitly considering phylogeny in analyses of functional groupings.

Fronts divide diazotroph communities in the Southern Indian Ocean.

Dinitrogen (N2) fixation represents a key source of reactive nitrogen in marine ecosystems. While the process has been rather well-explored in low latitudes of the Atlantic and Pacific Oceans, other higher latitude regions and particularly the Indian Ocean have been chronically overlooked. Here, we characterize N2 fixation and diazotroph community composition across nutrient and trace metals gradients spanning the multifrontal system separating the oligotrophic waters of the Indian Ocean subtropical gyre from the high nutrient low chlorophyll waters of the Southern Ocean. We found a sharp contrasting distribution of diazotroph groups across the frontal system. Notably, cyanobacterial diazotrophs dominated north of fronts, driving high N2 fixation rates (up to 13.96nmol N l-1 d-1) with notable peaks near the South African coast. South of the fronts non-cyanobacterial diazotrophs prevailed without significant N2 fixation activity being detected. Our results provide new crucial insights into high latitude diazotrophy in the Indian Ocean, which should contribute to improved climate model parameterization and enhanced constraints on global net primary productivity projections.

Moss species and precipitation mediate experimental warming stimulation of growing season N2 fixation in subarctic tundra.

Climate change in high latitude regions leads to both higher temperatures and more precipitation but their combined effects on terrestrial ecosystem processes are poorly understood. In nitrogen (N) limited and often moss-dominated tundra and boreal ecosystems, moss-associated N2 fixation is an important process that provides new N. We tested whether high mean annual precipitation enhanced experimental warming effects on growing season N2 fixation in three common arctic-boreal moss species adapted to different moisture conditions and evaluated their N contribution to the landscape level. We measured insitu N2 fixation rates in Hylocomium splendens, Pleurozium schreberi and Sphagnum spp. from June to September in subarctic tundra in Sweden. We exposed mosses occurring along a natural precipitation gradient (mean annual precipitation: 571-1155 mm) to 8 years of experimental summer warming using open-top chambers before our measurements. We modelled species-specific seasonal N input to the ecosystem at the colony and landscape level. Higher mean annual precipitation clearly increased N2 fixation, especially during peak growing season and in feather mosses. For Sphagnum-associated N2 fixation, high mean annual precipitation reversed a small negative warming response. By contrast, in the dry-adapted feather moss species higher mean annual precipitation led to negative warming effects. Modelled total growing season N inputs for Sphagnum spp. colonies were two to three times that of feather mosses at an area basis. However, at the landscape level where feather mosses were more abundant, they contributed 50% more N than Sphagnum. The discrepancy between modelled estimates of species-specific N input via N2 fixation at the moss core versus ecosystem scale, exemplify how moss cover is essential for evaluating impact of altered N2 fixation. Importantly, combined effects of warming and higher mean annual precipitation may not lead to similar responses across moss species, which could affect moss fitness and their abilities to buffer environmental changes.

Light drives nitrogen fixation in tropical montane cloud forests in Costa Rica

Tropical montane cloud forests are high altitude ecosystems characterized by very high ambient humidity, which favors organisms that depend on the environment for their water status, such as bryophytes and their nitrogen-fixing symbionts. Bryophyte-associated N2 fixation is a major source of new N in several northern environments, but their contributions to the N cycle in other ecosystems is still poorly understood. In this work, we evaluated N2 fixation rates associated with epiphytic bryophytes growing along the stems of pumpwood trees (Cecropia sp.) as well as in surrounding litter and soil from a primary and a secondary cloud forests in the Talamanca Mountain Range, Costa Rica. Nitrogen fixation was significantly higher in substrates from the secondary forest compared to those from the primary forest. Overall, N2 fixation rates associated with epiphytic bryophytes were 57 times those of litter and 270 times what was measured in soil. Further, light intensity was the major factor influencing N2 fixation rates in all substrates. Increased access to light in disturbed cloud forests may therefore favor bryophyte-associated N2 fixation, potentially contributing to the recovery of these ecosystems.

Marine Colloids Boost Nitrogen Fixation in Trichodesmium erythraeum by Photoelectrophy.

Nitrogen fixation by the diazotrophic cyanobacterium Trichodesmium contributes up to 50% of the bioavailable nitrogen in the ocean. N2 fixation by Trichodesmium is limited by the availability of nutrients, such as iron (Fe) and phosphorus (P). Although colloids are ubiquitous in the ocean, the effects of Fe limitation on nitrogen fixation by marine colloids (MC) and the related mechanisms are largely unexplored. In this study, we found that MC exhibit photoelectrochemical properties that boost nitrogen fixation by photoelectrophy in Trichodesmium erythraeum. MC efficiently promote photosynthesis in T. erythraeum, thus enhancing its growth. Photoexcited electrons from MC are directly transferred to the photosynthetic electron transport chain and contribute to nitrogen fixation and ammonia assimilation. Transcriptomic analysis revealed that MC significantly upregulates genes related to the electron transport chain, photosystem, and photosynthesis, which is consistent with elevated photosynthetic capacities (e.g., Fv/Fm and carboxysomes). As a result, MC increase the N2 fixation rate by 67.5-89.3%. Our findings highlight a proof-of-concept electron transfer pathway by which MC boost nitrogen fixation, broadening our knowledge on the role of ubiquitous colloids in marine nitrogen biogeochemistry.

Winter Convective Mixing Mediating Coupling of N‐Gain and ‐Loss in the Arabian Sea

AbstractMarine dinitrogen (N2) fixation fuels primary production and thereby influences the Earth's climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N2 fixation rates from the two spatially and physicochemically contrasting regions of the Arabian Sea during the winter monsoon: (a) the colder and nutrient‐rich waters in the northern region owing to winter convection and (b) the warmer and nutrient‐poor waters in the southern region unaffected by winter convection. We found higher N2 fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters. N2 fixation was favored by high nutrient concentrations in the euphotic waters, whereas remained unaffected by nutrient availability in the aphotic waters. We conclude that diazotrophs dwelling in the euphotic zone chose phosphate over fixed nitrogen‐poor waters. However, we found that among oligotrophic waters, anticyclonic eddy extremes the barrier of fixed nitrogen supply, and thereby, elevates N2 fixation. While the Arabian Sea loses about 20%–40% of the global ocean fixed nitrogen, we estimate that N2 fixation in the Arabian Sea offsets only up to 42% of its fixed nitrogen‐loss by denitrification, but this offset could be higher if diazotrophic activity is further examined up to the deeper depths of the Arabian Sea.

Short-term fate of nitrogen fixed by moss-cyanobacteria associations under different rainfall regimes

Nitrogen (N) fixation by moss-cyanobacteria associations has been recognized as an important N input pathway in many ecosystems from arctic tundra to tropical forests. However, the transfer of fixed N2 from mosses to the soil as well as the effects of rainfall frequency and volume on this N transfer has hardly been studied – even though mosses can leach nutrients upon rewetting. In this study, we investigated the transfer of fixed N2 by moss-cyanobacteria associations in one month under four watering regimes with a combination of high and low volume and frequency. For this, we used two morphologically similar moss species collected from ecosystems with different climate and N availability (subarctic - Hylocomium splendens; and tropical - Thuidium delicatulum). Acetylene reduction assays were conducted as a measure of N2 fixation rates in mosses, and 15N-N2 tracing was used to follow the fixed N2 from moss to the underlying substrate. Nitrogen fixation rates were higher in T. delicatulum than in H. splendens, but rainfall volume and frequency did not show strong effects on N2 fixation rates. Nonetheless, the extent of N leached from mosses was more sensitive to an increase in rainfall volume than to an increase in frequency, and more N was lost from T. delicatulum under high volume precipitation than from H. splendens. Both total nitrogen and 15N enrichment results demonstrate that the fixed N2 was mostly stored in moss tissues with less than 1 % leached to the substrate. Our results show that both moss species retain almost all fixed N2 within their tissues under small rainfall disturbances within one month, while increased N availability under higher precipitation volume renders some moss species an important N source for the soil.

Nitrogen Fixation at the Mid‐Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea

AbstractContinental shelves contribute a large fraction of the ocean's new nitrogen (N) via N2 fixation; yet, we know little about how physical processes at the ocean's margins shape diazotroph biogeography and activity. Here, we test the hypothesis that frontal mixing favors N2 fixation at the Mid‐Atlantic Bight shelfbreak. Using the 15N2 bubble release method, we measured N2 fixation rates on repeat cross‐frontal transects in July 2019. N2 fixation rates in shelf waters (median = 5.42 nmol N L−1 d−1) were higher than offshore (2.48 nmol N L−1 d−1) but did not significantly differ front frontal waters (8.42 nmol N L−1 d−1). However, specific N2 uptake rates, indicative of the relative contribution of diazotroph‐derived N to particulate N turnover, were significantly higher in frontal waters, suggesting that diazotroph‐derived N is of greater importance in supporting productivity there. This study furthered captured an ephemeral shelf‐water streamer, which resulted from the impingement of a warm core ring on the shelf. The streamer transported shelf‐water diazotrophs (including UCYN‐A and Richelia spp., as assessed by qPCR) offshore with sustained high N2 fixation rates. This feature injected >50 metric tons d−1 of newly fixed N to the Slope Sea—a rate equivalent to ∼4% of the total N flux estimated for the entire Mid‐Atlantic Bight. As intrusions of Gulf Stream meanders and eddies onto the shelf are increasing in frequency due to climate change, episodic lateral fluxes of new N into the Slope Sea may become increasingly important to regional budgets and ecosystem productivity.

Distribution of the N2 -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa in the Mexican Pacific upwelling system under two contrasting El Niño Southern Oscillation conditions.

The unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) is a key diazotroph in the global ocean owing to its high N2 fixation rates and wide distribution in marine environments. Nevertheless, little is known about UCYN-A in oxygen-deficient zones (ODZs), which may be optimal environments for marine diazotrophy. Therefore, the distribution and diversity of UCYN-A were studied in two consecutive years under contrasting phases (La Niña vs. El Niño) of El Niño Southern Oscillation (ENSO) along a transect in the ODZ of the Mexican Pacific upwelling system. Of the three UCYN-A sublineages found, UCYN-A1 and UCYN-A3 were barely detected, whereas UCYN-A2 was dominant in all the stations and showed a wide distribution in both ENSO phases. The presence of UCYN-A was associated with well-oxygenated waters, but it was also found for the first time under suboxic conditions (<20 μM) at the bottom of a shallow coastal station, within the oxygen-poor and nutrient-rich Subsurface Subtropical water mass. This study contributes to the understanding of UCYN-A distribution under different oceanographic conditions associated with ENSO phases in upwelling systems, especially because of the current climate change and increasing deoxygenation in many areas of the world's oceans.

Biogeochemical explanations for the world’s most phosphate-rich lake, an origin-of-life analog

Environmental phosphate concentrations are typically much lower (~10−6 M) than needed for prebiotic phosphorylation of nucleosides, critical for the origin of life. Here, we tested hypotheses explaining highly concentrated dissolved phosphate in carbonate-rich “soda” lakes by examining phosphorus and nitrogen cycling in Last Chance Lake and Goodenough Lake, Canada. We find a lack of geochemical phosphorus precipitation, that sedimentary calcium is in dolomite rather than apatite, and that N2-fixation rates, probably suppressed by high salinity, are too low to create significant biological phosphate demand. Thus, nitrogen-limitation of biological production and precipitation of calcium-rich carbonate instead of apatite combine to allow unimpeded evaporative phosphate buildup in Last Chance Lake to the highest known natural levels (37 mM) due to small biological and geochemical phosphorus sinks. Forming on basaltic rock, which was likely common on early Earth, evaporative soda lakes were consequently plausible origin-of-life settings with sufficient phosphate for prebiotic synthesis.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50–500 μg cyanobacterial chlorophyll-a L−1) and lake CO2 levels were undersaturated (≤300 μatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Biological nitrogen fixation and the role of soil diazotroph niche breadth in representative terrestrial ecosystems

Biological nitrogen fixation (BNF) driven by diazotrophs is a fundamental process underpinning the primary productivity of terrestrial ecosystems and plays a pivotal role in the global nitrogen cycle. However, the relationships between BNF function and soil diazotroph taxa with different niche breadths remain unclear. This study investigated the N2 fixation rate and community structure of diazotrophs in soils collected from representative terrestrial ecosystems in China—agricultural, desert, forest, grassland and wetland—and explored the specific contributions of groups of diazotrophs with different niche breadths to the N2 fixation rate. Among all the ecosystems, the N2 fixation rate, nifH gene abundance and diazotroph diversity were highest in the wetland ecosystems. The N2 fixation rate was significantly (P < 0.05) positively correlated with diazotroph diversity, indicating a relationship between microbial community diversity and ecosystem functionality. Furthermore, the N2 fixation rate and Shannon index were significantly (P < 0.05) positively correlated with the abundances of diazotrophs classified as specialists and opportunists. Structural equation modeling indicated that geographical factors and climatic factors affected the abundance of specialists and opportunists directly or indirectly through soil physicochemical properties, which in turn affected N2 fixation rates. Collectively, these valuable insights into the intrinsic relationship between nitrogen-fixing function and groups of soil diazotroph taxa with different niche breadths advance our understanding of BNF across terrestrial ecosystems.

Legume versus non-legume foliar litter decomposition in regularly burned loblolly pine forests

While understory prescribed burning is central to the ecological and economic management of many pine forests, there is concern that burning may cause excessive losses of N, thereby decreasing soil fertility and forest productivity. Symbiotic N2-fixation by fire-adapted native and naturalized herbaceous legumes may mitigate N losses due to burning. However, the N gain via legumes depends not only on the rates of N2-fixation, but also on the rates of litter decay between burn events. At present, the rates and controls of relatively N-rich legume litter decay in forests are poorly understood. In this study, foliar litter decomposition was assessed for 6 legume species (average [N] = 1.83 ± 0.52 %) and 4 non-legume species (average [N] = 0.80 ± 0.27 %) in “early” and “late” stage loblolly pine (Pinus taeda) forests for 3 years. We hypothesized that the N-rich legumes would exhibit greater mass loss, particularly in the early sites, and would thus release a greater percentage of their original N compared to the non-legumes. In contrast to our hypothesis, average mass loss rates were comparable for the legumes and non-legumes resulting in similar residual mass estimates (mean [95 % CI]) respectively in the early (11.9 % [9.8 %, 14.4 %] versus 15.5 % [12.2 %, 19.7 %]) and late (12.5 % [10.3 %, 15.1 %] versus 9.0 % [7.1 %, 11.3 %]) sites by the end of the study. Likewise, the average decay rate (k) for the legumes and non-legumes did not differ significantly in the early sites (0.73 ± 0.25 year−1 versus 0.69 ± 0.15 year−1, respectively), and the average legume decay rate was lower (P = 0.0080) than the average non-legume decay rate in the late sites (0.58 ± 0.18 year−1 versus 0.63 ± 0.13 year−1, respectively). Mass loss was typically greater in the early sites, but the differences were significant for less than 22 % of the comparisons. As hypothesized, legumes did release a greater percentage of their original N compared to non-legumes resulting in lower average N remaining estimates respectively in the early (34.2 % [29.4 %, 39.8 %] versus 51.3 % [40.2 %, 65.5 %]) and late (28.9 % [24.2 %, 34.5 %] versus 42.5 % [29.6 %, 61.1 %]) sites after 3 years. The results suggest that N, P, and S were co-regulators of litter decay, and that pine litter may be an important sink for these limiting macronutrients. The strong interplay among N, P, and S should be considered in the use of prescribed burning to reduce nutrient limitations in fire-maintained pine forests.