High Rates Of Nitrogen Fixation Research Articles

Nitrate Uptake and Primary Production Along the Amazon River Plume Continuum

AbstractThe Amazon River plume (ARP) has been shown to support high rates of nitrogen fixation and primary production. However, nitrogen fixation alone cannot account for total primary production determined in the region, hinting that other nitrogen uptake processes might play a role. For the first time, we measured nitrate uptake rates in the ARP during three cruises in May 2018, June 2019 and April/May 2021, along with primary production rates and an analysis of phytoplankton community composition via high performance liquid chromatography. Based on a classification according to the salt content the region was divided into estuarine (ES), mesohaline (MH) and oceanic (OC) stations. Primary production was light limited near the river mouth at ES stations and was maximal off the coasts of French Guiana and Suriname, where also nitrate uptake was highest with rates of 11.4 mmol m−2 d−1. The role of eddies pinching off a deflecting plume are discussed as possible reason for higher nutrient concentrations at the MH stations. Surprisingly, at most MH stations north of 5°N, nitrate uptake rates were low despite the presence of sufficient substrate concentration (up to 1.44 μM nitrate). Diatoms, dinoflagellates or Synechococcus sp. dominated phytoplankton communities. OC stations showed lowest productivity rates in accordance with oligotrophic conditions. However, rates seem to be sufficient to completely deplete the remaining riverine nitrate, preventing its export to the open ocean.

Sulfur oxidation and reduction are coupled to nitrogen fixation in the roots of the salt marsh foundation plant Spartina alterniflora

Heterotrophic activity, primarily driven by sulfate-reducing prokaryotes, has traditionally been linked to nitrogen fixation in the root zone of coastal marine plants, leaving the role of chemolithoautotrophy in this process unexplored. Here, we show that sulfur oxidation coupled to nitrogen fixation is a previously overlooked process providing nitrogen to coastal marine macrophytes. In this study, we recovered 239 metagenome-assembled genomes from a salt marsh dominated by the foundation plant Spartina alterniflora, including diazotrophic sulfate-reducing and sulfur-oxidizing bacteria. Abundant sulfur-oxidizing bacteria encode and highly express genes for carbon fixation (RuBisCO), nitrogen fixation (nifHDK) and sulfur oxidation (oxidative-dsrAB), especially in roots stressed by sulfidic and reduced sediment conditions. Stressed roots exhibited the highest rates of nitrogen fixation and expression level of sulfur oxidation and sulfate reduction genes. Close relatives of marine symbionts from the Candidatus Thiodiazotropha genus contributed ~30% and ~20% of all sulfur-oxidizing dsrA and nitrogen-fixing nifK transcripts in stressed roots, respectively. Based on these findings, we propose that the symbiosis between S. alterniflora and sulfur-oxidizing bacteria is key to ecosystem functioning of coastal salt marshes.

Nitrogen fixation rates in the Guinea Dome and the equatorial upwelling regions in the Atlantic Ocean

AbstractBiological nitrogen fixation is a key process balancing the loss of combined nitrogen in the marine nitrogen cycle. Its relevance in upwelling or high nutrient regions is still unclear, with the few available studies in these regions of the ocean reporting rates that vary widely from below detection limit to > 100 nmol N L−1 d−1. In the eastern tropical Atlantic Ocean, two open ocean upwelling systems are active in boreal summer. One is the seasonal equatorial upwelling, where the residual phosphorus associated with aged upwelled waters is suggested to enhance nitrogen fixation in this season. The other is the Guinea Dome, a thermal upwelling dome. We conducted two surveys along 23° W across the Guinea Dome and the Equator from 15° N to 5° S in September 2015 and August–September 2016 with high latitudinal resolution (20–60 nm between stations). The abundance of Trichodesmium colonies was characterized by an Underwater Vision Profiler 5 and the total biological nitrogen fixation in the euphotic layer was measured using the 15N2 technique. The highest abundances of Trichodesmium colonies were found in the area of the Guinea Dome (9°–15° N) with a maximum of 3 colonies L−1 near the surface. By contrast, colonies were almost absent in the Equatorial band between 2° N and 5° S. The highest nitrogen fixation rate was measured at the northern edge of the Guinea Dome in 2016 (ca. 31 nmol N L−1 d−1). In this region, where diazotrophs thrived on a sufficient supply of both phosphorus and iron, a patchy distribution was unveiled by our increased spatial resolution scheme. In the Equatorial band, rates were considerably lower, ranging from below detection limit to ca. 4 nmol N L−1 d−1, with a clear difference in magnitude between 2015 (rates close to zero) and 2016 (average rates around 2 nmol N L−1 d−1). This difference seemed triggered by a contrasting supply of phosphorus between years. Our study stresses the importance of surveys with sampling at fine-scale spatial resolution, and shows unexpected high variability in the rates of nitrogen fixation in the Guinea Dome, a region where diazotrophy is a significant process supplying new nitrogen into the euphotic layer.

Nitrogen fixation under the interaction of Kuroshio and upwelling in the northeastern South China Sea

Diazotrophic organisms are vulnerable to physical dynamic processes. However, the interactions and mechanisms of multiple dynamic processes on nitrogen fixation remain unclear. This study aimed to investigate one of these interactions; we conducted field investigations and bioassay experiments in the northeastern South China Sea, which faces the Kuroshio current and coastal upwelling. Using 15N tracer and molecular biology methods, we found that the eastern Guangdong upwelling, having high phosphorus and a lower N:P ratio, had higher nitrogen fixation rates as well as a higher abundance of diazotrophic cyanobacteria Trichodesmium spp. and UCYN-A compared with nonupwelling area. Additionally, a higher abundance of diazotrophic cyanobacteria occurred near the bottom of the euphotic zone along the shelf break that was affected by the Kuroshio current. Field incubation experiments confirmed that phosphorus increased the nitrogen fixation rates and stimulated the growth of cyanobacteria in the area affected by upwelling. Taken together, our results indicated that warmer surface water with a high N:P ratio likely mixed with upwelled phosphorus-rich water containing diazotrophic cyanobacteria, which were transported by the intensified Kuroshio current under the influence of the 2015/16 El Niño. This mixing then triggered a high abundance of diazotrophic cyanobacteria. These findings provide new insights into how the interaction of multiple dynamic processes can regulate nitrogen fixation.

Co-Compost Application of Magnesium Salts and Orthophosphate Adjusted Biochar and Cyanobacteria for Fixing Nitrogen, Improving Maize Quality, and Reducing Field Nutrient Loss

To solve the problem of nitrogen loss and harmful gas emissions in the co-composting of cyanobacteria with other wastes and agricultural non-point source pollution in the Chaohu Lake Basin (CHLB), this study selected cyanobacteria as the main material, spent mushroom and rice hulls as excipients, and biochar (B) and Mg2+/PO43− salt as an additive. The nitrogen fixation rate, maturity, toxicity, and fertilizer efficiency indexes were analyzed to explore the best rate of biochar application. Additionally, three different fertilization treatments (C0: No fertilization, CF: Chemical fertilizer, OF: Organic fertilizer) were applied to agricultural field experiments in the CHLB. The results showed that the cyanobacteria composting experiment with 10% B as additives had the lowest cumulative NH3 emission (603.00 g) and the highest nitrogen fixation rate (61.54%). We used this compost product for maize cultivation in field experiments. The results show that OF treatment can reduce field nutrient loss, and the increase in the rate of soil TN fixation was as follows: OF (18.62%) > CF (10.34%) > C0 (−9.66%). Moreover, the TN and TK losses in runoff were 33.33% and 22.74% lower than CF in farmland runoff. The OF treatment increased the content of crude fat, crude protein, and crude starch by 4.40%, 8.34%, and 2.66% compared with the CF treatment, respectively. Overall, the N, P, and K source agronomic coefficients of cyanobacteria organic fertilizer were 1.424, 1.001, and 1.16, respectively. These results indicated that cyanobacterial organic fertilizer could replace chemical fertilizer to reduce agricultural non-point source pollution and improve maize quality. Meanwhile, the amendment of B and Mg/P salt additives could reduce air pollution and improve the quality of compost products. This study provides a theoretical basis and reference for nitrogen fixation and extension of multiple wastes co-composting field experiments.

Soil Properties under Artificial Mixed Forests in the Desert-Yellow River Coastal Transition Zone, China

Mixed forests play a key role in the environmental restoration of desert ecosystems and in order to address the improvement of soil properties by different mixed vegetation types. We selected four typical mixed vegetation types (including: Populus alba var. pyramidalis × Caragana korshinskii, P. pyramidalis × Hedysarum mongdicum, P. pyramidalis × Hedysarum scoparium and Hedysarum scoparium × Salix cheilophila) that have been restored for 22 years and the moving sandy land in the transition zone between the desert and the Yellow River in northern China. We compared the differences in soil properties using a total of 45 soil samples from the 0–30 cm soil layer (10 cm units). We found that revegetation had a significant positive effect on fine particles, soil nutrients, soil bulk density (SBD), and soil fractal dimension (D) values. Soil D values under different types of vegetation range from 2.16 to 2.37. Soil nutrients and fractal dimension showed highly significant or stronger negative correlations with SBD and sand and highly significant or stronger positive correlations with clay and silt. The construction of P. pyramidalis × C. korshinskii improved the soil texture better than other vegetation restoration types. Compared to the mobile sandy land, organic carbon (SOC), available phosphorus (AP), available potassium (AK), alkaline hydrolysis nitrogen (AN), total nitrogen (TN), total potassium (TK), clay, and silt increased by 161%, 238%, 139%, 30%, 125%, 69%, 208%, and 441% respectively. As mentioned above, P. pyramidalis × C. korshinskii is a suitable type of mixed vegetation restoration for the area. In addition, establishing vegetation with high nitrogen fixation rates in desert ecosystems tolerant to drought and aeolian conditions is beneficial in reversing the trend of desertification. This research will suggest vegetation building strategies for controlling desertification.

Nitrogen-fixing cyanobacteria in the mountain-tundra ecosystems of the northern regions of the Urals (diversity, functional characteristics)

In biological soil crusts (BSC) of mountain tundras of three northern regions of the Urals, 131 species of cyanobacteria from 46 genera were identified. Nitrogen fixers make up about 40% of the total diversity. The complex of dominants is formed by representatives of the genera Nostoc, Stigonema, Scytonema. Based on the study of the nitrogen-fixing activity of biological crusts dominated by cyanobacteria, two groups were distinguished: those with high and low rates of nitrogen fixation. High rates of nitrogen fixation, on average 3.17 mg C2H4 m-2h-1, are typical for BSC dominated by Nostoc commune Vaucher ex Bornet & Flahault or Stigonema ocellatum Thuret ex Bornet & Flahault, low rates, on average 1.01 mg C2H4 m-2h-1, for BC dominated by Stigonema minutum Hassall ex Bornet et Flahault. It has been shown that the nitrogen fixation of the studied BSCs depends to a greater extent on the composition of the dominant cyanobacteria species in soil crusts than on the geographic zone in which the samples were collected.

Temperate southern Australian coastal waters are characterised by surprisingly high rates of nitrogen fixation and diversity of diazotrophs.

Biological dinitrogen (N2) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N2 fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N2 fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (nifH), quantitative nifH PCR, and 15N2 stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N2 fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N2 fixation (10–64 nmol L−1 d−1) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N2 fixation (2 nmol L−1 d−1) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO3/NO2 were significantly correlated with the highest N2 fixation rates, suggesting that N2 fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by nifH sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to Pseudomonas stutzeri, and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO4 concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N2 fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N2 fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N2 fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.

Drought and soil nutrients effects on symbiotic nitrogen fixation in seedlings from eight Neotropical legume species

AbstractSymbiotic nitrogen fixation is a dominant source of nitrogen to many terrestrial ecosystems and thus may influence their responses to global change. High legume species diversity and abundance are thought to lead to high rates of symbiotic nitrogen fixation in Neotropical forests. However, how changes in water and nutrient availability will affect symbiotic nitrogen fixation has only recently been explored, even as droughts begin to increase in severity and frequency in the Neotropics. To explore these effects, we grew eight species of Neotropical woody legume seedlings in a shadehouse for four months while manipulating soil water, phosphorus, and molybdenum availability. Overall, drought reduced nodule biomass, nitrogenase activity (acetylene reduction g‐1 nodule), and acetylene reduction per seedling by 33%, 27%, and 41%, respectively, but reduced seedling biomass by only 18%. Species varied in the manifestation of drought effects. For example, drought reduced the probability that nodules formed in some species, but in others reduced the nitrogenase activity or acetylene reduction per seedling. In contrast, the effects of phosphorus and molybdenum availability were more species‐specific. However, fertilization did affect (both positively and negatively) one or more symbiotic nitrogen fixation response variables in two of the eight species. Our results indicate drought reduces symbiotic nitrogen fixation in Neotropical legume seedlings, but the mechanism of response may be species‐specific. Therefore, predicting the response of symbiotic nitrogen fixation in the Neotropics to a changing climate, with expected increases in drought frequency and severity, may require grappling with the diversity of responses among nitrogen‐fixing legumes.Abstract in Portuguese is available with online material

Molybdenum, phosphorus, and pH do not constrain nitrogen fixation in a tropical forest in the southeastern Amazon.

High rates of biological nitrogen fixation (BNF) are commonly reported for tropical forests, but most studies have been conducted in regions that receive substantial inputs of molybdenum (Mo) from atmospheric dust and sea-salt aerosols. Even in these regions, the low availability of Mo can constrain free-living BNF catalyzed by heterotrophic bacteria and archaea. We hypothesized that in regions where atmospheric inputs of Mo are low and soils are highly weathered, such as the southeastern Amazon, Mo would constrain BNF. We also hypothesized that the high soil acidity, characteristic of the Amazon Basin, would further constrain Mo availability and therefore soil BNF. We conducted two field experiments across the wet and dry seasons, adding Mo, phosphorus (P), and lime alone and in combination to the forest floor in the southeastern Amazon. We sampled soils and litter immediately, and then weeks and months after the applications, and measured Mo and P availability through resin extractions and BNF with the acetylene reduction assay. The experimental additions of Mo and P increased their availability and the lime increased soil pH. While the combination of Mo and P increased BNF at some time points, BNF rates did not increase strongly or consistently across the study as a whole, suggesting that Mo, P, and soil pH are not the dominant controls over BNF. In a separate short-term laboratory experiment, BNF did not respond strongly to Mo and P even when labile carbon was added. We postulate that high nitrogen (N) availability in this area of the Amazon, as indicated by the stoichiometry of soils and vegetation and the high nitrate soil stocks, likely suppresses BNF at this site. These patterns may also extend across highly weathered soils with high N availability in other topographically stable regions of the tropics.

Nitrogen Fertilization Reduces Nitrogen Fixation Activity of Diverse Diazotrophs in Switchgrass Roots

The use of synthetic nitrogen fertilizers to grow biofuel crops adds to the economic and environmental costs of biomass production. Biological nitrogen fixation provides an alternative, eco-friendly source of nitrogen for leguminous plants and some nonlegumes. With the objective of characterizing and eventually harnessing nitrogen-fixing bacteria in switchgrass roots, we assayed nitrogenase activity and surveyed nifH-expressing bacteria in roots of switchgrass grown under agricultural conditions. Plants were cultivated at two separate locations in Oklahoma in different soil types for 5 years with three nitrogen fertilizer regimes (0, 90, and 180 kg of N ha−1 year−1). Nitrogenase activity associated with roots cleaned of soil was found to be highest in plants grown with no N fertilizer and lowest or not detectable for plants grown at the highest level of N fertilizer. A total of 454 abundant nifH operational taxonomic units (OTUs) were identified in the switchgrass roots. Diversity analysis of active nifH-expressing bacteria showed that the most common orders were Burkholderiales (171 OTUs), Desulfovibrionales (55 OTUs), and Rhizobiales (44 OTUs). OTUs belonging to Azonexus, Pseudacidovorax, Desulfovibrio, Dechloromonas, and Bradyrhizobium were enriched in roots of plants grown under low nitrogen conditions. Analysis of significant OTUs showed distinct nifH-expressing microbial communities at the two field locations, as well as a unique community in roots of unfertilized soil at Frederick that exhibited the highest rates of nitrogen fixation. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Enhanced Nitrogen Fixation in a glgX-Deficient Strain of Cyanothece sp. Strain ATCC 51142, a Unicellular Nitrogen-Fixing Cyanobacterium.

Cyanobacteria are oxygenic photosynthetic prokaryotes with important roles in the global carbon and nitrogen cycles. Unicellular nitrogen-fixing cyanobacteria are known to be ubiquitous, contributing to the nitrogen budget in diverse ecosystems. In the unicellular cyanobacterium Cyanothece sp. strain ATCC 51142, carbon assimilation and carbohydrate storage are crucial processes that occur as part of a robust diurnal cycle of photosynthesis and nitrogen fixation. During the light period, cells accumulate fixed carbon in glycogen granules to use as stored energy to power nitrogen fixation in the dark. These processes have not been thoroughly investigated, due to the lack of a genetic modification system in this organism. In bacterial glycogen metabolism, the glgX gene encodes a debranching enzyme that functions in storage polysaccharide catabolism. To probe the consequences of modifying the cycle of glycogen accumulation and subsequent mobilization, we engineered a strain of Cyanothece 51142 in which the glgX gene was genetically disrupted. We found that the ΔglgX strain exhibited a higher growth rate than the wild-type strain and displayed a higher rate of nitrogen fixation. Glycogen accumulated to higher levels at the end of the light period in the ΔglgX strain, compared to the wild-type strain. These data suggest that the larger glycogen pool maintained by the ΔglgX mutant is able to fuel greater growth and nitrogen fixation ability.IMPORTANCE Cyanobacteria are oxygenic photosynthetic bacteria that are found in a wide variety of ecological environments, where they are important contributors to global carbon and nitrogen cycles. Genetic manipulation systems have been developed in a number of cyanobacterial strains, allowing both the interruption of endogenous genes and the introduction of new genes and entire pathways. However, unicellular diazotrophic cyanobacteria have been generally recalcitrant to genetic transformation. These cyanobacteria are becoming important model systems to study diurnally regulated processes. Strains of the Cyanothece genus have been characterized as displaying robust growth and high rates of nitrogen fixation. The significance of our study is in the establishment of a genetic modification system in a unicellular diazotrophic cyanobacterium, the demonstration of the interruption of the glgX gene in Cyanothece sp. strain ATCC 51142, and the characterization of the increased nitrogen-fixing ability of this strain.

Disruption of acdS gene reduces plant growth promotion activity and maize saline stress resistance by Rahnella aquatilis HX2.

Rahnella aquatilis HX2 was isolated from Beijing vineyard soil and used as a plant growth-promoting rhizobacterium in the field. Previous studies have shown that it has a broad in vitro antimicrobial spectrum and could inhibit a variety of plant pathogenic bacteria and fungi. In this study, a gene, acdS, encoding 1-aminocyclopropane-1-carboxylic acid-deaminase was disrupted by in-frame deletion in the HX2 strain. Compared to the wild-type, the acdS-mutant had higher rates of nitrogen fixation, reduced indole-3-acetic acid production, lowered efficacy as a biological control agent against the grape crown gall pathogen Agrobacterium vitis. Under saline stress conditions, plant height, above-ground fresh weight, root fresh weight of corn plants were increased by treatment with HX2 but this increase was compromised by the disruption of acdS gene. Our data confirmed the function of HX2 on plant growth promoting and demonstrated that acdS gene plays a major role in its PGPR activities.

Asymbiotic nitrogen fixation on woody roots of Norway spruce and silver birch

High rates of asymbiotic nitrogen (N) fixation have been measured in woody roots in temperate forests, but this rate has not been quantified in boreal forests. We studied the asymbiotic N2fixation associated with living and decomposing woody roots of Norway spruce (Picea abies (L.) H. Karst.) in three sites in Finland. In addition, tree species effect was studied in one site that included Norway spruce and silver birch (Betula pendula Roth.) monocultures and mixed stands. The rate of N2fixation measured as nitrogenase activity was affected by host tree species; spruce roots were the most active (0.67 C2H4·day–1·(g dry mass)–1in spruce monocultures). The activity was not statistically different in decayed and living root samples, and moisture content did not explain the observed high variability in nitrogenase activity. In a birch–spruce mixed stand, the average N2fixation in woody roots was 0.17 kg N·ha−1·year−1, whereas in Norway spruce dominated sites, the activity ranged from 0.06 to 0.15 kg N·ha−1·year−1. The N2fixation in decaying and living woody roots is an important contributor to the long-term total N balance of the forest. However, the estimated rate of N2fixation is low compared with atmospheric N deposition.

Nitrogen fluxes in Western streams

Nitrogen pollution to streams is altering the nitrogen cycling in unknown ways, causing challenges for predicting nitrogen fixation fluxes within aquatic ecosystems. Increasing nitrate pollution decreases the amount of nitrogen fixation occurring in streams. However, the relationship between stream nitrate concentration and the rate of nitrogen fixation is unknown. We predict that lower nitrate streams will have the highest rates of nitrogen fixation. Additionally, there will be much more energy produced in streams with nitrogen fixation compared to the amount required to fix the nitrogen. We estimated whole-stream gross primary production and nitrogen fixation fluxes using the diel change in dissolved nitrogen and oxygen gases compared to the expected dissolved gas saturation. Our whole-stream method is preferable to chamber estimates to understand the relationship between energy requirements for nitrogen fixation and gross primary production, but additional data is needed to distinguish between relationship types and make our measurements generalizable.
 
 Featured photo by Intermountain Forest Service, USDA Region 4 Photography on Flickr. https://flic.kr/p/jbTRUj

Stable symbiotic nitrogen fixation under water-deficit field conditions by a stress-tolerant alfalfa microsymbiont and its complete genome sequence

We here characterized the stress-tolerant alfalfa microsymbiont Sinorhizobium meliloti B401. B401-treated plants showed high nitrogen fixation rates under humid and semiarid environments. The production of glycine betaine in isolated bacteroids positively correlated with low precipitation levels, suggesting that this compound acts as a critical osmoprotectant under field conditions. Genome analysis revealed that strain B401 contains alternative pathways for the biosynthesis and uptake of glycine betaine and its precursors. Such genomic information will offer substantial insight into the environmental physiology of this biotechnologically valuable nitrogen-fixing bacterium.

Exploring survival strategies of African Savanna trees by partial ordering techniques

The resilience of savanna ecosystems to climate and land-use changes is an important ecological and management question. The term ‘resilience’ is used to refer to the ability of a tree species to survive in a specific location, even under changing environmental conditions. In this study, vectors of functional traits of selected savanna tree species are studied by applying partial order algorithms to them. Some ecological interpretations are obtained and are compared to published research. One finding is the high rates of nitrogen fixation for the leaves of Acacia nigrescens. In opposition to this well-known fact, we discovered that Sclerocarya birrea has a bigger average leaf area than the other four tree species. Additionally, we found high carbonate values within the leaf from Colophospermum mopane, Combretum apiculatum, and Terminalis sericea. These results correspond to different ecological strategies for the tree species in question. It became obvious that geometric structures gained from partial ordering show a very good correspondence to ecological strategies of these tree species. Concepts of partial order theory may therefore be helpful in ecosystem research.

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

Abstract. In pristine Sphagnum-dominated peatlands, (di)nitrogen (N2) fixing (diazotrophic) microbial communities associated with Sphagnum mosses contribute substantially to the total nitrogen input, increasing carbon sequestration. The rates of symbiotic nitrogen fixation reported for Sphagnum peatlands, are, however, highly variable, and experimental work on regulating factors that can mechanistically explain this variation is largely lacking. For two common fen species (Sphagnum palustre and S. squarrosum) from a high nitrogen deposition area (25 kg N ha−1 yr−1), we found that diazotrophic activity (as measured by 15 − 15N2 labeling) was still present at a rate of 40 nmol N gDW−1 h−1. This was surprising, given that nitrogen fixation is a costly process. We tested the effects of phosphorus availability and buffering capacity by bicarbonate-rich water, mimicking a field situation in fens with stronger groundwater or surface water influence, as potential regulators of nitrogen fixation rates and Sphagnum performance. We expected that the addition of phosphorus, being a limiting nutrient, would stimulate both diazotrophic activity and Sphagnum growth. We indeed found that nitrogen fixation rates were doubled. Plant performance, in contrast, did not increase. Raised bicarbonate levels also enhanced nitrogen fixation, but had a strong negative impact on Sphagnum performance. These results explain the higher nitrogen fixation rates reported for minerotrophic and more nutrient-rich peatlands. In addition, nitrogen fixation was found to strongly depend on light, with rates 10 times higher in light conditions suggesting high reliance on phototrophic organisms for carbon. The contrasting effects of phosphorus and bicarbonate on Sphagnum spp. and their diazotrophic communities reveal strong differences in the optimal niche for both partners with respect to conditions and resources. This suggests a trade-off for the symbiosis of nitrogen fixing microorganisms with their Sphagnum hosts, in which a sheltered environment apparently outweighs the less favorable environmental conditions. We conclude that microbial activity is still nitrogen limited under eutrophic conditions because dissolved nitrogen is being monopolized by Sphagnum. Moreover, the fact that diazotrophic activity can significantly be upregulated by increased phosphorus addition and acid buffering, while Sphagnum spp. do not benefit, reveals remarkable differences in optimal conditions for both symbiotic partners and calls into question the regulation of nitrogen fixation by Sphagnum under these eutrophic conditions. The high nitrogen fixation rates result in high additional nitrogen loading of 6 kg ha−1 yr−1 on top of the high nitrogen deposition in these ecosystems.

Romul_Hum model of soil organic matter formation coupled with soil biota activity. III. Parameterisation of earthworm activity

Quantitative description of the role of soil fauna in soil organic matter (SOM) formation and dynamics is necessary for further development of SOM modelling. This is especially true for the role of anecic earthworms in SOM formation, which was first observed in the classical investigations of Charles Darwin in 1881. Despite the large number of earthworm studies available in the literature, more attention has been paid to the role of these organisms in loss of carbon through decomposition and nutrient release than to their role in SOM formation. A set of anecic earthworm parameters were compiled or developed for ease of incorporation as a module into the SOM dynamics model Romul_Hum, which also represents the SOM forming activities of soil micro- and meso-biota. An approach was developed for initialization of earthworm biomass in the absence of measured biomass, and parameters were developed for food palatability, ingestion and egestion. The eco-physiological parameters of food consumption, excretion efficiency, assimilation efficiency, lifespan, and mortality for anecic earthworms were developed using an approach similar to that for food web mesofauna in the Romul_Hum. Parameters for processes unique to freshly excreted casts were developed to include rapid mineralization of SOM and nitrogen, SOM formation, and high nitrogen fixation rates. The addition of the earthworm module to the Romul_Hum model allows assessment of the combined effects of earthworms cast production, and micro- and meso-faunal food web activity within the casts, on the formation of stable SOM.

Major cereal crops benefit from biological nitrogen fixation when inoculated with the nitrogen‐fixing bacterium Pseudomonas protegens Pf‐5 X940

A main goal of biological nitrogen fixation research has been to expand the nitrogen-fixing ability to major cereal crops. In this work, we demonstrate the use of the efficient nitrogen-fixing rhizobacterium Pseudomonas protegens Pf-5 X940 as a chassis to engineer the transfer of nitrogen fixed by BNF to maize and wheat under non-gnotobiotic conditions. Inoculation of maize and wheat with Pf-5 X940 largely improved nitrogen content and biomass accumulation in both vegetative and reproductive tissues, and this beneficial effect was positively associated with high nitrogen fixation rates in roots. 15 N isotope dilution analysis showed that maize and wheat plants obtained substantial amounts of fixed nitrogen from the atmosphere. Pf-5 X940-GFP-tagged cells were always reisolated from the maize and wheat root surface but never from the inner root tissues. Confocal laser scanning microscopy confirmed root surface colonization of Pf-5 X940-GFP in wheat plants, and microcolonies were mostly visualized at the junctions between epidermal root cells. Genetic analysis using biofilm formation-related Pseudomonas mutants confirmed the relevance of bacterial root adhesion in the increase in nitrogen content, biomass accumulation and nitrogen fixation rates in wheat roots. To our knowledge, this is the first report of robust BNF in major cereal crops.