N2-fixing Plants Research Articles

The \u03b415N value of N2 fixing actinorhizal plants and legumes grown with N2 as the only nitrogen source

The aim of this study was to investigate the effects of different plant parts and the age of plants at harvest as well as N2 fixing bacterial strains on the N concentration in symbiotic plant parts, especially on the δ15N signature of the actinorhizal plants and legumes. The 15N natural abundance method was used. Two actinorhizal plants were studied: Alnus incana (L.) infected with the Frankia strains ArI3 or “lsF” (local source of Frankia) and Hippophaë rhamnoides (L.) infected with the Frankia strains T1 or E15b. Two legume species were studied: Hedysarum coronarium (L.), infected with a soil suspension, and Robinia pseudoacacia (L.), infected with a crushed nodule suspension. It was particularly interesting that in A. incana, the two Frankia strains affected not only N concentration and δ15N signature of leaves and roots, but also had an impact on plant growth at first harvest. In Hippophaë rhamnoides plants inoculated with the Frankia strains T1 and E15b, N concentrations and δ15N values did not differ at any harvest time. However, plants nodulated by the Frankia strain T1 showed a higher nitrogen fixation rate and higher plant dry matter at all harvesting times. Based on our results for the quantification of N2 fixation with the “B” value, that is the δ15N value of the N2 fixing plants relying only on N2 fixation, plant parts, ages and strains should be carefully considered.

Nitrogen fertilization and its legacy have inconsistent and often negative effect on plant growth in undeveloped post mining soils

Nitrogen supplement by adding N-fertilizer and/or growing of N-fixing trees are considered as important stimulus for the development of early vegetation. We performed pot experiments to determine the responses of legumes (N-fixing) and grasses (non-N fixing) in respect to their biomass to N fertilization, legacy of N, and combination of these two in various ages of post mining soils. N legacy was achieved either by long-term mineral N fertilization (legacy of inorganic N) (experiment 1) or growing of N2 fixing plant (legacy of N-fixing) (experiment 2). The first pot experiment was conducted to test N fertilization and legacy of inorganic N and these two on growth of Festuca rubra and Lotus corniculatus when grown separately. Following this, plant-soil feedback was defined as positive or negative by comparing the second generation of plant biomass in soils with and without plants in the previous generation. In the second pot experiment, we compared the performance of P. compressa and L. corniculatus in the presence of legacy of N-fixing and N fertiliaztion when grown together, and at the same time, plant-soil feedback was determined.N fertilization increased the growth of grasses, but only increased biomass of legumes in young soils. N-legacy induced variable plant growth, in particular often decreased the biomass of legumes. Legacy of inorganic N was only beneficial for grasses rather than legumes in medium soils. When growing in the same pot, legacy of N-fixing showed positive effects on grasses, depending on soil age and N fertilization. Repeated growing of plants in the same soil mostly resulted in negative plant-soil feedback. Negative plant-soil feedback was not alleviated by N fertilization and legacy of inorganic N for both grasses and legumes when grown separately. However, growing grasses and legumes together can induce positive plant-soil feedback with no legacy of N-fixing.In conclusion, N addition often stimulates plant growth more than long-term N effect. Legacy of N tends to be only beneficial for non-N fixing plants in medium aged soils. While negative plant-soil feedback occurs for grasses and legumes both grown separately or together, legumes can induce positive plant-soil feedback when grown with grasses in no legacy of N-fixing soils.

Alnus sibirica encroachment promotes dissolved organic carbon biodegradation in a boreal peatland

Symbiotic dinitrogen (N2)-fixing trees have been expanding to boreal peatlands, yet its influence on dissolved organic carbon (DOC) biodegradation is unclear. Here, we measured DOC, ammonium‑nitrogen (NH4+-N), nitrate‑nitrogen (NO3−-N), dissolved inorganic nitrogen (DIN), and dissolved total nitrogen (DTN) concentrations, specific ultraviolet absorbance at 254 nm (SUVA254), and humification index in the extracts obtained from peats in the 0–10 cm, 10–20 cm, and 20–40 cm depths in the open peatlands and Alnus sibirica islands in a boreal peatland, Northeast China. Afterwards, the peat extracts were used to assess the effect of N2-fixing woody plant expansion on DOC biodegradation with a 42-day incubation experiment. The expansion of A. sibirica significantly increased NH4+-N, NO3−-N, DIN, and DTN concentrations, but did not produce a significant effect on SUVA254 and humification index in the extracts in each depth. Following A. sibirica expansion, DOC biodegradation was enhanced by 24.5%, 15.4%, and 38.3% at 0–10 cm, 10–20 cm, and 20–40 cm depths, respectively. Furthermore, DOC biodegradation was significantly and negatively correlated with DOC:DIN and DOC:DTN ratios, but exhibited no significant relationship with SUVA254 and humification index. This implied that improved N availability and associated shifts in C:N stoichiometry determined the increase in DOC biodegradation following A. sibirica expansion. Our findings suggest that N2-fixing tree encroachment promotes microbial decomposition of DOC through improved N availability in boreal peatlands, which may cause organic C loss from soils in these C-enriched ecosystems.

The response of mesophyll conductance to short- and long-term environmental conditions in chickpea genotypes.

. Mesophyll conductance (gm) has been shown to vary between genotypes of a number of species and with growth environments, including nitrogen availability, but understanding of gm variability in legumes is limited. We might expect gm in legumes to respond differently to limited nitrogen availability, due to their ability to fix atmospheric N2. Using online stable carbon isotope discrimination method, we quantified genetic variability in gm under ideal conditions, investigated gm response to N source (N2-fixation or inorganic N) and determined the effects of N source and water availability on the rapid response of gm to photosynthetic photon flux density (PPFD) and radiation wavelength in three genotypes of chickpea (Cicer arietinum). Genotypes varied 2-fold in gm under non-limiting environments. N-fed plants had higher gm than N2-fixing plants in one genotype, while gm in the other two genotypes was unaffected. gm response to PPFD was altered by N source in one of three genotypes, in which the gm response to PPFD was statistically significant in N-fed plants but not in N2-fixing plants. There was no clear effect of moderate water stress on the gm response to PPFD and radiation wavelength. Genotypes of a single legume species differ in the sensitivity of gm to both long- and short-term environmental conditions, precluding utility in crop breeding programmes.

The Common Bean (Phaseolus vulgaris) Basic Leucine Zipper (bZIP) Transcription Factor Family: Response to Salinity Stress in Fertilized and Symbiotic N2-Fixing Plants

The basic leucine zipper (bZIP) transcription factor family regulates plant developmental processes and response to stresses. The common bean (Phaseolus vulgaris), an important crop legume, possesses a whole set of 78 bZIP (PvbZIP) genes, the majority of these (59%) are most highly expressed in roots and nodules, root-derived new organs formed in the rhizobia N2-fixing symbiosis. Crop production is highly affected by salinity in Cuba and other countries. In this work we characterized the adverse effect of salinity to common bean plants of the Cuban CC-25-9-N cultivar grown in fertilized (full N-content) or symbiotic N-fixation (rhizobia inoculated) conditions. We assessed if PvbZIP TF participate in CC-25-9-N common bean response to salinity. Quantitative reverse-transcriptase-PCR (qRT-PCR) expression analysis showed that 26 out of 46 root/nodule-enhanced PvbZIP, that responded to salt stress in roots and/or nodules from fertilized and N2-fixing CC-25-9-N plants. From public common bean transcriptomic data, we identified 554 genes with an expression pattern similar to that of salt-responsive PvbZIP genes, and propose that the co-expressed genes are likely to be involved in the stress response. Our data provide a foundation for evaluating the individual roles of salt-responsive genes and to explore the PvbZIP-mediated improvement of salt tolerance in common bean.

Invasive Nitrogen-Fixing Plant Amplifies Terrestrial–Aquatic Nutrient Flow and Alters Ecosystem Function

Nitrate pollution is a global issue threatening the health and function of many lowland freshwater ecosystems. Quantifying nitrate loads and instream attenuation associated with land use is a critical requirement for improving freshwater management. One often overlooked nitrate source in catchments is invasive N-fixing trees such as Ulex europaeus (European gorse). This study compared nitrate concentrations in conjunction with stable isotope analyses (nitrate δ15N and δ18O) to investigate the effects of catchment gorse cover on stream nitrate export relative to three other land uses. These were regenerating native forest, low-intensity (dry-stock) and high-intensity (dairying) agriculture. We tested two hypotheses: (1) gorse is a regionally significant nitrate source; (2) instream nitrate attenuation is land-use dependent. The study was conducted in 24 reaches across six small, mixed land-use coastal catchments located on Banks Peninsula, New Zealand. Our results demonstrated that gorse-dominated stream reaches had significantly higher nitrate concentrations than all other land uses. Within the gorse-dominated reaches, nitrate concentration was significantly correlated with upstream catchment gorse cover. Furthermore, nitrate oxygen and nitrogen stable isotope compositions demonstrated that elevated nitrate concentrations in gorse streams were associated with decomposition of dead gorse foliage. The isotope data revealed sub-catchment-scale land-use-specific patterns of nitrate attenuation within streams. All three anthropogenic land uses (gorse, dry stock and dairy) showed distinctly different N-cycling from native-forested reaches where nitrate was efficiently cycled with evidence for highly localised nitrification. Stable isotope data demonstrated that overall nitrate attenuation became less efficient with higher nitrate loads. Our research demonstrates the significant impact N-fixing plants have on nitrate concentrations and instream attenuation. Quantifying the effects of N-fixing plants on water quality is an important step in achieving globally significant goals of sustaining ecosystem health and (sub)catchment-scale nutrient management.

Rising CO2 accelerates phosphorus and molybdenum limitation of N2-fixation in young tropical trees

Nitrogen fixation may be critical for supplying the nitrogen (N) needed to maintain the tropical carbon sink in a world of rising atmospheric CO2. However, we do not know whether increased CO2 acts to exacerbate nutrient limitation on the fixation process itself. We experimentally test this idea by growing N2-fixing plants in pre-Industrial (280 ppm), present-day (400 ppm), and doubled (800 ppm) atmospheric CO2. In a greenhouse experiment, we grew tree seedlings from N2-fixing species and a non-fixing species at three CO2 concentrations with control, +P (phosphorus), +Mo (molybdenum), and + P +Mo nutrient treatments. We found nutrient limitation to be minimal at pre-Industrial CO2, but with increasing CO2 fixer growth and fixation became increasingly limited by P and by a P-by-Mo interaction. At 400 ppm, plants with +P grew ~50% faster and fixed 10-15× more N2 based on nodule mass and nitrogenase activity. At 800 ppm, plants with +P +Mo grew 200% more, and fixed 25× more N2, suggesting Mo-P co-limitation at elevated CO2. Our findings imply that complex patterns of nutrient limitation can develop as CO2 rises, potentially suppressing tropical N2-fixation and the new inputs of N needed to sustain the tropical carbon sink.

Ancient environmental DNA reveals shifts in dominant mutualisms during the late\xa0Quaternary

DNA-based snapshots of ancient vegetation have shown that the composition of high-latitude plant communities changed considerably during the late Quaternary. However, parallel changes in biotic interactions remain largely uninvestigated. Here we show how mutualisms involving plants and heterotrophic organisms varied during the last 50,000 years. During 50–25 ka BP, a cool period featuring stadial-interstadial fluctuations, arbuscular mycorrhizal and non-N-fixing plants predominated. During 25-15 ka BP, a cold, dry interval, the representation of ectomycorrhizal, non-mycorrhizal and facultatively mycorrhizal plants increased, while that of N-fixing plants decreased further. From 15 ka BP, which marks the transition to and establishment of the Holocene interglaciation, representation of arbuscular mycorrhizal plants decreased further, while that of ectomycorrhizal, non-mycorrhizal, N-fixing and wind-pollinated plants increased. These changes in the mutualist trait structure of vegetation may reflect responses to historical environmental conditions that are without current analogue, or biogeographic processes, such as spatial decoupling of mutualist partners.

How belowground interactions contribute to the coexistence of mycorrhizal and non-mycorrhizal species in severely phosphorus-impoverished hyperdiverse ecosystems

Mycorrhizal strategies are very effective in enhancing plant acquisition of poorly-mobile nutrients, particularly phosphorus (P) from infertile soil. However, on very old and severely P-impoverished soils, a carboxylate-releasing and P-mobilising cluster-root strategy is more effective at acquiring this growth-limiting resource. Carboxylates are released during a period of only a few days from ephemeral cluster roots. Despite the cluster-root strategy being superior for P acquisition in such environments, these species coexist with a wide range of mycorrhizal species, raising questions about the mechanisms contributing to their coexistence. We surmise that the coexistence of mycorrhizal and non-mycorrhizal strategies is primarily accounted for by a combination of belowground mechanisms, namely (i) facilitation of P acquisition by mycorrhizal plants from neighbouring cluster-rooted plants, and (ii) interactions between roots, pathogens and mycorrhizal fungi, which enhance the plants’ defence against pathogens. Facilitation of nutrient acquisition by cluster-rooted plants involves carboxylate exudation, making more P available for both themselves and their mycorrhizal neighbours. Belowground nutrient exchanges between carboxylate-exuding plants and mycorrhizal N2-fixing plants appear likely, but require further experimental testing to determine their nutritional and ecological relevance. Anatomical studies of roots of cluster-rooted Proteaceae species show that they do not form a complete suberised exodermis. The absence of an exodermis may well be important to rapidly release carboxylates, but likely lowers root structural defences against pathogens, particularly oomycetes. Conversely, roots of mycorrhizal plants may not be as effective at acquiring P when P availability is very low, but they are better defended against pathogens, and this superior defence likely involves mycorrhizal fungi. Taken together, we are beginning to understand how an exceptionally large number of plant species and P-acquisition strategies coexist on the most severely P-impoverished soils.

Evolution of nitrogen-fixing symbioses on the basis of bacterial migration from mycorrhizal fungi and soil into plant tissues

A hypothesis is proposed on the emergence of N2-fixing plant symbionts from soil diazotrophs and satellites of Glomeromycota fungi that form arbuscular mycorrhizae (AM). This universal form of plant-microbe symbiosis possibly appeared due to the integration of ancestral land plants (rhyniophytes, psylophytes) and microbial consortia composed of AM-fungi assimilating soil phosphates and bacteria fixing atmospheric CO2 and/or N2. The release of these bacteria from AM-fungal hyphae into plant tissues elicited the selection of genotypes capable of fungi-independent multiplication in plants, as well as the fixation in bacterial genomes of the genes for the synthesis of chitin-like signal factors stimulating the development of symbiotic structures. An early stage of this evolution might been represented by the formation of N2-fixing syncyanoses, and the late stage might have been realized by the formation of nodular symbioses of dicots from the Eurosid I clade with rhizobia (α- and β-proteobacteria) and with the actinobacteria Frankia. The emergence of these symbioses was possibly based on the migration of soil and endophytic bacteria into the storage organs (modified stems or lateral roots), where the optimal conditions were established not only for N2 fixation but also for the evolution of bacteria towards increased symbiotic activity. This evolution resulted in the emergence of primary rhizobia (Bradyrhizobium, Burkholderia), which acted as donors of sym genes for a broad spectrum of microbes transformed into secondary rhizobia (Rhizobium, Sinorhizobium). The subsequent evolution of nodular symbioses was directed at an increased efficiency of symbiotrophic nitrogen nutrition in host plants following two scenarios: (a) “expensive,” which is based on an increase in N2-fixing activity via the transformation of bacteria into nonreproducible bacteroids; (b) “economical,” based on the acquisition of the determinate nodule structure and ureide nitrogen assimilation.

Mycorrhizae enhance nitrogen fixation and photosynthesis in phosphorus-starved soybean (Glycine max L. Merrill)

Nitrogen (N) and phosphorus (P) are nutrients that frequently limit plant performance and crop yield. Currently, awareness of the importance of plant-microbe interactions in plant nutrient acquisition and growth promotion persist, particularly under stressful conditions, such as those resulting from P scarcity. The aim of the present study was to evaluate the effects of P starvation on the influence of arbuscular mycorrhiza (AM) symbiosis on N2-fixing capacities, plant growth, nutrient uptake and photosynthetic performance of soybean plants at flowering and grain filling stages. To this end, soybean (Glycine max) plants were nodulated with Bradyrhizobium elkanii and inoculated or not with the AM fungus Glomus macrocarpum. The +AM and −AM plants were grown under low P conditions (50μM), and a control treatment with −AM plants under sufficient P concentration (500μM) was also performed (−AM+P). Plants were grown until the beginning of the flowering and another set till the grain filling stage. The results showed that mycorrhization improved soybean plant performance under P scarcity compared with non-AM associated plants under the same conditions. AM positively influenced growth and plant nodulation at both flowering and grain-filling stages, without significant changes in plant P content. AM symbiosis increased nitrogenase activity, P content in nodules and leaf N content, the maximum carboxylation rate of Rubisco and reduced the metabolic limitation of photosynthesis under P starvation, which indicated the AM stimulus to symbiotic N2 fixation (SNF) and photosynthesis. Under these conditions, mycorrhizal symbiosis could not completely meet soybean P demand compared with well-nourished soybean plants, which produced higher plant and nodules biomass. In conclusion, AM association in nodulated soybean plants is likely an important strategy for maintaining SNF functioning, conferring increased plant growth and better N status under P scarcity conditions and significantly attenuating the negative effects of low Pi in nodulated-soybean plants.

Spatial heterogeneity can resolve the nitrogen paradox of tropical forests.

Many tropical forests are characterized by large losses of plant-available forms of nitrogen (N), indicating that they are N rich, and by an abundance of plants capable of symbiotic N fixation. These N-fixing plants can fix enough N to drive N richness. However, biological N fixation (BNF) is more expensive than using plant-available N, so sustained BNF in N-rich soils appears to be a paradox. Here, we use spatially explicit ecosystem models to analyze the conditions under which spatial heterogeneity can induce simultaneous BNF and loss of plant-available N (hereafter, we call this combination "N-rich BNF"). Spatial movement of litter to neighboring plants' rooting zones can maintain N-rich BNF under a variety of conditions. For example, when N-fixers have higher N demand than non-fixers, N-fixers export N-rich litter to non-fixers, inducing large losses of plant-available N from the ecosystem, and receive N-poor litter from non-fixers, inducing BNF. BNF and N loss fluxes increase in proportion to the ratio of N-fixer litter N:P to non-fixer litter N:P, and also in proportion to the fraction of litter transferred out of a tree's rooting zone. Stoichiometric variability augments N-rich BNF, as does increasing the fraction of the landscape occupied by N-fixers, at least when they are rare. On the contrary, greater root overlap between neighbors and clumping of N-fixers diminish N-rich BNF. Finally, we examined how spatial litter transfer interacts with another mechanism that can sustain N-rich BNF, incomplete down-regulation of BNF. Spatial transfer and incomplete down-regulation can both sustain N-rich BNF, but they are compensatory rather than additive. These mechanisms can be distinguished by examining where N losses occur. Incomplete down-regulation of BNF leads to greater N loss under N-fixing trees, whereas spatial litter transfer leads to greater N loss under non-fixing trees. Along with time lags in regulating BNF, these results comprise a series of hypotheses that could help understand the N paradox of tropical forests.

Preinoculation of Soybean Seeds Treated with Agrichemicals up to 30 Days before Sowing: Technological Innovation for Large-Scale Agriculture.

The cultivation of soybean in Brazil experienced an expressive growth in the last decades. Soybean is highly demanding on nitrogen (N) that must come from fertilizers or from biological fixation. The N supply to the soybean crop in Brazil relies on the inoculation with elite strains of Bradyrhizobium japonicum, B. elkanii, and B. diazoefficiens, which are able to fulfill the crop's N requirements and enrich the soil for the following crop. The effectiveness of the association between N2-fixing bacteria and soybean plants depends on the efficacy of the inoculation process. Seed treatment with pesticides, especially fungicides or micronutrients, may rapidly kill the inoculated bacteria, affecting the establishment and outcome of the symbiosis. The development of technologies that allow inoculation to become a successful component of industrial seed treatment represents a valuable tool for the seed industry, as well as for the soybean crop worldwide. In this article, we report the results of new technologies, developed by the company Total Biotecnologia Indústria e Comércio S/A of Brazil, for preinoculation of soybean seeds with bradyrhizobia, in the presence of agrichemicals. Our results demonstrate improved bacterial survival for up to 30 days after inoculation, without compromising nodulation, N2-fixation, and yield in the field.

Carbon and nutrient dynamics in short-rotation coppice of poplar and willow in a converted marginal land, a case study in central France

Bioenergy woody biomass production shortens the life cycle of carbon and secures energy supplies. However, using fast growing trees in these woody crop systems such as in short rotation coppices (SRCs), implies an increasing risk of depleting the soil nutrient stocks by direct biomass removal and low nutrient return. The aim of this study was to quantify the impact of these woody crop systems after 4 years of growth on the soil quality (C and nutrient cycling) by monitoring tree components (i.e. shoot, litterfall, root system) instead of soils; C and nutrient contents in weed strips were also included into these analyses. Our results indicated that shoot biomass was similar in SRC-P and SRC-W 2 years after planting (4 t ha−1). However, after 4 years, willow outperformed poplar (21.9 vs. 8.6 t ha−1, respectively) removing until 30 % of K soil pool. Litterfall increased between first and second rotation coppices (0.85 vs. 4 t ha−1) similarly for SRC-P and SRC-W, indicating biomass exportation and return were strongly imbalanced in SRCs crop systems. N pool in weed strips represented a significant contribution to N cycling in these SRC woody crop systems due to the presence of N2-fixing plants. However, no differences were detected between poplar and willow crops. In low quality agricultural soils, which are the likely host for SRC implementation, we demonstrated that willow has a higher nutrient accumulation rate than poplar, particularly for K, Mg and P. Weed strips constitute an important reservoir of C and nutrients and should be studied further.

Nitrogen and phosphorus availability at early stages of soil development in the Damma glacier forefield, Switzerland; implications for establishment of N2-fixing plants

Di-nitrogen (N2)-fixing plants are absent in the pioneer stages of glacial forefields in the European Alps despite low amounts of nitrogen (N) in the soil. We aimed to evaluate whether symbiotic N2-fixation is needed to meet the N demand of plants during the early stages of soil formation, and how phosphorus (P) availability affects plant establishment. We measured total and available N and P in soil and N and P in the vegetation along the 137 year chronosequence in front of the Damma glacier (Switzerland). Available N as determined by in situ resin N decreased from the pioneer stage (<16 years since deglaciation) to the intermediate (57–80 years), likely resulting from increased plant N uptake and reduced funnelling of N by rocks. N concentrations in the vegetation were positively correlated to in situ resin N, and the N:P-ratio of the vegetation was negatively correlated with P concentrations in the vegetation. The N requirement of plants in the pioneer stage of soil development in the forefield is fulfilled by atmospheric N deposition being funnelled between rocks. N2-fixation is not needed as the vegetation is often limited by P, or co-limited by N and P.

Soil fertility and disturbance interact to drive contrasting responses of co-occurring native and nonnative species.

Some plant functional groups such as nonnative invasive and nitrogen (N)-fixing plants are widely thought to have consistent, coordinated differences in their functional traits relative to other groups such as native and non -N-fixing plants. Recent evidence suggests that these trait differences between groups can be context dependent, varying with environmental factors such as resource availability and disturbance. However, many previous comparisons among plant groups differing in invasion status have not standardized growth form between groups or have compared species that do not co-occur, which could result in invasion status per se being confounded with other factors. We determined growth and leaf functional trait responses of 20 co-occurring woody species, that is, five species within each of four functional groups (native N-fixers, native non -N-fixers, nonnative [invasive] N-fixers and nonnative [invasive] non-N-fixers), to factorial combinations of soil fertility and defoliation treatments in a mesocosm experiment to test each of two hypotheses. First, we hypothesized that nonnative invasive and N-fixing species will have functional traits associated with rapid resource acquisition whereas natives and non -N-fixing species will have traits linked to resource conservation. Second, we hypothesized that plant growth and leaf traits of nonnative and N-fixing species will be more strongly influenced by environmental factors (i.e., soil fertility and disturbance) than will natives and non-N-fixers. Plant growth, foliar nutrients, and leaf structural traits varied among plant functional groups in a manner consistent with our first hypothesis. Support for our second hypothesis was mixed; origin (native vs. nonnative) and soil fertility rarely interacted to determine plant growth or variation in leaf traits whereas interactions involving N-fixing ability and soil fertility were common. Further, there were no consistent interactive effects between plant groupings and disturbance. Our results demonstrate that variation in growth and functional traits among plant species were driven primarily by the relatively large responses of nonnative N-fixers to soil fertility, rather than by consistent differences between other plant functional groups. These findings highlight the importance of resource availability in determining trait or performance differences among plant functional groups, and provide insights into the assembly of plant functional traits in novel communities of co-occurring native and nonnative species.

Fertility of the Early Post-Eruptive Surfaces of Kasatochi Island Volcano

ABSTRACTIn the four years after the 2008 eruption and burial of Kasatochi Island volcano, erosion and the return of bird activity have resulted in new and altered land surfaces and initiation of ecosystem recovery. We examined fertility characteristics of the recently deposited pyroclastic surfaces, patches of legacy pre-eruptive surface soil (LS), and a post-eruptive surface with recent bird roosting activity. Pyroclastic materials were found lacking in N, but P, K, and other macronutrients were in sufficient supply for plants. Erosion and leaching are moving mobile P and Fe downslope to deposition fan areas. Legacy soil patches that currently support plants have available-N at levels (10–22 mg N kg−1) similar to those added by birds in a recent bird roosting area. Roosting increased surface available N from <1 mg N kg−1 in the new pyroclastic surfaces to up to 42 mg N kg−1 and increased soil biological respiration of CO2 from essentially zero to a level about 40% that of the LS surface. Laboratory plant growth trials using Lupinus nootkatensis and Leymus mollis indicated that the influence of eroded and redeposited LS in amounts as little as 10% by volume mixed with new pyroclastic materials could aid plant recovery by supplying vital N and soil biota to plants as propagules are introduced to the new surface. Erosion-exposure of fertile pre-eruptive soils and erosion-mixing of pre-eruptive soils with newly erupted materials, along with inputs of nutrients from bird activities, each will exert significant influences on the surface fertility and recovery pattern of the new post-eruptive Kasatochi volcano. For this environment, these influences could help to speed recovery of a more diverse plant community by providing N (LS and bird inputs) as alternatives to relying most heavily on N-fixing plants to build soil fertility.

Characterization of N2-fixing plant growth promoting endophytic and epiphytic bacterial community of Indian cultivated and wild rice (Oryza spp.) genotypes

The diversity of endophytic and epiphytic diazotrophs in different parts of rice plants has specificity to the niche (i.e. leaf, stem and root) of different genotypes and nutrient availability of the organ. Inoculation of the indigenous, polyvalent diazotrophs can facilitate and sustain production of non-leguminous crops like rice. Therefore, N2-fixing plant growth promoting bacteria (PGPB) were isolated from different parts of three Indian cultivated [Oryza sativa L. var. Sabita (semi deep/deep water)/Swarna (rain fed shallow lowland)/Swarna-Sub1(submergence tolerant)] and a wild (O. eichingeri) rice genotypes which respond differentially to nitrogenous fertilizers. Thirty-five isolates from four rice genotypes were categorized based on acetylene reduction assay on nitrogenase activity, biochemical tests, BIOLOG and 16S rRNA gene sequencing. The bacteria produced 9.36-155.83 nmole C2H4 mg(-1) dry bacteria h(-1) and among them nitrogenase activity of 11 potent isolates was complemented by nifH-sequence analysis. Phylogenetic analysis based on 16S rDNA sequencing divided them into five groups (shared 95-100 % sequence homology with type strains) belonging to five classes-alpha (Ancylobacter, Azorhizobium, Azospirillum, Rhizobium, Bradyrhizobium, Sinorhizobium, Novosphingobium, spp.), beta (Burkholderia sp.), gamma (Acinetobacter, Aeromonas, Azotobacter, Enterobacter, Klebsiella, Pantoea, Pseudomonas, Stenotrophomonas spp.) Proteobacteria, Bacilli (Bacillus, Paenibacillus spp.) and Actinobacteria (Microbacterium sp.). Besides, all bacterial strains possessed the intrinsic PGP traits of like indole (0.44-7.4 µg ml(-1)), ammonia (0.18-6 mmol ml(-1)), nitrite (0.01-3.4 mol ml(-1)), and siderophore (from 0.16-0.57 μmol ml(-1)) production. Inoculation of rice (cv. Swarna) seedlings with selected isolates had a positive impact on plant growth parameters like shoot and root elongation which was correlated with in vitro PGP attributes. The results indicated that the diverse polyvalent phytonic PGP bacteria, which may be exploited as bio-inoculants to improve rice production.

Do techniques based on 15N enrichment and 15N natural abundance give consistent estimates of the symbiotic dependence of N2-fixing plants?

The primary aim of this review is to determine if methods based on 15N enrichment (E) and 15N natural abundance (NA) give consistent estimates of the proportional dependence of N2-fixing species on biological N2 fixation (P atm), and secondly to attempt to explain any inconsistencies that may be found. Published estimates of the symbiotic dependence of N2-fixing plants based on E and NA techniques applied in the same experiment were compared across scales from glasshouse pots to field plots to landscapes in agricultural and forest ecosystems, which included grain legumes, pasture and forage legumes, and woody perennials. A meta-analysis of the published data was based on correlation coefficients, box-plots and confidence intervals of means. In some studies, estimates were reference plant dependent for both E and NA techniques, indicating temporal and/or spatial variations in the natural and artificial distribution of 15N, which can sometimes result in erroneous negative estimates of symbiotic dependence. While significant correlations were obtained between E and NA estimates of P atm for each of the three groups of N2-fixing species, the probability that the methods provided estimates of P atm within −5 to +5 % of each other was 0.29 or was 0.54 within −10 to +10 % of each other. We have identified a number of interacting factors that may contribute to the inconsistent agreement between estimates of P atm by E and NA techniques, which underlines the need for a re-examination of the fundamental assumptions on which each method is based, and whether those assumptions are valid in any given situation.

Influence of Forest Disturbance on Stable Nitrogen Isotope Ratios in Soil and Vegetation Profiles

Soil and plant stable N isotope ratios (δ15N) are influenced by atmospheric N2 inputs and processes that regulate organic matter (OM) transformation and N cycling. The resulting δ15N patterns may be useful for discerning ecosystem differences in N cycling. We studied two ecosystems, longleaf pine (Pinus palustris Mill.)–wiregrass (Aristida stricta Michx.) (LLP) and Appalachian hardwood (AHW) forests in the US Southeast under different management regimes. In LLP, burning removes OM. In AHW, clearcutting creates large OM pulses of logging residue. Although burning removes OM and clearcutting creates a pulse addition of OM, both management regimes increase soil N availability and N2–fixing plants. The LLP treatments included burning every 2 yr with N2 fixers and reference fire exclusion sites without N2 fixers. The AHW included 25-yr-old clearcut plots with and without N2 fixers, and uncut reference without N2 fixers. We hypothesized that: (i) compared with the reference, OM removal (LLP) would enrich δ15N values while OM addition (AHW) would deplete δ15N in soil and vegetation pools; and (ii) N2 fixers would mitigate δ15N enrichment in LLP response and exacerbate δ15N depletion in AHW. We examined total δ15N in soil profiles, tree increment cores, and foliage. The LLP soil and vegetation δ15N values showed no treatment effect. In AHW, δ15N values in clearcut subsurface soils (20–60 cm) were lower than the reference, but N2 fixers had no effect. Wood δ15N differed with treatment; N2 fixers had no effect. Our data suggest that AHW soil profile δ15N patterns may indicate past disturbance; however, wood and foliar δ15N response is species specific. Additionally, N2–fixing plants respond to ecosystem disturbance, but the data suggest that they play little role in soil or plant δ15N values.