N2-fixing Plants Research Articles

Accumulation in nutrient acquisition strategies of arbuscular mycorrhizal fungi and plant roots in poor and heterogeneous soils of karst shrub ecosystems

BackgroundArbuscular mycorrhizal (AM) fungi and roots play important roles in plant nutrient acquisition, especially in nutrient poor and heterogeneous soils. However, whether an accumulation strategy of AM fungi and root exists in such soils of karst shrubland ecosystems remains unclear. Root traits related to nutrient acquisition (root biomass, AM colonisation, root acid phosphatase activity and N2 fixation) were measured in two N2-fixing plants (i.e. Albizia odoratissima (Linn. f.) Benth. and Cajanus cajan (Linn.) Millsp.) that were grown in heterogeneous or homogeneous nutrient (ammonium) soil with and without AM fungi inoculation.ResultsBoth of these plants had higher AM colonisation, root biomass and relative growth rate (RGR), but lower N2 fixation and root acid phosphatase activity in the rhizosphere in the heterogeneous soil environment, than that in the homogeneous soil environment. Plants grown in the AM fungi-inoculated heterogeneous soil environment had increased root biomass and root acid phosphatase activity compared with those grown in soil without inoculation. AM colonisation was negatively correlated with the N2 fixation rate of A. odoratissima, while it was not significantly correlated with the root phosphatase activity.ConclusionsOur results indicated that enhanced AM symbiosis and root biomass increased the absorptive surfaces for nutrient acquisition, highlighting the accumulation strategies of AM and root traits for plant nutrient acquisition in nutrient poor and heterogeneous soils of the karst shrubland ecosystem.

Contrasting responses of soil phosphorus pool and bioavailability to alder expansion in a boreal peatland, Northeast China

Dinitrogen (N2)-fixing woody plants, mainly alder (Alnus) species, have widely expanded to boreal peatlands. However, little is known about the effect of N2-fixing plant expansion on soil phosphorus (P) dynamics and its potential mechanisms in these ecosystems. To clarify the response of soil P status to N2-fixing plant expansion, we compared the differences in aboveground plant biomass, soil total P pool, acid phosphatase activity, P sorption index, and four bioavailable P fractions (CaCl2, citrate, phosphatase enzyme, and HCl extractable P fractions) in the 0–10 cm, 10–20 cm, and 20–40 cm depths between A. sibirica islands and adjacent open peatlands in a boreal peatland, Northeast China. While alder expansion hardly affected bulk density and associated volume of the peats, it strongly decreased total P pool in the 0–40 cm soil depth because of approximately ninefold increases in aboveground plant biomass and associated P transfer from soils to plants. Soil total P concentration increased in the 0–10 cm depth, but declined in the 10–20 cm and 20–40 cm depths. At each depth, alder expansion increased soil acid phosphatase activity, and CaCl2 and enzyme extractable P fractions, despite unchanged P sorption index and citrate extractable P fraction. Both CaCl2 and enzyme extractable P fractions correlated positively with acid phosphatase activity, indicating that increased P bioavailability was primarily caused by elevated microbial mineralization of organic P. These findings highlight the contrasting responses of soil P pool and bioavailability to N2-fixing tree expansion, and suggest that N2-fixing tree expansion would reduce soil P pool via enhanced P bioavailability and subsequently increased plant P uptake in boreal peatlands.

Terrestrial-type nitrogen-fixing symbiosis between seagrass and a marine bacterium

Symbiotic N2-fixing microorganisms have a crucial role in the assimilation of nitrogen by eukaryotes in nitrogen-limited environments1–3. Particularly among land plants, N2-fixing symbionts occur in a variety of distantly related plant lineages and often involve an intimate association between host and symbiont2,4. Descriptions of such intimate symbioses are lacking for seagrasses, which evolved around 100 million years ago from terrestrial flowering plants that migrated back to the sea5. Here we describe an N2-fixing symbiont, ‘Candidatus Celerinatantimonas neptuna’, that lives inside seagrass root tissue, where it provides ammonia and amino acids to its host in exchange for sugars. As such, this symbiosis is reminiscent of terrestrial N2-fixing plant symbioses. The symbiosis between Ca. C. neptuna and its host Posidonia oceanica enables highly productive seagrass meadows to thrive in the nitrogen-limited Mediterranean Sea. Relatives of Ca. C. neptuna occur worldwide in coastal ecosystems, in which they may form similar symbioses with other seagrasses and saltmarsh plants. Just like N2-fixing microorganisms might have aided the colonization of nitrogen-poor soils by early land plants6, the ancestors of Ca. C. neptuna and its relatives probably enabled flowering plants to invade nitrogen-poor marine habitats, where they formed extremely efficient blue carbon ecosystems7.

The Effect of Low Irradiance on Leaf Nitrogen Allocation and Mesophyll Conductance to CO2 in Seedlings of Four Tree Species in Subtropical China.

Low light intensity can lead to a decrease in photosynthetic capacity. However, could N-fixing species with higher leaf N contents mitigate the effects of low light? Here, we exposed seedlings of Dalbergia odorifera and Erythrophleum fordii (N-fixing trees), and Castanopsis hystrix and Betula alnoides (non-N-fixing trees) to three irradiance treatments (100%, 40%, and 10% sunlight) to investigate the effects of low irradiance on leaf structure, leaf N allocation strategy, and photosynthetic physiological parameters in the seedlings. Low irradiance decreased the leaf mass per unit area, leaf N content per unit area (Narea), maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), light compensation point, and light saturation point, and increased the N allocation proportion of light-harvesting components in all species. The studied tree seedlings changed their leaf structures, leaf N allocation strategy, and photosynthetic physiological parameters to adapt to low-light environments. N-fixing plants had a higher photosynthesis rate, Narea, Vcmax, and Jmax than non-N-fixing species under low irradiance and had a greater advantage in maintaining their photosynthetic rate under low-radiation conditions, such as under an understory canopy, in a forest gap, or when mixed with other species.

Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation.

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N2 -fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N2 -fixation with the functioning of vascular plants. Neglecting nonvascular N2 -fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N2 -fixing plants are more common. Adequately accounting for nonvascular N2 -fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N2 -fixation responses to global change factors (elevated CO2 , N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N2 -fixation using nine Earth system models. The majority of empirical studies found positive effects of CO2 , warming, precipitation, or light on nonvascular N2 -fixation, but N deposition strongly downregulated N2 -fixation in most empirical studies. Furthermore, we found that the responses of N2 -fixation to elevated CO2 were generally consistent between models and very limited empirical data. In contrast, empirical-model comparisons suggest that all models we assessed, and particularly those that scale N2 -fixation with net primary productivity or evapotranspiration, may be overestimating N2 -fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N2 -fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N2 -fixation and that field studies include more targeted measurements to improve model structures and parameterization.

Activity of phosphatases and microbial phosphorus under various tree species growing on reclaimed technosols

Tree species forming a symbiosis with N-fixing bacteria (N-fixing plants) may alleviate P deficiency in afforested technosols by increasing activities of phosphatases. The objective of this study was to compare the effect of rhizobial and actinorhizal N-fixing tree species and non-N-fixing species on the activity of phosphatases in various technosols. Soil samples were taken at two depths (0 – 5 cm and 5 – 20 cm) under black locust (Robinia pseudoaccaccia), black alder (Alnus glutinosa), silver birch (Betula pendula) and Scots pine (Pinus sylvestris) stands growing on technosols developing from Quaternary sands, fly ashes after lignite combustion as well as acidic and alkaline Miocene clays. The samples were measured for the contents of organic carbon (Corg), total nitrogen (Nt) and total phosphorus (Pt), texture and pH in H2O and KCl. Microbial analyses included determination of microbial biomass carbon (Cmic) and phosphorus (Pmic), basal respiration rate (RESP) as well as activities of acid and alkaline phosphatase and inorganic pyrophosphatase. Enzyme activities were expressed per soil dry (AcdPho, AlkPho, PyroPho for acid phosphatase, alkaline phosphatase and and inorganic pyrophosphatase, respectively) and per unit of Cmic as specific enzyme activities (AcdPhoSP, AlkPhoSP and PyroPhoSP for acid, alkaline and inorganic pyrophosphatase, respectively). The soils under black locust and alder exhibited higher AcdPho than the soils under non-N-fixing trees. The soils under black locust had also higher AcdPhoSP, AlkPhoSP and PyroPhoSP values and the soils under alder AcdPhoSP compared with the soils under pine and birch. The high Nt content was the major factor positively affecting absolute activities of phosphatases. However, high values of specific activities suggest a strong competition for soil P between N-fixing plants and soil microorganisms. In particular, the high P demand of black locust resulted in more intense microbial release of phosphatases and greater retention of P in microbial biomass. Our study indicated that both N-fixing tree species affected microbial release of enzymes involved in P cycling but the influence of rhizobial black locust was stronger than the influence of actinorhizal alder.

Stand age, degree of encroachment and soil characteristics modulate changes of C and N cycles in dry grassland soils invaded by the N2-fixing shrub Amorpha fruticosa

The N2-fixing shrub Amorpha fruticosa L. is rapidly spreading in the dry riparian natural grasslands of Europe, altering ecosystem functions and depleting plant diversity. Alteration of the N cycle represents the key factor involved in invasions by N2-fixing plants with cascading effects on plant species richness. We hypothesized that A. fruticosa encroachment strongly impacts not only the N but also the C cycle and that the magnitude of such alterations may be modulated by soil characteristics. To test these hypotheses, we selected four river floodplains in North East of Italy and compared natural uninvaded grasslands with half invaded and completely invaded sites, based on A. fruticosa stand characteristic and relevant leaf traits and on soil properties related to soil texture and to C and N cycles. Soil organic matter mineralisation, ammonification and nitrification rates were determined. Soil nitrification increased remarkably with plant invasion while ammonification was significantly higher only in half invaded sites. Soil organic matter mineralisation, microbial biomass C sustained per soil organic C unit and nitrification positively correlated with stand age, regardless to the stage of the encroachment. Mineralisation and nitrification increased with soil organic C and total N in uninvaded and completely invaded sites, but decreased in half invaded sites. At the half invasion stage, trends in nitrification and CO2 mineralisation were transitionally reverted and remediation may be facilitated by less pronounced changes in soil properties compared to completely invaded sites. Direct effects of plant invasion are modulated by the action of soil characteristics such as soil organic C and clay contents, with soils rich in organic C showing larger nitrification and mineralisation rates.

Biodiversity effects on grape quality depend on variety and management intensity.

Interactions between plants can be beneficial, detrimental or neutral. In agricultural systems, competition between crop and spontaneous vegetation is a major concern. We evaluated the relative support for three non‐exclusive ecological hypotheses about interactions between crop and spontaneous plants based on competition, complementarity or facilitation.The study was conducted in Swiss vineyards with different vegetation management intensities. In all, 33 vineyards planted with two different grape varieties were studied over 3 years to determine whether low‐intensity vegetation management might provide benefits for grape quality parameters. Management intensity varied with the degree of control of spontaneous inter‐row vegetation. Features of spontaneous vegetation measured included total cover, total species richness and abundance of nitrogen‐fixing plants. Grape quality parameters of known importance to wine making (yeast assimilable nitrogen, sugars, tartaric acid and malic acid) were determined by Fourier‐transform infrared spectroscopy (FTIR). Using structural equation modelling, we evaluated hypotheses about the multivariate responses of grape quality parameters as well as the direct and indirect (plant‐mediated) effects of management.Observed effects of management differed between grape varieties. Management intensity and abundance of N‐fixing plants significantly influenced grape quality parameters while total richness of spontaneous plants did not have detectable effects. Abundance of N‐fixing plants was enhanced by low‐intensity management resulting in increased N content in the red grape variety Pinot noir, potentially enhancing grape quality, while measured soil N content did not explain the increase.Synthesis and applications. Our study shows that crop quality can be enhanced by spontaneous plants, in this case by the abundance of a key functional group (N‐fixers), most likely through plant–plant or plant–microbe facilitation. However, beneficial interactions may have a high specificity in terms of facilitation partners and may have contrasting effects at low taxonomic resolutions such as crop varieties. Generally, increasing plant biodiversity in agricultural systems may increase competition with crops. Thus, the identification of suitable interaction partners and a careful balance between crop variety and spontaneous plant species may be necessary to utilize beneficial interactions and to reduce the trade‐off between agricultural production and biodiversity to achieve a sustainable ecological benefit in agricultural systems.

Biogeochemical stoichiometry of soil and plant functional groups along a primary successional gradient following glacial retreat on the eastern Tibetan plateau

Di-nitrogen (N2)-fixing plants are crucial for vegetation development following glacial retreat; however, there is currently limited understanding of how the shifting of N2-fixing and non N2-fixing plant groups may influence the stoichiometric composition of the developing plant community and associated soils. Carbon (C), nitrogen (N), and phosphorus (P) concentrations and elementary stoichiometry were investigated in soil and the N2-fixing and non-N2-fixing plant communities across a 127-yr glacial primary succession chronosequence on the eastern slope of the Qinghai-Tibetan Plateau. Soil organic C (SOC), total N (TN), and available P (AP) gradually increased along succession, but total P (TP) was greatest at the 37-yr site, resulting in an increasing trend of soil N:P and a decreasing trend of N:AP ratios along the chronosequence. N2-fixing species had higher foliar N and P concentrations than non-N2-fixing species and exhibited a higher leaf N:P ratios in early stages (37 yr and younger) of the succession. Elementary concentration and C, N, P stoichiometry in plant leaves were strongly associated with both N2-fixing and non-N2 fixing plant biomasses, while those in soils were related only with the biomass of non-N2 fixing species. The community-weighted leaf N:P ratios increased from 14.0 at 7-yr to 19.9 at 59-yr and then decreased to 12.9 at 127-yr after glacial retreat, suggests the ecosystem as a whole is not N- or P-limited at the beginning of primary succession, but may become P-limited at the middle stages and then shift towards N limitation at the late successional stages. The results confirm that both N2-fixing and non-N2-fixing species can overcome N limitation in the early primary succession and highlights the different roles of pioneer N2-fixing and non-N2-fixing species in mediating vegetation and soil C, N and P stoichiometry with primary succession following glacial retreat.

Optimizing foliar allocation of limiting nutrients and fast‐slow economic strategies drive forest succession along a glacier retreating chronosequence in the eastern Tibetan Plateau

Nitrogen (N) and phosphorus (P) availabilities limit plant productivity, especially in primary succession; however, our understanding of species-specific strategies regarding their allocation and coordination with other functional traits remains limited. Community-weighted mean traits were compared to decipher the ecophysiological mechanisms of forest succession in nine dominant species along 120-y successional stages across a glacier-retreating chronosequence. High foliar N and P concentrations in N2-fixing plants on a 12-year-old surface did not result in high photosynthetic capacity due to the inefficient allocation, as indicated by their low photosynthetic N- and P-use efficiencies. On a 40-year-old surface, exploitative strategies, manifested in a higher specific leaf area and greater N allocation to Rubisco, as well as quick-return energy economics, helped deciduous forests dominate. When P availability decreased on a 120-year-old surface, evergreens maintained high photosynthetic P-use efficiency, by reducing overall P concentration and its allocation to structural fraction. Efficient P allocation and a higher ratio of leaf lifespan to payback time facilitated the dominance of evergreens in low P-sites. Optimizing allocation of limiting N or P among foliar fractions and fast–slow economic strategies drive primary succession after glacier retreat. Integrating the above- and below-ground subsystems through food webs will provide further insights into ecosystem dynamics.

Energetic supply regulates heterotrophic nitrogen fixation along a glacial chronosequence

Ice-free areas are expanding on the Tibetan Plateau and in its surroundings, and these recently exposed lands are typically deficient in nitrogen (N). Free-living N fixation (FLNF) represents a vital source of N input, especially when symbiotic N-fixing plants are absent. However, we know little about how FLNF changes with ecosystem succession after glacier retreat. In 2018, we measured the potential rates of FLNF in soil profiles at four sites (exposed since 2012, 1980, 1970 and 1930, respectively) along the Hailuogou glacier chronosequence and at a reference site with well-developed Podosols. We also tested the role of nifH gene abundance and resource availability in regulating FLNF rates. The results showed that photosynthetic cyanobacteria likely contributed the majority of FLNF in the biological soil crusts (exposed since 2012), while heterotrophic FLNF may prevail at later successional stages with closed canopies. FLNF rates were the highest in the litter layer of the soil profiles and decreased with soil depth. In the litter layer, FLNF rates were lower at the site dominated by N-fixing shrubs (exposed since 1980) than at the older sites dominated by non-N-fixing trees (exposed since 1970 and 1930, and the reference site). Carbon (C) availability was the major factor explaining variation in heterotrophic FLNF rates measured in the lab without light, while the effect of nifH gene abundance was nonsignificant. Overall, our results highlighted the energetic control on heterotrophic FLNF rates in relatively young ecosystems, which likely resulted in the poor relationship between nifH gene abundance and heterotrophic FLNF rates. The findings will help in understanding and modeling the coupled C and N cycles in glacier forefield ecosystems which are being exposed at accelerated rates.

Exploiting genetic diversity and gene synthesis to identify superior nitrogenase NifH protein variants to engineer N2-fixation in plants

Engineering nitrogen fixation in eukaryotes requires high expression of functional nitrogenase structural proteins, a goal that has not yet been achieved. Here we build a knowledge-based library containing 32 nitrogenase nifH sequences from prokaryotes of diverse ecological niches and metabolic features and combine with rapid screening in tobacco to identify superior NifH variants for plant mitochondria expression. Three NifH variants outperform in tobacco mitochondria and are further tested in yeast. Hydrogenobacter thermophilus (Aquificae) NifH is isolated in large quantities from yeast mitochondria and fulfills NifH protein requirements for efficient N2 fixation, including electron transfer for substrate reduction, P-cluster maturation, and FeMo-co biosynthesis. H. thermophilus NifH expressed in tobacco leaves shows lower nitrogenase activity than that from yeast. However, transfer of [Fe4S4] clusters from NifU to NifH in vitro increases 10-fold the activity of the tobacco-isolated NifH, revealing that plant mitochondria [Fe-S] cluster availability constitutes a bottleneck to engineer plant nitrogenases.

INOCULATION WITH N2-FIXING PLANT GROWTH PROMOTING RHIZOBACTERIA TO REDUCE NITROGEN FERTILIZER REQUIREMENT OF LETTUCE

This study was undertaken to determine the effect of three different mixtures of some N2-fixing plant growth-promoting rhizobacteria (PGPR) on growth, yield, element content and nitrate accumulation as well as the effect on the reduction of nitrogen fertilization of lettuce (Lactuca sativa L.). The measurements were made in two separate experiments in 2015 between 6 June – 5 August (Experiment 1) and 2 July – 3 September (Experiment 2) under the field conditions. Butterhead form and heat tolerant summer cultivar ‘Luna’ was used as a plant material. Agrobacterium rubi RK-34, Pantoea agglomerans RK-79 and RK-92, Pseudomonas putida RK-142 and TV-42A, Bacillus megaterium TV-6D, TV-60D and TV-91C, Pseudomonas flourescens TV-11D and Paenibacillus polymyxa TV-12E were used as N2-fixing plant growth promoting rhizobacteria. The treatments were 150 kg N/ha (available dose of AS) as ammonium sulphate (AS) [(NH4)2SO4), (21% N)] and three different mixtures of PGPR. Further, combined uses of decreasing doses of AS (50%, 75 kg ha–1AS and 75%, 112.5 kg ha–1 AS) and PGPR mixtures (M) such as M-1 + 75 AS, M-1 + 112.5 AS, M-2 + 75 AS, M-2 + 112.5 AS, M-3 + 75 AS and M-3 + 112.5 AS were used as additional treatments. All treatments increased the yield and the growth of lettuce according to the control. While inoculation with PGPR mixtures decreased the accumulation of heavy metals such as Cd, Ni, and Pb in lettuce, increased nutrient uptake of lettuce. It was determined that the nitrate accumulation of lettuce (cv. ‘Luna’) in PGPR mixtures were lower than the available dose of AS but higher than control. The yield in M-3 + 112.5 AS (48431 kg ha–1) was similar and in the same statistical group with the available dose of AS (48225 kg ha–1) in both experiments. Furthermore, according to the results of cost analyses, using 25% less of AS (112.5 kg ha–1) with M-3 will supply the same income instead of using AS (150 kg ha–1). It can be clearly said that the mixtures with some N2-fixing plant growth promoting rhizobacteria (PGPR), especially M-3 (P. putida RK-142 + P. flourescens TV-11D + B. megaterium TV-91C), have a great potential to decrease the nitrogen use (25%) for environmentally friendly crop production of lettuce.

Effects of invasive N2-fixing Acacia mearnsii on sediment nutrient concentrations in mountain streams: Implications of sediment geochemistry for ecosystem recovery

Widespread invasive nitrogen-fixing plant species pose major threats to water resources, biodiversity and nutrient dynamics of river catchments around the world. However, the impacts of invasive N2-fixing plants on in-stream sediment nutrient dynamics remain poorly understood. Here, we quantified the impacts of invasive N2-fixing Acacia mearnsii and A. mearnsii clearing on two mountain streams in South Africa’s Cape Floristic Region. Nutrients were measured in fine sediment that had infiltrated into the gravel bed and surface water in three reaches associated with natural fynbos shrubs, A. mearnsii invaded and cleared riparian ecotones. Results showed that the impacts of A. mearnsii invasions and Acacia clearing in riparian areas on in-stream sediment-associated nutrient concentrations are context-dependent. The difference in channel morphometry and sediment geochemical attributes at reach-scale contributed to the contrasting spatial patterns of sediment-adsorbed nutrients within these rivers. There was also an indication of a long-lasting effect of invasion on total phosphorus that could persist in river sediments ≥10 years after the removal of dense A. mearnsii stands in close proximity to streams. This could be a result of the complexation between phosphorus and immobile iron in sediment, resulting in the retention of ‘legacy phosphorus’. This underlines the importance of fine sediments in capturing nutrients and ultimately regulating nutrient concentrations in the water column.

Nitrate-mediated maintenance of photosynthetic process by modulating hypoxic metabolism of common bean plants

Nitrate has been reported to improve tolerance of plants under flooding by modulating carbon and nitrogen metabolism of root cells; however, the extent to which nitrate modulates the photosynthetic process is not well understood. This work aimed to evaluate the photosynthetic process by nitrate-mediated modulation of hypoxic metabolism in the root of common bean plants (Phaseolus vulgaris L.) under flooding and recovery conditions. Three groups of common beans [N2-fixing (N2)], N2-fixing pre-hypoxic nitrate-treated (N2 + NO3−), and nitrate (NO3−)-supplied plants] were grown in vermiculite and the root system was subjected to flooding for 48 h at the early reproductive stage. After flooding, plants returned to normoxic conditions per 24 and 72 h for recovery. Plants from N2 + NO3− and mainly NO3− groups maintained transpiration rate (E) and CO2 assimilation under root-flooding, which was not shown by N2-fixing plants. No changes in the dynamic dissipation of photosynthetic energy were evidenced in leaves of NO3− plants upon flooding, besides an increase in nitrate reductase (NR) activity and a decrease in fermentative enzymes in roots. Reactive oxygen species (ROS) and antioxidative enzymes increased in leaves and decreased in roots. Nitrate-mediated maintenance of the photosynthetic process may be related to induction in NR activity in roots to alleviate the toxic effects of fermentation through a decrease of fermentative enzyme activity.

Differences in the soil microbiomes of Pentaclethra macroloba across tree size and in contrasting land use histories

Legacy attributes from land-use history have lingering effects on soil and its below-ground components undergoing succession that has important consequences for regenerating tropical secondary forests. Yet, even landscapes of similar origins with analogous land-use histories have exhibited differing routes of forest recovery with different outcomes. There is increasing evidence that tree species-generated soil microbial heterogeneity is an important factor in facilitating regeneration, particularly nitrogen (N)-fixing tree species. However, it is unclear how land-use history influences the soil microbiome of important N-fixing plants developing under these conditions; at different life stages of N-fixing plant development; and how this compares to a primary forest. We examined differences in composition of the soil bacterial and fungal communities and their determinants (i.e. soil environmental factors) associated with large-, medium-, and small-sized Pentaclethra macroloba trees in a primary forest and in a 23-year-old secondary forest with contrasting land-use histories. We show that as Pentaclethra increases in size (and/or age), the soil microbiome associated with Pentaclethra also changes, and that these soil microbiomes can become similar even when developed in soils of contrasting land-use histories. We found that soil NH4+ and NO3− explained 61% of the variation in the soil bacterial community composition of small trees between the primary and secondary forest. This highlights the importance of inorganic N during tree soil microbiome development in contrasting land-use history of soils. Our findings suggest that legacy effects on may be mediated through size (and/or age) of Pentaclethra and its associated soil microbiome.

PRDI can maintain aboveground biomass and increase economic benefits in alfalfa through regulating N:P ratios in roots and leaves

N:P ratios is often used to identify nitrogen and/or phosphorus limitation status of crop growth and further provides optimal fertilizer management for crop production. Although partial root-zone drying irrigation (PRDI) has been verified to influence N:P ratios in nitrophilic and nitrogen-demanding plants, the effect of PRDI on N:P ratios in N-fixing plants is not well documented. This study, conducted in 2014–2016, focused on alfalfa (Medicago sativa L.) to investigate the effect of PRDI on N:P ratios in roots, stems and leaves of N-fixing plant and on the resulting net economics. The field experiment was designed with two irrigation modes: PRDI and conventional furrow irrigation (CFI); and four irrigation volumes: 70 % ETc (evapotranspiration), 85 % ETc, 100 % ETc, 115 % ETc. PRDI maintained aboveground biomass, and it increased both economic benefits and N:P ratios in alfalfa roots, stems and leaves. PRDI transitioned alfalfa plants from N and P co-limitation to P limitation. The N:P ratios in roots were most useful for identifying N and/or P limitation status in alfalfa plants in a PRDI environment. A structural equation model (SEM) showed that irrigation mode indirectly impacted aboveground biomass by positively affecting N:P ratios in roots and leaves, and irrigation volume directly affected aboveground biomass. This study further found that the optimal combination were PRDI with 97 % ETc irrigation volumes in alfalfa production. These results suggest a mechanism for N-fixing plant adaptation to the changing environment caused by PRDI and provide useful information for water and nutrient management in alfalfa production.

Concurrent carbon and nitrogen transfer between hemiparasite Santalum album and two N2-fixing hosts in a sandalwood plantation

Root hemiparasites feed on host root xylem and prefer N2-fixing hosts. Carbon (C) and nitrogen (N) translocation from hosts to hemiparasites affect growth and development, but detailed C and N economies between hemiparasites and N2-fixing hosts are unknown. In a 6-year-old Santalum album (sandalwood) plantation, concurrent movement of C and N between plants over 14 days were traced among N2-fixing Acacia confusa, N2-fixing Dalbergia odorifera (rosewood) and non-N2-fixing hemi-parasitic sandalwood by adding 13C-labeled and 15N-labeled urea to leaves of one species (donor) and tracing the label into other two species (receivers) within three triple-tree combinations. Haustorial connections and host selectivity were also examined on roots and nodules within adjacent plants. N2 fixation by acacia and rosewood was quantified in the stand by 15N natural abundance. Parasitic haustoria of sandalwood on roots and nodules of N2-fixing acacia and rosewood were more frequent than on other non-N2-fixing herbaceous plants, indicating strong host selectivity and preference. N2 fixation supplied 19.7% and 52.3% of the total N in acacia and rosewood, respectively. During 14 days of labeling, more 13C and 15N was transferred from acacia or rosewood to sandalwood than from sandalwood to these two N2 fixers. Net C and N transfer between tree pairs indicated that hemiparasite sandalwood was the C sink or N sink when associated with N2-fixing acacia or rosewood, while N2-fixing acacia was the C sink or N sink when associated with N2-fixing rosewood. This study indicated that mixed plantations had high rates of interplant nutrient transfer. Our results also showed that the herbaceous plants might acquire some C and N from sandalwood and two N2-fixing plants via both direct and indirect transfers, suggesting a nutrient translocationbetween plants within this community. This mixed plantation of acacia/rosewood/sandalwood could replace low-valued single species or mixed plantations for growing both high-value sandalwood and rosewood heartwoods. Our study quantified nutrient transfer among plants in a sandalwood plantation. These results have improved our understanding of the physiological interactions in resource acquisition between host species and parasitic plants and provided insights into promising mixed-plantation patterns for valuable tropical trees in south China and globally.

Effects of pioneer N2-fixing plants on the resource status and establishment of neighboring non-N2-fixing plants in a newly formed glacier floodplain, eastern Tibetan Plateau

The ecological roles of dinitrogen-fixing plants (N2-fixers) in the early stage of primary succession remain unclear. We aimed to examine the effects of pioneer N2-fixers on the resource status and establishment of neighboring non-N2-fixing plants (non-fixers) in a newly formed glacier floodplain, eastern Tibetan Plateau. We set up three plots in the floodplain and assessed the abundance, height, and coverage of two N2-fixing and three non-fixing species. We also collected plant and soil samples for the analyses of nutrient status and stable isotope signatures. The two N2-fixing species acquired > 80% of the nitrogen (N) from N2 fixation. The cushion-forming N2-fixer Astragalus mahoshanicus increased the soil nutrient availability compared with the non-fixers and improved the microclimate beneath the cushions. Natural 15N abundance indicated significant transfer of N from A. mahoshanicus to its neighboring non-fixers. Consequently, the neighboring non-fixers were significantly higher in N concentration, above-ground to below-ground biomass ratio and height and density than the non-fixers growing alone, but more deficient in some rock-derived nutrients. The other N2-fixer Hippophae rhamnoides (in seedling stage) generally did not increase the resource availability, and its neighboring non-fixers were similar in height and lower in density than the non-fixers growing alone. The effects of pioneer N2-fixers on the resource status and establishment of neighboring non-fixers in the subalpine floodplain were species dependent and were related to the life history traits of the N2-fixers. A. mahoshanicus has the potential to be used in the restoration of newly exposed land in subalpine regions.

High Temperatures During the Seed-Filling Period Decrease Seed Nitrogen Amount in Pea (Pisum sativum L.): Evidence for a Sink Limitation.

Higher temperatures induced by the on-going climate change are a major cause of yield reduction in legumes. Pea (Pisum sativum L.) is an important annual legume crop grown in temperate regions for its high seed nitrogen (N) concentration. In addition to yield, seed N amount at harvest is a crucial characteristic because pea seeds are a source of protein in animal and human nutrition. However, there is little knowledge on the impacts of high temperatures on plant N partitioning determining seed N amount. Therefore, this study investigates the response of seed dry matter and N fluxes at the whole-plant level (plant N uptake, partitioning in vegetative organs, remobilization, and accumulation in seeds) to a range of air temperature (from 18.4 to 33.2°C) during the seed-filling-period. As pea is a legume crop, plants relying on two different N nutrition pathways were grown in glasshouse: N2-fixing plants or NO3−-assimilating plants. Labeled nitrate (15NO3−) and intra-plant N budgets were used to quantify N fluxes. High temperatures decreased seed-filling duration (by 0.8 day per °C), seed dry-matter and N accumulation rates (respectively by 0.8 and 0.032 mg seed−1 day−1 per °C), and N remobilization from vegetative organs to seeds (by 0.053 mg seed−1 day−1 per °C). Plant N2-fixation decreased with temperatures, while plant NO3− assimilation increased. However, the additional plant N uptake in NO3−-assimilating plants was never allocated to seeds and a significant quantity of N was still available at maturity in vegetative organs, whatever the plant N nutrition pathway. Thus, we concluded that seed N accumulation under high temperatures is sink limited related to a shorter seed-filling duration and a reduced seed dry-matter accumulation rate. Consequently, sustaining seed sink demand and preserving photosynthetic capacity of stressed plants during the seed-filling period should be promising strategies to promote N allocation to seeds from vegetative parts and thus to maintain crop N production under exacerbated abiotic constraints in field due to the on-going climate change.