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Summary We are facing major biodiversity loss and there is evidence that such loss can alter ecosystem functioning. However, the effects of plant diversity on decomposition – a key component of the global carbon cycle – are still unclear. A recent study suggested that a plant trait – their nitrogen (N)‐fixing capacity – could mediate effects of litter diversity on decomposition by means of a microbial transfer of N from N‐fixers to non‐fixers. We explored this possibility in a microcosm experiment in which we manipulated litter species richness (one, two or four species), N‐fixing capacity (N‐fixer or non‐fixer species), the presence of detritivores (Sericostoma pyrenaicum larvae present or absent) and water N concentration [natural stream water (0·366 mg L−1 of NO3‐N) or elevated N concentration (five times the natural concentration: 1·835 mg L−1)]. We show that litter diversity accelerated decomposition by micro‐organisms and detritivores (by 7% and 15% respectively), mostly through complementarity effects. However, enhanced decomposition did not result in higher detritivore growth, possibly because all litter combinations provided sufficient resources for their maximum growth. The plant N‐fixing capacity had no effect on decomposition, which varied among species most likely because of differences in a combination of litter traits. Detritivores maximized the consumption of their preferred resource in litter mixtures, but also exploited less preferred resources, and their C : N ratios increased during the experiment regardless of litter type or water N concentration. Microbial decomposition of litter with low N content was enhanced at elevated water N concentration, suggesting that micro‐organisms used nutrients from the water when those nutrients were limiting in leaf litter. In contrast, detritivore growth was impaired at elevated water N concentration, possibly because a stoichiometric imbalance entails metabolic costs. Our findings suggest that loss of plant diversity in riparian forests would mostly affect decomposition in streams of high nutrient status, where effects on microbial decomposition would be more evident and detritivore populations may be reduced. A lay summary is available for this article. Lay Summary

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 -Nfixers, 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.

Nitrogen fixation by leguminous plants is especially important when farmers are trying to minimise fertilizer use for cost or environmental reasons. This second edition of the highly successful book, first published in 1991, contains thoroughly updated and revised material on the theory and practice of nitrogen fixation in tropical cropping systems.

Improving the ability of plants and plant-associated organisms to fix and assimilate atmospheric nitrogen has inspired plant biotechnologists for decades, not only to alleviate negative effects on nature from increased use and availability of reactive nitrogen, but also because of apparent economic benefits and opportunities. The combination of recent advances in synthetic biology and increased knowledge about the biochemistry and biosynthesis of the nitrogenase enzyme has made the seemingly remote and for long unreachable dream more possible. In this review, we will discuss strategies how this could be accomplished using biotechnology, with a special focus on recent progress on engineering plants to express its own nitrogenase.

In agroecosystems, nitrogen is one of the major nutrients limiting plant growth. To meet the increased nitrogen demand in agriculture, synthetic fertilizers have been used extensively in the latter part of the twentieth century, which have led to environmental challenges such as nitrate pollution. Biological nitrogen fixation (BNF) in plants is an essential mechanism for sustainable agricultural production and healthy ecosystem functioning. BNF by legumes and associative, endosymbiotic, and endophytic nitrogen fixation in non-legumes play major roles in reducing the use of synthetic nitrogen fertilizer in agriculture, increased plant nutrient content, and soil health reclamation. This review discusses the process of nitrogen-fixation in plants, nodule formation, the genes involved in plant-rhizobia interaction, and nitrogen-fixing legume and non-legume plants. This review also elaborates on current research efforts involved in transferring nitrogen-fixing mechanisms from legumes to non-legumes, especially to economically important crops such as rice, maize, and wheat at the molecular level and relevant other techniques involving the manipulation of soil microbiome for plant benefits in the non-legume root environment.

Aim Plant invasions are a major threat to native biodiversity, particularly on island ecosystems, which host unique species often less equipped for defence and competition. Native floras on islands have been shown to be underrepresented in plant species associating with nitrogen‐fixing bacteria (N‐fixing plants), consistent with limited availability of N‐fixing bacterial symbionts (i.e., the N‐fixing mutualist filter). As a result, naturalised N‐fixing plants may present a significant invasion risk on these islands. Location Global. Methods In this study, we used global plant species checklists to test whether naturalised N‐fixing plant species were able to overcome the N‐fixing mutualist filter experienced by native plant species. We did this by examining both patterns and drivers of N‐fixing presence and proportion in native and naturalised floras. Results We find that naturalised N‐fixing plant species no longer experience the N‐fixing mutualist filter seen in native species. Naturalised N‐fixing plant species are more successful on oceanic islands than on mainlands, with islands showing a nearly two‐fold increase in the likelihood of harbouring, and an increased proportion of flora composed of naturalised N‐fixing plants compared to mainlands. Moreover, naturalised N‐fixing plants are more likely to invade remote islands, where native N‐fixing species are underrepresented, suggesting that non‐native N‐fixing plants may fill the N‐fixing niche space left behind by the native flora. Main Conclusions Our results show the increased vulnerability of island ecosystems to invasions by N‐fixing plants, particularly at distant islands, informing management strategies to protect these unique ecosystems.

Aims The ecological roles of dinitrogen-fixing plants (N.sub.2-fixers) in the early stage of primary succession remain unclear. We aimed to examine the effects of pioneer N.sub.2-fixers on the resource status and establishment of neighboring non-N.sub.2-fixing plants (non-fixers) in a newly formed glacier floodplain, eastern Tibetan Plateau. Methods We set up three plots in the floodplain and assessed the abundance, height, and coverage of two N.sub.2-fixing and three non-fixing species. We also collected plant and soil samples for the analyses of nutrient status and stable isotope signatures. Results The two N.sub.2-fixing species acquired > 80% of the nitrogen (N) from N.sub.2 fixation. The cushion-forming N.sub.2-fixer Astragalus mahoshanicus increased the soil nutrient availability compared with the non-fixers and improved the microclimate beneath the cushions. Natural .sup.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 N.sub.2-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. Conclusions The effects of pioneer N.sub.2-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 N.sub.2-fixers. A. mahoshanicus has the potential to be used in the restoration of newly exposed land in subalpine regions.

Nitrogen (N) and phosphorus (P) concentrations and N:P ratios in terrestrial plants and their patterns of change along environmental gradients are important traits for plant adaptation to changes. We determined the leaf N and P concentrations of Chinese sea-buckthorn (Hippophae rhamnoides L. subsp. sinensis Rousi), a non-legume species with symbiotic N fixation (SNF), at 37 sites across northern China and explored their geographical patterns in relation to climate and soil factors. (1) The mean leaf N, P, and N:P ratio were 36.5, 2.1 mg g⁻¹, and 17.6, respectively, higher than the mean values of most shrub species in the region. (2) Leaf N was correlated with soil mineral N in cool areas (mean annual temperature MAT <3 °C) but with temperature in warm areas (MAT >3 °C). The high leaf N and divergent leaf N–soil N relationship suggested the importance of SNF in plant N uptake; SNF increases with temperature and is probably the major N source in warm areas. (3) Leaf P was positively related to mean annual precipitation. Leaf N:P ratio was primarily driven by changes in leaf P. The high leaf P reflected the greater requirements of the N-fixing species for P. Our results represent a major advance in understanding the elemental stoichiometry of non-legume N-fixing plants, indicating high P and N requirements and a shift in N source from SNF to soil as temperature declines. This knowledge will help in assessing the habitat suitability for the species and predicting the species dynamics under environmental changes.

Dinitrogen (N2) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N2 fixation across major sectors of the land biosphere. A question remains as to why symbiotic N2-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here we present a unifying framework for terrestrial N2 fixation that can explain the geographic occurrence of N2 fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, we find a clear advantage to symbiotic N2 fixers in phosphorus-limited tropical savannas and lowland tropical forests. The ability of N2 fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N2 fixation in phosphorus-limited tropical ecosystems. In contrast, modern-day temperatures seem to constrain N2 fixation rates and N2-fixing species from mature forests in the high latitudes. We propose that an analysis that couples biogeochemical cycling and biophysical mechanisms is sufficient to explain the principal geographical patterns of symbiotic N2 fixation on land, thus providing a basis for predicting the response of nutrient-limited ecosystems to climate change and increasing atmospheric CO2.

Fire and nutrients interact to influence the global distribution and dynamics of the savanna biome, but the results of these interactions are both complex and poorly known. A critical but unresolved question is whether short-term losses of carbon and nutrients caused by fire can trigger long-term and potentially compensatory responses in the nutrient stoichiometry of plants, or in the abundance of dinitrogen-fixing trees. There is disagreement in the literature about the potential role of fire on savanna nutrients, and, in turn, on plant stoichiometry and composition. A major limitation has been the lack of fire manipulations over time scales sufficiently long for these interactions to emerge. We use a 58-year, replicated, large-scale, fire manipulation experiment in Kruger National Park (South Africa) in savanna to quantify the effect of fire on (1) distributions of carbon, nitrogen, and phosphorus at the ecosystem scale; (2) carbon : nitrogen : phosphorus stoichiometry of above- and belowground tissues of plant species; and (3) abundance of plant functional groups including nitrogen fixers. Our results show dramatic effects of fire on the relative distribution of nutrients in soils, but that individual plant stoichiometry and plant community composition remained unexpectedly resilient. Moreover, measures of nutrients and carbon stable isotopes allowed us to discount the role of tree cover change in favor of the turnover of herbaceous biomass as the primary mechanism that mediates a transition from low to high soil carbon and nutrients in the absence of fire. We conclude that, in contrast to extra-tropical grasslands or closed-canopy forests, vegetation in the savanna biome may be uniquely adapted to nutrient losses caused by recurring fire.