Nitrogen Acquisition Strategies Research Articles

Uptake patterns for nitrogen and sulfur source by aquatic plants and various nitrogen acquisition strategies: Affected by mining activities

Understanding the nitrogen and sulfur uptake strategies of mine plants, including sources and preferences for nitrogen forms (ammonium nitrogen (NH4+) vs nitrate nitrogen (NO3−)), is critical to improving understanding of the role of plants in participating in the biogeochemical cycles of nitrogen and sulfur in mining areas. In this study, the stable N and S isotopic compositions of two species of aquatic plants (calamus and reed) in Linhuan mining area were analyzed to determine their absorption strategies for different nitrogen and sulfur sources. The results showed that river water was the largest source of nitrogen and sulfur, contributing 54.6% and 53.9% respectively. NO3− is the main form of nitrogen uptake by reed and calamus, followed by NH4+. In order to adapt to the change of nitrogen form in the environment, reed and calamus tend to absorb and utilize NO3− to maintain their absorption of nitrogen. Mine effluents from mining activities provide at least 12.9% and 16.8% sulfate to reed and calamus respectively, and the effect of mine effluents on reed and calamus sulfur has been underestimated. This study reveals the key factors controlling plant isotope composition, and the use of nitrogen and sulfur isotope composition of aquatic plants can help quantify the level of influence of mining activities, and understand the biogeochemical cycle of nitrogen and sulfur in mining areas.

A genomic view of environmental and life history controls on microbial nitrogen acquisition strategies.

Microorganisms have evolved diverse strategies to acquire the vital element nitrogen (N) from the environment. Ecological and physiological controls on the distribution of these strategies among microbes remain unclear. In this study, we examine the distribution of 10 major N acquisition strategies in taxonomically and metabolically diverse microbial genomes, including those from the Genomic Catalogue of Earth's Microbiomes dataset. We utilize a marker gene-based approach to assess relationships between N acquisition strategy prevalence and microbial life history strategies. Our results underscore energetic costs of assimilation as a broad control on strategy distribution. The most prevalent strategies arethe uptake of ammonium and simple amino acids, which have relatively low energetic costs, while energy-intensive biological nitrogen fixation is the least common. Deviations from the energy-based framework include the higher-than-expected prevalence of the assimilatory pathway for chitin, a large organic polymer. Energy availability is also important, with aerobic chemoorganotrophs and oxygenic phototrophs notably possessing ~2-fold higher numbers of total strategies compared to anaerobic microbes. Environmental controls are evidenced by the enrichment of inorganic N assimilation strategies among free-living taxa compared to host-associated taxa. Physiological constraints such as pathway incompatibility add complexity to N acquisition strategy distributions. Finally, we discuss the necessity for microbially-relevant spatiotemporal environmental metadata for improving mechanistic and prediction-oriented analyses of genomic data.

The effect of moisture on nitrogen acquisition strategies of ephemeral desert plants in north-western China are regulated by temperature

AimsSymbiotic plants achieve partitioning of shared resources by chemical nutrient niche separation to escape competition. Understanding whether the nitrogen(N) nutrient acquisition strategies of ephemeral plants are influenced by temperature and moisture can facilitate the elucidation of plant coexistence strategies in desert ecosystems under environmental change. This study aimed to see whether changes in temperature and moisture affected the preferential uptake of different N chemotypes by ephemerals. MethodsIn this study, four dominant ephemerals in the Gurbantunggut Desert, Eremopyrum orientale, Erodium oxyrhinchum, Centaurea pulchella, and Lappula patula, were selected for controlled experiments in indoor pots. We conducted a short-term 15N follow-up experiment on potted treatments by setting two temperature gradients (low temperature(T1): 15 °C during the day and 5 °C at night; high temperature(T2): 25 °C during the day and 15 °C at night) and three moisture gradients (drought treatment(W1): soil moisture content of 2%; normal moisture treatment(W2): soil moisture content of 8%; wet treatment(W3): soil moisture content of 14%). Results Four ephemerals preferentially absorbed 15N-NH4 at low temperature treatment (T1), with a contribution ranging from 17.33% to 55.11%. The greatest uptake rate of 15N-NH4 was at the low temperature-normal moisture treatment (T1W2), and the fastest N uptake rate among the four plants could reach 13.84 μg·h–1, while at the high temperature-drought treatment (T2W1), the uptake was the smallest. However, plants showed the greatest preference for 15N-NO3 uptake at the high temperature treatment, with contributions ranging from 16.09% to 54.06%. The uptake rate of 15N-NO3 was greatest at the high temperature-wet treatment (T2W3), where the fastest N uptake rate among the four plants could reach 12.48 μg·h–1, and the smallest rate of 15N-NO3 at the low temperature-drought treatment (T1W1). The four ephemerals could also absorb organic N, 15N-Glycine, especially at the low temperatures. ConclusionsThe results showed that the four plants exhibited “Plasticity” in the uptake of the three N forms under different hydrothermal conditions. The effect of temperature and moisture interaction is more reflective of the N acquisition pattern of ephemeral plants in the desert. In low temperature environments, the uptake preference of ephemerals for N forms was mainly regulated by water, while in high temperature environments, the uptake of N forms by ephemerals was mainly regulated by temperature.

Soil water conditions together with plant nitrogen acquisition strategies control vegetation dynamics in semi-arid wetlands undergoing land management changes

Land management changes can significantly alter wetland soil environment, vegetation, and their interactions. Although the relationships between plant communities and environmental factors have been widely studied, the intrinsic driving mechanisms of vegetation dynamics are poorly understood because nutrient acquisition strategies play an important role in plant community assembly. This study compared the plant community characteristics of wetlands under different land management practices in the semi-arid western Songnen Plain, Northeast China, and investigated soil properties and plant community nutrient contents. Results showed compared with those in natural wetlands, the values of growth characteristics of plant communities were generally lower, whereas species diversity was higher in grazed or mowed wetlands. Furthermore, soil carbon (C), nitrogen (N), phosphorus (P) contents, their stoichiometric ratios, and soil water content were lower, and N contents in leaves and stems, soil bulk density, and pH were higher compared with those in natural wetlands. The plant community characteristics were significantly correlated with soil properties and plant nutrient contents. A redundancy analysis showed that soil water content, bulk density, and organic carbon, accounted for 59.3%, 15.6%, and 2.9%, respectively, the N in leaves, stems, and roots accounted for 62.8%, and the P in leaves and roots accounted for 12.2% of the variation in the plant community. Variation partitioning analysis further demonstrated that soil properties, plant nutrient contents, and their interactions explained 19.1 %, 11.8%, and 59.4% of the variation in the plant community, respectively. Therefore, the wetland vegetation dynamics are combinedly controlled by soil properties (especially soil water conditions) and plant nutrient acquisition strategies (especially plant nitrogen acquisition) under different land management practices. This study provides the theoretical basis for maintaining the stability of wetland ecosystems by designing plant resources-related managements in semi-arid regions.

Nitrogen Acquisition Strategies Mediated by Insect Symbionts: A Review of Their Mechanisms, Methodologies, and Case Studies.

Simple SummaryNitrogen acquisition strategies mediated by insect symbionts through biological nitrogen fixation (BNF) and nitrogenous waste recycling (NWR) were reviewed and compared in our paper, and a model for nitrogen provisioning in insects was then constructed. In our model, (1) insects acquired nitrogen nutrition from food stuffs directly, and the subprime channels (e.g., BNF or NWR) for nitrogen provisioning were accelerated when the available nitrogen in diets could not fully support the normal growth and development of insects; (2) the NWR strategy was more accessible to more insects due to its energy conservation and mild reaction conditions; (3) ammonia produced by different channels was used for essential nitrogenous metabolites synthesis via the glutamine synthetase and glutamate synthase pathways.Nitrogen is usually a restrictive nutrient that affects the growth and development of insects, especially of those living in low nitrogen nutrient niches. In response to the low nitrogen stress, insects have gradually developed symbiont-based stress response strategies—biological nitrogen fixation and nitrogenous waste recycling—to optimize dietary nitrogen intake. Based on the above two patterns, atmospheric nitrogen or nitrogenous waste (e.g., uric acid, urea) is converted into ammonia, which in turn is incorporated into the organism via the glutamine synthetase and glutamate synthase pathways. This review summarized the reaction mechanisms, conventional research methods and the various applications of biological nitrogen fixation and nitrogenous waste recycling strategies. Further, we compared the bio-reaction characteristics and conditions of two strategies, then proposed a model for nitrogen provisioning based on different strategies.

The Effect of Moisture on Nitrogen Acquisition Strategies of Ephemeral Desert Plants in North-Western China are Regulated by Temperature

The Effect of Moisture on Nitrogen Acquisition Strategies of Ephemeral Desert Plants in North-Western China are Regulated by Temperature

Root mass carbon costs to acquire nitrogen are determined by nitrogen and light availability in two species with different nitrogen acquisition strategies.

Plant nitrogen acquisition requires carbon to be allocated belowground to build roots and sustain microbial associations. This carbon cost to acquire nitrogen varies by nitrogen acquisition strategy; however, the degree to which these costs vary due to nitrogen availability or demand has not been well tested under controlled conditions. We grew a species capable of forming associations with nitrogen-fixing bacteria (Glycine max) and a species not capable of forming such associations (Gossypium hirsutum) under four soil nitrogen levels to manipulate nitrogen availability and four light levels to manipulate nitrogen demand in a full-factorial greenhouse experiment. We quantified carbon costs to acquire nitrogen as the ratio of total root carbon to whole-plant nitrogen within each treatment combination. In both species, light availability increased carbon costs due to a larger increase in root carbon than whole-plant nitrogen, while nitrogen fertilization generally decreased carbon costs due to a larger increase in whole-plant nitrogen than root carbon. Nodulation data indicated that G. max shifted relative carbon allocation from nitrogen fixation to direct uptake with increased nitrogen fertilization. These findings suggest that carbon costs to acquire nitrogen are modified by changes in light and nitrogen availability in species with and without associations with nitrogen-fixing bacteria.

Carbon and nitrogen acquisition strategies by wood decay fungi influence their isotopic signatures in Picea abies forests

We examined whether sporocarp carbon and nitrogen isotope ratios (δ 13 C and δ 15 N values) reflected different functional strategies in 15 species of wood decay fungi. In Finnish Picea abies forests, we compared sporocarp δ 13 C and δ 15 N against log diameter, proximity to ground, and three wood decay types, specifically brown rot, nonselective white rot, and selective white rot (targeting hemicellulose and lignin preferentially). In regression analysis (adjusted r 2 = 0.576), species accounted for 31% of variability in δ 13 C, with factors influencing wood δ 13 C accounting for the remainder. Brown rot fungi and three white rot fungi that selectively attacked hemicellulose ( Heterobasidion parviporum , Phellopilus nigrolimitatus , and Trichaptum abietinum ) were higher in δ 13 C than nonselective white rot fungi. This was attributed to greater assimilation of 13 C-enriched pentoses from hemicellulose by these fungi. The pathogenic white rot fungus Heterobasidion parviporum had higher δ 15 N with proximity to ground and increasing log diameter. This suggested that 15 N-enriched soil N contributed to decomposing logs and that Heterobasidion growing from a bigger resource base had increased access to soil N. These isotopic patterns accordingly reflected both functional diversity of wood decay fungi and site-specific factors. • We investigated strategies of C and N use in wood decay fungi using stable isotopes. • Selective white rot fungi were higher in δ 13 C than nonselective white rot fungi. • Taxa of uncertain white rot type resembled nonselective white rot fungi in δ 13 C. • Spatial arrangements of hemicellulose and cellulose fibers influenced fungal δ 13 C. • If close to ground on big logs, Heterobasidion parviporum accessed soil N.

Nitrogen Pools in Tropical Plantations of N2-Fixing and Non-N2-Fixing Legume Trees under Different Tree Stand Densities

We investigated the nitrogen pools in monocultures of legume species widely used in reforestation in Brazil that have contrasting growth and nitrogen acquisition strategies. The plantations were established with the slow-growing and N2-fixing tree Anadenanthera peregrina var. peregrina, and the fast-growing and non-fixing tree Schizolobium parahyba var. amazonicum. The measurements of N pools in the tree biomass and the soil followed standard methods and were carried out on 54 experimental plots. The N2 fixation pools were evaluated by abundance natural of 15N and the N accretion methods. The soil N content was of similar magnitude between species and stand densities. The species showed similar amounts of N in the biomass, but divergent patterns of N accumulation, as well as the 15N signature on the leaves. S. parahyba accumulated most N in the stem, while A. peregrina accumulated N in the roots and leaves. However, the N accumulation in biomass of A. peregrina stand was less constrained by environment than in S. parahyba stands. The percentage of N derived from N2 fixation in A. peregrina stands decreased with the increase of stand density. The biological N2 fixation estimates depended on the method and the response of tree species to environment.

Convergent nitrogen uptake patterns and divergent nitrogen acquisition strategies of coexisting plant species in response to long-term nitrogen enrichment in a temperate grassland

Nitrogen (N) deposition can significantly alter plant diversity and soil N status and cycling. However, there is rare information about the responses of plant N uptake of coexisting species to N deposition, which could facilitate the understanding of these N-induced ecosystem changes, particularly when taking N addition frequency into consideration. We assessed the uptake rates of N of various chemical forms (both inorganic and organic N) among coexisting plant species in response to N enrichment, using in situ stable isotope labeling techniques (15N-labeled ammonium, 15N-labeled nitrate and 13C-15N-labeled glycine), through a long-term N deposition experiment with monthly and biannual N addition frequencies in a temperate grassland. The plants of all three targeted species took up much greater inorganic N than amino acid N (93.4–99.4 % vs. 0.6–6.6 % in relative contributions to the total N uptake). The most abundant soil N form that originally was NH4+, was shifted to be NO3− under low and moderate N additions and back to be NH4+ at high addition level. Plants of not only dominant but also minor species predominantly took up N from the most abundant soil N form along N addition gradient. The dominant species Leymus chinensis, which continued to maintain its dominance under N enrichment, exhibited the ability to capture N of all different forms to tolerate and survive in a wide range of soil N environments. In contrast, another dominant species Stipa grandis, which was losing its dominance with N addition, showed the acquisition capacity of single N form in response to increased N inputs. It should be noted that there were interaction effects of N addition frequency and level on plant N uptake rates. Plant N uptake patterns and acquisition strategies could have important consequences for changes in ecosystem biodiversity and function under current and future N deposition scenarios.

Towards a trait-based understanding of Symbiodiniaceae nutrient acquisition strategies

Symbiodiniaceae is a diverse group of dinoflagellates that form symbioses with marine invertebrates, provisioning energy and nutrients for their hosts. Symbiont diversity is a well-known predictor of host fitness and stress tolerance. Yet, we have a limited understanding of the mechanisms by which in hospite symbiont communities are structured. Therefore, we hypothesized that phylogenetic differences in nitrogen assimilation may affect symbiont dominance, as has been demonstrated in phytoplankton communities. Here, we quantified species-specific rates of nitrate assimilation using stable isotope labeling, and investigated key traits that have been adopted in phytoplankton ecology to explain the fundamental concept of nitrogen acquisition strategies and size-related trade-offs in Symbiodiniaceae. Traits related to structure and function were measured to look for convergent ecological strategies. Despite the limited sample size, we could distinguish two groups among the five species of Symbiodiniaceae with features described by the phytoplankton’s competition theories for resources: the affinity-adapted species which were associated with larger size, higher nitrogen content and nitrate affinity; and the velocity-adapted species which were small size, efficient in maximum nitrate assimilation, specific carbon assimilation rate, and growth. Our work supports the relevance of a functional trait-based approach to describe Symbiodinaceae diversity. The two contrasting nitrogen acquisition strategies identified may be fundamental to explain the composition and dynamics of Symbiodinaceae in hospite, stressing the importance of bottom-up mechanisms in shaping symbiont composition.

Nitrogen acquisition strategies of mature Douglas‐fir: a case study in the northern Rocky Mountains

AbstractNitrogen (N) limits plant growth in temperate ecosystems, yet many evergreens exhibit low photosynthetic N use efficiency, which can be explained in part by their tendency to store more N than to use it in photosynthesis. However, it remains uncertain to what extent mature conifers translocate internal N reserves or take up N from soils to support new growth. In this study, we explored N dynamics within mature Douglas‐fir (Pseudotsuga menziesii var. glauca) trees by linking N uptake in field‐grown trees with seasonal soil available N. We used a branch‐level mass balance approach to infer seasonal changes in total N among multiple needle and stem cohorts and bole tissue, and used foliar δ15N to evaluate N translocation/uptake from soils. Soil resin‐exchangeable N and net N transformation rates were measured to assess whether soils had sufficient N to support new needle growth. We estimated that after bud break, new needle biomass in Douglas‐fir trees accumulated an average of 0.20 ± 0.03 mg N/branch and 0.17 ± 0.03 mg N/branch in 2016 and 2017, respectively. While we did find some evidence of translocation of N from older stems to buds prior to bud break, we did not detect a significant drawdown of N from previous years’ growth during needle expansion. This suggests that the majority of N used for new growth was not reallocated from aboveground storage, but originated from the soils. This finding was further supported by the δ15N data, which showed divergent δ15N patterns between older needles and buds prior to leaf flushing (indicative of translocation), but similar patterns of depletion and subsequent enrichment following leaf expansion (indicative of N originating from soils). Overall, in order to support new growth, our study trees obtained the majority of N from the soils, suggesting tight coupling between soil available N and N uptake in the ecosystem.

Isotopic constraints on plant nitrogen acquisition strategies during ecosystem retrogression.

Plant root associations with microbes such as mycorrhizal fungi or N-fixing bacteria enable ecosystems to tap pools of nitrogen (N) that might otherwise be inaccessible, including atmospheric N or N in large soil organic molecules. Such microbially assisted N-foraging strategies may be particularly important in late-successional retrogressive ecosystems where productivity is low and soil nutrients are scarce. Here, we use natural N-stable isotopic composition to constrain pathways of N supplies to different plant functional groups across a well-studied natural soil fertility gradient that includes a highly retrogressive stage. We demonstrate that ectomycorrhizal fungi, ericoid mycorrhizal fungi, and N-fixing bacteria support forest N supplies at all stages of ecosystem succession, from relatively young, N-rich/phosphorus (P)-rich sites, to ancient sites (ca. 500 ky) where both N supplies and P supplies are exceedingly low. Microbially mediated N sources are most important in older ecosystems with very low soil nutrient availability, accounting for 75-96% of foliar N at the oldest, least fertile sites. These isotopically ground findings point to the key role of plant-microbe associations in shaping ecosystem processes and functioning, particularly in retrogressive-phase forest ecosystems.

Involvement of aquaporins on nitrogen-acquisition strategies of juvenile and adult plants of an epiphytic tank-forming bromeliad.

Depending on the N source and plant ontogenetic state, the epiphytic tank-forming bromeliad Vriesea gigantea can modulate aquaporin expression to maximize the absorption of the most available nitrogen source. Epiphytic bromeliads frequently present a structure formed by the overlapping of leaf bases where water and nutrients can be accumulated and absorbed, called tank. However, this structure is not present during the juvenile ontogenetic phase, leading to differences in nutrient acquisition strategies. Recent studies have shown a high capacity of the bromeliad Vriesea gigantea, an epiphytic tank-forming bromeliad, to absorb urea by their leaves. Since plant aquaporins can facilitate the diffusion of urea through the membranes, we cloned three foliar aquaporin genes, VgPIP1;1, VgPIP1;2 and VgTIP2;1 from V. gigantea plants. Through functional studies, we observed that besides water, VgTIP2;1 was capable of transporting urea while VgPIP1;2 may facilitate ammonium/ammonia diffusion. Moreover, aiming at identifying urea and ammonium-induced changes in aquaporin expression in leaves of juvenile and adult-tank plants, we showed that VgPIP1;1 and VgPIP1;2 transcripts were up-regulated in response to either urea or ammonium only in juvenile plants, while VgTIP2;1 was up-regulated in response to urea only in adult-tank plants. Thereby, an ontogenetic shift from juvenile to adult-tank-forming-plant appears to occur with metabolic changes regarding nitrogen metabolism regulation. Investigating urea metabolism in wild species that naturally cope with organic N sources, such as V. gigantea, may provide the knowledge to modify nitrogen use efficiency of crop plants.

A plant perspective on nitrogen cycling in the rhizosphere

Abstract Nitrogen is the major nutrient limiting plant growth in terrestrial ecosystems, and the transformation of inert nitrogen to forms that can be assimilated by plants is mediated by soil micro‐organisms. The last decade has witnessed many significant advances in our understanding of plant–microbe interactions with evidence that plants have evolved multiple strategies to cope with nitrogen limitation by shaping and recruiting nitrogen‐cycling microbial communities. However, most studies have typically focused on the impact of plants on only one, or relatively few, processes within the nitrogen cycle. This review synthesizes recent advances in our understanding of the various routes by which plants influence the availability of nitrogen via an array of interactions with different guilds of nitrogen‐cycling micro‐organisms. We also propose a plant trait‐based framework for linking plant nitrogen acquisition strategies to the activities of nitrogen‐cycling microbial guilds. In doing so, we provide a more comprehensive picture of the ecological relationships between plants and nitrogen‐cycling micro‐organisms in terrestrial ecosystems. Finally, we identify previously overlooked processes within the nitrogen cycle that could be targeted in future research and be of interest for plant health or for improving plant nitrogen acquisition, while minimizing nitrogen inputs and losses in sustainable agricultural systems. A plain language summary is available for this article.

Functional groups differ in trait means, but not in trait plasticity to species richness in local grassland communities.

Despite growing interest in incorporating intraspecific variation of functional traits in community-level studies, it remains unclear whether species classified into functional groups based on interspecific trait differences are similar regarding their variation in trait expression in response to varying plant diversity and composition in local communities. In a large biodiversity experiment (Jena Experiment) designed on a trait-based a priori definition of functional groups (grasses, legumes, small herbs, tall herbs), we studied means, extent of variation (coefficient of variation across communities) and plasticity to increased plant diversity (slopes over a logarithmic species richness ranging from 1, 2, 4, 8 and 16 to 60 species) for nine functional traits. Species means and extent of variation in traits related to nitrogen (N) acquisition and N use differed among functional groups and were more similar in phylogenetically closely related species than expected by chance. Species in the same functional group showed a weak phylogenetic signal and varied widely in means and extent of variation in traits related to shoot architecture and to a smaller extent in leaf traits related to carbon acquisition. This indicated that functional groups were less distinguishable in light than in nitrogen acquisition strategies. The direction and degree of trait plasticity to increasing species richness did not show a phylogenetic signal and were not different among functional groups, but varied largely among species within functional groups. Correlation structures in trait means, extent of trait variation and trait plasticity revealed functional tradeoffs in the acquisition of nitrogen and light across species. While correlations between trait means and extent of trait variation varied from trait to trait (positive, negative or unrelated), trait means and trait plasticity were mostly unrelated. Our results suggest that the concept of functional groups is viable, but context-specific trait measurements are required to improve our understanding about the functional significance of intraspecific trait variation and interspecific trait differences in local plant communities.

Continental‐scale nitrogen pollution is shifting forest mycorrhizal associations and soil carbon stocks

Most tree roots on Earth form a symbiosis with either ecto- or arbuscular mycorrhizal fungi. Nitrogen fertilization is hypothesized to favor arbuscular mycorrhizal tree species at the expense of ectomycorrhizal species due to differences in fungal nitrogen acquisition strategies, and this may alter soil carbon balance, as differences in forest mycorrhizal associations are linked to differences in soil carbon pools. Combining nitrogen deposition data with continental-scale US forest data, we show that nitrogen pollution is spatially associated with a decline in ectomycorrhizal vs. arbuscular mycorrhizal trees. Furthermore, nitrogen deposition has contrasting effects on arbuscular vs. ectomycorrhizal demographic processes, favoring arbuscular mycorrhizal trees at the expense of ectomycorrhizal trees, and is spatially correlated with reduced soil carbon stocks. This implies future changes in nitrogen deposition may alter the capacity of forests to sequester carbon and offset climate change via interactions with the forest microbiome.

Nitrogen acquisition strategies during the winter-spring transitional period are divergent at the species level yet convergent at the ecosystem level in temperate grasslands

Nitrogen (N) is a major limiting element for productivity in temperate grasslands, particularly during early spring when soil N availability is low and the vegetative demand for it is high. Therefore, knowing whether and how plant species adopt different N acquisition strategies during the winter-spring transitional period is essential for understanding ecosystem functioning in temperate grasslands. In this study, parallel experiments with 15N tracer were conducted to examine plant N acquisition strategies during winter-spring transition in a meadow and a typical steppe in northern China. We found that soil microbes immobilized ∼20% of the 15N tracer during the spring thawing period at both sites, and then released half of it back to the soil before late spring, confirming that soil microbes competed effectively with the plant roots for mineral N in early spring. Perennial bunch grasses adopted an active N acquisition strategy at the beginning of the spring thawing period. In contrast, perennial forbs and rhizome grasses began to take up N in the middle of the spring thawing period, and they acquired more N than the bunch grasses. However, sagebrushes and legumes accounted for little 15N recovery, indicating their dependence on internal N accumulation or N fixation. At the ecosystem level, no significant difference in the magnitude of plant 15N uptake was observed between the meadow steppe and typical steppe, although the plant biomass N in the meadow steppe was twice that of the typical steppe during the thawing period. This was attributed to the higher soil inorganic N and faster net N mineralization rate in the meadow steppe than in the typical steppe. Our results suggest that temporal niche differentiation in N acquisition during early spring may facilitate species coexistence in temperate grasslands despite strong plant-microbe or plant-plant competition for N. The divergent N acquisition strategies at the species level and convergent N acquisition strategies at the ecosystem level should be considered for model development to better simulate vegetation growth particularly under spring N stress.

Single-Cell View of Carbon and Nitrogen Acquisition in the Mixotrophic Alga Prymnesium parvum (Haptophyta) Inferred From Stable Isotope Tracers and NanoSIMS

Nutritional modes of unicellular eukaryotes range from pure photoautotrophy of some phytoplankton to pure heterotrophy of species typically called protozoa. Between these two extremes lies a functional continuum of nutrient and energy acquisition modes termed mixotrophy. Prymnesium parvum is an ecologically important mixotrophic haptophyte alga that can produce toxins and form ecosystem disruptive blooms that result in fish kills and changes in planktonic food web structure. We investigated carbon and nitrogen acquisition strategies of single cells of P. parvum using a combined experimental-imaging approach employing labeling of live cells with stable isotope tracers (13C and 15N) followed by measurement of cellular isotopic ratios using nanometer-scale secondary ion mass spectrometry (NanoSIMS). With this method, we were able to quantify the relative contributions of photosynthesis and heterotrophy to the nutrition of the alga. Our results suggest that P. parvum relies on predation primarily for nitrogen, while most carbon for cellular building blocks is obtained from inorganic sources. Our analysis further revealed that nitrogen assimilation can vary up to an order of magnitude among individual cells, a finding that would be difficult to determine using other methods. These results help to improve our understanding of mixotrophy across the enormous diversity of eukaryotes, one cell and one species at a time.

Phosphorus- and nitrogen-acquisition strategies in two Bossiaea species (Fabaceae) along retrogressive soil chronosequences in south-western Australia.

During long-term ecosystem development and its associated decline in soil phosphorus (P) availability, the abundance of mycorrhizal plant species declines at the expense of non-mycorrhizal species with root specialisations for P-acquisition, such as massive exudation of carboxylates. Leaf manganese (Mn) concentration has been suggested as a proxy for such a strategy, Mn concentration being higher in non-mycorrhizal plants that release carboxylates than in mycorrhizal plants. Shifts in nitrogen (N)-acquisition strategies also occur; nodulation in legumes is expected at low N availability, when sufficient P is available. We investigated whether two congeneric legume species (Bossiaea linophylla and Bossiaea eriocarpa) occurring along two long-term chronosequences on the south-western Australian coast and grown in a glasshouse at varying N and P supply exhibited plasticity in nutrient-acquisition strategies. We hypothesised that the shifts in nutrient limitation and nutrient-acquisition strategies at the community level would also be found at the species level. Leaf N: P ratios and the responses to nutrient availability suggested that growth of both species exhibited P-limitation in all treatments, due to the very high leaf [N] of legumes afforded by symbiotic N-fixation. Mycorrhizal colonisation was not greater at higher P supply, and root exudation of carboxylates was not stimulated at low P supply; both were unrelated to leaf [Mn]. However, nodule production declined with increasing N supply. We conclude that intraspecific variation in nutrient-acquisition and use is low in these species, and that the variation at the community level, observed in previous studies, is likely driven by high-species turnover.