Resource-based niches provide a basis for plant species diversity and dominance in arctic tundra

Plant nitrogen uptake preference and drivers in natural ecosystems at the global scale

Elucidating plant nitrogen (N) acquisition is crucial for understanding plant N strategies and ecosystem productivity. However, the variation in plant N uptake preference and its controlling factors on a global scale remain unclear. We conducted a global synthesis to explore plant N preference patterns and driving factors. Globally, the average contributions of ammonium (NH4 +), nitrate (NO3 -), and glycine N to the total plant N uptake were 41.6 ± 1.1%, 32.8 ± 1.2%, and 25.6 ± 0.9%, respectively. However, plant N uptake preferences differed significantly among climatic regions and vegetation types. Soil NH4 + was the most preferred N form by plants in (sub)tropical regions, whereas NO3 - preference was significantly higher in high-latitude than low-latitude regions. Plant functional type was one of the most important factors driving NO3 - preference, with significantly higher NO3 - preference of nonwoody species than broadleaf-evergreen, conifer, and shrub species. Organic N preference was lowest in (sub)tropics and significantly lower than that in temperate and alpine regions. This study shows clear climatic patterns and different influencing factors of plant NH4 + and NO3 - preference, which can contribute to the accurate prediction of N constraints on ecosystem productivity and soil carbon dynamics.

Effects of Freeze-Thaw Cycles on Uptake Preferences of Plants for Nutrient: A Review

Freeze-thawing is an abiotic climatic force prevalent at mid-to-high latitudes or high altitudes, significantly impacting ecosystem nitrogen (N) and phosphorus (P) cycling, which is receiving increasing attention due to ongoing global warming. The N and P nutrients are essential for plant growth and development, and the uptake and utilization of these nutrients by plants are closely linked to external environmental conditions. Additionally, the availability of N and P nutrients influences the ecological adaptability of plants. Adapting plants to diverse external environments for the efficient uptake and utilization of N and P nutrients represents a main focus in contemporary ecological research on plant nutrient utilization in the ecosystems of mid-to-high latitudes or high altitudes. Through a comprehensive analysis of the experimental results regarding plant nutrient uptake and utilization in mid-to-high-latitude or high-altitude ecosystems, this paper discussed the processes of soil N and P cycling and the different utilization strategies of nutrient forms employed by plants during freezing and thawing. Freeze-thaw cycles affect the availability of N and P in the soil. Under freeze-thaw conditions, plants preferentially take up readily available N sources (e.g., nitrate (NO3--N) or ammonium (NH4+-N)) and adjust their root growth and timing of N uptake, developing specific physiological and biochemical adaptations to meet their growth needs. When nutrient conditions are poor or N sources are limited, plants may rely more on low-molecular-weight organic nitrogen (e.g., amino acids) as N sources. Plants adapt to changes in their environment by adjusting root growth, making changes in root secretions, and utilizing microbial communities associated with the P cycle to support more efficient P utilization. Future research should (i) enhance the monitoring of plant roots and nutrient dynamics in the subterranean layers of the soil; (ii) incorporate a broader range of nutrients; (iii) examine specific freeze-thaw landscape types, along with the spatial and temporal heterogeneity of climate change within seasons, which is essential for minimizing uncertainty in our understanding of plant nutrient utilization strategies.

Seasonal water level changes affect plant diversity and littoral widths at different elevation zones in the Erhai Lake

The littoral width of lakeshores is crucial for maintaining and promoting plant diversity. However, it remains unclear how changes in seasonal water level affect littoral widths by regulating plant diversity and soil nutrient content. This study selected three elevation ranges in the lakeshore of Erhai: supralittoral, eulittoral, and infralittoral. We explored the effects of hydrological changes on littoral widths and their potential relationships by analyzing seasonal differences in plant communities and soil physicochemical properties during an extremely drought year. Our results indicated that the most significant seasonal differences in diversity indices, biomass, and soil physicochemical properties were observed in the eulittoral, followed by the infralittoral and supralittoral. The niche breadths of perennials was significantly decreased by 44.4% and the width of the eulittoral was significantly decreased by 48.6% during the winter. Generalized Additive Models (GAMs) were applied to analyze the elevation distribution ranges of dominant species. The results revealed that species with monotonically increasing distributions had the widest niche breadths, followed by symmetric unimodal species, while monotonically decreasing species exhibited the narrowest. Structural equation modeling revealed a positive and significant correlation between flooding days and soil water content and pH, and a negative correlation with plant parameters (species number, biomass, and coverage). Moreover, plant parameters showed a significant positive correlation with plant diversity. Importantly, plant diversity and soil nutrients were significantly positively correlated with littoral widths, suggesting their key roles in influencing littoral widths. This study highlights the significant impact of hydrological seasonal changes on the littoral widths of lakeshore zones, providing valuable guidance for managing wetland water levels in response to extreme drought events.

Nitrogen Acquisition by Invasive Plants: Species Preferential N Uptake Matching with Soil N Dynamics Contribute to Its Fitness and Domination

Plant invasions play a significant role in global environmental change. Traditionally, it was believed that invasive plants absorb and utilize nitrogen (N) more efficiently than native plants by adjusting their preferred N forms in accordance with the dominant N forms present in the soil. More recently, invasive plants are now understood to optimize their N acquisition by directly mediating soil N transformations. This review highlights how exotic species optimize their nitrogen acquisition by influencing soil nitrogen dynamics based on their preferred nitrogen forms, and the various mechanisms, including biological nitrification inhibitor (BNI) release, pH alterations, and changes in nutrient stoichiometry (carbon to nitrogen ratio), that regulate the soil nitrogen dynamics of exotic plants. Generally, invasive plants accelerate soil gross nitrogen transformations to maintain a high supply of NH4+ and NO3- in nitrogen-rich ecosystems irrespective of their preference. However, they tend to minimize nitrogen losses to enhance nitrogen availability in nitrogen-poor ecosystems, where, in such situations, plants with different nitrogen preferences usually affect different nitrogen transformation processes. Therefore, a comprehensive understanding requires more situ data on the interactions between invasive plant species' preferential N form uptake and the characteristics of soil N transformations. Understanding the combination of these processes is essential to elucidate how exotic plants optimize nitrogen use efficiency (NUE) and minimize nitrogen losses through denitrification, leaching, or runoff, which are considered critical for the success of invasive plant species. This review also highlights some of the most recent discoveries in the responses of invasive plants to the different forms and amounts of N and how plants affect soil N transformations to optimize their N acquisition, emphasizing the significance of how plant-soil interactions potentially influence soil N dynamics.

Nitrogen availability in soil controls uptake of different nitrogen forms by plants

Nitrogen (N) uptake by plant roots from soil is the largest flux within the terrestrial N cycle. Despite its significance, a comprehensive analysis of plant uptake for inorganic and organic N forms across grasslands is lacking. Here we measured in situ plant uptake of 13 inorganic and organic N forms by dominant species along a 3000 km transect spanning temperate and alpine grasslands. To generalize our experimental findings, we synthesized data on N uptake from 60 studies encompassing 148 plant species world-wide. Our analysis revealed that alpine grasslands had faster NH4 + uptake than temperate grasslands. Most plants preferred NO3 - (65%) over NH4 + (24%), followed by amino acids (11%). The uptake preferences and uptake rates were modulated by soil N availability that was defined by climate, soil properties, and intrinsic characteristics of the N form. These findings pave the way toward more fully understanding of N cycling in terrestrial ecosystems, provide novel insights into the N form-specific mechanisms of plant N uptake, and highlight ecological consequences of chemical niche differentiation to reduce competition between coexisting plant species.

Different nitrogen uptake patterns of plant and soil microorganisms in the forest-grassland transition zone on the Loess Plateau

Introduction

It is unclear whether plants and microorganisms achieve niche complementarity by taking up different inorganic nitrogen (N) forms to alleviate N competition, particularly in N-limited regions.

Methods

This paper conducted a 15-day 15N tracer study (15NH4NO3 or 15NH4NO3) in situ to quantitatively calculate the uptake rates of plants and microorganisms in four stands (pure Hippophae rhamnoides L, pure Pinus tabuliformis Carrière, mixed H. rhamnoides-P. tabuliformis, and Artemisia gmelinii Weber ex Stechm grassland) in the forest-grassland transition zone on the Loess Plateau during the growing season. Among them, H. rhamnoides and P. tabuliformis can associated with arbuscular mycorrhizal and ectomycorrhizal, respectively.

Results

The results indicated that H. rhamnoides in the pure stand and A. gmelinii preferred to take up 15NO3 -, whereas P. tabuliformis in the pure stand preferred 15NH4 +. Compared to pure stands, mixed afforestation decreased the NH4 + and NO3 - uptake rate of H. rhamnoides by 87% and 70%, respectively, but did not alter the N preference of plants. Plants and microorganisms differed in their N preferences in the pure stand, whereas this was not the case in the mixed stand. The proportional similarity index between H. rhamnoides and P. tabuliformis (0.90 ± 0.01) was higher than that between plants and microorganisms in forest stands, except for P. tabuliformis and microorganisms in the mixed stand (0.90 ± 0.02).

Discussion

Those results indicated that niche complementarity by preferring different N forms can alleviate N competition. This study helped to gain a deeper understanding of the plasticity of N uptake patterns by plants and microorganisms in the forest-grassland transition zone, and provides theoretical support for vegetation restoration during the implementation of the Grain for Green program on the Loess Plateau.

Loss of taxonomic and functional diversity and decrease in primary productivity with nitrogen enrichment after a long-term release from grazing in an alpine grassland

Moderate grazing can sustain high species diversity and productivity. However, nitrogen enrichment often reduces species richness while promoting primary productivity, which contradicts the traditional understanding of the positive effect of plant diversity on productivity. Whether the responses of diversity and productivity to N enrichment on a long-term scale conform to those on short-term scale. Furthermore, chemical N forms may alter species richness or functional diversity because of species preferences for different N forms. We used data from a 19-year field experiment with manipulated livestock exclusion and the addition of N in three chemical forms (ammonium, nitrate, and their mixture) in an alpine grassland to test their effects on diversity and aboveground productivity. Productivity in no N without grazing plots (Ctr) initially increased by 66 % during the first five years after livestock exclusion (0.53 kg m-2), following which productivity fluctuated at a similar level as that in winter livestock grazing (0.32 kg m-2) for the next eight years and ultimately decreased to the level of 0.24 kg m-2, below that in Gr with continued species loss. The addition of N forms in grazing exclusion increased productivity, accompanied by the loss of rare species in the first five years. Afterward, productivity decreased, and it took longer than the control to reach the same level as that seen in winter grazing. Then, productivity continued to decline with more species loss. The functional diversity of leaf traits and plant growth forms decreased. Ammonium favored grasses with guerrilla and phalanx rhizomes, nitrate favored sedges and forbs with guerrilla rhizomes, and ammonium-nitrate sustained a relatively higher diversity of plant growth forms. Our general conclusion is that grasslands with moderate grazing are beneficial for sustaining diverse species and high productivity. N enrichment reduces plant diversity, and the increase in productivity driven by N is only transitional, as productivity declines in the long run, with continuous N addition.

Independent and differential effects of arbuscular mycorrhizal fungal composition and plant pathogens on plant traits and nitrogen uptake

Arbuscular mycorrhizal fungi (AMF), as microbial mutualists, interact with various microbial taxa, including pathogens, and significantly shape the ecology and evolution of their host species. However, how AMF and pathogens jointly or independently affect plant traits and nitrogen uptake remains unclear. Here, we conducted a factorial experiment with three AMF treatments (AMF-free-control, Funneliformis mosseae, and a mixture of AMF species of F. mosseae and C. etunicatum), four plant-pathogen pairs, each under two pathogen treatments (one pathogen and a pathogen-free control). After 65 days of growth, we measured AMF colonization, pathogen infection, plant functional traits and ammonium and nitrate uptake. Our findings reveal that AMF and pathogens independently influence plant traits and nitrogen uptake, with no observed interactions between them. Specifically, colonization by F. mosseae or a mixed AMF species reduced nitrate uptake and plant height, without affecting root traits or ammonium uptake. In contrast, pathogen infection enhanced acquisitive root traits, such as increased specific root length and area but did not impact shoot traits or nitrogen uptake. These results broaden our understanding of the tripartite interactions among plants, AMF and pathogens, offering insights into how plant-microbial relationships influence plant health, growth and nitrogen cycling.

Competitive exclusion among self-replicating molecules curtails the tendency of chemistry to diversify

The transition of chemistry into biology is poorly understood. Key questions include how the inherently divergent nature of chemical reactions can be curtailed, and whether Darwinian principles from biology extend to chemistry. Addressing both questions simultaneously, we now show that the evolutionary principle of competitive exclusion, which states that a single niche can be stably occupied by only one species, also applies to self-replicating chemical systems, and that this principle diminishes the tendency of chemistry to diversify. Specifically, we report two systems in which three different self-replicator quasi-species emerge in a largely stochastic fashion from a mixture of two building blocks (resources). Competitive exclusion leads to the selection of only a single quasi-species when all replicators rely to the same extent on both resources. When one of the quasi-species preferentially uses one resource and another quasi-species specializes in the other (resource partitioning), these replicator quasi-species effectively occupy different niches and were found to coexist in an evolutionary stable steady state.

Microbial nitrogen transformations in tundra soil depend on interactive effects of seasonality and plant functional types

Nitrogen (N) cycling in organic tundra soil is characterised by pronounced seasonal dynamics and strong influence of the dominant plant functional types. Such patterns in soil N-cycling have mostly been investigated by the analysis of soil N-pools and net N mineralisation rates, which, however, yield little information on soil N-fluxes. In this study we investigated microbial gross N-transformations, as well as concentrations of plant available N-forms in soils under two dominant plant functional types in tundra heath, dwarf shrubs and mosses, in subarctic Northern Sweden. We collected organic soil under three dwarf shrub species of distinct growth form and three moss species in early and late growing season. Our results showed that moss sites were characterised by significantly higher microbial N-cycling rates and soil N-availability than shrub sites. Protein depolymerisation, the greatest soil N-flux, as well as gross nitrification rates generally did not vary significantly between early and late growing season, whereas gross N mineralisation rates and inorganic N availability markedly dropped in late summer at most sites. The magnitude of the seasonal changes in N-cycling, however, clearly differed among plant functional types, indicating interactive effects of seasonality and plant species on soil N-cycling. Our study highlights that the spatial variation and seasonal dynamics of microbial N transformations and soil N availability in tundra heath are intimately linked with the distinct influence of plant functional types on soil microbial activity and the plant species-specific patterns of nutrient uptake and carbon assimilation. This suggests potential strong impacts of future global change-induced shifts in plant community composition on soil N-cycling in tundra ecosystems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10533-024-01176-6.

Global distribution and drivers of relative contributions among soil nitrogen sources to terrestrial plants

Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (fNO3-), ammonium (fNH4+), and organic N (fEON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of fNO3-, fNH4+, and fEON. The fNO3- increases with MAT, reaching 46% at 28.5 °C. The fNH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the fEON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems.

Species-specific responses of C and N allocation to N addition: evidence from dual 13C and 15N labeling in three tree species

Soil nitrogen (N) availability affects plant carbon (C) utilization. However, it is unclear how various tree functional types respond to N addition in terms of C assimilation, allocation, and storage. Here, a microcosm experiment with dual 13C and 15N labeling was conducted to study the effects of N addition (i.e., control, 0 g N kg-1; moderate N addition, 1.68 g N kg-1; and high N addition, 3.36 g N kg-1 soil) on morphological traits, on changes in nonstructural carbohydrates (NSC) in different organs, as well as on C and N uptake and allocation in three European temperate forest tree species (i.e., Acer pseudoplatanus, Picea abies and Abies alba). Our results demonstrated that root N uptake rates of the three tree species increased by N addition. In A. pseudoplatanus, N uptake by roots, N allocation to aboveground organs, and aboveground biomass allocation significantly improved by moderate and high N addition. In A. alba, only the high N addition treatment considerably raised aboveground N and C allocation. In contrast, biomass as well as C and N allocation between above and belowground tissues were not altered by N addition in P. abies. Meanwhile, NSC content as well as C and N coupling (represented by the ratio of relative 13C and 15N allocation rates in organs) were affected by N addition in A. pseudoplantanus and P. abies but not in A. alba. Overall, A. pseudoplatanus displayed the highest sensitivity to N addition and the highest N requirement among the three species, while P. abies had a lower N demand than A. alba. Our findings highlight that the responses of C and N allocation to soil N availability are species-specific and vary with the amount of N addition.

Subsurface wastewater infiltration systems for nitrogen pollution control

Subsurface wastewater infiltration systems (SWISs) are suggested to be a cost-effective and environmentally friendly method for sewage treatment. However, a comprehensive summary of the relevant mechanisms and optimization methods for nitrogen (N) removal in SWIS is currently lacking. In this review, we first summarize the N transformation mechanisms in SWIS. The impact of operational parameters on the N removal efficiency is then delineated. To enhance pollutant removal and minimize resource wastage, it is advisable to maintain a wet-dry ratio of 1:1 and a hydraulic loading rate of 8-10 cm/day. The organic load should be determined based on influent characteristics to optimize the balance between sewage treatment and nitrous oxide (N2O) emission. Finally, various strategies and modifications have been suggested to enhance pollutant removal efficiency and reduce N2O emissions in SWIS, such as artificial aeration, supply electron donors, and well-designed structures. Overall, greater emphasis should be placed on the design and management of SWIS to optimize their co-benefits while effectively controlling N pollution. PRACTITIONER POINTS: SWISs are often considered black boxes with their efficiency depending on hydraulic characteristics, biological characteristics, and substrate properties. Biological nitrification coupled with denitrification is considered to be the major N removal process. Increasing the reduction of N2O to the inert N2 form is a potential mechanism to mitigate global warming. Strategies such as artificial aeration, supply electron donors, and well-designed structures are suggested to improve N removal performance.

Climate change disrupts the seasonal coupling of plant and soil microbial nutrient cycling in an alpine ecosystem

The seasonal coupling of plant and soil microbial nutrient demands is crucial for efficient ecosystem nutrient cycling and plant production, especially in strongly seasonal alpine ecosystems. Yet, how these seasonal nutrient cycling processes are modified by climate change and what the consequences are for nutrient loss and retention in alpine ecosystems remain unclear. Here, we explored how two pervasive climate change factors, reduced snow cover and shrub expansion, interactively modify the seasonal coupling of plant and soil microbial nitrogen (N) cycling in alpine grasslands, which are warming at double the rate of the global average. We found that the combination of reduced snow cover and shrub expansion disrupted the seasonal coupling of plant and soil N-cycling, with pronounced effects in spring (shortly after snow melt) and autumn (at the onset of plant senescence). In combination, both climate change factors decreased plant organic N-uptake by 70% and 82%, soil microbial biomass N by 19% and 38% and increased soil denitrifier abundances by 253% and 136% in spring and autumn, respectively. Shrub expansion also individually modified the seasonality of soil microbial community composition and stoichiometry towards more N-limited conditions and slower nutrient cycling in spring and autumn. In winter, snow removal markedly reduced the fungal:bacterial biomass ratio, soil N pools and shifted bacterial community composition. Taken together, our findings suggest that interactions between climate change factors can disrupt the temporal coupling of plant and soil microbial N-cycling processes in alpine grasslands. This could diminish the capacity of these globally widespread alpine ecosystems to retain N and support plant productivity under future climate change.

Uneven distributions of unique species promoting N niche complementarity explain the stability of degraded alpine meadow

Alpine meadow degradation, usually involving decreased soil nitrogen (N) and patchy landscapes, is a challenge for natural restoration. However, the mechanism underlying plant species coexistence under degradation is unclear. In this study, we evaluated plant N niche complementarity in degraded alpine meadows on the Qinghai-Tibet Plateau using a 15N-labeling (15NO3-, 15NH4+, and 15N-glycine) experiment. At the community level, the concentration of 15NO3- in the degraded alpine meadow was 1.5 times higher than that in the undegraded alpine meadow; both alpine meadows had a significant preference for NO3- (60.72 % and 66.84 % for the degraded and undegraded alpine meadows, respectively), and the degree of glycine preference was significantly higher in the degraded alpine meadow (30.77 %) relative to the undegraded alpine meadow (21.85 %). At the species level, dominant species in both alpine meadows consistently preferred NO3-; the generalist species that can be found in both meadows and unique species of the two alpine meadows generally showed NO3- preferences, while the other plant species that were unevenly distributed in the degraded alpine meadow tended to show increased utilization of glycine, which could reduce N competition. We observed that differentiation among N sources and the uneven distribution of unique species may explain the stability of degraded alpine meadows. Our results suggested that uneven distributions of plants could have strong impacts on community stability and highlighted the importance of considering fine-scale analysis in studies of niche theory. This study has important implications for the restoration of degraded alpine meadows.

Rapid migration of Mongolian oak into the southern Asian boreal forest

The migration of trees induced by climatic warming has been observed at many alpine treelines and boreal-tundra ecotones, but the migration of temperate trees into southern boreal forest remains less well documented. We conducted a field investigation across an ecotone of temperate and boreal forests in northern Greater Khingan Mountains of northeast China. Our analysis demonstrates that Mongolian oak (Quercus mongolica), an important temperate tree species, has migrated rapidly into southern boreal forest in synchrony with significant climatic warming over the past century. The average rate of migration is estimated to be 12.0 ± 1.0 km decade-1 , being slightly slower than the movement of isotherms (14.7 ± 6.4 km decade-1 ). The migration rate of Mongolian oak is the highest observed among migratory temperate trees (average rate 4.0 ± 1.0 km decade-1 ) and significantly higher than the rates of tree migration at boreal-tundra ecotones (0.9 ± 0.4 km decade-1 ) and alpine treelines (0.004 ± 0.003 km decade-1 ). Compared with the coexisting dominant boreal tree species, Dahurian larch (Larix gmelinii), temperate Mongolian oak is observed to have significantly lower capacity for light acquisition, comparable water-use efficiency but stronger capacity to utilize nutrients especially the most limiting nutrient, nitrogen. In the context of climatic warming, and in addition to a high seed dispersal capacity and potential thermal niche differences, the advantage of nutrient utilization, reflected by foliar elementomes and stable nitrogen isotope ratios, is also likely a key mechanism for Mongolian oak to coexist with Dahurian larch and facilitate its migration toward boreal forest. These findings highlight a rapid deborealization of southern Asian boreal forest in response to climatic warming.

Soil deterioration due to long-term grazing of desert-steppe promotes stress-tolerant ecological strategies in plants

Predicting the impact of grazing on desert steppes' vegetation is crucial for sustainable management. The competitor, stress-tolerator and ruderal (CSR) theory has offer valuable insights into plant adaptive capacity and predicting changes in vegetation structure. However, whether adaptive strategy of desert steppe change with grazing intensity is not understood yet. We conducted a comprehensive assessment of the effects of 19 years of varying grazing intensities (no grazing: CK; light grazing: LG; moderate grazing: MG; heavy grazing: HG) on plant CSR strategies. Within the desert steppe ecosystem, the prevailing plant community strategy leaned towards SR (Stress-tolerant/Ruderal) under non-grazed conditions (C:S:R = 2:48:50 %). However, as grazing intensity increased, the S-score increased significantly from 48 % to 62 % while the R-score decreased from 50 % to 36 %. This trend persisted across functional groups and species, except for shrubs, due to the convergent response of traits and ecological strategies of these species under intense grazing pressure. Furthermore, our findings reveal that the shift of CSR on the community level was primarily influenced by soil nutrients and soil density, but not by evolutionary history (phylogenetic conservatism of traits). This suggests that plant traits co-evolved under prolonged environmental stress, resulting consistent variation and ecological strategies in desert grasslands. Overall, long-term grazing reinforced selective pressure, creating a simplified plant community structure with low levels of diversity and productivity, dominated by conservative resource-use strategy. This study highlights the mechanisms of plant adaptation to the prolonged grazing disturbance in desert steppes.

Arctic rooting depth distribution influences modelled carbon emissions but cannot be inferred from aboveground vegetation type

The distribution of roots throughout the soil drives depth-dependent plant-soil interactions and ecosystem processes, particularly in arctic tundra where plant biomass, is predominantly belowground. Vegetation is usually classified from aboveground, but it is unclear whether such classifications are suitable to estimate belowground attributes and their consequences, such as rooting depth distribution and its influence on carbon cycling. We performed a meta-analysis of 55 published arctic rooting depth profiles, testing for differences both between distributions based on aboveground vegetation types (Graminoid, Wetland, Erect-shrub, and Prostrate-shrub tundra) and between 'Root Profile Types' for which we defined three representative and contrasting clusters. We further analyzed potential impacts of these different rooting depth distributions on rhizosphere priming-induced carbon losses from tundra soils. Rooting depth distribution hardly differed between aboveground vegetation types but varied between Root Profile Types. Accordingly, modelled priming-induced carbon emissions were similar between aboveground vegetation types when they were applied to the entire tundra, but ranged from 7.2 to 17.6 Pg C cumulative emissions until 2100 between individual Root Profile Types. Variations in rooting depth distribution are important for the circumpolar tundra carbon-climate feedback but can currently not be inferred adequately from aboveground vegetation type classifications.

Symbiotic control of canopy dominance in subtropical and tropical forests

Subtropical and tropical forests in Asia often comprise canopy dominant trees that form symbioses with ectomycorrhizal fungi, and species-rich understorey trees that form symbioses with arbuscular mycorrhizal fungi. We propose a virtuous phosphorus acquisition hypothesis to explain this distinct structure. The hypothesis is based on (i) seedlings being rapidly colonised by ectomycorrhizal fungi from established mycelial networks that generates positive feedback and resistance to pathogens, (ii) ectomycorrhizal fungi having evolved a suite of morphological, physiological, and molecular traits to enable them to capture phosphorus from a diversity of chemical forms, including organic forms, and (iii) allocation of photosynthate carbon from adult host plants to provide the energy needed to undertake these processes.

Nitrogen acquisition strategy and its effects on invasiveness of a subtropical invasive plant

Introduction

Preference and plasticity in nitrogen (N) form uptake are the main strategies with which plants absorb soil N. However, little effort has been made to explore effects of N form acquisition strategies, especially the plasticity, on invasiveness of exotic plants, although many studies have determined the effects of N levels (e.g. N deposition).

Methods

To address this problem, we studied the differences in N form acquisition strategies between the invasive plant Solidago canadensis and its co-occurring native plant Artemisia lavandulaefolia, effects of soil N environments, and the relationship between N form acquisition strategy of S. canadensis and its invasiveness using a 15N-labeling technique in three habitats at four field sites.

Results

Total biomass, root biomass, and the uptakes of soil dissolved inorganic N (DIN) per quadrat were higher for the invasive relative to the native species in all three habitats. The invader always preferred dominant soil N forms: NH4 + in habitats with NH4 + as the dominant DIN and NO3 - in habitats with NO3 - as the dominant DIN, while A. lavandulaefolia consistently preferred NO3 - in all habitats. Plasticity in N form uptake was higher in the invasive relative to the native species, especially in the farmland. Plant N form acquisition strategy was influenced by both DIN levels and the proportions of different N forms (NO3 -/NH4 +) as judged by their negative effects on the proportional contributions of NH4 + to plant N (f NH4 +) and the preference for NH4 + (β NH4 +). In addition, total biomass was positively associated with f NH4 + or β NH4 + for S. canadensis, while negatively for A. lavandulaefolia. Interestingly, the species may prefer to absorb NH4 + when soil DIN and/or NO3 -/NH4 + ratio were low, and root to shoot ratio may be affected by plant nutrient status per se, rather than by soil nutrient availability.

Discussion

Our results indicate that the superior N form acquisition strategy of the invader contributes to its higher N uptake, and therefore to its invasiveness in different habitats, improving our understanding of invasiveness of exotic plants in diverse habitats in terms of utilization of different N forms.

Shift of root nitrogen-acquisition strategy with tree age is mediated by root functional traits along the collaboration gradient of the root economics space

Root nitrogen (N)-uptake rate and uptake preference, and their association with root morphological and chemical traits are important to characterize root N-acquisition strategies of trees. However, how the root N-acquisition strategy varies with tree age, especially for those species that coexist at a common site, remains unknown. In this study, a field isotopic hydroponic method was used to determine the uptake rate and contribution of NH4+, NO3- and glycine, for three coexisting ectomycorrhizal coniferous species [Pinus koraiensis (Korean pine), Picea koraiensis (Korean spruce) and Abies nephrolepis (smelly fir)] at three age classes (young, middle-aged and mature) in a temperate forest. Concurrently, root morphological and chemical traits, as well as mycorrhizal colonization rate were determined. Our results show that the root uptake rate of total N and NH4+ gradually decreased across all three species with increasing tree age. The three species at all age classes preferred NH4+, except for middle-aged Korean spruce and mature smelly fir, which preferred glycine. In contrast, all three species showed the lowest acquisition of NO3-. According to the conceptual framework of 'root economics space', only a 'collaboration' gradient (i.e. dimension of root diameter vs specific root length or area) was identified for each species, in which root N-uptake rate loaded heavily on the side of 'do-it-yourself' (i.e. foraging N more by roots). Young trees of all species tended to exhibit the 'do-it-yourself' strategy for N uptake, and mature trees had an 'outsourcing' strategy (i.e. foraging N by a mycorrhizal partner), whereas middle-aged trees showed a balanced strategy. These findings suggest that shifts of root N-acquisition strategy with tree age in these species are mainly mediated by root traits along the 'collaboration' gradient, which advances our understanding of belowground competition, species coexistence and N cycling in temperate forests.

Shifts in the composition of nitrogen deposition in the conterminous United States are discernable in stream chemistry

Across the conterminous United States (U.S.), the composition of atmospheric nitrogen (N) deposition is changing spatially and temporally. Previously, deposition was dominated by oxidized N, but now reduced N (ammonia [NH3] + ammonium [NH4+]) is equivalent to oxidized N when deposition is averaged across the entire nation and, in some areas, reduced N dominates deposition. To evaluate if there are effects of this change on stream chemistry at the national scale, estimates of N deposition form (oxidized or reduced) from the National Atmospheric Deposition Program Total Deposition data were coupled with stream measurements from the U.S. Environmental Protection Agency (EPA) National Rivers and Streams Assessments (three stream surveys between 2000 and 2014). A recent fine-scaled N input inventory was used to identify watersheds (<1000 km2) where atmospheric deposition is the largest N source (n = 1906). Within these more atmospherically-influenced watersheds there was a clear temporal shift from a greater proportion of sites dominated by oxidized N deposition to a greater proportion of sites dominated by reduced forms of N deposition. We found a significant positive correlation between oxidized N deposition and stream NO3- concentrations, whereas the correlation between reduced N deposition and stream NO3- concentrations were significant but weaker. Sites dominated by atmospheric inputs of reduced N forms had higher stream total organic N and total N despite lower total N deposition on average. This higher stream concentration of total N is mainly driven by the higher concentration of total organic N, suggesting an interaction between elevated reduced N in deposition and living components of the ecosystem or soil organic matter dynamics. Regardless of the proportion of reduced to oxidized N forms in deposition, stream NH4+ concentrations were generally low, suggesting that N deposited in a reduced form is rapidly immobilized, nitrified and/or assimilated by watershed processes.

Lichens as spatially transferable bioindicators for monitoring nitrogen pollution

Excess nitrogen is a pollutant and global problem that harms ecosystems and can severely affect human health. Pollutant nitrogen is becoming more widespread and intensifying in the tropics. There is thus a requirement to develop nitrogen biomonitoring for spatial mapping and trend analysis of tropical biodiversity and ecosystems. In temperate and boreal zones, multiple bioindicators for nitrogen pollution have been developed, with lichen epiphytes among the most sensitive and widely applied. However, the state of our current knowledge on bioindicators is geographically biased, with extensive research effort focused on bioindicators in the temperate and boreal zones. The development of lichen bioindicators in the tropics is further weakened by incomplete taxonomic and ecological knowledge. In this study we performed a literature review and meta-analysis, attempting to identify characteristics of lichens that offer transferability of bioindication into tropical regions. This transferability must overcome the different species pools between source information - drawing on extensive research effort in the temperate and boreal zone - and tropical ecosystems. Focussing on ammonia concentration as the nitrogen pollutant, we identify a set of morphological traits and taxonomic relationships that cause lichen epiphytes to be more sensitive, or more resistant to this excess nitrogen. We perform an independent test of our bioindicator scheme and offer recommendations for its application and future research in the tropics.

Single-gene resolution of diversity-driven overyielding in plant genotype mixtures

In plant communities, diversity often increases productivity and functioning, but the specific underlying drivers are difficult to identify. Most ecological theories attribute positive diversity effects to complementary niches occupied by different species or genotypes. However, the specific nature of niche complementarity often remains unclear, including how it is expressed in terms of trait differences between plants. Here, we use a gene-centred approach to study positive diversity effects in mixtures of natural Arabidopsis thaliana genotypes. Using two orthogonal genetic mapping approaches, we find that between-plant allelic differences at the AtSUC8 locus are strongly associated with mixture overyielding. AtSUC8 encodes a proton-sucrose symporter and is expressed in root tissues. Genetic variation in AtSUC8 affects the biochemical activities of protein variants and natural variation at this locus is associated with different sensitivities of root growth to changes in substrate pH. We thus speculate that - in the particular case studied here - evolutionary divergence along an edaphic gradient resulted in the niche complementarity between genotypes that now drives overyielding in mixtures. Identifying genes important for ecosystem functioning may ultimately allow linking ecological processes to evolutionary drivers, help identify traits underlying positive diversity effects, and facilitate the development of high-performance crop variety mixtures.

Species richness drives selection of individuals within wetlands based on traits related to acquisition and utilization of light

Selection within natural communities has mainly been studied along large abiotic gradients, while the selection of individuals within populations should occur locally in response to biotic filters. To better leverage the role of the latter, we considered the hierarchal nature of environmental selection for the multiple dimensions of the trait space across biological levels, that is, from the species to the community and the ecosystem levels. We replicated a natural species richness gradient where communities included from two to 16 species within four wetlands (bog, fen, meadow, and marsh) contrasting in plant productivity. We sampled functional traits from individuals in each community and used hierarchical distributional modeling in order to analyze the independent variation of the mean and dispersion of functional trait space at ecosystem, community, and species levels. The plant productivity gradient observed between wetlands led to species turnover and selection of traits related to leaf nutrient conservation/acquisition strategy. Within wetlands, plant species richness drove trait variation across both communities and species. Among communities, variation of species richness correlated with the selection of individuals according to their use of vertical space and leaf adaptations to light conditions. Within species, intraspecific light-related trait variation in response to species richness was associated with stable population density for some species, while others reached low population density in more diverse communities. Within ecosystems, variation in biotic conditions selects individuals along functional dimensions that are independent of those selected across ecosystems. Within-species variations of light-related traits are related to demographic responses, linking biotic selection of individuals within communities to eco-evolutionary dynamics of species.

Ecological niche overlap in the Arctic vegetation influenced by seabirds

In the High Arctic, nutrients are the most limiting resources, so terrestrial vegetation is of low complexity and grows slowly. However, locally, large seabird colonies increase soil fertility by deposition of faeces, supporting the development of rich and fast-growing plant communities. Here, we assessed how seabird colonies affected ecological niche segregation of plants, across the fertilisation gradient. Study sites were located near five little auk colonies, distributed longitudinally across the Svalbard archipelago. We described vascular plant composition and identified 13 environmental variables, based on which, we calculated and tested the niche overlap (NO) between the 18 most frequent species. Based on the hierarchical classification of the NO matrix, we distinguished typical High Arctic Vegetation (HAV), and Bird-Cliff Vegetation (BCV). The BCV was characterised by higher average NO and soil δ15N compared to HAV. The highest NO values across the fertilisation gradient were found on the border between the distinguished communities and were positively correlated with species diversity. We suggest that in the High Arctic, seabirds-delivered nutrients lead to the development of separate plant communities through the mechanism of avoiding inter-species competition, while simultaneous high species diversity and NO are related to high facilitation between plants on the border between the communities.

Metabolite-Mediated Responses of Phyllosphere Microbiota to Rust Infection in Two Malus Species

Plants recruit beneficial microbes to enhance their ability to fight pathogens. However, the current understanding of microbial recruitment is largely limited to belowground systems (root exudates and the rhizosphere). It remains unclear whether the changes in leaf metabolites induced by infectious pathogens can actively recruit beneficial microbes to mitigate the growth of foliar pathogens. In this study, we integrated microbiome and metabolomic analyses to systematically explore the dynamics of phyllosphere fungal and bacterial communities and key leaf metabolites in two crabapple species (Malus sp. "Flame" and Malus sp. "Kelsey") at six stages following infection with Gymnosporangium yamadae. Our results showed that the phyllosphere microbiome changed during lesion expansion, as highlighted by a reduction in bacterial alpha-diversity and an increase in fungal alpha-diversity; a decreasing and then an increasing complexity of the microbial co-occurrence network was observed in Kelsey and a decreasing complexity occurred in Flame. In addition, nucleotide sugars, diarylheptanoids, and carboxylic acids with aromatic rings were more abundant in early stages of collection, which positively regulated the abundance of bacterial orders Pseudomonadales (in Kelsey), Acidimicrobiales, Bacillales, and Flavobacteriales (in Flame). In addition, metabolites such as flavonoids, lignin precursors, terpenoids, coumarins, and quaternary ammonium salts enriched with the expansion of lesions had a positive regulatory effect on fungal families Rhynchogastremataceae and Golubeviaceae (in Flame) and the bacterial order Actinomycetales (in Kelsey). Our findings highlight that plants may also influence phyllosphere microorganisms by adjusting leaf metabolites in response to biotic stress. IMPORTANCE Our findings demonstrate the response patterns of bacterial and fungal communities in the Malus phyllosphere to rust fungus G. yamadae infection, and they also reveal how the phyllosphere microbiome changes with the expansion of lesions. We identified several metabolites whose relative abundance varied significantly with lesion expansion. Using a framework for assessing the role of leaf metabolites in shaping the phyllosphere microbiome of the two Malus species, we identified several specific metabolites that have profoundly selective effects on the microbial community. In conclusion, our study provides new evidence of the ecological niche of the phyllosphere in supporting the "cry for help" strategy for plants.

Co-occurring epiphytic orchids have specialized mycorrhizal fungal niches that are also linked to ontogeny

Mycorrhizal symbiosis has been related to the coexistence and community assembly of coexisting orchids in few studies despite their obligate dependence on mycorrhizal partners to establish and survive. In hyper-diverse environments like tropical rain forests, coexistence of epiphytic orchids may be facilitated through mycorrhizal fungal specialization (i.e., sets of unique and dominant mycorrhizal fungi associated with a particular host species). However, information on the role of orchid mycorrhizal fungi (OMF) in niche differentiation and coexistence of epiphytic orchids is still scarce. In this study, we sought to identify the variation in fungal preferences of four co-occurring epiphytic orchids in a tropical rainforest in Costa Rica by addressing the identity and composition of their endophytic fungal and OMF communities across species and life stages. We show that the endophytic fungal communities are formed mainly of previously recognized OMF taxa, and that the four coexisting orchid species have both a set of shared mycorrhizal fungi and a group of fungi unique to an orchid species. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time. This study provides evidence for the utilization of specific OMF that may be involved in niche segregation, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.

Nitrogen enrichment and foliar fungal pathogens affect the mechanisms of multispecies plant coexistence

Changes in resources (e.g. nitrogen) and enemies (e.g. foliar pathogens) are key drivers of plant diversity and composition. However, their effects have not been connected to the niche and fitness differences that determine multispecies coexistence. Here, we combined a structuralist theoretical approach with a detailed grassland experiment factorially applying nitrogen addition and foliar fungal pathogen suppression to evaluate the joint effect of nitrogen and pathogens on niche and fitness differences, across a gradient from two to six interacting species. Nitrogen addition and pathogen suppression modified species interaction strengths and intrinsic growth rates, leading to reduced multispecies fitness differences. However, contrary to expected, we also observed that they promote stabilising niche differences. Although these modifications did not substantially alter species richness, they predicted major changes in community composition. Indirect interactions between species explained these community changes in smaller assemblages (three and four species) but lost importance in favour of direct pairwise interactions when more species were involved (five and six). Altogether, our work shows that explicitly considering the number of interacting species is critical for better understanding the direct and indirect processes by which nitrogen enrichment and pathogen communities shape coexistence in grasslands.

Contrasting effects of nitrogen and phosphorus additions on nitrogen competition between coniferous and broadleaf seedlings

Nitrogen (N) is a major element limiting plant growth and metabolism. Nitrogen addition can influence plant growth, N uptake, and species interactions, while phosphorus (P) addition may affect N acquisition. However, knowledge of how nutrient availability influences N uptake and species interactions remains limited and controversial. Here, pot experiments were conducted for 14 months, in which conifers (Pinus massoniana and Pinus elliottii) and broadleaved trees (Michelia maudiae and Schima superba) were planted in monoculture or mixture, and provided additional N and P in a full-factorial design. Nitrogen addition increased the biomass, but P addition did not significantly affect the biomass of the four subtropical species. Combined N and P (NP) addition had no additive effect on plant biomass over N addition. Total plant biomass was significantly positively correlated to root traits (branching intensity and root tissue density) and leaf traits (net photosynthetic rate, stomatal conductance, and transpiration rate), but negatively correlated to root diameter in response to nutrient addition. Plant uptake rates of NH₄⁺ or NO₃^- were not altered by N addition, but P or NP additions decreased NH₄⁺ uptake rates and increased NO₃^- uptake rates. Neighboring conifers significantly inhibited NH₄⁺ and NO₃^- uptake rates of the two broadleaf species, but neighboring broadleaves had no effects on the N uptake rates of pine species. The effects of nutrient additions on interspecific interactions differed among species. Nitrogen addition altered the interaction of P. elliottii and M. maudiae from neutral to competition, while P addition altered the interaction of P. massoniana and M. maudiae from neutral to favorable effects. Increasing nutrient availability switched the direction of interspecific interaction in favor of pines. This study provides insights into forest management for productivity improvement and optimizing the selection of broadleaf species regarding differences in soil fertility of subtropical plantations.

Nitrogen Preference of Dominant Species during Hailuogou Glacier Retreat Succession on the Eastern Tibetan Plateau

Plant nitrogen (N) uptake preference is a key factor affecting plant nutrient acquisition, vegetation composition and ecosystem function. However, few studies have investigated the contribution of different N sources to plant N strategies, especially during the process of primary succession of a glacial retreat area. By measuring the natural abundance of N isotopes (δ¹⁵N) of dominant plants and soil, we estimated the relative contribution of different N forms (ammonium-NH₄⁺, nitrate-NO₃^- and soluble organic N-DON) and absorption preferences of nine dominant plants of three stages (12, 40 and 120 years old) of the Hailuogou glacier retreat area. Along with the chronosequence of primary succession, dominant plants preferred to absorb NO₃^- in the early (73.5%) and middle (46.5%) stages. At the late stage, soil NH₄⁺ contributed more than 60.0%, In addition, the contribution of DON to the total N uptake of plants was nearly 19.4%. Thus, the dominant plants' preference for NO₃^- in the first two stages changes to NH₄⁺ in the late stages during primary succession. The contribution of DON to the N source of dominant plants should not be ignored. It suggests that the shift of N uptake preference of dominant plants may reflect the adjustment of their N acquisition strategy, in response to the changes in their physiological traits and soil nutrient conditions. Better knowledge of plant preferences for different N forms could significantly improve our understanding on the potential feedbacks of plant N acquisition strategies to environmental changes, and provide valuable suggestions for the sustainable management of plantations during different successional stages.

Salt marsh restoration surprise: A subordinate species accumulates and shares nitrogen while outcompeting salt marsh dominants

Selectively planting native species could guide ecosystem development toward wetland restoration targets, once we understand how influential species function, alone and in combination. Knowing that Triglochin concinna (arrow grass, Juncaceae) accumulates N in its perennial roots, we asked how it would influence N dynamics on an excavated salt marsh plain at Tijuana Estuary, in southern California. We hypothesized that it would (a) accumulate N in roots and shoots, (b) reduce biomass of other marsh plain plants or, alternatively, (c) share N with neighbors as its litter decomposed and released N. We used 15N stable isotope enrichment to quantify N transfer between Triglochin and the marsh plain’s seven-species halophyte assemblage in field and greenhouse experiments. We also examined the effect of Triglochin on individual marsh plain species’ biomass and N accumulation. Triglochin had low shoot biomass (0.96 ± 0.5 g m−2 in field plots and 17.64 ± 2.2 g m−2 in greenhouse pots), high root:shoot ratios (4.3 in the field and 2.0 in the greenhouse), and high tissue N content (1.9 ± 0.2% in the field and 1.7 ± 0.1% in the greenhouse). Two productive perennials, Sarcocornia pacifica (pickleweed) and Frankenia salina (alkali heath), outgrew Triglochin; yet these biomass dominants produced 44%–45% less shoot biomass in greenhouse pots with Triglochin than without. However, we did not find this reduction in the field where roots were unconfined. In the greenhouse, δ15N values were higher for species grown with 15N-enriched Triglochin, indicating that this species made N available to its neighbors. The δ15N values for plants grown in the field exceeded background levels, also indicating that the marsh plain assemblage took up N released by Triglochin. We conclude that Triglochin can influence the restoration of salt marsh vegetation by accumulating N and releasing its tissue N to neighbors as leaves and roots decompose, while simultaneously reducing the biomass of neighbors. The seasonally deciduous Triglochin is low in shoot biomass, yet competitively superior in N uptake. Because this often-ignored species has limited tidal dispersal, we suggest restoration plantings, including tests of its ability to facilitate diversity where S. pacifica, the marsh plain dominant, might otherwise form monocultures.

Plant nitrogen-use strategies and their responses to the urban elevation of atmospheric nitrogen deposition in southwestern China

The elevation of nitrogen (N) deposition by urbanization profoundly impacts the structure and function of surrounding forest ecosystems. Plants are major biomass sinks of external N inputs into forests. Yet, the N-use strategies of forest plants in many areas remain unconstrained in city areas, so their responses and adapting mechanisms to the elevated N deposition are open questions. Here we investigated concentrations and N isotope (δ¹⁵N) of total N (TN) and nitrate (NO₃^-) in leaves and roots of four plant species in subtropical shrubberies and pine forests under N deposition levels of 13 kg-N ha^-1 yr^-1 and 29 kg-N ha^-1 yr^-1 at the Guiyang area of southwestern China, respectively. The δ¹⁵N differences between plant NO₃^- and soil NO₃^- revealed a meager NO₃^- reduction in leaves but a preferentially high NO₃^- reduction in roots. δ¹⁵N mass-balance analyses between plant TN and soil dissolved N suggested that soil NO₃^- contributed more than reduced N, and dissolved organic N contributed comparably with ammonium to plant TN, and the study plants preferred NO₃^- over reduced N. The elevation of N deposition induced root but not leaf NO₃^- reduction and enhanced the contribution of soil NO₃^- to plant TN, but plant NO₃^- preference decreased due to much higher magnitudes of soil NO₃^- enrichment than plant NO₃^- utilization. We conclude that plants in subtropical forests of southwestern China preferred NO₃^- over reduced N, and NO₃^- was reduced more in roots than in leaves, anthropogenic N pollution enhanced soil NO₃^- enrichment and plant NO₃^- utilization but reduced plant NO₃^- preference.

Dung-visiting beetle diversity is mainly affected by land use, while community specialization is driven by climate

Dung beetles are important actors in the self-regulation of ecosystems by driving nutrient cycling, bioturbation, and pest suppression. Urbanization and the sprawl of agricultural areas, however, destroy natural habitats and may threaten dung beetle diversity. In addition, climate change may cause shifts in geographical distribution and community composition. We used a space-for-time approach to test the effects of land use and climate on α-diversity, local community specialization (H 2') on dung resources, and γ-diversity of dung-visiting beetles. For this, we used pitfall traps baited with four different dung types at 115 study sites, distributed over a spatial extent of 300 km × 300 km and 1000 m in elevation. Study sites were established in four local land-use types: forests, grasslands, arable sites, and settlements, embedded in near-natural, agricultural, or urban landscapes. Our results show that abundance and species density of dung-visiting beetles were negatively affected by agricultural land use at both spatial scales, whereas γ-diversity at the local scale was negatively affected by settlements and on a landscape scale equally by agricultural and urban land use. Increasing precipitation diminished dung-visiting beetle abundance, and higher temperatures reduced community specialization on dung types and γ-diversity. These results indicate that intensive land use and high temperatures may cause a loss in dung-visiting beetle diversity and alter community networks. A decrease in dung-visiting beetle diversity may disturb decomposition processes at both local and landscape scales and alter ecosystem functioning, which may lead to drastic ecological and economic damage.

Acquisition pattern of nitrogen by microorganisms and plants affected by gravel mulch in a semiarid Tibetan grassland

As an important coarse inorganic fraction of soil, gravel may regulate the effects of the interaction between above- and belowground communities and affect the relationship between microorganisms and plants in alpine ecosystems. However, comparatively little is known about the effects of gravel on the acquisition pattern of nitrogen (N) by microorganisms and plants in alpine ecosystems. In this study, a ¹⁵N-labelling experiment was conducted to investigate the acquisition pattern of organic (¹⁵N-glycine) and inorganic N (¹⁵N-NO₃^- and ¹⁵N-NH₄⁺) by microorganisms and plants under three particle sizes of gravel mulch (fine: 2-10 mm, medium: 10-20 mm, coarse: 20-40 mm) on a semiarid Tibetan grassland. Gravel mulch significantly improved the ¹⁵N recovery of Stipa purpurea, but had no significant impacts on A. nanschanica. Therefore, gravel mulch decreased the ratio of microbial biomass ¹⁵N recovery to plant biomass ¹⁵N recovery for S. purpurea, but caused little effect on the state of N competition between plants and soil microbes for A. nanschanica. The N absorption preference of plants from both species shifted from an individual preference for ¹⁵N-NO₃^- in the natural (i.e., control) microplots to a common preference for ¹⁵N-NO₃^-and ¹⁵N-NH₄⁺ in the fine- and medium-sized gravel mulch microplots, while there were no significant differences in microbial N recovery between ¹⁵N-NO₃^- and ¹⁵N-NH₄⁺ across all treatments. The results helped to improve the understanding of the acquisition pattern of N by microorganisms and plants under the influence of gravel mulch in alpine ecosystems, and provide theoretical support for revegetation in alpine ecosystems in the future.

Precipitation, Not Land Use, Primarily Determines the Composition of Both Plant and Phyllosphere Fungal Communities

Plant communities and fungi inhabiting their phyllospheres change along precipitation gradients and often respond to changes in land use. Many studies have focused on the changes in foliar fungal communities on specific plant species, however, few have addressed the association between whole plant communities and their phyllosphere fungi. We sampled plant communities and associated phyllosphere fungal communities in native prairie remnants and post-agricultural sites across the steep precipitation gradient in the central plains in Kansas, USA. Plant community cover data and MiSeq ITS2 metabarcode data of the phyllosphere fungal communities indicated that both plant and fungal community composition respond strongly to mean annual precipitation (MAP), but less so to land use (native prairie remnants vs. post-agricultural sites). However, plant and fungal diversity were greater in the native remnant prairies than in post-agricultural sites. Overall, both plant and fungal diversity increased with MAP and the communities in the arid and mesic parts of the gradient were distinct. Analyses of the linkages between plant and fungal communities (Mantel and Procrustes tests) identified strong correlations between the composition of the two. However, despite the strong correlations, regression models with plant richness, diversity, or composition (ordination axis scores) and land use as explanatory variables for fungal diversity and evenness did not improve the models compared to those with precipitation and land use (ΔAIC < 2), even though the explanatory power of some plant variables was greater than that of MAP as measured by R2. Indicator taxon analyses suggest that grass species are the primary taxa that differ in the plant communities. Similar analyses of the phyllosphere fungi indicated that many plant pathogens are disproportionately abundant either in the arid or mesic environments. Although decoupling the drivers of fungal communities and their composition - whether abiotic or host-dependent - remains a challenge, our study highlights the distinct community responses to precipitation and the tight tracking of the plant communities by their associated fungal symbionts.

Interactions between species change the uptake of ammonium and nitrate in Abies faxoniana and Picea asperata

Plant nitrogen (N) uptake is affected by plant-plant interactions, but the mechanisms remain unknown. A 15N-labeled technique was used in a pot experiment to analyze the uptake rate of ammonium (NH4+) and nitrate (NO3-) by Abies faxoniana Rehd. et Wils and Picea asperata Mast. in single-plant mode, intraspecific and interspecific interactions. The results indicated that the effects of plant-plant interactions on N uptake rate depended on plant species and N forms. Picea asperata had a higher N uptake rate of both N forms than A. faxoniana, and both species preferred NO3-. Compared with single-plant mode, intraspecific interaction increased NH4+ uptake for A. faxoniana but reduced that for P. asperata, while it did not change NO3- uptake for the two species. The interspecific interaction enhanced N uptake of both N forms for A. faxoniana but did not affect the P. asperata compared with single-plant mode. NH4+ and NO3- uptake rates for the two species were regulated by root N concentration, root nitrate reductase activity, root vigor, soil pH and soil N availability under plant-plant interactions. Decreased NH4+ uptake rate for P. asperata under intraspecific interaction was induced by lower root N concentration and nitrate reductase activity. The positive effects of interspecific interaction on N uptake for A. faxoniana could be determined mainly by positive rhizosphere effects, such as high soil pH. From the perspective of root-soil interactions, our study provides insight into how plant-plant interactions affect N uptake, which can help to understand species coexistence and biodiversity maintenance in forest ecosystems.

Temporal Variations Rather than Long-Term Warming Control Extracellular Enzyme Activities and Microbial Community Structures in the High Arctic Soil

In Arctic soils, warming accelerates decomposition of organic matter and increases emission of greenhouse gases (GHGs), contributing to a positive feedback to climate change. Although microorganisms play a key role in the processes between decomposition of organic matter and GHGs emission, the effects of warming on temporal responses of microbial activity are still elusive. In this study, treatments of warming and precipitation were conducted from 2012 to 2018 in Cambridge Bay, Canada. Soils of organic and mineral layers were collected monthly from June to September in 2018 and analyzed for extracellular enzyme activities and bacterial community structures. The activity of hydrolases was the highest in June and decreased thereafter over summer in both organic and mineral layers. Bacterial community structures changed gradually over summer, and the responses were distinct depending on soil layers and environmental factors; water content and soil temperature affected the shift of bacterial community structures in both layers, whereas bacterial abundance, dissolved organic carbon, and inorganic nitrogen did so in the organic layer only. The activity of hydrolases and bacterial community structures did not differ significantly among treatments but among months. Our results demonstrate that temporal variations may control extracellular enzyme activities and microbial community structure rather than the small effect of warming over a long period in high Arctic soil. Although the effects of the treatments on microbial activity were minor, our study provides insight that microbial activity may increase due to an increase in carbon availability, if the growing season is prolonged in the Arctic.

Plants Play Stronger Effects on Soil Fungal than Bacterial Communities and Co-Occurrence Network Structures in a Subtropical Tree Diversity Experiment

Increasing biodiversity loss profoundly affects community structure and ecosystem functioning. However, the differences in community assembly and potential drivers of the co-occurrence network structure of soil fungi and bacteria in association with tree species richness gradients are poorly documented. Here, we examined soil fungal and bacterial communities in a Chinese subtropical tree species richness experiment (from 1 to 16 species) using amplicon sequencing targeting the internal transcribed spacer 2 and V4 hypervariable region of the rRNA genes, respectively. Tree species richness had no significant effect on the diversity of either fungi or bacteria. In addition to soil and spatial distance, tree species richness and composition had a significant effect on fungal community composition but not on bacterial community composition. In fungal rather than bacterial co-occurrence networks, the average degree, degree centralization, and clustering coefficient significantly decreased, but the modularity significantly increased with increasing tree species richness. Fungal co-occurrence network structure was influenced by tree species richness and community composition as well as the soil carbon: nitrogen ratio, but the bacterial co-occurrence network structure was affected by soil pH and spatial distance. This study demonstrates that the community assembly and potential drivers of the co-occurrence network structure of soil fungi and bacteria differ in the subtropical forest. IMPORTANCE Increasing biodiversity loss profoundly affects community structure and ecosystem functioning. Therefore, revealing the mechanisms associated with community assembly and co-occurrence network structure of microbes along plant species diversity gradients is very important for understanding biodiversity maintenance and community stability in response to plant diversity loss. Here, we compared the differences in community assembly and potential drivers of the co-occurrence network structure of soil fungi and bacteria in a subtropical tree diversity experiment. In addition to soil and spatial distance, plants are more strongly predictive of the community and co-occurrence network structure of fungi than those of bacteria. The study highlighted that plants play more important roles in shaping community assembly and interactions of fungi than of bacteria in the subtropical tree diversity experiment.

Effects of Pedicularis kansuensis Expansion on Plant Community Characteristics and Soil Nutrients in an Alpine Grassland

Pedicularis kansuensis is an indicator species of grassland degradation. Its population expansion dramatically impacts the production and service function of the grassland ecosystem, but the effects and mechanisms of the expansion are still unclear. In order to understand the ecological effects of P. kansuensis, three P. kansuensis patches of different densities were selected in an alpine grassland, and species diversity indexes, biomasses, soil physicochemical properties, and the mechanism among them were analyzed. The results showed that P. kansuensis expansion increased the richness index, the Shannon−Wiener index significantly, and the aboveground biomass ratio (ABR) of the Weed group (p < 0.05), but reduced the total biomass of the community and the ABR of the Gramineae and Cyperaceae decreased insignificantly (p > 0.05); soil moisture, soil AOC, and NO3−·N decreased significantly (p < 0.05), while soil pH and total soil nutrients did not change significantly, and available phosphorus (AP) decreased at first and then increased (p < 0.05). The structural equation model (SEM) showed that P. kansuensis expansion had a significant positive effect on the community richness index, and a significant negative effect followed on the soil AOC from the increase of the index; the increase of pH had a significant negative effect on the soil AOC, NO3−·N, and AP. It indicated that P. kansuensis expansion resulted in the increase of species richness, the ABR of the Weed group, and the community’s water demand, which promoted the over-utilization of soil available nutrients in turn, and finally caused the decline of soil quality. This study elucidated a possible mechanism of poisonous weeds expansion, and provided a scientific and theoretical basis for grassland management.

Designing a Robust and Versatile System to Investigate Nutrient Exchange in, and Partitioning by, Mycorrhiza (Populus x canesces x Paxillus involutus) Under Axenic or Greenhouse Conditions

Phosphorus (P) bioavailability affects plant nutrition. P can be present in soils in different chemical forms that are not available for direct plant uptake and have to be acquired by different mechanisms, representing different resource niches. These mechanisms, of which many seem to be attributed to mycorrhiza, likely influence the diversity and stability of plant communities in natural ecosystems, as they also might help to overcome a future shortage of P supply in agro-ecosystems. In order to understand the mechanisms of P acquisition, the associated carbon costs, and the resource partitioning by mycorrhizal fungi, the ecosystem situation has to be mimicked in smaller scaled experiments. Here, different experimental setups are evaluated using plantlets of Populus x canescens and its functional ectomycorrhizal (ECM) fungus Paxillus involututs strain MAJ. To investigate resource partitioning involving mycorrhizae, the protocols of this study describe preparation of an in vitro and a rhizotrone culture systems for studies under axenic conditions as well as a mesocosm culture system for greenhouse conditions. We also describe the construction of separate compartments containing nutrients and excluding plant roots as well as the progress that has been made in in vitro propagation of plant and ECM fungal material. The practical experience made in our study shows that the in vitro culture system is prone to desiccation and its construction and maintenance are more time consuming and complicated. In contrast, with the axenic rhizotrone culture system and the mesocosms we have created more robust and very versatile systems that are also suitable for greenhouse conditions.

Effects of Habitat and Competition on Niche Partitioning and Community Structure in Neotropical Ants

Competition for limited resources can yield two contrasting outcomes in community structure, namely, either (i) dominance of most competitive species (with functional convergence of the traits conferring this ability), or (ii) niche partitioning of species using distinct resources. In addition, varying resource availability in different environmental contexts is expected to yield varying community dynamics and composition between the contexts (habitat filtering). We addressed resource-based ant community structure in a tropical ecosystem. We expected ant species to display varying trophic preferences and foraging behaviors, allowing habitat selection and niche differentiation in ant assemblages. Furthermore, we expected habitat filtering to occur between open and forested areas in the landscape mosaic, and competition to further influence local species co-occurrence. We assessed resource use in nine ant assemblages distributed in two habitats (i.e., forests and croplands), devising two separate experiments using bait-traps to characterize ant species’ trophic preference (e.g., eating prey, seeds, sugars) and their ability to obtain a same resource in heterogeneous forms (e.g., on vegetation, litter, with variable amounts…). The majority of baits offered were rapidly exploited in the two habitats suggesting important resource limitations. Forest and cropland ant communities differed, however, in the proportions of resources exploited, suggesting different competitive pressures toward specific resources between habitats. Within each habitat, ants preferentially exploited the same resources, suggesting habitat filtering, but locally, interspecific resource partitioning resulted in a reduction of resource overlap compared to habitat scale. Our study provides evidence of the effects of habitat filtering and competition for resource in tropical ant community structure. Our findings also suggest that niche filtering and niche partitioning are co-variant forces determining the identity of the species present in local assemblages.

Niche partitioning in nitrogen uptake among subtropical tree species enhances biomass production

Nitrogen (N) is a main nutrient limiting plant growth in most terrestrial ecosystems, but so far it remains unknown which role plant N uptake plays for the positive relationship between species richness and productivity. An in situ¹⁵N labeling experiment was carried out by planting four subtropical tree species (i.e., Koelreuteria bipinnata, Lithocarpus glaber, Cyclobalanopsis myrsinaefolia and Castanopsis eyrei) in pots, at richness levels 1, 2 and 4 species per pot. Plant N uptake preference for inorganic N form of NO₃^- to NH₄⁺ and organic N form of glycine, as well as biomass and plant functional traits was evaluated under different tree species richness level. Overall, pot biomass productivity increased with tree species richness. Biomass of the most productive species, K. bipinnata increased, but not at the expense of a decreased growth of the other species. In mixtures, the species shifted their preference for the inorganic N form, from NO₃^- to NH₄⁺ or vice versa. The uptake preference for glycine remained stable along the species richness gradient. Plant N uptake was well correlated with numerous functional traits, both aboveground, such as height and shoot diameter, and belowground, such as root diameter and root length. We conclude that increased ecosystem biomass production with tree species richness could be largely explained by niche partitioning in N uptake among tree species. Our findings highlight that niche partitioning for N uptake should be a possible important mechanism maintaining species diversity and ecosystem production in subtropical forests.

Eutrophication decrease compositional dissimilarity in freshwater plankton communities

Human activities, such as land use change and eutrophication, threaten freshwater biodiversity and ecosystem function. In this study, we examined both the α- and β-diversity of plankton communities, that is, bacteria/prokaryotic algae, eukaryotic algae, and zooplankton/metazoans, using both classical microscopy and high-throughput sequencing methods across 40 lakes of the Yangtze River Basin. The spatial variations in plankton communities were explained by environmental variables such as trophic status index (TSI) and environmental heterogeneity according to non-metric multidimensional scaling analyses, mantel tests, and structural equation model. Our results showed that the compositional dissimilarities of bacteria, cyanobacteria, eukaryotic algae, and metazoans all decreased with the increasing TSI values, and were significantly positively related to environmental dissimilarity. Both the species richness and compositional dissimilarity of zooplankton had positive effects on zooplankton/phytoplankton biomass ratio. Zooplankton diversity was not directly affected by TSI and environmental dissimilarity; however, it was indirectly affected by the biotic interactions with cyanobacteria or eukaryotic algae. In addition, there were significant positive relationships between bacteria/cyanobacteria and eukaryotic algae dissimilarities. Our results indicated that increased trophic status and decreased environmental dissimilarity as consequences of eutrophication may weaken the trophic cascading effects of planktonic food chain via reducing the top-down effects of zooplankton on phytoplankton.

Distinct kin strategies of the legume soybean and the non-legume balsam by accomplishing different nitrogen acquisition and rhizosphere microbiome composition

Kin selection has been proposed vvto be an important mechanism for plant relatives growing together. To reveal kin recognition, we used 15N labeling to assess the short‐term nitrogen (N) acquisition (uptake of nitrate and ammonium), long‐term N utilization (recovery of added urea), N‐use efficiency (NUE) and rhizosphere microbiome in leguminous Glycine max and non‐leguminous Impatiens balsamina. Individuals of each species were planted pairwise with either a sibling or a stranger. Enzyme activity and soil microbial composition were compared between kinship groups. Compared with strangers, G. max siblings increased aboveground biomass, NUE, and nitrogenase activity, whereas I. balsamina siblings decreased root biomass and increased uptake rate of nitrate and potential nitrification rate. Plant kinship affected soil bacterial communities by enriching specific groups possessing explicit eco‐functions (Rhizobiales for G. max and Nitrospira for I. balsamina). Kinship‐sensitive operational taxonomic units formed independent modules in the bacterial co‐occurrence network and were positively correlated with plant growth performance, N acquisition and enzymatic activity. Plant kin recognition may depend on the growth strategies of the plant species. Kin selection was dominant in G. max by enhancing biological N fixation through the enrichment of symbiotic rhizobia (demonstrated by aboveground growth and NUE superiority among siblings). Kin selection and niche partitioning occurred simultaneously in I. balsamina, expressed through reduced root allocation but increased nitrate uptake, and enhanced soil N nitrification, by enriching functional microbial groups. Kin recognition responses are the consequence of complex interactions among the host plant, the microbiome, and soil nutrient cycling and utilization processes.