The allometry of plant height explains species loss under nitrogen addition

Response of ecological stoichiometry and homeostasis characteristic to nitrogen addition in Hippophae rhamnoides L

Nitrogen (N) in the atmosphere frequently affects plant growth, ecological stoichiometric equilibrium, and homeostasis stability. However, the effect of N addition application on the growth of Hippophae rhamnoides seedlings remains ambiguous. We investigated the effects of N addition on the ecological stoichiometry and homeostatic characteristics of H. rhamnoides seedlings. Greenhouse cultivation experiments were conducted at five N application levels: 0 kg ha-1 yr-1(CK), 100 kg ha-1 yr-1 (N10), 200 kg ha-1 yr-1 (N20), 400 kg ha-1 yr-1 (N40), 800 kg ha-1 yr-1 (N80). The results showed that pH and available phosphorus (AP) significantly decreased with increasing N, whereas soil C:P and N:P ratios significantly increased under the N40 and N80 treatments. The leaf C:N ratio significantly decreased with increasing N, whereas the N:P ratio increased. With N addition, the C:N ratio of plant stems and roots significantly decreased, whereas the C:P and N:P ratios significantly increased. N addition was significantly correlated with the ecological stoichiometry of plant leaves and soil properties (0.38 and 0.84, respectively). Homeostasis of the organs of H. rhamnoides seedlings exhibited an absolute steady state. The C, N, and C:P ratios of the roots exhibited insensitive states under the N40 treatment. N addition significantly modified both the soil ecological stoichiometry and the stoichiometry of H. rhamnoides seedlings. However, it did not demonstrate a pronounced negative effect on the homeostasis of H. rhamnoides seedlings. This study offers new insights into the ecological adaptation process of H. rhamnoides, particularly concerning its nutrient distribution, utilization strategies, and stability.

Mechanisms of biodiversity loss under nitrogen enrichment: unveiling a shift from light competition to cation toxicity

The primary mechanisms contributing to nitrogen (N) addition induced grassland biodiversity loss, namely light competition and soil cation toxicity, are often examined separately in various studies. However, their relative significance in governing biodiversity loss along N addition gradient remains unclear. We conducted a 4-yr field experiment with five N addition rates (0, 2, 10, 20, and 50 g N m-2 yr-1) and performed a meta-analysis using global data from 239 observations in N-fertilized grassland ecosystems. Results from our field experiment and meta-analysis indicate that both light competition and soil cation (e.g. Mn2+ and Al3+) toxicity contribute to plant diversity loss under N enrichment. The relative importance of these mechanisms varied with N enrichment intensity. Light competition played a more significant role in influencing species richness under low N addition (≤ 10 g m-2 yr-1), while cation toxicity became increasingly dominant in reducing biodiversity under high N addition (>10 g m-2 yr-1). Therefore, a transition from light competition to cation toxicity occurs with increasing N availability. These findings imply that the biodiversity loss along the N gradient is regulated by distinct mechanisms, necessitating the adoption of differential management strategies to mitigate diversity loss under varying intensities of N enrichment.

Explaining the mechanisms behind niche dimensionality and light-driving species diversity based on functional traits

Two prevalent ecological mechanisms, niche dimensionality and light asymmetry, may well explain species loss with fertilization gradients in grassland communities. Although there is still controversy surrounding the two competitive mechanisms that maintain species coexistence, few studies have examined the patterns of change in dissimilarity in species composition (β-diversity) and the relative explanatory contributions of plant functional traits to α- and β-diversity when multiple resources are added. To clarify this knowledge gap, we conducted a 6-year experiment of resource addition in an alpine meadow on the Qinghai-Tibet Plateau to assess how species richness and spatial β-diversity are affected by increasing numbers of added resources (NAR) and light limitation. Our results found that both NAR and light limitation led to decreased species richness, suggesting that niche dimensionality and light asymmetry may contribute equally to species loss, rather than either alone. Moreover, NAR is the primary factor responsible for the increase in β-diversity, which exhibits a negative relationship with species richness. Furthermore, the increase in height is the most likely explanation for β-diversity, while the increase in SLA is the most likely explanation for species richness, thereby indicating the changes in species richness and composition can be effectively explained by the response of certain morphological functional traits with the addition of multiple resources. Future research should focus on the complex interactions of different ecological mechanisms that contribute to the maintenance of biodiversity in grassland ecosystems all over the world.

Responses in species diversity in the Hulunbuir grassland to phosphorus addition under nitrogen-limiting and non-limiting conditions

The phenomenon of nitrogen deposition resulting in species loss in terrestrial ecosystems has been demonstrated in several experiments. Nitrogen (N) and phosphorus (P), as major nutrients required for plant growth, exhibit ecological stoichiometric coupling in many ecosystems. The increased availability of nitrogen can exacerbate the ecological effects of phosphorus. To reveal the ecological effects of phosphorus under nitrogen-limiting and non-limiting conditions, we conducted a controlled N-P interaction experiment over 5 years in the Hulunbuir meadow steppe, where two nitrogen addition levels were implemented: 0 g N·m-2·a-1 (nitrogen-limiting condition) and 10 g N·m-2·a-1 (nitrogen-non-limiting condition), together with six levels of phosphorus addition (0, 2, 4, 6, 8, and 10 g P·m-2·a-1). The results showed that nitrogen addition (under nitrogen-non-limiting conditions) significantly decreased species diversity in the steppe community, which was exacerbated under phosphorus addition. Under nitrogen-limiting conditions, phosphorus addition had no marked impact on species diversity compared to the control; however, there were substantial differences between different levels of phosphorus addition, exhibiting a unimodal change. Under both experimental nitrogen conditions, the addition of 6 g P·m-2·a-1 was the threshold for affecting the community species diversity. Nitrogen addition reduced the relative biomass of legumes, bunch grasses, and forbs, but substantially increased the relative biomass of rhizomatous grasses. In contrast, phosphorus addition only markedly affected the relative biomass of forbs and rhizomatous grasses, with the former showing a unimodal pattern of first increasing and then decreasing with increasing phosphorus addition level, and the latter exhibiting the opposite pattern. The different responses of rhizomatous grasses and other functional groups to nitrogen and phosphorus addition were observed to have a regulatory effect on the changes in grassland community structure. Phosphorus addition may increase the risk of nitrogen deposition-induced species loss. Both nitrogen and phosphorus addition lead to soil acidification and an increase in the dominance of the already-dominant species, and the consequent species loss in the forb functional group represents the main mechanism for the reduction in community species diversity.

Nitrogen deposition differentially regulates the sensitivity of gross primary productivity to extreme drought versus wetness

Global hydroclimatic variability is increasing with more frequent extreme dry and wet years, severely destabilizing terrestrial ecosystem productivity. However, what regulates the consequence of precipitation extremes on productivity remains unclear. Based on a 9-year field manipulation experiment on the Qinghai-Tibetan Plateau, we found that the responses of gross primary productivity (GPP) to extreme drought and wetness were differentially regulated by nitrogen (N) deposition. Over increasing N deposition, extreme dry events reduced GPP more. Among the 12 biotic and abiotic factors examined, this was mostly explained by the increased plant canopy height and proportion of drought-sensitive species under N deposition, making photosynthesis more sensitive to hydraulic stress. While extreme wet events increased GPP, their effect did not shift over N deposition. These site observations were complemented by a global synthesis derived from the GOSIF GPP dataset, which showed that GPP sensitivity to extreme drought was larger in ecosystems with higher N deposition, but GPP sensitivity to extreme wetness did not change with N deposition. Our findings indicate that intensified hydroclimatic variability would lead to a greater loss of land carbon sinks in the context of increasing N deposition, due to that GPP losses during extreme dry years are more pronounced, yet without a synchronous increase in GPP gains during extreme wet years. The study implies that the conservation and management against climate extremes merit particular attention in ecosystems subject to N deposition.

Responses of Intraspecific and Interspecific Trait Variations to Nitrogen Addition in a Tibetan Alpine Meadow

A community functional structure may respond to environmental changes such as nitrogen (N) enrichment by altering intraspecific and interspecific trait variations. However, the relative contributions of both components in determining the community response to N enrichment are unclear. In this study, we measured the plant height (H), leaf area (LA), leaf dry matter content (LDMC), and specific leaf area (SLA) based on a nine-year N addition gradient experiment in an alpine meadow on the Tibetan Plateau. We examined the intraspecific and interspecific variations within and among the communities, the responses of traits in terms of community weighted mean (CWM) and non-weighted mean (CM) to N addition, and the effects of these trait variations on aboveground net primary productivity (ANPP). Our results show that N addition increased the interspecific variation in H while decreasing that of LA within the community, whereas it had no significant effects on the intraspecific variations in the four traits within the community. In contrast, N addition significantly increased the intraspecific variation in H and decreased that of LA among the communities. Moreover, the contribution of intraspecific variation was greater than that of the interspecific variation in terms of CWM for all traits, while the opposite contribution was observed in terms of CM, suggesting that the dominant species would have greater resilience while subdominant species would become less resistant to N addition. Further, intraspecific variations of LA and LDMC within the community played an important role in explaining community productivity. Our results highlight the importance of both intraspecific and interspecific variations in mediating functional trait responses to N enrichment, and intraspecific variation within the communities has important implications for community functioning that should be considered to better understand and predict the responses of the alpine grasslands to N enrichment.

Plant height as an indicator for alpine carbon sequestration and ecosystem response to warming

Growing evidence indicates that plant community structure and traits have changed under climate warming, especially in cold or high-elevation regions. However, the impact of these warming-induced changes on ecosystem carbon sequestration remains unclear. Using a warming experiment on the high-elevation Qinghai-Tibetan Plateau, we found that warming not only increased plant species height but also altered species composition, collectively resulting in a taller plant community associated with increased net ecosystem productivity (NEP). Along a 1,500 km transect on the Plateau, taller plant community promoted NEP and soil carbon through associated chlorophyll content and other photosynthetic traits at the community level. Overall, plant community height as a dominant trait is associated with species composition and regulates ecosystem C sequestration in the high-elevation biome. This trait-based association provides new insights into predicting the direction, magnitude and sensitivity of ecosystem C fluxes in response to climate warming.

Plant size-dependent influence of foliar fungal pathogens promotes diversity through allometric growth

The effect of pathogens on host diversity has attracted much attention in recent years, yet how the influence of pathogens on individual plants scales up to affect community-level host diversity remains unclear. Here, we assessed the effects of foliar fungal pathogens on plant growth and species richness using allometric growth theory in population-level and community-level foliar fungal pathogen exclusion experiments. We calculated growth scaling exponents of 24 species to reveal the intraspecific size-dependent effects of foliar fungal pathogens on plant growth. We also calculated the intercepts to infer the growth rates of relatively larger conspecific individuals. We found that foliar fungal pathogens inhibited the growth of small conspecific individuals more than large individuals, resulting in a positive allometric growth. After foliar fungal pathogen exclusion, species-specific growth scaling exponents and intercepts decreased, but became positively related to species' relative abundance, providing a growth advantage for individuals of abundant species with a higher growth scaling exponent and intercept compared with rare species, and thus reduced species diversity. By adopting allometric growth theory, we elucidate the size-dependent mechanisms through which pathogens regulate species diversity and provide a powerful framework to incorporate antagonistic size-dependent processes in understanding species coexistence.

Fast-slow traits predict competition network structure and its response to resources and enemies

Plants interact in complex networks but how network structure depends on resources, natural enemies and species resource-use strategy remains poorly understood. Here, we quantified competition networks among 18 plants varying in fast-slow strategy, by testing how increased nutrient availability and reduced foliar pathogens affected intra- and inter-specific interactions. Our results show that nitrogen and pathogens altered several aspects of network structure, often in unexpected ways due to fast and slow growing species responding differently. Nitrogen addition increased competition asymmetry in slow growing networks, as expected, but decreased it in fast growing networks. Pathogen reduction made networks more even and less skewed because pathogens targeted weaker competitors. Surprisingly, pathogens and nitrogen dampened each other's effect. Our results show that plant growth strategy is key to understand how competition respond to resources and enemies, a prediction from classic theories which has rarely been tested by linking functional traits to competition networks.

Contrasting roles of fungal and oomycete pathogens in mediating nitrogen addition and winter grazing effects on biomass

Both bottom-up and top-down processes modulate plant communities. Fungal and oomycete pathogens are most common in global grasslands, and due to differences in their physiology, function, host range, and life cycles, they may differentially affect plants (in both intensity and direction). However, how fungal and oomycete pathogens regulate bottom-up and top-down effects on plant community biomass remains unclear. To this end, we conducted a 3-year field experiment in an alpine meadow incorporating mammalian herbivore exclosure, fungicide/oomyceticide application, and nitrogen addition treatments. We arranged 12 blocks with half randomly assigned to be mammalian herbivore exclosures (fenced to exclude grazing sheep), and the other half were fenced most of the year but not in winter (winter grazing control). Six 2.5 × 2.5 m square plots were established in each block, with each of the six plots assigned as control, nitrogen addition, fungicide application, oomyceticide application, nitrogen addition + fungicide application, and nitrogen addition + oomyceticide application. We found that fungicide application significantly increased plant community biomass (mainly Poaceae species) under nitrogen addition and promoted the bottom-up effect of nitrogen addition on plant community biomass by altering the community-weighted mean of plant height (via species turnover). Meanwhile, oomyceticide application significantly increased plant community biomass (mainly Poaceae species) when mammalian herbivores were excluded and weakened the top-down effect of winter grazing on plant community biomass by driving intraspecific variation in plant height. Our results highlight that fungal and oomycete pathogens play important (but differing) roles in mediating the effects of nutrient availability and higher trophic levels on plant community biomass. Mechanistically, we demonstrated that plant pathogen-related modulation of plant community biomass is achieved by alterations to plant height. Overall, this study combines both community and disease ecology to reveal complex interactions among higher trophic levels and their potential impacts on terrestrial ecosystem functioning under human disturbance.

Reproductive height determines the loss of clonal grasses with nitrogen enrichment in a temperate grassland

Tall clonal grasses commonly display competitive advantages with nitrogen (N) enrichment. However, it is currently unknown whether the height is derived from the vegetative or reproductive module. Moreover, it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization, and determines species diversity. In this study, the impacts on clonal grasses were studied in a field experiment employing two frequencies (twice a year vs. monthly) crossing with nine N addition rates in a temperate grassland, China. We found that the N addition decreased species frequency and increased extinction probability, but did not change the species colonization probability. A low frequency of N addition decreased species frequency and colonization probability, but increased extinction probability. Moreover, we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions. The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity, suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition. Overall, this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.

Fungal pathogens increase community temporal stability through species asynchrony regardless of nutrient fertilization

Natural enemies and their interaction with host nutrient availability influence plant population dynamics, community structure, and ecosystem functions. However, the way in which these factors influence patterns of community stability, as well as the direct and indirect processes underlying that stability, remains unclear. Here, we investigated the separate and interactive roles of fungal/oomycete pathogens and nutrient fertilization on the temporal stability of community biomass and the potential mechanisms using a factorial experiment in an alpine meadow. We found that fungal pathogen exclusion reduced community temporal stability mainly through decreasing species asynchrony, while fertilization tended to reduce community temporal stability by decreasing species stability. However, there was no interaction between pathogen exclusion and nutrient fertilization. These effects were largely due to the direct effects of the treatments on plant biomass and not due to indirect effects mediated through plant diversity. Our findings highlight the need for a multitrophic perspective in field studies examining ecosystem stability.

Host plant height explains the effect of nitrogen enrichment on arbuscular mycorrhizal fungal communities

Nitrogen (N) enrichment is widely known to affect the root-associated arbuscular mycorrhizal fungal (AMF) community in different ways, for example, via altering soil properties and/or shifting host plant functional structure. However, empirical knowledge of their relative importance is still lacking. Using a long-term N addition experiment, we measured the AMF community taxonomic and phylogenetic diversity at the single plant species (roots of 15 plant species) and plant community (mixed roots) levels. We also measured four functional traits of 35 common plant species along the N addition gradient. We found divergent responses of AMF diversity to N addition for host plants with different innate heights (i.e. plant natural height under unfertilized treatment). Furthermore, our data showed that species-specific responses of AMF diversity to N addition were negatively related to the change in maximum plant height. When scaling up to the community level, N addition affected AMF diversity mainly through increasing the maximum plant height, rather than altering soil properties. Our results highlight the importance of plant height in driving AMF community dynamics under N enrichment at both species and community levels, thus providing important implications for understanding the response of AMF diversity to anthropogenic N deposition.

Assessing community assembly controls over community-scale nutrient resorption responses to nitrogen deposition

Nutrient resorption is a fundamental physiological process in plants, with important ecological controls over numerous ecosystem functions. However, the role of community assembly in driving responses of nutrient resorption to perturbation remains largely unknown. Following the Price equation framework and the Community Assembly and Ecosystem Function framework, we quantified the contribution of species loss, species gain, and shared species to the reduction of community-level nutrient resorption efficiency in response to multi-level nitrogen (N) addition in a temperate steppe, after continuous N addition for seven years. Reductions of both N and phosphorus (P) resorption efficiency (NRE and PRE, respectively) were positively correlated with N addition levels. The dissimilarities in species composition between N-enriched and control communities increased with N addition levels, and N-enriched plots showed substantial species losses and gains. Interestingly, the reduction of community-scale NRE and PRE mostly resulted from N-induced decreases in resorption efficiency for the shared species in the control and N-enriched communities. There were negative correlations between the contributions of species richness effect and species identity effect and between the number and identity of species gained for the changes in both NRE and PRE following N enrichment. By simultaneously considering N-induced changes in species composition and in species-level resorption, our work presents a more complete picture of how different community assembly processes contribute to N-induced changes in community-level resorption.

Compensatory responses of vital rates attenuate impacts of competition on population growth and promote coexistence

Competition is among the most important factors regulating plant population and community dynamics, but we know little about how different vital rates respond to competition and jointly determine population growth and species coexistence. We conducted a field experiment and parameterised integral projection models to model the population growth of 14 herbaceous plant species in the absence and presence of neighbours across an elevation gradient (284 interspecific pairs). We found that suppressed individual growth and seedling establishment contributed the most to competition-induced declines in population growth, although vital rate contributions varied greatly between species and with elevation. In contrast, size-specific survival and flowering probability and seed production were frequently enhanced under competition. These compensatory vital rate responses were nearly ubiquitous (occurred in 92% of species pairs) and significantly reduced niche overlap and stabilised coexistence. Our study highlights the importance of demographic processes for regulating population and community dynamics, which has often been neglected by classic coexistence theories.

Spatial scale-dependent dilution effects of biodiversity on plant diseases in grasslands

The rapid biodiversity losses of the Anthropocene have motivated ecologists to understand how biodiversity affects infectious diseases. Spatial scale is thought to moderate negative biodiversity-disease relationships (i.e., dilution effects) in zoonotic diseases, whereas evidence from plant communities for an effect of scale remains limited, especially at local scales where the mechanisms (e.g., encounter reduction) underlying dilution effects actually work. Here, we tested how spatial scale affects the direction and magnitude of biodiversity-disease relationships. We utilized a 10-year-old nitrogen addition experiment in a Tibetan alpine meadow, with 0, 5, 10, and 15 g/m² nitrogen addition treatments. Within the treatment plots, we arranged a total of 216 quadrats (of either 0.125 × 0.125 m, 0.25 × 0.25 m or 0.5 × 0.5 m size) to test how the sample area affects the relationship between plant species richness and foliar fungal disease severity. We found that the dilution effects were stronger in the 0.125 × 0.125 m and 0.25 × 0.25 m quadrats, compared with 0.5 × 0.5 m quadrats. There was a significant interaction between species richness and nitrogen addition in the 0.125 × 0.125 m and 0.25 × 0.25 m quadrats, indicating that a dilution effect was more easily observed under higher levels of nitrogen addition. Based on multigroup structural equation models, we found that even accounting for the direct impact of nitrogen addition (i.e., "nitrogen-disease hypothesis"), the dilution effect still worked at the 0.125 × 0.125 m scale. Overall, these findings suggest that spatial scale directly determines the occurrence of dilution effects, and can partly explain the observed variation in biodiversity-disease relationships in grasslands. Next-generation frameworks for predicting infectious diseases under rapid biodiversity loss scenarios need to incorporate spatial information.

Positive Effects of Land Use Change on Wintering Bar-Headed Geese between 2010 and 2021

Human-induced land use change often drives species losses, yet some species can derive benefits from particular land use changes. Thus, case studies of how specific land use changes affect population size for species of interest are essential to their conservation. In this study, wintering bar-headed geese in Caohai, in Guizhou Province in China, were fitted with satellite trackers to assess their use of different land types and the impact of land use changes occurring between 2010 and 2021. We found that bar-headed geese preferentially spent time in arable lands, grasslands, and open water; most foraging occurred in cropland (59.5%) and grasslands (26.4%), while resting occurred in open water (68.3%) and in grasslands (43.5%). The population of wintering bar-headed geese in Caohai increased in size from 1366 to 2803 between 2010 and 2021. A concomitant decrease in cropland area (10.7%) and increase in open water (5.52%) and grasslands (48.45%) positively affected population growth. The use of abandoned croplands reduced human disturbance of goose foraging, while larger water and grassland areas provided more foraging and resting opportunities for bar-headed geese. Our study reveals a positive impact of recent land use changes on waterbird populations and provides a case study for managing human-wildlife relationships and protecting waterbirds and other wildlife.

Evolutionary effects of nitrogen are not easily predicted from ecological responses

PREMISE

Anthropogenic nitrogen (N) addition alters the abiotic and biotic environment, potentially leading to changes in patterns of natural selection (i.e., trait-fitness relationships) and the opportunity for selection (i.e., variance in relative fitness). Because N addition favors species with light acquisition strategies (e.g., tall species), we predicted that N would strengthen selection favoring those same traits. We also predicted that N could alter the opportunity for selection via its effects on mean fitness and/or competitive asymmetries.

METHODS

We quantified the strength of selection and the opportunity for selection in replicated populations of the annual grass Setaria faberi (giant foxtail) growing in a long-term N addition experiment. We also correlated these population-level parameters with community-level metrics to identify the proximate causes of N-mediated evolutionary effects.

RESULTS

N addition increased aboveground productivity, light asymmetry, and reduced species diversity. Contrary to expectations, N addition did not strengthen selection for trait values associated with higher light acquisition such as greater height and specific leaf area (SLA); rather, it strengthened selection favoring lower SLA. Light asymmetry and species diversity were associated with selection for height and SLA, suggesting a role for these factors in driving N-mediated selection. The opportunity for selection was not influenced by N addition but was negatively associated with species diversity.

CONCLUSIONS

Our results indicate that anthropogenic N enrichment can affect evolutionary processes, but that evolutionary changes in plant traits within populations are unlikely to parallel the shifts in plant traits observed at the community level.

Belowground Root Competition Alters the Grass Seedling Establishment Response to Light by a Nitrogen Addition and Mowing Experiment in a Temperate Steppe

Predicting species responses to climate change and land use practices requires understanding both the direct effects of environmental factors as well as the indirect effects mediated by changes in belowground and aboveground competition. Belowground root competition from surrounding vegetation and aboveground light competition are two important factors affecting seedling establishment. However, few studies have jointly examined the effect of belowground root and light competition on seedling establishment, especially under long-term nitrogen addition and mowing. Here, we examined how belowground root competition from surrounding vegetation and aboveground light competition affect seedling establishment within a long-term nitrogen addition and mowing experiment. Seedlings of two grasses (Stipa krylovii and Cleistogenes squarrosa) were grown with and without belowground root competition under control, nitrogen addition, and mowing treatments, and their growth characteristics were monitored. The seedlings of the two grasses achieved higher total biomass, height, mean shoot and root mass, but a lower root/shoot ratio in the absence than in the presence of belowground root competition. Nitrogen addition significantly decreased shoot biomass, root biomass, and the survival of the two grasses. Regression analyses revealed that the biomass of the two grass was strongly negatively correlated with net primary productivity under belowground root competition, but with the intercept photosynthetic active radiation in the absence of belowground root competition. This experiment demonstrates that belowground root competition can alter the grass seedling establishment response to light in a long-term nitrogen addition and mowing experiment.

Above- and below-ground resource acquisition strategies determine plant species responses to nitrogen enrichment

BACKGROUND AND AIMS

Knowledge of plant resource acquisition strategies is crucial for understanding the mechanisms mediating the responses of ecosystems to external nitrogen (N) input. However, few studies have considered the joint effects of above-ground (light) and below-ground (nutrient) resource acquisition strategies in regulating plant species responses to N enrichment. Here, we quantified the effects of light and non-N nutrient acquisition capacities on species relative abundance in the case of extra N input.

METHODS

Based on an N-manipulation experiment in a Tibetan alpine steppe, we determined the responses of species relative abundances and light and nutrient acquisition capacities to N enrichment for two species with different resource acquisition strategies (the taller Stipa purpurea, which is colonized by arbuscular mycorrhizal fungi, and the shorter Carex stenophylloides, which has cluster roots). Structural equation models were developed to explore the relative effects of light and nutrient acquisition on species relative abundance along the N addition gradient.

KEY RESULTS

We found that the relative abundance of taller S. purpurea increased with the improved light acquisition along the N addition gradient. In contrast, the shorter C. stenophylloides, with cluster roots, excelled in acquiring phosphorus (P) so as to elevate its leaf P concentration under N enrichment by producing large amounts of carboxylate exudates that mobilized moderately labile and recalcitrant soil P forms. The increased leaf P concentration of C. stenophylloides enhanced its light use efficiency and promoted its relative abundance even in the shade of taller competitors.

CONCLUSIONS

Our findings highlight that the combined effects of above-ground (light) and below-ground (nutrient) resources rather than light alone (the prevailing perspective) determine the responses of grassland community structure to N enrichment.