Different inter-annual responses to availability and form of nitrogen explain species coexistence in an alpine meadow community after release from grazing

Root pH variation of herbaceous plants among plant functional groups in response to climate and soil gradients on the Tibetan alpine grasslands

Plant pH is an emerging functional trait that plays important roles in physiological processes and nutrient cycling. However, how root pH varies among plant functional groups (PFGs) and the regulatory factors on a large scale remain unclear. Therefore, we quantified root pH variation of herbaceous plants in four PFGs from 20 sites on the Tibetan Plateau along a 1600 km transect and explored the correlations between root pH and different PFGs, climate and soil conditions. The results showed that the root pH of herbaceous plants was slightly acidic (6.46 ± 0.05). Grasses had the highest root pH (6.91 ± 0.10) across all functional groups (p < .05), whereas legumes had the lowest (5.90 ± 0.08; p < .05). The root pH decreased with mean annual precipitation, aridity index, soil water content and soil stress coefficient, whereas the significant positive correlation with soil pH. PFGs, climate and soil explained 5.39, 11.15 and 24.94% of the root pH variance, respectively. This study provided a comprehensive analysis of root pH patterns in herbaceous plants over a large spatial scale. Root pH was controlled by the combined influence of PFGs, climate and soil properties, with moisture status being the main influential factor. In contrast to the leaf pH, the root pH of herbaceous plants is strongly affected by the soil pH along environmental gradients. Our findings provide new insights into root functional traits and survival strategies of herbaceous plants in alpine ecosystems.

Divergent impacts of fertilization regimes on below-ground prokaryotic and eukaryotic communities in the Tibetan Plateau

Chemical nutrient amendment by human activities can lead to environmental impacts contributing to global biodiversity loss. However, the comprehensive understanding of how below- and above-ground biodiversity shifts under fertilization regimes in natural ecosystems remains elusive. Here, we conducted a seven-year field experiment (2011-2017) and examined the effects of different fertilization on plant biodiversity and soil belowground (prokaryotic and eukaryotic) communities in the alpine meadow of the Tibetan Plateau, based on data collected in 2017. Our results indicate that nitrogen addition promoted total plant biomass but reduced the plant species richness. Conversely, phosphorus enrichment did not promote plant biomass and exhibited an unimodal pattern with plant richness. In the belowground realm, distinct responses of soil prokaryotic and eukaryotic communities were observed under fertilizer application. Specifically, soil prokaryotic diversity decreased with nitrogen enrichment, correlating with shifts in soil pH. Similarly, soil eukaryotic diversity decreased with increased phosphorous inputs, aligning with the equilibrium between soil available and total phosphorus. We also established connections between these soil organism communities with above-ground plant richness and biomass. Overall, our study contributes to a better understanding of the sustainable impacts of human-induced nutrient enrichment on the natural environment. Future research should delve deeper into the long-term effects of fertilization on soil health and ecosystem functioning, aiming to achieve a balance between agricultural productivity and environmental conservation.

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.

Functional Groups Dominate Aboveground Net Primary Production under Long-Term Nutrient Additions in a Tibetan Alpine Meadow

Anthropogenic nutrient additions are influencing the structure and function of alpine grassland ecosystems. However, the underlying mechanisms of the direct and indirect effects of nutrient additions on aboveground net primary productivity (ANPP) are not well understood. In this study, we conducted an eight-year field experiment to explore the ecological consequences of nitrogen (N) and/or phosphorous (P) additions on the northern Tibetan Plateau. ANPP, species diversity, functional diversity, and functional groups were used to assess species' responses to increasing nutrients. Our results showed that nutrient additions significantly increased ANPP due to the release in nutrient limitations. Although N addition had a significant effect on species richness and functional richness, and P and N + P additions altered functional diversity, it was functional groups rather than biodiversity that drove changes in ANPP in the indirect pathways. We identified the important roles of N and P additions in begetting the dominance of grasses and forbs, respectively. The study highlights that the shift of functional groups should be taken into consideration to better predict the structure, function, and biodiversity-ANPP relationship in grasslands, particularly under future multifaceted global change.

Advantages of growth and competitive ability of the invasive plant Solanum rostratum over two co-occurring natives and the effects of nitrogen levels and forms

Introduction

Atmospheric nitrogen (N) deposition has often been considered as a driver of exotic plant invasions. However, most related studies focused on the effects of soil N levels, and few on those of N forms, and few related studies were conducted in the fields.

Methods

In this study, we grew Solanum rostratum, a notorious invader in arid/semi-arid and barren habitats, and two coexisting native plants Leymus chinensis and Agropyron cristatum in mono- and mixed cultures in the fields in Baicheng, northeast China, and investigated the effects of N levels and forms on the invasiveness of S. rostratum.

Results

Compared with the two native plants, S. rostratum had higher aboveground and total biomass in both mono- and mixed monocultures under all N treatments, and higher competitive ability under almost all N treatments. N addition enhanced the growth and competitive advantage of the invader under most conditions, and facilitated invasion success of S. rostratum. The growth and competitive ability of the invader were higher under low nitrate relative to low ammonium treatment. The advantages of the invader were associated with its higher total leaf area and lower root to shoot ratio compared with the two native plants. The invader also had a higher light-saturated photosynthetic rate than the two native plants in mixed culture (not significant under high nitrate condition), but not in monoculture.

Discussion

Our results indicated that N (especially nitrate) deposition may also promote invasion of exotic plants in arid/semi-arid and barren habitats, and the effects of N forms and interspecific competition need to be taken into consideration when studying the effects of N deposition on invasion of exotic plants.

Soil Microbe-Mediated N:P Stoichiometric Effects on Solidago canadensis Performance Depend on Nutrient Levels

Both soil microbes and soil N:P ratios can affect plant growth, but it is unclear whether they can interact to alter plant growth and whether such an interactive effect depends on nutrient levels. Here, we tested the hypothesis that soil microbes can ameliorate the negative effects of nutrient imbalance caused by low or high N:P ratios on plant growth and that such an ameliorative effect of soil microbes depends on nutrient supply levels. We grew individuals of six populations of the clonal plant Solidago canadensis at three N:P ratios (low (1.7), intermediate (15), and high (135)), under two nutrient levels (low versus high) and in the presence versus absence of soil microbes. The presence of soil microbes significantly increased biomass of S. canadensis at all three N:P ratios and under both nutrient levels. Under the low-nutrient level, biomass, height, and leaf number of S. canadensis did not differ significantly among the three N:P ratio treatments in the absence of soil microbes, but they were higher at the high than at the low and the intermediate N:P ratio in the presence of soil microbes. Under the high-nutrient level, by contrast, biomass, height, and leaf number of S. canadensis were significantly higher at the low than at the high and the intermediate N:P ratio in the absence of soil microbes, but increased with increasing the N:P ratio in the presence of soil microbes. In the presence of soil microbes, number of ramets (asexual individuals) and the accumulation of N and P in plants were significantly higher at the high than at the low and the intermediate N:P ratio under both nutrient levels, whereas in the absence of soil microbes, they did not differ significantly among the three N:P ratio regardless of the nutrient levels. Our results provide empirical evidence that soil microbes can alter effects of N:P ratios on plant performance and that such an effect depends on nutrient availability. Soil microbes may, therefore, play a role in modulating ecosystem functions such as productivity and carbon and nutrient cycling via modulating nutrient imbalance caused by low and high N:P ratios.

Allometry and Distribution of Nitrogen in Natural Plant Communities of the Tibetan Plateau

Nitrogen (N) is an important element for most terrestrial ecosystems; its variation among different plant organs, and allocation mechanisms are the basis for the structural stability and functional optimization of natural plant communities. The nature of spatial variations of N and its allocation mechanisms in plants in the Tibetan Plateau-known as the world's third pole-have not been reported on a large scale. In this study, we consistently investigated the N content in different organs of plants in 1564 natural community plots in Tibet Plateau, using a standard spatial-grid sampling setup. On average, the N content was estimated to be 19.21, 4.12, 1.14, and 10.86 mg g^-1 in the leaf, branch, trunk, and root, respectively, with small spatial variations. Among organs in communities, leaves were the most active, and had the highest N content, independent of the spatial location; as for vegetation type, communities dominated by herbaceous plants had higher N content than those dominated by woody plants. Furthermore, the allocation of N among different plant organs was allometric, and not significantly influenced by vegetation types and environmental factors; the homeostasis of N was also not affected much by the environment, and varied among the plant organs. In addition, the N allocation strategy within Tibet Plateau for different plant organs was observed to be consistent with that in China. Our findings systematically explore for the first time, the spatial variations in N and allometric mechanisms in natural plant communities in Tibet Plateau and establish a spatial-parameters database to optimize N cycle models.

Heterogeneous Nitrogen Supply With High Frequency and Ramet Damage Increases the Benefits of Clonal Integration in Invasive Hydrocotyle vulgaris

Nitrogen (N) deposition significantly affects the growth and the function of invasive clonal plants. However, the effects of heterogeneous N supply with different frequencies on the growth and the potential contribution of clonal integration in invasion plants are still unclear, especially in the complex environment considering ramet damage. To address this question, apical and basal ramets of the clonal invader Hydrocotyle vulgaris were connected or disconnected, N was added to the basal ramets with a high frequency, a low frequency, or no supply, and the total N quantity was the same for the different frequency. Furthermore, 8 aphids were placed on the apical ramets, and 30% of each leaf was cut off to cause damage. The connection between ramets significantly increased the biomass, total carbon (C), and total N of the basal and apical ramets. Higher frequency N supply significantly increased the biomass, total C, and total N of the basal ramets and the entire clonal fragment biomass. The damage had no significant effect on the growth of basal and apical ramets. Especially, under the high N frequency and ramet damage condition, the connection between ramets more significantly increased the biomass, total C, and total N of the apical ramets and the entire clonal fragment biomass. In addition, the uptake rates of ¹⁵ NH 4 + and ¹⁵ NO 3 - in H. vulgaris had no significant difference, and N supply increased the uptake rates of ¹⁵ NH 4 + and ¹⁵ NO 3 - of the basal ramets. Our results suggest that both higher frequency N supply and clonal integration are beneficial to the growth of H. vulgaris. Moreover, the heterogeneous N supply with high frequency and ramet damage increases the benefits of clonal integration in H. vulgaris. These findings improve our understanding of the response of clonal invader H. vulgaris to nitrogen deposition and ramet damage.

Nitrogen deposition magnifies the positive response of plant community production to precipitation: Ammonium to nitrate ratio matters

The impacts of atmospheric nitrogen (N) deposition amount on plant communities have been extensively explored. However, the responses of plant communities to the ratio of reduced (NH₄⁺-N) and oxidized (NO₃^--N) forms remain unclear in natural ecosystems. A field N enrichment experiment using different NH₄⁺-N/NO₃^--N ratios was conducted in a natural semi-arid grassland in northern China from 2014 to 2019. Nitrogen addition tended to reduce plant species richness and significantly enhanced plant community aboveground net primary productivity (ANPP). Neither plant species richness nor plant ANPP at species and community levels was significantly affected by NH₄⁺-N/NO₃^--N ratios. At the plant functional group level, ANPP of grasses was not significantly affected by the NH₄⁺-N/NO₃^--N ratios examined, whereas ANPP of forbs was significantly increased at 1:1 NH₄⁺-N/NO₃^--N. Regardless of N supplied using the different ratios of NH₄⁺-N/NO₃^--N examined, plant community ANPP was positively associated with growing season precipitation. Unexpectedly, 1:1 NH₄⁺-N/NO₃^--N (NH₄NO₃) significantly improved the positive response of plant community ANPP to precipitation (it had the biggest slope value). Our results suggest that precipitation was the main determinant of the influence of NH₄⁺-N/NO₃^--N ratios on plant community ANPP. Therefore, the results of our study showed that without referring to NH₄⁺-N/NO₃^--N ratios and precipitation, models using NH₄NO₃ enrichment may overestimate the positive effect of atmospheric N deposition on ecosystem ANPP in semi-arid ecozones.

Silicon addition improves plant productivity and soil nutrient availability without changing the grass:legume ratio response to N fertilization

Silicon (Si) plays an important role in plant nutrient capture and absorption, and also promotes plant mechanical strength and light interception in alpine meadows. In this study, we conducted a field experiment to examine the effect of nitrogen (N) application, with (N + Si) and without Si (N-only), on the potential for soil nutrient and the growth of grass and legume plant functional types (PFTs) in an alpine meadow. It was found that N + Si resulted in higher soil nutrient contents, leaf N and P concentrations, abundance and biomass of legume and grass PFTs than N-only. The aboveground biomass of grass (598 g m-2) and legume (12.68 g m-2) PFTs under 600 kg ha-1 ammonium nitrate (NH4NO3) per year addition with Si was significantly higher than that under the same level of N addition without Si (515 and 8.68 g m-2, respectively). The grass:legume biomass ratio did not differ significantly between the N + Si and N-only. This demonstrates that Si enhances N fertilization with apparently little effect on grass:legume ratio and increases plant-available nutrients, indicating that Si is essential for the plant community in alpine meadows.

Different sensitivity and threshold in response to nitrogen addition in four alpine grasslands along a precipitation transect on the Northern Tibetan Plateau

The increase in atmospheric nitrogen (N) deposition has resulted in some terrestrial ecological changes. In order to identify the response of sensitive indicators to N input and estimate the sensitivity and saturation thresholds in alpine grasslands, we set up a series of multilevel N addition experiments in four types of alpine grasslands (alpine meadow [AM], alpine meadow-steppe [AMS], alpine steppe [AS], and alpine desert-steppe [ADS]) along with a decreasing precipitation gradient from east to west on the Northern Tibetan Plateau. N addition only had significant effects on species diversity in AMS, while had no effects on the other three alpine grasslands. Aboveground biomass of grasses and overall community in ADS were enhanced with increasing N addition, but such effects did not occur in AS. Legume biomass in ADS and AS showed similar unimodal patterns and exhibited a decreasing tend in AM. Regression fitting showed that the most sensitive functional groups were grasses, and the N saturation thresholds were 103, 115, 136, and 156 kg N hm-2 year-1 in AM, AMS, AS, and ADS, respectively. This suggests that alpine grasslands become more and more insensitive to N input with precipitation decrease. N saturation thresholds also negatively correlated with soil N availability. N sensitivity differences caused by precipitation and nutrient availability suggest that alpine grasslands along the precipitation gradient will respond differently to atmospheric N deposition in the future global change scenario. This different sensitivity should also be taken into consideration when using N fertilization to restore degraded grasslands.

Nitrogen and phosphorus enrichment accelerates soil organic carbon loss in alpine grassland on the Qinghai-Tibetan Plateau

Anthropogenic activities have substantially increased soil nutrient availability, which in turn affects ecosystem processes and functions, especially in nutrient-limited ecosystems such as alpine grasslands. Although considerable efforts have been devoted to understanding the responses of plant productivity and community composition to nitrogen (N) and phosphorus (P) enrichment, the nutrient enrichment effects on soil organic carbon (SOC) and microbial functions are not well understood. A four-year field experiment was established to evaluate the influence of continuous N and P enrichment on plant growth and SOC content in an alpine grassland of the Qinghai-Tibetan Plateau. The study included four treatments: Control without addition, N addition, P addition, and N plus P addition. N addition strongly increased aboveground plant biomass and decreased species richness by promoting growth of the dominant grasses species. In contrast, N and P enrichment significantly decreased SOC, especially the recalcitrant organic C content in the surface layer (0-10 cm) by reducing the slow C pool and enlarging the active C pool. Microbial biomass and activities of C-degrading enzymes (β-glucosidase, cellulase and polyphenol oxidase) and an N-degrading enzyme (chitinase) increased with nutrient inputs. The CO₂ emissions during a 300 d incubation period were positively correlated with the cellulase and chitinase activities, while the slow C pool was negatively correlated with the cellulase and polyphenol oxidase activities. Consequently, N and P enrichment accelerated decomposition of the recalcitrant C by stimulating microbial growth and increasing enzyme activities, leading to negative impacts on soil C sequestration. Overall, the results indicate that alpine grassland soils of the Qinghai-Tibetan Plateau may be changing from a C sink to a C source under increasing N and P availability, and improvement of alpine grassland management through nutrient inputs should consider not only the aboveground biomass for grazing, but also the soil C sequestration and ecosystem functioning.

Mowing mitigates the negative impacts of N addition on plant species diversity

Increasing availability of reactive nitrogen (N) threatens plant diversity in diverse ecosystems. While there is mounting evidence for the negative impacts of N deposition on one component of diversity, species richness, we know little about its effects on another one, species evenness. It is suspected that ecosystem management practice that removes nitrogen from the ecosystem, such as hay-harvesting by mowing in grasslands, would mitigate the negative impacts of N deposition on plant diversity. However, empirical evidence is scarce. Here, we reported the main and interactive effects of N deposition and mowing on plant diversity in a temperate meadow steppe with 4-year data from a field experiment within which multi-level N addition rates and multiple N compounds are considered. Across all the types of N compounds, species richness and evenness significantly decreased with the increases of N addition rate, which was mainly caused by the growth of a tall rhizomatous grass, Leymus chinensis. Such negative impacts of N addition were accumulating with time. Mowing significantly reduced the dominance of L. chinensis, and mitigated the negative impacts of N deposition on species evenness. We present robust evidence that N deposition threatened biodiversity by reducing both species richness and evenness, a process which could be alleviated by mowing. Our results highlight the changes of species evenness in driving the negative impacts of N deposition on plant diversity and the role of mowing in mediating such negative impacts of N deposition.

Shifts in priming partly explain impacts of long-term nitrogen input in different chemical forms on soil organic carbon storage

Input of labile organic carbon can enhance decomposition of extant soil organic carbon (SOC) through priming. We hypothesized that long-term nitrogen (N) input in different chemical forms alters SOC pools by altering priming effects associated with N-mediated changes in plants and soil microbes. The hypothesis was tested by integrating field experimental data of plants, soil microbes and two incubation experiments with soils that had experienced 10 years of N enrichment with three chemical forms (ammonium, nitrate and both ammonium and nitrate) in an alpine meadow on the Tibetan Plateau. Incubations with glucose-¹³ C addition at three rates were used to quantify effects of exogenous organic carbon input on the priming of SOC. Incubations with microbial inocula extracted from soils that had experienced different long-term N treatments were conducted to detect effects of N-mediated changes in soil microbes on priming effects. We found strong evidence and a mechanistic explanation for alteration of SOC pools following 10 years of N enrichment with different chemical forms. We detected significant negative priming effects both in soils collected from ammonium-addition plots and in sterilized soils inoculated with soil microbes extracted from ammonium-addition plots. In contrast, significant positive priming effects were found both in soils collected from nitrate-addition plots and in sterilized soils inoculated with soil microbes extracted from nitrate-addition plots. Meanwhile, the abundance and richness of graminoids were higher and the abundance of soil microbes was lower in ammonium-addition than in nitrate-addition plots. Our findings provide evidence that shifts toward higher graminoid abundance and changes in soil microbial abundance mediated by N chemical forms are key drivers for priming effects and SOC pool changes, thereby linking human interference with the N cycle to climate change.

Can nutrient enrichment influence the invasion of Phragmites australis?

Plant invasion and nutrient enrichment because of anthropogenic landscape modifications seriously threaten native plant community diversity in aquatic and wetland ecosystems. It is poorly understood, however, whether these two disturbances interact with the functional identity of recipient native plants to drive community change. We performed combined studies in the fields and greenhouse to examine whether nutrient enrichment may trigger the invasion of Phragmites australis in wetlands through competitive advantage over native Melaleuca ericifolia. Chemical characterizations of rhizosphere water were distinguished in two different nutrient enriched wetlands associated with and without Phragmites over the seasons. Significant changes in rhizosphere water were observed in invaded area compared to uninvaded area at both sites. High nitrogen (NO₃^-), phosphorous (PO₄^3-), dissolved organic carbon, phenolics contents, with low pH were found in invaded areas compared to uninvaded areas. Total biomass of Phragmites was positively regressed with rhizosphere water nitrogen (NO₃^-) and phosphorous (PO₄^3-) content. Nutrient addition significantly enhanced the growth and competitive ability of Phragmites over Melaleuca. In contrast, Melaleuca was significantly less competitive than Phragmites. There was a significantly positive correlation between the growth of Phragmites grown alone and its competitive ability. The findings in greenhouse studies coupled with characteristics of Phragmites and its' rhizosphere chemistry in the nutrient enriched fields suggest that nutrient enrichment may enhance Phragmites invasion through correspondingly increasing growth and maintaining inherent competitive advantages of Phragmites. Nutrient management could limit the vigorous growth of Phragmites in wetlands and thereby reduce invasion through competitive advantages over natives, which might have important management implications for wetland managers.

Nitrogen addition and clonal integration alleviate water stress of dependent ramets of Indocalamus decorus under heterogeneous soil water environment

Water and nitrogen are two of the most important factors for plant growth and development. However, little is known about effects of N on water translocation between connected bamboo ramets. We performed experiment connected Indocalamus decorus ramets in adjacent pots with different soil water contents and three N levels. We determined antioxidase activities, concentration of osmotic adjustment products, O₂·⁻, MDA and photosynthetic pigments, and electrolyte leakage rate in paired unit. When N supply to supporting ramets increased, their electrolyte leakage rates and contents of O₂·⁻ and MDA significantly increased, while antioxidase activities and contents of osmotic adjustment products and photosynthetic pigments in connected dependent ramets increased markedly as their electrolyte leakage rates and contents of O₂·⁻ and MDA decreased greatly. When N addition to dependent ramets increased, antioxidant enzyme activity and contents of osmotic adjustment products and photosynthetic pigments decreased in both ramets, but electrolyte leakage rates and O₂·⁻ and MDA contents increased significantly. Therefore, N addition to either supporting or dependent ramets can improve water integration among I. decorus ramets. N addition to supporting ramets promotes water translocation and alleviates water stress of dependent ramets, but N addition to dependent ramets exacerbates drought stress damage to dependent ramets.

Nutrient enrichment alters impacts of Hydrocotyle vulgaris invasion on native plant communities

Nutrients may affect the invasiveness of alien plants and the invasibility of native plant communities. We performed a greenhouse experiment to investigate the interactive effect of invasion by a clonal herb Hydrocotyle vulgaris and nutrient enrichment on biomass and evenness of native plant communities. We established three types of plant communities (H. vulgaris alone, native plant communities without or with H. vulgaris) under low and high levels of nutrients. Native communities consisted of eight native, terrestrial species of three functional groups, i.e. four grasses, two legumes, and two forbs. Invasion of H. vulgaris had no effect on biomass of the native community, the functional groups, or the individual species. High nutrients increased biomass of grasses, but reduced evenness of the community. High nutrients also decreased the competitive effect, and the relative dominance index of H. vulgaris. Therefore, high nutrients reduced the competitive ability of H. vulgaris and enhanced the resistance of the native community to invasion. The results provide a basis for management strategies to control the invasion and spread of H. vulgaris by manipulating resource availability to support native communities.

Impacts of nitrogen addition on plant biodiversity in mountain grasslands depend on dose, application duration and climate: a systematic review

Although the influence of nitrogen (N) addition on grassland plant communities has been widely studied, it is still unclear whether observed patterns and underlying mechanisms are constant across biomes. In this systematic review, we use meta-analysis and metaregression to investigate the influence of N addition (here referring mostly to fertilization) upon the biodiversity of temperate mountain grasslands (including montane, subalpine and alpine zones). Forty-two studies met our criteria of inclusion, resulting in 134 measures of effect size. The main general responses of mountain grasslands to N addition were increases in phytomass and reductions in plant species richness, as observed in lowland grasslands. More specifically, the analysis reveals that negative effects on species richness were exacerbated by dose (ha(-1) year(-1) ) and duration of N application (years) in an additive manner. Thus, sustained application of low to moderate levels of N over time had effects similar to short-term application of high N doses. The climatic context also played an important role: the overall effects of N addition on plant species richness and diversity (Shannon index) were less pronounced in mountain grasslands experiencing cool rather than warm summers. Furthermore, the relative negative effect of N addition on species richness was more pronounced in managed communities and was strongly negatively related to N-induced increases in phytomass, that is the greater the phytomass response to N addition, the greater the decline in richness. Altogether, this review not only establishes that plant biodiversity of mountain grasslands is negatively affected by N addition, but also demonstrates that several local management and abiotic factors interact with N addition to drive plant community changes. This synthesis yields essential information for a more sustainable management of mountain grasslands, emphasizing the importance of preserving and restoring grasslands with both low agricultural N application and limited exposure to N atmospheric deposition.

Reduced compensatory effects explain the nitrogen-mediated reduction in stability of an alpine meadow on the Tibetan Plateau

Many ecosystems are facing strong perturbations such as nitrogen (N) fertilization, which can greatly alter ecosystem stability via different mechanisms. Understanding such mechanisms is critical for predicting how ecosystems will function in the face of global changes. We examined how 8 yr of N fertilization with different N rates (no N addition or N addition at a low, medium or high rate) and different forms of N (ammonium, nitrate or ammonium nitrate) affected the temporal stability of the aboveground biomass of an alpine meadow on the Tibetan Plateau, and tested four mechanisms (diversity effect, mean-variance scaling, compensatory dynamics and dominance effect) that may alter stability. Compared with the control (no N addition), a high N rate did not affect the diversity effect, the mean-variance scaling or the dominance effect, but significantly decreased compensatory dynamics among species and functional groups, which contributed to the reduction in community stability of the alpine meadow. The form of N did not affect any of the four mechanisms and thus did not affect community stability. A high N rate can change community stability by altering compensatory dynamics, whereas the form of N may not have an effect.

Differential responses of arbuscular mycorrhizal fungi to nitrogen addition in a near pristine Tibetan alpine meadow

Elucidating the responses of soil microbial abundance and community composition to nitrogen (N) addition is important for predicting ecosystem function under increased atmospheric N deposition. We examined the arbuscular mycorrhizal (AM) fungal community under three N forms (NH4(+)-N, NO3(-)-N, and NH4NO3-N) and two N rates (1.5 and 7.5 g N m(-2) year(-1)) in an alpine meadow of the Qinghai-Tibetan Plateau. AM fungal extraradical hyphal density was significantly decreased by NH4(+)-N in May, but was not affected by N form nor N rate in August. N rate, but not N form, significantly affected AM fungal spore density; high N rate decreased spore density. No direct N addition effect was observed on AM fungal community; however, soil available phosphorus, pH, and NO3(-)-N were considered as important factors that influenced AM fungal community composition. Structural equation model results showed that N rate, not N form, strongly affected soil characteristics, which directly influenced community compositions of plants and AM fungi, as well as spore density. Therefore, AM fungal community was influenced by N addition, primarily because of altered soil characteristics, and partially by a modified plant community, but not or just slightly by direct N addition effects in this alpine meadow ecosystem.