Limited inorganic N niche partitioning by nine alpine plant species after long-term nitrogen addition

Effects of simulated litter inputs on ecological niches and interspecific connectivity of alpine meadow plants

Introduction

Plant litter enters the soil as the main nutrient for alpine meadow ecosystems, and the amount of litter input has a significant effect on alpine meadow plant diversity, ecological niches, and interspecific connectivity.

Methods

The ecological niche width, ecological niche overlap and interspecific associations of alpine meadow species in the Sanjiangyuan area of the Qinghai-Tibetan Plateau were investigated using ecological statistical methods, and the competitive linkages between species and limited resources within the community and the stable coexistence among populations under different levels of plant litter inputs were explored.

Results

(1) Litter inputs could significantly increase the plant diversity and aboveground biomass of alpine meadows, and the species with the highest importance value was Kobresia humilis Clarke. (2) compared with the control, with the increase of litter inputs, the ecological niche widths of the major plant species and ecological niche overlap values were significantly increased. And the F2 treatment had 15 major species totaling 105 pairs, and there were 82 pairs of ecological niche overlap values ≥0.950, accounting for 78.1%. (3) The correlation analysis between species showed that the negative correlation between species decreased and then increased with the increase of litter input. There were 3 pairs with Ac ≥ 0.25 under F2.

Discussion

A moderate amount of litter input (F2) is beneficial to the structural stability of alpine meadow ecosystems. Excessive litter inputs may break the original balance of alpine meadow ecosystems and affect plant growth strategies. This study lays a foundation for the conservation of vegetation diversity and long-term use of grassland resources in degraded alpine meadows on the Tibetan Plateau.

Reseeding Native Species Promotes Community Stability by Improving Species Diversity, Niche, and Interspecific Relationships in the Desert Steppe of Northwest China

The mechanism of community stability is a hot topic in the field of ecology. Research on the stability of the grassland community has gradually increased, and the reseeding of native species is one of the main measures to restore the degraded desert steppe in northwest China. However, little is known about the changes in the stability of the plant community in the desert steppe after reseeding native species. This study established a long-term observation site for native species reseeding in the desert steppe. We established reseeding and grazing exclusion plots in May 2017 and conducted surveys on degraded grasslands (0YEX(RS)) before setting up reseeding and grazing exclusion treatment experimental plots. After 3 and 6 years of setting up the test, the vegetation restoration status of the plot was investigated, respectively. The results showed that reseeding native species increased the Shannon-Wiener index and Margalef index of the community. At the same time, the importance value and the breadth of the niche of gramineous plants improved, while the proportion of pairs of high niche overlaps and the logarithm of significant association decreased. The general association of reseeding of the desert steppe was positively correlated, and the stability of the community gradually increased. The results of partial least squares path modelling show that reseeding has a highly significant positive effect on community stability. Both the EX (grazing exclusion grassland) and RS (reseeded grassland) models indicate that niche and diversity indices influence community stability to varying degrees, while interspecific linkage coefficients affect mainly niche overlap. Our research has shown that reseeding native species can improve the intensity of competition between species for resources, leading to a more stable community and ultimately increasing species diversity and community stability. These findings provide valuable theoretical support for vegetation restoration and sustainable management in the desert steppe.

Diversity loss and light limitation threaten the sustainability of ecosystem productivity gains under nitrogen enrichment

Plant photosynthesis significantly regulates atmospheric CO₂ but is often limited by nitrogen (N) availability. While N deposition could alleviate this limitation and enhance gross ecosystem productivity (GEP), its long-term effects are uncertain due to potential negative impacts like biodiversity loss and soil acidification. Yet, many long-term N addition experiments emphasize community biomass over gross GEP. Here, we conducted a six-year N addition experiment in an alpine meadow, frequently monitoring GEP, community structure, aboveground net primary productivity (ANPP)and plant traits. We found that N addition significantly enhanced GEP in the first three years, but during 4-6 years this effect disappeared. We further disentangled the mechanisms affecting GEP into biomass-based and non-biomass-based processes. The latter is expressed as biomass-specific GEP, defined as GEP per unit biomass. Differing with GEP, biomass-specific GEP provides a metric of carbon assimilation efficiency normalized to biomass. Unlike previous studies, we found that it was not ANPP, but specific GEP that determined the loss of the short-term N effect. ANPP showed a consistent increase under N addition, whereas specific GEP decreased in the last three years. This specific GEP reduction was primarily regulated by biodiversity loss and increased light limitation under N addition. Overall, our findings suggest that short-term benefits of N deposition on GEP are not sustained in long term, highlighting the need to explore the non-biomass-based mechanisms to better predict ecosystem responses to prolonged N enrichment.

Different responses of individuals, functional groups and plant communities in CSR strategies to nitrogen deposition in high-altitude grasslands

The Competitor, Stress Tolerator, and Ruderal (CSR) theory delineates the ecological strategies of plant species. Nevertheless, how these ecological strategies shift at the levels of individuals, functional groups and plant communities to cope with increasing nitrogen deposition remains unclear. In this study, simulated nitrogen deposition experiments were performed in high-altitude grasslands of alpine meadows and alpine steppe on the Qinghai-Tibetan Plateau (QTP) by employing the strategy and functional type framework (StrateFy) methodology to evaluate plant CSR strategies. Our results indicated that the dominant ecological strategy of the high-altitude grassland on the QTP were predominantly aligned with the R-strategy. In both alpine meadow and alpine steppe grasslands, the community-weighted mean (CWM) of C scores were increased with nitrogen addition, while CWM of R and S scores were not significantly correlated with nitrogen addition. Remarkably, the increase in C scores due to nitrogen enrichment was observed solely in non-legumes, suggesting an enhanced competitive capability of non-legumes in anticipation of future nitrogen deposition. Leymus secalinus was dominated in both alpine meadow and alpine steppe grasslands across all levels of nitrogen deposition, with increasing C scores along the nitrogen gradients. Furthermore, the sensitivity of C scores of individual plant, functional group and plant community to nitrogen deposition rates was more pronounced in alpine steppe grassland than in alpine meadow grassland. These findings furnish novel insights into the alterations of ecological strategies in high-altitude alpine grasslands on the QTP and similar regions worldwide in cope with escalating nitrogen deposition.

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.

Effect of slaughter age on environmental efficiency on beef cattle in marginal area including soil carbon sequestration: A case of study in Italian Alpine area

The production of beef carries significant environmental repercussions on a worldwide level. Considering that the production of beef in Alpine mountainous regions, such as South Tyrol (Italy), constitutes a modest yet progressively growing segment within the local agricultural sector focus must be put on minimizing the environmental impact of producing one kilogram of meat, while also accounting for the carbon sequestered by Alpine pastures in such marginal areas. To this end 20 beef farms distributed in the South Tyrolean region (Italy) were divided based on the age at slaughter of the beef cattle: 10 farms with a slaughter age of 12 months (SA12) and 10 farms with a slaughter age of 24 months (SA24). Live cycle assessment (LCA) approach was used, and the impact was estimated using two functional units (FU): 1 kg of live weight (LW) and 1 kg of carcass weight (CW). Global warming potential (GWP100, kg CO2-eq), acidification potential (AP, g SO2-eq), and eutrophication potential (EP, g PO4-eq) were investigated. Furthermore, within the account, the carbon sequestered by pastures and permanent grassland has been included for estimated the overall carbon footprint. In terms of GWP100, the SA12 system proved to be significantly lower for both two functional units under studies, with reductions of 8.5 % and 7.4 % in terms of LW and CW, respectively, compared to the SA24 system, specifically, the SA12 system showed an environmental impact in terms of GWP100 of 19.5 ± 1.1 kg CO2-eq/kg LW, which was significantly lower than the SA24 system that exhibited a value of 22.9 ± 1.1 kg CO2-eq/kg LW (P < 0.05). When accounting for the carbon sequestered within the system, the observed values in terms of GWP100 are significantly lower for SA12 compared to SA24, 17.6 ± 1.5 vs. 20.9 ± 1.5 kg CO2-eq/Kg LW (P < 0.05), and 29.2 ± 2.5 vs. 38.7 ± 2.5 kg CO2-eq/Kg CW (P < 0.01). These differences are due to less purchase of concentrated feed and greater use of natural resources such as pastures and permanent grasslands. The research indicated that the production of beef in the Alpine region of South Tyrol predominantly occurs within extensive parameters, leading to a satisfactory environmental profile, also including the C sequestration.

Response of Aboveground Net Primary Production, Species and Phylogenetic Diversity to Warming and Increased Precipitation in an Alpine Meadow

The uncertain responses of aboveground net primary productivity (ANPP) and plant diversity to climate warming and increased precipitation will limit our ability to predict changes in vegetation productivity and plant diversity under future climate change and further constrain our ability to protect biodiversity and ecosystems. A long-term experiment was conducted to explore the responses of ANPP, plant species, phylogenetic α-diversity, and community composition to warming and increased precipitation in an alpine meadow of the Northern Tibet from 2014 to 2019. Coverage, height, and species name were obtained by conventional community investigation methods, and ANPP was obtained using observed height and coverage. Open-top chambers with two different heights were used to simulate low- and high-level climate warming. The low- and high-level increased precipitation treatments were achieved by using two kinds of surface area funnel devices. The high-level warming reduced sedge ANPP (ANPPsedge) by 62.81%, species richness (SR) by 21.05%, Shannon by 13.06%, and phylogenetic diversity (PD) by 14.48%, but increased forb ANPP (ANPPforb) by 56.65% and mean nearest taxon distance (MNTD) by 33.88%. Species richness, Shannon, and PD of the high-level warming were 19.64%, 9.67%, and 14.66% lower than those of the low-level warming, respectively. The high-level warming-induced dissimilarity magnitudes of species and phylogenetic composition were greater than those caused by low-level warming. The low- rather than high-level increased precipitation altered species and phylogenetic composition. There were significant inter-annual variations of ANPP, plant species, phylogenetic α-diversity and community composition. Therefore, climate warming and increased precipitation had non-linear effects on ANPP and plant diversity, which were due to non-linear changes in temperature, water availability, and/or soil nutrition caused by warming and increased precipitation. The inter-annual variations of ANPP and plant diversity were stronger than the effects of warming and especially increased precipitation on ANPP and plant diversity. In terms of plant diversity conservation and related policy formulation, we should pay more attention to regions with greater warming, at least for the northern Tibet grasslands. Besides paying attention to the responses of ANPP and plant diversity to climate change, the large inter-annual changes of ANPP and plant diversity should be given great attention because the large inter-annual variation indicates the low temporal stability of ANPP and plant diversity and thus produces great uncertainty for the development of animal husbandry.

Soil Fungal Composition Drives Ecosystem Multifunctionality after Long-Term Field Nitrogen and Phosphorus Addition in Alpine Meadows on the Tibetan Plateau

An ecosystem can provide multiple functions and services at the same time, i.e., ecosystem multifunctionality (EMF). Above- and belowground biodiversity and abiotic factors have different effects on EMF. Human activities increase atmospheric nitrogen (N) and phosphorus (P) deposition, but the mechanism of how atmospheric N and P deposition affect EMF in alpine meadows on the Tibetan Plateau is still unclear. Here, we measured eleven ecosystem parameters to quantify EMF by averaging method and explored the impact of plant and microbial species diversity and abiotic factors on EMF after long-term field N and P addition in alpine meadows on the Tibetan Plateau. Results showed that N addition reduced EMF by 15%, NP increased EMF by 20%, and there was no change due to P addition. N and P addition reduced pH, relative light conditions (RLC), and plant species richness and modified plant and fungal community composition. Structural equation model (SEM) analysis confirmed that fungal community composition was an important and positive driver on EMF. These results provided an understanding of how N and P addition affect EMF directly and indirectly through biotic and abiotic pathways, which was important for predicting the response of EMF to atmospheric N and P deposition in the future. Furthermore, the findings suggested that soil fungal composition was more important driving factors than abiotic factors in the response of EMF to N and P addition and the importance of the interactions between plant and soil microbial species diversity in supporting greater EMF.

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.

Nitrogen Addition Affects Ecosystem Carbon Exchange by Regulating Plant Community Assembly and Altering Soil Properties in an Alpine Meadow on the Qinghai-Tibetan Plateau

Nitrogen (N) deposition can affect the global ecosystem carbon balance. However, how plant community assembly regulates the ecosystem carbon exchange in response to the N deposition remains largely unclear, especially in alpine meadows. In this study, we conducted a manipulative experiment to examine the impacts of N (ammonium nitrate) addition on ecosystem carbon dioxide (CO2) exchange by changing the plant community assembly and soil properties at an alpine meadow site on the Qinghai-Tibetan Plateau from 2014 to 2018. The N-addition treatments were N0, N7, N20, and N40 (0, 7, 20, and 40 kg N ha-1year-1) during the plant growing season. The net ecosystem CO2 exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured by a static chamber method. Our results showed that the growing-season NEE, ER and GEP increased gradually over time with increasing N-addition rates. On average, the NEE increased significantly by 55.6 and 65.2% in N20 and N40, respectively (p < 0.05). Nitrogen addition also increased forage grass biomass (GB, including sedge and Gramineae) by 74.3 and 122.9% and forb biomass (FB) by 73.4 and 51.4% in N20 and N40, respectively (p < 0.05). There were positive correlations between CO2 fluxes (NEE and GEP) and GB (p < 0.01), and the ER was positively correlated with functional group biomass (GB and FB) and soil available N content (NO3 --N and NH4 +-N) (p < 0.01). The N-induced shift in the plant community assembly was primarily responsible for the increase in NEE. The increase in GB mainly contributed to the N stimulation of NEE, and FB and the soil available N content had positive effects on ER in response to N addition. Our results highlight that the plant community assembly is critical in regulating the ecosystem carbon exchange response to the N deposition in alpine ecosystems.

Synchronous Responses of Plant Functional Traits to Nitrogen Deposition From Dominant Species to Functional Groups and Whole Communities in Alpine Grasslands on the Qinghai-Tibetan Plateau

Nitrogen deposition is recognized as one of the major threats to the ecosystem function of alpine grasslands on the Qinghai-Tibetan Plateau (QTP). However, few studies have documented the gradient responses of plant species, functional groups, and communities in alpine grassland ecosystems to various levels of N deposition on the QTP. We applied eight linear mixed-effect models combing acidification, eutrophication, and phosphorus availability to explore if the responses of functional traits (particularly plant height and specific leaf area) of plants from dominant species to functional groups and whole communities in different types of grassland to nitrogen deposition were consistent with the same or different models. We found that the specific leaf area of Stipa capillata, non-forb, and community-weighted mean value in the alpine steppe were synchronous and related to acidification with nitrogen addition; the height of Stipa capillata, non-forb, and community-weighted mean value in the alpine steppe was synchronous and related to acidification, eutrophication, and phosphorus availability with nitrogen addition; the height and specific leaf area of Elymus breviaristatus to functional groups and community-weighted mean value in cultivated grasslands (CGs) were synchronous and related to acidification, eutrophication, and phosphorus availability with nitrogen addition. Most of the responses of functional traits of plants to acidification, eutrophication, and phosphorus availability associated with nitrogen deposition in the alpine steppe and the CG were synchronous, while only the response of the specific leaf area of forb functional groups to eutrophication associated with N deposition in the alpine steppe was asynchronous.

Mobilization of soil phosphate after 8 years of warming is linked to plant phosphorus-acquisition strategies in an alpine meadow on the Qinghai-Tibetan Plateau

Phosphorus (P) is essential for productivity of alpine grassland ecosystems, which are sensitive to global warming. We tested the hypotheses that (1) mobilized 'calcium-bound inorganic P' (Ca-P_i ) is a major source of plant-available P in alpine meadows with alkaline soils after long-term warming, (2) mobilization of Ca-P_i is linked to effective plant carboxylate-releasing P-acquisition strategies under warming, and (3) the mobilization is also related to plant nitrogen (N)-acquisition. We conducted an 8-year warming experiment in an alpine meadow (4635 m above sea level) on the Qinghai-Tibetan Plateau. A significant increase in P concentration in both aboveground and belowground biomass indicates an increased mobilization and assimilation of P by plants under warming. We observed a significant decrease in Ca-P_i , no change in moderately-labile organic P, and an increase in highly resistant organic P after warming. There was no increase in phosphatase activities. Our results indicate that Ca-P_i , rather than organic P was the major source of plant-available P for alpine meadows under warming. Higher leaf manganese concentrations of sedges and forbs after warming indicate that carboxylates released by these plants are a key mechanism of Ca-P_i mobilization. The insignificant increase in Rhizobiales after warming and the very small cover of legumes show a minor role of N-acquisition strategies in solubilizing phosphate. The insignificant change in relative abundance of mycorrhizal fungi and bacteria related to P cycling after warming shows a small contribution of microorganisms to Ca-P_i mobilization. The significant increase in leaf N and P concentrations and N:P ratio of grasses and no change in sedge leaf N:P ratio reflect distinct responses of plant nutrient status to warming due to differences in P-acquisition strategies. We highlight the important effects of belowground P-acquisition strategies, especially plant carboxylate-releasing P-acquisition strategies on responses of plants to global changes in alpine meadows.

Ammonium application mitigates the effects of elevated carbon dioxide on the carbon/nitrogen balance of Phoebe bournei seedlings

The study of plant responses to increases in atmospheric carbon dioxide (CO2) concentration is crucial to understand and to predict the effect of future global climate change on plant adaptation and evolution. Increasing amount of nitrogen (N) can promote the positive effect of CO2, while how N forms would modify the degree of CO2 effect is rarely studied. The aim of this study was to determine whether the amount and form of nitrogen (N) could mitigate the effects of elevated CO2 (eCO2) on enzyme activities related to carbon (C) and N metabolism, the C/N ratio, and growth of Phoebe bournei (Hemsl.) Y.C. Yang. One-year-old P. bournei seedlings were grown in an open-top air chamber under either an ambient CO2 (aCO2) (350 ± 70 μmol•mol-1) or an eCO2 (700 ± 10 μmol•mol-1) concentration and cultivated in soil treated with either moderate (0.8 g per seedling) or high applications (1.2 g per seedling) of nitrate or ammonium. In seedlings treated with a moderate level of nitrate, the activities of key enzymes involved in C and N metabolism (i.e., Rubisco, Rubisco activase and glutamine synthetase) were lower under eCO2 than under aCO2. By contrast, key enzyme activities (except GS) in seedlings treated with high nitrate or ammonium were not significantly different between aCO2 and eCO2 or higher under eCO2 than under aCO2. The C/N ratio of seedlings treated with moderate or high nitrate under eCO2was significantly changed compared with the seedlings grown under aCO2, whereas the C/N ratio of seedlings treated with ammonium was not significantly different between aCO2 and eCO2. Therefore, under eCO2, application of ammonium can be beneficial C and N metabolism and mitigate effects on the C/N ratio.

The allometry of plant height explains species loss under nitrogen addition

Light asymmetry, with a higher light acquisition per unit biomass for larger plants, has been proposed as a major mechanism of species loss after nitrogen addition. However, solid evidence for this has been scarce. We measured the allometric size-height relationships of 25 plant species along a nitrogen addition gradient manipulated annually for eight years in a speciose alpine meadow and found that the positive relationship between species relative abundance and the height scaling exponent in natural conditions disappeared after nitrogen addition. Those species with lower height scaling exponents increased in relative abundance after nitrogen addition, thereby decreasing the community weighted mean and dispersion of the height scaling exponent and ultimately the species richness. Our results provided some unique evidence for light asymmetry induced species loss after nitrogen addition and a new insight from the perspective of allometric scaling to explain biodiversity maintenance in the face of global changes.