Effect of Nitrogen Application Rate on the Relationships between Multidimensional Plant Diversity and Ecosystem Production in a Temperate Steppe.

Nitrogen (N) deposition, as one of the global change drivers, can alter terrestrial plant diversity and ecosystem function. However, the response of the plant diversity-ecosystem function relationship to N deposition remains unclear. On one hand, in the previous studies, taxonomic diversity (i.e., species richness, SR) was solely considered the common metric of plant diversity, compared to other diversity metrics such as phylogenetic and functional diversity. On the other hand, most previous studies simulating N deposition only included two levels of control versus N enrichment. How various N deposition rates affect multidimensional plant diversity-ecosystem function relationships is poorly understood. Here, a field manipulative experiment with a N addition gradient (0, 1, 2, 4, 8, 16, 32, and 64 g N m-2 yr-1) was carried out to examine the effects of N addition rates on the relationships between plant diversity metrics (taxonomic, phylogenetic, and functional diversity) and ecosystem production in a temperate steppe. Production initially increased and reached the maximum value at the N addition rate of 47 g m-2 yr-1, then decreased along the N-addition gradient in the steppe. SR, functional diversity calculated using plant height (FDis-Height) and leaf chlorophyll content (FDis-Chlorophyll), and phylogenetic diversity (net relatedness index, NRI) were reduced, whereas community-weighted means of plant height (CWMHeight) and leaf chlorophyll content (CWMChlorophyll) were enhanced by N addition. N addition did not affect the relationships of SR, NRI, and FDis-Height with production but significantly affected the strength of the correlation between FDis-Chlorophyll, CWMHeight, and CWMChlorophyll with biomass production across the eight levels of N addition. The findings indicate the robust relationships of taxonomic and phylogenetic diversity and production and the varying correlations between functional diversity and production under increased N deposition in the temperate steppe, highlighting the importance of a trait-based approach in studying the plant diversity-ecosystem function under global change scenarios.

Analyzing Spatio-Temporal Dynamics of Grassland Resilience and Influencing Factors in the West Songnen Plain, China, for Eco-Restoration.

Grassland plays an indispensable role in the stability and development of terrestrial ecosystems. Quantitatively assessing grassland resilience is of great significance for conducting research on grassland ecosystems. However, the quantitative measurement of resilience is difficult, and research on the spatio-temporal variation of grassland resilience remains incomplete. Utilizing the Global Land Surface Satellite (GLASS) leaf area index (LAI) product derived from MODIS remote sensing data, along with land cover and meteorological data, this paper constructed the grassland resilience index (GRI) in the west Songnen Plain, China, a typical region with salt and alkali soils. This paper analyzed the spatio-temporal changes of the GRI and explored the contribution of climate factors, human activities, and geographical factors to the GRI. The results revealed that from 2000 to 2021, the GRI in the study area ranged from 0.1 to 0.22, with a multi-year average of 0.14. The average GRI exhibited a pattern of high-value aggregations in the north and low-value distributions in the south. Trend analysis indicated that areas with an improved GRI accounted for 59.09% of the total grassland area, but there were still some areas with serious degradation. From 2000 to 2015, the latitude and mean annual temperature (MAT) were principal factors to control the distribution of the GRI. In 2020, the mean annual precipitation (MAP) and MAT played important roles in the distribution of the GRI. From 2000 to 2021, the influence of human activities was consistently less significant compared to geographical location and climate variables.

Soil fungal community structure and function response to rhizoma perennial peanut cultivars.

BACKGROUND: Crop-associated microorganisms play a crucial role in soil nutrient cycling, and crop growth, and health. Fine-scale patterns in soil microbial community diversity and composition are commonly regulated by plant species or genotype. Despite extensive reports in different crop or its cultivar effects on the microbial community, it is uncertain how rhizoma peanut (RP, Arachis glabrata Benth.), a perennial warm-season legume forage that is well-adapted in the southern USA, affects soil microbial community across different cultivars. RESULTS: This study explored the influence of seven different RP cultivars on the taxonomic composition, diversity, and functional groups of soil fungal communities through a field trial in Marianna, Florida, Southern USA, using next-generation sequencing technique. Our results showed that the taxonomic diversity and composition of the fungal community differed significantly across RP cultivars. Alpha diversity (Shannon, Simpson, and Pielou's evenness) was significantly higher in Ecoturf but lower in UF_Peace and Florigraze compared to other cultivars (p < 0.001). Phylogenetic diversity (Faith's PD) was lowest in Latitude compared to other cultivars (p < 0.0001). The dominant phyla were Ascomycota (13.34%), Mortierellomycota (3.82%), and Basidiomycota (2.99%), which were significantly greater in Florigraze, UF_Peace, and Ecoturf, respectively. The relative abundance of Neocosmospora was markedly high (21.45%) in UF_Tito and showed large variations across cultivars. The relative abundance of the dominant genera was significantly greater in Arbrook than in other cultivars. There were also significant differences in the co-occurrence network, showing different keystone taxa and more positive correlations than the negative correlations across cultivars. FUNGuild analysis showed that the relative abundance of functional guilds including pathogenic, saprotrophic, endophytic, mycorrhizal and parasitic fungi significantly differed among cultivars. Ecoturf had the greatest relative abundance of mycorrhizal fungal group (5.10 ± 0.44), whereas UF_Peace had the greatest relative abundance of endophytic (4.52 ± 0.56) and parasitic fungi (1.67 ± 0.30) compared to other cultivars. CONCLUSIONS: Our findings provide evidence of crop cultivar's effect in shaping fine-scale fungal community patterns in legume-based forage systems.

Vegetation communities and soil properties along the restoration process of the Jinqianghe mine site in the Qilian Mountains, China.

The study explores the impact of mine grassland restoration on plant communities and soil properties in alpine grasslands, a subject of significant interest due to the observed relationship between grassland changes, plant communities, and soil properties. While prior research has mainly focused on the consequences of grassland degradation on plant diversity and soil characteristics, the specific effects of varying restoration degrees in alpine mining grasslands at the regional scale remain poorly understood. To address this knowledge gap, we established 15 sampling plots (0.5m×0.5m) across five different restoration degrees within alpine mining grasslands in the Qilian Mountains, China. Our objective was to assess the variations in plant diversity and soil properties along these restoration gradients. We conducted comprehensive analyses, encompassing soil properties [soil water content (SWC), available nitrogen (AN), total phosphorus (TP), nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), total nitrogen (TN), available phosphorus (AP), soil organic carbon (SOC), nitrate nitrogen, soil pH, and electrical conductivity (EC)], plant characteristics (height, density, frequency, coverage, and aboveground biomass), and plant diversity indices (Simpson, Shannon-Wiener, Margalef, Dominance, and Evenness indexes). Our findings included the identification and collection of 18 plant species from 11 families and 16 genera across the five restoration degrees: Very Low Restoration Degree (VLRD), Low Restoration Degree (LRD), Moderate Restoration Degree (MRD), High Restoration Degree (HRD), and Natural Grassland (NGL). Notably, species like Carex duriuscula, Cyperus rotundus, and Polygonum viviparum showed signs of recovery. Principal component analysis and Pearson correlation analysis revealed that soil pH, SWC, SOC, NO3-N, and AN were the primary environmental factors influencing plant communities. Specifically, soil pH and EC decreased as restoration levels increased, while SWC, AN, TP, NH4-N, TN, AP, SOC, and NO3-N exhibited a gradual increase with greater restoration efforts. Furthermore, the HRD plant community demonstrated similarities to the NGL, indicating the most effective natural recovery. In conclusion, our study provides valuable insights into the responses of plant community characteristics, plant diversity, and soil properties across varying restoration degrees to environmental factors. It also elucidates the characteristics of plant communities along recovery gradients in alpine grasslands.

Predicting the responses of European grassland communities to climate and land cover change.

European grasslands are among the most species-rich ecosystems on small spatial scales. However, human-induced activities like land use and climate change pose significant threats to this diversity. To explore how climate and land cover change will affect biodiversity and community composition in grassland ecosystems, we conducted joint species distribution models (SDMs) on the extensive vegetation-plot database sPlotOpen to project distributions of 1178 grassland species across Europe under current conditions and three future scenarios. We further compared model accuracy and computational efficiency between joint SDMs (JSDMs) and stacked SDMs, especially for rare species. Our results show that: (i) grassland communities in the mountain ranges are expected to suffer high rates of species loss, while those in western, northern and eastern Europe will experience substantial turnover; (ii) scaling anomalies were observed in the predicted species richness, reflecting regional differences in the dominant drivers of assembly processes; (iii) JSDMs did not outperform stacked SDMs in predictive power but demonstrated superior efficiency in model fitting and predicting; and (iv) incorporating co-occurrence datasets improved the model performance in predicting the distribution of rare species. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Enhancing the component intra- and interrelationship of Elymus nutans mono- and mixed sowing communities via adjusting sowing patterns in the Qinghai Tibetan Plateau.

Spatial arrangement is a key factor in maintaining community yield and stability via regulating component intra-/interspecific competition in an alpine climate environment. A 2-yr field trial was conducted on the Qinghai Tibetan Plateau, including cross row (S_C), double row (S_D), single row (S_R), broadcast (M_B), dependent row (M_D) and independent row (M_I). Our results showed that S_C could avoid intraspecific competition by reasonable spatial arrangement, which favored the dominant component growth (1st year: leaf; 2nd year: stem and reproductive organ). For mixed communities, RII (relative interaction intensity) implied that interspecific competition also embodied on dominant component, and higher Elymus nutans component advantages seriously limited Onobrychis viciifolia's components growth in the 2nd year. More details displayed that E. nutans in M_B or M_D produced the maximum system yield via increasing leaf investment at the initial stages and stem investment after July 2019. Besides, M_I possessed lower component numbers than M_B and M_D in the unit area. PCA analysis revealed that component numbers or biomasses changed synchronously, besides the E. nutans of S_C, M_B, and M_D presented significant discrepancies compared to other treatments in September 2019, which verified the effect of sowing patterns on component growth (P < 0.05), but O. viciifolia in different sowing patterns was similar in the 2nd year. Considering the adaptability and production for the environment of the Qinghai Tibetan Plateau, S_C is recommended for the promoted effect on component biomasses. M_B and M_D, with the merit of spacing utilization as well as higher resistance to variation in seasonal growth conditions via optimizing interspecific relationships for mixed communities, are adapted for increasing yield via component harvesting. Our results unveiled the potential of optimizing spatial usage efficiency via controlling component growth characteristics and stressed the importance of dynamic change of dominant components to enhance forage system production in alpine regions.

Grassland health assessment based on indicators monitored by UAVs: a case study at a household scale.

Grassland health assessment (GHA) is a bridge of study and management of grassland ecosystem. However, there is no standardized quantitative indicators and long-term monitor methods for GHA at a large scale, which may hinder theoretical study and practical application of GHA. In this study, along with previous concept and practices (i.e., CVOR, the integrated indexes of condition, vigor, organization and resilience), we proposed an assessment system based on the indicators monitored by unmanned aerial vehicles (UAVs)-UAVCVOR, and tested the feasibility of UAVCVOR at typical household pastures on the Qinghai-Tibetan Plateau, China. Our findings show that: (1) the key indicators of GHA could be measured directly or represented by the relative counterpart indicators that monitored by UAVs, (2) there was a significantly linear relationship between CVOR estimated by field- and UAV-based data, and (3) the CVOR decreased along with the increasing grazing intensity nonlinearly, and there are similar tendencies of CVOR that estimated by the two methods. These findings suggest that UAVs is suitable for GHA efficiently and correctly, which will be useful for the protection and sustainable management of grasslands.

Benefits, potential and risks of China's grassland ecosystem conservation and restoration.

Severe threats from ongoing degradation undermine the grasslands to support ecosystem services, biodiversity, and human well-being. Unfortunately, grasslands are often underappreciated and ignored in sustainable development agendas. Despite a series of projects for Grassland Ecosystem Conservation and Restoration (GECR) been implemented in China, the effects and cost-effectiveness of these efforts remain uncertain and untested. Therefore, we developed an integrated assessment framework to evaluate the benefits of GECR, considering ecological value accounting and input-output efficiency estimation. Additionally, we projected potential and risk areas for GECR in the future. The results showed that in 2020, the annual ecological value of China's grassland ecosystem was CNY 246 trillion. The investment in GECR exceeded CNY 7 billion, leading to an ecological benefit of CNY 3478 billion, with an input-output ratio of 1:446. Over the past 20 years, GECR positively impacted nearly 90 % of China's grassland. Furthermore, grasslands in southern provinces with favorable hydrothermal conditions exhibited significantly higher GECR efficiency, boasting an input-output ratio of >1:2000. The arid and semi-arid northern grasslands and the alpine grasslands on the Tibetan Plateau, despite being the main regions for animal husbandry development and GECR, exhibited comparatively lower efficiency and input-output ratio in GECR. Moreover, the central and northwest parts of Tibet showed higher potential and lower risk, indicating their greatest likelihood of benefiting from GECR in the future. Meanwhile, Hulunbeier and Inner Mongolia deserve more special attention to reverse degradation and mitigate climate change due to their lower potential and higher risks. Our study provides an important basis for prioritizing and implementing effective and sustainable GECR treatment methods.

Characteristics of net ecosystem exchange and source distribution of Xilinhot grassland, China.

To understand carbon sequestration capacity of grasslands, the changes of CO2 flux in Xilinhot grasslands and the influence of environmental factors were analyzed by using the eddy data of Xilinhot National Climate Observatory in 2018-2021, and the distribution of flux source areas was analyzed. The results showed that the southwest wind prevailed in the study area throughout the year, the source area in the growing season was larger than that in the non-growing season, and the source area under stable atmospheric conditions was larger than that under unstable conditions. The maximum length of source region with a contribution rate of 90% was close to 400 m, which was consistent with the length estimated by the classical law. The net ecosystem exchange (NEE) of Xilinhot grasslands had obvious diurnal and seasonal dynamics, which was manifested as a carbon sink in the daytime and a carbon source at night during the growing season and weak carbon source in the non-growing season. From 2018 to 2021, the annual total NEE were -15.59, -46.28, -41.94, and -78.14 g C·m-2·a-1, respectively, with an average value of -45.49 g C·m-2·a-1, indicating that Xilinhot grassland had strong carbon sequestration capacity. Vapor pressure deficit and photosynthetically active radiation helped grasslands absorb atmospheric CO2. At night, when temperature was above 0 ℃, the increases in air and soil temperature promoted vegetation respiration to release CO2.

Drought is threatening plant growth and soil nutrients of grassland ecosystems: A meta-analysis.

As a widespread direct effect of global warming, drought is currently wreaking havoc on terrestrial ecosystems' structure and function, however, the synthesized analysis is lacked to explore the general rules between drought changes and main functional factors of grassland ecosystems. In this work, meta-analysis was used to examine the impacts of drought on grassland ecosystems in recent decades. According to the results, drought greatly reduced aboveground biomass (AGB), aboveground net primary production (ANPP), height, belowground biomass (BGB), belowground net primary production (BNPP), microbial biomass nitrogen (MBN), microbial biomass carbon (MBC) and soil respiration (SR), and increased dissolved organic carbon (DOC), total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO3--N), and the ratio of microbial biomass carbon and nitrogen (MBC/MBN). The drought-related environmental factor mean annual temperature (MAT) was negatively correlated with AGB, height, ANPP, BNPP, MBC, and MBN, however, mean annual precipitation (MAP) had positive effect on these variables. These findings indicate that drought is threatening the biotic environment of grassland ecosystem, and the positive steps should be taken to address the negative effects of drought on grassland ecosystems due to climate change.

Grassland biodiversity response to livestock grazing, productivity, and climate varies across biome components and diversity measurements.

Livestock overgrazing and climate change have been identified as the primary causes of grassland degeneration and biodiversity decline, yet the underlying mechanism remains unclear. To gain a better understanding of this, we conducted a meta-analysis of 91 local or regional field studies from 26 countries across all inhabited continents. Using concise statistical analyses, we assessed five theoretical hypotheses for grazing intensity, grazing history, grazing animal type, productivity, and climate, and decomposed the individual contributions of each factor in regulating multiple components of grassland biodiversity. After controlling for confounding effects, we found that: no significant linear or binomial pattern for the effect-size of grassland biodiversity as grazing intensity increased; the effect-size of producer richness was relatively lower (negative biodiversity response) in grasslands with a short grazing history, grazed by large livestock, high productivity, or high climate suitability; additionally, significant difference for the effect-size of consumer richness was only detected across grazing animal groups; and the effect-size of consumer abundance, and decomposer abundance all displayed significant differences with respect to grazing characters, grassland productivity, and climate suitability. Besides, results of hierarchical variance partitioning suggested that the total and individual contribution of predictors varied across biome components and diversity measurements. Specifically, grassland productivity acted as a key factor in driving producer richness. The findings presented here collectively suggest that the response of grassland biodiversity to livestock grazing, productivity, and climate varies across different components of the biome and measurements of diversity.

Research Progress of Grassland Ecosystem Structure and Stability and Inspiration for Improving Its Service Capacity in the Karst Desertification Control.

The structure and stability of grassland ecosystems have a significant impact on biodiversity, material cycling and productivity for ecosystem services. However, the issue of the structure and stability of grassland ecosystems has not been systematically reviewed. Based on the Web of Science (WOS) and China National Knowledge Infrastructure (CNKI) databases, we used the systematic-review method and screened 133 papers to describe and analyze the frontiers of research into the structure and stability of grassland ecosystems. The research results showed that: (1) The number of articles about the structure and stability of grassland ecosystems is gradually increasing, and the research themes are becoming increasingly diverse. (2) There is a high degree of consistency between the study area and the spatial distribution of grassland. (3) Based on the changes in ecosystem patterns and their interrelationships with ecosystem processes, we reviewed the research progress and landmark results on the structure, stability, structure-stability relationship and their influencing factors of grassland ecosystems; among them, the study of structure is the main research focus (51.12%), followed by the study of the influencing factors of structure and stability (37.57%). (4) Key scientific questions on structural optimization, stability enhancement and harmonizing the relationship between structure and stability are explored. (5) Based on the background of karst desertification control (KDC) and its geographical characteristics, three insights are proposed to optimize the spatial allocation, enhance the stability of grassland for rocky desertification control and coordinate the regulation mechanism of grassland structure and stability. This study provided some references for grassland managers and relevant policy makers to optimize the structure and enhance the stability of grassland ecosystems. It also provided important insights to enhance the service capacity of grassland ecosystems in KDC.

Could green infrastructure supplement ecosystem service provision from semi-natural grasslands?

Ancient semi-natural grasslands in Europe are important for ecosystem service (ES) provision. Often, the surrounding matrix contains 'Grassland Green Infrastructure' (GGI) that contain grassland species which have the potential to supplement grassland ES provision across the landscape. Here we investigate the potential for GGI to deliver a set of complementary ES, driven by plant composition.We surveyed 36 landscapes across three European countries comprising core grasslands and their surrounding GGI. We calculated community-level values of plant species characteristics to provide indicators for four ES: nature conservation value, pollination, carbon storage and aesthetic appeal.Inferred ES delivery for GGI was substantially lower than in core grasslands for conservation, pollination and aesthetic appeal indicators, but not for carbon storage. These differences were driven by the GGI having 17% fewer plant species, and compositional differences, with 61% of species unique to the core grasslands. In addition, connectivity to the core, the amount of GGI and inferred seed dispersal distances by livestock, were strongly positively correlated with conservation value, pollination and aesthetic indicators. All ES indicators showed similar responses to the GGI spatial structure and distance to the core, suggesting robust effects of these drivers on ES. We projected that improved landscape-wide delivery of nature conservation value and pollination could be achieved through targeted GGI management. Reductions in the distances seeds would need to disperse, more GGI, along with a diversification of the GGI elements, were predicted to enhance service credits.We conclude that for vegetation-related ES, species surveys can be employed to assess potential ES delivery. Creating and enhancing GGI is a useful landscape management strategy to supplement the ES delivered by ancient grasslands.

Canopy structure: An intermediate factor regulating grassland diversity-function relationships under human disturbances.

Grasslands are one of the largest coupled human-nature terrestrial ecosystems on Earth, and severe anthropogenic-induced grassland ecosystem function declines have been reported recently. Understanding factors influencing grassland ecosystem functions is critical for making sustainable management policies. Canopy structure is an important factor influencing plant growth through mediating within-canopy microclimate (e.g., light, water, and wind), and it is found coordinating tightly with plant species diversity to influence forest ecosystem functions. However, the role of canopy structure in regulating grassland ecosystem functions along with plant species diversity has been rarely investigated. Here, we investigated this problem by collecting field data from 170 field plots distributed along an over 2000 km transect across the northern agro-pastoral ecotone of China. Aboveground net primary productivity (ANPP) and resilience, two indicators of grassland ecosystem functions, were measured from field data and satellite remote sensing data. Terrestrial laser scanning data were collected to measure canopy structure (represented by mean height and canopy cover). Our results showed that plant species diversity was positively correlated to canopy structural traits, and negatively correlated to human activity intensity. Canopy structure was a significant indicator for ANPP and resilience, but their correlations were inconsistent under different human activity intensity levels. Compared to plant species diversity, canopy structural traits were better indicators for grassland ecosystem functions, especially for ANPP. Through structure equation modeling analyses, we found that plant species diversity did not have a direct influence on ANPP under human disturbances. Instead, it had a strong indirect effect on ANPP by altering canopy structural traits. As to resilience, plant species diversity had both a direct positive contribution and an indirect contribution through mediating canopy cover. This study highlights that canopy structure is an important intermediate factor regulating grassland diversity-function relationships under human disturbances, which should be included in future grassland monitoring and management.

Microbial network structure, not plant and microbial community diversity, regulates multifunctionality under increased precipitation in a cold steppe.

It is known that the dynamics of multiple ecosystem functions (i. e., multifunctionality) are positively associated with microbial diversity and/or biodiversity. However, how the relationship between microbial species affects ecosystem multifunctionality remains unclear, especially in the case of changes in precipitation patterns. To explore the contribution of biodiversity and microbial co-occurrence networks to multifunctionality, we used rainfall shelters to simulate precipitation enhancement in a cold steppe in Northeast China over two consecutive growing seasons. We showed that an increased 50% precipitation profoundly reduced bacterial diversity and multidiversity, while inter-annual differences in precipitation did not shift microbial diversity, plant diversity, or multidiversity. Our analyses also revealed that increased annual precipitation significantly increased ecosystem, soil, nitrogen, and phosphorous cycle multifunctionality. Neither increased precipitation nor inter-annual differences in precipitation had a significant effect on carbon cycle multifunctionality, probably due to the relatively short period (2 years) of our experiment. The co-occurrence network of bacterial and fungal communities was the most dominant factor affecting multifunctionality, the numbers of negative interactions but not positive interactions were linked to multifunctionality. In particular, our results provided evidence that microbial network topological features are crucial for maintaining ecosystem functions in grassland ecosystems, which should be considered in related studies to accurately predict the responses of ecosystem multifunctionality to predicted changes in precipitation patterns.

Compensatory growth as a response to post-drought in grassland.

Grasslands are structurally and functionally controlled by water availability. Ongoing global change is threatening the sustainability of grassland ecosystems through chronic alterations in climate patterns and resource availability, as well as by the increasing frequency and intensity of anthropogenic perturbations. Compared with many studies on how grassland ecosystems respond during drought, there are far fewer studies focused on grassland dynamics after drought. Compensatory growth, as the ability of plants to offset the adverse effects of environmental or anthropogenic perturbations, is a common phenomenon in grassland. However, compensatory growth induced by drought and its underlying mechanism across grasslands remains not clear. In this review, we provide examples of analogous compensatory growth from different grassland types across drought characteristics (intensity, timing, and duration) and explain the effect of resource availability on compensatory growth and their underlying mechanisms. Based on our review of the literature, a hypothetic framework for integrating plant, root, and microbial responses is also proposed to increase our understanding of compensatory growth after drought. This research will advance our understanding of the mechanisms of grassland ecosystem functioning in response to climate change.

Variation characteristics of different plant functional groups in alpine desert steppe of the Altun Mountains, northern Qinghai-Tibet Plateau.

In grassland ecosystems, the plant functional group (PFG) is an important bridge connecting individual plants to the community system. The grassland ecosystem is the main ecosystem type on the Qinghai-Tibet Plateau. Altun Mountain is located in the key grassland transcontinental belt of the northern Qinghai-Tibet Plateau. The composition and changes in the PFG in this ecosystem reflect the community characteristics in the arid and semi-arid extreme climate regions of the Plateau. The main PFGs were forbs and grasses, and the importance values (IVs) accounted for more than 50%. Plant species diversity of the community was influenced by the IV of the legumes, and the increase in legumes would promote the increase in plant community diversity. The C, N, and P contents of plant communities were mainly influenced by forbs and grasses, and the relationship between forbs and C, N, and P was opposite to that of grasses. However, under the influence of different hydrothermal conditions, forbs and grasses as dominant functional groups had a stronger correlation with community and soil nutrients. This indicates that the dominant PFGs (forbs and grasses) can dominate the C, N, and P contents of the community and soil, and legumes affect community composition and succession. In this study, we analyzed the changing characteristics of functional groups in dry and cold extreme environments and the difference in their impacts on community development compared with other grassland ecosystem functional groups.

Characteristics and driving conditions of flash drought in different grassland ecosystems.

At present, flash droughts are poorly understood. Mature prevention and control measures are not available. This study aims to thoroughly explore the evolution characteristics of flash droughts in grassland ecosystems and to determine the meteorological driving conditions for inducing and relieving flash droughts. We propose the concept of the strong evapotranspiration flash drought (SEFD), a new type of flash drought in mid-temperate grasslands. The frequency of SEFDs is lower than that of heat wave flash droughts (HWFDs), but the intensity and impact of SEFDs are greater than those of HWFDs. Flash droughts in grasslands exhibit a high outbreak period from May to August, with the intensity basically above that of moderate drought. HWFDs occur most frequently in June and SEFDs in May. Meadow grasslands are the type of grassland with the highest risk of flash drought. Typical grasslands are more prone to HWFDs, while desert grasslands are more prone to SEFD outbreaks. In this study, a multifactor method was established to quantitatively evaluate the key influencing factors inducing flash droughts of different intensities in different time periods and to quantitatively predict the evolution of flash droughts into seasonal droughts. The temperature, water vapor pressure, precipitation, and wind speed were the key influencing factors of flash droughts and that the most important period in terms of inducing flash droughts is one pentad before the onset of flash drought. The outbreak rate and threshold of key influencing factors are the driving conditions for inducing flash droughts, and the recovery rate can be used as the basis for predicting whether an flash drought will evolve into a seasonal drought. The results showed that the characteristics and driving conditions of flash droughts are different in different types of grasslands and different time periods and are different for different types of flash droughts. In the future, the impact of flash droughts will become more severe.

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Nitrogen is the most limiting nutrient for ecosystems. The natural abundance of δ15N (15N/14N) can efficiently indicate ecosystem nitrogen cycling processes. We investigated the interannual variations in natural abundance of δ15N in soil-plant system and soil net nitrogen mineralization in a meadow steppe of Inner Mongolia. Results across the four sampling years (2017-2020) showed that the content of soil NO3--N (9.83-14.79 mg·kg-1) was significantly higher than that of NH4+-N (3.92-5.00 mg·kg-1) and that δ15N value of soil NH4+ (13.3‰-18.3‰) was significantly higher than that of NO3-(3.76‰-6.14‰). The δ15N value of soil NO3- was negatively correlated with soil NO3- content. The δ15N value of soil NH4+ was relatively higher in the dry years, while the δ15N value of soil NO3- significantly decreased in the wetter and drier years. Soil net mineralization and ammonification rates were significantly higher in the dry years than that of the wet years, while soil nitrification rates showed no correlation with annual precipitation. The δ15N values of plants were not related to that of soils, but nega-tively correlated with plant nitrogen content. Both δ15N values and nitrogen contents were significantly and positively correlated between the leguminous and non-leguminous plants, suggesting that legume could facilitate nitrogen uptake of non-leguminous plants. These results could provide supporting data for nitrogen cycling and their responses to changes in precipitation in grassland soil-plant systems.