Effects of nitrogen addition on vegetation and soil and its linkages to plant diversity and productivity in a semi-arid steppe

Coordination of plant functional traits under nitrogen deposition with phosphorus addition in a desert steppe ecosystem

Understanding how plant functional traits respond to nutrient enrichment becomes more crucial for predicting changes in grassland community composition and functions under global changes. However, it remains unclear how nitrogen (N) and phosphorus (P) additions jointly influence a variety of leaf traits and how they coordinate with contrastingly adaptive mechanisms in arid ecosystems. A two-year field experiment with five N levels and two P treatments was conducted to examine the effects of N and P additions on leaf/community functional traits in a desert steppe. We found N addition significantly affected the other six leaf morphological and nutrient traits except leaf thickness (LT); nitrogen addition remarkably increased leaf nitrogen concentration (Nmass) and decreased C/N with or without P; nitrogen addition profoundly elevated stomatal conductance (gs) but did not obviously affect photosynthetic rate (Aarea) except Tribulus terrestris. Compared to grasses, the annual forb T. terrestris exhibited stronger competitiveness (Nmass, Aarea) with increased N application. Nitrogen addition obviously increased community-weighted means (CWMs) of Nmass, specific leaf area (SLA), plant height, gs and Aarea, improving aboveground biomass (AGB), whereas P addition significantly enhanced CWM of SLA but reduced CWMs of transpiration rate and LT. With increasing N addition rates, dominant S-strategy species (higher LT and C/N) in low-nutrient environments were gradually substituted by R-strategy species (higher Nmass and Aarea) in high-nutrient environments. Our results highlight differential responses of plant functional traits to nutrient enrichment and divergent adaptive strategies among species should be considered when assessing the impacts of global environmental changes on community assembly and functioning.

High plant diversity alleviates the negative effects of nitrogen deposition on soil nitrogen cycling multifunctionality

Introduction

Changes in plant diversity and increased atmospheric nitrogen deposition independently influence soil nitrogen cycling in terrestrial ecosystems. However, the interactive effects of plant diversity and nitrogen deposition on soil nitrogen cycling multifunctionality (NCMF) in grassland ecosystems remain poorly understood.

Methods

We conducted a fully factorial microcosm experiment to quantify the responses and underlying mechanism of soil NCMF to nitrogen addition (0, 5, and 10 g N m-2 yr.-1) and plant diversity gradients (1, 3, and 6 species).

Results

Our results revealed a significant interactive effect between plant diversity and nitrogen addition on soil NCMF. Specifically, high plant diversity alleviated the negative effects of nitrogen addition on soil NCMF. The addition of nitrogen reduced the soil pH, which imposed microbial stress by limiting carbon availability. In contrast, higher plant diversity increased soil organic matter via below-ground carbon inputs, thereby reducing the soil carbon limitation of microorganims and enhancing the soil NCMF.

Discussion

Overall, our findings suggest that maintaining or enhancing plant diversity in grasslands could be a key strategy to mitigate the adverse effects of atmospheric nitrogen deposition on soil nitrogen cycling, highlighting the crucial role of plant diversity in regulating ecosystem nutrient cycling under global change.

Leaf adaptation strategy of non-tree plants altered by community structure implies vegetation degradation risk in alpine rocky desertification areas

In alpine rocky desertification areas, environmental stress poses challenges to vegetation restoration and protection. Merely observing the changes in specific leaf area driven by environmental factors may overlook the risk of non-tree vegetation degradation. The leaf resource allocation strategies of non-tree plants need to be focused on. In the alpine rocky desertification areas of the Jinsha River Basin, three vegetation types were investigated. The leaf traits, vegetation coverage, species diversity of non-tree plants, and soil total nitrogen, rock bareness degree were measurement. An increase in altitude led to a decrease in vegetation coverage and an increase in species diversity. In grasslands with exceeded 35% rock bareness degree, the increase in species diversity intensified competition pressure, resulting in a decrease in the specific leaf area. In forests with less than 20% rock bareness degree, the species of shrubs have become homogeneous, resulting in a decrease in vegetation coverage but an increase in the specific leaf area. But due to environmental stress, the leaf resource allocation of different species may have favored leaf dry weight (allometric index < 1.0). An increase in soil total nitrogen alleviated environmental stress, causing leaf resources to be allocated to both leaf dry weight and leaf area (allometric index ≁1.0). However, it enhanced the above-ground competitiveness of few dominant species, squeezing out the living space of auxiliary species, and vegetation degradation risk increased. Species with similar specific leaf areas can have different leaf resource allocation strategies. By combining the changes in specific leaf area with leaf resource allocation strategies, the development of vegetation under environmental stress can be accurately revealed.

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.

Dominant species modulates nitrogen effects on the temporal stability of above- and below-ground biomass in a temperate desert steppe

The temporal stability of above-ground biomass (AGB) and below-ground biomass (BGB) in grasslands is crucial for maintaining a continuous supply of ecosystem functions and services, particularly in the context of global changes. Nitrogen (N) addition is well known to affect AGB stability, however, we still lack knowledge of how N addition affect BGB stability. Furthermore, a crucial knowledge gap remains regarding which underlying mechanisms drive AGB and BGB stability, which obstructs our comprehensive awareness of biomass stability from both above- and below-ground perspectives simultaneously. Through a five-year manipulative experiment with seven N addition levels, we tested how AGB and BGB stability responded to N addition and which factors modulated the effects of N addition on stability. Using a framework developed to quantify changes in biomass stability, we investigated the relative contributions of dominant species, taxonomic diversity, functional diversity, species asynchrony, species stability and soil properties in driving AGB and BGB stability. We found that N addition enhanced AGB stability directly and indirectly through the modulatory effects of dominant species. N addition increased dominance of fast species, causing high community-weighted mean (CWM) fast-slow and further high species asynchrony. Large increases in species asynchrony and weak decreases in dominant species stability, rather than decreases in species richness, were crucial factors driving AGB stability. Additionally, increased CWM fast-slow enhanced BGB stability, although this positive effect was partially offset by a slight decrease in soil water content (SWC). Our results broaden the insurance hypothesis and the mass ratio hypothesis, providing new insights into how N addition affects biomass stability and underlying driving mechanisms in a temperate desert steppe. These findings emphasize that dominant species and CWM fast-slow play crucial modulatory roles in driving biomass stability. Therefore, it is very necessary to pay attention to dominant species and functional diversity, in order to provide guidance for the sustainable functions and services in species-poor drylands.

Effects of climate warming on soil nitrogen cycles and bamboo growth in core giant panda habitat

Climate change can pose a significant threat to terrestrial ecosystems by disrupting the circulation of soil nitrogen. However, experimental analyses on the effect of climate change on soil nitrogen cycles and the implications for the conservation of key wildlife species (i.e., the giant panda, Ailuropoda melanoleuca) remain understudied. We investigated the effects of a 1.5 °C, 3 °C, and 4.5 °C temperature increase on nitrogen distribution in different soil layers of bamboo forest via an in-situ experiment and assessed the implications for the growth and survival of arrow bamboo (Bashania faberi), a critical food resource for giant pandas. Our results showed that warming treatments generally increased soil N content, while effects differed between surface soil and subsurface soil and at different warming treatments. Particularly an increase of 1.5 °C raised the subsurface soil NO3-N content, as well as the content of N in bamboo leaves. We found a significant positive correlation between the subsurface soil NO3-N content and the N content of arrow bamboo. An increase of 3-4.5 °C raised the content of total N and NO3-N in the surface soil and led to a reduction in the total aboveground biomass and survival rate of arrow bamboo. Limited warming (e.g., the increase of 0-1.5 °C) may promote the soil N cycle, raise the N-acetylglucosaminidase (NAG) enzyme activity, increase NO3-N in subsurface soil, increase the N content of bamboo, and boost the biomass of bamboo - all of which could be beneficial to giant panda survival. However, higher warming (e.g., an increase of 3-4.5 °C) resulted in mass death of bamboo and a large reduction in aboveground biomass. Our findings provide a cautiously optimistic scenario for bamboo forest ecosystems under low levels of warming over a short period of time, but risks from higher levels of warming may be serious, especially considering the unpredictability of global climatic change.

Effects of different soil and water conservation measures on plant functional traits in the Loess Plateau

Soil and water conservation measures (SWCM) have wide-ranging effects on vegetation and soil, and their effects on the ecosystem are multifaceted, with complex mechanisms. While numerous studies have focused on the impact of such measures on soil, the improvement of plant functional traits is a major factor in the ecological recovery of the Loess Plateau. This survey extensively investigated no measure plots, vegetation measure plots, and engineering measure plots in the Loess Plateau. The impact of SWCM on plant functional traits was investigated using structural equation modeling. We examined six plant functional traits-leaf dry weight (LD), specific leaf area (SLA), leaf tissue density (LTD), leaf total phosphorus (LTP), leaf total nitrogen (LTN), and leaf volume (LV)-correlated with resource acquisition and allocation. In 122 plots, we explored the effects of measures, soil, diversity, and community structure on the weighted average of plant functional traits. The findings showed substantial positive correlations between LD and SLA, LD and LV, SLA and LV, SLA and LTP, and LTP and LTN. LTD has a substantial negative correlation with LD, LTD with SLA, and LTD with LV. SWCM limits diversity, and the mechanisms by which it affects plant functional traits vary. In the structural equation model (SEM) of vegetation measures, improving community structure enhances plant functional traits, but soil factors have the greatest influence on plant functional traits in SEM engineering measures. Plant functional trait differences on the Loess Plateau result are due to differential plant responses to diverse soil properties and community structure. Vegetation measures enhance the chemical properties of plant functional traits, while engineering measures improve physical properties. The study provides a theoretical foundation for vegetation restoration and management following the implementation of diverse SWCM.

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 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.

Interactive effects of grassland utilization and climatic factors govern the plant diversity-soil C relationship in steppe of North China

The relationship between plant diversity and the ecosystem carbon pool is important for understanding the role of biodiversity in regulating ecosystem functions. However, it is not clear how the relationship between plant diversity and soil carbon content changes under different grassland use patterns. In a 3-year study from 2013 to 2015, we investigated plant diversity and soil total carbon (TC) content of grasslands in northern China under different grassland utilization methods (grazing, mowing, and enclosure) and climatic conditions. Shannon-Wiener and Species richness index of grassland were significantly decreased by grazing and mowing. Plant diversity was positively correlated with annual precipitation (AP) and negatively correlated with annual mean temperature (AMT). AP was the primary regulator of plant diversity. Grazing and mowing decreased TC levels in grasslands compared with enclosures, especially in topsoil (0-20 cm). The average TC content was decreased by 58 % and 36 % in the 0-10 cm soil layer, while it was decreased by 68 % and 39 % in 10-20 cm soil layer. TC was positively correlated with AP and negatively correlated with AMT. Principal component analysis (PCA) showed that plant diversity was positively correlated with soil TC, and the correlation decreased with an increase in the soil depth. Overall, this study provides a theoretical basis for predicting soil carbon storage in grasslands under human disturbances and climate change impacts.

Effects of Short-Term Nitrogen Additions on Biomass and Soil Phytochemical Cycling in Alpine Grasslands of Tianshan, China

The nitrogen deposition process, as an important phenomenon of global climate change and an important link in the nitrogen cycle, has had serious and far-reaching impacts on grassland ecosystems. This study aimed to investigate the survival adaptation strategies of plants of different functional groups under nitrogen deposition, and the study identified the following outcomes of differences in biomass changes by conducting in situ simulated nitrogen deposition experiments while integrating plant nutrient contents and soil physicochemical properties: (1) nitrogen addition enhanced the aboveground biomass of grassland communities, in which Poaceae were significantly affected by nitrogen addition. Additionally, nitrogen addition significantly influenced plant total nitrogen and total phosphorus; (2) nitrogen addition improved the plant growth environment, alleviated plant nitrogen limitation, and promoted plant phosphorus uptake; and (3) there was variability in the biomass responses of different functional groups to nitrogen addition. The level of nitrogen addition was the primary factor affecting differences in biomass changes, while nitrogen addition frequency was an important factor affecting changes in plant community structure.

Nitrogen fertilization reduces plant diversity by changing the diversity and stability of arbuscular mycorrhizal fungal community in a temperate steppe

Nitrogen (N) deposition resulting from anthropogenic activities poses threats to ecosystem stability by reducing plant and microbial diversity. However, the role of soil microbes, particularly arbuscular mycorrhizal fungi (AMF), as mediators of N-induced shifts in plant diversity remains unclear. In this study, we conducted 6 and 11 years of N addition field experiments in a temperate steppe to investigate AMF richness and network stability and their associations with plant species richness in response to N deposition. The N fertilization, especially in the 11 years of N addition, profoundly decreased the AMF richness and plant species richness. Furthermore, N fertilization significantly decreased the AMF network complexity and stability, with these effects becoming more enhanced with the increase in N addition duration. AMF richness and network stability showed positive associations with plant diversity, and these associations were stronger after 11 than 6 years of N addition. Our findings suggest that N deposition may lead to plant diversity loss via a reduction of AMF richness and network stability, with these effects strengthened over time. This study provides a better understanding of plant-AMF interactions and their response to the prevailing global N deposition.

Effects of different soil and water conservation measures on plant diversity and productivity in Loess Plateau

Many soil and water conservation measures (SWCM) have been implemented in the Loess Plateau of China, and they have an impact on ecosystems all levels and involve complicated mechanisms. Previously, studies typically focused on a single factor's effect on diversity or productivity. With this background, the current investigation embarked on an extensive study, with vegetation survey conducted in the no measure plots (NM), vegetation measure plots (VM) and engineering measure plots (EM) in the Loess Plateau of China. We used structural equation models (SEM) to explain the mechanism by which SWCM affects plant productivity and diversity. VM have direct effects on plant diversity, and EM have direct effects on soil properties and community structure. The two measures also had indirect effects on plant functional traits and community structure. The results show that the changes in plant functional traits and community structure by SWCM decreased plant diversity, whereas the increase of productivity was primarily dominated by improvements in community structure, and we conclude that variability in plant diversity and productivity across different measures on the Loess Plateau was primarily due to the responses of different plants to variable soil properties and the community responses. It was also emphasized that vegetation measures were beneficial to the increase of biomass per plant, while engineering measures were more beneficial to the growth of dominant species. These findings provide a theoretical foundation for vegetation management and restoration after the application of different SWCM.

Effects of nitrogen addition on species composition and diversity of early spring herbs in a Korean pine plantation

Under the background of global nitrogen deposition, temperate forest ecosystems are suffering increasing threats, and species diversity is gradually decreasing. In this study, nitrogen addition experiments were conducted on Korean pine (Pinus koraiensis) plantations in Northeast China to explore the effect of long-term nitrogen addition on herb species diversity to test the following hypothesis: long-term nitrogen addition further reduced plant species diversity by affecting plant growth, which may be due to soil acidification caused by excessive nitrogen addition. Experimental nitrogen addition was conducted from 2014 to 2021, and the nitrogen treatment levels were as follows: N0 (control treatment, 0/(kg N ha-1 year-1)), N20 (low nitrogen treatment, 20/(kg N ha-1 year-1)), N40 (medium nitrogen treatment, 40/(kg N ha-1 year-1)) and N80 (high nitrogen treatment, 80/(kg N ha-1 year-1)). A herb community survey was conducted in the region from 2015 to 2021. The results showed that long-term nitrogen addition decreased soil pH, changed the species and composition of herbaceous plants, and decreased the species diversity of understory herbaceous plants. With the increase in nitrogen application years, middle- and high-nitrogen treatments significantly reduced the diversity of early-spring flowering herbs and early-spring foliating herbs, and their diversity decreased with the decrease in soil pH, indicating that soil acidification caused by long-term nitrogen addition may lead to the decrease of plant diversity. However, for early-spring growing herbs, adequate nitrogen addition may promote their growth. Our results show that plants have evolved different life-history strategies based on their adaptation mechanisms to the environment, and different life-history strategies have different responses to long-term nitrogen addition. However, for most plants, long-term nitrogen application will have a negative impact on the growth and diversity of herbs in temperate forests.

Effects of Grazing, Extreme Drought, Extreme Rainfall and Nitrogen Addition on Vegetation Characteristics and Productivity of Semiarid Grassland

Grassland use patterns, water and nutrients are the main determinants of ecosystem structure and function in semiarid grasslands. However, few studies have reported how the interactive effects of rainfall changes and nitrogen deposition influence the recovery of semiarid grasslands degraded by grazing. In this study, a simulated grazing, increasing and decreasing rainfall, nitrogen deposition test platform was constructed, and the regulation mechanism of vegetation characteristics and productivity were studied. We found that grazing decreased plant community height (CWMheight) and litter and increased plant density. Increasing rainfall by 60% from May to August (+60%) increased CWMheight; decreasing rainfall by 60% from May to August (-60%) and by 100% from May to June (-60 d) decreased CWMheight and coverage; -60 d, +60% and increasing rainfall by 100% from May to June (+60 d) increased plant density; -60% increased the Simpson dominance index (D index) but decreased the Shannon-Wiener diversity index (H index); -60 d decreased the aboveground biomass (ABG), and -60% increased the underground biomass (BGB) in the 10-60 cm layer. Nitrogen addition decreased species richness and the D index and increased the H index and AGB. Rainfall and soil nitrogen directly affect AGB; grazing and rainfall can also indirectly affect AGB by inducing changes in CWMheight; grazing indirectly affects BGB by affecting plant density and soil nitrogen. The results of this study showed that in the semiarid grassland of Inner Mongolia, grazing in the nongrowing season and grazing prohibition in the growing season can promote grassland recovery, continuous drought in the early growing season will have dramatic impacts on productivity, nitrogen addition has a certain impact on the species composition of vegetation, and the impact on productivity will not appear in the short term.

Long-term application of nitrogen fertilizer alters the properties of dissolved soil organic matter and increases the accumulation of polycyclic aromatic hydrocarbons

Soil is a key component of terrestrial ecosystems, as it provides nutrients and energy for all terrestrial organisms and is the site of various physical, chemical, and biological processes. Soil organic matter is particularly important for the role that it plays in element cycling, as well as the adsorption and degradation of soil pollutants. Nitrogen (N) fertilizer is an important nutrient element in the soil microenvironment. Applications of N fertilizer can improve soil quality, but the long-term excessive application of N fertilizer can lead to the deterioration of the soil environment, alter the properties of organic matter, and affect the adsorption and accumulation of soil pollutants. In recent years, several pollutants, especially polycyclic aromatic hydrocarbons (PAHs), have accumulated in farmland soil due to long-term sewage irrigation. However, few studies have examined the response of soil PAHs accumulation to long-term N application, as well as the relationship between this response and changes in soil microenvironmental indicators caused by N application. Here, we conducted field experiments to study changes in soil pH, total organic carbon, and dissolved organic matter (DOM) under long-term N application, as well as their effects on PAHs accumulation. The application of N fertilizer resulted in the aromatization and humification of soil DOM, enhanced the accumulation response ratio (-0.05-0.32) and the amount of PAHs accumulated in soil (more than 30%), and exacerbated the environmental risks of PAHs. Our findings provide new insights that could aid the management and control of PAHs pollution of soil in sewage-irrigated areas.

Differential responses of the properties of soil humic acid and fulvic acid to nitrogen addition in the North China Plain

Humus (HS) is an important component of soil organic matter. Humic acid (HA) and fulvic acid (FA) are two of the most important components of HS, as they substantially affect biogeochemical processes and the migration and transformation of pollutants in soil. Long-term nitrogen (N) addition can lead to changes in soil physical and chemical properties, affect the structural characteristics of soil HS (HA and FA), cause changes in the adsorption and migration of pollutants, and ultimately result in the continuous deterioration of the soil ecological environment. However, few studies have examined the effects of N addition on the structural characteristics of soil HS, including the responses of soil HA and FA to N addition. Here, we conducted a long-term positioning experiment with different levels of N addition (CK: 0 kg N ha^-1 yr^-1, LN: 100 kg N ha^-1 yr^-1, and HN: 300 kg N ha^-1 yr^-1) in typical farmland soils of the North China Plain to study the response of soil HA and FA to N addition. N addition altered the physical and chemical properties of soil (e.g., pH, SOC, TN, and enzyme activity), which affected the responses of the chemical structure, quality indexes, and composition distribution of soil HA and FA to N addition. Differences in the response to N addition between HA and FA were observed. The structural characteristics of FA were stronger in response to HN compared with those of soil HA. As the level of N added increased, soil FA degradation increased, the composition distribution changed, the aromatization degree and molecular weight decreased, and the molecular structure became simpler. The properties of soil HA did not significantly respond to N addition. Given increases in the global N input (N addition and N deposition), our results have implications for agricultural fertilization, soil management, and other activities.

Agents Affecting the Plant Functional Traits in National Soil and Water Conservation Demonstration Park (China)

Plant functional traits (PFTs) can reflect the response of plants to environment, objectively expressing the adaptability of plants to the external environment. In previous studies, various relationships between various abiotic factors and PFTs have been reported. However, how these factors work together to influence PFTs is not clear. This study attempted to quantify the effects of topographic conditions, soil factors and vegetation structure on PFTs. Four categories of variables were represented using 29 variables collected from 171 herb plots of 57 sites (from different topographic and various herb types) in Xindian SWDP. The partial least squares structural equation modeling showed that the topographic conditions and soil properties also have a direct effect on plant functional traits. Among the topographic conditions, slope (SLO) has the biggest weight of 0.629, indicating that SLO contributed the most to plant functional traits and vegetation structure. Among soil properties, maximum water capacity (MWC) contributes the most and is followed by soil water content (SWC), weighted at 0.588 and 0.416, respectively. In a word, the research provides new points into the quantification of the correlation between different drivers that may be important for understanding the mechanisms of resource utilization, competition and adaptation to the environment during plant recovery.

Nitrogen addition alters plant growth in China's Yellow River Delta coastal wetland through direct and indirect effects

In the coastal wetland, nitrogen is a limiting element for plant growth and reproduction. However, nitrogen inputs increase annually due to the rise in nitrogen emissions from human activity in coastal wetlands. Nitrogen additions may alter the coastal wetlands' soil properties, bacterial compositions, and plant growth. The majority of nitrogen addition studies, however, are conducted in grasslands and forests, and the relationship between soil properties, bacterial compositions, and plant growth driven by nitrogen addition is poorly understood in coastal marshes. We conducted an experiment involving nitrogen addition in the Phragmites australis population of the tidal marsh of the Yellow River Delta. Since 2017, four nitrogen addition levels (N0:0 g • m^-2 • year^-1, N1:5 g • m^-2 • year^-1, N2:20 g • m^-2 • year^-1, N3:50 g • m^-2 • year^-1) have been established in the experiment. From 2017 to 2020, we examined soil properties and plant traits. In 2018, we also measured soil bacterial composition. We analyzed the effect of nitrogen addition on soil properties, plant growth, reproduction, and plant nutrients using linear mixed-effect models. Moreover, structural equation modeling (SEM) was utilized to determine the direct and indirect effects of nitrogen addition, soil properties, and bacterial diversity on plant growth. The results demonstrated that nitrogen addition significantly affected plant traits of P. australis. N1 and N2 levels generally resulted in higher plant height, diameter, leaf length, leaf breadth, and leaf TC than N0 and N3 levels. Nitrogen addition had significantly impacted soil properties, including pH, salinity, soil TC, and soil TS. The SEM revealed that nitrogen addition had a direct and positive influence on plant height. By modifying soil bacterial diversity, nitrogen addition also had an small indirect and positive impact on plant height. However, nitrogen addition had a great negative indirect impact on plant height through altering soil properties. Thus, nitrogen inputs may directly enhance the growth of P. australis at N1 and N2 levels. Nonetheless, the maximum nitrogen addition (N3) may impede P. australis growth by reducing soil pH. Therefore, to conserve the coastal tidal marsh, it is recommended that an excess of nitrogen input be regulated.

Heterogeneity of Spatial-Temporal Distribution of Nitrogen in the Karst Rocky Desertification Soils and Its Implications for Ecosystem Service Support of the Desertification Control—A Literature Review

In recent years, the study of soil nitrogen distribution (SND) in rocky desertification control ecosystems has increased exponentially. Rocky desertification experiences severe environmental degradation due to its fragile nature, and understanding rocky desertification soil nitrogen (SN) is critical for ecosystem services (ES) to support sustainable development. From the perspective of bibliometrics, this paper systematically, comprehensively, qualitatively and quantitatively describes the progress, trends and hotspots of SND in the field of rocky desertification environment. The results show that: 97.40% of the document type is “Article”; the study of rocky desertification SND shows the characteristics of rapid growth, the volume of published articles in the past three years accounted for 34.30% of the total; active countries are mainly China, Germany, United States, Sweden, Finland, etc. The research hotspots in this field include karst and nitrogen, and the future research hotspots tend to focus on karst rocky desertification ecosystem, soil nutrients and vegetation diversity in south China. It is suggested to construct SN management strategy suitable for rocky desertification fragile ecosystems in the future, strengthen theoretical research and comprehensively understand the characteristics of rocky desertification control ecosystem to put forward sustainable management strategy according to local conditions.

Nitrogen addition promotes soil microbial beta diversity and the stochastic assembly

Nitrogen (N) deposition is one of major environmental concerns and alters the microbial communities in the pedosphere. A central debate in governing microbial community is on the relative importance of deterministic (ecological selection) vs. stochastic processes (dispersal, drift, diversification or speciation), which consequently limited our understanding of microbial assembly in response to N addition. Here, we conducted a global analysis of high-throughput sequencing data to reveal the mechanisms of N-addition effects on soil microbial communities. The results show that N addition significantly shifted the microbial community structure and promoted microbial beta diversity, particularly in the N-limited ecosystems. Changes in microbial structure and beta diversity increased significantly as the N addition rate, study duration, and the degree of soil acidification increased. The stochastic processes are more important than the deterministic processes for microbial community assembly, while N addition significantly increase the importance of stochastic processes whether the phylogenetic relationship is considered or not. Overall, the current study highlights the important of ecological stochasticity in regulating microbial assembly under N deposition scenarios.