Levels Of Nitrogen Addition Research Articles

Intraspecific trait variability mediates the effect of nitrogen addition and warming on aboveground productivity

Recent studies have shown that intraspecific trait variability is an important source of total trait variation. However, the contribution of intraspecific variability to ecosystem functions in the face of global change remains unknown. We quantified the relative contribution of intra‐ and inter‐specific functional changes on productivity in 48 plots subjected to eight years of nitrogen addition and warming in a Tibetan alpine meadow. The change of the mean (community weighted mean) and the variation (Rao's quadratic entropy) in trait values in response to nitrogen addition and warming were separated into the changes driven by interspecific and intraspecific trait variations using a variance partitioning method. We found that productivity showed a hump‐shaped response to nitrogen addition, with the highest productivity at intermediate levels of nitrogen addition. This hump‐shaped response was mediated by the changes in plant functional structure. A community having higher interspecific variation in plant height and individuals producing bigger leaf area can increase productivity via niche complementary and dominance effects, respectively. Warming reduced productivity directly and marginally decreased individuals' leaf area which suppressed productivity indirectly. Our research suggests a non‐negligible role of plant intraspecific trait variability in maintaining ecosystem functions, especially in the face of global change.

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

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

Effect of Nitrogen Addition on Soil Fungal Diversity in a Degraded Alpine Meadow at Different Slopes

This study proposed nitrogen addition experiments to analyze the effects of exogenous nitrogen addition on soil fungal diversity in alpine meadow. All the experiments were performed in degraded alpine meadow with two different slopes (gentle slope and steep slope) in Guoluo Prefecture of the Sanjiangyuan Region, and the sequence and analysis of ITS of soil fungi were performed using MiSeq PE250 sequencing technology. Comparative analysis was carried out with three nitrogen addition levels on soil fungal diversity in degraded grassland with different slopes, which included low nitrogen (LN, 2 g·m-2), middle nitrogen (MN, 5 g·m-2), and high nitrogen (HN, 10 g·m-2). The results showed that:① the distribution groups of fungi in the soil were Ascomycota, Basidiomycota, Mortierellomycota, and Glomomycota, and the dominant bacteria was Ascomycota. ② The dominant genera were Mortierella and Archaeorhizomyces, and there were no differences in response to different slopes and nitrogen addition levels. ③ A total of 95 genera (Gibberellum, Preussia, etc.) were identified and significantly differed between two different slopes (P<0.05). ④ Bacteria with a relative abundance less than 1% had significant differences in nitrogen addition at different levels on the same slope (P<0.05). 5 In addition, the analyses of α and β diversities showed that soil fungal community structure was stable under different slopes and nitrogen addition levels. Exogenous nitrogen supplementation significantly improved the relative abundance of non-dominant fungal communities without destroying soil fungal community structure.

Nitrogen addition weakens the biodiversity [sbnd] multifunctionality relationships across soil profiles in a grassland assemblage

The dynamics of multiple ecosystem functions (that is, multifunctionality) are closely associated with biodiversity, often show positive relationships, but the dynamics of the biodiversitymultifunctionality relationship along soil profiles remain unclear. Eutrophication significantly affects biodiversity and soil functions, whereas the mechanisms underlying the impact of nitrogen addition on the biodiversitymultifunctionality relationship are poorly understood. Here, we conducted a common garden experiment manipulating plant diversity and nitrogen addition to quantify the effects of biodiversity on soil multifunctionality in the topsoil and subsoil under different nitrogen addition levels. We showed that nitrogen addition had a minor impact on soil multifunctionality but weakened the positive biodiversitymultifunctionality relationship. We also found that the positive plant diversity effect on soil multifunctionality decreased with soil depths, resulting in a weaker biodiversitymultifunctionality relationship in the subsoil. The weak biodiversitymultifunctionality relationship in the subsoil was caused by weakening the relationship between aboveground biomass and soil multifunctionality; nitrogen addition weakened the biodiversitymultifunctionality relationship through the attenuation of the association of aboveground biomass and soil microbes with multifunctionality. Our study demonstrates that nitrogen addition undermines the biodiversitysoil multifunctionality relationship and the relationship is weakened along the soil profiles, suggesting that Earth system models must represent these heterogeneous soil dynamics to accurately predict future feedbacks to global changes.

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.

Nitrogen Addition Affects Nitrous Oxide Emissions of Rainfed Lucerne Grassland.

Nitrous oxide (N2O) is a potent greenhouse gas. Assessing the N2O emission from lucerne grasslands with nitrogen addition will aid in estimating the annual N2O emissions of such agriculture areas, particularly following summer rainfall events in light of precipitation variation associated with global change. Here, we measured soil N2O emissions, soil temperature and water content of lucerne grasslands with four levels of nitrogen addition over 25 days, which included 10 rainfall events. Results showed that nitrogen addition was observed to increase soil NO3−-N content, but not significantly improve dry matter yield, height or leaf area index. Nitrogen addition and rainfall significantly affected N2O emissions, while the response of N2O emissions to increasing nitrogen input was not linear. Relative soil gas diffusivity (Dp/Do) and water-filled pore space (WFPS) were good indicators of N2O diurnal dynamics, and Dp/Do was able to explain slightly more of the variation in N2O emissions than WFPS. Collectively, nitrogen addition did not affect lucerne dry matter yield in a short term, while it induced soil N2O emissions when rainfall events alter soil water content, and Dp/Do could be a better proxy for predicting N2O emissions in rainfed lucerne grasslands.

Leaf traits mediate herbivory across a nitrogen gradient differently in extirpated vs. extant prairie species.

Increasing nitrogen deposition threatens many grassland species with local extinction. In addition to the direct effects of nitrogen deposition, nitrogen can indirectly affect plant populations via phenotypic shifts in plant traits that influence plant susceptibility to herbivory. Here, I test how herbivory varies across an experimental nitrogen gradient and whether differences in susceptibility to herbivory might explain patterns of local species loss. Specifically, I examine how increasing nitrogen availability in a restored prairie influences leaf traits and subsequent herbivory (by leaf-chewers like insects/small mammals versus deer) and the severity of herbivore damage on confamiliar pairs of extirpated versus extant species from Michigan prairies. Nitrogen increased herbivory by both leaf-chewers and deer as well as herbivore damage (proportion of leaves damaged). Leaf hairiness and specific leaf area affected patterns of herbivory following nitrogen addition, although patterns varied between extirpated vs. extant taxa and herbivory type. Nitrogen increased leaf hairiness. At high levels of nitrogen addition, hairy extant plants experienced less herbivory and damage than smooth-leaved plants. In contrast, hairy extirpated plants were more likely to experience leaf-chewer herbivory. Extirpated plants with thin leaves (high specific leaf area) were less likely to experience leaf-chewer herbivory; the opposite was true for extant species. Generally, extant species experienced more herbivory than locally extirpated species, particularly at high levels of nitrogen addition, suggesting that increasing herbivory under nutrient addition likely does not influence extirpation in this system. This study suggests that trait-mediated responses to nitrogen addition and herbivory differ between extant and extirpated species.

长白山苔原带凋落物生态化学计量特征及其对模拟氮沉降的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 长白山苔原带凋落物生态化学计量特征及其对模拟氮沉降的响应 DOI: 10.5846/stxb202110212976 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（41571078） Response of chemical composition and ecological stoichiometric characteristics of three types of litter to simulated nitrogen deposition in the Changbai Mountain tundra Author: Affiliation: Fund Project: The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan) 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:长白山苔原是我国乃至欧亚大陆东部独有的高山苔原，根据前人调查植被以灌木苔原为主要类型。在全球变暖背景下，近30年来，长白山岳桦林下的草本植物侵入苔原带，原生灌木苔原分化为灌木苔原、灌草苔原和草本苔原，形成了灌木、灌草混合和草本3种不同类型的凋落物，凋落物数量和质量发生显著改变。与此同时长白山苔原氮沉降量也在逐年增加，导致了土壤中氮的累积，势必影响凋落物的分解。凋落物作为连接植物和土壤的纽带，其分解过程中碳（C）、氮（N）、磷（P）等化学组分和化学计量比的变化直接和间接影响着土壤养分有效性和植物养分利用策略。为揭示氮沉降增加对长白山苔原带不同类型凋落物化学组分及生态化学计量特征早期变化的影响，开展了为期8个月的模拟氮沉降室内凋落物分解实验。在苔原带采集灌木优势种牛皮杜鹃和草本优势种小叶章的凋落物带回实验室，模拟灌木牛皮杜鹃群落、灌草混合的牛皮杜鹃-小叶章群落和草本小叶章群落的3种不同类型凋落物，设置三个施氮处理：对照（CK，0 g N m-2 a-1）、低氮（LN，10 g N m-2 a-1）、高氮（HN，20 g N m-2 a-1）。研究表明：（1）不施氮处理时，3种凋落物的C、P均呈释放状态，木质素（Li）呈先累积再略有降解趋势；牛皮杜鹃凋落物的N元素富集而其余两种凋落物N元素呈释放状态；灌草混合和草本凋落物比原生的灌木凋落物C和N元素释放快、Li累积少；而灌木凋落物的P释放略快于灌草和草本凋落物。3种植被类型凋落物的C/N、C/P、Li/N大小表现为：牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物>小叶章凋落物；N/P表现为：小叶章凋落物>牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物。（2）氮沉降促进3种类型凋落物分解过程中C、N和P化学组分的释放，且氮浓度越高促进作用越显著。在牛皮杜鹃凋落物分解过程中，氮素添加到达某一阈值后，其C/N、C/P、N/P、Li/N的降幅最大，后续若再增加氮素，其对化学计量比的影响均会减弱；本实验中的氮素添加量增加促进了小叶章凋落物的C/N、Li/N下降。（3）草本植物入侵引起凋落物类型的变化带来凋落物分解加快，将导致长白山苔原带养分循环的变化；氮沉降增加对小叶章凋落物化学组分的释放及C/N、Li/N的下降更为促进，小叶章凋落物内难分解化合物减少，分解受到促进。高氮沉降加快了小叶章凋落物与土壤、草本植物之间的养分循环。因此，随着未来苔原带氮沉降量的增加，将更有利于小叶章在与牛皮杜鹃的竞争中获胜，使苔原带呈现草甸化趋势。 Abstract:The Changbai Mountain tundra is the only typical alpine tundra of eastern China and eastern Eurasia. In the recent 30 years, herbaceous plants have invaded from low altitude to the alpine tundra, which transformed shrub tundra to shrub-herb-mixed tundra and herb tundra, causing significant changes to the litter components. To understand the response of chemical composition and ecological stoichiometric characteristics of three types of litter to simulated nitrogen deposition in the Changbai Mountain tundra, an artificial simulated litter decomposition experiment was designed and conducted. Rhododendron aureum (native dominant shrub species) litter, Deyeuxia angustifolia (invasion herb species) litter and Rhododendron aureum-Deyeuxia angustifolia-mixed litter collected from the Changbai alpine tundra were used to set up nitrogen addition experiment at three levels under laboratory condition:control group (CK:without nitrogen addition), low N level group (LN:10 g N m-2 a-1)), and high N level group (HN:20 g N m-2 a-1)). Then we collected litter after decomposition and measured the content of C, N, P and lignin (Li), aiming to study the differences of chemical composition and ecological stoichiometric characteristics of the litter with different components and nitrogen addition levels. The results showed that:(1) Without nitrogen addition, the content of C and P in the litter reduced significantly while the content of Li increased at the beginning and then reduced; the content of N in the R. aureum litter increased significantly while the content of N in the R. aureum litter and the mixed litter reduced. C/N, C/P, Li/N of the litter of three types decreased by R. aureum>R. aureum-D. angustifolia-mixed>D. angustifolia; N/P of the litter of three types decreased by D. angustifolia>R. aureum> R. aureum-D. angustifolia-mixed. (2) The simulated nitrogen deposition promoted the release of nutrients during decomposition process and the promoting effects of high nitrogen addition turned out to be more significant. C/N, C/P, N/P, Li/N of the R. aureum litter decreased rapidly at a certain nitrogen addition threshold then the effects on ecological stoichiometric characteristics would be weakened if more nitrogen was added. In our study, high nitrogen addition promoted the decline of C/N and Li/N of the D. angustifolia litter. (3) Change of litter components caused by herbaceous species invasion would promote the decomposition of litter and affect the nutrient cycle mechanisms of Changbai Mountain alpine tundra. As nitrogen deposition increased, the contents of C, N, P, Li and C/N, Li/N of the D. angustifolia litter decreased further and refractory compounds decreased. Nitrogen deposition accelerated nutrients cycle between the D. angustifolia litter, soils and herbaceous plants. Thus, we suggest that the increase of nitrogen deposition will be more favorable to the win of D. angustifolia in the competition with R. aureum, which will impulse meadowization trend in the Changbai Mountain alpine tundra. 参考文献 相似文献 引证文献

Species-level identity of Pisolithus influences soil phosphorus availability for host plants and is moderated by nitrogen status, but not CO2

Trees are dependent on the activity of soil microorganisms, including mutualistic ectomycorrhizal (ECM) fungi and soil-dwelling bacteria, for access to phosphorus (P). While P is a key limiting nutrient in temperate and other forest ecosystems, our understanding of the contributions of ECM fungi to plant P nutrition and cycling are unclear. Further complicating our understanding of these processes are the combined effects of fungal species and future climate scenarios and soil nutrient availability.In this study we characterised how the ECM fungi Pisolithus albus and Pisolithus microcarpus influenced the amount of plant-available P in soils and plant P content of Eucalyptus grandis. We explored how these fungi influence P cycling by studying their relative P-solubilising and P-mineralising abilities and examining their impact on P cycling gene abundance in the soil bacterial community. These were investigated under different levels of nitrogen addition and atmospheric CO2 to understand how these processes may be impacted by future anthropogenic and climactic change. While inoculation with either P. albus or P. microcarpus resulted in an increase in plant-available P in soil, the amount of plant-available P mobilised was species-specific. This observation was supported by differences in the in vitro P-mobilising abilities of the two fungi. P. albus and P. microcarpus also favoured bacterial communities characterised by a greater abundance of glucose dehydrogenase gene copies for inorganic P solubilisation, complementing the strengths of the ECM fungi in organic P mineralisation and suggesting a distinction in the roles of ECM fungi and bacteria in P cycling. Furthermore, both nitrogen and CO2 levels impacted these P cycling outcomes, often in a species-specific manner. Our findings expand the current understanding of P cycling between forest trees, ECM fungi and soil bacteria, and have important implications for estimations of future anthropogenic impacts on forest ecosystems.

Trait–Abundance Relationships of Annual Ephemerals in Response to Nitrogen Addition in Gurbantunggut Desert

Understanding the effect of nitrogen addition on species trait–abundance relationships is one of the central focuses of community ecology and can offer us insights into the mechanisms of community assembly under atmospheric nitrogen deposition. However, few studies have focused on desert ecosystems. In this study, we measured the abundance and ecological stoichiometric traits, leaf carbon content (LCC), nitrogen content (LNC), and phosphorus content (LPC) for all annual ephemerals in all plots subjected to nitrogen addition in early spring in Gurbantunggut Desert, northern Xinjiang, China. We found a significant relationship between traits (LNC, N:P, and C:N) and abundance, indicating that ecological stoichiometry is a good proxy for explaining and predicting species abundance. We further found that significant trait–abundance relationships still existed under different nitrogen addition levels. The result suggests that trait-based niche-assembly theory plays an important role in determining species abundance under atmospheric nitrogen deposition.

Effects of Soil Nitrogen Addition on Crown CO2 Exchange of Fraxinus mandshurica Rupr. Saplings

The impact of atmospheric nitrogen deposition on carbon exchange between forest and atmosphere is one of the research hotspots of global change ecology, past researchers have extensively studied the impacts on leaf level, while the impacts on crown CO2 exchange are still unclear. Therefore, we explored the impacts of different nitrogen addition levels on crown CO2 exchange of Fraxinus mandshurica saplings and their responses to the changes of major meteorological factors (photosynthetically active radiation, PAR; vapor pressure deficiency, VPD; and air temperature, Tair) with a novel automated chamber system. There are four levels of nitrogen addition treatments: control (no nitrogen addition, CK), 23 (low nitrogen addition, LN), 46 (medium nitrogen addition, MN), and 69 kgN·hm−2·a−1 (high nitrogen addition, HN). Our results showed that all nitrogen addition treatments increased daily average and accumulated gross primary production (GPP), crown respiration (R), and net crown CO2 exchange (Ne), especially at medium and high nitrogen levels. Similarly, maximum net photosynthetic rate (Nemax) and apparent quantum efficiency (α) were promoted. The change of Ne with PAR, Tair, and VPD showed that nitrogen addition postponed the appearance of photosynthesis midday depression. In addition, the monthly accumulation of R with all nitrogen addition treatments showed an increasing trend (June to July), and then decreased (July to September) during the growing season, while the Ne and GPP decreased gradually with seasonal vegetation senescence. Finally, the crown shifted from carbon sink to carbon source at the end of the growing season, however, the change under high nitrogen treatment occurred 3 days later. The crown CO2 exchange measurements provide a new perspective to better understand the response of forest ecosystem CO2 exchange to elevated nitrogen deposition and provide a basis for related carbon model parameter correction under the influence of nitrogen deposition.

Effects of excessive nitrogen on nitrogen uptake and transformation in the wetland soils of Liaohe estuary, northeast China

It remains unclear whether excessive nitrogen additions lead to the degradation of Suaeda salsa (S. salsa) by affecting the nitrogen pool, enzyme activities, and bacterial community structure of wetland soils. This study investigated the effect of five added nitrogen concentrations (0, 1, 2, 4, and 6 mmol L−1 N with NH4NO3 = group C, group L, group M, group H, and group G, respectively) on nitrogen uptake by S. salsa and nitrogen transformation in the wetland soils of the Liaohe estuary. The height, weight, and total nitrogen (TN) of S. salsa in group G was significantly lower than in the other groups (p <0.05). The NH4+-N concentration in the soil tended to increase with increasing nitrogen addition, but the TN concentration in the soil tended to decrease. The nitrogenase, protease, urease, ammonia monooxygenase (AMO), nitrous oxide reductase (NOR), and dehydrogenase (DHA) activities increased with increasing nitrogen addition within the range of 0 to 4 mmol L−1. We identified 30 phyla and 48 known genera across all five groups. The predominant phyla were Proteobacteria (52.68%), Bacteroidetes (22.58%), and Planctomycetes (3.94%). The most abundant genus was Acinetobacter (13.38%), followed by Proteiniphilum (11.88%) and Brevundimonas (6.03%). The total number of soil bacterial species increased with increasing nitrogen addition. Group G had lower soil bacterial activity and diversity than the other groups. It was concluded that appropriate levels of nitrogen addition could promote nitrogen uptake by S. salsa and nitrogen transformation in the wetland soils of the Liaohe estuary by affecting soil enzyme activities and soil bacterial activity, diversity, abundance, and composition, while excessive nitrogen additions may be one of the reasons for the degradation of S. salsa.

Investigating the effects of nitrogen deposition and substrates on the microbiome and mycobiome of the millipede Cherokia georgiana georgiana (Diplopoda: Polydesmida)

Millipedes are ubiquitous soil invertebrates that play major roles in physical and chemical processes in soil. Despite their importance to soil ecology and their presumed interactions with soil microorganisms, little is known about how millipedes influence and are influenced by soil microbes. Furthermore, it is not fully understood how these millipede-microbe interactions are influenced by available soil nitrogen, which is predicted to increase over the foreseeable future with increased anthropogenic production. Here, using the millipede Cherokia georgiana georgiana as a model and using a manipulative mesocosm and metabarcoding approach, we examine (1) the impacts of millipedes on soil microbial communities (fungi and bacteria) with varying nitrogen addition levels, and (2) the temporal impacts of nitrogen on millipede fecal communities (fungi and bacteria). This research demonstrates that millipede presence strongly alters soil communities and that alterations in nitrogen levels do not impact millipede gut communities. This work also provides the first evidence suggesting that millipede gut communities are predominantly derived from soils rather than leaf litter, though both contribute to gut composition. This work advances our current poor understanding of millipede-soil interactions and provides a framework for further investigations to disentangle the interactive effects of substrate, nitrogen, and time to better understand ecological impacts of these interactions.

Effects of low temperature and nitrogen addition during cold stratification on seed germination of Korean pine (Pinus koraiensis)

Pinus koraiensis Siebold &amp; Zucc. seeds are primarily dormant following dispersal, restricting the natural regeneration of populations. The aim of this study was to determine the combined effects of increasing low temperature and nitrogen addition during cold stratification on seed germination. We stratified the seeds at 0 °C, 5 °C, and 10 °C for 3.5 months. At each cold stratification temperature, seeds were subjected to 0 (0 nitrogen), 0.08 (low nitrogen), 0.16 (medium nitrogen), and 0.25 mol·L–1 (high nitrogen) nitrate additions. Seed germination response was tested at 25/15 °C and 10/5 °C. Under 0 nitrogen and high nitrogen levels, increasing cold stratification temperature did not markedly affect seed germination at 25/15 °C, but significantly inhibited seed germination at 10/5 °C. At medium nitrogen levels, seed germination at both 25/15 °C and 10/5 °C significantly decreased. At low nitrogen levels, increasing cold stratification temperature significantly decreased germination percentage at 25/15 °C from 86.7% (0 °C) to 50.2% (10 °C) but did not markedly change seed germination percentage at 10/5 °C. We suggest that the effect of increasing cold stratification temperature on seed germination of P. koraiensis depends on both the level of nitrogen addition during cold stratification and subsequent incubation temperature after cold stratification.

Soil Nitrogen Transformations Respond Diversely to Multiple Levels of Nitrogen Addition in a Tibetan Alpine Steppe

AbstractElevated reactive nitrogen (N) input could modify soil N transformations, regulating ecosystem functions such as soil N retention and loss. Although multiple hypotheses advocate nonlinear variations in soil N transformations with continuous N input, there still lacks empirical evidences for the responses of soil N transformations to multiple N additions. Here, based on a manipulative N addition experiment and a 15N pool dilution approach, we explored changes in soil gross N transformations with eight N addition levels and associated mechanisms in a Tibetan alpine steppe. Our results showed that soil gross N mineralization rate (GNM) increased first and then stabilized with increasing N additions. Meanwhile, soil microbial immobilization rate (MIM) exhibited an initially increased and subsequently declined pattern under various N addition levels. In contrast, soil gross nitrification rate (GN) increased linearly across multiple N addition levels. Our results also revealed that variations in GNM were mainly regulated by aboveground vegetation N pool‐induced changes in dissolved organic N content along the N addition gradient. Meanwhile, changes in GN were dominantly modified by soil pH‐induced variations in ammonia‐oxidizing archaea abundance across multiple N addition levels. Additionally, alterations in MIM under various N input levels were primarily controlled by microbial biomass which was regulated by dissolved organic carbon content under low N input and NH4+‐N content at high N level, respectively. Overall, patterns and drivers of soil N transformations observed in this study provide valuable benchmark for Earth system models to better predict ecosystem N dynamics under global N‐enrichment scenarios.

Nitrogen-enhanced herbaceous competition threatens woody species persistence in a desert ecosystem

Water and nitrogen availability are primary determinants of woody communities in dryland ecosystems and are strongly affected by global change. However, the mechanisms by which resource availability and plant demographic responses regulate woody-herb composition are unclear. We investigated plant productivity, shrub demography, and resource availability in a 5-year field experiment involving three levels of precipitation (ambient, + 20% precipitation, + 40% precipitation) and two levels of nitrogen addition (0, 60 kg N ha− 1 yr− 1) in a desert shrubland of northern China. Increased precipitation had similar positive effects on shrub and herbaceous productivity and, thus, did not alter the community composition. However, nitrogen enrichment dramatically increased herbaceous productivity but tended to decrease shrub productivity. Five years after the experimental manipulation, increased precipitation and nitrogen enrichment significantly altered shrub demography. Increased precipitation decreased the number and cover of small shrubs but increased those of large shrubs. Nitrogen enrichment largely decreased the number and cover of small shrubs, whereas large shrubs showed a higher resistance. Structural equation modeling revealed that nitrogen-enhanced herbaceous competition for water and light potentially reduced shrub growth, especially the growth of small shrubs. Our results suggest that large shrubs can coexist with herbs, whereas small shrubs tend to be competitively excluded from the system under enhanced precipitation and nitrogen deposition. Nitrogen enrichment may lead to continued declines in shrub recruitment and growth, and a conversion to grassland. Our findings highlight the competitive and demographic mechanisms underlying the responses of woody communities to global change.

黄土区不同恢复年限草地群落生物量及根冠比对氮添加的响应

大气氮沉降增加作为全球变化的主要环境问题之一，已引发人们的广泛关注，持续的氮沉降对草地生态系统的组成、结构和功能产生重要影响。为深入了解草地恢复进程中群落生物量和根冠比对氮沉降的响应，以黄土区3个不同恢复年限（初期12a、中期28a和后期37a）的天然草地为研究对象，通过设置6个氮添加水平，CK （0）、N1（2.34g m^-2a^-1）、N2（4.67g m^-2a^-1）、N3（9.34g m^-2a^-1）、N4（18.68g m^-2a^-1）、N5（37.35g m^-2a^-1）来测定草地群落地上生物量、地下生物量和总生物量，并计算根冠比和氮响应效率（NRE）。结果表明：（1）地上生物量在恢复中期最大，随氮添加梯度增加，地上生物量在恢复初期和恢复后期呈不显著上升趋势，对氮添加表现为非线性的正响应（ΔNRE>0），在恢复中期呈不显著下降趋势，对氮添加表现为非线性的负响应（ΔNRE<0）。（2）群落地下生物量对氮添加无显著响应，总生物量只有在恢复后期的N4添加水平下，与对照存在显著差异。（3）根冠比在恢复初期时，N3添加水平下显著高于对照和其他氮添加水平，其余恢复年限对氮添加无显著响应。综上所述，通过分析比较黄土区不同恢复年限草地群落的地上、地下及总生物量和根冠比对氮添加的响应。建议对该区域开展试点实验，实行适应性草地管理，如进行两年一次刈割或轻度放牧（2只羊/hm²），来探寻更科学有效的管理措施，使草地实现系统性恢复，进而满足生态系统容量和社会需求的变化。;As one of the main environmental problems of global change, the increase of atmospheric nitrogen deposition has aroused widespread concern. Continuous nitrogen deposition has an important impact on the composition, structure and function of grassland ecosystems. In order to deeply understand the response of community biomass and root-shoot ratio to nitrogen deposition in the process of grassland restoration, this study took 3 natural grasslands with different recovery years (early 12 years, middle 28 years, and late 37 years) on the Loess Plateau as the research object. Six nitrogen addition levels, CK (0), N1(2.34 g m^-2a^-1),N2(4.67g m^-2a^-1),N3(9.34g m^-2a^-1),N4(18.68g m^-2a^-1),N5(37.35g m^-2a^-1)were used to measure the aboveground biomass, underground biomass, and total biomass of grassland communities, and the root-shoot ratio and nitrogen response efficiency (NRE). The results showed that:(1) The aboveground biomass was the largest in the middle stage of restoration. With the increase of nitrogen addition gradient, the aboveground biomass showed an insignificant upward trend in the early and late stages of restoration, a non-linear positive response to nitrogen addition (ΔNRE>0), an insignificant downward trend in the middle stages of restoration, and a non-linear negative response to nitrogen addition (ΔNRE<0). (2) The underground biomass of the community had no significant response to nitrogen addition, and the total biomass was significantly different from that of the control only at the N4 addition level in the later stage of restoration. (3) At the early stage of restoration, the root-shoot ratio at the N3 addition level was significantly higher than that of the control and other nitrogen addition levels, while the other restoration years had no significant response to nitrogen addition. In conclusion, the responses of aboveground, underground, total biomass and root-shoot ratio of grassland communities with different restoration years on the Loess Plateau to nitrogen addition were analyzed and compared. We suggest that pilot experiments should be carried out in this region to implement adaptive grassland management, such as biennial cutting or light grazing (2 sheep/hm²), to explore more scientific and effective management measures to achieve systematic restoration of grassland, so as to meet the changes of ecosystem capacity and social needs.

Effects of Nitrogen Deposition on Nitrogen-Mineralizing Enzyme Activity and Soil Microbial Community Structure in a Korean Pine Plantation.

To predict the effects of nitrogen deposition on nitrogen-mineralizing enzyme activity and soil microbial community structure in artificial temperate forests in northern China, we studied the soil properties, nitrogen-mineralizing enzyme activity, and microbial community structure in the soil of a Korean pine plantation in which different concentrations (0, 20, 40, 80 kg N ha-1 year-1) of ammonium nitrate were applied for 5 consecutive years. The results showed that nitrogen addition at different concentrations did not significantly affect the soil pH. High nitrogen addition (80 kg N ha-1 year-1) significantly increased the soil organic matter, ammonium nitrogen, and nitrate nitrogen content in the Korean pine plantation, and ammonium nitrogen was the key factor that influenced the soil fungal community structure. The urease activity under the moderate nitrogen addition treatment (40 kg N ha-1 year-1) was significantly lower than that under the control (0 kg N ha-1 year-1), and the protease activity in the three treatments was also significantly lower than that in the control. There was no significant correlation between microbial community structure and the four mineralizing enzymes. After nitrogen addition at different concentrations, the Simpson and Shannon indexes of soil bacteria decreased significantly under low nitrogen addition (20 kg N ha-1 year-1), but the α-diversity index of soil fungi did not show significant differences under nitrogen addition. The microbial community composition was significantly changed by the different treatments. PLS-DA analysis showed that Tardiphaga was an important genus that made the greatest contribution to the differences in bacterial community composition among treatments, as was Taeniolella for fungal community composition. The low level of nitrogen addition inhibited nitrogen mineralization in the Korean pine plantation by reducing the relative abundances of Nitrosomonadaceae and Betaproteobacteriales and by reducing the abundances of symbiotrophic fungi. Berkelbacteria and Polyporales were bacteria and fungi, respectively, that changed significantly under the high nitrogen addition treatment (80 kg N ha-1 year-1). This study provides more data to support predictions of the changes in nitrogen-mineralizing enzyme activity and microbial community structure in artificial temperate forest soils in response to increased nitrogen deposition.

Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China

Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China

Resource-use efficiency drives overyielding via enhanced complementarity.

Overyielding, the primary metric for assessing biodiversity effects on ecosystem functions, is often partitioned into "complementarity" and "selection" components, but this reveals nothing about the role of increased resource use, resource-use efficiency, or trait plasticity. We obtained multiple overyielding values by comparing productivity in a five-species mixture to expected values from its component monocultures ata)six levels of nitrogen addition (spanning 0-500kgNha-1year-1) andb)across four seasons. We also measured light, water, and nitrogen use, resource-use efficiency, and three functional traits-leaf nitrogen content, specific leaf area, and leaf area ratio-n mixtures and monocultures. We found strong evidence for non-transgressive overyielding. This was strongest in spring, with mixture productivity exceeding expected values by 20kg dry matter ha-1day-1. Peak overyielding was driven by enhanced complementarity, with the two non-N2-fixing forb species far exceeding expected productivity in mixtures. Peak overyielding also coincided with higher water use in the mixture than for any monoculture, and enhanced mixture-resource-use efficiency. There was only weak evidence that trait plasticity influenced overyielding or resource use. Our findings suggest that when complementarity drives overyielding in grassland mixtures, and this is made possible both by increased water use and enhanced efficiency in water, nitrogen, and light use.Our results also suggest that mixtures offer a viable compromise between productivity, resource-use efficiency, and reduced environmental impacts (i.e., nitrate leaching) from intensive agriculture.