Nitrogen (N) deposition is widely considered an environmental problem that leads to biodiversity loss and reduced ecosystem resilience; but, N fertilization has also been used as a management tool for enhancing primary production and ground cover, thereby promoting the restoration of degraded lands. However, empirical evaluation of these contrasting impacts is lacking. We tested the dual effects of N enrichment on biodiversity and ecosystem functioning at different organizational levels (i.e., plant species, functional groups, and community) by adding N at 0, 1.75, 5.25, 10.5, 17.5, and 28.0 g N m612 yr611 for four years in two contrasting field sites in Inner Mongolia: an undisturbed mature grassland and a nearby degraded grassland of the same type. N addition had both quantitatively and qualitatively different effects on the two communities. In the mature community, N addition led to a large reduction in species richness, accompanied by increased dominance of early successional annuals and loss of perennial grasses and forbs at all N input rates. In the degraded community, however, N addition increased the productivity and dominance of perennial rhizomatous grasses, with only a slight reduction in species richness and no significant change in annual abundance. The mature grassland was much more sensitive to N-induced changes in community structure, likely as a result of higher soil moisture accentuating limitation by N alone. Our findings suggest that the critical threshold for N-induced species loss to mature Eurasian grasslands is below 1.75 g N m612 yr611, and that changes in aboveground biomass, species richness, and plant functional group composition to both mature and degraded ecosystems saturate at N addition rates of approximately 10.5 g N m612 yr611. This work highlights the tradeoffs that exist in assessing the total impact of N deposition on ecosystem function.

Understanding how the aboveground net primary production (ANPP) of arid and semiarid ecosystems of the world responds to variations in precipitation is crucial for assessing the impacts of climate change on terrestrial ecosystems. Rain-use efficiency (RUE) is an important measure for acquiring this understanding. However, little is known about the response pattern of RUE for the largest contiguous natural grassland region of the world, the Eurasian Steppe. Here we investigated the spatial and temporal patterns of ANPP and RUE and their key driving factors based on a long-term data set from 21 natural arid and semiarid ecosystem sites across the Inner Mongolia steppe region in northern China. Our results showed that, with increasing mean annual precipitation (MAP), (1) ANPP increased while the interannual variability of ANPP declined, (2) plant species richness increased and the relative abundance of key functional groups shifted predictably, and (3) RUE increased in space across different ecosystems but decreased with increasing annual precipitation within a given ecosystem. These results clearly indicate that the patterns of both ANPP and RUE are scale dependent, and the seemingly conflicting patterns of RUE in space vs. time suggest distinctive underlying mechanisms, involving interactions among precipitation, soil N, and biotic factors. Also, while our results supported the existence of a common maximum RUE, they also indicated that its value could be substantially increased by altering resource availability, such as adding nitrogen. Our findings have important implications for understanding and predicting ecological impacts of global climate change and for management practices in arid and semiarid ecosystems in the Inner Mongolia steppe region and beyond.

Abstract Niche dimensionality provides a general theoretical explanation for biodiversity-more niches, defined by more limiting factors, allow for more ways that species can coexist. Because plant species compete for the same set of limiting resources, theory predicts that addition of a limiting resource eliminates potential trade-offs, reducing the number of species that can coexist. Multiple nutrient limitation of plant production is common and therefore fertilization may reduce diversity by reducing the number or dimensionality of belowground limiting factors. At the same time, nutrient addition, by increasing biomass, should ultimately shift competition from belowground nutrients towards a one-dimensional competitive trade-off for light. Here we show that plant species diversity decreased when a greater number of limiting nutrients were added across 45 grassland sites from a multi-continent experimental network. The number of added nutrients predicted diversity loss, even after controlling for effects of plant biomass, and even where biomass production was not nutrient-limited. We found that elevated resource supply reduced niche dimensionality and diversity and increased both productivity and compositional turnover. Our results point to the importance of understanding dimensionality in ecological systems that are undergoing diversity loss in response to multiple global change factors.

Understanding the response of the plant community to increasing nitrogen (N) deposition is helpful for improving pasture management in semi-arid areas. We implemented a 5-year N addition experiment in aStipa kryloviisteppe of Inner Mongolia, northern China. The aboveground biomass (AGB) and species richness were measured annually. Along with the N addition levels, the species richness declined significantly, and the species composition changed noticeably. However, the total AGB did not exhibit a noticeable increase. We found that compensatory effects of the AGB occurred not only between the grasses and the forbs but also among Gramineae species. The plant responses to N addition, from the community to species level, lessened in dry years compared to wet or normal years. The N addition intensified the reduction of community productivity in dry years. Our study indicated that the compensatory effects of the AGB among the species sustained the stability of grassland productivity. However, biodiversity loss resulting from increasing N deposition might lead the semi-arid grassland ecosystem to be unsustainable, especially in dry years.

以内蒙古多伦县恢复生态学试验示范研究站弃耕10余年的草地为研究对象,于2006年起分别设置对照、氮素添加、刈割和氮素添加+刈割4种处理,每种处理6次重复,研究弃耕草地氮素添加和刈割对土壤氮矿化的影响,结合土壤理化性质和植被地上生产力的动态变化,分析弃耕草地土壤氮矿化对植被恢复的响应,为当地草地恢复与重建提供理论依据和数据支持。实验结果表明:1氮素添加显著增加了植物地上净初级生产力(ANPP)和土壤无机氮库,与对照相比分别提高115%和196%,同时显著提高了土壤总硝化速率;但是氮素添加对总氨化速率、土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳氮比(MBC/MBN)、微生物呼吸(MR)以及呼吸熵(q CO2)均无显著影响;2总氨化速率和硝化速率对刈割处理的响应均不显著,但是刈割处理显著降低了土壤MR(P0.05);3氮素添加+刈割处理5—7a后,土壤总氨化和硝化速率均无显著变化;但是氮素添加+刈割处理显著增加了ANPP、土壤无机氮库和q CO2,同时显著降低了MBC和MBC/MBN。这说明在弃耕草地适应性管理中,氮素添加可以显著提高草地生产力,但是长期的氮添加对土壤微生物氮的转化是否有利还值得我们进一步研究。

以内蒙古多伦县恢复生态学试验示范研究站弃耕10余年的草地为研究对象,于2006年起分别设置对照、氮素添加、刈割和氮素添加+刈割4种处理,每种处理6次重复,研究弃耕草地氮素添加和刈割对土壤氮矿化的影响,结合土壤理化性质和植被地上生产力的动态变化,分析弃耕草地土壤氮矿化对植被恢复的响应,为当地草地恢复与重建提供理论依据和数据支持。实验结果表明:1氮素添加显著增加了植物地上净初级生产力(ANPP)和土壤无机氮库,与对照相比分别提高115%和196%,同时显著提高了土壤总硝化速率;但是氮素添加对总氨化速率、土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳氮比(MBC/MBN)、微生物呼吸(MR)以及呼吸熵(q CO2)均无显著影响;2总氨化速率和硝化速率对刈割处理的响应均不显著,但是刈割处理显著降低了土壤MR(P0.05);3氮素添加+刈割处理5—7a后,土壤总氨化和硝化速率均无显著变化;但是氮素添加+刈割处理显著增加了ANPP、土壤无机氮库和q CO2,同时显著降低了MBC和MBC/MBN。这说明在弃耕草地适应性管理中,氮素添加可以显著提高草地生产力,但是长期的氮添加对土壤微生物氮的转化是否有利还值得我们进一步研究。

Topsoil characteristics and their determinants in the steppe on the central Inner Mongolian Plateau were investigated. Percentages of different grain size fractions of topsoil samples from 236 plots and corresponding standardized data of four environmental/human-activity factors, namely mean annual temperature (MAT), mean annual precipitation (MAP), human disturbance index (HDI) and land use/land cover (LULC) types were used to perform canonical correspondence analysis (CCA). The roles of both human land uses and climatic factors on topsoil grain size distribution are highlighted. Climatic factors seem to have played a significant role in determining the coarse sand (0.2-1 mm in size) percentages in the steppe and meadow by affecting the vegetation cover, while human disturbance indicated by species composition appears to have significantly influenced the percentages of clay (<0.002 mm in size) and fine-medium silt (0.002-0.016 mm in size) in the typical steppe (P < 0.01). Soil Organic Carbon (SOC) content is negatively correlated with coarse sand percentages (P < 0.01), implying that reduction of soil fertility is linked to soil coarsening. Percentages of fine-medium silts (0.002-0.016 mm in size) are still more abundant in the area where artificial vegetation are planted and remains as the major source of dust storms. This study implies that afforestation in the steppe region has not improved soil conditions and thus reduced dust storms as earlier expected.

Abstract Increased atmospheric nitrogen (N) deposition and altered precipitation regimes have profound impacts on ecosystem functioning in semiarid grasslands. The interactions between those two factors remain largely unknown. A field experiment with N and water additions was conducted in a semiarid grassland in northern China. We examined the responses of aboveground net primary production (ANPP) and plant N use during two contrasting hydrological growing seasons. Nitrogen addition had no impact on ANPP, which may be accounted for by the offset between enhanced plant N uptake and decreased plant nitrogen use efficiency (NUE). Water addition significantly enhanced ANPP, which was largely due to enhanced plant aboveground N uptake. Nitrogen and water additions significantly interacted to affect ANPP, plant N uptake and N concentrations at the community level. Our observations highlight the important role of plant N uptake and use in mediating the effects of N and water addition on ANPP.

1. Grassland on lowland peat has considerable potential for agriculture and conservation. 2. The limited use of fertilizer on the Somerset Moors, designated as an Environmentally Sensitive Area has enabled the impact on vegetation of applying a range of nitrogen levels to be investigated using a randomized block experiment with large plots at Tadham Moor. 3. From 1986 to 1990, the plot measurements each year included the individual percentage cover of some 100 species; these enabled the derivation of a number of measures of plant community structure. 4. The applications of fertilizer encouraged grasses (notably Lolium perenne and Holcus lanatus) which came to dominate the sward at the expense of most other species. All measures of species diversity declined significantly and the sward became taller in plots receiving high levels of nitrogen. Only a very few forbs (e.g. Rumex acetosa) maintained or increased their cover with increased nitrogen. 5. All Carex species, Juncus species and mosses became significantly less common in plots treated with nitrogen, producing an apparently more mesotrophic sward. Short-lived species and low-growing wetland forbs also declined with nitrogen, perhaps due to shading by tall grasses. Legumes were suppressed by high levels of nitrogen, but were common in plots receiving 25 kg ha-1year-1N, where replacement levels of phosphorus and potassium were applied after cutting. 6. Except for grasses, many species common in the experimental site in 1986 declined in cover, particularly in plots receiving high levels of nitrogen. However, the data showed no instances of extinction by 1990. The application of 25 kg ha-1N encouraged the spread of agriculturally productive grasses within 2 years and 50 kg ha-1year-1N significantly reduced species richness in 3 years. 7. Certain detailed findings from this large-scale study of nitrogen application in meadows enable contrasts with previous results to be identified.

Abstract Increasing atmospheric nitrogen (N) deposition may affect plant biodiversity, subsequently altering ecosystem stability. While a few studies have explored how simulated N deposition affects community stability and its underlying mechanisms, the experimental levels of N addition used are usually higher than current and future N deposition rates. Thus, their results could produce highly uncertain predictions of ecosystem function, especially if the responses to N deposition are nonlinear. We conducted a manipulative experiment that simulated elevated atmospheric N deposition with several N addition levels to evaluate the effect of N deposition on ecosystem stability and its underlying mechanisms in a semiarid grassland in northern China. Here we show that N addition altered community diversity, reducing species richness, evenness, diversity and dominance. In addition, we found that N addition at current N deposition levels had no significant impact on community stability. In contrast, N addition at levels from 4.6 to 13.8gNm -2 yr -1 significantly decreased community stability, although community stability for the 13.8gNm -2 yr -1 treatment was higher than that for the 4.6gNm -2 yr -1 treatment. These results indicate that the response of community stability to N enrichment is nonlinear. This nonlinear change in community stability was positively correlated with species asynchrony, species richness, and species diversity as well as the stability of dominant species and the stability of the grass functional group. Our data suggest a need to re-evaluate the mechanisms responsible for the effects of N deposition on natural ecosystem stability across multiple levels of N enrichment and that additional experimentation with gradients of N loads more similar to future atmospheric N deposition rates is needed. Copyright 2017 Elsevier B.V. All rights reserved.

A rapid increase in grazing intensity since the 1980s has caused large areas of the Inner Mongolian grasslands to become degraded. Increasing atmospheric nitrogen (N) deposition might exert an important influence on vegetation restoration in these degraded grasslands by increasing available N and relieving N limitations on productivity. However, no previous studies have tested the assumption that increasing N deposition promotes vegetation restoration in degraded grasslands. By conducting a 4-year field restoration experiment with four N addition treatments (0, 5, 10, and 20gNm/2 year/1) and two grazing treatments (grazed and ungrazed), we investigated the effects of N enrichment and grazing on the restoration of patches in which vegetation had been degraded. N addition significantly accelerated the restoration of vegetation-degraded grassland patches regarding both plant cover and diversity. Moderate grazing also promoted the restoration of degraded-vegetation patches in term of both plant diversity and species similarity. Importantly, the positive effects of N addition on the restoration of degraded patches may be augmented by grazing. This study demonstrate that low levels of N enrichment (or increasing atmospheric N deposition) positively impact vegetation restoration in degraded grassland patches, particularly under moderate grazing practices. Our findings provide new insights into the management of severely degraded grasslands through the regulation of N inputs and grazing practices.

1 Plant community theory often invokes competition to explain why species diversity declines as productivity increases. Competition for all resources might become more intense and lead to greater competitive exclusion or, alternatively, competition for light only could become more intense and exclude poor light competitors. 2 To test these hypotheses, we constructed communities of seven old-field species using combined monocultures. Constructs experienced no interspecific competition, only shoot competition or only root competition, with and without fertilizer. Diversity in these limited interaction communities was compared to diversity in unfertilized and fertilized mixtures of the seven species. 3 Diversity decreased with fertilization in mixtures and in communities with only root competition. Shoot competition had small effects on the community and did not contribute to changes in diversity with fertilization. 4 Root competition may strongly impact plant community structure in unproductive communities where light never becomes limiting, or under non-equilibrium conditions following human disturbances.

[Aims] Theories based on resource additions indicate that plant species richness is mainly determined by the number of limiting resources. However, the individual effects of various limiting resources on species richness and aboveground net primary productivity (ANPP) are less well understood. Here, we analyzed potential linkages between additions of limiting resources, species loss and ANPP increase and further explored the underlying mechanisms. [Methods] Resources (N, P, K and water) were added in a completely randomized block design to alpine meadow plots in the Qinghai-Tibetan Plateau....

The challenge to identify the biospheric sinks for about half the total carbon emissions from fossil fuels must include a consideration of below-ground ecosystem processes as well as those more easily measured above-ground. Recent studies suggest that tropical grasslands and savannas may contribute more to the 'missing sink' than was previously appreciated, perhaps as much as 0.5 Pg (= 0.5 Gt) carbon per annum. The rapid increase in availability of productivity data facilitated by the Internet will be important for future scaling-up of global change responses, to establish independent lines of evidence about the location and size of carbon sinks.

中国草地面积广阔,自然资源丰富,准确评估草地资源既是合理开发和利用草地的基础,也对生态环境的保护具有重要意义.但是,关于中国草地面积、生产力和承载力等本底数据至今为止尚没有系统的梳理.本文收集、整理了过去几十年来我国草地资源研究的各类数据,并利用1982~2011年的遥感影像（NOAA/AVHRR-NDVI）和气候数据重新估算了我国天然草地生物量和生产力及其近30年的变化.由于草地的定义、数据来源和分析方法不同,现存资料对中国天然草地面积的估算差异很大,变动范围达2倍以上（1.67×10~6~4.31×10~6 km~2）,这些资料也表明,我国目前的天然草地面积在2.80×10~6~3.93×10~6 km~2之间比较合适.草地生物量的估算值也存在显著差异,平均地上生物量在79~123 g m~（-2）之间,但本文对最近30年（1982~2011年）天然草地地上生物量的重新估算结果为178 g m~（-2）,在此期间平均每年增加0.4 gm~（-2）.我国天然草地平均净初级生产力的估算差异更大,为89~320 g Cm~（-2）a~（-1）（平均176 g Cm~（-2） a~（-1））,但许多研究都发现近年来有增加趋势.而基于过去30年平均气候估算,我国天然草地的潜在生产力可高达348 gCm~（-2） a~（-1）.另一方面,我国人工草地面积比较小,约为2.09×10~7 hm~2,但生产力高,可达天然草地的2.7~12.1倍.由于我国对天然草地缺乏有效管理,加上人工草地的比例低,目前我国草地对放牧家畜的承载力比较低,很多地方的超载现象较为严重,平均超载率估计为20%.此外,降水的不足始终是影响我国草地生物多样性、生物量和生产力的重要因素,进一步探讨气候变化和过度放牧等人为活动对我国草地资源的数量和质量的影响是十分必要的.

中国草地面积广阔,自然资源丰富,准确评估草地资源既是合理开发和利用草地的基础,也对生态环境的保护具有重要意义.但是,关于中国草地面积、生产力和承载力等本底数据至今为止尚没有系统的梳理.本文收集、整理了过去几十年来我国草地资源研究的各类数据,并利用1982~2011年的遥感影像（NOAA/AVHRR-NDVI）和气候数据重新估算了我国天然草地生物量和生产力及其近30年的变化.由于草地的定义、数据来源和分析方法不同,现存资料对中国天然草地面积的估算差异很大,变动范围达2倍以上（1.67×10~6~4.31×10~6 km~2）,这些资料也表明,我国目前的天然草地面积在2.80×10~6~3.93×10~6 km~2之间比较合适.草地生物量的估算值也存在显著差异,平均地上生物量在79~123 g m~（-2）之间,但本文对最近30年（1982~2011年）天然草地地上生物量的重新估算结果为178 g m~（-2）,在此期间平均每年增加0.4 gm~（-2）.我国天然草地平均净初级生产力的估算差异更大,为89~320 g Cm~（-2）a~（-1）（平均176 g Cm~（-2） a~（-1））,但许多研究都发现近年来有增加趋势.而基于过去30年平均气候估算,我国天然草地的潜在生产力可高达348 gCm~（-2） a~（-1）.另一方面,我国人工草地面积比较小,约为2.09×10~7 hm~2,但生产力高,可达天然草地的2.7~12.1倍.由于我国对天然草地缺乏有效管理,加上人工草地的比例低,目前我国草地对放牧家畜的承载力比较低,很多地方的超载现象较为严重,平均超载率估计为20%.此外,降水的不足始终是影响我国草地生物多样性、生物量和生产力的重要因素,进一步探讨气候变化和过度放牧等人为活动对我国草地资源的数量和质量的影响是十分必要的.

Chinese grasslands are extensive natural ecosystems that comprise 40 % of the total land area of the country and are sensitive to N deposition. A field experiment with six N rates (0, 30, 60, 120, 240, and 480 kg N halt;supgt;amp;minus;1lt;/supgt; yrlt;supgt;amp;minus;1lt;/supgt;) was conducted at Duolun, Inner Mongolia, during 2005 and 2010 to identify some effects of N addition on a temperate steppe ecosystem. The dominant plant species in the plots were divided into two categories, grasses and forbs, on the basis of species life forms. Enhanced N deposition, even as little as 30 kg N halt;supgt;amp;minus;1lt;/supgt; yrlt;supgt;amp;minus;1lt;/supgt; above ambient N deposition (16 kg N halt;supgt;amp;minus;1lt;/supgt; yrlt;supgt;amp;minus;1lt;/supgt;), led to a decline in species richness. The cover of grasses increased with N addition rate but their species richness showed a weak change across N treatments. Both species richness and cover of forbs declined strongly with increasing N deposition as shown by linear regression analysis (lt;igt;plt;/igt; lt; 0.05). Increasing N deposition elevated aboveground production of grasses but lowered aboveground biomass of forbs. Plant N concentration, plant amp;delta;lt;supgt;15lt;/supgt;N and soil mineral N increased with N addition, showing positive relationships between plant amp;delta;lt;supgt;15lt;/supgt;N and N concentration, soil mineral N and/or applied N rate. The cessation of N application in the 480 kg N halt;supgt;amp;minus;1lt;/supgt; yrlt;supgt;amp;minus;1lt;/supgt; treatment in 2009 and 2010 led to a slight recovery of the forb species richness relative to total cover and aboveground biomass, coinciding with reduced plant N concentration and soil mineral N. The results show N deposition-induced changes in soil N transformations and plant N assimilation that are closely related to changes in species composition and biomass accumulation in this temperate steppe ecosystem.

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

61Soil ammonium and nitrate concentrations frequently respond relatively quickly to reduced N.61The response of plant tissue N concentrations varied between habitats.61Soil processes are often slow to recover from reduced N inputs.61Vegetation species composition is also often slow to recover from reduced N inputs.

A transect of 68 acid grasslands across Great Britain, covering the lower range of ambient annual nitrogen deposition in the industrialized world (5 to$35 kg N ha^{-1} year^{-1}$), indicates that long-term, chronic nitrogen deposition has significantly reduced plant species richness. Species richness declines as a linear function of the rate of inorganic nitrogen deposition, with a reduction of one species per 4-m2quadrat for every$2.5 kg N ha^{-1} year^{-1}$of chronic nitrogen deposition. Species adapted to infertile conditions are systematically reduced at high nitrogen deposition. At the mean chronic nitrogen deposition rate of central Europe ($17 kg N ha^{-1} year^{-1}$), there is a 23% species reduction compared with grasslands receiving the lowest levels of nitrogen deposition.

Abstract Humans dominate many important Earth system processes including the nitrogen (N) cycle. Atmospheric N deposition affects fundamental processes such as carbon cycling, climate regulation, and biodiversity, and could result in changes to fundamental Earth system processes such as primary production. Both modelling and experimentation have suggested a role for anthropogenically altered N deposition in increasing productivity, nevertheless, current understanding of the relative strength of N deposition with respect to other controls on production such as edaphic conditions and climate is limited. Here we use an international multiscale data set to show that atmospheric N deposition is positively correlated to aboveground net primary production (ANPP) observed at the 1-m2 level across a wide range of herbaceous ecosystems. N deposition was a better predictor than climatic drivers and local soil conditions, explaining 16% of observed variation in ANPP globally with an increase of 1 kg N·ha611·yr611 increasing ANPP by 3%. Soil pH explained 8% of observed variation in ANPP while climatic drivers showed no significant relationship. Our results illustrate that the incorporation of global N deposition patterns in Earth system models are likely to substantially improve estimates of primary production in herbaceous systems. In herbaceous systems across the world, humans appear to be partially driving local ANPP through impacts on the N cycle.

Overgrazing is one of the main causes of desertification in the semiarid Horqin sandy grassland of northern China. Excluding grazing livestock is considered as an alternative to restore vegetation in degraded sandy grassland in this region. However, few data are available concerning the impacts of continuous grazing and livestock exclusion on soil properties. In this paper, characteristics of vegetation and soil properties under continuous grazing and exclusion of livestock for 5 and 10 years were examined in representative degraded sandy grassland. Continuous grazing resulted in a considerable decrease in ground cover, which accelerates soil erosion by wind, leading to a further coarseness in surface soil, loss of soil organic C and N, and a decrease in soil biological properties. The grassland under continuous grazing is in the stage of very strong degradation. Excluding livestock grazing enhances vegetation recovery, litter accumulation, and development of annual and perennial grasses. Soil organic C and total N concentrations, soil biological properties including some enzyme activities and basal soil respiration improved following 10-year exclusion of livestock, suggesting that degradation of the grassland is being reversed. The results suggest that excluding grazing livestock on the desertified sandy grassland in the erosion-prone Horqin region has a great potential to restore soil fertility, sequester soil organic carbon and improve biological activity. Soil restoration is a slow process although the vegetation can recover rapidly after removal of livestock. A viable option for sandy grassland management should be to adopt proper exclosure in a rotation grazing system in the initial stage of grassland degradation.

Abstract Loss of plant diversity with increased anthropogenic nitrogen (N) deposition in grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of light or space is invoked as a key mechanism. Here, we provide evidence from a 9-yr N-addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply shifted plant community composition from grassorb codominance (primarily Stipa krylovii and Artemisia frigida , respectively) to exclusive dominance by grass, with associated declines in overall species richness. Reduced abundance of forbs was linked to soil acidification that increased mobilization of soil Mn, with a 10-fold greater accumulation of Mn in forbs than in grasses. The enhanced accumulation of Mn in forbs was correlated with reduced photosynthetic rates and growth, and is consistent with the loss of forb species. Differential accumulation of Mn between forbs and grasses can be linked to fundamental differences between dicots and monocots in the biochemical pathways regulating metal transport. These findings provide a mechanistic explanation for N-induced species loss in temperate grasslands by linking metal mobilization in soil to differential metal acquisition and impacts on key functional groups in these ecosystems.

Enhanced deposition of atmospheric nitrogen (N) resulting from anthropogenic activities has negative impacts on plant diversity in ecosystems. Several mechanisms have been proposed to explain the species loss. Ion toxicity due to N deposition-induced soil acidification has been suggested to be responsible for species loss in acidic grasslands, while few studies have evaluated the role of soil-mediated homeostasis of ions in species loss under elevated N deposition in grasslands with neutral or alkaline soils. To determine whether soil-mediated processes are involved in changes in biodiversity induced by N deposition, the effects of 9-year N addition on soil properties, aboveground biomass (AGB) and species richness were investigated in an Inner Mongolia steppe. Low to moderate N addition rate (2, 4, 8 g N myr) significantly enhanced AGB of graminoids, while high N addition rate ( 16 g N myr) reduced AGB of forbs, leading to an overall increase in AGB of the community under low to moderate N addition rates. Forb richness was significantly reduced by N addition at rates greater than 8 g N myr, while no effect of N addition on graminoid richness was observed, resulting in decline in total species richness. N addition reduced soil pH, depleted base cations (Ca, Mgand K) and mobilized Mn, Fe, Cuand Alions in soils. Soil inorganic-N concentration was negatively correlated with forb richness and biomass, explaining 23.59% variation of forb biomass. The concentrations of base cations (Caand Mg) and metal ions (Mn, Cuand, Fe) showed positively and negatively linear correlation with forb richness, respectively. Changes in the metal ion concentrations accounted for 42.77% variation of forb richness, while reduction of base cations was not associated with the reduction in forb richness. These results reveal that patterns of plant biodiversity in the temperate steppe of Inner Mongolia are primarily driven by increases in metal ion availability, particularly enhanced release of soil Mn.

Abstract It has long been recognized that alternative vegetation states may occur in terrestrial grazing systems. This phenomenon may be of great importance as small environmental fluctuations may lead to relatively sudden and irreversible jumps between vegetation states. Early theoretical studies emphasized saturation of herbivore feeding to explain multiple stable states and catastrophic behaviour. Recent studies on semi-arid grasslands and arctic salt marshes, however, relate catastrophic events in these systems to plant-soil interactions.

The dynamics of belowground net primary productivity (BNPP) is of fundamental importance in understanding carbon (C) allocation and storage in grasslands. However, our knowledge of the interannual variability in response of BNPP to ongoing global warming is limited. In this study, we explored temporal responses of BNPP and net primary productivity (NPP) partitioning to warming and clipping in a tallgrass prairie in Oklahoma, USA. Infrared heaters were used to elevate soil temperature by approximately 2 C since November 1999. Annual clipping was to mimic hay harvest. On average from 2005 to 2009, warming increased BNPP by 41.89% in the unclipped subplots and 66.93% in the clipped subplots, with significant increase observed in wet years. Clipping also had significant positive impact on BNPP, which was mostly found under warming. Overall, fBNPP, the fraction of BNPP to NPP, increased under both warming and clipping treatments, more in dry years. Water availability (either precipitation or soil moisture) was the most limiting factor for both BNPP and fBNPP. It strongly dominated the interannual variability in NPP, fBNPP, and their responses to warming and clipping. Our results suggest that water availability regulates tallgrass prairie's responses to warming and land use change, which may eventually influence the global C cycle. With increasing variability in future precipitation patterns, warming effects on the vegetation in this region may become less predictable.

Although nitrogen addition and recovery from degradation can both promote production of grassland biomass, these two factors have rarely been investigated in combination. In this study, we established a field experiment with six N-treatment (CK, 10, 20, 30, 40, 5065g65N65m61265yr611) on five fields with different degradation levels in the Inner Mongolian steppe of China from 2011–2013. Our observations showed that while the external nitrogen increased the aboveground biomass in all five grasslands, the magnitude of the effects differed with the severity of degradation. Fields with a higher level of degradation tended to have a higher saturation value (2065g65N65m61265yr611) than those with a lower degradation level (65<651065g65N m61265yr611). After three years of experimentation, species richness showed little change across degradation levels. Among the four functional groups of grasses, sedges, forbs and legumes, grasses shared the most similar response patterns with those of the whole community, demonstrating the predominant role that they play in the restoration of grassland under a stimulus of nitrogen addition.

Anthropogenic perturbations may affect biodiversity and ecological stability as well as their relationships. However, diversity–stability patterns and associated mechanisms under human disturbances have rarely been explored. We conducted a 7-year field experiment examining the effects of mowing and nutrient addition on the diversity and temporal stability of herbaceous plant communities in a temperate steppe in northern China. Mowing increased population and community stability, whereas nutrient addition had the opposite effects. Stability exhibited positive relationships with species richness at population, functional group and community levels. Treatments did not alter these positive diversity–stability relationships, which were associated with the stabilising effect of species richness on component populations, species asynchrony and portfolio effects. Despite the difficulty of pinpointing causal mechanisms of diversity–stability patterns observed in nature, our results suggest that diversity may still be a useful predictor of the stability of ecosystems confronted with anthropogenic disturbances.

青藏高原正经历着明显的温暖化过程，由此引起的土壤温度的升高促进了土壤中微生物的活性，同时青藏高原东缘地区大气氮沉降十分明显，并呈逐年增加的趋势，这些环境变化均促使土壤中可利用营养元素增加，因此深入了解青藏高原高寒草甸植物生物量对可利用营养元素增加的响应，是准确预测未来全球变化背景下青藏高原高寒草甸碳循环过程的重要基础。该研究基于在青藏高原高寒草甸连续4年（2009-2012年）氮、磷添加后对不同功能群植物地上生物量、群落地上和地下生物量的测定，探讨高寒草甸生态系统碳输入对氮、磷添加的响应。结果表明：（1）氮、磷添加均极显著增加了禾草的地上绝对生物量及其在群落总生物量中所占的比例，同时均显著降低了杂类草在群落总生物量中的比例，此外磷添加极显著降低了莎草地上绝对生物量及其在群落总生物量中所占的比例。（2）氮、磷添加均显著促进了青藏高原高寒草甸的地上生物量增加，分别增加了24％和52％。（3）氮添加对高寒草甸地下生物量无显著影响，而磷添加后地下生物量有增加的趋势。（4）氮添加对高寒草甸植物总生物量无显著影响，而磷添加后植物总生物量显著增加。研究表明，氮、磷添加可缓解青藏高原高寒草甸植物生长的营养限制，促进植物地上部分的生长，然而高寒草甸植物的生长极有可能更受土壤中可利用磷含量的限制。

青藏高原正经历着明显的温暖化过程，由此引起的土壤温度的升高促进了土壤中微生物的活性，同时青藏高原东缘地区大气氮沉降十分明显，并呈逐年增加的趋势，这些环境变化均促使土壤中可利用营养元素增加，因此深入了解青藏高原高寒草甸植物生物量对可利用营养元素增加的响应，是准确预测未来全球变化背景下青藏高原高寒草甸碳循环过程的重要基础。该研究基于在青藏高原高寒草甸连续4年（2009-2012年）氮、磷添加后对不同功能群植物地上生物量、群落地上和地下生物量的测定，探讨高寒草甸生态系统碳输入对氮、磷添加的响应。结果表明：（1）氮、磷添加均极显著增加了禾草的地上绝对生物量及其在群落总生物量中所占的比例，同时均显著降低了杂类草在群落总生物量中的比例，此外磷添加极显著降低了莎草地上绝对生物量及其在群落总生物量中所占的比例。（2）氮、磷添加均显著促进了青藏高原高寒草甸的地上生物量增加，分别增加了24％和52％。（3）氮添加对高寒草甸地下生物量无显著影响，而磷添加后地下生物量有增加的趋势。（4）氮添加对高寒草甸植物总生物量无显著影响，而磷添加后植物总生物量显著增加。研究表明，氮、磷添加可缓解青藏高原高寒草甸植物生长的营养限制，促进植物地上部分的生长，然而高寒草甸植物的生长极有可能更受土壤中可利用磷含量的限制。

Abstract Humans are both intentionally (fertilization) and unintentionally (atmospheric nutrient deposition) adding nutrients worldwide. Increasing availability of biologically reactive nitrogen (N) is one of the major drivers of plant species loss. It remains unclear, however, whether plant diversity will be equally reduced by inputs of reactive N coming from either small and frequent N deposition events or large and infrequent N fertilization events. By independently manipulating the rate and frequency of reactive N inputs, our study teases apart these potentially contrasting effects. Plant species richness decreased more quickly at high rates and at low frequency of N addition, which suggests that previous fertilization studies have likely over-estimated the effects of N deposition on plant species loss. N-induced species loss resulted from both acidification and ammonium toxicity. Further study of small and frequent N additions will be necessary to project future rates of plant species loss under increasing aerial N deposition.