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Backgrounds: Climate change is well-known to alter the structure and function of grassland ecosystems, and multifunctionality contributes to a comprehensive understanding of the impacts of climate change on ecosystem functions. Warming and humidification are predicted to be the climate change trend on the northeastern Qinghai-Tibetan Plateau. However, understanding of how long-term warming and increased precipitation affect ecosystem multifunctionality in alpine meadows is still limited. Methods: Here, we conducted an 8-year field experiment involving warming and increased precipitation in an alpine meadow to explore how warming, increased precipitation, and their interaction affect ecosystem multifunctionality. Results: The results indicated that increased precipitation had a positive effect on ecosystem multifunctionality. However, warming and the interaction of warming and precipitation had no significant effects on it. Warming decreased species richness and plant coverage. Increased precipitation enhanced aboveground carbon (C), nitrogen (N), and phosphorus (P) pools of plant community, and soil moisture, but decreased soil pH. Aboveground P and N pools of plant community and microbial biomass nitrogen (MBN) were important predictors of ecosystem multifunctionality. Conclusion: This study demonstrated long-term increased precipitation can enhance ecosystem multifunctionality by indirectly affecting the individual functions (aboveground P and N pools of plant community and MBN), soil moisture, and pH in an alpine meadow. These findings highlighted that increased precipitation is more critical than warming for enhancing ecosystem multifunctionality in semi-arid alpine meadows. [ABSTRACT FROM AUTHOR]

Aims: Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of meadow degradation on soil BNF and diazotrophic communities in alpine meadows. Methods: We investigated changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Results: Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g−1 dry soil d−1 across all sites. The highest BNF rate in the non-degraded meadow was 1.5–4.6-fold higher than that in the degraded meadows. The abundance and diversity of diazotrophs measured by nifH abundance and Shannon diversity was also decreased in the degraded meadows, accompanied by decreases in plant biomass, soil moisture, and nutrient content (C, N). Soil BNF rate was correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, and unidentified genera within the Rhizobiales and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, with both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributing significantly to BNF. The plant functional groups, particularly the sedge family, were the primary drivers of soil BNF rates via mediating soil moisture, nutrient content (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Conclusions: Our results reveal the main drivers of decreased BNF during alpine meadow degradation and emphasize the importance of plant functional groups in shaping the diazotrophic community and regulating the BNF rate. This information can be applied to the restoration of degraded meadow ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Viable seeds in herbivore dung constitute the dung seed bank, and the contribution of livestock dung to this seed bank in grazing pastures is often overlooked. Grazing season (warm and cold), seed characteristics (size and shape), and forage preference are the main factors that affect the size and composition of the dung seed bank and ultimately affect grassland ecology. However, how these three factors interact is unclear. Methods: We collected yak dung as well as seeds of the common plant species from warm- and cold-season alpine meadows in northeastern Qinghai-Tibetan Plateau, and explored how grazing season (warm and cold), seed characteristics (size and shape) and foraging preferences (temporary cages method) affects yak dung seedling density, richness and diversity in an alpine pasture. Results: Forty-three plant species (mainly perennials) germinated from yak dung. Dung seedling density, richness, and diversity did not differ significantly between the two grazing seasons. Small to medium-sized spherical seeds (seed size < 10 mg, shape index < 0.5) had the greatest germination potential. Conclusions: Yaks vary their forage preference depending on the season (phenological period), and endozoochory occurs throughout both grazing seasons. Seed shape and size directly regulate the dung seedling density, richness, and diversity. Dung seedlings increase the heterogeneity of the aboveground vegetation near the microsites of the dung pieces and therefore promote grassland patching. Our study demonstrates that grazing season, seed characteristics, and yak forage preferences affect the dung seed bank in grazing ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Root dynamics plays a fundamental role in determining carbon allocation and other main ecological processes in grasslands. Understanding the responses of root activities to ongoing warming in grazed alpine meadows enable us to predict the potential changes in the carbon budget and ecosystem functions in alpine regions. Methods: We conducted a controlled 4-year field experiment with warming and grazing in an alpine meadow on the Qinghai-Tibetan Plateau. Our objective was to explore the responses of root standing crop, production, mortality and turnover rate to warming and grazing using minirhizotrons. Results: Warming only significantly inhibited root mortality while grazing promoted all the four root metrics, which were also significantly influenced by the interaction of warming and grazing. Warming oppositely affected the four metrics under grazing versus non-grazing conditions. It significantly reduced root mean standing crop, annual production and mortality without grazing, but significantly stimulated only mean standing crop under grazing. Grazing had significantly negative and positive effects on mean standing crop in the no warming and warming plots. It promoted annual root production and mortality regardless of warming, whereas the effects were significant only under warming condition. Moreover, grazing significantly increased turnover rate in no warming plots but slightly decreased it in warming plots. Conclusions: These findings highlight the non-additive interactions on the alpine meadow root dynamics between warming and grazing. Therefore, grazing regimes should be considered to better model the ecosystem feedback to global warming and to improve the prediction of future ecosystem functions. [ABSTRACT FROM AUTHOR]

Aims: Fencing to prevent livestock grazing has widely been implemented to restore vegetation and soils of degraded alpine meadows on the Zoige Plateau in recent decades, but its efficacy is still unclear. This study was designed to investigate the responses of plant community and soil properties to long-term fencing. Methods: We surveyed vegetation structure and production, soil physical and chemical properties in three sites, each having paired adjacent plots (i.e. one was fenced for 11–17 years and the other was allowed for regular grazing). Results: Long-term fencing resulted in species loss at the community level and decreased plant species richness at the plot level. Fencing increased aboveground plant biomass and plant litter accumulation but reduced root biomass, and in particular it dramatically increased the aboveground biomass of grasses at the expense of legumes and sedges. Moreover, fencing decreased soil organic carbon and total N concentrations, and soil bulk density, but increased soil water infiltration rate, soil total P and soluble N concentrations, while soil soluble P concentration remained unchanged. Conclusions: These results indicate that long-term (> 11 years) fencing is not beneficial to plant diversity and soil organic carbon sequestration of the Zoige alpine meadows. [ABSTRACT FROM AUTHOR]

Background and aims: Long-term nitrogen (N) addition can affect soil organic carbon (SOC) pool within different soil fractions with different turnover rates. However, the mechanisms of these effects, particularly in alpine grassland ecosystems, are not clear. Methods: We studied the responses of SOC content in different soil fractions to N addition based on a six-year N addition field experiment in an alpine meadow ecosystem on the Tibetan Plateau. We measured soil chemical and microbial properties, and SOC content in bulk soil, particular organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. Results: N addition increased soil N availability, decreased soil pH and microbial biomass, but had minimal effect on plant biomass, soil enzyme activity, and SOC content in bulk soil. With increasing levels of N addition, SOC in the POM fraction (POC) showed a significant negative trend, while SOC in the MAOM fraction (MAOC) did not change significantly. Conclusions: As plant biomass input and soil enzyme activity were not significantly altered with N addition, the decline in POC was likely caused by changes in microbial physiology (carbon use efficiency), while the insignificant change in MAOC may be determined by the balance between input (from microbial necromass) and output (from microbial decomposition). Taken together, our study showed that the less-protected POC fraction is more vulnerable to N addition than the more-protected MAOC fraction in the alpine grassland. This finding may improve the prediction of soil C dynamics in response to N deposition in alpine grassland ecosystems on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Background and aims: Responses of grassland carbon (C) cycling to changes in precipitation and land use remain a major uncertainty in model prediction of future climate. It remains even far from clear on how does clipping regulate the response sensitivity of grassland C sequestration to precipitation change. Methods: We conducted a manipulative field experiment with five precipitation treatments (1/12P, 1/4P, 1/2P, 3/4P, P, and 5/4P, where P is the growing season precipitation) interacting with clipping treatments in an alpine meadow to examine net ecosystem C uptake and its associated processes in response to clipping and precipitation gradients as well as their interaction. Results: (1) clipping significantly increased soil temperature in 2016 and 2017, while had no effect on soil moisture; (2) clipping also stimulated the net ecosystem productivity (NEP) and gross ecosystem productivity (GEP) during the two growing seasons; (3) although there were no significantly interactive effects of clipping with precipitation changes, clipping increased the response sensitivity of C uptakes to soil moisture changes. NEP increased more with soil moisture under clipping (0.21 μmol m−2 s−1 v/v−1) than unclipping treatments (0.11 μmol m−2 s−1 v/v−1). This was due to the compensatory growth and the positive effect of clipping on soil temperature, which stimulated plant growth and productivity. Conclusions: Our findings suggest that clipping will aggravate grassland's sensitivity to precipitation change. These findings highlight the important role of land use change in regulating ecosystem C sequestration under further climate conditions in an alpine meadow. [ABSTRACT FROM AUTHOR]

Background and aims: In addition to improving vegetation, grazing prohibition changes the soil micro-environment. Soil enzymes and microbes, which are the chief constituents responsible for mediation of most of the processes involved in energy and material fluxes of ecosystems, are very sensitive to changing environments. This study aimed to illustrate the effects of grazing prohibition on vegetation and soil enzyme characteristics, and to elucidate the responses of soil fungal and bacterial function and community composition on grazing prohibition for 10 years in a Qinghai-Tibetan alpine meadow. Methods: About 6 hm2 lightly degraded alpine meadows were selected in 2006, and half of which were fenced to completely prohibit grazing throughout the year, while the other half were subjected to free yak grazing. Four different plots in each site were selected as experimental fields in 2016. Soil microbial characteristics were analyzed by high-throughput gene detection. Results: Grazing prohibition had no significant impact on vegetable Shannon-Wiener diversities or below-ground biomass, but dramatically changed the above-ground biomass and soil phosphatase activity. In the Qinghai-Tibetan alpine meadow, bacteria are dominant members of the soil microbial community. Grazing prohibition significantly increased the fungal species diversity and the abundance of pathotrophs, saprotrophs and symbiotrophs, while it had no effect on soil bacterial diversity and functional structure. A total of 100.0% of the variations in fungi and bacteria could be explained by plant variables. The reciprocal action among plant biomass (i.e. above-ground biomass, below-ground biomass), plant diversities (i.e. Shannon-wiener diversity, species number, coverage) and species important value have a great impact on soil microbial composition variations. Conclusion: Long-term grazing prohibition restrained the phosphatase activity and increased the abundance of pathotrophic fungi but had no effect on the abundance of functional bacteria, which participate in the soil biogeochemistry cycling process. Grazing prohibition for 10 years did not improve the community structure of functional microorganisms related to carbon, nitrogen and sulfur, and appropriate grazing could maintain the sustainable utilization of grassland resources. [ABSTRACT FROM AUTHOR]

Aims: This study investigated the influences of herbivore grazing intensity and grazing season on decomposition and nutrient release of dung and litter, which aimed to improve our understandings of grazing affecting nutrient cycling in alpine meadows on the Qinghai-Tibetan Platean. Methods: A factorial design experiment comprising 3 grazing intensities (non-grazing, moderate grazing, and heavy grazing) and 2 grazing seasons (summer and winter), was applied to quantify the decomposition and chemistry of dung and litter in an alpine pasture using the litterbag technique. Litterbags were retrieved for analysis of mass loss and nutrient release with 180, 360, 540, and 720 days after placement. Results: Grazing activity accelerated the decomposition of dung and litter and increased nutrient release from dung and litter by increasing soil temperature compared with non-grazing pastures, whereas grazing season had no effect on decomposition. The decomposition time was shorter for dung than that for litter. Conclusions: Herbivores grazing benefited dung and litter decomposition and nutrient cycling directly by increasing soil temperature, which is likely to promote soil microbial activity due to low temperatures in alpine meadows, and indirectly through herbage ingestion and dung deposition which increase the organic debris concentration used for microorganisms growth and reproduction. This study provides insights into the mechanisms of grazing regulating nutrient cycling in alpine ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: In recent years, ecologists have tried to determine the importance of intraspecific trait variation (ITV) versus community composition change (CCC) in shifts in community functional composition. However, results to date have not provided generality.Methods: A six-year nitrogen (N) fertilization experiment was conducted in species-rich alpine meadow communities. We focused on five key functional traits (height, leaf dry mass content LDMC, specific leaf area SLA, leaf nitrogen content LNC, and leaf phosphorus content LPC) that belong to three trait categories (whole-plant level trait, leaf morphology trait and leaf chemical trait) and measured biomass of each species in community. A sum of squares decomposition method was used to distinguish the relative contribution of ITV versus CCC to community weighted mean (CWM) traits.Results: Our results showed that N fertilization led to increasing heightCWM and LNCCWM due to light competition intensification and soil nutrient enrichment. However, the responses of community-wide SLA, LDMC and LPC were highly inconsistent and depended on the balance of different opposing processes. Moreover, during short-term fertilization, ITV played a more important role in mediating functional composition changes in all traits, but the effects of CCC overwhelmed ITV and became more important in determining community-wide whole-plant level trait and leaf morphology traits (height, LDMC and SLA) in subsequent fertilization years. On the other hand, ITV always played a more important role than CCC in determining the community-wide leaf chemical traits (LNC and LPC), but CCC had a greater contribution than ITV in terms of explaining shifts in whole-plant traits (height) and leaf morphology traits (LDMC and SLA). In addition, both positive and negative covariations appeared in our study, indicating that community-wide trait shifts due to ITV and CCC may either reinforce or oppose one another, depending on different trait categories.Conclusions: These findings highlight the importance of ITV, CCC and their covariation in mediating community functional composition, and the relative importance of ITV effects and CCC effects depended on fertilization duration and trait category. This study significantly improve our understanding of the mechanisms governing the shifts in community functional composition under N deposition scenarios. [ABSTRACT FROM AUTHOR]

Aims: A lacking of understanding about cooling effects on methane (CH) fluxes and potential asymmetrical responses to warming and cooling causes uncertainty about climate change effects on the atmospheric CH concentration. We investigated CH fluxes in an alpine meadow on the Tibetan Plateau in response to climate warming and cooling. Methods: A 2-year reciprocal translocation experiment was implemented to simulate climate warming (i.e. downward translocation) and cooling (i.e. upward translocation) along an elevation gradient with four different vegetation types (at 3200, 3400, 3600 and 3800 m elevation) during the growing season (May to October) in 2008 and 2009. Results: Although the effects of warming and cooling varied depending on vegetation type, elevation and timescale (i.e., daily and seasonally), warming increased average seasonal CH uptake by 60 %, while cooling reduced it by 19 % across all vegetation types, based on a 1.3-5.1 °C difference in soil temperature at 20 cm depth. Soil temperature over the range of 4-10 °C explained 11-25 % of the variation in average seasonal CH fluxes, while there was no relationship with soil moisture over the range of 13-39 % and soil NH-N and NON content. Methane uptake was more sensitive to warming than to cooling. Conclusions: Because of warming and cooling spells in the alpine region, warming effects on CH uptake would be over-estimated by 64 % if cooling effects on it are not considered. Our findings suggest that asymmetrical responses of CH fluxes to warming and cooling should be taken into account when evaluating the effects of climate change on CH uptake in the alpine meadow on the Tibetan plateau. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the effect of grazing season on net ecosystem CO exchange (NEE) and its components is crucial to predict the feedback of grazing management to climate change. Methods: We estimated NEE, gross primary productivity (GPP) and ecosystem respiration (Re) under different seasonal grazing practices (i.e. no-grazing (NG), warm season grazing (WG) and cold season grazing (CG)) by sheep during the growing seasons from 2008 to 2012 on the Tibetan Plateau. Results: Our results show that the impacts of seasonal grazing on daily GPP, Re and NEE in the alpine meadow ecosystem varied with sampling date and year. Compared with NG and CG, WG significantly reduced average seasonal NEE by 22.7 %, because grazing impact was exacerbated by drought in July in 2010. Soil temperature only explained 19-31 % of the variation in daily GPP, Re and NEE for all grazing treatments. The interannual variabilities of GPP, Re and NEE were mainly determined by root biomass and/or average soil temperature during the growing season. Conclusions: Our results suggest that although WG may decrease sequestration of CO under continuous drought conditions after grazing, it would have little impact on CO sequestration during the growing season under conditions of future warming with greater rainfall in alpine meadows on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Background and aims: Niche complementarity arising from divergence in resource use is an important mechanism underlying species coexistence. We hypothesized fertilization with different N forms would generate plastic divergence among species with regard to their N form uptake and preference. Methods: In the eighth year of a long-term N fertilization experiment in an alpine meadow on the Tibetan plateau, we labeled 11 common plant species with ammonium-N or nitate-N in subplots without fertilization (control) or fertilized with 7.5 g N m yr in the form of ammonium, nitrate, or ammonium nitrate to trace N form uptake. Results: Depending on species, fertilization with nitrate or ammonium nitrate had positive, negative or neutral effects on NO-N uptake rate, although ammonium fertilization showed little impact. By contrast, fertilization with any N form had little impact on NH-N uptake rate. Consequently, effects of nitrate fertilization and ammonium nitrate fertilization on relative N form preference diverged among the species and the functional groups (grasses, sedges, legumes and forbs). Conclusions: Alpine plant species can diverge in N form uptake and preference in response to long-term N fertilization, and such divergence may contribute to species coexistence after long-term fertilization. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the role of resource availability in structuring biotic communities is of importance in community ecology. This study investigates how light and soil nutrient availability drive assemblages of both plants and their root-associated arbuscular mycorrhizal fungi (AMF). Methods: We conducted a 4-year light [full light or shade] and soil fertility [unfertilized or fertilized with (NH)HPO] interactive manipulations in an alpine meadow ecosystem. Species and phylogenetic compositions of plant and AMF communities were simultaneously measured, and the primary ecological processes structuring both communities were inferred from the community phylogenetic analysis. Results: Reducing light and/or increasing soil fertility significantly reduced species richness and changed community compositions of both plant and AMF. Plant community phylogenetic structure shifted from random in untreated control to overdispersion in other treatments, whereas AMF communities were phylogenetically clustered and random in unfertilized and fertilized plots, respectively. These results suggest that plant communities in treated plots were mainly determined by competitive exclusion, and that AMF communities in unfertilized and fertilized plots were determined by environmental filtering and random process, respectively. Conclusions: We observed strong effects of light and soil nutrient availability on both plant and AMF communities, and our findings highlight that the primary ecological processes that drive plant and AMF assemblages should be highly dependent on the level of resource availability. [ABSTRACT FROM AUTHOR]

Aims: We evaluated the effects of temperature and addition of leaf litter of Androsace tapete MaximWe-a dominant cushion plant species of alpine meadows on the Tibetan Plateau-on carbon (C) and nitrogen (N) mineralization, microbial biomass C (MBC) and N (MBN). Methods: A laboratory incubation experiment with and without cushion plant litter addition was conducted for 112 days at three temperature regimes (−1, 5 and 11 °C). C and net N mineralization were simultaneously measured during the incubation period. Results: C and N mineralization were affected by interactions between litter addition and temperature. Litter addition increased C mineralization and MBN but shifted N mineralization to immobilization at higher temperature. The positive relationship between net N mineralization and MBC and MBN was shifted to a negative one through cushion plant litter addition. Cushion plant litter also changed the relationship between C mineralization and net N mineralization from insignificantly positive to significantly negative. Conclusions: These findings indicate that low temperature in winter could be important for alpine plants because low temperature can increase net N mineralization and supply plants with available N for their growth in the early growing season. During the growing season, climate warming-either directly through a temperature effect or indirectly through triggering increased cushion plant litter production-might lead to stronger competition for N between alpine plants and microorganisms. [ABSTRACT FROM AUTHOR]

Background and Aims: Information on soil seed bank processes is crucial for understanding vegetation dynamics. Despite the documented importance of soil seed banks in many ecosystems, their role is not fully understood in some sensitive habitats, such as the alpine meadows of the Tibetan Plateau. Methods: We studied the seasonal dynamics of the germinable soil seed bank under four disturbance intensities in an alpine meadow on the Tibetan Plateau as well as seed size distribution relative to disturbance intensity. Composition of the seed bank was compared with that of the standing vegetation. Results: Density of buried seeds increased with disturbance intensity, but species richness and species diversity decreased. Seed density and species richness of the seed bank varied seasonally in all layers (0-2, 2-7, 7-12 cm) and the whole (0-12 cm). The species composition of seed bank was not significantly influenced by season. There was no trend in seed size distribution as disturbance increased. Seasonal seed bank turnover rates increased with increase in disturbance. The result of the NMDS showed that species composition of seed bank and vegetation exhibited a fairly uniform pattern in each season. Conclusions: Although as a whole the species composition of the vegetation and seed bank showed a relatively low degree of similarity in each season, similarity was highest in the most disturbed habitat. There was no alteration in species composition of seed bank regardless of disturbance intensity, but seed density decreased as disturbance increased. Disturbances in alpine plant communities might increase persistence of regeneration niches. Regeneration from the seed bank together with vegetative reproduction contributed to aboveground vegetation in highly disturbed habitats. Clonal species played an important role in regeneration of vegetation in slightly disturbed areas, where there was little contribution of ruderals from soil seed banks. [ABSTRACT FROM AUTHOR]

Background and aim: Because the indigenous burrowing lagomorph plateau pika ( Ochotona curzoniae) is considered to have negative ecological impacts on alpine meadow steppe grasslands of the Headwaters Region of the Yellow, Yangtze and Mekong Rivers we investigated its effects on ecosystem productivity and soil properties, and especially net ecosystem carbon flux. Methods: We measured net ecosystem CO exchange (NEE) and its components gross ecosystem productivity (GEP) and ecosystem respiration (ER) at peak aboveground biomass by the chamber method with reference to plant and soil characteristics of areas of alpine meadow steppe with different densities of pika burrows. Results: Higher burrow density decreased NEE, GEP and ER. Above-ground biomass, species number, plant cover and leaf area index decreased with increasing pika density. Higher burrow density was associated with lower soil moisture and higher soil temperature. Responses of NEE were related to changes of abiotic and biotic factors affecting its two components. NEE was positively related to soil moisture, soil ammonium nitrogen, plant cover, leaf area index and above-ground biomass but was negatively correlated with higher soil nitrate nitrogen. Conclusion: Decrease of NEE by plateau pika may reduce the carbon sink balance of Qinghai-Tibet plateau grassland. Such effects may be influenced by grazing pressure from domestic livestock, population levels of natural predators, and climate change. [ABSTRACT FROM AUTHOR]

Aims: To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO) exchange to N addition. Methods: Based on a multi-form, low-level N addition experiment, soil CO effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO efflux. Results: The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH-N pool. N applications significantly increased aboveground biomass and soil CO efflux; moreover, this effect was more significant from NH-N than from NO-N fertilizer. In addition, the soil CO efflux was mainly driven by soil temperature, followed by aboveground biomass and NH-N pool. Conclusions: These results suggest that chronic atmospheric N deposition will stimulate soil CO efflux in the alpine meadow on the Qinghai-Tibetan Plateau by increasing available N content and promoting plant growth. [ABSTRACT FROM AUTHOR]

We studied seasonal dynamics of seed banks along an altitudinal gradient in three alpine meadows on the Tibetan Plateau, as well as seed size distribution relative to depth. Seed bank density and species richness decreased significantly with altitude increase in both April and July. The highest elevations showed highest seed bank depletion. Although species composition of the vegetation changed along the altitudinal gradient, seed bank composition was almost unvaried from April to July. This indicated that seed bank density has an obvious seasonal change but composition does not. We found a low degree of similarity between the species composition of vegetation and of the seed bank along the whole gradient, and this similarity decreased with altitude increase. These suggest that the role of the seed bank decreased gradually with altitude increase. The hypothesis that a species seed bank strategy is an inherited trait was not confirmed because for some species seed bank strategy changed with altitude. We found that persistent seed banks were the most frequent strategy at all three altitudes. Species with persistent seeds tended to have smaller seeds than those with transient seeds only in highest altitude, while the other two altitudes did not show difference. There was no trend in seed size distribution with altitudes and soil depths. [ABSTRACT FROM AUTHOR]

We conducted a field experiment in two alpine meadows to investigate the short-term effects of nitrogen enrichment and plant litter biomass on plant species richness, the percent cover of functional groups, soil microbial biomass, and enzyme activity in two alpine meadow communities. The addition of nitrogen fertilizer to experimental plots over two growing seasons increased plant production, as indicated by increases in both the living plant biomass and litter biomass in the Kobresia humilis meadow community. In contrast, fertilization had no significant effect on the amounts of living biomass and litter biomass in the K. tibetica meadow. The litter treatment results indicate that litter removal significantly increased the living biomass and decreased the litter biomass in the K. humilis meadow; however, litter-removal and litter-intact treatments had no impact on the amounts of living biomass and litter biomass in the K. tibetica meadow. Litter production depended on the degree of grass cover and was also influenced by nitrogen enrichment. The increase in plant biomass reflects a strong positive effect of nitrogen enrichment and litter removal on grasses in the K. humilis meadow. Neither fertilization nor litter removal had any impact on the grass biomass in the K. tibetica meadow. Sedge biomass was not significantly affected by either nutrient enrichment or litter removal in either alpine meadow community. The plant species richness decreased in the K. humilis meadow following nitrogen addition. In the K. humilis meadow, microbial biomass C increased significantly in response to the nitrogen enrichment and litter removal treatments. Enzyme activities differed depending on the enzyme and the different alpine meadow communities; in general, enzyme activities were higher in the upper soil layers (0–10 cm and 10–20 cm) than in the lower soil layers (20–40 cm). The amounts of living plant biomass and plant litter biomass in response to the different treatments of the two alpine meadow communities affected the soil microbial biomass C, soil organic C, and soil fertility. These results suggest that the original soil conditions, plant community composition, and community productivity are very important in regulating plant community productivity and microbial biomass and activity. [ABSTRACT FROM AUTHOR]