Moderate grazing increased alpine meadow soils bacterial abundance and diversity index on the Tibetan Plateau

The response of grassland soil bacterial community characteristics to different grazing intensities is central ecological topics. However, the underlying mechanisms between bacterial abundance, diversity index, and grazing intensity remain unclear. We measured alpine meadow soil bacterial gene richness and diversity index under four grazing intensities using 16S rDNA sequence analysis on the Tibetan Plateau. The results suggest that extreme grazing significantly decreased alpine meadow both bacterial gene abundance and diversity index (p < .05). The lowest operational taxonomic unit numbers were 3,012 ± 447 copies under heavy grazing in the growing season. It was significantly lower than heavy grazing with approximately 3,958 ± 119 copies (p < .05). The Shannon index for medium and high grazing grassland bacterial diversity was slightly higher than for light grazing in the growing season. Furthermore, the lowest index was approximately 9.20 ± 0.50 for extreme grazing of grassland in the growing season. The average bacterial gene abundance and diversity index in the dormancy period were slightly higher than that in the growing season. Soil bulk density, pH, ammonium, and nitrate nitrogen were the main positive factors driving grazed grassland bacterial communities. Our study provides insight into the response of alpine meadows to grazing intensity, demonstrating that moderate grazing increases bacterial community diversity in grazed grasslands.     

Impact of grazing regime on a Mongolian forest steppe

Question: What is the impact of grazing regime on plant species abundance, plant growth form, plant productivity and plant nutrient concentrations in a forest steppe? Location: Hustai National Park in the forest steppe region of Mongolia. Methods: On the Stipa steppe we applied three different grazing regimes by using; (1) one type of exclosure which excluded grazing by large mammalian herbivores, mainly takh (Przewalski horse), (2) another type of exclosure that excluded both large and small (Siberian marmots) mammalian herbivores, and (3) control plots which were freely grazed. We measured species frequencies, tiller densities, plant biomass and nitrogen concentrations of the vegetation. Results: Exclusion from grazing by takh and marmots significantly increased plant standing crop, but marmot grazing and full grazing did not show significant differences. Protection from grazing decreased forage quality, shown by a lower N-concentration of the standing crop. However, this was solely the result of the lower live-dead ratio of the vegetation. The frequency of the rhizomatous Leymus chinensis decreased under reduced grazing, as did the frequency of the total of rhizomatous species. The frequency of Stipa krylovii increased under reduced grazing, as did its basal areas, tiller density and tussock height. Conclusion: Reduced grazing leads to a lower abundance of rhizomatous species and an increase in tussock species.     

Phytosociological and palynological studies of alpine steppe communities on the northern Tibetan Plateau,Qinghai Province,China

The dry cold climate in the highland of the eastern Kunlun Mountains in Qinghai Province, north‐western part of China supports the establishment of alpine grassland, consisting of Poaceae and Cyperaceae species and dwarf shrub communities dominated by Potentilla and Krascheninnikovia shrubs. The phytosociological vegetation studies carried out around Lake Heihai (36°00′N/93°26′E, 4446 m a.s.l.) reveals a complex pattern of high mountainous vegetation, including three major vegetation communities. A Polygonum sibiricum community occupies wet and slightly saline sites close to the Heihai lake shoreline and Kobresia robusta and Poa pachyantha communities are characteristic for the drier slopes. These communities differ in ecological conditions, exposure and several characteristic species that form local subunits. Main ecological factors influencing plant growth are moisture coming from glacial melt water and the composition and texture of the surface sediments. The atmospheric circulation (i.e. monsoonal wind system) and the different amount of insolation of the southern and northern slopes are of minor importance for the establishment of the different communities. The reflection of the vegetation composition in the modern pollen rain is generally poor, since the pollen spectrum is highly influenced by the local appearance of taxa. Though the vegetation unit of a alpine steppe can be detected. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Embedded rock fragments affect alpine steppe plant growth,soil carbon and nitrogen in the northern Tibetan Plateau

Background and Aims

Rock fragments within topsoil have important effects on soil properties and plant growth. This study mainly aimed to investigate the relationships between rock fragments, soil carbon (C) and nitrogen (N) densities and vegetation biomass in an alpine steppe.

Methods

Rock fragments, plant and soil samples were collected from four topographic positions (top, upper, lower, and bottom) on a hillslope.

Results

Volumetric rock fragment content within the 0–30 cm soil profile varied from 17.8 to 30.5%, the upper position value was significantly greater (P < 0.05) than those at other positions. The highest aboveground biomass was observed at the lower position (921 kg ha^?1), while the highest belowground biomass within the 0–30 cm profile was found at the upper position (4460 kg ha^?1). More fine earth and plant litter input accompanied by lower C and N losses induced by rainfall erosion resulted in higher soil organic C and total N densities (28.6 Mg C ha^?1 and 2.87 Mg N ha^?1) at the lower position.

Conclusions

Rock fragments may promote root growth but limit aboveground biomass production, and can therefore change the biomass distribution pattern. Our findings provide more evidence for scientifically assessing alpine steppe productivity.

     

增温对青藏高原高寒草原生态系统碳交换的影响

碳交换是影响草地生态系统碳汇功能的关键过程,对气候变暖极为敏感。青藏高原分布着大面积的高寒草原,其碳汇功能对气候变暖的响应对区域碳循环过程具有重要的影响。为探究高寒草原生态系统碳交换过程对增温的响应,2012—2014年,在青藏高原班戈县进行了模拟增温对高寒草原生态系统碳交换过程影响的研究。结果表明,增温对高寒草原碳交换各组分的影响存在年际差异,但总体上对碳交换存在负面影响。3年平均结果显示,增温显著降低了高寒草原地上生物量、总生态系统生产力(GEP)、生态系统呼吸(ER)和净生态系统碳交换量(NEE)(P0.05),平均降幅分别为15.1%、36.8%、19.2%和51.5%。增温条件下3年平均土壤呼吸(SR)较对照无显著变化(P0.05),但2013年增温显著降低了SR(P0.05),降幅达18.1%。增温对SR与ER的比值具有一定的促进作用,最高增幅达到40.0%。GEP、ER、SR和NEE与土壤温度和土壤水分无显著相关(P0.05),而GEP、ER和NEE与空气温度呈显著的负相关关系(P0.05)。增温引起的干旱胁迫以及地上生物量降低是导致高寒草原NEE降低的主要原因。研究表明,全球变暖会一定程度降低青藏高原高寒草原的碳汇功能。     

Impact of grazing on soil carbon and microbial biomass in typical steppe and desert steppe of Inner Mongolia

The potential of grazing lands to sequester carbon must be understood to develop effective soil conservation measures and sustain livestock production. Our objective was to evaluate the effects of grazing on soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon (MBC) in Typical steppe and Desert steppe ecosystems, which are both important grassland resources for animal grazing and ecological conservation in China, and to derive region-specific soil C changes associated with different stocking rates (ungrazed, UG; lightly grazed, LG; moderately grazed, MG; heavily grazed, HG). This study substantiated that significant higher SOC, TN and MBC appeared with the treatment of LG in typical steppe. From 2004 to 2010, grazing treatments increased soil carbon storage in desert steppe, which was partly due to the grazing history. The higher MBC concentration and MBC/SOC suggest a great potential for carbon sequestration in the desert steppe ecosystem. The greater MBC in desert steppe than typical steppe was mainly the result of higher precipitation and temperature, instead of soil substrate. The change of MBC and the strong positive relationships between MBC and SOC indicated that MBC in the soil was a sensitive index to indicate the dynamics of soil organic carbon in both steppes in Inner Mongolia of China.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

围栏对青藏高原不同类型草地土壤原核微生物多样性的影响

草地退化是草地植被的倒退演替,导致生物多样性丧失和生态系统功能退化,围栏是恢复退化草地生态系统功能的有效管理措施。微生物是土壤中的重要组成部分,在维持草地生态系统稳定性和功能方面发挥着重要作用。然而,目前尚不清楚围栏如何影响不同类型草地土壤微生物群落。以青藏高原草甸、草原和荒漠草地三种草地类型的退化草地为研究对象,设置围栏和放牧两种处理,采用Illumina HiSeq高通量测序技术研究了围栏对土壤原核微生物群落多样性和群落结构的影响。结果表明：围栏未显著影响草甸土壤原核微生物的丰富度、Shannon多样性和均匀度,但显著增加了草原土壤的原核微生物的丰富度、Shannon多样性和均匀度（P<0.05）,稍降低了荒漠草地土壤原核微生物的丰富度、Shannon多样性和均匀度（P=0.086、0.072和0.099）。在围栏处理的草地中,土壤原核微生物丰富度、Shannon多样性和均匀度与年均温、干旱度和pH显著负相关（P<0.01）,与年平均降水量、溶解性有机碳、地上生物量和植物多样性显著正相关（P<0.01）。在放牧处理的草地中,土壤原核微生物丰富度、Shannon多样性和均匀度与年均温和干旱度显著负相关（P<0.05）,但原核微生物丰富度和Shannon多样性与所有土壤理化和植被因素均无显著相关性。冗余分析（RDA）表明,不同类型草地土壤原核微生物群落结构发生了显著的变化,并沿草甸、草原和荒漠草地的过渡逐渐转变（P<0.001）。方差分解分析（VPA）进一步表明,原核微生物群落结构变化主要受年均温、年平均降水量、干旱度和pH的驱动。围栏显著改变了不同类型草地中部分样点土壤原核微生物群落结构。三种草地类型的主要原核微生物优势门均为放线菌门（Actinobacteria）、变形菌门（Proteobacteria）和酸杆菌门（Acidobacteria）。放线菌门（Actinobacteria）的相对丰度在荒漠草地土壤中最高,而变形菌门（Proteobacteria）和酸杆菌门（Acidobacteria）的相对丰度在草甸土壤中最高。此外,不同类型围栏和放牧草地土壤原核微生物类群的相对丰度均无显著差异。研究表明不同类型草地土壤原核微生物群落对围栏的响应不同,这为因地制宜制定草地管理措施提供了数据支持,为草地退化的防治提供了理论支持。     

The effect of grazing on bumblebees in the high rangelands of the eastern Tibetan Plateau of Sichuan

Many Tibetans who were formerly nomadic yak herders are settling near towns. However, yak remain a traditional symbol of wealth, which can lead to increased local stocking densities and severe over-grazing. We used belt-transect counts to study the area around Hongyuan in Sichuan. We find that intensive summer grazing significantly reduces the vegetation height and is associated with significant reductions in bumblebee-food-plant abundance and bumblebee diversity. For the significantly reduced bumblebee species, we identify the most frequently used and preferred bumblebee-food plants. For the food plants, we identify changes in absolute flower availability and changes in bumblebee visits. In particular, reductions in the bumblebees Bombus supremus, B. filchnerae, B. humilis and B. impetuosus are associated with reductions in flowers of the food plants Hedysarum and Saussurea.     

Methanotrophic community structure and activity under warming and grazing of alpine meadow on the Tibetan Plateau

Knowledge about methanotrophs and their activities is important to understand the microbial mediation of the greenhouse gas CH₄ under climate change and human activities in terrestrial ecosystems. The effects of simulated warming and sheep grazing on methanotrophic abundance, community composition, and activity were studied in an alpine meadow soil on the Tibetan Plateau. There was high abundance of methanotrophs (1.2–3.4 × 10⁸ pmoA gene copies per gram of dry weight soil) assessed by real-time PCR, and warming significantly increased the abundance regardless of grazing. A total of 64 methanotrophic operational taxonomic units (OTUs) were obtained from 1,439 clone sequences, of these OTUs; 63 OTUs (98.4%) belonged to type I methanotrophs, and only one OTU was Methylocystis of type II methanotrophs. The methanotroph community composition and diversity were not apparently affected by the treatments. Warming and grazing significantly enhanced the potential CH₄ oxidation activity. There were significantly negative correlations between methanotrophic abundance and soil moisture and between methanotrophic abundance and NH₄–N content. The study suggests that type I methanotrophs, as the dominance, may play a key role in CH₄ oxidation, and the alpine meadow has great potential to consume more CH₄ under future warmer and grazing conditions on the Tibetan Plateau.     

藏北高原草甸土壤固碳微生物群落特征随海拔和季节的变化

研究土壤固碳微生物丰度、群落结构、多样性差异及其影响因子对了解青藏高原土壤碳循环和固碳潜力具有重要意义。采用定量PCR(qPCR)、末端限制性片段分析(T-RFLP)、克隆文库和测序方法,研究了青藏高原草甸土壤固碳微生物丰度与群落结构随海拔和季节的变化,主要结果如下:1)随海拔升高高寒草甸土壤固碳微生物丰度显著升高,但季节变化不明显,不同类别微生物固碳基因cbbL丰度依次为:Form ICForm IABForm ID,其中Form IC类固碳微生物可达10~8拷贝数/g土壤,cbbL基因丰度与海拔、土壤含水量和铵态氮含量(NH_4~+-N)呈正相关关系,与土壤温度和pH值负相关;2)固碳微生物多样性和丰富度随海拔升高而升高,在4800m达到最大,且二者受季节影响较小,其群落结构随海拔升高而逐渐变化,主要受土壤pH值、海拔和土壤水分影响;3)Form IC类固碳微生物主要包括放线菌门和和变形菌门,其中α变形菌门是高寒草甸土壤优势固碳微生物类群。本研究有助于理解土壤微生物群落功能及其在土壤碳循环过程中的作用,为更准确评估高寒草甸土壤碳循环过程提供了科学依据。     

Impact of grazing on germination trait selection in an alpine grassland on the Tibet Plateau

放牧对青藏高原高寒草地种子萌发性状选择的影响 以前的研究表明放牧能够引起草地生物与非生物环境的显著变化,但这种变化影响草地群落对种子萌发特征的选择机制尚不清楚。因此,我们旨在回答：放牧是否对草地群落中萌发特征的组成和多样性产生显著影响。我们在实验室检测了研究草地群落内主要物种种子的萌发特性,并比较了这些植物在放牧和非放牧草地上的表现。在此基础上,比较了放牧草地和非放牧草地的各萌发性状的群落加权平均值和萌发性状多样性,从而了解放牧草地和非放牧草地是否存在不同的萌发性状结构。研究结果表明,在物种水平上,放牧和非放牧草地各物种的多度变化与物种的萌发性状无显著关系。但在群落水平上,与非放牧草地相比,放牧草地的物种普遍具有较高的种子萌发率;放牧草地种子萌发对变温的正响应显著大于非放牧草地,而且放牧草地种子萌发温度生态位宽度小于非放牧草地。与非放牧草地相比,放牧草地种子萌发性状多样性增加,萌发性状均匀度降低。放牧可以改变微生境,从而通过环境过滤改变草地群落对萌发性状的选择,导致草地群落的萌发性状于放牧前后在群落水平上存 在显著差异。     

青藏高原高寒草原生态系统土壤碳氮比的分布特征

利用67个样点数据,研究了青藏高原高寒草原生态系统土壤碳氮比的分布特征。结果表明:(1)在水平方向上,土壤碳氮比呈现出西北高、东南低的总体态势和斑块状交错分布的格局,碳氮比的高值区主要集中在藏北高原腹地和喜马拉雅山北麓湖盆区,不同草地型和不同自然地带土壤碳氮比差异显著;(2)土壤剖面自上而下,不同草地型碳氮比可分为低-高-低型、由高到低型、由低到高型、高-低-高-低型和高-低-高型等5个类型。表土层(0—20 cm)与底土层(30—40 cm)土壤碳氮比差异显著;(3)土壤碳氮比与与最冷月均气温、年均蒸发量、年均相对湿度和土壤全氮含量呈极显著正相关关系,而与年均日照时数、年均气温、速效钾含量呈极显著负相关关系,这些环境因素对土壤碳氮比影响从大到小的顺序是年均相对湿度年均日照时数最冷月均气温年均气温年均蒸发量土壤全氮含量土壤速效钾含量。     

Elevational diversity and distribution of ammonia-oxidizing archaea community in meadow soils on the Tibetan Plateau

Unraveling elevational diversity patterns of plants and animals has long been attracting scientific interests. However, whether soil microorganisms exhibit similar elevational patterns remains largely less explored, especially for functional microbial communities, such as ammonia oxidizers. Here, we investigated the diversity and distribution pattern of ammonia-oxidizing archaea (AOA) in meadow soils along an elevation gradient from 4400 m to the grassline at 5100 m on the Tibetan Plateau using terminal restriction fragment length polymorphism (T-RFLP) and sequencing methods by targeting amoA gene. Increasing elevations led to lower soil temperature and pH, but higher nutrients and water content. The results showed that AOA diversity and evenness monotonically increased with elevation, while richness was relatively stable. The increase of diversity and evenness was attributed to the growth inhibition of warm-adapted AOA phylotypes by lower temperature and the growth facilitation of cold-adapted AOA phylotypes by richer nutrients at higher elevations. Low temperature thus played an important role in the AOA growth and niche separation. The AOA community variation was explained by the combined effect of all soil properties (32.6%), and 8.1% of the total variation was individually explained by soil pH. The total AOA abundance decreased, whereas soil potential nitrification rate (PNR) increased with increasing elevations. Soil PNR positively correlated with the abundance of cold-adapted AOA phylotypes. Our findings suggest that low temperature plays an important role in AOA elevational diversity pattern and niche separation, rising the negative effects of warming on AOA diversity and soil nitrification process in the Tibetan region.     

Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

Impact of different sheep grazing intensities on salt marsh vegetation in northern Germany

Abstract. Mainland salt marshes in Schleswig-Holstein (northern Germany) have been grazed intensively by sheep for several decades. In 1988 experimental sites were established in the lower and middle salt marsh of Sönke-Nissen-Koog and subjected to different grazing intensities. From 1989 to 1993 the impact of sheep grazing on the composition and structure of the salt marsh vegetation was studied through the yearly analysis of permanent plots, vegetation mapping and measurements of the vegetation height. The intensively grazed site (10 sheep/ha) is covered by a short monotonous Puccinellia maritima sward with Salicornia europaea and Suaeda maritima. Halimione portulacoides and Aster tripolium, especially flowering plants, are rare. On the sites with 1.5 and 3 sheep/ha Puccinellia maritima remained dominant. The population density of Salicornia europaea decreases after reduction of the grazing intensity, whereas Suaeda maritima finds optimal growing conditions. Stands of Halimione portulacoides and flowering Aster tripolium plants are rare near the sea dike but their cover and size increases further away from the dike. In the plot with 1.5 sheep/ha the height of the vegetation increases along the gradient from the dike towards the tidal flats, due to local differences in actual grazing intensity. When grazing is stopped, Puccinellia maritima is successively replaced by Festuca rubra, Halimione portulacoides and Aster tripolium. High variability of vegetation height indicates structural diversity. Patches of higher and lower vegetation correspond with the distribution pattern of different plant species. In terms of nature conservation cessation of grazing is recommended.     

Feedback of grazing on gross rates of N mineralization and inorganic N partitioning in steppe soils of Inner Mongolia

Plant-microbe interactions are crucial regulators of belowground nitrogen cycling in terrestrial ecosystems. However, such interactions have mostly been excluded from experimental setups for the investigation of gross inorganic N fluxes and N partitioning to plants and microorganisms. Ungulate grazing is likely to feed back on soil N fluxes, and hence it is of special importance to simultaneously investigate grazing effects on both plant and microbial N fluxes in intact plant-soil systems, where plant-microbe interactions persist during the experimental incubation. Based on the homogenous ¹⁵NH ₄ ⁺ labelling of intact plant-soil monoliths we investigated how various stocking rates (0, 2.35, 4.8 and 7.85 sheep ha^?1 grazing season^?1) in steppe of Inner Mongolia feedback on gross rates of N mineralization and short-term inorganic N partitioning between plant, microbial and soil N pools. Our results showed that the effect of grazing on gross N mineralization was non-uniform. At low stocking rate gross N mineralization tended to decrease but increased with higher grazing pressure. Hence, there was no significant correlation between stocking rate and gross N mineralization across the investigated grazing intensities. Grazing decreased ¹⁵N recovery both in plant and microbial N pools but strongly promoted NO ₃ ^? accumulation in the soil and thus negatively affected potential ecosystem N retention. This appeared to be closely related to the grazing-induced decline in easily degradable soil C availability at increasing stocking rate.     

Different response of transient and persistent seed bank of alpine wetland to grazing disturbance on the Tibetan Plateau

Plant and Soil - With increased grazing disturbance of wetlands, soil moisture decreases, which could have an effect on the seed bank. The seed bank is a crucial resource for vegetation...