Spatial and Temporal Variations in Available Soil Nitrogen—A Case Study in Kobresia Alpine Meadow in the Qinghai-Tibetan Plateau, China

Elucidating the factors that determine the effects of temporal and spatial variation of nutrients is important for analyzing the characteristics of an ecosystem. The goal of this paper was to estimate how values obtained using a particular sampling approach correlated with the actual data for an entire plot. The mesh partition method was employed to divide an integrated observing field (IOF) located at the Haibei National Field Research Station of an alpine grassland ecosystem, China, into 25 subplots. Five of the 25 subplots were randomly selected for soil sampling and to determine the source of variations in soil nutrient content from 2001 to 2012. The results showed that, contributions of temporal and spatial variation in available nitrogen in the 0 - 10 cm soil layer accounted for 47.3% and 52.7%, respectively. The contribution of spatial variance was higher than that of temporal variance especially in the surface soil layers. The available soil nitrogen content in the alpine meadow was not obviously affected by fluctuations in rainfall and temperature. Increasing the number of samples could reduce calculation errors in measuring available soil nitrogen content, while collecting a reasonable number of samples can save time and labor.

Effect of Spraying Liquid Mineral Mixed Fertilizers on the Mineral Elements of Potted Poa crymophila cv. Qinghai

Objective] The aim was to explain the accumulation characteristic of mineral elements in alpine grassland plants and the effect of supplementary supply on the nutrient changes of mineral elements. [Method] Potted Poa crymophila cv. Qinghai plants were regularly applied with liquid mineral mixed fertilizers, and samples were collected for analysis and detection. [Result] After the regular spraying of liquid mineral mixed fertilizers, the mineral elements in potted P. crymophila and its soil reduced （ P 〈0.05）, but the accumulation of corresponding mineral elements of P. crymophila in the control group was increased due to the ＂Starvation Effect＂, which was one of the endogenetic forces driving the increase in the accumulation of mineral elements. [Conclusion] This study is of great significance for the study of the accumulation of mineral elements in degraded grassland plants and its dynamic mechanism, as well as testing the hypothesis of the Starvation Effect of mineral elements. In addition, it also provides scientific bases and technical support for the restoration and remediation of degraded grassland and the development of ecological livestock husbandry.

高寒小嵩草草甸可能成为青藏高原相同地理和气候单元下多种类型草地的偏途演替状态

高寒嵩草(Kobresia)草甸是青藏高原冬春放牧草场的主体,在放牧强度分异的条件下可以形成特征迥异的稳态。然而,目前还不清楚在不同放牧强度下其他类型草地是否可以演替成为这些稳态。本研究通过土壤剖面特征推断其历史植物群落特征,以植物群落现场调查确定其现实植物群落特征,以非结构式问卷调查确定草地放牧强度,采用多元回归树模型分析导致草地稳态转化的节点。研究结果表明,高寒灌丛草甸和旱化沼泽草甸在过度放牧条件下均可转化成为高寒矮嵩草(K.humilis)草甸和高寒小嵩草(K.pygmaea)草甸,从而使高寒灌丛草甸和旱化沼泽化草甸进入高寒矮嵩草草甸退化演替序列,最终转化成为黑土滩-杂类草次生裸地。以放牧强度为分节点,最多可以将这些植物群落划分为4类稳态,各稳态转化的节点依次为11.0、8.0、5.5羊/ha。所有研究草地总体表现为随放牧强度的增高地上总生物量呈下降趋势。其中,11.0羊/ha是草地转化成为黑土滩杂类草次生裸地的节点。放牧强度改变可以影响植物群落特征及其演替方向,进而改变草地的畜牧生产服务能力,因此放牧强度可以作为反映牧场畜牧生产服务能力的表观指标。未来牧场的可持续发展策略应以草畜平衡为主,合理控制放牧强度,增加草地生产附加值和延伸畜牧业产业链以实现区域草场生态和生产效益的双赢。

Self-stabilizing maintenance process in plant communities of alpine meadows under different grazing intensities

Backgrounds:Grazing prohibition and reduced grazing intensity,as two important“vegetation close-to-nature recovery”methods,have been suggested as economical and effective technologies for enhancing forage production.However,numerous studies have found that the yield of forage could be increased by removing or reducing grazing in a short time in some steady stage of alpine Kobresia meadows,but not in others.To reveal the mechanism behind this phenomenon,we proposed a series of experiments.Methods:We monitored the plant and soil characteristics in the key steady stages of Kobresia meadows under reduced and prohibited grazing conditions in the same geographic and climatic environments in the northeastern Qinghai-Tibet Plateau for 6 years.We estimated the relationships between the plant community and soil nutrients and obtained the following results.Results:All measured variables were positively correlated with each other.The plant community structure had higher path coefficients to aboveground biomass,soil organic matter,total nitrogen,and nitrate nitrogen than to other factors.The plant community structure played an important role in response to grazing intensity.Different plant functional groups(PFGs)had different responses to grazing intensity,which led to plant community re-establishment or re-organization under different grazing intensities.Poaceae and Kobresia were more sensitive to grazing intensity than other PFGs,and the ratio of Kobresia biomass(including Kobresia humilis and Kobresia pygmaea)to the total biomass of Poaceae and Kobresia could be used as an indicator of regime shifts within plant communities.With Kobresia pygmaea as the dominant species,the prohibition of grazing was not an efficient approach to increase the yield in the steady stages because this treatment needed more time to recover aboveground biomass.If Poaceae is the dominant PFG,grazing should only be prohibited for 3 years in the steady stages because the aboveground biomass will decrease if grazing is prohibited for more than 3 years.Conclusions:Therefore,the different steady stages of alpine meadows require different recovery methods to increase recovery efficiency and speed.

Synergies between microsites of plant communities and steady-stage alpine meadows on the Qinghai-Tibetan Plateau

Background:Due to the effects of climate change and overgrazing in recent decades,alternative stable states in the alpine Kobresia meadow degradation process have coexisted in the same geographical and climatic environment,with variations occurring among microsites.Methods:We used a space-for-time substitution approach to explore the synergies of microsite variation according to its numerical characteristics and the proportion of each stable state at various stages of succession in alpine Kobresia meadows on the Qinghai-Tibetan Plateau.Results:(1)The highest average aboveground biomass in summer was 196.2±20.3 gm^(-2),with significantly higher levels of biomass in≤3.65 sheep unit ha^(-1) than in other levels of grazing intensity,while the parameters showed no significant differences among grazing intensity levels in>3.65 sheep unit ha^(-1).(2)The importance of plant functional groups,aboveground biomass,and niche breadth of Poaceae and Cyperaceae significantly decreased as the grazing intensity increased.(3)The effects of≥0°C accumulated temperature,total precipitation,altitude,longitude,and latitude cumulatively contributed less than 20%of the variation in the distribution of functional group characteristics across microsites.Conclusions:(1)Overgrazing decreases primary production in alpine Kobresia meadows,but ecosystem responses regulate plant community structure and botanical components so as to partially counteract grazing disturbance.(2)Overgrazing changed the proportion of microsites,which in turn led to regime shift in the plant community and subsequent synergies between the microsites of plant communities and their stable states.

Phosphorus does not alleviate the negative effect of nitrogen enrichment on legume performance in an alpine grassland

Aims Nitrogen(N)-fixing legumes,despite being highly phosphorus(P)-demanding,constitute an important plant functional group and play key roles in N-poor ecosystems such as alpine grasslands.However,legume performance,including biomass,abundance and species richness,is expected to change,because anthropogenic activities have drastically increased soil N and P availability world-wide.We conducted a field experiment to assess the effects of N and P addition,alone and in combination,on legume performance in an alpine grassland,and identified and clarified the mechanisms underlying these changes.Methods A three year field experiment of N addition(10 g N m−2 year−1),P addition(5 g P m−2 year−1),and N+P combined addition(both N and P,same amounts as solo treatments)was conducted in an alpine grassland on the tibetan Plateau in china from 2011 to 2013.Effects of nutrient addition were assessed at the community level(above-ground net primary production(ANPP),height and light intensity),functional group level(biomass,species richness,relative height,relative coverage and relative density of legumes)and species level(foliar N,P concentration of two legumes).Important findings Overall,adding N alone significantly increased ANPP by 20.82%,but adding P alone did not;whereas,addition of N and P together resulted in a large increase in ANPP(+37.03%)than addition of either alone,indicating potential co-limitation of alpine grasslands.In contrast,adding P alone significantly promoted legume perfor-mance as measured by 65.22%increase in biomass and 58.45%increase in relative abundance,while adding N alone reduced leg-ume performance as measured by 39.54%decrease in biomass and 50.36%in relative abundance.combining P and N addition did not mitigate the negative effect of N addition on legume performance and,surprisingly,suppressed legume biomass by 53.14%and relative abundance by 63.51%.N and P addition altered the balance of light competition between grasses and legumes as indicated by the changes in light levels,plant heights and litter accumulation.However,there were no obvious changes in legume species richness in response to N and P within our experimental timeframe.this study provides further evidence of the importance of P as a co-limiting nutrient in alpine grasslands,contrary to the traditional view that N limitation predominates in such regions.the contrasting effects of N and P addition on legume performance provide important insights into potential changes in legume performance in nutrient-limited grasslands following N and P enrichment under climate change,with implications for nutrient management in alpine grasslands.

Characterizing nitric oxide emissions from two typical alpine ecosystems

portion of alpine meadows has been and will continue to be cultivated due to the concurrent increasing demands for animal-and crop-oriented foods and global warming.However, it remains unclear how these long-term changes in land use will affect nitric oxide(NO) emission. At a field site with a calcareous soil on the Qinghai-Tibetan Plateau,the authors measured the year-round NO fluxes and related variables in a typically wintergrazed natural alpine meadow(NAM) and its adjacent forage oat field(FOF). The results showed that long-term plow tillage, fertilization and growing forage oats significantly yielded ca. 2.7 times more(p < 0.01) NO emissions from the FOF than the NAM(conservatively 208 vs. 56 g N/(ha·year) on average). The spring freeze–thaw period and non-growing season accounted for 17%-35% of the annual emissions, respectively. The Q10 of surface soil temperature(Ts) was 8.9 in the NAM(vs. 3.8 in the FOF), indicating increases of 24%–93% in NO emissions per 1–3 °C increase. However, the warming-induced increases could be smaller than those due to land use change and management practices. The Tsand concentrations of ammonium, nitrate and water-extractable organic carbon jointly explained 69% of the variance in daily NO fluxes from both fields during the annual period(p < 0.001). This result indicates that temporally and/or spatially distributed NO fluxes from landscapes with calcareous soils across native alpine meadows and/or fields cultivated with forage oats can be predicted by simultaneous observations of these four soil variables.

Seasonal changes in soil properties,microbial biomass and enzyme activities across the soil profile in two alpine ecosystems

Microbial biomass and extracellular enzyme activities control the rate of soil organic carbon decomposition,thereby affecting soil carbon pool.However,seasonal dynamics of soil microbial properties at different depths of the soil profile remain unclear.In this study,we sampled soils in the early,middle and late growing season at different soil depths(0–100 cm)in two alpine ecosystems(meadow and shrubland)on the Tibetan Plateau.We measured plant belowground biomass,soil properties,microbial biomass and extracellular enzyme activities.We found that soil properties changed significantly with sampling time and soil depth.Specifically,most of soil properties consistently decreased with increasing soil depth,but inconsistently varied with sampling time.Moreover,root biomass and microbial biomass decreased with increasing soil depth and increased with sampling time during the growing season.However,microbial extracellular enzyme activities and their vector properties all changed with depth,but did not vary significantly with time.Taken together,these results show that soil properties,microbial biomass and extracellular enzyme activities mostly decline with increasing depth of the soil profile,and soil properties and microbial biomass are generally more variable during the growing season than extracellular enzyme activities across the soil profile in these alpine ecosystems.Further studies are needed to investigate the changes in soil microbial community composition and function at different soil depths over the growing season,which can enhance our mechanistic understanding of whole-profile soil carbon dynamics of alpine ecosystems under climate change.