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Rare species can switch between abundant and rare, respond to various external disturbances, enhance community resilience, and reduce the lag in ecosystem recovery. However, the impact of N enrichment on microbial community dynamic changes in rare species during the growing season is still poorly understood. To clarify which taxa contribute most to the community variation under N enrichment and whether the responses of these taxa vary at different periods of the growing season, we set up a 6-years multi-level field N addition experiment in an alpine meadow on the Tibetan Plateau. The variation in soil bacterial community diversity, composition, species interactions, and assembly process of different abundance profiles were investigated in the early, mid, and late-growing seasons. Approximately 80% of bacteria were classified as rare taxa (including permanently and conditionally rare taxa). The influence of N enrichment on the bacterial community was seasonally dependent. Bacterial community diversity and composition dissimilarity significantly increased only in the early growing season, especially in permanently rare taxa and conditionally rare taxa, but remained stable in the mid-and late-growing seasons. Seasonal fluctuations led to a shift in community composition, and conditionally rare taxa were the primary component of seasonal variation in community composition. Simultaneously, microbial network complexity and stability gradually increased over time, as evidenced by decreased modularity and vulnerability and increased robustness, clustering coefficient, connectivity, and keystone nodes. The assembly of conditionally rare taxa showed a significant difference under N addition, presented as decreased drift and increased dispersal limitation, which are closely related to the composition of the entire bacterial community. Our findings emphasize that the conditionally rare taxa decisive contribute to changes in the bacterial community under N enrichment, but seasonal fluctuations can make bacterial communities resistant to the influence of N enrichment. [ABSTRACT FROM AUTHOR]

Phenology is an important indicator of plant responses to environmental changes and is closely correlated with biomass production. However, how changes in phenological events affect plant biomass production when exposed to changing temperature and precipitation remain unclear.We conducted a 4‐year manipulative experiment of warming and precipitation addition to explore phenology–biomass interactions under climate change in a dry alpine meadow on the central Qinghai–Tibetan Plateau from 2015 to 2018.In dry and warm years, warming delayed phenology and precipitation addition advanced them. Warming decreased the biomass of Kobresia pygmaea in 2018 and the biomass of Poa pratensis in 2015, 2017 and 2018. However, precipitation addition significantly increased the biomass of Poa pratensis and Potentilla multifida in most of the experimental years. Phenological changes regulated the responses of biomass to treatments. Specifically, delay of green up of P. pratensis and delay of withering of K. pygmaea induced by warming can increase biomass production, but it can be offset by the direct negative effects of warming on biomass.Synthesis. Here we show how warming‐induced drought tend to decrease the biomass production of graminoids and the negative effects of warming on the biomass of P. pratensis and K. pygmaea were partially offset by green up postponement and withering postponement respectively. Our results highlights phenology is a crucial regulator for biomass production under climate change. Hence, both direct and indirect effects of warming and precipitation addition on phenology and biomass cannot be ignored when predicting biomass responses to climate change. [ABSTRACT FROM AUTHOR]

• Alpine meadow had higher CUE and lower WUE than cultivated grassland under warming. • Warming decreased biomass of cultivated grassland in the later of the experiment. • Warming increased ET, did not affect GEP, and decreased WUE in alpine meadow. • Warming decreased NEE, did not alter ER, and decrease CUE in cultivated grassland. Carbon and water use efficiency (CUE and WUE) are crucial indicators of grassland ecosystem responses to climate change. Grass planting is one of the dominant methods of grassland restoration and agricultural production. However, how warming and reclamation impact grassland CUE and WUE remain unclear. Here we examined how warming affected carbon and water processes in natural alpine meadow and cultivated grassland, including gross ecosystem production (GEP), net ecosystem CO 2 exchange (NEE), evapotranspiration (ET), CUE and WUE. We deployed a seven-year warming experiment using open top chambers in alpine meadow and cultivated grassland on the Tibetan Plateau. Our results showed that the alpine meadow had a higher CUE and a lower WUE compared to the cultivated grassland under warming. Warming increased biomass production of the cultivated grassland in the beginning of the experiment but decreased it later in the experiment. Warming increased ET, did not affect GEP, and finally decreased WUE in the natural alpine meadow. In the cultivated grassland, warming decreased NEE, did not alter ER, and resulted the decrease in CUE. Our results highlight that reclamation decreases CUE and warming decreased WUE of alpine meadow. Although grass planting can increase biomass, the negative impacts of warming on carbon and water processes of cultivated grassland should be considered when reclaiming alpine meadow under warmer future. [ABSTRACT FROM AUTHOR]

Alpine grasslands on the Qinghai-Tibetan Plateau are sensitive and vulnerable to climate change and human activities. Climate warming and overgrazing have already caused degradation in a large fraction of alpine grasslands on this plateau. However, it remains unclear how human activities (mainly livestock grazing) regulates vegetation dynamics under climate change. Here, alpine grassland productivity (substituted with the normalized difference vegetation index, NDVI) is hypothesized to vary in a nonlinear trajectory to follow climate fluctuations and human disturbances. With generalized additive mixed modelling (GAMM) and residual-trend (RESTREND) analysis together, both magnitude and direction of climatic (in terms of temperature, precipitation, and radiation) and anthropogenic impacts on NDVI variation were examined across alpine meadows, steppes, and desert-steppes on the Qinghai-Tibetan Plateau. The results revealed that accelerating warming and greening, respectively, took place in 76.2% and 78.8% of alpine grasslands on the Qinghai-Tibetan Plateau. The relative importance of temperature, precipitation, and radiation impacts was comparable, between 20.4% and 24.8%, and combined to explain 66.2% of NDVI variance at the pixel scale. The human influence was strengthening and weakening, respectively, in 15.5% and 14.3% of grassland pixels, being slightly larger than any sole climatic variable across the entire plateau. Anthropogenic and climatic factors can be in opposite ways to affect alpine grasslands, even within the same grassland type, likely regulated by plant community assembly and species functional traits. Therefore, the underlying mechanisms of how plant functional diversity regulates nonlinear ecosystem response to climatic and anthropogenic stresses should be carefully explored in the future. • Generalized additive model revealed nonlinear climatic and anthropogenic impacts on ecosystems. • Climate change dominantly drove grassland productivity changes on the Qinghai-Tibetan Plateau. • Human impacts on grassland productivity on the Qinghai-Tibetan Plateau weakened in past decades. [ABSTRACT FROM AUTHOR]

• Responses of green up to warming were different among years. • Water addition did not advance community phenology except in a dry and warm year. • Warming increased annual variability of plant green up and flowering. • Flowering of K. pygmaea advanced under warming plus water addition in all three years. • Soil moisture plays a critical role in determining phenological response to warming. Temperature and precipitation are primary regulators of plant phenology. However, our knowledge of how these factors might interact to affect plant phenology is incomplete. The Qinghai-Tibetan Plateau, a cold and high region, has experienced no consistent changes in spring phenology, despite a significant warming trend. We conducted a manipulative experiment of warming and precipitation addition in an alpine meadow on the Qinghai-Tibetan Plateau in 2015 (cold and wet), 2016 (warm and dry) and 2017 (mild and very wet). We found that warming increased annual variability of plant spring phenology. Warming delayed green up of all monitored species in 2016, advanced green up of early flowering species in 2015, and did not alter green up in 2017. For example, green up of the shallow rooted Kobresia pygmaea advanced 8 (± 2) days in 2015 and was delayed by 23 (± 3) days in a dry year (2016) under warming compared with control. Early spring precipitation addition can offset the delaying effects of warming in a dry year on the Qinghai-Tibetan Plateau. Under warming plus precipitation addition, community average green up advanced compared to control plots in 2015 and 2016, and community average flowering advanced for all three years. In 2016, flowering of K. pygmaea (an early flowering species) advanced under warming plus precipitation addition compared to control while flowering of other species did not change. Our results highlight that annual variation of soil moisture condition plays a critical role in determining the magnitude and direction of spring phenology response to warming. We provide insights in how plant spring phenology might change in a warmer future in the presence or absence of precipitation increase. [ABSTRACT FROM AUTHOR]