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Leaf δ13C is an indicator of water‐use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ13C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ13C values among dominant species reflected ecosystem patterns controlled by large‐scale environmental gradients, and that within‐species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species (Picea crassifolia, Abies fabri) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ13C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous

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Sources of competition for limited soil resources, such as nitrogen, include competitive interactions among different plant species and between plants and soil microbes. We hypothesized that plant interactions intensified plant competition for inorganic nitrogen with soil microorganisms. To test these competitive interactions, one dominant species (Kobresia humilis Serg) and one less abundant gramineous herb (Elymus nutans Griseb) in an alpine ecosystem were selected as target species to grow under interactions with their neighboring plants and without interaction treatments in field plots.15 Nlabeled ammonium and nitrate were used to quantify their partition between plants and soil microorganisms for 48 h after tracer additions. Responses of K. humilis to interactions from their surrounding plants were negative, while those of E. nutans were positive. Species identity, inorganic nitrogen forms, and plant interactions significantly affected the total amount of nitrogen utilization by soil microorganisms and plants. Although

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h i g h l i g h t sEcosystem respiration and its components were mainly controlled by temperature. Q 10 values varied widely with time and among ecosystem respiratory components. Summer flood duration could largely alter drawdown period carbon sink intensity. a r t i c l e i n f o a b s t r a c tLittle is known about the components of ecosystem respiration from a subtropical littoral wetland with dramatic annual inundation dynamics. In this study, we investigated ecosystem respiration and its components in a Poyang lake Carex meadow during the drawdown periods from May 2009 to June 2011. Both ecosystem respiration and its components showed clear temporal variation pattern, with temperature being the dominant control. Ecosystem respiration ranged from 98.01 to 1359.25 mg CO 2 m À2 h À1. Shoot and root respiration contributed approximately 36% and 26% to the ecosystem respiration, respectively, whereas microbial respiration accounted for 38% of the ecosystem respiration. The ratio of total soil respiration to ecosystem respiration varied from 0.45 to 0.90, depending on growing season stages. Their Q 10 values ranged from 1.72 to 2.51, with the maximum for shoot respiration and the minimum for microbial respiration. In addition, the Q 10 values varied with time and among ecosystem respiratory components and hence could not be treated as a constant. None of the respiration measurements was significantly related to soil moisture, suggesting that soil moisture was not a limiting environmental factor for respiratory activity during the drawdown periods in this meadow. The Carex meadow acted as strong carbon sink during the drawdown periods due to double growing seasons, but the previous summer flood duration could substantially alter carbon sink intensity in the following drawdown period. The total carbon sink of the littoral zone of Poyang Lake during drawdown periods was estimated to be 0.17e0.59 Tg C yr À1.

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Plant water sources were estimated by two or three compartment linear mixing models using hydrogen and oxygen isotope (δD and δ Water sources for broad-leaved and coniferous species were different although they grew in the same environmental conditions. For example, the broad-leaved species Q. aquifolioides used mainly the water from deep soil, while 92.5% of xylem water of the coniferous species P. tabulaeformis was derived from rainwater during the growth season. The study will be helpful for us to fully understand responses of species on the Tibetan Plateau to changes in precipitation patterns, and to assess accurately changes of vegetation distribution in the future.