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

Much effort has been spent in the last few decades to reconstruct the climate over China using a variety of historical documents. However, differences in the results of reconstructions exist even when people are using similar documents. In order to address this issue, 14 published temperature series by different studies were analyzed for coherence and mutual consistency. The analyses on their temporal fluctuations indicate that for the individual time series (standardized) on the 10-years time scales, 57 of the 91 correlation coefficients reach the significance level of 99%. The spatial patterns among the different time series also show high coherency. In addition, consistency also exhibit when comparing the reconstructions with other available natural climate change indicators. Above information was subsequently used to synthesize the temperature series for the last 500 and 1000 years.

Nitrogen (N) limitation is common in most terrestrialPlants and microorganisms compete for the same soil resources, but they are mutually dependent on each other 1 . Soil microorganisms need labile organic substances from plants in the form of litter and root exudates 2-4 to mineralize nutrients from organic to inorganic forms. Plants rely on nutrient supplies mediated by soil microorganisms 4-6 . Plant productivity and soil microbial activities are often tightly coupled, especially in nutrient-poor ecosystems 2,6 . Understanding how plants and microorganisms acquire limited nutrients from soils is essential for understanding carbon (C) and nitrogen (N) cycles.Nitrogen is a fundamental nutrient for plant growth and metabolism but limited in most terrestrial ecosystems 7 , causing strong competition for available N between roots and soil microorganisms 8,9 . Studies have explored plant-microbial competition for N to understand the mechanisms responsible for plant productivity 6 , species coexistence 10,11 , and ecological consequences of this competition in various terrestrial ecosystems. The consequences of competition often lead to: i) limitation on plant growth, ii) reduced microbial mineralization, and iii) increased competition for N between coexisting plant species.The old paradigm for terrestrial N cycling assumed that plants were only capable of using inorganic N (i.e., NH 4 + and NO 3 − ), mineralized by microorganisms from organic N forms. Many studies had investigated competition for inorganic N between plants and microorganisms 12-14 . However, some studies 15,16 also showed that plants could utilize organic N, such as free amino acids and peptides, found in the soil 17,18 . To understand

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

Leymus chinensis and Stipa grandis are two important plant species of temperate steppes in Inner Mongolia of North China. They differ in their life forms, e.g., L. chinensis is a type of rhizomatous clonal grass, whereas S. grandis is a type of tussock grass. Here we hypothesize that both plant species possess distinct nitrogen (N) acquisition strategies for their growth and survival. To test this hypothesis, we conducted a four-factor experimental field study using a short-term (three hours) 15 N labeling technique in two plant communities mono-dominated by L. chinensis and S. grandis of the temperate steppes over two months (July and August) and at two soil depths. In both of communities, L. chinensis and S. grandis directly absorbed all three of the common forms of N, including substantial portions of N-derived from glycine (organic and inorganic forms) ranged from 2.7 to 17.8 %, although they absorbed more inorganic N. Nitrogen uptake rates showed significant effects of communities, months, soil depths, and N forms. The uptake rate was higher in August than in July and at 0-5 cm than at 5-15 cm soil depths. L. chinensis and S. grandis showed different preference on N form across months. L. chinensis shifted its uptake pattern from more nitrate (NO 3 − ) in July to more ammonium (NH 4 + ) in August, whereas S. grandis took up comparable NH 4 + and NO 3 − in both months. In general, L. chinensis showed a more flexible N acquisition strategy and S. grandis performed a more concentrated and relatively more stable N acquisition strategy. The distinct N acquisition strategies used by L. chinensis and S. grandis varied greatly across different months and soil depths. These findings are more helpful in further understanding the plasticity of nutrient utilization issues of different plant species in response to N-limited conditions of grassland ecosystems.

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After converting cropland to forest, carbon is sequestered in the aggrading biomass of the new forests, but the question remains, to what extent will the former arable soil contribute as a sink for CO 2 ? Quantifying changes in soil carbon is an important consideration in the large-scale conversion of cropland to forest. Extensive field studies were undertaken to identify a number of suitable sites for comparison of soil properties under pasture and forest. The present paper describes results from a study of the effects of first rotation larch on soil carbon in seven stands in an afforestation chronosequence compared with adjacent Korean pine, pasture, and cropland. An adjacent 250-year-old natural forest was included to give information on the possible long-term changes in soil carbon in northeast China in 2004. Soil carbon initially decreased during the first 12 yr before a gradual recovery and accumulation of soil carbon occurred. The initial (0-12 yr) decrease in soil carbon was an average 1.2% per year among case studies, whereas the increase in soil carbon (12-33 yr) was 1.90% per year. Together with the carbon sequestration of forest floors, this led to total soil carbon stores of approximately 101.83 Mg/hm 2 over the 33-year chronosequence. Within the relatively short time span, carbon sequestration occurred mainly in tree biomass, whereas soil carbon stores were clearly higher in the 250-year-old plantation (184 Mg/hm 2 ). The ongoing redistribution of mineral soil carbon in the young stands and the higher soil carbon contents in the 250-year-old afforested stand suggest that nutrient-rich afforestation soils may become greater sinks for carbon (C) in the long term.Key words: afforestation; carbon sequestration; China; forest litter; mineral soil; Olga Bay larch. 2003), has been undertaken and 14.7 million hectares of cropland and 17.3 million hectares of degraded land are planned to be converted to forest over a 10-year period (Zhang 2003). Establishment of plantations on exagricultural land (afforestation) is generally recognized as a valid and potentially useful means to offset greenhouse gas emissions and is an eligible activity under Article 3.3 of the Kyoto Protcol (Paul et al. 2002).Approximately 75% of total terrestrial carbon (C) is stored in Huntington 1995). Therefore, even if afforestation only slightly affects soil C stocks at the local level, it could have a significant effect on the global C budget if enough agricultural land is converted (Low 1972;de Jong 1981;Elliott 1986;Janzen 1987; Monreal et al.1995). Following afforestation, changes inevitably occur in the quality, quantity, timing, and spatial distribution of soil C inputs (Paul et al. 2002). There are also many abiotic factors affecting the extent of the change in soil C, including site preparation, previous land use, climate, soil texture, site management, and harvesting (Paul et al. 2002).Johnson (1992) concluded that the reversion of former agricultural land to forest usually results in substantial increases in soi...

Aims Kin selection and resource partitioning have been proposed to explain interactions between plants growing with siblings (from the same mother). These mechanisms have been examined by measurements of fitness, phenotype or allocation traits, but have seldom been tested with N acquisition traits. Methods We determine if kin selection and resource partitioning are occurring using two annual species (Sorghum vulgare and Glycine max) with a short-term 15 N experiment. A mixture of ammonium, nitrate and glycine (1:1:1) was injected into soils around plants after they grew for 47 days. Only one nitrogen (N) form was 15 N labeled in each labeling solution. Results S. vulgare increased root allocation when growing with strangers (from the different mother), but not increase their N uptake. Although G. max strangers did not increase their root allocation, they significantly increased uptake of total N and the most abundant N form (nitrate) and decreased uptake of the least abundant (glycine). Conclusions G. max siblings reduced competition due to chemical resource partitioning while S. vulgare showed kin selection. We concluded that processes related to kin selection and resource partitioning can occur simultaneously, resulting in different competitive ability. These findings can improve our understanding of plants growing with siblings or strangers.