首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21篇
  免费   0篇
  2014年   1篇
  2013年   1篇
  2011年   1篇
  2010年   4篇
  2008年   2篇
  2007年   1篇
  2006年   1篇
  2005年   1篇
  2004年   1篇
  2002年   1篇
  1997年   2篇
  1996年   1篇
  1992年   2篇
  1985年   1篇
  1980年   1篇
排序方式: 共有21条查询结果,搜索用时 15 毫秒
1.
Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water‐limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (θ), soil respiration (Rs), along with leaf‐level photosynthesis (Anet), predawn leaf water potential (Ψpd), and seasonal aboveground net primary productivity (ANPP) of the dominant C4 grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil θ which corresponded with a significant increase in Rs and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.  相似文献   
2.
Using an insoluble inorganic salt precipitation technique, the permeability of cell walls and especially of endodermal Casparian bands (CBs) for ions was tested in young roots of corn (Zea mays) and rice (Oryza sativa). The test was based on suction of either 100 µm CuSO4 or 200 µm K4[Fe(CN)6] into the root from its medium using a pump (excised roots) or transpirational stream (intact seedlings), and subsequent perfusion of xylem of those root segments with the opposite salt component, which resulted in precipitation of insoluble brown crystals of copper ferrocyanide. Under suction, Cu2+ could cross the endodermis apoplastically in both plant species (although at low rates) developing brown salt precipitates in cell walls of early metaxylem and in the region between CBs and functioning metaxylem vessels. Hence, at least Cu2+ did cross the endodermis dragged along with the water. The results suggested that CBs were not perfect barriers to apoplastic ion fluxes. In contrast, ferrocyanide ions failed to cross the mature endodermis of both corn and rice at detectable amounts. The concentration limit of apoplastic copper was 0.8 µm at a perfusion with 200 µm K4[Fe(CN)6]. Asymmetric development of precipitates suggested that the cation, Cu2+, moved faster than the anion, [Fe(CN)6]4–, through cell walls including CBs. Using Chara cell wall preparations (‘ghosts’) as a model system, it was observed that, different from Cu2+, ferrocyanide ions remained inside wall-tubes suggesting a substantially lower permeability of the latter which agreed with the finding of an asymmetric development of precipitates. In both corn and rice roots, there was a significant apoplastic flux of ions in regions where laterals penetrated the endodermis. Overall, the results show that the permeability of CBs to ions is not zero. CBs do not represent a perfect barrier for ions, as is usually thought. The permeability of CBs may vary depending on growth conditions which are known to affect the intensity of formation of bands.  相似文献   
3.
Nitrous oxide emissions from a cropped soil in a semi-arid climate   总被引:5,自引:0,他引:5  
Understanding nitrous oxide (N2O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N2O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha?1 yr?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N2O using gas chromatography. Daily N2O emissions were low (?1.8 to 7.3 g N2O‐N ha?1 day?1) and culminated in an annual loss of 0.11 kg N2O‐N ha?1 from N‐fertilized soil and 0.09 kg N2O‐N ha?1 from nonfertilized soil. Over half (55%) the annual N2O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N2O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N2O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N2O emission, demonstrating that a proportion of N2O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N2O emissions.  相似文献   
4.
1. Whether life‐history traits can determine community composition and structure is an important question that has been well explored theoretically, but has received scant empirical attention. Life‐history traits of a seven‐member community of galler and parasitoid fig wasp species (Chalcidoidea), developing within the inflorescences (syconia) of Ficus racemosa (Moraceae) in India, were determined and used to examine community structure and ecology. 2. Gallers were pro‐ovigenic (all eggs are mature upon adult emergence) whereas parasitoids were synovigenic (eggs mature progressively during adult lifespan). Initial egg load was correlated with body size for some species, and there was a trade‐off between egg number and egg size across all species. Although all species completed their development and left the syconium concurrently, they differed in their adult and pre‐adult lifespans. Providing sucrose solutions increased parasitoid lifespan but had no effect on the longevity of some galler species. While feeding regimes and body size affected longevity in most species, an interaction effect between these variables was detected for only one species. 3. Life‐history traits of wasp species exhibited a continuum in relation to their arrival sequence at syconia for oviposition during syconium development, and therefore reflected their ecology. The largest number of eggs, smallest egg sizes, and shortest longevities were characteristic of the earliest‐arriving galling wasps at the smallest, immature syconia; the converse characterised the later‐arriving parasitoids at the larger, already parasitised syconia. Thus life history is an important correlate of community resource partitioning and can be used to understand community structure. 4. This is the first comprehensive study of life‐history traits in a fig wasp community. The comparative approach revealed constraints and flexibility in trait evolution.  相似文献   
5.
6.
The turions of Potamogeton crispus L. develop in early summer and function in propagation and dispersal. Under natural conditions during longday periods, an average minimum air temperature of more than 12°C was found to be important for turion formation. Experiments with controlled environments indicate that both temperature and photoperiod regulate turion formation. Turions can be induced at 13°C or above but not at 8 or 10°C. At a temperature range of 13–24°C turions form in both 12- and 16-h days, but not in 8-h days. By increasing temperature from 24 to 30 or 35°C turions can be induced under 8-h days. Light intensity was found to be important in the formation of turions.  相似文献   
7.
The effect of nutrients and their availability on the diet of the herbivorous Malabar giant squirrel Ratufa indica (Sciuridae) was investigated at Magod and Bhimashankar in western India. The daily consumption of food items (percent wet diet) and the intake rates of these items and the contained nutrients (wet g s-1) were determined by continuous observation of focal animals. Water content was a significant positive predictor of relative food item consumption while mineral contents in general had the opposite effect. The intake rates of water and more digestible nutrients such as nonstructural carbohydrates, as well as the intake rate of minerals such as zinc (at Magod), were significant positive predictors of the relative contribution of an item to the daily diet, thus indicating considerations of feeding costs versus benefits. Tannins, some alkaloids, and other secondary metabolites may negatively influence food choice. Protein content, relative to digestibility reducers, influenced food item consumption only at Bhimashankar. There was seasonal variation in daily biomass consumption. At Bhimashankar, daily biomass consumption increased with the proportion of fruit in the diet while this did not occur at Magod. This is perhaps a result of the higher water content and the lower content of some soluble nutrients within fruit at Bhimashankar relative to Magod. There was also intra-month variation between individual squirrels in daily biomass of food consumed. Squirrels consumed ephemeral food items opportunistically and non-ephemeral items such as mature leaves and bark on a regular daily basis. Squirrels probably obtained minerals and nitrogen from both fruit (especially seeds) and non-fruit sources (mature leaves and bark in the case of minerals, young and mature leaves in the case of nitrogen). However, they obtained lipid and non-structural carbohydrates mostly from fruit. Squirrels were selective in their utilization of tree species on a monthly basis, but this selectivity was not evident over a longer time period because individuals were constrained to use different phenological stages of tree species present within their territories. Large body size in this squirrel permits dietary flexibility and enables an overall generalist feeding strategy.  相似文献   
8.
An ethogram was developed for the foraging Cape pangolin in order to construct activity budgets. Behaviour was classified into seven categories and nine types of feeding sites were recorded. Activity budgets of animals foraging in different vegetation types are presented to illustrate the importance of vegetation on behaviour patterns and feeding site utilization. Availability of feeding sites may depend on the vegetation type with trees, shrubs and woody debris comprising most feeding sites. Seven prey species were recorded. One animal displayed strictly diurnal activity which suggests greater variability in activity patterns than previously thought.  相似文献   
9.
Cell wall activities which are related to the final stages oflignin biosynthesis, that is the generation of hydrogen peroxideand peroxidase activity, were investigated using biochemicaland histochemical methods. Peroxidases involved in both reactionsappeared to be restricted to lignifying tissues. Isolated cellwalls exhibited a very high affinity for syringaldazine. Cellwalls were able, in vitro as well as in vivo, to oxidize guaiacolin the absence of H2O2 when NADH was present in the incubationmedium together with p-coumarate and MnCl2 as cofactors. Theorigin of the NADH used to form H2O2 is discussed. Key words: Lignin biosynthesis, Oxidative polymerization, Peroxidases  相似文献   
10.
ABSTRACT. The cytidine triphosphate synthetase genes from three diverse strains of Giardia duodenalis have been sequenced and found to vary significantly from one another. The isolates were chosen as representatives of three demes as determined by several criteria including divergence in the rDNA repeat unit. Inserts in the genes and protein are conserved in length but are the most divergent regions among the three sequences examined. Variation in the rest of the gene occurs primarily in the third base position resulting in many silent mutations. One of the isolates (1709) was found to contain two genes with high sequence homology.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号