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   258篇
  免费   28篇
  国内免费   1篇
林业   29篇
农学   15篇
基础科学   1篇
  117篇
综合类   18篇
农作物   6篇
水产渔业   16篇
畜牧兽医   68篇
园艺   4篇
植物保护   13篇
  2023年   6篇
  2021年   10篇
  2020年   14篇
  2019年   10篇
  2018年   5篇
  2017年   9篇
  2016年   7篇
  2015年   8篇
  2014年   16篇
  2013年   19篇
  2012年   21篇
  2011年   15篇
  2010年   14篇
  2009年   12篇
  2008年   17篇
  2007年   14篇
  2006年   9篇
  2005年   13篇
  2004年   7篇
  2003年   10篇
  2002年   8篇
  2001年   9篇
  1999年   2篇
  1998年   1篇
  1997年   4篇
  1995年   7篇
  1993年   2篇
  1992年   1篇
  1990年   2篇
  1989年   1篇
  1980年   2篇
  1979年   1篇
  1976年   4篇
  1974年   1篇
  1965年   2篇
  1961年   1篇
  1960年   1篇
  1932年   2篇
排序方式: 共有287条查询结果,搜索用时 0 毫秒
281.
282.
283.
Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied 15N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added 15N label. In all treatments, microbes immobilized a major part of the added 15N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved 15N (TD15N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.  相似文献   
284.
Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5 cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5 cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.  相似文献   
285.
Near infrared (NIR) reflectance spectroscopy was tested as a fast method for characterizing the toxic effects of air pollution on trees. Fagus sylvatica was exposed to known, different levels of ozone during summer of 2003 in plant chambers. Leaves were taken from the plants and NIR spectra were recorded. In order to derive calibration models, reference analyses were carried out and soluble carbohydrates were determined by an enzymatic method. Minor components, like α-tocopherol, were measured after solvent extraction using gas chromatography-mass spectrometry. A partial least squares (PLS) algorithm was used to perform the chemometric analyses. Good or in some cases very good calibration statistics expressed in terms of root mean square error of cross validation and R2 were obtained for glucose, fructose, sucrose, and α-tocopherol. In addition, a better than expected correlation between the NIR spectra of the leaves and the ozone concentration in the plant chambers was observed. NIR spectroscopy appears to be a very useful method to simultaneously determine many components in leaves that are related to the health of trees. It is obviously able to quantitatively describe the changing patterns of constituents in leaves of trees caused by toxic substances. Thus, NIR spectroscopy can be a very effective tool for environmental biomonitoring, especially for observation of forests.  相似文献   
286.
Parasites are an important but neglected component of ecosystems that can be used as indicators of host biology and ecology. In the present study, the metazoan parasite assemblage of Ruvettus pretiosus, an understudied but widely distributed predatory gempylid, caught off South Africa was surveyed. A total of seven parasite taxa, including four new infection records (Bolbosoma capitatum, Rhadinorhynchus sp., Hepatoxylon trichiuri and Anisakis sp.), two new locality records (B. capitatum and Rhipidocotyle sp.) as well as the ectoparasitic copepod Sagum foliaceus and the cestode Tentacularia coryphaenae were recovered from the eight specimens examined.  相似文献   
287.
Management intensity modifies soil properties, e.g., organic carbon (Corg) concentrations and soil pH with potential feedbacks on plant diversity. These changes might influence microbial P concentrations (Pmic) in soil representing an important component of the P cycle. Our objectives were to elucidate whether abiotic and biotic variables controlling Pmic concentrations in soil are the same for forests and grasslands, and to assess the effect of region and management on Pmic concentrations in forest and grassland soils as mediated by the controlling variables. In three regions of Germany, Schwäbische Alb, Hanich‐Dün, and Schorfheide‐Chorin, we studied forest and grassland plots (each n = 150) differing in plant diversity and land‐use intensity. In contrast to controls of microbial biomass carbon (Cmic), Pmic was strongly influenced by soil pH, which in turn affected phosphorus (P) availability and thus microbial P uptake in forest and grassland soils. Furthermore, Pmic concentrations in forest and grassland soils increased with increasing plant diversity. Using structural equation models, we could show that soil Corg is the profound driver of plant diversity effects on Pmic in grasslands. For both forest and grassland, we found regional differences in Pmic attributable to differing environmental conditions (pH, soil moisture). Forest management and tree species showed no effect on Pmic due to a lack of effects on controlling variables (e.g., Corg). We also did not find management effects in grassland soils which might be caused by either compensation of differently directed effects across sites or by legacy effects of former fertilization constraining the relevance of actual practices. We conclude that variables controlling Pmic or Cmic in soil differ in part and that regional differences in controlling variables are more important for Pmic in soil than those induced by management.  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