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   2964篇
  免费   193篇
  国内免费   1篇
  2024年   2篇
  2023年   21篇
  2022年   31篇
  2021年   68篇
  2020年   40篇
  2019年   47篇
  2018年   82篇
  2017年   43篇
  2016年   91篇
  2015年   141篇
  2014年   157篇
  2013年   221篇
  2012年   272篇
  2011年   228篇
  2010年   166篇
  2009年   132篇
  2008年   195篇
  2007年   171篇
  2006年   160篇
  2005年   148篇
  2004年   131篇
  2003年   133篇
  2002年   141篇
  2001年   26篇
  2000年   11篇
  1999年   24篇
  1998年   26篇
  1997年   21篇
  1996年   31篇
  1995年   20篇
  1994年   31篇
  1993年   22篇
  1992年   13篇
  1991年   13篇
  1990年   11篇
  1989年   4篇
  1988年   7篇
  1987年   8篇
  1986年   4篇
  1985年   4篇
  1984年   9篇
  1983年   13篇
  1982年   5篇
  1981年   2篇
  1980年   7篇
  1979年   6篇
  1978年   7篇
  1974年   4篇
  1973年   2篇
  1964年   2篇
排序方式: 共有3158条查询结果,搜索用时 15 毫秒
881.
882.
John Clifton-Brown  Astley Hastings  Moritz von Cossel  Donal Murphy-Bokern  Jon McCalmont  Jeanette Whitaker  Efi Alexopoulou  Stefano Amaducci  Larisa Andronic  Christopher Ashman  Danny Awty-Carroll  Rakesh Bhatia  Lutz Breuer  Salvatore Cosentino  William Cracroft-Eley  Iain Donnison  Berien Elbersen  Andrea Ferrarini  Judith Ford  Jörg Greef  Julie Ingram  Iris Lewandowski  Elena Magenau  Michal Mos  Martin Petrick  Marta Pogrzeba  Paul Robson  Rebecca L. Rowe  Anatolii Sandu  Kai-Uwe Schwarz  Danilo Scordia  Jonathan Scurlock  Anita Shepherd  Judith Thornton  Luisa M. Trindade  Sylvia Vetter  Moritz Wagner  Pei-Chen Wu  Toshihiko Yamada  Andreas Kiesel 《Global Change Biology Bioenergy》2023,15(5):538-558
Demand for sustainably produced biomass is expected to increase with the need to provide renewable commodities, improve resource security and reduce greenhouse gas emissions in line with COP26 commitments. Studies have demonstrated additional environmental benefits of using perennial biomass crops (PBCs), when produced appropriately, as a feedstock for the growing bioeconomy, including utilisation for bioenergy (with or without carbon capture and storage). PBCs can potentially contribute to Common Agricultural Policy (CAP) (2023–27) objectives provided they are carefully integrated into farming systems and landscapes. Despite significant research and development (R&D) investment over decades in herbaceous and coppiced woody PBCs, deployment has largely stagnated due to social, economic and policy uncertainties. This paper identifies the challenges in creating policies that are acceptable to all actors. Development will need to be informed by measurement, reporting and verification (MRV) of greenhouse gas emissions reductions and other environmental, economic and social metrics. It discusses interlinked issues that must be considered in the expansion of PBC production: (i) available land; (ii) yield potential; (iii) integration into farming systems; (iv) R&D requirements; (v) utilisation options; and (vi) market systems and the socio-economic environment. It makes policy recommendations that would enable greater PBC deployment: (1) incentivise farmers and land managers through specific policy measures, including carbon pricing, to allocate their less productive and less profitable land for uses which deliver demonstrable greenhouse gas reductions; (2) enable greenhouse gas mitigation markets to develop and offer secure contracts for commercial developers of verifiable low-carbon bioenergy and bioproducts; (3) support innovation in biomass utilisation value chains; and (4) continue long-term, strategic R&D and education for positive environmental, economic and social sustainability impacts.  相似文献   
883.
Perennial energy crops (PECs) can reduce the negative impacts of intensive silage maize cultivation on agroecosystems in Central Europe. Furthermore, the remaining vegetation of PECs after harvest may provide suitable habitat and more beneficial overwintering conditions for arthropods than maize. It was hypothesized that after harvest and in winter, arthropod abundance and biomass are higher in PECs than in silage maize. In a field experiment arranged in a factorial split-plot design of eight main plots (plot size: 240 m2), the two PECs cup plant (Silphium perfoliatum L.) and field grass were compared with silage maize (Zea mays L.) regarding their suitability as autumn (post-harvest) and overwintering habitats for arthropods. Soil temperature, moisture as well as biomass and abundance of autumn-active and overwintering arthropods were analyzed for these three crops. Suction sampling was used during autumn and emerging arthropods were sampled with emergence trap sets in spring. In PEC plots, soils were moister and less exposed to cold temperatures than in silage maize. Compared with silage maize, total arthropod abundance and biomass were higher in PEC plots for both sampling periods. Results were similar for most examined arthropod taxa. The results of this study demonstrate that, compared with silage maize, PECs provide suitable post-harvest habitats and constitute more suitable overwintering habitats for arthropods. Differences are likely to be based on lack of disturbance and the provision of vegetation structures after harvest that function as overwintering habitats for arthropods. It can be concluded that the positive effects of PECs on ground arthropods are not limited to their growing time but continue to a certain extend after harvest and during winter.  相似文献   
884.
Due to its versatility and storability, biomass is an important resource for renewable materials and energy. Miscanthus hybrids combine high yield potential, low input demand, tolerance of certain marginal land types and several ecosystem benefits. To date, miscanthus breeding has focussed on increasing yield potential by maximising radiation interception through: (1) selection for early emergence, (2) increasing the growth rate to reach canopy closure as fast as possible, and (3) delayed flowering and senescence. The objective of this study is to compare early season re-growth in miscanthus hybrids cultivated across Europe. Determination of differences in early canopy development on end-of-year yield traits is required to provide information for breeding decisions to improve future crop performance. For this purpose, a trial was planted with four miscanthus hybrids (two novel seed-based hybrids M. sinensis × sinensis [M sin × sin] and M. sacchariflorus × sinensis [M sac × sin], a novel rhizome-based M sac × sin and a standard Miscanthus × giganteus [M × g] clone) in the UK, Germany, Croatia and Italy, and was monitored in the third and fourth growing season. We determined differences between the hybrids in base temperature, frost sensitivity and emergence strategy. M × g and M sac × sin mainly emerged from belowground plant organs, producing fewer but thicker shoots at the beginning of the growing season but these shoots were susceptible to air frosts (determined by recording 0°C 2 m above ground surface). By contrast, M sin × sin emerged 10 days earlier, avoiding damage by late spring frosts and producing a high number of thinner shoots from aboveground shoots. Therefore, we recommend cultivating M sac × sin at locations with low risk and M sin × sin at locations with higher risk of late spring frosts. Selecting miscanthus hybrids that produce shoots throughout the vegetation period is an effective strategy to limit the risk of late frost damage and avoid reduction in yield from a shortened growing season.  相似文献   
885.
The development of multipurpose crops will drive the transformation of agricultural “waste” into added-value products, helping to meet biomass demands without competing with food production or increasing environmental pressure. Lupinus mutabilis, has been proposed not only as a valid source of protein and oil for Europe but also as a possible source of lignocellulosic feedstock for the biorefinery industry. In this study, the quality of L. mutabilis lignocellulosic biomass and its genetic architecture are investigated for the first time, using a panel of 223 accessions planted across three locations in two different European cropping conditions. Biomass quality was evaluated based on the estimation of neutral detergent fiber, cellulose, hemicellulose and acid detergent lignin fractions, and on the basis of the monosaccharide composition of cell wall polysaccharides. The broad variation in yield and composition of biomass encountered in the panel confirms the potential of L. mutabilis as lignocellulosic feedstock and points out the value of this panel as a breeding tool for the improvement of biomass quality. A genome-wide association study was conducted to identify single-nucleotide polymorphisms (SNPs) associated with biomass quality, both across locations and per specific location. Scanning of 16,781 SNPs across the whole genome identified 46 unique quantitative trait loci for biomass quality, 4 of which were detected as common either among traits or GWAS models. For each of the traits analyzed, between 3 and 10 SNPs were detected, explaining 2.7%–15.9% of the phenotypic variation. Underlying these loci, 28 genes were proposed as candidate genes for biomass quality. Important genes involved in cellulose and sucrose synthesis (CESA4, SPP1,WRKY33, GONST2), monolignol biosynthesis (SKIP31, WAT1, CCR-SNL6) and pectin degradation (RAV1, PE) were identified and will require validation to confirm their value for application in L. mutabilis breeding.  相似文献   
886.
Summary The transfer of a Chl element, causing resistance to chloramphenicol in Streptomyces coelicolor A3(2), was studied in NF x SCP1 superfertile crosses. When the Chl element is on the donor side (NF) its transfer to the recombinant cells was virtually total as if the element acted as a second concomitant transfer origin. When the Chl element was on the recipient side (SCP1) it was never displaced by the immigrant chromosome even when the region facing chl + was selected for. A fraction of the original Chl mutants presented a requirement for arginine (ArgB). A Chl mutant gave rise spontaneously to ArgB derivatives at high frequency. The same ArgB requirement come out at high frequency among Chl derivatives from a cross NFChl x SCP1Chl+ in which neither parent required arginine or produced spontaneously arginineless derivatives. It is suggested that the Chl element is a transposable element (Tn) presumably associated with insertion sequences (IS). The insertional inactivation of the Chl element may be accompanied or followed by a deletion in the adjacent ArgB gene.  相似文献   
887.
Summary Concentrations of airborne pollens recorded in Trieste in 1989 are evaluated in relation to allergic complaints in 113 patients with skin prick tests positive to one pollen species. Analysis of the result enable the two most important allergens to be confirmed as: Poaceae in 70.7% of patients and Parietaria in 18.6%. Few people had monosensitation to Compositae (Artemisia), Corylaceae and Fagaceae.Sympotoms are related to the flowering period when pollen levels climbed to daily averages of 15–20 grains/m3 but they started later than airborne peak concentrations and finished later than pollen decline. Pollen concentration recording can be a useful way to predict the clinical manifestation in sensitive patients but other factors are involved in determining symptoms like subjective mucosal reactions, polysensitization, patients living and working environment.  相似文献   
888.
Riassunto Nell'ambito del progetto «Flora Palinologica Italiana» e proseguendo lo studio delle Corylaceae, è qui presentata la scheda diCarpinus betulus L., con alcune osservazioni sul numero/posizione delle aperture e sulle principali differenze tra polline «fresco» e polline acetolizzato.
Summary According to the program «Palynological Italian Flora», and continuing the study of the Italian Corylaceae, the palynological card ofCarpinus betulus L. is shown. The study is carried out on pollen coming from three different localities and regards both «fresh» and acetolyzed pollen. For each sample, measurements are carried out on 30 «fresh» pollen grains in glicerol jelly and on 30 acetolyzed pollen grains in water/glicerol 1/1; general observations regard 500 «fresh» and 500 acetolyzed pollen grain/sample. Some morphological observations on the aperture number/position and differencies between «fresh» and acetolized pollen are pointed out.
  相似文献   
889.
Dogs are the domestic reservoir of Leishmania infantum, a vector-borne intracellular protozoan agent of human visceral leishmaniasis. The role of polymorphonuclear leukocytes (PMNs) in the immune defence against this parasite has been poorly studied. We have investigated the function of peripheral blood PMNs in naive beagle dogs that have been naturally exposed to phlebotomine vectors in an area highly endemic for canine leishmaniasis, and found infected by Leishmania at the end of the transmission season. Whole blood phagocyte oxidative metabolism was assessed by a rapid method that determines a luminol-amplified chemiluminescence (CL) emission. This was evaluated using either a soluble stimulant, phorbol mirystate acetate (PMA), or phagocytic stimuli, such as zymosan unopsonized (ZYM) or opsonized with autologous serum (OPZ). In blood samples taken 2 months after exposure to Leishmania transmission, data on CL emission revealed a significant decrease of reactive oxygen intermediates (ROI) production in the presence of both PMA and ZYM, compared with blood samples obtained from dogs before exposure. On the contrary, no variations in CL emission were detected in presence of OPZ. Our data indicate that immunological changes occur early in canine leishmaniasis and confirm that the role of PMNs and their products need to be clarified. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   
890.
A method for estimating the general rate of nonpaternity in a population was validated using phenotype data on seven blood groups (A1A2BO, MNSs, Rh, Duffy, Lutheran, Kidd, and P) on 396 mother, child, and legal father trios from Nuevo León, Mexico. In all, 32 legal fathers were excluded as the possible father based on genetic exclusions at one or more loci (combined average exclusion probability of 0.694 for specific mother-child phenotype pairs). The maximum likelihood estimate of the general nonpaternity rate in the population was 0.118 ± 0.020. The nonpaternity rates in Nuevo León were also seen to be inversely related with the socioeconomic status of the families, i.e., the highest in the low and the lowest in the high socioeconomic class. We further argue that with the moderately low (69.4%) power of exclusion for these seven blood group systems, the traditional critical values of paternity index (PI ≥ 19) were not good indicators of true paternity, since a considerable fraction (307/364) of nonexcluded legal fathers had a paternity index below 19 based on the seven markers. Implications of these results in the context of genetic-epidemiological studies as well as for detection of true fathers for child-support adjudications are discussed, implying the need to employ a battery of genetic markers (possibly DNA-based tests) that yield a higher power of exclusion. We conclude that even though DNA markers are more informative, the probabilistic approach developed here would still be needed to estimate the true rate of nonpaternity in a population or to evaluate the precision of detecting true fathers. Am J Phys Anthropol 109:281–293, 1999. © 1999 Wiley-Liss, Inc.  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