全文获取类型
收费全文 | 17489篇 |
免费 | 59篇 |
国内免费 | 2篇 |
专业分类
林业 | 3742篇 |
农学 | 1375篇 |
基础科学 | 152篇 |
3001篇 | |
综合类 | 784篇 |
农作物 | 2184篇 |
水产渔业 | 1865篇 |
畜牧兽医 | 1338篇 |
园艺 | 1128篇 |
植物保护 | 1981篇 |
出版年
2023年 | 8篇 |
2022年 | 32篇 |
2021年 | 34篇 |
2020年 | 36篇 |
2019年 | 34篇 |
2018年 | 2775篇 |
2017年 | 2734篇 |
2016年 | 1215篇 |
2015年 | 92篇 |
2014年 | 61篇 |
2013年 | 116篇 |
2012年 | 853篇 |
2011年 | 2193篇 |
2010年 | 2142篇 |
2009年 | 1298篇 |
2008年 | 1359篇 |
2007年 | 1597篇 |
2006年 | 68篇 |
2005年 | 120篇 |
2004年 | 125篇 |
2003年 | 185篇 |
2002年 | 90篇 |
2001年 | 39篇 |
2000年 | 63篇 |
1999年 | 21篇 |
1998年 | 10篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 5篇 |
1993年 | 15篇 |
1992年 | 8篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 22篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1977年 | 12篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1973年 | 9篇 |
1972年 | 5篇 |
1971年 | 6篇 |
1970年 | 6篇 |
1969年 | 8篇 |
1968年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Mary Nyawira Muchane Bashir Jama Caleb Othieno Robert Okalebo David Odee Joseph Machua Jan Jansa 《Agroforestry Systems》2010,78(2):139-150
A field study was carried out on a six-year-old on-farm field trial during long-rains season (April–August) 2003 to investigate
the effect of improved fallow systems and phosphorus application on arbuscular mycorrhiza fungi (AMF) symbiosis in maize.
The trial comprised of maize rotated with a fast growing leguminous Crotalaria
grahamiana fallow and a non-leguminous Tithonia diversifolia fallow for 3 years followed by continuous maize. The experiment was randomized complete block design with three cropping
(continuous maize, Crotalaria fallow and Tithonia fallow) systems and two phosphorus levels (0 and 50 kg P/ha). AMF colonization in maize roots, maize yield and macro-nutrients
uptake were recorded. Phosphorus applications improved (P < 0.05) early (<8 weeks old maize) AMF colonization, nutrient uptake and maize yield in improved fallow systems. Greater
differences due to phosphorus application were noted in maize in Tithonia fallow than in Crotalaria fallow. Following phosphorus application, a positive relationship existed between early AMF colonization and maize yield
(r = 0.38), and phosphorus and nitrogen uptake (r = 0.40 and r = 0.43, respectively), demonstrating the importance of phosphorus fertilization in enhancing low-input technologies (improved
fallows systems) in phosphorus deficient and acidic soils of western Kenya. 相似文献
992.
Fujio Hyodo Takashi F. Haraguchi Muneto Hirobe Ryunosuke Tateno 《Journal of Forest Research》2016,21(4):170-177
Forest development in temperate regions is considered to be a global carbon sink. Many studies have examined forest development after harvesting or fire from aboveground (e.g., biomass) or belowground (e.g., soil nutrient) perspectives. However, few studies have explored forest development from both perspectives simultaneously in cool-temperate forests in Japan. In this study, we examined changes over 105 years in both aboveground and belowground components during secondary natural succession. The aboveground biomass increased for 50 years and reached a plateau in a 105-year-old stand. The N mineralization rate increased during succession for 50 years, but showed a decline in the 105-year-old stand due to the decrease in the nitrification rate in late succession. The percent nitrification (i.e., relative contribution of nitrification to N mineralization) decreased significantly with increasing forest stand age. The N mineralization rates had significant relationships with N concentrations of the dominant tree foliage and litter fall and with the amount of litter fall N. Meanwhile, other belowground properties (i.e., soil pH, phenol concentration, soil microbial respiration, and litter mass loss) did not show any significant relationship with forest stand age. This may be because the soil at the study sites was heterogeneous and consisted of Cambisols and Andosols, the latter of which originally has high organic matter content, and thus may have buffered the effect of the aboveground development. These results indicate that belowground N dynamics are more closely associated with aboveground development than other belowground properties in these forests. 相似文献
993.
• Introduction
The annual development cycle of boreal and temperate trees results from an evolutionary trade-off between two opposing forces. These are namely, the adjustment of leaf phenology to the timing of frost occurrence at the beginning and/or the end of the growth season countered by an effective adjustment to the duration of the growth season to maximise photosynthesis and biomass production during the growing season. 相似文献994.
Dawn B. Woodard Robert L. Wolpert Michael A. O’Connell 《Journal of Agricultural, Biological & Environmental Statistics》2010,15(2):209-227
We develop a method for multiscale estimation of pollutant concentrations, based on a nonparametric spatial statistical model.
We apply this method to estimate nitrate concentrations in groundwater over the mid-Atlantic states, using measurements gathered
during a period of 10 years. A map of the fine-scale estimated nitrate concentration is obtained, as well as maps of the estimated
county-level average nitrate concentration and similar maps at the level of watersheds and other geographic regions. The fine-scale
and coarse-scale estimates arise naturally from a single model, without refitting or ad hoc aggregation. As a result, the
uncertainty associated with each estimate is available, without approximations relying on high spatial density of measurements
or parametric distributional assumptions. 相似文献
995.
E. Sunderasan Rusni A. Kadir Valérie Pujade-Renaud Frédéric de Lamotte H. Y. Yeang Sheila Nathan 《Journal of General Plant Pathology》2009,75(1):19-26
Corynespora leaf disease poses a serious threat to rubber cultivation because infected leaves develop necrotic lesions and
abscise, leaving the tree unproductive. The destructiveness of Corynespora cassiicola has been largely attributed to cassiicolin, a protein toxin secreted by the fungus. Recombinant antibody technology offers
hope to curtail the disease whereby single-chain variable fragments (scFv) specific to cassiicolin could bind and deactivate
the toxin in genetically modified rubber trees that harbour the antibody gene. A scFv phage library was constructed from heavy
and light variable chains of IgG from cassiicolin immunized Balb/C mice spleen. Biopanning of the phage library yielded a
scFv clone with high specificity to cassiicolin. The nucleotide sequence and deduced amino acid sequence information of the
scFv were obtained. Hemagglutinin (HA)-tagged scFv expressed in Escherichia coli is discerned as a band at ca. 30 kDa on SDS-PAGE, and the corresponding band was detected by anti-HA IgG on a Western immunoblot.
Deactivation of cassiicolin by the affinity-purified scFv was demonstrated in a detached-leaf bio-assay on selected susceptible
Hevea clones (PB 260, RRIM 2020, RRIM 901 and RRIM 929). The assay was also performed on clones that are relatively more resistant
to the fungus (RRIM 600 and GT-1), and the results were as expected. Thus, we have successfully demonstrated that the cassicolin-specific
scFv can effectively reduce cassicolin toxicity. 相似文献
996.
We investigated C management index (CMI; an indicator of sustainability of a management system and is based on total and labile
C) and soil aggregation in medium-textured soils (silt loam and silty clay loam) under different cropping systems as follows:
maize-wheat (M-W), rice-wheat (R-W), soybean-wheat (S-W), Guinea grass, and Setaria grass. Field experiments were 6–32 years
long and were located in the wet-temperate zone of northwest Himalayas. The plant nutrients were applied through chemical
fertilizers (urea, superphosphate, and muriate of potash) with or without organic materials (FYM, wheat straw, and Lantana spp.). The content of total C (CT), labile C (CL), CMI, mean weight diameter (MWD), and aggregate porosity varied significantly under different cropping systems. The range
was 1.59 (R-W)–4.29% (Setaria) for CT, 1.23 (R-W)–3.89 mg/kg (Guinea grass) for CL, 52.09 (R-W)–129.77 (Guinea grass) for CMI, 0.90 (R-W)–5.09 (Guinea grass) for MWD, and 41.5 (R-W)–56.8% (S-W) for aggregate
porosity. Aggregate porosity was highest (56.8%) under S-W, followed by grasses (50.1–51.2%), and M/R-W (41.5–50.0%). As per
these data, (a) continuous use of N alone as urea lowered soil sustainability over control (no fertilizers); (b) use of NPK
at recommended rates improved soil productivity over control; (c) the NPK + organic amendments further improved soil sustainability;
and (d) the sustainability under different cropping systems followed the order: perennial grasses > soybean-wheat > maize-wheat
> rice-wheat. 相似文献
997.
Ulrich Förstner 《Journal of Soils and Sediments》2007,7(4):270-270
998.
Shahbaz Khan Muhammad Nadeem Asghar Tariq Rana 《Water, air, and soil pollution》2007,182(1-4):131-148
Automated synoptic weather typing and robust orthogonal stepwise regression analysis (via principal components analysis) were applied together to develop within-weather-type air pollution prediction models for a variety of pollutants (specifically, carbon monoxide – CO, nitrogen dioxide – NO2, ozone – O3, sulphur dioxide – SO2, and suspended particles – SP) for the period 1974–2000 in south-central Canada. The SAS robust regression procedure was used to limit the influence of outliers on air pollution prediction algorithms. Six-hourly Environment Canada surface observed meteorological data and 6-hourly US National Centers for Environmental Prediction (NCEP) reanalysis data of various weather elements were used in the analysis. The models were developed using two-thirds of the total years for meteorological and air pollution data; the remaining one-third (randomly selected) was used for model validation. Robust stepwise regression analysis was performed to analytically determine the meteorological variables that might be used to predict air pollution concentrations. There was a significant correlation between observed daily mean air pollution concentrations and model predictions. About 20, 50, and 80% of the 80 prediction models across the study area possessed R 2 values ≥ 0.7, 0.6, and 0.5, respectively. The results of model validation were similar to those of model development, with slightly smaller model R 2 values. 相似文献
999.
Wataru Doi Than Than Lwin Masashi Yokota Carlos Augusto Strüssmann Seiichi Watanabe 《Fisheries Science》2007,73(2):331-340
ABSTRACT: Sexual maturity, morphological sexual dimorphism, and reproduction of the goneplacid crab Carcinoplax vestita were investigated in Tokyo Bay, Japan, from November 2002 to October 2003. The puberty molt in males was evidenced by changes in the relative size of the chelipeds and merus of the walking legs, and was estimated to occur at a size range of 13.20–18.85 mm carapace length. Post-pubertal females were identified by the relative size of the abdomen and puberty was estimated to occur at a size range of 12.81–15.46 mm carapace length. Sexual dimorphism in C. vestita was observed in all features that showed secondary sexual characteristics. Monthly changes of gonad index in males, and of seminal receptacle index and occurrence of sperm plugs in females were synchronized, and indicated that mating was intense in spring. Ovaries began developing in March. Ovigerous females were found in all months except December, but were clearly more abundant between August and October. Fecundity ranged from 7800 to 57 000 mature oocytes per female per batch and was highly correlated with body size. The results suggest that some females may spawn more than one batch per year. 相似文献
1000.
Biodiversity and its fragility in Yunnan,China 总被引:1,自引:0,他引:1
PU Ying-shan ZHANG Zhi-yi PU Li-na HUI Chao-mao 《林业研究》2007,18(1):39-47
In Yunnan, 8 major aspects of biodiversity and fragility in landforms, ecosystems, distribution populations, alien invasion, segregation, pollution and maladministration with various menace factors causing biodiversity loss have been described. It is revealed that the facts that the biodiversity and fragility coexists in this paper. Accordingly, 6 major countermeasures for effective conservation and rational utilization of the provincial biodiversity were suggested on the basis of the scientific development concepts, principles of nature protection, conservation biology, resource management and ethnobotany and present status in Yunnan with rich intangible resources such as climatic, ethnical and cultural diversity, etc. 相似文献