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   595篇
  免费   31篇
林业   61篇
农学   14篇
基础科学   4篇
  90篇
综合类   102篇
农作物   30篇
水产渔业   66篇
畜牧兽医   172篇
园艺   32篇
植物保护   55篇
  2022年   7篇
  2021年   12篇
  2020年   9篇
  2019年   21篇
  2018年   11篇
  2017年   11篇
  2016年   13篇
  2015年   17篇
  2014年   17篇
  2013年   23篇
  2012年   24篇
  2011年   28篇
  2010年   23篇
  2009年   19篇
  2008年   34篇
  2007年   30篇
  2006年   32篇
  2005年   28篇
  2004年   27篇
  2003年   21篇
  2002年   15篇
  2001年   5篇
  2000年   12篇
  1999年   7篇
  1998年   6篇
  1996年   7篇
  1995年   9篇
  1994年   4篇
  1993年   5篇
  1992年   5篇
  1991年   7篇
  1990年   8篇
  1989年   10篇
  1988年   3篇
  1987年   9篇
  1986年   8篇
  1985年   8篇
  1984年   12篇
  1983年   4篇
  1981年   4篇
  1980年   3篇
  1978年   8篇
  1977年   4篇
  1975年   3篇
  1974年   3篇
  1973年   6篇
  1972年   4篇
  1970年   4篇
  1969年   3篇
  1960年   3篇
排序方式: 共有626条查询结果,搜索用时 15 毫秒
121.
122.
We studied the influence of spent mushroom substrate (SMS) land application on water resources. Four study sites, including mushroom farms with low or high density land applications of SMS, and two controls, an alfalfa field and a woodland, were instrumented with soilwater lysimeters and groundwater monitoring wells. Water samples were collected during the dormant season (winter) and growing season (spring). Samples were analyzed for a number of water quality parameters, including dissolved organic carbon (DOC), dissolved organic nitrogen (DON), ammonia, chloride, nitrate, nitrite, phosphate, sulfate, aluminum, cadmium, calcium, chromium, copper, iron, lead, magnesium, manganese, mercury, nickel, potassium, silicon, sodium, and zinc. Additional analyses were performed for pesticides commonly used in the cultivation of alfalfa or corn, or for insect control, including methomyl, dimethoate, hexazinone, atrazine, diuron and permethrin.

All agricultural sites had elevated salt concentrations relative to the woodland site. The mushroom farm where SMS was applied in high concentrations had salt concentrations in the soilwater that were 10 to 100 times higher than the other agricultural sites. Of particular note were ammonium, nitrate, chloride, sulfate, calcium, magnesium, sodium, and potassium. Each of these were also elevated in the groundwater. The high salt concentrations were reflected in measurements of electrical conductivity. DOC and DON concentrations were also elevated in the soilwater and groundwater. Groundwater from each agricultural site, including the agricultural control, exceeded the primary drinking water standard for nitrate.

No pesticide residues were detected in well or lysimeter water collected at either site amended with SMS. Water samples collected from the woodland and at the alfalfa field not receiving SMS contained part per trillion quantities of a few pesticides.  相似文献   
123.
Soaking three beans cultivars (Phaseolus vulgaris L.: Great Northern, kidney, and pinto) in mixed salt solution (sodium chloride 2.5% + sodium bicarbonate 1.5% + sodium tripolyphosphate 1.0% + sodium carbonate 0.5%) resulted in 80%–85% reduction in cooking time over corresponding controls. Irradiation (-rays) at 500 krads of soaked and dehydrated beans caused a reduction of nearly 50% in cooking time. Water uptake and leaching losses for each treatment during soaking at 22°, 37°, and 45°C were investigated. High temperature (37° and 45°C) and pH (9.0) caused greater water imbibition and total solid loss than at room temperature (22°C). Organoleptic evaluation revealed that quick-cooking Great Northern beans appear to be more acceptable than kidney and pinto beans. Quick-cooking cooked beans had better in vitro protein digestibility than conventionally cooked beans. Phenolic content was found to be inversely related to in vitro digestibility.Utah Agricultural Experiment Station Journal article no. 2481.  相似文献   
124.
The present study was performed to investigate the possible effects of wheat germ supplementation on lipid metabolism in humans. Ten free-living adult subjects participated in the study. None was obese or diabetic. They all presented an, hypercholesterolemia (from 6.58 to 9.50 mM), associated in 6 over 10 cases to an hypertriglyceridemia (from 1.70 to 5.00 mM). The subjects were studied in three consecutive periods, during which they first were on their usual diet (first week), they then ingested a daily supplement of 30 g wheat germ (4 weeks) and then they returned to their usual basal diet (4 weeks follow-up). Dietary records were obtained for 7 and 3 consecutive days before and during wheat germ supplementation, respectively. Fasting blood samples were taken at the end of each period. After 4 weeks of wheat, germ intake, glycemia did not change while total plasma cholesterol significantly decreased (paired Student'st test,p0.05) from 7.80 to 7.15 mM. LDL and HDL cholesterol values did not show marked changes, but VLDL cholesterol significantly dropped by 40.6%. Thus, the plasma/HDL total cholesterol ratio was significantly lower. Apoprotein B and A1 decreased. In the hypertriglyceridemic subjects, this was accompanied by a significant reduction of plasma triglycerides (1.64 vs. 2.68 mM) and a marked drop of VLDL triglycerides (–51%). Taken as a whole, the present results obtained in humans are very close to those previously obtained in the rat [40] and point out that wheat germ may play a beneficial role in the dietary management of hyperlipidemia.Part of the data communicated in this paper was presented at the Fibre 90 Conference, 17–20 April 1990, Norwich, UK.  相似文献   
125.
Background : Rice production in low‐input systems of West Africa relies largely on nitrogen supply from the soil. Especially in the dry savanna agro‐ecological zone, soil organic N is mineralized during the transition period between the dry and the wet seasons. In addition, in the inland valley landscape, soil N that is mineralized on slopes may be translocated as nitrate into the lowlands. There, both in‐situ mineralized as well as the laterally translocated nitrate‐N will be exposed to anaerobic conditions and is thus prone to losses. Aim : We determined the dynamics of soil NO3‐N along a valley toposequence during the dry‐to‐wet season transition period and the effects of soil N‐conserving production strategies on the grain yield of rainfed lowland rice grown during the subsequent wet season. Methods : Field experiments in Dano (Burkina Faso) assessed during two consecutive years the temporal dynamics and spatial fluxes of soil nitrate along a toposequence. We applied sequential and depth‐stratified soil nitrate analysis and nitrate absorption in ion exchange resin capsules in lowlands that were open to subsurface interflow and in those where the interflow from the was intercepted. During one year only we also assessed the effect of pre‐rice vegetation on conserving this NO3‐N as well as on N addition by biological N2 fixation in legumes using δ15N isotope dilution. Finally, we determined the impact of soil N fluxes and their differential management during the transition season on growth, yield and N use of rainfed lowland rice. Results : Following the first rainfall event of the season, soil NO3‐N initially accumulated and subsequently decreased gradually in the soil of the valley slope. Much of this nitrate N was translocated by lateral sub‐surface flow into the valley bottom wetland. There, pre‐rice vegetation was able to absorb much of the in‐situ mineralized and the laterally‐translocated soil NO3‐N, reducing its accumulation in the soil from 40–43 kg N ha?1 under a bare fallow to 1–23 kg N ha?1 in soils covered by vegetation. Nitrogen accumulation in the biomass of the transition season crops ranged from 44 to 79 kg N ha?1 with a 36–39% contribution from biological N2 fixation in the case of legumes. Rice agronomic performance improved following the incorporation as green manure of this “nitrate catching” vegetation, with yields increasing up to 3.5 t ha?1 with N2‐fixing transition seasons crops. Conclusion : Thus, integrating transition season legumes during the pre‐rice cropping niche in the prevailing low‐input systems in inland valleys of the dry savanna zone of West Africa can temporarily conserve substantial amounts of soil NO3‐N. It can also add biologically‐fixed N, thus contributing to increase rice yields in the short‐term and, in the long‐term, possibly maintaining or improving soil fertility in the lowland.  相似文献   
126.
Papuamide A is representative of a class of marine derived cyclic depsipeptides, reported to have cytoprotective activity against HIV-1 in vitro. We show here that papuamide A acts as an entry inhibitor, preventing human immunodeficiency virus infection of host cells and that this inhibition is not specific to R5 or X4 tropic virus. This inhibition of viral entry was determined to not be due to papuamide A binding to CD4 or HIV gp120, the two proteins involved in the cell-virus recognition and binding. Furthermore, papuamide A was able to inhibit HIV pseudotype viruses expressing envelope glycoproteins from vesicular stomatitis virus or amphotropic murine leukemia virus indicating the mechanism of viral entry inhibition is not HIV-1 envelope glycoprotein specific. Time delayed addition studies with the pseudotyped viruses show that papuamide A inhibits viral infection only at the initial stage of the viral life cycle. Additionally, pretreatment studies revealed that the virus, and not the cell, is the target of papuamide A’s action. Together, these results suggest a direct virucidal mechanism of HIV-1 inhibition by papuamide A. We also demonstrate here that the other papuamides (B-D) are able to inhibit viral entry indicating that the free amino moiety of 2,3-diaminobutanoic acid residue is not required for the virucidal activity.  相似文献   
127.
The elongated encased spermatophores of the capsalid (entobdelline) monogeneans Neoentobdella diadema (Monticelli, 1902) Kearn et Whittington, 2005 and N. apiocolpos (Euzet et Maillard, 1967) Kearn et Whittington, 2005 have been found attached by their proximal ends to the region of the vaginal opening, with the bulk of the spermatophore projecting from the vagina and therefore lying outside the body. In spite of previous reports, no spermatophores were found projecting from the common genital opening and if spermatophore exchange is as rapid as it is in the related entobdelline Entobdella soleae, then the chances of finding a spermatophore in this location are small. In N. diadema and N. apiocolpos it is likely that sperm enters the vagina through the open proximal end of an attached spermatophore, after which the empty spermatophore case is probably discarded. There is no evidence for a previous proposal that the whole spermatophore is engulfed by the vagina followed by digestion of the case to release the sperm. Three specimens of N. diadema were found each with two spermatophore cases projecting from the vagina and a specimen of N. apiocolpos carried three cases. Assuming that each parasite is able to donate or receive only one spermatophore at each mating, then the presence of one spermatophore does not prevent a further mating and acceptance of a fresh spermatophore. In spite of differences between the spermatophores of E. soleae and N. diadema/N. apiocolpos, the events of spermatophore exchange may be similar.  相似文献   
128.
A 4-year-old female donkey residing in an open field in Indiana was admitted for evaluation of facial lesions of 2 years duration. Cytologic and histologic examination of exudate and tissue from the lesions revealed a pyogranulomatous inflammatory reaction with numerous yeasts. Sporothrix schenckii was suspected to be the infectious agent; however, multiple culture attempts did not provide positive identification of the organism. Serologic examination supported infection with S. schenckii. A specific direct immunofluorescent antibody test performed on paraffin-embedded tissue sections confirmed the organism as S. schenckii. Clinical signs resolved after appropriate iodide therapy.  相似文献   
129.
Nest predation is an important cause of mortality for many bird species, especially in grassland ecosystems where generalist predators have responded positively to human disturbance and landscape fragmentation. Our study evaluated the influence of the composition and configuration of the surrounding landscape on nest predation. Transects consisting of 10 artificial ground nests each were set up in 136 roadsides in six watersheds in south-central Iowa. Nest predation on individual roadside transects ranged from 0 to 100% and averaged 23%. The relationship of landscape structure within spatially-nested landscapes surrounding each roadside transect (within 200, 400, 800, 1200, and 1600 m of the transect line) to nest predation was evaluated by using multiple regression and canonical correlation analyses. The results of this multiscale landscape analysis demonstrated that predation on ground nests was affected by the surrounding landscape mosaic and that nest predators with different-sized home ranges and habitat affinities responded to landscapes in different ways. In general, wooded habitats were associated with greater nest predation, whereas herbaceous habitats (except alfalfa/pasture) either were associated with less nest predation or were not important. Different landscape variables were important at different spatial scales. Whereas some block-cover habitats such as woodland were important at all scales, others such as rowcrops and alfalfa/pasture were important at large scales. Some strip-cover habitats such as gravel roads and paved roads were important at small scales, but others such as wooded roadsides were important at all all scales. Most landscape metrics (e.g., mean patch size and edge density) were important at large scales. Our study demonstrated that the relationships between landscape structure and predator assemblages are complex, thus making efforts to enhance avian productivity in agricultural landscapes a difficult management goal.  相似文献   
130.
Soil enrichment was used to isolate soil microorganisms capable of degrading isopropyl carbanilate (propham), 3′,4′-dichloropropionanilide (propanil), 3′-chloro-2-methyl-p-valerotoluidide (solan), and methyl 3,4-dichlorocarbanilate (swep) in a muck and a silty clay loam. Degradation of the pesticides in enrichment solutions, and by pure cultures of effective microbial isolates was demonstrated by the production of the corresponding aniline, chloride ion liberation and disappearance of the parent compound. Degradation products were identified by gas-liquid and thin-layer chromatography.Organisms isolated include Pseudomonas striata Chester, Achromobacter sp., Aspergillus ustus (Bain) Thorn and Church, A. versicolor (Vuill. Tirabaschi), Fusarium oxysporum Schlecht, F. solani (Martius) Appel and Wollenweber, Penicillium chrysogenum Thorn, P. janthinellum Biourge, P. rugulosum Thorn and Trichoderma viride Pers. Each organism demonstrated a unique substrate specificity and was capable of degrading other aniline-based pesticides of the acetamide, acylanilide, carbamate, toluidine and urea classes.  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