全文获取类型
收费全文 | 142043篇 |
免费 | 6821篇 |
国内免费 | 73篇 |
专业分类
林业 | 6486篇 |
农学 | 4199篇 |
基础科学 | 929篇 |
18044篇 | |
综合类 | 24211篇 |
农作物 | 5909篇 |
水产渔业 | 6641篇 |
畜牧兽医 | 72054篇 |
园艺 | 1752篇 |
植物保护 | 8712篇 |
出版年
2018年 | 2104篇 |
2017年 | 2361篇 |
2016年 | 2224篇 |
2015年 | 1850篇 |
2014年 | 2205篇 |
2013年 | 5564篇 |
2012年 | 4156篇 |
2011年 | 4770篇 |
2010年 | 3066篇 |
2009年 | 2993篇 |
2008年 | 4455篇 |
2007年 | 4148篇 |
2006年 | 3798篇 |
2005年 | 4175篇 |
2004年 | 4148篇 |
2003年 | 3862篇 |
2002年 | 3371篇 |
2001年 | 4047篇 |
2000年 | 3990篇 |
1999年 | 3080篇 |
1998年 | 1293篇 |
1995年 | 1457篇 |
1994年 | 1348篇 |
1993年 | 1292篇 |
1992年 | 2784篇 |
1991年 | 2959篇 |
1990年 | 2939篇 |
1989年 | 2992篇 |
1988年 | 2696篇 |
1987年 | 2761篇 |
1986年 | 2892篇 |
1985年 | 2778篇 |
1984年 | 2273篇 |
1983年 | 2098篇 |
1982年 | 1365篇 |
1981年 | 1323篇 |
1979年 | 2290篇 |
1978年 | 1791篇 |
1977年 | 1536篇 |
1976年 | 1569篇 |
1975年 | 1654篇 |
1974年 | 2097篇 |
1973年 | 2085篇 |
1972年 | 2054篇 |
1971年 | 1904篇 |
1970年 | 1744篇 |
1969年 | 1691篇 |
1968年 | 1467篇 |
1967年 | 1561篇 |
1966年 | 1444篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Thomas R. Vale 《Biological conservation》1975,8(4):257-260
The Bureau of Land Management, a federal agency administering more than 72 million ha in the western United States, has often been criticised for allowing excessive livestock grazing and neglecting wildlife, watershed and recreation values on its lands. The first draft of a report by Bureau personnel evaluating the agency's policies in the State of Nevada supports the allegations. Congressional action is necessary to provide the Bureau with broader responsibilities, increased funding and greater authority, all of which are necessary to bring better balance to the Bureau's programme. 相似文献
992.
Frederick R. Gehlbach 《Biological conservation》1975,8(2):79-88
Texas experience suggests that amateur naturalists will be heavily involved in preliminary ecological surveys of natural areas, hence a simple key for identification of community-types is presented. Line intercept transects are suggested for amateur use in providing standardised quantitative data on the physiognomy of natural areas. A scheme for the evaluation of each area utilises the weighted values of climax condition, educational suitability, species significance, community representation, and human impact through multiplicative scoring to give a natural area score. With the weighted values-multiplicative scoring scheme, natural areas are clearly distinguished in priority for acquisition. 相似文献
993.
The inorganic forms of phosphorus in nine samples of surface soils from the Nainital Tarai of India were determined by a series of extractions with different reagents after incubation for 45 days under aerobic or anaerobic conditions. The forms of phosphorus in samples incubated under aerobic conditions and the pH range of 7.2–8.5 indicated that the soils had been subject to slight chemical weathering. Anaerobic incubation to simulate conditions in lowland rice paddies converted part of the inorganic phosphorus into more labile forms, based on changes in amounts of isotopically exchangeable phosphorus. Amounts of the element converted to more labile forms differed among the nine soil samples.A better correlation was found between amounts of phosphorus extracted by 0.5M NaHCO3 (Olsen's method) than by 0.03N NH4F in 0.25N HCl (Bray's method) and the isotopically exchangeable phosphorus. The better correlation suggests that the bicarbonate extraction would be the better method for estimating “availablelrd phosphorus in soils of lowland rice paddies. Recalibration of the test prior to such use seems desirable to allow for increases in amounts of phosphorus in labile forms after soils have been shifted from aerobic to anaerobic conditions. 相似文献
994.
Red and yellow earths are kaolinitic and generally sesquioxidic soils having massive B horizons with porous, earthy fabrics and weak profile differentiation but usually displaying a gradual increase in clay content with depth. They are widespread in tropical Queensland and northeast Brazil where comparable climates prevail although the vegetation is markedly different. Their parent materials are mostly quartzitic sedimentary rocks or unconsolidated sediments and the soils occur in a wide variety of topographic and geomorphic situations. In both regions it is probable that many of the soils have not formed under the present climatic conditions.Profile morphology, drainage, silt and clay contents, clay mineralogy, and soil chemistry have been compared and contrasted between the red and yellow earths within and between the two regions. Canonical variate analysis has shown that, on the basis of surface soil chemical attributes, the red earths can be separated from the yellow earths within each region and that the Queensland soils can be separated from the Brazilian soils. 相似文献
995.
Helvécio De-Polli Eiichi Matsui Johanna D?bereiner Eneas Salati 《Soil biology & biochemistry》1977,9(2):119-123
Intact soil cores containing plants of Paspalum notatum or Digitaria decumbens were selected with the acetylene reduction method, and then exposed to 15N15 to confirm nitrogen fixation in tropical grass-bacteria associations. In a preliminary experiment with P. notatum15N2 incorporation was slow but progressive during 24 h in roots but translocation to rhizomes and leaves ceased after 17h. With improved assay chambers, enrichments of 0.151 and 0.563 15N atom % excess were obtained in roots of D. decumbens cv transvala and P. notatum systems respectively, after 3 days. Enrichments in rhizomes were similar to those of roots; however in the leaves only 8% of root enrichment was observed. The addition of sucrose to the soil doubled N2-fixation in roots in both grass species studied, but did not result in increased incorporation into the leaves of P. notatum. 相似文献
996.
Amprolium, a coccidiostat added to poultry feed, occurs in excreta at concentrations of 204 μ g?1 and investigations were made of the effect of this quantity of amprolium on the biochemistry of soil to which manure had been applied.Greenhouse experiments showed that 0.8 μg g?1 amprolium was found in soil pots 80 days after treatment with manure at the equivalent of 56.1 t ha?1 and was detectable 20 days following treatment at the equivalent of 11.2 t ha?1.Laboratory experiments indicated that amprolium was differentially adsorbed to two complexing media, soil and manure. Since amprolium was a constituent of treated manure, it was expected that the amprolium manure-soil system would offer various sites for adsorption of amprolium. Mixing amprolium with soil and with soil plus untreated manure yielded approximately the same effect on amprolium adsorption based on recoveries in water solution, and as methanol extractable. However, total recovery of amprolium from treated manure added to soil was only a fraction of the above, indicating the high complexing capacity of the manure.No effect on soil respiration was observed by either pure amprolium or amprolium as a constituent of treated manure. The higher rate of manure application caused greater respiration due to the presence of more readily oxidizable organic matter, but the respiration pattern attributed to the manure component was not unlike the respiration pattern of the Guelph loam soil. 相似文献
997.
Reduction of N2O in moist soil was inhibited completely by 10?2 atm C2H2 and partially by 10?5 atm C2H2. The effect of C2H4 was 104 times less than that of C2H2. Denitrification of NO?3 occurred in anaerobically or aerobically incubated waterlogged soil and in anaerobic but not in aerobic moist soil. In the absence of C2H2 there was transient accumulation of N2O. In the presence of C2H2 there was stoichiometric conversion of NO?3 to N2O. Some kinetics of the reduction of N2O and of NO?3 to N2O are presented. Denitrification of 1 μg added NO?3-N.g? could be measured within 1 h. Stoichiometries of production of N2O from NO?2 and NO?3, respectively, and production of CO2 attributable to denitrification were consistent with reported energy yields. Reduction of C2H2 to C2H4 occurred immediately following complete denitrification of added NO?3. The incubation of soil in the presence and in the absence of C2H2 thus permits assay of both denitrification and N2 fixation and provides information on the mole fraction of N2O in the products of denitrification. 相似文献
998.
Water, Air, & Soil Pollution - The basis for the present energy dilemma is described. The current solution to the inadequate petroleum and natural gas supplies and possible long-term solutions... 相似文献
999.
A survey was made in 1972, of the status of Rhododendron pronticum L. at Winterton, Norfolk, and aerial photographs were used to assess its spread in recent years. The ecological implications of the continuing presence of this invsive alien shrub on the dunes are discussed. Comparisons with the highly successfull dune shrub Hippophaë rhamnoides L. help put into perspective the present and likely future role of Rhododendron at Winterton. The implications of controls are discussed together with methods by which this may be achieved. 相似文献
1000.