全文获取类型
收费全文 | 402篇 |
免费 | 19篇 |
专业分类
林业 | 44篇 |
农学 | 3篇 |
基础科学 | 1篇 |
245篇 | |
综合类 | 31篇 |
农作物 | 14篇 |
水产渔业 | 11篇 |
畜牧兽医 | 35篇 |
园艺 | 6篇 |
植物保护 | 31篇 |
出版年
2023年 | 5篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 3篇 |
2019年 | 7篇 |
2018年 | 2篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 13篇 |
2014年 | 13篇 |
2013年 | 17篇 |
2012年 | 22篇 |
2011年 | 23篇 |
2010年 | 19篇 |
2009年 | 29篇 |
2008年 | 41篇 |
2007年 | 32篇 |
2006年 | 25篇 |
2005年 | 16篇 |
2004年 | 21篇 |
2003年 | 19篇 |
2002年 | 14篇 |
2001年 | 10篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1983年 | 3篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有421条查询结果,搜索用时 93 毫秒
361.
362.
Rainer Petzold Kai Schwärzel Karl-Heinz Feger 《European Journal of Forest Research》2011,130(5):695-706
The aim of this study was to investigate transpiration and its main driving factors on the example of a hybrid poplar plantation
with the clone Populus maximowiczii × P. nigra, cv. Max 1 on a site in the hilly loess region of Saxony (Germany). Transpiration was measured using sap flow techniques
during the 2007 and 2008 growing season. At the same time, throughfall, soil moisture dynamics and soil physical properties
were also measured. Total transpiration rates amounted to 486 mm and 463 mm, respectively, during the 2 years. Maximum daily
transpiration rates reached 6.7 mm/day, while an average of 2.2 mm/day for the entire growing season was recorded. The main
controlling factors for stand transpiration included the evaporative demand, water availability and soil temperature. The
information was implemented into a simple empirical model for the prediction of transpiration. It can be concluded that large-scale
establishment of poplar plantations will result in a distinct reduction in groundwater recharge. On the other hand, surface
run-off and soil erosion may decrease. Due to limited water availability in the late growing season, the growth potential
of the tested clone cannot fully be exploited at many sites in Germany. 相似文献
363.
This study examines farmers’ knowledge and management of soil fertility and vegetation in Vitellaria paradoxa and Cordyla pinnata dominated agroforestry parklands in a Sudano-Sahelian region of Mali. Qualitative data from a series of semi-structured interviews
were used to construct an empirically based typology of farmer practices. These interrelated practices center on famers’ agroecological
knowledge, decision-making, and cropping strategies and are conceptualized as variable and adjustable socioecological responses
to environmental and socioeconomic contingencies. The typology categorizes farmers in terms of their capacity for managing
parkland soil and tree resources in an ecologically sustainable manner while negotiating livelihood challenges. It identifies
the ideal farmer as one who practices the following: (1) use of environmental knowledge that considers multidimensional relationships
among ecological and social components of parkland systems, (2) agricultural techniques that adaptively combine aspects of
intensive and traditional extensive agriculture, and (3) natural resource decision-making that explicitly includes women’s
environmental knowledge and livelihood strategies. 相似文献
364.
Martin-Jurado O Vogt R Kutter AP Bettschart-Wolfensberger R Hatt JM 《American journal of veterinary research》2008,69(10):1254-1261
OBJECTIVE: To determine the effect of inhalation of isoflurane at end-tidal concentrations greater than, equal to, and less than the minimum anesthetic concentration (MAC) on bispectral index (BIS) in chickens. Animals-10 chickens. PROCEDURES: For each chicken, the individual MAC of isoflurane was determined by use of the toe-pinch method. After a 1-week interval, chickens were anesthetized with isoflurane at concentrations 1.75, 1.50, 1.25, 1.00, and 0.75 times their individual MAC (administered from higher to lower concentrations). At each MAC multiple, a toe pinch was performed and BIS was assessed and correlated with heart rate, blood pressure, and an awareness score (derived by use of a visual analogue scale). RESULTS: Among the chickens, mean +/- SD MAC of isoflurane was 1.15 +/- 0.20%. Burst suppression was detected at every MAC multiple. The BIS and awareness score were correlated directly with each other and changed inversely with increasing isoflurane concentration. Median (range) BIS values during anesthesia at 1.75, 1.50, 1.25, 1.00, and 0.75 MAC of isoflurane were 25 (15 to 35), 35 (25 to 45), 35 (20 to 50), 40 (25 to 55), and 50 (35 to 65), respectively. Median BIS value at extubation was 70 +/- 9. Values of BIS correlated with blood pressure, but not with heart rate. Blood pressure changed with end-tidal isoflurane concentrations, whereas heart rate did not. CONCLUSIONS AND CLINICAL RELEVANCE: Assessment of BIS can be used to monitor the electrical activity of the brain and the degree of unconsciousness in chickens during isoflurane anesthesia. 相似文献
365.
William R. Caine Willem C. Sauer Guishan S. Huang Gerd Diebold Margit Schollenberger Rainer Mosenthin 《Livestock Science》2008,116(1-3):300-308
The objective of this study was to determine the effect of guanidination, the conversion of dietary lysine to homoarginine, on the apparent ileal digestibilities (AID) of amino acids (AA) from 3 protein sources in diets for pigs. Six barrows, initial body weight of 18 ± 1.5 kg, fitted with a simple T-cannula at the distal ileum, were fed one of three corn starch-based diets that contained 18% crude protein (CP) from soybean meal, rapeseed meal or peas for an experimental period of 10 d according to a repeated 3 × 3 Latin square design. On d 9 of each experimental period, the pigs were fed one meal of the guanidinated test meals, which corresponded to their respective diets. In the test meals, 50% of the protein source in each diet was replaced, on an equal weight basis, with a guanidinated source. Guanidination increased the content of CP by 190, 116 and 28 g kg− 1 for soybean meal, rapeseed meal and peas, respectively. With the exception of lysine, the AID of CP and AA were 2.2 to 8.2 percentage units (pu) higher in the test meal than in the soybean meal diet. Other than arginine and isoleucine, the differences were significant (P < 0.05) for all indispensable AA. With the exception of lysine, the AID of CP and AA were 3.8 to 11.9 pu higher in the test meal than in the diet with peas. The differences were significant (P < 0.05) for all indispensable AA. Other than lysine, glycine and proline, the AID of CP and AA were slightly higher, 0.1 to 4.3 pu, in the test meal than in the rapeseed meal diet. Of the indispensable AA, the differences were significant (P < 0.05) for arginine, leucine and phenylalanine. The AID of lysine was 10.9 pu lower (P < 0.05) in the test meal than in the rapeseed meal diet, 61.5 vs. 72.4%, indicating that guanidination of rapeseed meal was not uniform and random. The increase in the AID of CP and AA after guanidination may result from solubilisation of protein and degradation of antinutritional factors such as fiber, trypsin inhibitors and tannins. 相似文献
366.
Joachim Katzur Klaus Fischer Lutz Böcker Falk Liebner Rainer Schiene 《Archives of Agronomy and Soil Science》2013,59(6):639-653
Die bodenmeliorativen und ertragssteigernden Effekte des Einsatzes von N-modifizierter Weichbraunkohle als Humusdüngestoff bei der landwirtschaftlichen Rekultivierung humusfreier Kippenböden werden in mehreren Freilandversuchen überprüft. In diesem Beitrag wird über die zu Rekultivierungsbeginn mit W.-Roggen, W.-Weizen, Kartoffeln und Luzerne erzielten Ergebnisse berichtet. 相似文献
367.
Michael Dannenmann Rainer Gasche Astrid Ledebuhr Thomas Holst Helmut Mayer Hans Papen 《European Journal of Forest Research》2007,126(2):331-346
The effect of forest management (thinning) on in situ carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) trace gas exchange between soil and atmosphere was studied in three consecutive years at three beech forest sites, which
differ in aspect [southwest (SW), northeast (NE), northwest (NW)]. At all sites adjacent thinning plots (“T”) and untreated
control plots (“C”) were established. Measurements at the SW and NE sites covered the years 4–6 after thinning while at the
NW site measurements covered the year before and the first 2 years after thinning. Mean N2O fluxes were <3 μg N2O–N m−2 h−1 at all plots except for the newly thinned NWT plot. CH4 uptake was rather low, too. Very low CH4 oxidation rates during dry periods are explained by physiological drought stress for CH4 oxidizers. Heterotrophic litter decomposition constitutes the largest part of total soil respiration. On the whole, no significant
positive or negative effects of the silvicultural treatment on the magnitude of CO2-, CH4- and N2O-trace gas exchange could be observed at the SW site 4–6 years after thinning. Also at the NE site, no effects of thinning
on CO2 and N2O fluxes could be demonstrated. However, at this site a significant moisture-induced lower CH4 uptake could be shown. At the NW site forest management led to a dramatic increase in N2O emissions in the first two summers after thinning and to distinct effects on CO2 emissions and CH4 uptake in the first year after the felling. The unambiguous effects of thinning at the NW site are mainly related to higher
C input by dead residues leading to enhanced mineralization activity, to a shift in the competition for nutrients favoring
microorganisms as compared to trees and to changes in the soil water availability at the thinned plot. Considering the data
obtained from the NE and SW site we expect that with the development of an understorey vegetation at the NW site the observed
effects on the magnitude of trace gas exchange due to thinning will continue to decline in the following years. Our results
implicate that it is indispensable to take account of the effects of forest management in order to accurately calculate trace
gas emission inventories for the investigated forest ecosystem in case thinning took place immediately before. 相似文献
368.
Arthur Geßler Klaus Jung Rainer Gasche Hans Papen Anita Heidenfelder Eric Börner Berthold Metzler Sabine Augustin Ernst Hildebrand Heinz Rennenberg 《European Journal of Forest Research》2005,124(2):95-111
The effects of local climate and silvicultural treatment on the inorganic N availability, net N uptake capacity of mycorrhizal beech roots and microbial N conversion were assessed in order to characterise changes in the partitioning of inorganic N between adult beech and soil microorganisms. Fine root dynamics, inorganic N in the soil solution and in soil extracts, nitrate and ammonium uptake kinetics of beech as well as gross ammonification, nitrification and denitrification rates were determined in a beech stand consisting of paired sites that mainly differed in aspect (SW vs. NE) and stand density (controls and thinning treatments). Nitrate was the only inorganic N form detectable in the soil water. Its concentration was high in control plots of the NE aspect, but only in canopy gaps and not influenced by thinning. Neither thinning nor aspect affected the abundance of root tips in the soil. Maximum nitrate net uptake by mycorrhizal fine roots of beech, however, differed with aspect, showing significantly lower values at the SW aspect with warm–dry local climate. There were no clear-cut significant effects of local climate or thinning on microbial N conversion, but a tendency towards higher ammonification and nitrification and lower denitrification rates on the untreated controls of the SW as compared to the NE aspect. Apparently, the observed sensitivity of beech towards reduced soil water availability is at least partially due to impaired N acquisition. This seems to be mainly a consequence of reduced N uptake capacity rather than of limited microbial re-supply of inorganic N or of changed patterns of inorganic N partitioning between soil bacteria and roots. 相似文献
369.
Elevated concentrations of carbon dioxide ([CO2]) and ozone ([O3]) affect primary metabolism of trees in opposite ways. We studied their potential interactions on carbohydrate concentrations and contents. Two hypotheses currently under debate were tested. (1) Stimulation of primary metabolism by prolonged exposure to elevated [CO2] does not compensate for the adverse effects of O3 on carbohydrate accumulation and biomass partitioning to the root. (2) Growth in a mixed-species planting will repress plant responses to elevated [O3] and [CO2] relative to conditions in a monoculture. To this end, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) saplings grown under conditions of intra- and interspecific competition were pre-acclimated for 1 year to ambient or elevated [CO2]. In the following 2-year phytotron study, trees were exposed to factorial combinations of ambient and elevated [O3] and [CO2]. The total carbohydrate content (sugar and starch) of spruce was greater in plants exposed to elevated [CO2] than in plants exposed to ambient [CO2]. In beech, the opposite response was observed, especially when this species was grown in combination with spruce. Overall, the data did not support Hypothesis 1, because the adverse effects of O3 were counteracted by elevated [CO2]. Support for Hypothesis 2 was species-dependent. In beech saplings, reduction of carbohydrates by elevated [O3] and stimulation by elevated [CO2] were repressed by competitive interaction with spruce. In contrast, in spruce, stimulation of carbohydrates by elevated [CO2] was similar in mono- and mixed cultures. Thus Hypothesis 2 was supported for beech but not spruce. We conclude that, in juvenile beech and spruce, a 3-year exposure to elevated [CO2] counteracts the adverse effects of O3 on carbohydrate concentrations and contents. For beech, sensitivity to elevated [CO2] and [O3] was high in monoculture but was largely repressed by interspecific competition with spruce. In contrast, the response of spruce to perturbations of atmospheric chemistry was not significantly affected by either intra- or interspecific competition. 相似文献
370.
Meike S. Andersson Michael Peters Rainer Schultze-Kraft Gerardo Gallego Myriam C. Duque 《Agroforestry Systems》2006,68(3):231-245
Random amplified polymorphic DNA (RAPD) markers were used for assessing genetic diversity and its structure in a collection of the drought-tolerant, tropical multipurpose shrub legume Flemingia macrophylla (Willd.) Merrill. The species is especially suited to low-input smallholder production systems and is used as dry season forage supplement, live soil cover, mulch and living barrier, among others. Genetic groups identified by RAPD analysis related closely to four morphotypes revealed by multivariate analysis of morphological, agronomic and forage quality characteristics. Overall genetic diversity in the collection was moderate (H
T = 0.241), with 79% differentiation among and high genetic similarity (GS) within groups (0.672–0.965). Results indicate a closer relationship of the semi-erect morphotypes M3 and M4 with F. stricta rather than with the erect morphotype M1. The latter was genetically the most depauperate, followed by morphotypes M2 and M4. New evidence is provided that non-Asian accessions collected in Africa and South America are derived from introduced Southeast Asian material. Furthermore, RAPD data suggest the presence of duplicates in the collection, especially in the overrepresented erect morphotype M1. 相似文献