全文获取类型
收费全文 | 113篇 |
免费 | 8篇 |
专业分类
林业 | 3篇 |
农学 | 1篇 |
基础科学 | 6篇 |
31篇 | |
综合类 | 16篇 |
农作物 | 6篇 |
水产渔业 | 12篇 |
畜牧兽医 | 44篇 |
植物保护 | 2篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 1篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 2篇 |
2012年 | 11篇 |
2011年 | 11篇 |
2010年 | 5篇 |
2009年 | 7篇 |
2008年 | 4篇 |
2007年 | 8篇 |
2006年 | 2篇 |
2005年 | 7篇 |
2004年 | 6篇 |
2003年 | 6篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有121条查询结果,搜索用时 15 毫秒
71.
Characterizing water use and management in irrigated agriculture is a prerequisite for conserving agricultural water. We carried out a detailed analysis of irrigation performance by documenting the water use of about 840 parcels in an irrigation scheme (Genil–Cabra irrigation scheme; GCIS) located in Andalusia, southern Spain, from 1996 to 2000. Performance indicators based on the water balance detected two water-management strategies, depending on the crop: (1) cotton, garlic, maize and sugar beet had average ratios of measured irrigation supply to the simulated optimum demand (ARIS) ranging between 0.73 and 0.91 and (2) winter cereals, sunflower and olive had a much lower average ARIS (with a 4-year average of 0.28–0.39). We found a large variability in water usage among the management units in all cases. For instance, in cotton, even though the average ARIS was around 0.8, about 50% of the fields were not irrigated adequately (41% with deficit, 9% with excess). Water productivity (WP) in the GCIS was highest for the horticultural crops (garlic, olive; from 1.13 €/m3 to 6.52 €/m3) while it varied among the field crops, being lowest in maize (4-year average of 0.28 €/m3) and highest in sugar beet (4-year average of 1.04 €/m3). Large year-to-year variations in WP were observed in all crops, particularly in sunflower and garlic due either to fluctuating prices for garlic or to the effects of the 1998/1999 drought for sunflower. In fact, WP was lowest in all crops in that year, because seasonal irrigation depths were much higher than in the other 3 years. The combination of ARIS and other performance indicators allowed for determining performance levels and improvement measures. It was found that if more irrigation water is used in the GCIS, garlic and olive will be the crops that profit most from the additional supply. However, it was concluded that, given the wide range in water use and management encountered at the parcel level, improvement policies at the scheme level should always consider individual performance when designing measures for water conservation in irrigated agriculture. 相似文献
72.
The gradient of water vapor pressure between soil and air controls the rate of evaporation from soil, but it has not been possible to determine the humidity at the soil surface. We have devised an instrument to measure the surface humidity in the field. Thermocouple psychrometry is used to monitor air equilibrated with the soil surface in a chamber placed briefly on the soil. A programmable data logger scans the thermocouples and stores the data. We calibrated the instrument over saturated salt solutions, and over soils whose relative humidity was calculated from measured water potentials. The instrument worked well, but underestimated slightly at relative humidities above 90%. It can be used to distinguish between stage 1 and stage 2 evaporation for soils. In field trials, the surface psychrometer was used to monitor diurnal and day-to-day changes in soil surface humidity during the drying cycle after irrigation. The cumulative vapor pressure difference between the soil surface and the air at 1.5 m was highly correlated with soil evaporation from microlysimeters set between crop rows. Changes in the cumulative difference in vapor pressure were also closely related to the changes in the evaporation component of measured evapotranspiration, as estimated with a simulation model. 相似文献
73.
The formation of CH3 ONO in 11 soils treated with HNO2 or NaNO2 in a closed system, was studied by measuring the concentration in the gas space above the soil and by absorbing CH3 ONO in HI. The gaseous concentration of CH3 ONO increased and then decreased following additions of HNO2 or NaNO2 , and the production of CH3 ONO increased with increasing concentrations of HNO2 or NaNO2 added to soils.
The amounts of CH3 ONO trapped in HI were 13.5 to 20.4 times higher than those determined by integrating under the net production curves. The evolved CH3 ONO amounted to 0.4 to 3.5% of added NO2 − , and 4.2 to 50% of the gaseous forms of N absorbed by acidic KMnO4 solution. The CH3 ONO evolved from soils was positively correlated with the methoxy content of the soils, and inversely related to soil pH, with negligible amounts being evolved from alkaline soils. The results show that CH3 ONO is a product of NO2 − decomposition in soils, and indicate that small concentrations of the gas may be produced in N–fertilized soils in which NO2 − accumulates. 相似文献
The amounts of CH
74.
The reactions of methyl nitrite (CH3ONO), a gaseous product of NO?2 decomposition in soils, were studied by exposure of soils in closed vessels to the gas. The N transformations occurring in soils at different soil-water states were assessed by measuring CH3ONO and other gaseous forms of N in the gas space, soil inorganic N (NH4+, NO?2, NO3?) and incorporation of CH3O15NO into the soil organic N fraction. The initial rate of uptake of CH3ONO increased with decreasing soil-water content, but the rate of hydrolysis decreased as soil-water content decreased below – 33kPa matric potential. Uptake was not affected by y-irradiation of soils. Adsorption isotherms conformed to the Langmuir equation in each of 22 oven-dry soils studied. Langmuir adsorption maxima were positively correlated with the clay contents of the soils, and adsorption was reversible to some extent at all soil-water states. Small amounts of added CH3ONO were recovered as N2 and N2O and as 15NH4+ in γ-irradiated soils. From 60 to 72% of added CH3O15NO was recovered by Kjeldahl digestion; this was indicative of a chemical reaction with soil organic matter. The results suggest that the physical process of adsorption of CH3ONO by clay minerals and the chemical fixation of CH3ONO by soil organic matter are key factors controlling the atmospheric concentration of CH3ONO, and that the combined effect of these processes, together with hydrolysis in the soil solution, will inhibit the emission of CH3ONO formed in N-fertilized soils. 相似文献
75.
Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot 总被引:1,自引:0,他引:1
The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons. 相似文献
76.
77.
NaOCl浸泡对百喜草种子发芽率的影响 总被引:5,自引:1,他引:4
对NaCOl处理的百喜草(Paspalum notatum Fluegge)作形态观察,发现随浸泡时间的延长,其颖片、稃片、果皮及种皮等均有明显变化,首次报道果皮、种皮的结构。0.5、1hNaCOl浸泡及光中提高百喜草发芽率2%~3%,效果不明显。本对百喜草种子发芽率低的原因进行了讨论。 相似文献
78.
M. F. MAGALHÃES C. E. RAMALHO & M. J. COLLARES-PEREIRA 《Fisheries Management and Ecology》2008,15(4):273-289
Abstract Biological indicators for Mediterranean rivers are poorly developed. This study evaluates the effectiveness of the Index of Biotic Integrity approach (IBI) with fish assemblages in the Guadiana catchment, a typical Mediterranean watershed in Southern Portugal. Reference sites were selected from a set of 95 sites, using a multivariate approach. Fifty-five candidate metrics were screened for range, responsiveness, precision and redundancy. Final metrics included: proportion of native fish, number of intolerant and intermediate species, number of invertivore native fish, number of phyto-lithophilic and polyphilic species, and catches of exotics. The IBI scores correlated with composite gradients of human impact and differed significantly between reference and non-reference sites. Application of the IBI to an independent validation set with 123 sites produced results congruent with the development set and repeatable assessments at 22 sites showed concordance in IBI scoring. This application highlights the effectiveness of the IBI approach even with fish assemblages of limited diversity and ecological specialisation as in Mediterranean streams. 相似文献
79.
80.