全文获取类型
收费全文 | 3328篇 |
免费 | 312篇 |
国内免费 | 840篇 |
出版年
2024年 | 14篇 |
2023年 | 69篇 |
2022年 | 105篇 |
2021年 | 129篇 |
2020年 | 158篇 |
2019年 | 158篇 |
2018年 | 154篇 |
2017年 | 122篇 |
2016年 | 140篇 |
2015年 | 140篇 |
2014年 | 155篇 |
2013年 | 205篇 |
2012年 | 128篇 |
2011年 | 157篇 |
2010年 | 107篇 |
2009年 | 124篇 |
2008年 | 119篇 |
2007年 | 164篇 |
2006年 | 143篇 |
2005年 | 175篇 |
2004年 | 124篇 |
2003年 | 136篇 |
2002年 | 150篇 |
2001年 | 135篇 |
2000年 | 87篇 |
1999年 | 106篇 |
1998年 | 75篇 |
1997年 | 90篇 |
1996年 | 84篇 |
1995年 | 83篇 |
1994年 | 72篇 |
1993年 | 98篇 |
1992年 | 71篇 |
1991年 | 59篇 |
1990年 | 73篇 |
1989年 | 56篇 |
1988年 | 48篇 |
1987年 | 45篇 |
1986年 | 24篇 |
1985年 | 32篇 |
1984年 | 35篇 |
1983年 | 18篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1975年 | 7篇 |
1958年 | 4篇 |
排序方式: 共有4480条查询结果,搜索用时 15 毫秒
111.
The effects of slow and fast-release fertilizers (P, K, Mg) on the movement and availability of nutrients in acid forest soil were studied. Fast-release superphosphate, potassium chloride and magnesium sulphate and slow-release apatite (P) and biotite (K, Mg) were applied alone or together with urea or urea+limestone. The nutrient content in the organic horizon was determined one growing season and three growing seasons after the application, and in the mineral layer after one growing season. The movement of nutrient ions in the organic horizon was studied by an ion exchange resin bag method during a 5-month period following application. The fast-release salts immediately increased the soluble P and exchangeable K and Mg contents in the organic and mineral soils and in the resin bags. After three growing seasons the effect of K application in the organic layer was non-detectable and that of P had clearly diminished. Apatite gradually increased soluble P content in the organic layer, but biotite had only a minor effect on the K and Mg contents. The nutrients from the fast-release fertilizers had clearly become available and mobile in the year of application and were thus susceptible to leaching. The rate of nutrient release from apatite and biotite is slower and the added nutrients are retained in the organic horizon. Slow-release compounds, like apatite and biotite, might be potential fertilizers for counteracting acidic deposition and subsequent nutrient losses. 相似文献
112.
Acid atmospheric deposition can cause losses of metal nutrients from the organic layer of a soil. The size of these losses
depend on the sizes of the different pools in which the metals are present, as these pools differ in mobility. The metal pools
in an organic soil layer of a Douglas fir forest in the Netherlands subjected to acid deposition were determined by means
of extractions and percolations. Na was mainly dissolved and exchangeably adsorbed, K dissolved, exchangeably adsorbed and
present in the soil microbial biomass, Ca exchangeably adsorbed and present in organic precipitates, Mg exchangeably adsorbed
and present in the soil biomass, and Mn exchangeably adsorbed and present in inorganic precipitates. The main part of the
metals was exchangeably adsorbed. The adsorption affinity increased in the order Na < K < Mg < Mn ≈ Ca. The vertical distribution
of the metals in the organic layer showed that all metals were continuously lost from the organic layer. The differences between
the metals in retention and vertical distribution patterns were in agreement with their differences in deposition rate, pool
distribution, and exchange affinity. Since the metals were mainly exchangeably adsorbed, and the acidifying cations dominated
the atmospheric deposition, acid deposition and cation exchange must be processes that strongly affect the losses of metals
from this organic soil layer. R F Huettl Section editor 相似文献
113.
The paper contains a discussion of the interrelations between the sciences used by managers of forest land to improve their management, in particular with respect to the plant nutrient economy of the forest ecosystems. Both site studies and studies of nutrient cycling have been carried out for long periods without proper consideration of tree nutrition. Therefore these studies contributed less to the understanding of the role of nutrients as regulators of processes in ecosystems than might have been expected. This situation has improved, especially within the last decade. In addition the necessity to manage forest land for environmental values as well as for forest yield requires new interdisciplinary approaches in the study of the roles of plant nutrients in the forest. Even more branches of biological and environmental sciences than those just mentioned must be involved. 相似文献
114.
Phosphorus limitation of forest leaf area and net primary production on a highly weathered soil 总被引:11,自引:0,他引:11
We tested the hypothesis that P was the nutrient limiting net primary production of a nativeMetrosideros polymorpha forest on a highly weathered montane tropical soil in Hawaii. A factorial experiment used all combinations of three fertilizer treatments: nitrogen (N), phosphorus (P) and a mix of other essential nutrients (OE), consisting primarily of mineral derived cations and excluding N and P. P addition, but not N or OE, increased leaf area index within 12 months, foliar P concentration measured at 18 months, and stem diameter increment within 18 months. Stem growth at 18 months was even greater when trees fertilized with P also received the OE treatment. N and P additions increased leaf litterfall and N and P in combination further increased litterfall. The sequence of responses suggests that increased available P promoted an increase in photosynthetic area which led to increased wood production. P was the essential element most limiting to primary production on old volcanic soil in contrast to the N limitation found on young volcanic soils. 相似文献
115.
Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion 总被引:2,自引:2,他引:0
Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO4-Pmg–1C in zooplankton (C
zp
) h–1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg–1C
zp
h–1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO4-P mg–1 C h–1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO4-P mg–1 C hp–1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the real rates. 相似文献
116.
Barleygrass ( Hordeum leporinum ) from Australian low-P (phosphorus) soils and commercial barley ( H. vulgare ) with high fertilizer requirements were grown in solution culture at 3 levels of P supply. The high-P-adapted barley produced more biomass at all levels of P supply and was more responsive to added P in terms of rate of tillering, rate of leaf production, final leaf size, and therefore total shoot weight compared to barleygrass. In both species root: shoot ratio decreased in response to improved tissue P status, even at P levels where total biomass did not respond to P supply. Removal of endosperm reserves of barley reduced total biomass to a greater extent than it altered phosphate absorption rate, thus increasing tissue P status and making plants less responsive to added P. Similarly, barleygrass had a slower growth rate but a comparable P absorption rate to that of barley. Thus barleygrass also accumulated tissue P and was unresponsive to added P. All phosphorus chemical fractions increased in response to improved tissue P status, but to differing extents (inorganic-P > nucleic acid-P > lipid-P > ester-P), suggesting that all P fractions (particularly inorganic P) serve, in part, a storage function. Both barleygrass and barley without endosperm had higher concentrations of all P fractions (particularly inorganic P) than did unaltered barley, but this was due entirely to their higher P status (due to slow growth) rather than to any major difference in P metabolism between species. We conclude that slow growth is more important than interspecific differences in P metabolism, P absorption, or efficiency of P utilization in explaining the success of barleygrass and other low-P-adapted species on infertile soils. 相似文献
117.
Rolf Jess Jørgensen 《Experimental parasitology》1980,49(1):106-115
Dictyocaulus viviparus larvae were exposed to ox bile and CO2 at intervals during their cultivation to the infective stage. Preinfective and young infective larvae were stimulated by CO2. Bile slightly inhibited preinfective larvae, but stimulated the infective stage. Old coiled, resting infective larvae were stimulated by bile down to a concentration of 10 ppm of bile dry matter, by vertebrate biles of pig, sheep, newborn calf, cow, guinea pig, dog, and chicken, as well as by defatted bile dry matter and by glyco-, tauro-, glycodeoxy-, and taurodeoxycholates. Continuous bile exposure appeared necessary to maintain high larval activity. A high pCO2 as well as a low redox potential potentiated the effect of bile, but had no effect alone. Exposure to pepsin-HCl and to trypsin had only a minor stimulatory effect. 相似文献
118.
119.
Laminaria longicrucis De la Pylaie took up exogenous nitrate under both summer and winter conditions. During July and August no NO3- was detected in the ambient water or in algal tissues although it was present in both in February. Discs (2.3 cm diam.) of thin blade tissue were incubated with NO3- at four temperatures, with and without illumination. Similar values Jor NO3- uptake were found for both summer and winter collected plants when measured in light at 0 C. An apparent K of 4–6 μM was recorded for both types of plants; the Vmax ranged from 7 to 10 μmol h-1 g-1 dry wt measured in ca. 1800 μW cm-2 of cool-white fluorescent light. Uptake rates at 5 C were 66%, and at 0 C 30% of those for controls run at 15 C. The alga scavenged NO3- from solutions <0.5 μM. Ammonia did not inhibit NO3- uptake. Antibiotic pretreatment reduced NO3- uptake by a maximum of 12%. Nitrite uptake was inhibited in proportion to the concentration of NO3- in the medium. 相似文献
120.
The use of a partition locus to increase stability of tryptophan-operon-bearing plasmids in Escherichia coli 总被引:6,自引:0,他引:6
The stability of different derivatives of plasmid vectors pBR322 and pACYC184 carrying the tryptophan operon of Escherichia coli was monitored in various media. It was found that in the absence of any special selective pressure, all plasmids were lost from the culture. The stability varied depending both on the orientation of the inserted tryptophan fragment and the growth media used. The pBR322::trp+ plasmids were lost at an average frequency of 0.3 to 0.8% per cell generation, while the pACYC184::trp+ plasmid was lost at a rate higher than 5%. In all cases the whole plasmid was lost at a rate higher than 5%. In all cases the whole plasmid was lost, indicating a high stability of the plasmid::cloned DNA as such. To increase the stability of the cloning vectors, the partition locus of plasmid pSC101 was added to both the pBR322::trp+ and pACYC184::trp+ plasmids. The addition of this gene increased the replicon stability at least 3- to 10-fold, with the pBR322::trp+-par+ plasmids being the most stable. Also in this case, the stability was dependent on the plasmid type and on the growth medium. In no case was there a discoordinate loss of the antibiotic-resistance and tryptophan genes from the vectors. 相似文献