全文获取类型
收费全文 | 6341篇 |
免费 | 391篇 |
国内免费 | 446篇 |
出版年
2023年 | 64篇 |
2022年 | 76篇 |
2021年 | 153篇 |
2020年 | 159篇 |
2019年 | 188篇 |
2018年 | 142篇 |
2017年 | 146篇 |
2016年 | 170篇 |
2015年 | 197篇 |
2014年 | 224篇 |
2013年 | 273篇 |
2012年 | 179篇 |
2011年 | 240篇 |
2010年 | 161篇 |
2009年 | 299篇 |
2008年 | 297篇 |
2007年 | 357篇 |
2006年 | 292篇 |
2005年 | 243篇 |
2004年 | 267篇 |
2003年 | 235篇 |
2002年 | 145篇 |
2001年 | 141篇 |
2000年 | 137篇 |
1999年 | 156篇 |
1998年 | 150篇 |
1997年 | 122篇 |
1996年 | 115篇 |
1995年 | 121篇 |
1994年 | 138篇 |
1993年 | 128篇 |
1992年 | 120篇 |
1991年 | 132篇 |
1990年 | 105篇 |
1989年 | 97篇 |
1988年 | 120篇 |
1987年 | 105篇 |
1986年 | 122篇 |
1985年 | 125篇 |
1984年 | 112篇 |
1983年 | 45篇 |
1982年 | 82篇 |
1981年 | 85篇 |
1980年 | 68篇 |
1979年 | 54篇 |
1978年 | 13篇 |
1977年 | 27篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 9篇 |
排序方式: 共有7178条查询结果,搜索用时 84 毫秒
21.
Bioassays of nutrient limitation in a tropical rain forest soil 总被引:4,自引:0,他引:4
Summary Six speices of shrubs and one large herb with contrasting life history patterns were used as bioassays of nutrient availability in a Costa Rican lowland rain forest soil. Growth responses of the herb (Phytolacca rivinoides, Phytolaccaceae) confirmed soil measurements indicating high availability of N and potentially limiting levels of P, K, Mg and Ca. Growth responses of the shrub species (Miconia spp., Melastomataceae and Piper spp., Piperaceae) to a complete nutrient fertilizer were generally less than that of Phytolacca. Lack of a strong shrub response to +P fertilization is probably due to mycorrhizal associations and slower growth rates of woody species. In general, increased growth did not occur at the expense of phenolic production in the leaves. The results emphasize that assessment of specific nutrient limitations to plant growth vary depending on species selected for the bioassay, even among species from the same community. 相似文献
22.
The influence of earthworms (Lumbricidae) on the nitrogen dynamics in the soil litter system of a deciduous forest 总被引:2,自引:0,他引:2
S. Scheu 《Oecologia》1987,72(2):197-201
Summary The influence of earthworms (Aporrectodea caliginosa (Savigny) and Lumbricus castaneus (Savigny)) on the rate of nitrogen net mineralization of the soil was studied in the laboratory and in the field. The additional mineralization of nitrogen cause by the burrowing activity of the substrat feeding earthworm A. caliginosa (N
L
)was directly correlated to the biomass of the lumbricids independently of their number. A rise in temperature caused an exponential increase in N
L
values. The Q
10 value of this process (2.18) was found to be much higher than that of the nitrogen mineralization without earthworms (Q
10=1.22). At 15°C the N
L
value caused by A. caliginosa was calculated to be about 250 g N g-1 fresh body wt d-1. Using the experimentally determined exponential relationship between temperature and N
L
values, the additional nitrogen mineralization caused by a population of A. caliginosa in a beechwood on limestone was calculated to be 4.23 kg ha-1 a-1.In contrast to A. caliginosa the litter dwelling species L. castaneus lost considerable amounts of biomass (56%) during the 4 week incubation period. Only 1/3 of the nitrogen equivalent to the weight loss of the animals was recovered in the mineral nitrogen pool.The addition of litter (old beech leaf litter, freshly fallen beech and ash leaf litter) had a pronounced effect on both nitrogen net mineralization and N
L
values of the soil. Presence of old beech leaves caused an increase in both values, wheres the other litter types effected a decrease in nitrogen net mineralization. Fragmented ash litter was found to have the most distinct effect on N
L
values (-69%) and nitrogen net mineralization (-74%). 相似文献
23.
24.
G. Van den Eede B. Dreyfus K. Goethals M. Van Montagu M. Holsters 《Molecular & general genetics : MGG》1987,206(2):291-299
Summary After random Tn5 mutagenesis of the stem-nodulating Sesbania rostrata symbiont strain ORS571, Nif-, Fix- and Nod- mutants were isolated. The Nif- mutants had lost both free-living and symbiotic N2 fixation capacity. The Fix- mutants normally fixed N2 in the free-living state but induced ineffective nodules on S. rostrata. They were defective in functions exclusively required for symbiotic N2 fixation. A further analysis of the Nod- mutants allowed the identification of two nod loci. A Tn5 insertion in nod locus 1 completely abolished both root and stem nodulation capacity. Root hair curling, which is an initial event in S. rostrata root nodulation, was no longer observed. A 400 bp region showing weak homology to the nodC gene of Rhizobium meliloti was located 1.5 kb away from this nod Tn5 insertion. A Tn5 insertion in nod locus 2 caused the loss of stem and root nodulation capacity but root hair curling still occurred. The physical maps of a 20.5 kb DNA region of nod locus 1 and of a 40 kb DNA region of nod locus 2 showed no overlaps. The two nod loci are not closely linked to nif locus 1, containing the structural genes for the nitrogenase complex (Elmerich et al. 1982). 相似文献
25.
The fate of15N labeled nitrogen applied to mature citrus trees 总被引:1,自引:0,他引:1
Summary The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched
nitrogen fertilizer, K15NO3, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N3) and to an N-starved tree (N1). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were
excavated.
Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N1−61.7% N3−56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil
was much larger in the N3 treatment than in N1. The highest percentage of fertilizer N was found in the new organs,i.e. fruits, twigs and leaves. The roots and branches took up only 6–14% from the labeled fertilizer.
Only 20.9% of the leaf N and 23.4% of the fruit N in the N3 tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth.
Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled
fertilizer.
Contribution 1599E. 相似文献
26.
J. H. Becking 《Plant and Soil》1987,100(1-3):183-212
Summary The survival of Azolla was studied in an artificial system which simulated the soil/water interface and the desiccation of
soil during a fallow period in lowland rice culture. Tests with non-sporulating and sporulating Azolla fronds showed that
Azolla only survives with sporulated fronds. At their reappearance the Azolla fronds already harboured the Anabaena endophyte.
A detailed light microscopic and transmission electron microscopic study of macro- and micros-porocarp formation and development
revealed that the endophyte is transmitted by the macrosporocarps and not by the microsporocarps. The Anabaena cells within
the macrosporocarps are found just below the indusium cap. These cells are not nitrogen-fixing akinetes. The free-living Anabaena
cells at the stem apex and below the overarching developing leaves do not bear heterocysts and accordingly are non nitrogen-fixing.
During the development of the leaf the Anabaena enters the leaf cavity, but later the pore of this, cavity closes and the
imprisoned cyanobacteria are lysed before the leaf decays. As the Azolla leaves age a nitrogen-fixing capability is successively
built up concomittantly with the production of heterocysts. Heterocyst frequencies of 40–50% can be found inAnabaena azollae. Usually a gradient of nitrogen-fixing capacity occurs along the Azolla rhizome with two distinct peaks at leaf number 7/8
and at leaf number 13/14 from the apex. 相似文献
27.
Summary The alder has a perennial nodule cluster. The nodule amount on the roots increases with tree age. The N2-fixing activity of nodules decreases with nodule age. Purple coloured soils with various soil pHs and CaCO3 contents are, in the main, the ones which influence nodulation and N2-fixing. Higher N2-fixing capacity existed in the neutral and low calcium soils. High calcium soils and acid soils can restrain nodulation and
the N2-fixing rate significantly. On the slope, where calcarous light loams are found, the annual nitrogen fixation capacity of
alder and cypress mixed plantations, less than 10 years old, is 16 or 17 kg/ha yr, but in the valley, a pure alder plantation
can reach 40 kg/ha yr. 相似文献
28.
C. A. Atkins 《Plant and Soil》1987,100(1-3):157-169
Summary Nitrogen (N2) fixed by Rhizobium bacteroids in the legume nodule is excreted as ammonia to the surrounding host cell where it is efficiently
assimilated into the amide group of glutamine. Generally glutamine is a minor exported solute of nitrogen, being further metabolised
to asparagine in temperate species and to the ureides, allantoin and allantoic acid in tropical species. These solutes serve
as the principal translocated forms of nitrogen in xylem. Compartmentalisation of the pathways of nitrogen metabolism and
the role of ammonia in regulation of their activity is examined in nodules of both asparagine-forming (Lupinus albus L.) and ureide-forming (Vigna unguiculata L. Walp) symbioses. 相似文献
29.
M. R. Sarić 《Plant and Soil》1987,99(1):197-209
Summary The paper discusses the problems of genetic aspects of plant mineral nutrition in the light of the results presented at the
First and Second Symposia on ‘Genetic Aspects of Plant Mineral Nutrition’ organized in Beograd in 1982 and Madison in 1985,
respectively. On the basis of the results, future directions of research are discussed. The papers deal with the concentration
and content of mineral nutrients in different genotypes, physiological and biochemical aspects of the genetic specificity
of plant mineral nutrition, relations between plant genotypes and nitrogen fixing micro-organism strains, as well as with
some related problems which have been investigated to a lesser extent. Particular attention is paid to papers and problems
referring to genetic and breeding research work linked with genetic aspects of plant mineral nutrition as well as the possibilities
of developing new cultivars requiring certain soil and mineral nutrition conditions for their cultivation. 相似文献
30.
The relationship between nitrification potential and nitrogen accumulation was studied in an early successional sere on Mt.
Fuji. Soil organic nitrogen accumulated with the invasion ofPolygonum cuspidatum and successively withMiscanthus oligostachyus and other species. Laboratory incubation experiments showed a higher nitrification potential at theM. oligostachyus state. The numbers of nitrifying bacteria increased with the progress of succession. No significant difference in nitrate
reductase activity was found between pioneer and succeeding species. The soil solution at theM. oligostachyus stage contained a lower level of nitrate than rainwater, while that of the bare ground and theP. cuspidatum stage contained a higher nitrate level than rainwater. It was concluded that the high nitrate levels in the soil solution
of the bare ground and theP. cuspidatum stage were due to lower nitrate-absorbing activity, leading to loss of nitrogen with precipitation, while the lower nitrate
levels at theM. oligostachyus stage when higher nitrification activity occurred were due to higher nitrate-absorbing activity, preventing net loss of nitrogen
from the ecosystem. 相似文献