全文获取类型
收费全文 | 14074篇 |
免费 | 876篇 |
国内免费 | 1097篇 |
专业分类
林业 | 1017篇 |
农学 | 1063篇 |
基础科学 | 100篇 |
2101篇 | |
综合类 | 5928篇 |
农作物 | 1236篇 |
水产渔业 | 861篇 |
畜牧兽医 | 1538篇 |
园艺 | 522篇 |
植物保护 | 1681篇 |
出版年
2024年 | 68篇 |
2023年 | 251篇 |
2022年 | 400篇 |
2021年 | 511篇 |
2020年 | 470篇 |
2019年 | 551篇 |
2018年 | 379篇 |
2017年 | 622篇 |
2016年 | 776篇 |
2015年 | 638篇 |
2014年 | 734篇 |
2013年 | 888篇 |
2012年 | 1137篇 |
2011年 | 1137篇 |
2010年 | 1016篇 |
2009年 | 920篇 |
2008年 | 855篇 |
2007年 | 921篇 |
2006年 | 728篇 |
2005年 | 571篇 |
2004年 | 448篇 |
2003年 | 312篇 |
2002年 | 226篇 |
2001年 | 213篇 |
2000年 | 166篇 |
1999年 | 192篇 |
1998年 | 128篇 |
1997年 | 116篇 |
1996年 | 107篇 |
1995年 | 83篇 |
1994年 | 77篇 |
1993年 | 76篇 |
1992年 | 75篇 |
1991年 | 76篇 |
1990年 | 43篇 |
1989年 | 35篇 |
1988年 | 33篇 |
1987年 | 31篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1963年 | 1篇 |
1962年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 190 毫秒
991.
M. Curci M. D. R. Pizzigallo C. Crecchio R. Mininni P. Ruggiero 《Biology and Fertility of Soils》1997,25(1):1-6
Modification of soil environment by different farming practices can significantly affect crop growth. Tillage causes soil
disturbance, altering the vertical distribution of soil organic matter and plant nutrient supplies in the soil surface, and
it may affect the enzyme activity and microbial biomass which are responsible for transformation and cycling of organic matter
and plant nutrients. In this study, the influence of three conventional tillage systems (shallow plowing, deep plowing and
scarification) at different depths on the distribution and activity of enzymes, microbial biomass and nucleic acids in a cropped
soil was investigated. Analysis of variance for depth and tillage showed the influence of the different tillage practices
on the activity of some enzymes and on the nucleic acids. Glucosidase, galactosidase, nitrate reductase and dehydrogenase
activity were significantly affected by the three tillage modalities. Activity in the upper layer (0–20 cm) was higher in
the plots tilled by shallow plowing and scarification than in those tilled by deep plowing. Positive relationships were observed
between the soil enzymes themselves, with the exception of urease and pyrophosphatase activity. Moreover, significant correlations
were found between DNA and β-galactosidase, and between RNA and β-glucosidase, β-galactosidase, alkaline phosphatase and phosphodiesterase.
α-Glucosidase, β-galactosidase, alkaline phosphatase and phosphodiesterase were highly correlated with biomass C determined
by the fumigation-extraction method.
Received: 27 June 1996 相似文献
992.
Barbara Kraigher Janez Hacin Ivan Mahne Ines Mandic-Mulec 《Soil biology & biochemistry》2006,38(9):2762-2771
Fen peatlands are specific wetland ecosystems containing high soil organic carbon (SOC). There is a general lack of knowledge about the microbial communities that abound in these systems. We examined the microbial activity and community structure in two fen soils differing in SOC content sampled from the Ljubljana Marsh under different seasonal conditions. Substrate-induced respiration and dehydrogenase activity were used as indicators of total microbial activity. Both methods indicated higher microbial activities in the fen soil with the higher SOC content on all dates of sampling. To determine whether the differences in microbial activity were associated with differences in the microbial community structures, terminal restriction fragment length polymorphism (T-RFLP) of bacterial 16S rRNA genes was performed. Comparison of the T-RFLP profiles revealed very similar community structures in both fens and in the two seasonal extremes investigated. This suggested a stable community structure in the two fens, which is not affected by the SOC content or seasonal variation. In addition, a bacterial 16S ribosomal RNA gene based clone library was prepared from the fen soil with the higher SOC content. Out of 114 clones analysed, approximately 53% belonged to the Proteobacteria, 23% to the Acidobacteria, 21% to a variety of other taxa, and less than 3% were affiliated with the Firmicutes. 相似文献
993.
Little is known about the relationship between soil biological function and the physical and chemical characteristics of soil-feeding termite nests in the Lopé tropical rainforest (Gabon). We compared nine soil-feeding termite nests of Cubitermes of different ages (fresh to mature to old) and six surrounding soils that originated from three forests differing with respect to age and vegetative cover according to 14 physical and chemical variables and acid (pH 4) and alkaline (pH 9) phosphatase activities. Physical and chemical variables of the studied samples were influenced by the three factors tested: (1) forest age, (2) termite activity (nest versus soil), (3) termite nest age. Soils from the gallery forest were strongly discriminated from all the other soils studied notably due to their high organic matter contents. All mature nests showed significant increases in K, P, clay and fine silt, pH, and cationic exchange capacity compared to soils. Some nests also had increased amounts of organic matter and larger water retention capacities. Moreover, we observed that with age the termite nests possessed decreased values of these variables from fresh to mature to old. Likewise, phosphatase activities also differed according to the three factors tested. Due to its high organic matter contents, the highest phosphatase activities were noted in the gallery forest. Within each forest, phosphatase activities decreased in mature nests compared to soils and tended to be higher in fresh nests compared to mature nests. These differences might be due to an inhibition by high inorganic P contents, as mature nests were enriched in this element and to the quality of organic matter as nests are built with termite faeces. Termite activity has an important role in influencing physical and chemical variables and phosphatase activities. 相似文献
994.
M. Belén Hinojosa José A. Carreira Richard P. Dick 《Soil biology & biochemistry》2004,36(10):1559-1568
Heavy metal contamination can inhibit soil functions but it is often difficult to determine the degree of pollution or when soil reclamation is complete. Enzyme assays offer potential as indicators of biological functioning of soils. However, antecedent water content of soil samples may affect the outcome of biological measurements. In Mediterranean regions, for much of the year ‘field moist’ surface soil can have water content similar to that of air-dry samples. The objectives of this study were to: (1) determine the sensitivity of a range of enzyme assays to detect the degree of pollution from a heavy metal mine spill; (2) evaluate rewetting field-dry soil as a pre-treatment for enzyme assays; and (3) test multivariate analysis for improving discrimination between polluted, reclaimed and non-polluted soils. The Aznalcóllar mining effluent spill provided a unique opportunity to address these objectives. This accident released toxic, heavy metal-contaminated (As, Bi, Cd, Cu, Pb, Tl, Zn…) and acid tailings into the Guadiamar watershed (SW Spain) in 1998, severely affecting the riparian zone along more than 4000 ha. Contaminated soils were collected from the highly polluted upper watershed and less polluted lower watershed along with reclaimed soil at both sites. Enzyme activities (phosphatases, arylsulfatase, β-glucosidase, urease and dehydrogenase) were assessed on both field-moist samples and soils rewetted to 80% of water-holding capacity and then incubated at 21 °C for 7 d prior to the assay. The reclaimed soils had higher activities than polluted soils but, typically, 1.5-3 times lower levels of activity than the non-polluted soil. Regardless of the moisture pre-treatment, all enzymes showed significant effects due to pollution, with urease and β-glucosidase showing the greatest discrimination between degrees of contamination. In general, rewetting field-dried soils increased activities on non-polluted and reclaimed soils which improved discrimination with polluted soils. Another method to increase the potential of soil enzyme activities to detect soil contamination could be to combine them in multivariate analysis, which provides a more holistic representation of the biochemical and microbial functionality of a soil. 相似文献
995.
Summary The nitrogen metabolism of wheat plants inoculated with various Azospirillum brasilense strains and nitrate reductase negative (NR–) mutants was studied in two monoxenic test tube experiments. The spontaneous mutants selected with chlorate under anaerobic conditions with nitrite as terminal electron acceptor fixed N2 in the presence of 10 mM NO3
– and were stable after the plant passage. One strain (Sp 245) isolated from surface-sterilized wheat roots produced significant increases in plant weight at both NO3 levels (1 and 10 mM) which were not observed with the NR– mutants or with the two other strains. Similar effects were observed in a pot experiment with soil on dry weight and total N incorporation but only at the higher N fertilizer level. In the monoxenic test tube experiments plants inoculated with the mutants showed lower nitrogenase activities than NR+ strains at the low NO3
– level (1 = mM) but maintained the same level of activity with 10 mM NO3
– where the activity of all NR+ strains was completely repressed. The nitrate reductase activity of roots increased with the inoculation of the homologous strains and with the mutants at both NO3
– levels. At the low NO3
– level this also resulted in increased activity in the shoots, but at the high NO3
– level the two homologous strains produced significantly lower nitrate reductase activity in shoots while the mutants more than doubled it. The possible role of the bacterial nitrate reductase in NO3
– assimilation by the wheat plant is discussed. 相似文献
996.
S. C. Tiwari 《Biology and Fertility of Soils》1993,16(4):293-295
The effects of fertilization with N, P, K, and organic manure (alone or in combination) on earthworm populations, biomass, and casting activity were measured in a cultivated soil (organic C 1.5%, annual rainfall 2000–2300 mm). These applications of fertilizer caused significant increases in earthworm numbers, biomass, and casts. N alone or in combination with P and K also influenced these earthworm parameters significantly. The inorganic NPK fertilizer in combination with organic manure had a significantly greater effect on earthworm activities than NPK fertilizer alone, and therefore the addition of organic matter appears advisable in order to obtain maximum benefits from NPK fertilizer in this soil. 相似文献
997.
A. Teuben 《Biology and Fertility of Soils》1991,10(4):256-266
Summary The functional roles of the fungivorous collembolan Tomocerus minor and the detritivorous isopod Philoscia muscorum during the decomposition of Pinus nigra needles were studied in mesocosms filled with two different types of F1 litter, obtained from two different forest soils. The effects of the animals on the availability of K+, Ca2+, NO
inf3
sup-
, NH
inf4
sup+
, and PO
inf4
sup3-
and on the respiration, dehydrogenase, and cellulase activity of microorganisms were measured over one growing season. The animals were introduced into the F1 litter in three densities. The most important animal effect was a buffering effect, in that addition of the animals increased nutrient availability and microbial activity where the corresponding values in control mesocosms without animals were low, and decreased the nutrient availability and microbial activity where control values were high. This effect occurred for both species and was most evident in the substrate with the highest temporal fluctuations. The effects on nutrient availability are attributed to an animal effect on the activity of and successional stage reached the microbial community, with NH
inf4
sup+
availability seen as the most important factor. The concept of functional groups in relation to these animal effects is discussed. 相似文献
998.
本文研究了用氨气敏电极-标准加入法测定土壤脲酶活性的新方法。该法使样品液和标准溶液均处于同一条件下测定,消除了土壤中基体对测定的影响,提高了方法的准确性。方法简便、快速,结果可靠,应用于不同土壤样品中脲酶活性的测定,相对标准偏差为1.78%-3.94%,回收率在98.3%-101.8%之间。 相似文献
999.
1000.
Ion control of nutrient solutions to control nitrogen (N), phosphorus (P), and potassium (K) was developed for Superior (medium-to-early maturing) and Atlantic (mid-late) potato cultivars grown in closed hydroponic systems in which solutions were replenished and recirculated. Results were compared with conventional nutrient solution management strategies. In the “solution replacement” treatment, nutrient solutions were completely replaced each week. In the “electrical conductivity (EC) control” treatment, water use by potato plants was compensated by adding ground water to achieve the original volume (water replenishment) and the diluted EC of the solution was adjusted to the target levels using stock solution. In “ion control” treatment, ammonium dihydrogen phosphate (NH4H2PO4) and potassium nitrate (KNO3) were added to the EC-controlled nutrient solution. The amounts increased with plant age in both cultivars. The concentrations of nitrate (NO3), P, and K in the ion control nutrient solution could be maintained at target levels. In water replenishment, recycling of nutrient solution resulted in a progressive decrease in EC and an increase in pH. Root activity increased by 93% and 59% in the Superior and Atlantic cultivars, respectively, compared with the nutrient solution replacement. These changes decreased photosynthesis, plant growth, water use, and thus tuber growth in the Superior cultivar. Decreased growth of shoots and tubers occurred without affecting photosynthesis in the Atlantic cultivar. Although there were no significant differences in root activity, photosynthesis, or plant growth between the ion control treatment and the EC control treatment, increased tuber growth was observed in the ion control treatment, possibly as a result of the constant supply of nutrients. High tuber growth and the capability to maintain solution nutrient concentration in the ion control treatment are highly desirable for closed hydroponic systems. 相似文献