全文获取类型
收费全文 | 8087篇 |
免费 | 419篇 |
国内免费 | 657篇 |
专业分类
林业 | 443篇 |
农学 | 505篇 |
基础科学 | 304篇 |
2656篇 | |
综合类 | 3960篇 |
农作物 | 254篇 |
水产渔业 | 246篇 |
畜牧兽医 | 352篇 |
园艺 | 237篇 |
植物保护 | 206篇 |
出版年
2024年 | 57篇 |
2023年 | 150篇 |
2022年 | 227篇 |
2021年 | 282篇 |
2020年 | 232篇 |
2019年 | 312篇 |
2018年 | 199篇 |
2017年 | 335篇 |
2016年 | 387篇 |
2015年 | 363篇 |
2014年 | 423篇 |
2013年 | 510篇 |
2012年 | 599篇 |
2011年 | 632篇 |
2010年 | 575篇 |
2009年 | 602篇 |
2008年 | 511篇 |
2007年 | 562篇 |
2006年 | 447篇 |
2005年 | 388篇 |
2004年 | 280篇 |
2003年 | 188篇 |
2002年 | 140篇 |
2001年 | 109篇 |
2000年 | 97篇 |
1999年 | 86篇 |
1998年 | 57篇 |
1997年 | 70篇 |
1996年 | 65篇 |
1995年 | 52篇 |
1994年 | 41篇 |
1993年 | 46篇 |
1992年 | 28篇 |
1991年 | 30篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有9163条查询结果,搜索用时 156 毫秒
941.
为研究降水径流过程中非点源污染物氮、磷浓度的变化特征,分析降水作用下长沙县金井河小流域农业源头沟渠水中氮、磷的流失特征与生态拦截效应。结果表明:在降水初期非点源污染物氮、磷浓度与降水径流量的变化均呈逐渐递增趋势,雨后均呈递减变化;降水作用下沟渠系统总氮输出的最大值为4.67 mg/L,总磷输出的最大值为0.38 mg/L,其氮素输出的主要形态为氨态氮和硝态氮;雨后各断面氮、磷的输出随着时间的增加呈先增加后降低的趋势,其中总氮与总磷含量分别在雨后第3天和第2天达到最高;菖蒲–茭白段的生态拦截效果明显高于自然段的自然净化效果,表明植物拦截和吸收是植被段氮、磷去除的重要途径,菖蒲、茭白在6月吸收氮、磷分别达26.90、2.34 g/m~2。 相似文献
942.
943.
Variation in competitive abilities of plants and microbes for specific amino acids 总被引:21,自引:0,他引:21
D. A. Lipson T. K. Raab S. K. Schmidt R. K. Monson 《Biology and Fertility of Soils》1999,29(3):257-261
Microbes are assumed to possess strong competitive advantages over plants for uptake of nutrients from the soil. The finding
that non-mycorrhizal plants can obtain a significant fraction of their N requirement from soil amino acids contradicts this
assumption. The amino acid glycine (Gly) has been used as a model amino acid in many recent studies. Our preliminary studies
showed that Gly was a poor substrate for microbial growth compared to other amino acids. We tested the hypothesis that the
alpine sedge Kobresia myosuroides competes better for Gly than for other amino acids because of decreased microbial demand for this compound. Soil microbial
populations that could grow using Gly as a sole carbon source were about 5 times lower than those that could grow on glutamate
(Glu). Gly supported a significantly lower population than any of the ten other amino acids tested except serine. In contrast,
K. myosuroides took up Gly from hydroponic solution at faster rates than Glu. In plant-soil microcosms, plants competed with soil microbes
3.25 times better for Gly than for Glu. We conclude that the low microbial demand and the rapid plant uptake of Gly relative
to other amino acids allow Gly to be an especially important nitrogen source for K. myosuroides.
Received: 9 February 1998 相似文献
944.
L. Leita M. De Nobili C. Mondini G. Muhlbachova L. Marchiol G. Bragato M. Contin 《Biology and Fertility of Soils》1999,28(4):371-376
We studied the long-term effects (12 years) of municipal refuse compost addition on the total organic carbon (TOC), the amount
and activity of the microbial biomass (soil microbial biomass C, BC and metabolic quotient qCO2) and heavy metal bioavaiability in soils as compared to manuring with mineral fertilizers (NPK) and farmyard manure (FYM).
In addition, we studied the relationships between among the available fraction [Diethylenetriaminopentacetic acid (DTPA) extractable]
of heavy metals and their total content, TOC and BC. After 12 years of repeated treatments, the TOC and BC of control and mineral fertilized plots did not differ. Soils treated with FYM and composts showed a significant increase
in TOC and BC in response to the increasing amounts of organic C added. Values of the BC/TOC ratio ranged from 1.4 to 2, without any significative differences among soil treatments. The qCO2 increased in the organic-amended soil and may have indicated microbial stress. The total amounts of metals in treated soils
were lower than the levels permitted by the European Union in agricultural soils. DTPA-extractable metals increased in amended
soils in response to organic C. A multiple regression analysis with stepwise selection of variables was carried out in order
to discriminate between the influence exerted on DTPA-extractable metals by their total content, TOC and BC. Results showed that each metal behaved quite differently, suggesting that different mechanisms might be involved in metal
bioavailability
Received: 31 October 1997 相似文献
945.
P. F. A. M. Römkens Johannes van der Plicht Jan Hassink 《Biology and Fertility of Soils》1999,28(3):277-284
Conversion of arable land (maize) to pasture will affect the soil organic matter (SOM) content. Changes in the SOM content
were studied using a size- and density-fractionation method and 13C analysis. Twenty-six years of maize cropping had resulted in a depletion of carbon stored in the macro-organic fractions
(>150 μm) and an increase in the <20 μm fraction. Maize-derived carbon in the upper 20 cm increased from 10% in the finest
fraction (<20 μm) to 91% in the coarse (>250 μm), light (b.d. <1.13 g cm–3) fractions. Pasture installation resulted in a rapid recovery of the total SOM content. Up to 90% of the pasture-derived
carbon that was mineralized during maize cropping was replaced within 9 years. Especially the medium and coarse size (>150
μm) and light (b.d. <1.13 g cm–3) fractions were almost completely regenerated by input of root-derived SOM. The amount of medium-weight and heavy macro-organic
fractions (>150 μm; b.d. >1.13 g cm–3) in the 0- to 20-cm layer was still 40–50% lower than in the continuous pasture plots. Average half-life times calculated
from 13C analyses ranged from 7 years in the light fractions to 56 years in heavy fractions. Fractionation results and 13C data indicated that mechanical disturbance (plowing) during maize cropping had resulted in vertical displacement of dispersed
soil carbon from the 0- to 20-cm layer down to 60–80 cm. Conversion of arable land to pasture, therefore, not only causes
a regeneration of the soil carbon content, it also reduces the risk of contaminant transport by dispersed soil carbon.
Received: 10 March 1998 相似文献
946.
Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions 总被引:33,自引:0,他引:33
Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase
activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic
fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic
matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases
of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg–1 soil in unfertilized soil to 423 mg kg–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils
increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil
organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil,
are enhanced by use of organic amendments along with inorganic fertilizers.
Received: 6 May 1998 相似文献
947.
An open incubation technique was used to measure S mineralization in a range of upland soils of north China. Six mineralization
patterns were examined, and a soil S-exhaustion experiment with ryegrass (Lolium multiflorum L.) was conducted to investigate the availability of various organic S pools to plants. For all of the 12 soils tested, the
release of S as SO4
2– was curvilinear with time, and during a 28-week incubation at 30 °C the amount of S mineralized ranged from 14.0 mg S kg–1 soil to 37.4 mg S kg–1 soil. A first-order model and Gompertz model appeared to best describe S mineralization. Examination of the soils after incubation
revealed the bulk of the mineralized S was mainly derived from the C-bonded S pool, while the majority of mineralized S under
soil S exhaustion by ryegrass was derived from the HI-reducible S pool.
Received: 9 July 1998 相似文献
948.
论水土保持在解决中国水问题中的战略地位 总被引:13,自引:3,他引:10
洪涝灾害、淡水资源短缺和水污染问题是中国水问题中的三大主要问题,已引起社会各界的高度重视。影响中国三大水问题的因素甚多,但水土保持是解决水问题之根本性、战略性措施。 相似文献
949.
950.
南宁市郊区土壤中汞污染状况调查 总被引:5,自引:0,他引:5
李俊芳 《广西农业生物科学》1999,18(1):80-83
分析了来自南宁郊区5个不同地点的土壤汞污染状况。结果表明,这5个不同地点的土壤中的汞含量明显高于全国平均水平。其中汞含量最高的是安吉地区,达到1.647mg/kg。同时还对种植在这些土壤中的蔬菜的汞含量进行了调查。结果表明,这些地区的蔬菜的汞含量明显偏高。研究结果也说明,蔬菜中汞的含量与土壤中的汞污染程度有明显的相关性。 相似文献