全文获取类型
收费全文 | 7400篇 |
免费 | 411篇 |
国内免费 | 736篇 |
专业分类
林业 | 444篇 |
农学 | 753篇 |
基础科学 | 345篇 |
2510篇 | |
综合类 | 2586篇 |
农作物 | 743篇 |
水产渔业 | 126篇 |
畜牧兽医 | 540篇 |
园艺 | 152篇 |
植物保护 | 348篇 |
出版年
2024年 | 18篇 |
2023年 | 153篇 |
2022年 | 230篇 |
2021年 | 298篇 |
2020年 | 266篇 |
2019年 | 295篇 |
2018年 | 250篇 |
2017年 | 402篇 |
2016年 | 399篇 |
2015年 | 340篇 |
2014年 | 375篇 |
2013年 | 567篇 |
2012年 | 594篇 |
2011年 | 542篇 |
2010年 | 383篇 |
2009年 | 387篇 |
2008年 | 364篇 |
2007年 | 413篇 |
2006年 | 354篇 |
2005年 | 283篇 |
2004年 | 233篇 |
2003年 | 199篇 |
2002年 | 129篇 |
2001年 | 124篇 |
2000年 | 123篇 |
1999年 | 109篇 |
1998年 | 91篇 |
1997年 | 82篇 |
1996年 | 75篇 |
1995年 | 67篇 |
1994年 | 59篇 |
1993年 | 59篇 |
1992年 | 62篇 |
1991年 | 51篇 |
1990年 | 30篇 |
1989年 | 36篇 |
1988年 | 37篇 |
1987年 | 34篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1977年 | 4篇 |
1962年 | 2篇 |
排序方式: 共有8547条查询结果,搜索用时 46 毫秒
991.
Eric Paterson Barry Thornton Andrew J. Midwood Shona M. Osborne Allan Sim Pete Millard 《Soil biology & biochemistry》2008,40(9):2434-2440
Plants link atmospheric and soil carbon pools through CO2 fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous 13C-labelling to soil-grown Lolium perenne, imposing atmospheric CO2 concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific 13C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO2 concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO2 and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study. 相似文献
992.
Hardsetting soil properties are undesirable in agricultural soils because they hamper crop production by limiting seedling emergence and root growth via increased mechanical soil resistance at low moisture contents. The objective of this study was to determine the effect of additions of organic matter on the penetration resistance of a hardsetting soil for the entire water tension range. Investigations were carried out on Saalian glacial till, which is used as a reclamation substrate in post-lignite-mining reclamation. Proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter (OM) were used. The remoulded samples were saturated under a constant load of 2.4 kPa to achieve bulk densities equivalent to a soil depth of 15–20 cm via water-induced consolidation. Subsequently, the mixtures were adjusted to water tensions between 100 and 107 hPa and penetrated using a small cone penetrometer. Compared to 0% OM, the addition of 1% OM led to a very small but significant (P < 0.01) increase in the bulk density, while between 1% and 4% OM bulk density was seen to decrease in a linear fashion. At moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% OM to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots. At moisture contents smaller than field capacity, penetration resistance values were inversely related to the bulk density, supporting the concept that the type of organic matter added contributed to soil cohesion. Modeling the relation between water tension and penetration resistance using a sigmoidal equation showed a high consistency between the observed data and the model. 相似文献
993.
Mechanisms of real and apparent priming effects and their dependence on soil microbial biomass and community structure: critical review 总被引:6,自引:1,他引:5
The number of studies on priming effects (PE) in soil has strongly increased during the last years. The information regarding
real versus apparent PE as well as their mechanisms remains controversial. Based on a meta-analysis of studies published since
1980, we evaluated the intensity, direction, and the reality of PE in dependence on the amount and quality of added primers,
the microbial biomass and community structure, enzyme activities, soil pH, and aggregate size. The meta-analysis allowed revealing
quantitative relationships between the amounts of added substrates as related to microbial biomass C and induced PE. Additions
of easily available organic C up to 15% of microbial biomass C induce a linear increase of extra CO2. When the added amount of easily available organic C is higher than 50% of the microbial biomass C, an exponential decrease
of the PE or even a switch to negative values is often observed. A new approach based on the assessment of changes in the
production of extracellular enzymes is suggested to distinguish real and apparent PE. To distinguish real and apparent PE,
we discuss approaches based on the C budget. The importance of fungi for long-term changes of SOM decomposition is underlined.
Priming effects can be linked with microbial community structure only considering changes in functional diversity. We conclude
that the PE involves not only one mechanism but a succession of processes partly connected with succession of microbial community
and functions. An overview of the dynamics and intensity of these processes as related to microbial biomass changes and C
and N availability is presented. 相似文献
994.
995.
The abandonment of cultivated wetland soil increased the contents of light fraction organic matter (LFOM), heavy fraction
organic matter (HFOM) and soil organic matter (SOM). The LFOM and HFOM content increased to 13.3 g kg−1 and 62.4 g kg−1 after 5 years whereas they were 8.4 and 47.9 g kg−1 after 9 years of cropping, respectively. Fourteen years after abandonment, HFOM content increased to 104.3 g kg−1. LFOM was positively correlated with HFOM (p < 0.001). A Langmuir equation was used to calculate the highest HFOM value. The value for the natural wetland soil was closed
to this theoretical value (140.8 g kg−1). After 14 years of abandonment, the HFOM maximum (HFOMMax) value was lower than the equilibrium value suggesting that a further increase in HFOM can occur after abandonment. Assuming
a linear accumulation (3.87 Mg C ha−1yr−1), it would take approximately 24 years after the abandonment to reach the HFOMMax value. 相似文献
996.
J.M. De la Rosa J.A. González-Pérez R. González-Vázquez H. Knicker E. López-Capel D.A.C. Manning F.J. González-Vila 《CATENA》2008
In Mediterranean ecosystems, forest fires are a common phenomenon. They involve the transformation of vegetation and litter, leaving charred residues and so influencing the carbon cycle by changing (a) the amounts of soil organic matter and (b) the proportions within it of pools with differing stability. In addition to affecting C cycles, fires also affect the amounts of N within soil organic matter, and its availability. 相似文献
997.
DEM栅格分辨率对多元线性土壤—景观模型及其制图应用的影响 总被引:7,自引:1,他引:6
以亚热带丘陵地区为对象,以该区4 km×3 km的5 m、10 m、15 m、20 m、25 m、30 m数字高程模型(DEM)为基础,建立多元线性土壤景观模型,并应用该模型预测研究区内土壤表层有机质含量分布,进而比较不同分辨率DEM中土壤景观模型及其预测制图的精度。结果表明:在本研究区11 km2范围内,随着DEM栅格分辨率降低,坡度、曲率、比汇水面积(对数)频度均表现出了向其中值区集中的趋势;地形因子的这一变化规律对土壤景观模型的影响较小,例如模型的变量、变量系数及R2在不同分辨率DEM中的差异很小;但地形因子的这一变化规律对模型预测制图的精度具有较大影响,各项指标均说明,模型在10~25 m DEM中的制图精度较高,而在更高分辨率(5 m)或更低分辨率(30 m)DEM中较低。本研究结果对其他亚热带丘陵地区具有一定的指示意义。 相似文献
998.
不同风沙土壤颗粒的分形特征 总被引:6,自引:1,他引:5
应用土壤颗粒的重量与粒径分布原理来描述了古尔班通古特沙漠地风沙土壤颗粒的分形特征。通过对10种样品颗粒的机械组成进行分析,分别计算出了它们的分形维数(D=2.3237~2.9347),并分析了其与流动风沙土、半固定风沙土和固定风沙土之间的关系。分析结果表明,风沙土壤结构具有明显的分形特征,其粒径分布分形维数为2~3。土粒表面分形维数与2~0.2mm间的土粒含量存在显著的负相关;而与0.02~0.002mm和<0.002mm的颗粒含量存在显著的正相关,表现为随着土壤质地从流动风沙土、半固定风沙土到固定风沙土的变化,其土粒表面的平均分形维数呈依次增高。土粒表面分形维数与三种典型风沙土壤有机质含量为极显著正相关,而与其硬度为显著负相关。 相似文献
999.
1000.
研究了珠穆朗玛峰北坡地区沿绒布河谷海拔4400~5550m的夏季土壤中水解酶类脲酶、中性磷酸酶活性,并探讨了这两种土壤酶与土壤有机质和海拔高度的相互关系。结果表明:(1)随着海拔升高,土壤脲酶、中性磷酸酶活性呈高低波动变化,在研究区域出现多个峰值,总的趋势是上升的,其最大值分别出现在海拔5500m和5450m处。(2)偏相关分析表明,土壤脲酶活性与有机质含量呈正相关,土壤中性磷酸酶活性与有机质含量呈显著正相关;土壤脲酶活性与海拔的相关性不具显著性意义,中性磷酸酶活性与海拔呈极显著正相关。 相似文献