全文获取类型
收费全文 | 4062篇 |
免费 | 277篇 |
国内免费 | 401篇 |
专业分类
林业 | 517篇 |
农学 | 264篇 |
基础科学 | 405篇 |
852篇 | |
综合类 | 1331篇 |
农作物 | 141篇 |
水产渔业 | 248篇 |
畜牧兽医 | 328篇 |
园艺 | 105篇 |
植物保护 | 549篇 |
出版年
2024年 | 25篇 |
2023年 | 91篇 |
2022年 | 105篇 |
2021年 | 126篇 |
2020年 | 143篇 |
2019年 | 164篇 |
2018年 | 125篇 |
2017年 | 180篇 |
2016年 | 212篇 |
2015年 | 167篇 |
2014年 | 224篇 |
2013年 | 235篇 |
2012年 | 280篇 |
2011年 | 346篇 |
2010年 | 244篇 |
2009年 | 255篇 |
2008年 | 242篇 |
2007年 | 228篇 |
2006年 | 190篇 |
2005年 | 169篇 |
2004年 | 132篇 |
2003年 | 112篇 |
2002年 | 82篇 |
2001年 | 82篇 |
2000年 | 65篇 |
1999年 | 60篇 |
1998年 | 71篇 |
1997年 | 53篇 |
1996年 | 56篇 |
1995年 | 57篇 |
1994年 | 44篇 |
1993年 | 50篇 |
1992年 | 34篇 |
1991年 | 35篇 |
1990年 | 17篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有4740条查询结果,搜索用时 234 毫秒
991.
992.
This study tested the hypothesis that incorporation of green leaf manure (GLM) from leguminous trees into agroforestry systems
may provide a substitute for inorganic N fertilisers to enhance crop growth and yield. Temporal and spatial changes in soil
nitrogen availability and use were monitored for various cropping systems in southern Malawi. These included Gliricidia sepium (Jacq.) Walp. trees intercropped with maize (Zea mays L.), with and without pigeonpea (Cajanus cajan L.), sole maize, sole pigeonpea, sole gliricidia and a maize + pigeonpea intercrop. Soil mineral N was determined before
and during the 1997/1998, 1998/1999 and 1999/2000 cropping seasons. Total soil mineral N content (NO3− + NH4+) was greatest in the agroforestry systems (p<0.01). Pre-season soil mineral N content in the 0–20 cm horizon was greater in treatments containing trees (≤85 kg N ha−1) than in those without (<60 kg ha−1; p<0.01); however, soil mineral N content declined rapidly during the cropping season. Uptake of N was substantially greater
in the agroforestry systems (200–270 kg N ha−1) than in the maize + pigeonpea and sole maize treatments (40–95 kg N ha−1; p<001). Accumulation of N by maize was greater in the agroforestry systems than in sole maize and maize + pigeonpea (p<0.01); grain accounted for 55% of N uptake by maize in the agroforestry systems, compared to 41–47% in sole maize and maize + pigeonpea.
The agroforestry systems enhanced soil fertility because mineralisation of the applied GLM increased pre-season soil mineral
N content. However, this could not be fully utilised as soil N declined rapidly at a time when maize was too small to act
as a major sink for N. Methods for reducing losses of mineral N released from GLM are therefore required to enhance N availability
during the later stages of the season when crop requirements are greatest. Soil mineral N levels and maize yields were similar
in the gliricidia + maize and gliricidia + maize + pigeonpea treatments, implying that addition of pigeonpea to the tree-based
system provided no additional improvement in soil fertility. 相似文献
993.
M. Browaldh 《Agroforestry Systems》1996,35(2):131-138
The effect of tree harvesting on soil mineral nitrogen and microbiological activity were investigated in an agrisilvicultural system consisting of wheat cultivated along the sides of a poplar plantation in Sweden from 1993 to 1995. Sampling for mineral nitrogen was carried out in three layers down to 90 cm at two distances, near (0.5–1.5 m) and far (4.0–5.0 m) from rows of standing, ST, and harvested trees, HT. Sampling for basic respiration and substrate-induced respiration was carried out in the 0–10 cm layer in 1993 and in the 0–10 cm and 10–20 cm layers in 1994 at the same distances from trees.There was a higher concentration of ammonium and lower concentration of nitrate closer to trees, indicating an efficient uptake of nitrate by trees and enhanced N mineralization close to trees. Shortly after tree harvesting, there were higher concentrations of nitrate and ammonium in the 0–30 cm soil layer near the harvested trees than near standing trees, suggesting a derease in nitrogen uptake by tree roots. The soil microbiological activity was lower in the harvested than the standing plots of trees, which is considered as an indication of the important role of root exudates in maintaining a larger microbial biomass close to trees. 相似文献
994.
运用W.比特利希的可变样圆半径的角规标准地法,在广东封开黑石顶自然保护区选择一种亚热带常绿阔叶林群落,随机布设20个可变半径为50D(D为林木胸径)的调查样圆。调查研究该群落的植物区系及各个树种的密度、相对密度、频度、相对频度、显著度、相对显著度及重要值并据此确定群落的优势及亚优势种;运用分层株数及频度的方法思想把群落按树种分为下层:Ⅰ层,10m以下;Ⅱ层,10—20m;Ⅲ层,20m以上。根据同一树种在以上三层中频度和株数的变化规律及该树种的生态学和生物学特性,初步把该群落的主要树种划分为四种类型:衰退型:如马尾松、福建青冈、米椎;进展型:如黄果厚壳桂、红鳞蒲桃、显脉新木姜;稳定型:如绒英红豆、短花序楠;待变型:如小硬叶楠、铁榄等。应用分层频度(株数)法不仅可以预测某一植物群落区系的未来群落优势种,及未来群落上层树种的结构动态,还可以作为群落是否达到顶极状态的一项数量判别方法。 相似文献
995.
996.
997.
1981~1990连续10年进行稻瘟病菌生理小种类群及分布的研究,先后对总计435个菌株鉴定为7群12~18个小种。在明确各地区小种分布的同时,确定各地优势小种一直为ZG群,与生产上栽培品种抗性保持相对稳定的情况相一致。用日本单基因品种对多数菌株进行致病力测定,具有Pi—Z~t,Pi—b,Pi—ta,Pi—ta~2,Pi—K~m基因型的品种致病率较低,对其它基因型品种致病率较高。稻瘟病菌变异的研究表明,从不同地区,不同品种的穗颈稻瘟病斑上分离的单孢,接种鉴定所属小种有差异;同一病斑上多次继代分离的单孢,其生理小种差异也很大。由此证明稻瘟病菌生理小种有很大变异性。 相似文献
998.
By using the chemical vapor deposition (CVD), a surface with gradient surface energy is fabricated based on the treated silicon chip. The dodecyltrichlorosilane (C_(12)H_(25)Cl_3Si) and Octrytrichlorosilane (C_(8)H_(17)Cl_3Si) are used as the source of silanization. By using the measurement of sessile drop method, the surface with gradient surface energy was characterized by measuring the contact angle of small water which lied on the surface of the treated silicon chip. The experiment shows the liquid droplets are driven to move from hydrophobic to hydrophilic zone on the gradient surface which is placed horizontally, the diameter of the droplets are 1~3 mm, and the single droplet reached the maximum velocity of up to 0.9 m/s. Based on the experimental results, the mechanical of the movement of the liquid droplet on the gradient surface is researched. 相似文献
999.
ZHANG Wen-pu~ 《保鲜与加工》2004,(1):21-24
The simple principle of Contact Image Sensor (CIS) and the noise types existed in CIS are introduced. The theory of suppressing these noise types for CIS by using Correlated Double Sampling(CDS) is described,and the timing relationship between driving signals CP/SP that CIS required and sampling-holding signals SH1/SH2 are also analyzed,these signals are produced by using Complex Programmable Logic Device(CPLD). CDS circuit principle is analyzed,and experiment of suppressing noise for CIS based on CDS is made. The experiment shows that satisfying results are obtained by using the technique of suppressing noise for CIS based on CDS. 相似文献
1000.
The depth of coal mining in Yanshitai mine is over 600 m,dynamics phenomena have been occurred of ter during mining ahd excavating.On the basis of research in Yanshitai mine,this paper recorded the geology and mechanics model of typical dynamics phenomena based on the mining and geology conditions,which have been applied for the mechanism of dynamics phenomena and studied with catastrophe.Meanwhile,the features and influence factor have been studied,and they can be references for mine hazards. 相似文献