全文获取类型
收费全文 | 16565篇 |
免费 | 968篇 |
国内免费 | 1279篇 |
专业分类
林业 | 1596篇 |
农学 | 946篇 |
基础科学 | 364篇 |
6718篇 | |
综合类 | 6328篇 |
农作物 | 559篇 |
水产渔业 | 381篇 |
畜牧兽医 | 885篇 |
园艺 | 371篇 |
植物保护 | 664篇 |
出版年
2024年 | 201篇 |
2023年 | 524篇 |
2022年 | 658篇 |
2021年 | 646篇 |
2020年 | 650篇 |
2019年 | 733篇 |
2018年 | 584篇 |
2017年 | 945篇 |
2016年 | 1072篇 |
2015年 | 864篇 |
2014年 | 979篇 |
2013年 | 1347篇 |
2012年 | 1541篇 |
2011年 | 1330篇 |
2010年 | 928篇 |
2009年 | 873篇 |
2008年 | 762篇 |
2007年 | 823篇 |
2006年 | 677篇 |
2005年 | 554篇 |
2004年 | 372篇 |
2003年 | 301篇 |
2002年 | 206篇 |
2001年 | 187篇 |
2000年 | 165篇 |
1999年 | 128篇 |
1998年 | 139篇 |
1997年 | 97篇 |
1996年 | 82篇 |
1995年 | 90篇 |
1994年 | 48篇 |
1993年 | 59篇 |
1992年 | 50篇 |
1991年 | 43篇 |
1990年 | 37篇 |
1989年 | 34篇 |
1988年 | 28篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1962年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
181.
Soil chemical, biochemical, biological and structural properties were measured in two New Zealand loessial soils that were topsoil-mined 10 and 25 years ago respectively. Measurements at the 10-year site were compared to some earlier measurements made at this site and the data combined in a chronological sequence for analysis. Topsoil mining had a large, detrimental impact on the soil microbial biomass, the earthworm populations, easily mineralizable N and soil enzyme activities. However, most of these properties substantially recovered, to 80-90 per cent of the levels in unmined soils, within 10-25 years of restoration under pasture. In contrast, while total soil C and N were less affected by topsoil mining, their recovery was much slower. Stabilities of macro-aggregates of soil had fully recovered within 10-25 years after topsoil mining. The apparent changes in all the measured properties between 10 and 25 years of restoration were small in comparison with changes between 0-10 years of restoration after topsoil mining. The total C content of both soils under pasture appeared unlikely to attain the levels present in unmined soils. In soils undergoing restoration, the ratio of microbial C/total soil C may be a useful index of soil ‘biological stability’. Sulphatase activity may reflect the recovery of pasture production. 相似文献
182.
R. G. Barber 《Land Degradation \u0026amp; Development》1995,6(2):95-107
This paper examines the soil degradation processes that are threatening the productivity of the mechanized annual cropping areas in the Santa Cruz lowlands of tropical eastern Bolivia. The dominant process is that of machinery-induced and natural compaction, which has resulted in an estimated 50 per cent of the soils in the Central Zone being moderately to severely compacted, causing serious root restriction and the loss of both transmission and water-storage pores. Degradation has made the soils increasingly susceptible to moisture stress due to the combined effect of (i) restricted rooting as a result of compaction and the hardsetting characteristics of the soils, (ii) reduced rainfall infiltration due to the loss of transmission pores and surface crusting, and (iii) a decrease in available soil moisture caused by the loss of storage pores, the incorporation of wind-blown fine sand deposits, and soil organic matter losses due to accelerated decomposition rates. The loss of transmission pores has also made the soils more prone to waterlogging in periods of high rainfall. The degradation tendencies of these soils are exacerbated by the greater variability of seasonal rainfall during the last 20 years that has led to a greater frequency or extremely high or extremely low rainfall events than hitherto. 相似文献
183.
采用杉木屑为原料,氢氧化钠为活化剂制备木质活性炭产品,探讨活化时间、活化温度、氢氧化钠浓度等工艺参数对木质活性炭的得率和吸附性能的影响.结果 表明,随活化温度、氢氧化钠浓度和活化时间的增大,木质活性炭的得率呈不断下降的趋势,木质活性炭的吸附性能呈先上升后下降的趋势.较优的工艺条件为:活化温度850℃、活化时间1.0h、... 相似文献
184.
185.
旅游循环经济与低碳经济之间存在耦合机制,为减少碳源和增加碳汇,循环经济下的乡村旅游资源低碳化开发方法主要有梯次开发、变废为宝、资源共享、循环经济技术、生态补偿、地方性利用六种。自然环境美化、示范效应和融合发展是具体开发模式,其中,低碳型的融合模式以产业外部循环和要素内部循环为主要表现。 相似文献
186.
187.
正交法优化白平菇深层培养基的研究 总被引:1,自引:0,他引:1
采用液体深层培养的方法,研究了白平菇对碳氮源的利用情况,并以三因素三水平的正交试验进行了培养基配方的优化。结果表明:最适碳源为红糖、乳糖、淀粉,最适氮源为麦麸、酵母粉、蛋白胨,最适碳氮比为20~30∶1,最适培养基为红糖1.5%、麦麸3%、硫酸镁0.1%。 相似文献
188.
189.
有机磷农药与小麦酯酶作用关系研究 总被引:3,自引:1,他引:2
本研究以小麦为植物酯酶的酶源,α-乙酸萘酯为底物,考察了小麦酯酶与有机磷农药之间作用的关系,建立了10个以吸光值变化表示的酶活性与有机磷农药浓度对数之间的线性方程,并确定了函数的单调区间和极值。 相似文献
190.
土壤有机碳库及其控制因子的研究进展 总被引:3,自引:0,他引:3
土壤碳库的动态平衡影响作物产量和土壤肥力的高低,是土壤肥力保持和提高的重要研究内容。本文简要评述了气候、植被、土壤理化特性和农业生产对土壤有机碳分解的影响,介绍了土壤有机碳库及其控制因子的研究进展。 相似文献