全文获取类型
收费全文 | 4164篇 |
免费 | 125篇 |
国内免费 | 297篇 |
专业分类
林业 | 264篇 |
农学 | 141篇 |
基础科学 | 128篇 |
1084篇 | |
综合类 | 2575篇 |
农作物 | 42篇 |
水产渔业 | 19篇 |
畜牧兽医 | 75篇 |
园艺 | 118篇 |
植物保护 | 140篇 |
出版年
2024年 | 11篇 |
2023年 | 40篇 |
2022年 | 55篇 |
2021年 | 60篇 |
2020年 | 68篇 |
2019年 | 89篇 |
2018年 | 63篇 |
2017年 | 211篇 |
2016年 | 215篇 |
2015年 | 209篇 |
2014年 | 232篇 |
2013年 | 291篇 |
2012年 | 347篇 |
2011年 | 430篇 |
2010年 | 383篇 |
2009年 | 342篇 |
2008年 | 311篇 |
2007年 | 330篇 |
2006年 | 246篇 |
2005年 | 145篇 |
2004年 | 80篇 |
2003年 | 91篇 |
2002年 | 54篇 |
2001年 | 37篇 |
2000年 | 26篇 |
1999年 | 33篇 |
1998年 | 32篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1995年 | 17篇 |
1994年 | 18篇 |
1993年 | 38篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有4586条查询结果,搜索用时 596 毫秒
71.
Neutral models for testing landscape hypotheses 总被引:5,自引:0,他引:5
Neutral landscape models were originally developed to test the hypothesis that human-induced fragmentation produces patterns
distinctly different from those associated with random processes. Other uses for neutral models have become apparent, including
the development and testing of landscape metrics to characterize landscape pattern. Although metric development proved to
be significant, the focus on metrics obscured the need for iterative hypothesis testing fundamental to the advancement of
the discipline. We present here an example of an alternative neutral model and hypothesis designed to relate the process of
landscape change to observed landscape patterns. The methods and program, QRULE, are described and options for statistical
testing outlined. The results show that human fragmentation of landscapes results in a non-random association of land-cover
types that can be describe by simple statistical methods. Options for additional landscape studies are discussed and access
to QRULE described in the hope that these methods will be employed to advance our understanding of the processes that affect
the structure and function in human dominated landscapes. 相似文献
72.
Land-use and vegetation cover have been linked to the nutrient levels (nitrogen, phosphorus) of surface waters in several
countries. However, the links generally relate to streams and rivers, or to specific types of standing water, for example
shallow lakes in a geologically defined region. We measured physical variables and nutrient chemistry of 45 water bodies representative
of the wide range of lentic wetland environments (swamps, riverine wetlands, estuaries, reservoirs, shallow lakes, deep lakes)
in Otago, New Zealand, and related these to catchment variables and land-use in order to assess the potential influence of
catchment modification on water quality of these diverse wetlands. Catchment boundaries and land cover were derived from maps
using ArcView GIS software. Our predictions that concentrations of nutrients and other components of water quality would correlate
positively with the nature and intensity of catchment modification were confirmed in multivariate analyses. Physical and chemical
measures were positively related to the extent of modification in the catchment (percentage of the catchment in pasture, planted
forest, scrub and urban areas), and negatively related to lack of catchment modification (more of the catchment in bare ground,
tussock grassland and indigenous forest). The strong negative correlations between nutrient concentrations, suspended sediment,
water colour and the percentage of tussock cover in the catchment imply that increased conversion of the␣native tussock grassland
to pastoral farming in␣Otago will increase nutrient concentrations and␣reduce water quality of the diverse lentic ecosystems. 相似文献
73.
74.
从农民土地利用偏好以及居民地域性认同出发,将农民土地利用偏好分为农业生产功能偏好和经济生产功能偏好2类;并运用数学方法求取居民地域性认同综合指数,建立Logistic二项回归模型,研究农民土地利用偏好和居民地域性认同的相关性。结果表明,地域性认同程度高的农民更偏好于土地的经济生产功能。地方在开展土地整理或制定土地利用计划时,可根据当地居民地域性认同程度选择增大农地面积或是商铺、工厂的面积,提高人民的满意度。 相似文献
75.
Urbanization is a process that is undergoing all over the world, which will speed up in the forthcoming years, especially in China as the boom of economy. On average, urbanization level is not only depended on the speed, but theefficiency, particularly efficiency of using land resource which affects urbanization directly. This paper provided status quo of land resource utilization efficiency, indictors, methods and factors, and illustrated the reference of well land utilization, aiming at fostering urbanization in China. 相似文献
76.
简析土地行政公益诉讼制度的设立 总被引:1,自引:0,他引:1
分析了土地行政公益诉讼制度的内涵与特征,阐释了设立土地行政公益诉讼制度的法律依据,明确设立土地行政公益诉讼制度的意义。 相似文献
77.
吉林省耕地压力指数时空分异特征研究及其预测研究 总被引:1,自引:0,他引:1
笔者运用耕地压力指数模型和灰色系统理论GM(1,1)模型,对吉林省2003—2012 年间的耕地利用压力指数时空分异特征进行分析,并对其动态变化和未来发展趋势进行了预测。研究结果显示:随着时间的推移,加之国家放开“二胎政策”人口不断在增加,未来人口会出现增加趋势,即使在规定未来十年内人们的人均食物需求量也会不增加的基础上,最小人均耕地面积的增加幅度越来越小,以及大量耕地向建设用地转换,使得耕地压力指数有过大的趋向,而且2017—2027 年吉林省的耕地压力指数处于上升的趋向,即使耕地压力不是很明显,但仍不可忽视。过大的趋向,而且2017-2027年吉林省的耕地压力指数处于上升的趋向,即使耕地压力不是很明显,但仍不可忽视。 相似文献
78.
79.
80.
技术进步是农业发展的必要条件之一。但技术的发展受到资源禀赋和市场需求的影响制约。在近代由于我国和西方在耕地资源禀赋和市场发育等方面存在显著差异,导致了我国和西方在农业技术进步和社会发展方面的巨大不同。 相似文献