全文获取类型
收费全文 | 24430篇 |
免费 | 358篇 |
国内免费 | 646篇 |
专业分类
林业 | 4351篇 |
农学 | 2297篇 |
基础科学 | 638篇 |
3634篇 | |
综合类 | 3628篇 |
农作物 | 2597篇 |
水产渔业 | 2063篇 |
畜牧兽医 | 2339篇 |
园艺 | 1627篇 |
植物保护 | 2260篇 |
出版年
2024年 | 17篇 |
2023年 | 64篇 |
2022年 | 232篇 |
2021年 | 295篇 |
2020年 | 284篇 |
2019年 | 291篇 |
2018年 | 2882篇 |
2017年 | 2945篇 |
2016年 | 1400篇 |
2015年 | 463篇 |
2014年 | 390篇 |
2013年 | 421篇 |
2012年 | 1376篇 |
2011年 | 2754篇 |
2010年 | 2739篇 |
2009年 | 1827篇 |
2008年 | 1842篇 |
2007年 | 2147篇 |
2006年 | 493篇 |
2005年 | 510篇 |
2004年 | 331篇 |
2003年 | 300篇 |
2002年 | 188篇 |
2001年 | 159篇 |
2000年 | 148篇 |
1999年 | 152篇 |
1998年 | 96篇 |
1997年 | 94篇 |
1996年 | 87篇 |
1995年 | 71篇 |
1994年 | 53篇 |
1993年 | 79篇 |
1992年 | 68篇 |
1991年 | 44篇 |
1990年 | 44篇 |
1989年 | 47篇 |
1988年 | 35篇 |
1987年 | 13篇 |
1986年 | 19篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1977年 | 4篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
992.
高浓度CO2下红松幼苗根系呼吸对土壤呼吸的贡献 总被引:4,自引:0,他引:4
本文于2003年5月至10月在长白山森林生态系统定位站内研究了高浓度CO2(500和700靘olmol-1)对红松幼苗土壤呼吸以及根系呼吸对土壤呼吸的贡献。经过4个生长季高浓度CO2的处理,利用LI-6400-09土壤呼吸室对红松幼苗土壤总呼吸和根系呼吸进行了测定。为了区分根系呼吸对土壤总呼吸的贡献,本文采用了PVC管断根法,即每种处理下将三根PVC管插入土壤中30cm以切断根系,从而终止了植物冠层对根系碳水化合物的供应。分别于6月16日、8月20日和10月8日对管内外土壤呼吸进行了测定。结果表明大气和土壤5cm温度都存在明显的日变化,但不同处理之间没有显著差别(P>0.05)。土壤总呼吸和断根土壤呼吸也有明显的日变化和季节变化。不同处理之间土壤总呼吸和断根土壤呼吸差异显著(P<0.01)。6月16日、8月20日和10月8日不同处理下土壤总呼吸和根系呼吸的贡献的平均值分别为3.26、4.78和1.47靘olm-2s-1以及11.5%、43.1%和27.9%。图5表1参38。 相似文献
993.
994.
在盆栽和大田试验条件下测定了最新引入我国的盐草属(Distichlis spicata)植物NyPa宽叶和NyPa窄叶草的抗逆性。结果表明:在用盐水灌溉的条件下,盆栽NyPa草的高生长量明显大于对照植物高羊茅,用含盐量0~30‰范围的盐水灌溉(即从淡水到海水灌溉)对其高生长均没有明显影响,而高羊茅在20‰盐水灌溉后死亡;大田条件下,NyPa草在5‰~6‰含盐量的土壤上植株高生长正常,而高羊茅则受到明显影响;NyPa草可在-20℃低温下安全越冬,对早霜和晚霜有极强的抵抗能力;夏季高温天气下NyPa草可耐10多天的地面水淹环境;在试验点气候及盐碱土壤条件下,连续2年不进行灌水施肥,对其生长未见明显影响。说明NyPa草是一个具有较强抗逆特性的饲料和草坪兼用型草本植物。 相似文献
995.
1993~ 1994年 ,在广东省开平市国营镇海林场建立了 2个巨尾桉组培苗瓶内接种外生菌根菌的野外造林试验。 2个试验中 ,共采用 17个外生菌根菌菌株。造林后定期观测了 12或 2 4个月树高、地径和胸径等生长指标。试验结果表明 :除试验 2造林初期的树高外 ,其他所有生长指标在菌根菌处理间均有极显著差异或显著差异 ,菌根菌间对巨尾桉生长有不同的影响 ;从多重比较和接种效果分析看 ,菌株E4 0 70、H4 5 0 9、E4 2 4 0、H6 177、H4 339、E4 72 6、C930 1、C92 13和C92 0 3,在造林后 6个月开始可明显地促进巨尾桉树木生长 ,说明组培苗瓶内菌根化技术是完全可行的 ,有关菌株对巨尾桉幼树生长具有促进作用 相似文献
996.
本文通过介绍基于J2EE技术框架的林业经济信息系统的构建,指出了为林业经济信息的集成与共享提供一个基础平台的重要作用与意义,以及其构建的方法与步骤。 相似文献
997.
Sun Hong-gang Zhang Jian-guo Duan Ai-guo He Cai-yun 《中国林学(英文版)》2007,9(1):85-94
Growth and yield modeling has a long history in forestry. The methods of measuring the growth of stand basal area have evolved from those developed in the U.S.A. and Germany during the last century. Stand basal area modeling has progressed rapidly since the first widely used model was published by the U.S. Forest Service. Over the years, a variety of models have been developed for predicting the growth and yield of uneven/even-aged stands using stand-level approaches. The modeling methodology has not only moved from an empirical approach to a more ecological process-based approach but also accommodated a variety of techniques such as: 1) simultaneous equation methods, 2) difference models, 3) artificial neural network techniques, 4) linear/nonlinear regres-sion models, and 5) matrix models. Empirical models using statistical methods were developed to reproduce accurately and precisely field observations. In contrast, process models have a shorter history, developed originally as research and education tools with the aim of increasing the understanding of cause and effect relationships. Empirical and process models can be married into hybrid mod-els in which the shortcomings of both component approaches can, to some extent, be overcome. Algebraic difference forms of stand basal area models which consist of stand age, stand density and site quality can fully describe stand growth dynamics. This paper reviews the current literature regarding stand basal area models, discusses the basic types of models and their merits and outlines recent progress in modeling growth and dynamics of stand basal area. Future trends involving algebraic difference forms, good fitting variables and model types into stand basal area modeling strategies are discussed. 相似文献
998.
Wang Wei Zhao Lin-sen 《中国林学(英文版)》2007,9(3):213-216
The importance of ecological benefit analyses about green space systems has been widely discussed on the basis of a per-ceived bias towards the landscaping effect of green spaces to be built in urban areas as opposed to the perception of comprehensive ecological benefits of designed plant communities. Given the basic principle of ecological benefit analyses and evaluation methods widely applied today, the methodology to calculate ecological benefits and to model the expansion of urban green spaces by CITY-green 5.0 and its conditions for application were explored. We propose that aerial images can be substituted by AutoCAD graphics of a green space containing detailed information of plant dispositions by digitizing the key features during the working process in order to evaluate the ecological benefits quickly and to offer proper guidance to the establishment of small scale green spaces in urban areas. The theoretical foundation, potential application range and prospects for extension of the method are introduced by using the analysis of ecological benefits of the green spaces on the campus of Southwest Forestry College, Kunming, as an example. 相似文献
999.
1000.
Reproductive phenology was observed over three years in a Eucalyptus grandis seed orchard in Madagascar to determine the impact of geographic differences of parental selections on the expected genetic composition of seed crops. Pollen flow (observed pollen cloud) was deduced from a paternity assignment based on sampling from one year’s seed production. The two approaches were used to verify whether knowledge of phenology is sufficient to predict the genetic quality of the seeds collected. Despite the high flowering level, with a constant cycle over three years, the results demonstrated wide reproductive phenological differences associated with the parents’ origin, suggesting putative pollination disequilibrium. From both observed and expected pollen clouds, the results showed preferential mating among different provenances, which has consequences for seed crop composition. Phenological observations, which can be made inexpensively, can be used to promote methods of effective seed orchard management in order to improve the genetic quality of seeds. 相似文献