全文获取类型
收费全文 | 4044篇 |
免费 | 294篇 |
国内免费 | 416篇 |
专业分类
林业 | 273篇 |
农学 | 222篇 |
基础科学 | 245篇 |
1669篇 | |
综合类 | 1368篇 |
农作物 | 134篇 |
水产渔业 | 350篇 |
畜牧兽医 | 245篇 |
园艺 | 67篇 |
植物保护 | 181篇 |
出版年
2024年 | 32篇 |
2023年 | 121篇 |
2022年 | 149篇 |
2021年 | 143篇 |
2020年 | 167篇 |
2019年 | 184篇 |
2018年 | 134篇 |
2017年 | 242篇 |
2016年 | 264篇 |
2015年 | 168篇 |
2014年 | 187篇 |
2013年 | 215篇 |
2012年 | 308篇 |
2011年 | 275篇 |
2010年 | 246篇 |
2009年 | 213篇 |
2008年 | 193篇 |
2007年 | 271篇 |
2006年 | 182篇 |
2005年 | 140篇 |
2004年 | 138篇 |
2003年 | 109篇 |
2002年 | 107篇 |
2001年 | 85篇 |
2000年 | 64篇 |
1999年 | 52篇 |
1998年 | 52篇 |
1997年 | 40篇 |
1996年 | 38篇 |
1995年 | 39篇 |
1994年 | 59篇 |
1993年 | 33篇 |
1992年 | 33篇 |
1991年 | 22篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有4754条查询结果,搜索用时 46 毫秒
991.
以27个分属MGⅠ~MGⅧ的北美大豆生育期组标准品种为参照,于2013-2015年在广西南宁市广西农业科学院明阳基地春播条件下,对广西春大豆主栽品种,新选育的春大豆品种及广西春大豆区域试验和热带亚热带地区春大豆区域试验对照种的生育期组归属进行了鉴定与划分。结果表明:在南宁春播条件下所有北美大豆生育期组标准品种均正常成熟,生育日数趋势基本是由短到长,但不同年份间却有所差异。不同年份不同生育期组内生育日数差异非常接近,同一年份不同生育期组间生育日数存在交叉现象。热带、亚热带南方春大豆区域试验对照品种华春6号生育期组归属于MGⅠ,春大豆晚熟品种华春1号生育期归属于MGⅤ,华春2号和华春5号生育期归属于MGⅣ。广西春大豆品种生育期组归属于MGⅢ和MGⅣ,夏大豆品种生育期组归属于MGⅦ和MGⅧ。鉴定结果可为我国大豆生育期组系统建立、大豆育种、引种及品种布局提供依据。 相似文献
992.
993.
994.
文章首先对无障碍设计的含义进行了分析,通过对南京市道路和公共建筑无障碍设施方面的实地调查研究,总结现存的问题,在借鉴国内外无障碍建设经验,提出了一系列的规划设计方法,以期望达到无障碍建设达标、使用良好的效果。 相似文献
995.
介绍了新时期农村群体性事件的特征,运用经济学的相关理论,从经济学的角度探求群体利益受损的原因,以及群体为何采取这种极端的利益诉求方式,并在此基础上提出农村群体性事件的预防与应对建议。 相似文献
996.
997.
Interstitial water conditions in the hyporheic zone of the stream bed are determinants of salmonid egg hatching success. We used standardised egg exposures to develop and validate discriminant analysis and generalised linear model models linking the hatching success of brown trout (Salmo trutta) with physicochemical factors of the interstitial zone (e.g., oxygen, specific conductance, nitrate, nitrite, ammonium, pH and redox potential). Interstitial water quality was identified as a limiting factor for egg development (median of relative hatching rates = 0.23). Hatching success was unimodal in hatchery and field references incubated in free‐flowing water, but bimodal (very high or very low hatching success) in natural sediment exposures. The effects of physicochemical factors on the hatching success of Salmo trutta strongly depended on both the time and spatial scale analysed. The variables retained in the models differed between the macroscale (over all rivers), the river‐specific scale (within a river) and the microscale (at different sediment depths). Egg hatching success decreased with increased substratum depth (decrease of 26% in 150 mm compared with 50 mm). Increasingly more variable interstitial water conditions (e.g., oxygen) throughout the egg incubation period suggest progressive degradation rates in the stream substratum during the incubation period at the microscale level. Consequently, consideration of different spatial and temporal scales is necessary for the evaluation of habitat quality in salmonid conservation and catchment management plans. 相似文献
998.
Xiang ZHAO 《干旱区科学》2019,11(2):267-279
The aboveground primary production is a major source of carbon(C) and nitrogen(N) pool and plays an important role in regulating the response of ecosystem and nutrient cycling to natural and anthropogenic disturbances. To explore the mechanisms underlying the effect of spring fire and topography on the aboveground biomass(AGB) and the soil C and N pool, we conducted a field experiment between April 2014 and August 2016 in a semi-arid grassland of northern China to examine the effects of slope and spring fire, and their potential interactions on the AGB and organic C and total N contents in different plant functional groups(C_3 grasses, C_4 grasses, forbs, Artemisia frigida plants, total grasses and total plants).The dynamics of AGB and the contents of organic C and N in the plants were examined in the burned and unburned plots on different slope positions(upper and lower). There were differences in the total AGB of all plants between the two slope positions. The AGB of grasses was higher on the lower slope than on the upper slope in July. On the lower slope, spring fire marginally or significantly increased the AGB of C_3 grasses, forbs, total grasses and total plants in June and August, but decreased the AGB of C_4 grasses and A.frigida plants from June to August. On the upper slope, however, spring fire significantly increased the AGB of forbs in June, the AGB of C_3 grasses and total grasses in July, and the AGB of forbs and C_4 grasses in August. Spring fire exhibited no significant effect on the total AGB of all plants on the lower and upper slopes in 2014 and 2015. In 2016, the total AGB in the burned plots showed a decreasing trend after fire burning compared with the unburned plots. The different plant functional groups had different responses to slope positions in terms of organic C and N contents in the plants. The lower and upper slopes differed with respect to the organic C and N contents of C_3 grasses, C_4 grasses, total grasses, forbs, A. frigida plants and total plants in different growing months. Slope position and spring fire significantly interacted to affect the AGB and organic C and N contents of C_4 grasses and A. frigida plants. We observed the AGB and organic C and N contents in the plants in a temporal synchronized pattern. Spring fire affected the functional AGB on different slope positions, likely by altering the organic C and N contents and, therefore,it is an important process for C and N cycling in the semi-arid natural grasslands. The findings of this study would facilitate the simulation of ecosystem C and N cycling in the semi-arid grasslands in northern China. 相似文献
999.
Soil erosion is widespread in agricultural lands of the US Corn Belt. The objective of this study was to examine the impact of antecedent erosion on loss of soil under laboratory simulated rainfall. The soil was obtained from the surface layer of eroded (ER) and uneroded (UN) sites within a conservation agro‐ecosystem in central Ohio, USA. Air‐dried soil was subjected to a rainfall simulation for 60 min (dry run), and to another simulation (wet run) 24 h after the dry run. In the dry run, the cumulative water runoff, sediment yield, and soil organic carbon loss were higher in ER (12.3 L/m2, 169.3 g/m2, and 5.6 g/m2, respectively) than in the UN (7.3 L/m2, 22.6 g/m2, and 0.9 g/m2 respectively). An opposite trend was observed for the cumulative water infiltration (0.9 and 3.9 L/m2, respectively). In the wet run, despite a similar cumulative water runoff from the two erosional phases (20.1 and 19.6 L/m2 in ER and UN respectively), sediment yield and soil organic carbon loss were higher in ER (484.4 g/m2, and 16.3 g/m2 respectively) than in the UN (146.6 g/m2, and 5.3 g/m2 respectively). Also for the wet run, an opposite trend was observed for the cumulative infiltration (0.8 and 5.8 L/m2 respectively). This study suggests that past erosional processes increase the susceptibility of remaining soil to accelerated erosion. 相似文献
1000.
基于香蕉、粉蕉成熟过程中硬度变化建立成熟度随机森林模型 总被引:1,自引:0,他引:1
【目的】研究香蕉和粉蕉成熟过程中果实柄部、中部和端部的硬度变化,并据此建立随机森林模型预测其成熟度。【方法】通过GY-4型果实硬度计分别测量香蕉和粉蕉成熟过程中果实柄部、中部、端部3个部位的硬度,并用相关性分析研究果实硬度与其他相关成熟指标之间的关系。使用相似性分析(ANOSIM)分别对香蕉与粉蕉成熟过程中果实3个部位的硬度进行分级后,通过随机森林算法(Random Forest)构建模型预测其成熟度,再比较果实各部位硬度对模型的重要性。【结果】香蕉和粉蕉成熟过程中果实硬度变化趋势相似,果实3个部位的硬度变化不一致。利用相关性分析发现香蕉与各种酶类活性和乙烯释放量呈显著负相关关系。香蕉和粉蕉成熟度随机森林模型的错误率分别为4.94%和0%,说明此模型能准确预测香蕉与粉蕉的成熟度,且香蕉果实中部与粉蕉果实柄部的硬度对模型的影响最大。【结论】利用随机森林模型,香蕉和粉蕉成熟过程中果实硬度能准确预测其成熟度,且果实3个部位的硬度变化不一致,根据果实不同部位的成熟度差异可提高利用率,香蕉果实中部与粉蕉果实柄部的硬度更能反映自身的成熟度,为鲜食蕉成熟度量化与快速监测分级提供参考,促进食品加工工业的发展。 相似文献