全文获取类型
收费全文 | 1817篇 |
免费 | 120篇 |
国内免费 | 219篇 |
专业分类
林业 | 173篇 |
农学 | 161篇 |
基础科学 | 279篇 |
665篇 | |
综合类 | 534篇 |
农作物 | 44篇 |
水产渔业 | 17篇 |
畜牧兽医 | 127篇 |
园艺 | 69篇 |
植物保护 | 87篇 |
出版年
2024年 | 9篇 |
2023年 | 31篇 |
2022年 | 47篇 |
2021年 | 58篇 |
2020年 | 57篇 |
2019年 | 79篇 |
2018年 | 70篇 |
2017年 | 83篇 |
2016年 | 98篇 |
2015年 | 92篇 |
2014年 | 84篇 |
2013年 | 136篇 |
2012年 | 148篇 |
2011年 | 151篇 |
2010年 | 106篇 |
2009年 | 103篇 |
2008年 | 102篇 |
2007年 | 114篇 |
2006年 | 102篇 |
2005年 | 65篇 |
2004年 | 70篇 |
2003年 | 55篇 |
2002年 | 36篇 |
2001年 | 31篇 |
2000年 | 32篇 |
1999年 | 29篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 20篇 |
1994年 | 25篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 17篇 |
1990年 | 6篇 |
1989年 | 13篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有2156条查询结果,搜索用时 15 毫秒
71.
【目的】土壤盐渍化是限制新疆南部棉花高产的主要因子,准确获取区域尺度土壤剖面盐分信息。【方法】以南疆阿拉尔垦区为研究区,以田间尺度采集的30个不同盐渍化程度棉田的540个样点的0~0.375、0~0.750、0~1.000 m的土壤剖面电导率数据和对应的电磁感应数据为数据源,采用线性模型和非线性模型分别构建了田间尺度和区域尺度的土壤剖面电导率的电磁感应反演模型,并采用缩减建模样本量方法进一步检验了区域尺度模型的可靠性和稳定性。【结果】多元线性回归(MLR)、偏最小二乘回归(PLSR)和主成分回归(PCR)建模方法的田间尺度模型R2在0.88~0.95,而对应的区域尺度模型R2在0.34~0.53。基于随机森林(RF)、神经网络(NN)和支持向量机(SVM)非线性建模方法构建的土壤剖面电导率的区域尺度电磁感应反演模型R2在0.60~0.85,其中RF模型的精度最高。0~0.375、0~0.750、0~1.000 m土壤剖面电导率的RF反演模型R2分别为0.80、0.85和0.84,相较于线性建模方法的区域尺度模型精度有明显的提高。RF区域尺度模型的样本数量由540个缩减到240个,模型精度没有明显变化,表明采用区域尺度模型,可大幅度降低土壤剖面样本采集数量,从而可显著提高采样效率和降低采样成本。【结论】区域尺度下构建土壤剖面电导率反演模型时,随机森林建模方法效果较优,模型预测能力具有较高的可靠性。 相似文献
72.
电气照明设计在建筑工程设计中起着重要作用,如何利用高科技、现代化灯光特技功能效果优势,使电气照明达到最佳效果,是从事电气设计工作者值得探讨的课题。笔者依据电气照明设计实际,探讨设计中应注意的几个问题,可供同行参考。 相似文献
73.
74.
E Lambooij J W van de Vis H Kuhlmann W Münkner J Oehlenschläger R J Kloosterboer & C Pieterse 《Aquaculture Research》2002,33(9):643-652
It was observed that farmed eels could be rendered unconscious and insensible instantaneously by passing an electrical current through fresh water. The general epileptiform insult on the EEG was characterized by a tonic/clonic and an exhaustion phase. After stunning, the ECG (electro‐cardiogram) revealed fibrillation. The electrical stunning parameters were on average 194 ± 4 V and 0.636 ± 0.040 A/dm2 for 1.6 ± 0.4 s. Within a confidence level of 95%%, taking into account the number of animals with a reliable EEG (n = 29), at least 93% of all eels are effectively stunned in fresh water by an average current of 0.636 ± 0.040 A/dm2. The behaviour of groups of five eels, which were able to move freely in the water was observed before and after stunning with 50 V and 0.17 A/dm2. After 3‐s stunning, two eels were turned upside down. They changed to a normal position after 10 and 13 s respectively. Subsequently, all eels were very active in swimming behaviour and stopped swimming after 75 s. When stunned for a longer duration, all eels were turned upside down and stopped breathing for a limited period of time. In the last experiment the eels were stunned in fresh water (500 μS) with a voltage of 200 V for approximately 1 s, which was followed by 50 V for 5 min. As soon as the stunning started the water was de‐oxygenated by flushing nitrogen to kill the eels by suffocation during the period of unconsciousness and insensitivity. The oxygen saturation decreased from 74 ± 10 to 23 ± 11% at 22 °C. After stunning no brain activity and no responses to pain stimuli were observed on the EEG. The heart rate increased (P < 0.05) after stunning, which was followed by a significant decrease. Only 1 out of 18 eels returned partially from upside down to a normal position 2 h after stunning; however, the eel did not respond to pain stimuli in behaviour. The developed stunning procedure can be recommended for humane slaughter of 50‐kg batches of eels. 相似文献
75.
Bo-Seong SEO Young-Jae JEONG Nu-Ri BAEK Hyun-Jin PARK Hye In YANG Se-In PARK Woo-Jung CHOI 《土壤圈》2022,32(6):905-915
Electrical conductivity (EC) of soil-water extracts is commonly used to assess soil salinity. However, its conversion to the EC of saturated soil paste extracts (ECe), the standard measure of soil salinity, is currently required for practical applications. Although many regression models can be used to obtain ECe from the EC of soil-water extracts, the application of a site-specific model to different sites is not straightforward due to confounding soil factors such as soil texture. This study was conducted to develop a universal regression model to estimate a conversion factor (CF) for predicting ECe from EC of soil-water extracts at a 1:5 ratio (EC1:5), by employing a site-specific soil texture (i.e., sand content). A regression model, CF=8.910 5e0.010 6sand/1.298 4 (r2=0.97, P < 0.001), was developed based on the results of coastal saline soil surveys (n=173) and laboratory experiments using artificial saline soils with different textures (n=6, sand content=10%-65%) and salinity levels (n=7, salinity=1-24 dS m-1). Model performance was validated using an independent dataset and demonstrated that ECe prediction using the developed model is more suitable for highly saline soils than for low saline soils. The feasibility of the regression model should be tested at other sites. Other soil factors affecting EC conversion factor also need to be explored to revise and improve the model through further studies. 相似文献
76.
The transient analysis of mini disc infiltrometer (MDI) measurements is an established method for characterising near-surface hydraulic characteristics of soils. The reliability of hydraulic characteristics obtained from transient analysis depends on the (1) adequacy of model, (2) adequacy of data, (3) measurement time and (4) measurement footprint. The measurement time dependence recommendations are reported only for a few soil textures, initially wet samples and tension infiltrometer (TI) with a higher measurement footprint than the MDI. This study investigated the adequacy of infiltration data (using cumulative linearization [CL] and differentiated linearization [DL]) and measurement time influence on the hydraulic parameters determined from the transient analysis of MDI measurements for six soil textures. The objective of the study is to identify suitable MDI measurement durations for different soil textures for the initially dry state, considering both adequacy of data and time fractionation (measurement time influence). The data adequacy time obtained from the DL (TDL) was found to be 0.8 times less than the value obtained from CL (TCL). The marginal difference in TDL and TCL had a significant influence on the determination of infiltration equation coefficient C1 and negligible influence on coefficient C2. The time fractionation procedure adopted for identifying adequate MDI measurement time (Tm) was found to be comparable based on sorptivity (S0) and hydraulic conductivity (K0). The average Tm was also comparable with TDL and TCL with a strong positive correlation. The C2 values obtained based on Tm, TDL, and TCL were in better agreement than the corresponding C1 values. The adequate MDI measurement times identified by considering Tm, TDL, and TCL were texture dependent, ranging from 45 min for silt to 120 min for silt loam and silty clay loam. For loamy sand, it was 50 min; for sand, it was 70 min, followed by 60 min for loam. 相似文献
77.
[目的]探讨中华蟾蜍(Bufo bufo gargarizans)胚后嗅球神经元自发电活的电生理学特性的发育变化。[方法]应用微电极电生理技术,对中华蟾蜍胚后嗅球神经元的自发放电活动进行在体胞外记录。[结果]随着嗅球的发育,其神经元的放电形式逐渐增多,单个放电振幅逐渐增大,且在变态期达最大;连续放电振幅在胚后发育的中期变化不大,但高于发育的早期和成体期;连续簇状放电频率随着发育时期而降低,但在成体期再次升高,而连续单个放电频率只在成体期增大。[结论]随着嗅球的发育,神经元的兴奋性逐步提高,神经元电活动形式逐渐呈现多样化。 相似文献
78.
[目的]研究不同工艺参数(竹造纸固剩物粒度、固剩物与漂珠配比、板材厚度)对复合板材各种性能的影响.[方法]采用正交试验设计,利用竹造纸固剩物和漂珠制造复合板材,并通过对力学性能和导热系数的测定,探讨固剩物粉末粒度、固剩物与漂珠配比、板材厚度对板材性能的影响.[结果]随着粒度和板材厚度的增大,板材的物理力学性能MOE、24hTS呈现下降趋势;随着配比的增大,MOR、MOE增大,24hTS减小.[结论]最佳工艺为固剩物粒度为20~40目、固剩物与漂珠配比为8∶2、板材厚度为14 mm时,此时板材的各种性能达到最优. 相似文献
79.
电导法对32个辣椒品种组合抗寒性的测定及配合力分析 总被引:2,自引:0,他引:2
为了比较辣椒品种的抗寒性,以32个线椒杂交组合为试材,在人工气候箱下进行低温处理,应用电导法测定叶片相对电导率(REC)。通过Logistic曲线方程拟合求出拐点温度(即半致死温度)(LT50),作为各试材抗寒性评价的参考指标,并将32个线椒组合的半致死温度(LT50)进行比较。结果表明:随处理温度的降低,相对电导率不断升高,变化过程呈"S"型曲线。32个线椒组合抗寒性顺序依次为:H2H13≈H28H31≈H1≈H23H24H26H30H8H27≈H19H5≈H12H21H3H18H15≈H14H22H17H7≈H29H4H11H10≈H16H20H6H9H25H32。配合力分析表明,相对电导率的GCA效应值以亲本P1和P2最低,SCA效应值以P1×P2最低。 相似文献
80.