全文获取类型
收费全文 | 2751篇 |
免费 | 619篇 |
国内免费 | 289篇 |
专业分类
林业 | 264篇 |
农学 | 122篇 |
基础科学 | 119篇 |
1390篇 | |
综合类 | 1090篇 |
农作物 | 36篇 |
水产渔业 | 13篇 |
畜牧兽医 | 223篇 |
园艺 | 20篇 |
植物保护 | 382篇 |
出版年
2024年 | 4篇 |
2023年 | 88篇 |
2022年 | 137篇 |
2021年 | 110篇 |
2020年 | 83篇 |
2019年 | 66篇 |
2018年 | 86篇 |
2017年 | 122篇 |
2016年 | 145篇 |
2015年 | 120篇 |
2014年 | 162篇 |
2013年 | 206篇 |
2012年 | 218篇 |
2011年 | 221篇 |
2010年 | 254篇 |
2009年 | 202篇 |
2008年 | 217篇 |
2007年 | 233篇 |
2006年 | 186篇 |
2005年 | 182篇 |
2004年 | 127篇 |
2003年 | 85篇 |
2002年 | 57篇 |
2001年 | 73篇 |
2000年 | 35篇 |
1999年 | 25篇 |
1998年 | 26篇 |
1997年 | 25篇 |
1996年 | 19篇 |
1995年 | 20篇 |
1994年 | 34篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 20篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有3659条查询结果,搜索用时 31 毫秒
1.
中国三大自然区域紫花苜蓿土壤速效钾丰缺指标和推荐施钾量 总被引:2,自引:0,他引:2
为给内蒙古高原区、黄土高原区和西北荒漠绿洲区紫花苜蓿测土施肥奠定科学基础,采用零散实验数据整合法以及养分平衡-地力差减法,开展了三大自然区域紫花苜蓿土壤速效钾丰缺指标和推荐施钾量研究。结果表明,内蒙古高原区紫花苜蓿土壤速效钾第1~4级指标依次为≥342mg/kg、89~342mg/kg、24~89mg/kg和<24mg/kg,黄土高原区第1~6级指标依次为≥171mg/kg、96~171mg/kg、54~96mg/kg、30~54mg/kg、17~30mg/kg和<17mg/kg,西北荒漠绿洲区第1~4级指标依次为≥303mg/kg、140~303mg/kg、65~140mg/kg和<65mg/kg;当目标产量9~27t/hm2、钾肥利用率50%时,第1~6级土壤的推荐施钾量分别为0、54~162kg/hm2、108~324kg/hm2、162~486kg/hm2、216~648kg/hm2和270~810kg/hm2。 相似文献
2.
3.
设置标准径流小区进行长期定位监测,探讨天然降雨条件下,流域内不同土地利用方式对坡面产流产沙的影响,以期为该流域土地利用规划、水土流失综合治理、生态环境改善提供理论依据。基于汾河上游阳坡小流域内2种坡度、6种不同土地利用方式的8个径流小区,开展自然降雨观测、径流泥沙观测。结果表明:(1)试验期降雨超过多年同期平均雨量,属于丰水年;且年内雨量分布较多年平均差异较大;(2)不同土地利用方式条件下,坡面产流量差异明显,坡面为15°时,各径流小区的产流量大小顺序为裸坡地>坡耕地>草地;而18°各径流小区的产流量大小为裸坡地>草地>油松林地>灌木林地(黄刺玫);(3)土地利用方式不同,坡面产沙量差异明显。15°径流小区坡面产沙量裸坡地>胡麻>土豆>草地;相比之下,18°径流小区产沙量以灌木林地和油松林地最少,均为0.30 t/hm2;裸坡地产沙量最多,是林地的92.08倍;林草地减流减沙效果明显。研究结果可为该流域的生态重建,土地利用以及汾河上游其他小流域的土地利用规划,从而减少水土流失提供借鉴。 相似文献
4.
《国际水土保持研究(英文)》2021,9(4):520-531
This research aims to improve erosion control practice in the Loess Plateau, by studying the surface erosion processes, including splash, sheet/interrill and rill erosion in four contrasting soils under high rainfall intensity (120 mm h−1) with three-scale indoor artificial experiments. Four contrasting soils as sandy loam, sandy clay loam, clay loam and loamy clay were collected from different parts of the Loess Plateau. The results showed that sediment load was significantly impacted by soil properties in all three sub-processes. Splash rate (4.0–21.6 g m−2∙min−1) was highest in sandy loam from the north part of the Loess Plateau and showed a negative power relation with the mean weight diameter of aggregates after 20 min of rainfall duration. The average sediment load by sheet/interrill erosion (6.94–42.86 g m−2∙min−1) was highest in clay loam from middle part of the Loess Plateau, and the stable sediment load after 20 min showed a positive power relation with the silt content in soil. The average sediment load increased dramatically by rill and interrill erosion (21.03–432.16 g m−2∙min−1), which was highest in loamy clay from south part of the Loess Plateau. The average sediment load after the occurrence of rill showed a positive power relation with clay content and a negative power relation with soil organic matter content. The impacts of slope gradient on the runoff rate and sediment load also changed with soil properties. The critical factors varied for different processes, which were the aggregate size for splash erosion, the content of silt particles and slope gradient for sheet/interrill erosion, and the content of clay particles, soil organic matter and slope gradient for rill erosion. Based on the results of the experiments, specific erosion control practices were proposed by targeting certain erosion processes in areas with different soil texture and different distribution of slope gradient. The findings from this study should support the improvement of erosion prediction and cropland management in different regions of the Loess Plateau. 相似文献
5.
黄土高原高寒区不同人工林土壤养分及生态化学计量特征 总被引:1,自引:1,他引:0
6.
为了解苹果复合种植即果-蔬[MB]、果-草[MH]、果-荒[MW]、果-粮[MZ]模式下土壤动物群落结构特征,采用手拣法和改良干、湿生漏斗法,对黄土残塬沟壑区苹果园4种复合种植模式下的土壤动物群落组成及特征进行调查研究。4种模式下共分离得到土壤动物57.33百只/m~2,隶属4门11纲23目42个类群。4种复合种植模式下土壤动物个体密度和类群数的垂直分布特征表现出明显的表聚特征,水平分布特征表现为果-草[MH]果-蔬[MB]果-荒[MW]≥果-粮[MZ]。果-草[MH]复合种植模式下土壤动物的多样性指数、均匀度指数及丰富度指数最高,优势度指数最低,表明4种复合种植模式对果园土壤动物类群多样性的影响呈现不同的特征。研究结果可为黄土残塬沟壑区苹果果园生物多样性保护提供土壤动物生态学依据。 相似文献
7.
为了研究枣(Ziziphus jujuba Mill.)和酸枣(Z.jujuba var.spinosa)叶脉序的关系,选用黄土高原丘陵枣区的枣(55个)、酸枣(28个)和过渡型(7个)共计90个样品,利用LEAF GUI叶脉序分析技术,测定了叶脉密度(VLA)、叶脉间距离(IVD)和节点数(Nodes)等14个指标,主成分分析选出8个枣叶脉序特征指标,研究了枣和酸枣叶脉序指标特点及特征。结果表明,在干旱、贫瘠生境中生长的酸枣,其叶脉间距离(0.094 mm)、网眼面积(2.84×10~(-4)mm~2)和节点数(17 823个)都显著低于黄土高原丘陵区枣园栽培条件下的枣(p0.05),而酸枣的叶脉密度(7.14 mm·mm~(-2))显著高于栽培条件下的枣,表现出很高的生态适应性。并且,枣叶脉序特征指标的变异系数范围(15.24%~74.69%)大于酸枣(20.37%~52.84%)。因此,枣和酸枣可以通过权衡叶脉序指标之间的关系,采取不同的适应策略。酸枣相比枣,更倾向于选择高叶脉密度、低网眼面积、低节点数的适应策略。对陕北样品聚类分析,在相似系数为1.8时,分为I、II两大类群;对黄土高原丘陵区的样品聚类分析,在1.69时,分为I、II两大类群,分别为酸枣和枣,过渡型在两个类群中都有。说明酸枣经过渡型向枣的演化,也说明叶脉序可以区分枣和酸枣。而栽培枣被分为4个类群,相似系数为1.29,种间结构较为复杂,是由长期的人工选择和营养繁殖模式所决定。 相似文献
8.
HUANG Laiming 《干旱区科学》2021,13(7):688-698
Soil water deficit is increasingly threatening the sustainable vegetation restoration and ecological construction on the Loess Plateau of China due to the climate warming and human activities. To determine the response thresholds of Amygdalus pedunculata (AP) and Salix psammophila (SP) to soil water availability under different textural soils, we measured the changes in net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), leaf water potential (ψw), water use efficiency (WUE) and daily transpiration rate (Td) of the two plant species during soil water content (SWC) decreased from 100% field capacity (FC) to 20% FC in the sandy and loamy soils on the Loess Plateau in the growing season from June to August in 2018. Results showed that Pn, Gs, WUE and Td of AP and SP remained relatively constant at the beginning of soil water deficit but decreased rapidly as plant available soil water content (PASWC) fell below the threshold values in both the sandy and loamy soils. The PASWC thresholds corresponding to Pn, Gs and Ci of AP in the loamy soil (0.61, 0.62 and 0.70, respectively) were lower than those in the sandy soil (0.70, 0.63 and 0.75, respectively), whereas the PASWC thresholds corresponding to Pn, Gs and Ci of SP in the loamy soil (0.63, 0.68 and 0.78, respectively) were higher than those in the sandy soil (0.58, 0.62 and 0.66, respectively). In addition, the PASWC thresholds in relation to Td and WUE of AP (0.60 and 0.58, respectively) and SP (0.62 and 0.60, respectively) in the loamy soil were higher than the corresponding PASWC thresholds of AP (0.58 and 0.52, respectively) and SP (0.55 and 0.56, respectively) in the sandy soil. Furthermore, the PASWC thresholds for the instantaneous gas exchange parameters (e.g., Pn and Gs) at the transient scale were higher than the thresholds for the parameters (e.g., Td) at the daily scale. Our study demonstrates that different plant species and/or different physiological parameters exhibit different thresholds of PASWC and that the thresholds are affected by soil texture. The result can provide guidance for the rational allocation and sustainable management of reforestation species under different soil conditions in the loess regions. 相似文献
9.
10.
Evaluation of the Low-Temperature Tolerance of Cotton Varieties in the Huang-Huai Region during Seed Germination 总被引:1,自引:0,他引:1
[Objective] To provide a theoretical foundation and germplasm resources for low-temperature-germination cotton breeding, the germination characteristics of 38 cotton varieties novel to or formerly widely planted in the Huang-Huai Region were evaluated under different low temperature stress conditions. [Method] Analysis of variance, the fuzzy membership function method, and cluster analysis were used to comprehensively evaluate the tolerance of these varieties to low temperature stress. [Result] As the temperature was lowered, germination rate, germination index, and vigor index exhibited a decreasing trend in all cotton varieties, but resistance to low temperature was significantly different among varieties. Cluster analysis divided the 38 varieties into three categories: highly resistant, resistant, and non-resistant. [Conclusion] Early-introduced varieties and those bred from early-introduced germplasm resources had weaker resistance to low temperature stress, with Stoneville and Deltapine series showing the lowest tolerance. The new transgenic insect-resistant cotton varieties SCRC 37 and SCRC 36 had the strongest resistance to low temperature stress. 相似文献