全文获取类型
收费全文 | 8087篇 |
免费 | 421篇 |
国内免费 | 654篇 |
专业分类
林业 | 443篇 |
农学 | 505篇 |
基础科学 | 304篇 |
2655篇 | |
综合类 | 3960篇 |
农作物 | 254篇 |
水产渔业 | 246篇 |
畜牧兽医 | 352篇 |
园艺 | 237篇 |
植物保护 | 206篇 |
出版年
2024年 | 56篇 |
2023年 | 150篇 |
2022年 | 227篇 |
2021年 | 282篇 |
2020年 | 232篇 |
2019年 | 312篇 |
2018年 | 199篇 |
2017年 | 335篇 |
2016年 | 387篇 |
2015年 | 363篇 |
2014年 | 423篇 |
2013年 | 510篇 |
2012年 | 599篇 |
2011年 | 632篇 |
2010年 | 575篇 |
2009年 | 602篇 |
2008年 | 511篇 |
2007年 | 562篇 |
2006年 | 447篇 |
2005年 | 388篇 |
2004年 | 280篇 |
2003年 | 188篇 |
2002年 | 140篇 |
2001年 | 109篇 |
2000年 | 97篇 |
1999年 | 86篇 |
1998年 | 57篇 |
1997年 | 70篇 |
1996年 | 65篇 |
1995年 | 52篇 |
1994年 | 41篇 |
1993年 | 46篇 |
1992年 | 28篇 |
1991年 | 30篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有9162条查询结果,搜索用时 93 毫秒
931.
Mitigation options for diffuse phosphorus loss to water 总被引:4,自引:0,他引:4
Abstract. Agriculture contributes significant loads of P to surface waters. The reductions in these diffuse P inputs necessary to help prevent eutrophication problems and/or assist in the restoration of water quality will require controls over both nutrient inputs and their subsequent transport in land runoff. Specific mitigation options include nutrient budgeting, input management, soil conservation, land use management and the establishment of riparian, and other buffer zones. The variable nature of diffuse P loss suggests that the best approach to control is through integrated management at a range of scales. Critical control concepts at the farm level include targeting source areas adequately, maintaining P input loading rates within recommended limits and avoiding high-risk management actions. Since eutrophication is a natural phenomenon and with potential conflicts with the need to meet production targets and/or minimize loss of other nutrients (N), some assessment of acceptable levels of P loss, of cost effectiveness of options and some prioritization of goals are necessary to find optimal solutions. As the requirements of individual waterbodies differ, these solutions need to be site specific and their successful adoption requires an appreciation by farmers of the importance of minimizing agricultural P loss both as individuals and collectively within a catchment. 相似文献
932.
933.
934.
本文以陕西某一火力发电厂为例,通过高斯公式对SO_2扩散着地最大浓度和最大浓度距离的计算及探讨,为工业卫生防护林带绿化位置设计和环境保护提供一定的理论依据。 相似文献
935.
千岛湖底泥重金属污染的生态风险评价 总被引:13,自引:0,他引:13
水体底泥污染是世界范围内的一个环境问题,污染物通过大气沉降、废水排放、雨水淋溶与冲刷等途径进入水体,最后沉积到底泥中并逐渐富集,使底泥受到污染。千岛湖不同监测点底泥Cu、Zn、M n、Pb、Cd、A s、Hg等重金属含量存在差异。大坝前、毛竹源和三潭岛监测点M n、A s和Hg含量较高。不同采样点,底泥重金属污染度有差异,表现为毛竹源>三潭岛>航头岛>茅头尖>大坝前>排岭水厂>街口,毛竹源测点底泥重金属污染风险最高,街口重金属污染风险最低。 相似文献
936.
PAH decrease in soil samples from different polluted industrial sites by activating the autochthonous microflora In pot experiments under field conditions the PAH decrease in ten different soil samples from former industrial sites with differing PAH load were investigated over a period of 74 and 168 weeks. 15 out of 16 PAH according to US EPA (without acenaphthylene) were determined. Easily degradable organic matter, mineral fertilizers, synthetic surfactants and in some experiments also lime were added to the soil samples in varying amounts. Depending on the nature and quantity of the amendments, the biological activity and (co-)metabolic decomposition of PAH by soil microorganisms could considerably be increased. In the different soil samples a decrease of the initial PAH contents between 12 and 90% was achieved within 74 weeks. Even from 74 up to 168 weeks for some soil samples a remarkable further decrease of the PAH contents could be observed. The decrease of the extractable PAH with time is mainly caused by microbial decomposition and formation of nonextractable residues. This behavior can be fitted by two coupled exponential functions, one for an initial phase of rapid decomposition and the other for a subsequent phase of slow decomposition. Therefore, two different processes (I, II) determine the decrease of PAH. In the first week of the experiment the decomposition rates for process I amount to 4.2–88.3 and for process II to 0.06–5.3 mg PAH ? kg?1 soil ? week?1; in the 168th week they are no longer determinable for process I and amount to 0.05–2.3 mg ? kg?1 ? week' for process II. Higher initial PAH contents often led to higher relative PAH decreases, but also to absolute higher contents of residues. The persistence of PAH is mainly determined by their bioavailability. PAH degradation is increased by the soil treatments. The addition of easily degradable organic matter (C/N ratio < 20) in a quantity of 30g ? kg?1 (w/w) combined with the addition of nitrate and a surfactant most effectively increased the degradation of PAH. Raising the pH of a very strongly acidic soil sample revealed a maximum PAH decomposition at a pH of 6. 相似文献
937.
为了研究含氟废气对农业生态环境的影响,采用环境监测和农业生态学方法进行了全面系统的调查与研究,结果表明,砖厂含氟废气排放点周围的大气,土壤,农作物已受到不同程度的污染,砖厂周围种植的农作物含氟量明显高于对照点,附近的家禽(鸡)死亡率也明显高于建砖厂前,因此,建砖厂时应周围环境污染影响予以重视。 相似文献
938.
江西红壤性水稻土肥力特性及其管理 总被引:5,自引:1,他引:4
江西红壤性水稻土为202.8827万hm2。其肥力特征:1.土壤三要素营养不平衡,中低产田占70.00%;2.缺硼钼少硅硫锌,影响土壤养分平衡;3.灌溉水能提供稍多Ca、Mg,少量S、Si、K、N,以补充土壤养分不足;4.耕层浅薄,障碍因素较多。管理:1.深耕培肥,增加土壤有机质;2.坚持有机无机肥配合,提高稻田肥力;3.合理施肥,协调养分平衡;4.开沟排水、搞好稻田沟系建设;5.水旱轮作、改革耕作制度。 相似文献
939.
Cd Pb Cu Zn As复合污染对水稻的影响 总被引:6,自引:0,他引:6
应用田间试验和盆栽试验,研究Cd、Pb、Cu、Zn、As复合污染在草甸棕壤上对水稻产量及元素含量的影响。结果表明,在接近土壤环境质量标准低浓度水平下,水稻发育正常,减产幅度<10%,但土壤Cd、Pb临界值下降,籽实中Cd、Pb、Zn吸收系数提高,Cd、Pb含量超标,低剂量时Cd超标37.5%,高剂量时超标100%,增加了对人体健康潜在危险。主要原因在于5元素间存在协同作用,Pb、Cu、Zn、As的存在增加了Cd的活性与解吸率,使其易迁移,为防止土壤污染增加了困难。 相似文献
940.
蔬菜铅污染状况及其防治对策 总被引:4,自引:0,他引:4
在金属冶炼厂周围和公路两侧200m范围内的蔬菜均受到铅污染,污染源为工业企业排放的烟尘废气和汽车尾气。蔬菜食用部位中的铅含量与菜地距公路的距离成负相关;相同距离点上生长的不同蔬菜中铅含量有明显差别,表明不同种类的蔬菜吸收和积累大气中铅的能力有强弱之分,一般是普通叶菜的铅含量大于结球叶菜、根菜及果菜,且普通叶菜的铅含量超过国家食品卫生标准。减少蔬菜铅污染的措施主要是:1.根治污染源;2.调整菜田布局;3.绿化带间隔;4.食用前多次水洗;5.实行蔬菜售前铅检测。 相似文献