全文获取类型
收费全文 | 1150篇 |
免费 | 47篇 |
国内免费 | 65篇 |
专业分类
林业 | 83篇 |
农学 | 58篇 |
基础科学 | 16篇 |
322篇 | |
综合类 | 456篇 |
农作物 | 113篇 |
水产渔业 | 27篇 |
畜牧兽医 | 131篇 |
园艺 | 47篇 |
植物保护 | 9篇 |
出版年
2024年 | 8篇 |
2023年 | 11篇 |
2022年 | 20篇 |
2021年 | 21篇 |
2020年 | 22篇 |
2019年 | 17篇 |
2018年 | 14篇 |
2017年 | 47篇 |
2016年 | 43篇 |
2015年 | 40篇 |
2014年 | 57篇 |
2013年 | 52篇 |
2012年 | 115篇 |
2011年 | 93篇 |
2010年 | 85篇 |
2009年 | 81篇 |
2008年 | 60篇 |
2007年 | 73篇 |
2006年 | 73篇 |
2005年 | 42篇 |
2004年 | 43篇 |
2003年 | 34篇 |
2002年 | 23篇 |
2001年 | 17篇 |
2000年 | 22篇 |
1999年 | 17篇 |
1998年 | 12篇 |
1997年 | 13篇 |
1996年 | 19篇 |
1995年 | 23篇 |
1994年 | 19篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 11篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 2篇 |
排序方式: 共有1262条查询结果,搜索用时 15 毫秒
111.
长期施肥对酸性土壤磷形态及有效性的影响 总被引:4,自引:0,他引:4
为明确长期施肥对集约化果园土壤磷形态、分布特征及其有效性的影响,本研究对琯溪蜜柚主产区29个果园的土壤进行调查研究,采用张守敬和Jackson的酸性土壤无机磷分级方法,研究蜜柚果园土壤磷素累积对土壤磷形态及有效性的影响。结果表明:在集约化蜜柚果园中,土壤磷素累积丰富,随树龄增加土壤速效磷含量上升显著,且土壤磷形态在不同土层存在显著性差异;当全磷含量≤0.5 g/kg时,土壤磷形态主要以有机磷、铁磷为主,随着全磷含量的上升,铝磷含量、占比均显著上升;多元线性回归和逐步回归分析结果表明本试验中铝磷与速效磷相关性最好。果园土壤已经形成一个巨大的磷库,且有效性较高,可适当减少磷肥投入,降低土壤磷含量,减少经济成本,提高生态效益。 相似文献
112.
113.
Prolonged simulated acid rain treatment in the subarctic: Effect on the soil respiration rate and microbial biomass 总被引:2,自引:0,他引:2
Humus chemistry and respiration rate, ATP, ergosterol, and muramic acid concentration as measures of chemical properties, microbial activity, biomass, and indicators of fungal and bacterial biomass were studied in a long-term acid rain experiment in the far north of Finnish Lapland. The treatments used in this study were dry control, irrigated control (spring water, pH 6), and two levels of simulated acid rain (pH 4 and pH 3). Originally (1985–1988), simulated acid rain was prepared by adding both H2SO4 and HNO3 (1.9:1 by weight). In 1989 the treatments were modified as follows. In subarea 1 the treatments continued unchanged (H2SO4+HNO3 in rain to pH 4 and pH 3), but in subarea 2 only H2SO4 was applied. The plots were sampled in 1992. The acid application affected humus chemistry by lowering the pH, cation exchange capacity, and base saturation (due to a decrease in Ca and Mg) in the treatment with H2SO4+HNO3 to pH 4 (total proton load over 8 years 2.92 kmol ha-1), whereas the microbial variables were not affected at this proton load, and only the respiration rate decreased by 20% in the strongest simulated acid rain treatment (total proton load 14.9 kmol ha-1). The different ratios of H2SO4+HNO3 in subareas 1 and 2 did not affect the results. 相似文献
114.
M. von Lützow L. Zelles I. Scheunert J. C. G. Ottow 《Biology and Fertility of Soils》1992,13(3):130-134
Summary Seasonal effects of liming, irrigation, and acid precipitation on microbial biomass N and some physicochemical properties of different topsoil horizons in a spruce forest (Picea abies L.) were measured throughout one growing season. The highest biomass N was recorded in autumn and spring in the upper soil horizons, while the lowest values were obtained in summer and in deeper horizons. The clearest differences between the different soil treatments were apparent in autumn and in the upper horizons. Liming increased the microbial biomass N from 1.7% of the total N content to 6.8% (Olf1 layer) and from 1% to 2% of the total N content in the Of2 layer. The main inorganic-N fraction in the deeper horizons was NO
inf3
sup-
. An increase in cation exchange capacity was observed down to the Oh layer, while soil pH was only slightly higher in the Olf1 and Of2 layers after liming. The effects of irrigation were less marked. The microbial biomass N increased from 1.7% of total N to 4.8% in the Olf1 layer and from 1% to 2% of total N in the Of2 layer. In the Olf1 layer an increase in C mineralization was observed. Acid precipitation decreased the microbial biomass N in the upper horizons from 4.8% of total N to 1.8% in the Olf1 layer and from 2% to 0.5% in the Of2 layer. No significant changes in soil pH were observed, but the decrease in cation exchange capacity may result in a decrease in the proton buffering capacity in the near future. 相似文献
115.
Fang You Ram Dalal David Mulligan 《Communications in Soil Science and Plant Analysis》2015,46(4):375-389
The present study aims to evaluate carbon recovery and analytical precision of two methods based on dry combustion for organic carbon (OC) quantification in base metal mine tailings: (1) IC subtraction method (inorganic C(IC) measured and subtracted from total C) and (2) direct OC quantification after acid pretreatment. Results showed IC subtraction method effectively hydrolyzed a range of IC minerals [calcium carbonate (CaCO3), magnesium carbonate (MgCO3), dolomite, magnesite, calcite] with satisfactory IC (as CO2 released) recovery (87–103 percent). In the direct OC quantification method, 5 M hydrochloric acid (HCl) pretreatment resulted in a satisfactory recovery (76–92 percent) of most organic compounds [ethylenediaminetetraacetic acid (EDTA), cellulose, plant litter, and charcoal], except for water-soluble OC (sucrose, 40 percent recovery). The precision of both methods declined when C levels were <5 g kg?1 with RSD (Relative Standard Deviation) > 10 percent. The OC values in test samples of base metal mine tailings were comparable between the two methods. However, IC subtraction method is not applicable for tailings with low OC levels (<5 g kg?1) until the precision is substantially improved. Moreover, compared to the IC subtraction method, OC values are significantly lower in direct OC quantification method for tailings with high OC levels. 相似文献
116.
不同产地和生长年限伊贝母地上部位与鳞茎生物碱含量研究 总被引:1,自引:0,他引:1
[目的]研究不同产地和生长年限伊贝母地上部位与地下鳞茎总生物碱含量。[方法]采用酸性染料比色法测定总生物碱的含量。[结果]在不同产地和生长年限的伊贝母中,其地上部位与地下鳞茎的生物碱含量存在差异,4年生的伊贝母总生物碱含量比3年和5年的含量高,地上部位中总生物碱含量为地下鳞茎中的1.258~2.870倍,新疆伊犁州新源县中的伊贝母总生物碱含量均比其他产地的总生物碱含量低。[结论]不同产地和生长年限伊贝母中总生物碱含量因产地及生长年限不同而存在差异,但总体上地上部分的总生物碱含量高于地下鳞茎,存在进一步研究与开发的价值。 相似文献
117.
[目的]研究臭氧对4种偶氮染料的处理效果.[方法]利用臭氧在相同条件下处理4种偶氮染料(酸性黑ATT、酸性大红3R、酸性品红、碱性品绿)废水,研究臭氧对这4种染料的降解效果.[结果]臭氧对4种偶氮染料的脱色效果分别是酸性大红3R>酸性品红>酸性黑ATT>碱性品绿.在既定条件下,臭氧对酸性大红3R的脱色效果好于酸性品红,臭氧对酸性大红3R和酸性品红溶液的处理时间短且都能达到很好的COD去除率.但是,臭氧对酸性黑ATT和碱性品绿尤其是碱性品绿的处理不是十分的理想,所需时间较长.[结论]利用臭氧处理不同的染料需要根据不同的染料性质去改变其相应的条件. 相似文献
118.
污泥衍生吸附剂对废水中酸性雪青的吸附特性研究 总被引:2,自引:0,他引:2
[目的]研究污泥衍生吸附剂对废水中酸性雪青的吸附特性.[方法]以污泥粉为原料,加入锯末,用氯化锌活化,高温热解制得一种污泥衍生吸附剂(碘值为462.36 mg/g),并研究了污泥衍生吸附剂对废水中酸性雪青的吸附特性.[结果]污泥衍生吸附剂对酸性雪青吸附动力学模型拟合结果表明,100、300、400 mg/L的酸性雪青溶液都符合假二级动力学模型,其R2都>0.99; 100、300、400 mg/L的酸性雪青溶液到达平衡的时间分别是30、90、150 min,低浓度到达平衡的时间短,高浓度到达平衡时间长.根据热力学试验可以得出,其吸附过程在10、20、30℃下都可用Langmuir和Freundlich吸附模型很好地描述(其R2都>0.99).根据其吸附过程与液膜扩散模型和颗粒内扩散模型的拟合情况,得到液膜扩散是其吸附过程的速率控制步骤,但不是唯一速率控制步骤,而颗粒内扩散并不是其过程的速率控制步骤.[结论]使用污泥衍生吸附剂吸附废水中酸性雪青具有可行性. 相似文献
119.
120.
本文用半经验分子轨道法对亚硝酸分子进行构象分析,同最小分子能量对应,平衡几何构型以顺式稳定,以E_(LUMO)和E_(HUMO)—E_(LUMO)作为得电子指标,对氧化还原稳定性来说,反式的构型大于顺式。 相似文献