全文获取类型
收费全文 | 8846篇 |
免费 | 1183篇 |
国内免费 | 5520篇 |
出版年
2024年 | 27篇 |
2023年 | 352篇 |
2022年 | 412篇 |
2021年 | 484篇 |
2020年 | 602篇 |
2019年 | 701篇 |
2018年 | 663篇 |
2017年 | 659篇 |
2016年 | 614篇 |
2015年 | 618篇 |
2014年 | 606篇 |
2013年 | 754篇 |
2012年 | 618篇 |
2011年 | 577篇 |
2010年 | 498篇 |
2009年 | 659篇 |
2008年 | 579篇 |
2007年 | 676篇 |
2006年 | 576篇 |
2005年 | 517篇 |
2004年 | 472篇 |
2003年 | 460篇 |
2002年 | 359篇 |
2001年 | 337篇 |
2000年 | 304篇 |
1999年 | 310篇 |
1998年 | 236篇 |
1997年 | 215篇 |
1996年 | 224篇 |
1995年 | 205篇 |
1994年 | 177篇 |
1993年 | 138篇 |
1992年 | 132篇 |
1991年 | 107篇 |
1990年 | 113篇 |
1989年 | 107篇 |
1988年 | 80篇 |
1987年 | 61篇 |
1986年 | 50篇 |
1985年 | 43篇 |
1984年 | 46篇 |
1983年 | 12篇 |
1982年 | 54篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1979年 | 26篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1973年 | 5篇 |
1958年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
红壤丘陵区土地利用方式变更后土壤有机碳动态变化的模拟 总被引:43,自引:6,他引:37
采用双组分模型模拟土地利用方式变更后土壤有机碳储量的变化,并用一些调查和监测数据进行了初步验证.此双组分模型将土壤有机碳分为新形成有机碳和原有有机碳两个组分.每个组分有机碳的形成转化用一级动力学方程描述.本文用此模型对亚热带土壤开垦利用为马尾松林地、湿地松林地、柑桔园和牧草地4种方式10年来土壤有机碳储量的变化过程进行了模拟,初步结果表明,模拟值与实测值拟合较好.可见,此方法适于用来模拟不同土壤类型下土地利用系统变更初期的土壤有机碳储量动态变化过程. 相似文献
992.
松嫩平原盐碱化草地治理方法的比较研究 总被引:22,自引:0,他引:22
采用生物、化学和物理方法对盐碱化草地治理进行了比较研究.生物方法采用枯草处理;化学方法采用石膏处理;物理方法采用铺沙处理,种植羊草、野大麦和碱茅3种牧草.结果表明,3种方法均能降低土壤pH值、电导率和增加土壤含水率.在生物治理中,羊草和野大麦在枯草量达1500g·m-2,碱茅在枯草量达1000g·m-2时,便可良好生长.在化学治理中,石膏施用量达1.25kg·m-2,即可达到改良效果.在物理方法治理中,羊草和野大麦在沙层厚度为13cm,碱茅在11cm时即可正常生长发育. 相似文献
993.
994.
Yoshikazu Horie Dong Mei Li Toshimitu Fukiharu Ruoyu Li Paride Abliz Kazuko Nishimura Duan Li Wang 《Mycoscience》1998,39(2):161-165
Emericella appendiculata, a new species isolated from soil of the Pamire Plateau, is described and illustrated. It is characterized by grayish green
non-ostiolate ascomata surrounded by a thick layer of hülle cells, membranaceous peridium, prototunicate asci, violet-brown,
lenticular ascospores which are ornamented by two stellate equatorial crests, capitate convex surfaces, and long filiform
appendages, and anAspergillus anamorph with biseriate conidiogenous cells. 相似文献
995.
The availability of O2 is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N2O. The availability of O2 in soil is controlled by the O2 partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O2 partial pressures and soil moisture contents on the NO and N2O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N2O release by denitrification increased with decreasing O2 partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N2O were only released by denitrification. NO and N2O release by nitrification also increased with decreasing O2 partial pressure, but reached a maximum at 0.1–0.5% O2 and then decreased again. Nitrification was the main source of NO and N2O at O2 partial pressures higher than 0.1–0.5% O2. At lower O2 partial pressures denitrification was the main source of NO and N2O. With decreasing O2 partial pressure N2O release increased more than NO release, indicating that the N2O release was more sensitive against O2 than the NO release. At ambient O2 partial pressure (20.5% O2) NO and N2O release by denitrification increased with increasing soil moisture content. The maximum NO and N2O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N2O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N2O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N2O release by denitrification. Soil texture had also an effect on the release of NO and N2O. The coarse-textured sandy silt released more NO than N2O compared with the fine-textured loamy silt. At high soil moisture contents (80–100% whc) the fine-textured soil showed a higher N2O release by denitrification than the coarse-textured soil. We assume that the fine-textured soil became anoxic at a lower soil moisture content than the coarse-textured soil. In conclusion, the effects of O2 partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N2O when conditions for soil microorganisms become increasingly anoxic. 相似文献
996.
Effects of climate change on nitrogen dynamics in upland soils. 2. A soil warming study 总被引:7,自引:0,他引:7
Philip Ineson David G. Benham Janet Poskitt Anthony F. Harrison Kenneth Taylor Clive Woods 《Global Change Biology》1998,4(2):153-161
A new warming technique has been developed in a field experimental study of the potential effects of climatic change on N leaching from hill land plant/soil systems. Thermocouple compensating cable has been utilized to provide a small cross-section, flexible, low voltage heating cable, mounted on a framework of stainless steel mesh, to provide uniform heating at the vegetation/soil interface of zero-tension lysimeters and surrounding turf. We describe a specially designed heat controller capable of maintaining a temperature differential of 3 °C above ambient at a soil depth of 0.8 cm. The equipment raises temperatures down the soil profile and within the grass sward, whilst tracking normal diurnal temperature variation. Results presented here illustrate the efficacy of the warming technique, together with the consequences for the release of nitrate from lysimeters. The responses of soil solution concentrations of nitrate varied markedly between soil types, but showed a significant decrease in the brown earth during the first 5 months of additional heating. This suggests that increased nutrient release is masked by plant uptake in this soil, but the responses in the other two soils were less marked. 相似文献
997.
The effect of CO2 concentration on plant growth and the size of the rhizosphere denitrifier population was investigated for ryegrass grown at 3 different soil pH values (pH 4.3, 5.9 and 7.0). Soil microcosms were planted with ryegrass and maintained under constant growth conditions at either ambient (450ppm) or elevated (720ppm) CO2 concentration. At harvest, the rhizosphere soil was collected and subjected to a potential denitrification assay to provide an estimate of the size of the denitrifier population present. Ryegrass dry matter production varied across the pH range studied and contrary to other studies, elevated CO2 concentration did not consistently increase growth. Plant growth was reduced by ≈ 35% and 23% at pH 4.3 and pH 5.9, respectively, under elevated CO2 concentration. At pH 7.0, however, plant growth was increased by ≈ 45% under elevated CO2. Potential denitrification rates within the rhizosphere followed a similar pattern to plant growth in the different treatments, suggesting that plant growth and the size of denitrifier population within the rhizosphere are coupled. This study investigates the relationship between plant growth and rhizosphere denitrification potential, thereby providing an estimate of the size of the denitrifier population under increased CO2 concentration and soil pH. 相似文献
998.
Preliminary estimates of the potential for carbon mitigation in European soils through no-till farming 总被引:9,自引:0,他引:9
PETE. SMITH DAVID S. POWLSON MARGARET J. GLENDINING JO. U. SMITH 《Global Change Biology》1998,4(6):679-685
In this paper we estimate the European potential for carbon mitigation of no-till farming using results from European tillage experiments. Our calculations suggest some potential in terms of (a) reduced agricultural fossil fuel emissions, and (b) increased soil carbon sequestration. We estimate that 100% conversion to no-till farming would be likely to sequester about 23 Tg C y–1 in the European Union or about 43 Tg C y–1 in the wider Europe (excluding the former Soviet Union). In addition, up to 3.2 Tg C y–1 could be saved in agricultural fossil fuel emissions. Compared to estimates of the potential for carbon sequestration of other carbon mitigation options, no-till agriculture shows nearly twice the potential of scenarios whereby soils are amended with organic materials. Our calculations suggest that 100% conversion to no-till agriculture in Europe could mitigate all fossil fuel-carbon emissions from agriculture in Europe. However, this is equivalent to only about 4.1% of total anthropogenic CO2-carbon produced annually in Europe (excluding the former Soviet Union) which in turn is equivalent to about 0.8% of global annual anthropogenic CO2-carbon emissions. 相似文献
999.
Plant Effects on Spatial and Temporal Patterns of Nitrogen Cycling in Shortgrass Steppe 总被引:6,自引:0,他引:6
Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties
are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns
of nutrient cycling between plant and soil. Plants having the carbon-3 (C3) or carbon-4 (C4) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net
N mineralization rates and traced nitrogen-15 (15N) additions among plant and soil components during May, July, and September of 1995 in native plots of C3 plants, C4 plants, or mixtures of C3 and C4. Net N mineralization was significantly greater in C3 plots than in C4 plots during both July and September. C3 plots retained significantly more 15N in May than did mixed and C4 plots; these differences in
15N retention were due to greater 15N uptake by C3 plants than by C4 plants during May. There were no significant differences in total 15N retention among plant communities for July and September. Soil 15N was influenced more by presence or absence of plants than by type of plant; greater quantities of
15N remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate
that plant functional type (C3 versus C4) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to
determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention,
and storage.
Received 8 December 1997; accepted 6 May 1998. 相似文献
1000.
Distribution of resting spores of the Lymantria dispar pathogen Entomophaga maimaiga in soil and on bark 总被引:1,自引:0,他引:1
Cadavers of late instar Lymantria dispar (gypsy moth) larvae killed by the fungal pathogen Entomophaga maimaiga predominantly contain resting spores (azygospores). These cadavers frequently remain attached to tree trunks for several weeks before they detach and fall to the ground. Density gradient centrifugation was used to quantify resting spores in the soil and on tree bark. Titers of resting spores were extremely high at 0–10 cm from the base of the tree and the number decreased with distance from the trunk of the tree. Titers were also highest in the organic layer of the soil with numbers decreasing precipitously with increasing depth in the soil. While resting spores were obtained from tree bark, densities per unit area were much lower than those found in the organic soil layer at the base of the tree. Field bioassays were conducted with caged L. dispar larvae to compare infection levels with distance from the tree trunk as well as on the trunk. Highest infection levels were found at 50cm from the tree base with lowest infection on the tree trunk at 0.5 m height, although we expected the highest infection levels among larvae caged at the bases of trees, where highest spore titers occurred. Laboratory experiments demonstrated that L. dispar larvae exposed to resting spore- bearing soil at the soil surface became infected while larvae exposed to soil with resting spores buried at least 1 cm below the surface did not become infected. 相似文献