全文获取类型
收费全文 | 10523篇 |
免费 | 1169篇 |
国内免费 | 5614篇 |
出版年
2024年 | 109篇 |
2023年 | 389篇 |
2022年 | 455篇 |
2021年 | 509篇 |
2020年 | 623篇 |
2019年 | 746篇 |
2018年 | 716篇 |
2017年 | 668篇 |
2016年 | 611篇 |
2015年 | 611篇 |
2014年 | 762篇 |
2013年 | 873篇 |
2012年 | 738篇 |
2011年 | 751篇 |
2010年 | 636篇 |
2009年 | 792篇 |
2008年 | 727篇 |
2007年 | 753篇 |
2006年 | 626篇 |
2005年 | 580篇 |
2004年 | 471篇 |
2003年 | 448篇 |
2002年 | 363篇 |
2001年 | 321篇 |
2000年 | 293篇 |
1999年 | 300篇 |
1998年 | 236篇 |
1997年 | 214篇 |
1996年 | 215篇 |
1995年 | 198篇 |
1994年 | 185篇 |
1993年 | 150篇 |
1992年 | 119篇 |
1991年 | 113篇 |
1990年 | 117篇 |
1989年 | 107篇 |
1988年 | 81篇 |
1987年 | 60篇 |
1986年 | 57篇 |
1985年 | 53篇 |
1984年 | 86篇 |
1983年 | 51篇 |
1982年 | 104篇 |
1981年 | 57篇 |
1980年 | 50篇 |
1979年 | 59篇 |
1978年 | 26篇 |
1977年 | 26篇 |
1974年 | 22篇 |
1973年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
221.
Land use induced changes of organic carbon storage in soils of China 总被引:29,自引:0,他引:29
Using the data compiled from China's second national soil survey and an improved method of soil carbon bulk density, we have estimated the changes of soil organic carbon due to land use, and compared the spatial distribution and storage of soil organic carbon (SOC) in cultivated soils and noncultivated soils in China. The results reveal that ~ 57% of the cultivated soil subgroups ( ~ 31% of the total soil surface) have experienced a significant carbon loss, ranging from 40% to 10% relative to their noncultivated counterparts. The most significant carbon loss is observed for the non‐irrigated soils (dry farmland) within a semiarid/semihumid belt from northeastern to southwestern China, with the maximum loss occurring in northeast China. On the contrary, SOC has increased in the paddy and irrigated soils in northwest China. No significant change is observed for forest soils in southern China, grassland and desert soils in northwest China, as well as irrigated soils in eastern China. The SOC storage and density under noncultivated conditions in China are estimated to ~ 77.4 Pg (1015 g) and ~ 8.8 kg C m?2, respectively, compared to a SOC storage of ~ 70.3 Pg and an average SOC density of ~ 8.0 kg C m?2 under the present‐day conditions. This suggests a loss of ~ 7.1 Pg SOC and a decrease of ~ 0.8 kg C m?2 SOC density due to increasing human activities, in which the loss in organic horizons has contributed to ~ 77%. This total loss of SOC in China induced by land use represents ~ 9.5% of the world's SOC decrease. This amount is equivalent to ~ 3.5 ppmv of the atmospheric CO2 increase. Since ~ 78% of the currently cultivated soils in China have been degraded to a low/medium productivities and are responsible for most of the SOC loss, an improved land management, such as the development of irrigated and paddy land uses, would have a considerable potential in restoring the SOC storage. Assuming a restoration of ~ 50% of the lost SOC during the next 20–50 years, the soils in China would absorb ~ 3.5 Pg of carbon from the atmosphere. 相似文献
222.
CREIGHTON M. LITTON MICHAEL G. RYAN† DENNIS H. KNIGHT PETER D. STAHL‡ 《Global Change Biology》2003,9(5):680-696
The effects of fire on soil‐surface carbon dioxide (CO2) efflux, FS, and microbial biomass carbon, Cmic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (~110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season FS increased with tree density in young stands (1.0 µmol CO2 m?2 s?1 in low‐density stands, 1.8 µmol CO2 m?2 s?1 in moderate‐density stands and 2.1 µmol CO2 m?2 s?1 in high‐density stands) and with stand age (2.7 µmol CO2 m?2 s?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g?1 dry soil over the growing season; Cmic was significantly greater in mature stands (0.5–0.8 mg C g?1 dry soil). Soil‐surface CO2 efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but Cmic pools and overall FS rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of FS and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances. 相似文献
223.
Feeding by the pine weevil Hylobius abietis in relation to sun exposure and distance to forest edges
Abstract 1 The intensity of feeding by adult pine weevils Hylobius abietis (L.) on the stem bark of Norway spruce Picea abies (L.) Karst. seedlings planted in rows with a north–south orientation across a clear‐cutting, was measured throughout a growth season. The feeding was then correlated to light interception, soil temperature and distance to the nearest forest edge. 2 Feeding was at least twice as intense on seedlings in the central part of the clear‐cutting compared to those at the edges. The decline began approximatety 15 m from the edge and was of similar proportions on both the sun‐exposed and shaded sides. 3 Measures of global radiation and soil temperature correlated well with consumption on the shaded side. However, on the sun‐exposed side, there were no apparent correlations with global radiation or soil temperature that could explain the decline in consumed bark area. 4 We conclude that the decline in feeding towards the forest edges was mainly due to factors other than the microclimate variables we monitored. We suggest that the presence of roots of living trees along the forest edge may reduce damage to seedlings, since they provide an alternative source of food for the weevils. This alternative‐food hypothesis may also explain why seedlings in shelterwoods usually suffer less damage from pine weevils than seedlings in clear‐cuttings. 相似文献
224.
Mercury evaporation from undisturbed iron‐humus podzol lysimeters was measured over 3 months after treatment with HgCl2 spiked with radioactive 203Hg. The relative evaporation rate from HgCl2 treated soils followed the sum of two exponential functions. Because evaporation asymptotically approaches zero with time, the integral of the fit curve represents the evaporative loss in percent of atmospheric deposition. For the soil investigated, about 5% of atmospheric Hg deposition was reemitted into the atmosphere. It is hypothesized that mercury evaporation can decrease the leaching of mercury in and from soil significantly; this effect is probably increasing with decreasing rain acidity or soil acidity. Mercury deposited as soluble salt remains susceptible to reemission to air for 300 d after incorporation into the soil matrix. Indications are found that Hg evaporation from soils in geological background areas predominantly derives from recent atmospheric Hg deposition and not from geological sources. 相似文献
225.
H. B. Rogers C. A. Beyrouty T. D. Nichols D. C. Wolf C. M. Reynolds 《Soil & Sediment Contamination》1996,5(2):171-186
A mixture of organic chemicals (MOC) containing equal molar amounts of benzoic acid, hexadecane, 2,2‐dimethyl 4,n‐propyl‐benzene, phenanthrene, pyrene, and either cycloheptane or cis‐decahydronaphthalene (cis‐decalin) was applied to soil at rates of 0 to 8000 mg/kg. In a plant‐screening experiment, growth responses of four legume and five nonlegume species were determined at 10 and 25°C. The MOC applied at 2000 mg/kg reduced the growth of several species without resulting in significant seedling death. At 10°C, the growth of alpine bluegrass (Poa alpina L.) in the 1000 and 2000 mg/kg treatments of soil increased by more than 185%. In a plant growth response experiment, alpine bluegrass and alfalfa (Medicago sativa L.) were grown in soil that had been contaminated at rates of 0 and 2000 mg/kg. At 14 weeks, the shoot and root dry weights of alfalfa were 97% lower in the contaminated soil, while the shoot dry weight, root dry weight, and root length of alpine bluegrass were 135,235 and 268% higher, respectively. Except for pyrene, <23% of the compounds comprising the MOC remained in the soil after 4 weeks and <5% after 14 weeks. The disappearance of the MOC was not significantly influenced by the presence of alfalfa or alpine bluegrass. 相似文献
226.
C. Marjorie Aelion J. N. Shaw R. P. Ray M. A. Widdowson H. W. Reeves 《Soil & Sediment Contamination》1996,5(3):225-241
Portable meters and simplified gas Chromatographic (GC) techniques were investigated for monitoring volatile hydrocarbon (HC), CO2, and O2, concentrations in groundwater, exhaust gases, and soil vapor during in situ remediation using soil vapor extraction (SVE) and air sparging (AS). Results of groundwater samples analyzed in‐house using a headspace technique compared well to split samples analyzed by a certified analytical laboratory (r2 = 0.94). SVE exhaust gas HC and CO2 concentrations measured using a GT201 portable HC/O2 meter and a RA‐411A meter (GasTech), respectively, were highly correlated with in‐house laboratory GC analyses (r2 = 0.91). O2 concentrations fell in a small range and meter analyses were not well correlated with laboratory analyses. Results of soil gas monitoring were not as well correlated as those for exhaust gases for HC, CO2, or O2, perhaps due to environmental conditions such as changes in relative humidity or the wider range of soil gas values. Overall, the meters were good indicators of vapor contamination, they greatly simplified estimates of total HC mass removal, and they allowed estimates of the biological contribution to contaminant removal during the remediation process. 相似文献
227.
以质粒pRK404为载体亚克隆含大豆根瘤菌吸氢酶结构基因(hup)的片段,构建成嵌合质粒pHR11、pHR4和pHR10。通过三亲本杂交将这些嵌合质粒导入无吸氢活性的Rhodobactersphaeroides241菌株(Hup-),均获得Hup+的接合子。利用启动子检测质粒pMP220证明,在hup对结构基因上游1.2kb内存在hup启动基因片段。以pRK2013为助质粒可将pHR11导入Enterobactercloacae和Klebsiellaoxytoca等土壤固氮菌株。本文以接合子E.cloacaeEH1113为例,通过对基因组DNASouthern杂交分析证明,嵌合质粒pHR11在EH1113中稳定贮存和复制。H2诱导接合子EH1113吸氢酶活性高表达,吸氢活性与放氢活性比值约为对照的两倍。当以延胡索酸为电子受体时,吸氢酶的吸氢作用支持菌株固氮酶活性的提高。 相似文献
228.
Biodiversity of soil biota and plants in abandoned arable fields and grasslands under restoration management 总被引:2,自引:0,他引:2
The currently widespread abandoning of agricultural land use in Western Europe offers new opportunities for ecological restoration and nature conservation. This is illustrated for abandoned arable fields and for permanent grasslands cut for hay after the cessation of fertilizer application. Although initiated by a sudden reduction of nutrient input to the system, the changing nutrient supply from the soil is considered to be the main driving force of succession. The soil nutrient supply is affected by soil organisms, both directly (root symbionts and herbivores) and indirectly (nutrient mineralization from dead organic matter). It is argued that because of the close association of changes in species diversity with changes in the functioning of ecosystems, biodiversity has to be studied in an ecosystem ecology context. 相似文献
229.
J. H. Lawton D. E. Bignell G. F. Bloemers P. Eggleton M. E. Hodda 《Biodiversity and Conservation》1996,5(2):261-273
Theoretically, there are three principal ways in which ecosystem processes might respond to reductions in species richness. These theories are reviewed, and then considered in the context of a study of the diversity of soil nematodes and termites in near-primary forest sites at Mbalmayo, Cameroon, and the contribution made by these two taxa to carbon fluxes (CO2 and CH4) from the forest floor. Nematode abundances average 2.04 × 106 m-2, and termites between 2933 and 6957 m-2. The site is the most species-rich yet investigated for both groups anywhere in the world, so that a very large number of species contribute to carbon fluxes. We speculate about how much redundancy might be built into the functioning of both assemblages, and point out the enormous difficulties of resolving such questions, and of producing such detailed species-inventories. 相似文献
230.
利用32P同位素示踪法对白浆土中P肥利用率进行了研究.结果表明,在岗地白浆土上,表层土壤表现出一定程度的供P不足现象,白浆层土壤有效P含量严重缺乏.表层土壤中当季P肥利用率的变幅范围为6.09~12.35%,白浆层土壤P肥利用率为13%左右.施用有机肥能明显提高白浆土Olsen P的含量,加速土壤本身P的活化,各种有机物中,以猪粪对土壤潜在P的活化效果最好.将Olsen P与X值、A值比较,认为Olsen P是评价白浆上P肥力简便易行的可靠指标. 相似文献