全文获取类型
收费全文 | 3264篇 |
免费 | 113篇 |
国内免费 | 259篇 |
出版年
2023年 | 15篇 |
2022年 | 32篇 |
2021年 | 39篇 |
2020年 | 38篇 |
2019年 | 58篇 |
2018年 | 37篇 |
2017年 | 49篇 |
2016年 | 48篇 |
2015年 | 48篇 |
2014年 | 81篇 |
2013年 | 106篇 |
2012年 | 53篇 |
2011年 | 133篇 |
2010年 | 75篇 |
2009年 | 180篇 |
2008年 | 196篇 |
2007年 | 207篇 |
2006年 | 177篇 |
2005年 | 150篇 |
2004年 | 158篇 |
2003年 | 119篇 |
2002年 | 70篇 |
2001年 | 62篇 |
2000年 | 64篇 |
1999年 | 76篇 |
1998年 | 82篇 |
1997年 | 62篇 |
1996年 | 59篇 |
1995年 | 62篇 |
1994年 | 72篇 |
1993年 | 71篇 |
1992年 | 72篇 |
1991年 | 74篇 |
1990年 | 61篇 |
1989年 | 53篇 |
1988年 | 60篇 |
1987年 | 74篇 |
1986年 | 102篇 |
1985年 | 76篇 |
1984年 | 90篇 |
1983年 | 34篇 |
1982年 | 63篇 |
1981年 | 54篇 |
1980年 | 52篇 |
1979年 | 43篇 |
1978年 | 9篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1975年 | 2篇 |
1974年 | 4篇 |
排序方式: 共有3636条查询结果,搜索用时 15 毫秒
881.
882.
Summary In a pot culture study, copper addition to soil increased the crop yield only in presence of nitrogen. The latter increased
the utilization of both native as well as applied copper but more that of applied. It also minimised the adverse effect of
applied phosphorus on copper utilization. Phosphorus at the rate 45 ppm had the tendency of decreasing copper uptake by wheat
if applied without nitrogen or with its low level. 相似文献
883.
Summary Concentrations of P, N, K, Ca, and Mg in above-ground tissues ofP. caribaea were sampled in the species'native savanna habitat. Concentrations were relatively low, but some evidence of higher consumption of K and Ca was found in trees grown on more fertile soils. Regressions were developed to predict the quantities of nutrients sequestered in above-ground tree tissues, and estimates made of the quantities stored in above-ground stands of this species in its native habitat and in several plantations elsewhere. Estimates were also made of the nutrient removals to be expected by harvesting these stands in different ways. Nutrient quantities stored in stands generally exceed those extractable from savanna surface soils, and it is suggested that inputs from the atmosphere are the most probable alternate nutrient source. A comparison of these inputs for tropical areas with the quantities required for stand growth in the savanna, and harvesting removals, suggests that an adequate supply of all elements except P exists, provided that capture by pine is effective. However, atmospheric inputs generally fall below the storage and harvest removal rates for fast growing exotic plantations of this species suggesting that multiple rotations of these plantations at current growth rates may not be feasible without artifical fertilization. 相似文献
884.
Summary Under high atmospheric humidity, Rhodes grass plants responded favourably to an increase in nitrate fertilization. Under low atmospheric humidity an optimum point was reached at lower N-treatment.Plants' growth was improved by a salinity treatment of up to 100 mM, at high atmospheric humidity. A higher salt concentration cancelled the favourable effect of added nitrate.The rise in yield which follows salt or nitrate treatments is apparently combined with an increase in activity of the key photosynthetic enzymes, Phosphoenol pyruvate carboxylase and Ribulose biphosphate carboxylase. A similar rise in activity is seen in nitrate reductase, a key enzyme in nitrogen metabolism. Evidently, all three enzymatic systems are not damaged in high salt treatments, and the potential photosynthetic capacity remained practically uneffected in all treatments. As no correlation could be found between transpiration and growth curves, it is assumed that the supply of CO2 is also unhampered. Thus, the major negative effect of salinity, seems to be on protein synthesis, which eventually leads to disturbed growth.Agricultural Research Organization. The Volcanic Center. 相似文献
885.
N. B. Comerford 《Plant and Soil》1981,63(3):345-353
Summary The vertical distribution of inorganic nutrient concentrations in red pine were dependent on the foliage age. Older foliage
did not show an average vertical gradient while younger foliage did show a significant gradient. Horizontal gradients across
foliage age classes in a branch whorl were consistent for all branch whorls, but the relative difference between the concentration
of the current foliage and foliage three years-old or older was dependent on crown position. Coefficients of variation (CV)
did not show a variability gradient in the crown for nitrogen and phosphorus. Variability of potassium tended to decrease
as foliage age increased.
Contribution of the University of Florida, Soil Science Department, Gainesville, FL and State University of New York, College
of Environmental Science and Forestry, Syracuse, New York. Florida Agricultural Experiment Station Journal Series No. 3017. 相似文献
886.
Nutrients such as bioavailable iron, phosphorus and nitrogen have been hypothesised to stimulate nuisance blooms of the toxic, marine cyanobacterium Lyngbya majuscula. The current study used two laboratory based biological assay techniques to test whether the addition of iron, phosphorus and nitrogen enhanced L. majuscula growth. A three-way analysis of variance showed all nutrients stimulated L. majuscula filament growth and biomass under laboratory conditions, with organically chelated iron (FeEDTA) phosphorus > nitrogen. An in situ field experiment was also conducted concurrently on naturally occurring L. majuscula in eastern Moreton Bay (Amity Banks), Australia, to determine any growth response to phosphorus and iron additions. Field results showed substantial and significant increases in biomass occurred with the addition of organically chelated iron and phosphorus (15.9 and 6.4 times the control, respectively)—confirming the laboratory responses to iron and phosphorus. Furthermore, results from the field and laboratory experiments were highly correlated, showing the applicability of results obtained in laboratory experiments to the natural environment. These results show that nutrients, particularly organically chelated iron, phosphorus and nitrogen can promote prolific growth of the bloom forming cyanobacterium L. majuscula. The current study shows that a precautionary approach should be taken to limit or reduce nutrient additions to streams, estuaries and coastal waters, otherwise the magnitude of L. majuscula blooms is likely to increase in Moreton Bay in the future. 相似文献
887.
Nitrogenous preference of toxigenic Pseudo-nitzschia australis (Bacillariophyceae) from field and laboratory experiments 总被引:1,自引:0,他引:1
Meredith D. Armstrong Howard William P. Cochlan Nicolas Ladizinsky Raphael M. Kudela 《Harmful algae》2007,6(2):206-217
Field and laboratory experiments were designed to determine the differential growth and toxin response to inorganic and organic nitrogen additions in Pseudo-nitzschia spp. Nitrogen enrichments of 50 μM nitrate (KNO3), 10 μM ammonium (NH4Cl), 20 μM urea and a control (no addition) were carried out in separate carboys with seawater collected from the mouth of the San Francisco Bay (Bolinas Bay), an area characterized by high concentrations of macronutrients and iron. All treatments showed significant increases in biomass, with chlorophyll a peaking on days 4–5 for all treatments except urea, which maintained exponential growth through the termination of the experiment. Pseudo-nitzschia australis Frenguelli abundance was 103 cells l−1 at the start of the experiment and increased by an order of magnitude by day 2. Particulate domoic acid (pDA) was initially low but detectable (0.15 μg l−1), and increased throughout exponential and stationary phases across all treatments. At the termination of the experiment, the urea treatment produced more than double the amount of pDA (9.39 μg l−1) than that produced by the nitrate treatment (4.26 μg l−1) and triple that of the control and ammonium treatments (1.36 μg l−1 and 2.64 μg l−1, respectively). The mean specific growth rates, calculated from increases in chlorophyll a and from cellular abundance of P. australis, were statistically similar across all treatments.These field results confirmed laboratory experiments conducted with a P. australis strain isolated from Monterey Bay, CA (isolate AU221-a) grown in artificial seawater enriched with 50 μM nitrate, 50 μM ammonium or 25 μM of urea as the sole nitrogen source. The exponential growth rate of P. australis was significantly slower for cells grown on urea (ca. 0.5 day−1) compared to the cells grown on either nitrate or ammonium (ca. 0.9 day−1). However the urea-grown cells produced more particulate and dissolved domoic acid (DA) than the ammonium- or nitrate-grown cells. The field and laboratory experiments demonstrate that P. australis is able to grow effectively on urea as the primary source of nitrogen and produced more pDA when grown on urea in both natural assemblages and unialgal cultures. These results suggest that the influence of urea from coastal runoff may prove to be more important in the development or maintenance of toxic blooms than previously thought, and that the source of nitrogen may be a determining factor in the relative toxicity of west coast blooms of P. australis. 相似文献
888.
D. Paredes P. Kuschk T. S. A. Mbwette F. Stange R. A. Müller H. Köser 《Engineering in Life Science》2007,7(1):13-25
Over the past few years, new technologies for nitrogen removal have been developed mainly because of the increasing financial costs of the traditional wastewater treatment technologies. Newly discovered pathways, like the anaerobic oxidation of ammonium (ANAMMOX), and uses for nitrogen removal technologies are under discussion. Processes and technologies such as: Partial nitrification; Single reactor systems for High Ammonium Removal Over Nitrite (SHARON); Anammox; Aerobic/anoxic deammonification; Oxygen Limited Autotrophic Nitrification‐Denitrification (OLAND); Completely Autotrophic Nitrogen Removal Over Nitrite (CANON); wetland based systems, all have a high potential for nitrogen removal. However, the pathways of nitrogen transformation processes are very complex. An understanding of how various environmental factors affect these processes and a sound knowledge of existing, worldwide experience pertaining to these novel technologies are the key if the nitrogen removal rates are to be improved and success is to be realized in full‐scale applications. This review describes the present knowledge of the new treatment technologies for wastewater with high nitrogen loads. Special emphasis is given to the influence of environmental factors and the reactor configuration on the nitrogen transformation process and microbial activity. 相似文献
889.
Hauser F Pessi G Friberg M Weber C Rusca N Lindemann A Fischer HM Hennecke H 《Molecular genetics and genomics : MGG》2007,278(3):255-271
890.
Species richness and soil properties in Pinus ponderosa forests: A structural equation modeling analysis 总被引:1,自引:0,他引:1
Question: How are the effects of mineral soil properties on understory plant species richness propagated through a network of processes involving the forest overstory, soil organic matter, soil nitrogen, and understory plant abundance? Location: North‐central Arizona, USA. Methods: We sampled 75 0.05‐ha plots across a broad soil gradient in a Pinus ponderosa (ponderosa pine) forest ecosystem. We evaluated multivariate models of plant species richness using structural equation modeling. Results: Richness was highest at intermediate levels of understory plant cover, suggesting that both colonization success and competitive exclusion can limit richness in this system. We did not detect a reciprocal positive effect of richness on plant cover. Richness was strongly related to soil nitrogen in the model, with evidence for both a direct negative effect and an indirect non‐linear relationship mediated through understory plant cover. Soil organic matter appeared to have a positive influence on understory richness that was independent of soil nitrogen. Richness was lowest where the forest overstory was densest, which can be explained through indirect effects on soil organic matter, soil nitrogen and understory cover. Finally, model results suggest a variety of direct and indirect processes whereby mineral soil properties can influence richness. Conclusions: Understory plant species richness and plant cover in P. ponderosa forests appear to be significantly influenced by soil organic matter and nitrogen, which are, in turn, related to overstory density and composition and mineral soil properties. Thus, soil properties can impose direct and indirect constraints on local species diversity in ponderosa pine forests. 相似文献