全文获取类型
收费全文 | 21495篇 |
免费 | 266篇 |
国内免费 | 602篇 |
专业分类
林业 | 4201篇 |
农学 | 2217篇 |
基础科学 | 601篇 |
3662篇 | |
综合类 | 2095篇 |
农作物 | 2333篇 |
水产渔业 | 1961篇 |
畜牧兽医 | 1800篇 |
园艺 | 1300篇 |
植物保护 | 2193篇 |
出版年
2024年 | 6篇 |
2023年 | 77篇 |
2022年 | 131篇 |
2021年 | 225篇 |
2020年 | 193篇 |
2019年 | 195篇 |
2018年 | 2839篇 |
2017年 | 2889篇 |
2016年 | 1377篇 |
2015年 | 326篇 |
2014年 | 251篇 |
2013年 | 312篇 |
2012年 | 1083篇 |
2011年 | 2476篇 |
2010年 | 2415篇 |
2009年 | 1531篇 |
2008年 | 1647篇 |
2007年 | 1845篇 |
2006年 | 338篇 |
2005年 | 357篇 |
2004年 | 244篇 |
2003年 | 245篇 |
2002年 | 127篇 |
2001年 | 88篇 |
2000年 | 151篇 |
1999年 | 137篇 |
1998年 | 115篇 |
1997年 | 103篇 |
1996年 | 81篇 |
1995年 | 71篇 |
1994年 | 76篇 |
1993年 | 78篇 |
1992年 | 63篇 |
1991年 | 54篇 |
1990年 | 31篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 25篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1972年 | 4篇 |
1970年 | 6篇 |
1968年 | 5篇 |
1967年 | 3篇 |
1924年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
To improve its overall performance, fast-growing poplar was modified using the vacuum-pressure-vacuum impregnation method
with a urea-formaldehyde resin-sodium montmorillonite intercalation as the modification solution. The results showed that
considerable amounts of urea-formaldehyde resin and montmorillonites entered the poplar tracheid, and some entered the microporous
wood. These substances formed bonds with the active groups in timber, causing reduced crystallinity in the amorphous region
of the poplar, a decreased level of free hydroxyl, and an enhanced association with hydroxyl and ether bonds. The density,
dimensional stability, and mechanical properties of poplar were markedly improved. The best results were obtained with 14%
sodium montmorillonite and 20% ureaformaldehyde resin: the bending resistance, compressive resistance, and elastic modulus
increased by 19.37%, 30.24%, and 50.06%, respectively. With elevated levels of sodium montmorillonite, the impact toughness
and wear rate decreased. 相似文献
992.
To improve the properties of particleboard, boards were produced using a sealed press. With the sealed press, boards were
processed under high-temperature and high-pressure steam. This increased the saturation temperature, causing a dramatic rise
in temperature inside the board, faster curing of the binder, and a shorter pressing time. The boards were bonded with urea
formaldehyde resin, melamine urea formaldehyde resin, or poly(methylene diphenyl diisocyanate) (PMDI). The sealed press improved
the internal bond strength and thickness swelling of boards regardless of the binder used during the reduced pressing time.
The increased bonding strength improved the board properties, allowing PMDI with a lower resin content to be used for bonding
the boards. 相似文献
993.
994.
Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability 总被引:1,自引:0,他引:1
Zakaria Solaiman Petra Marschner Dongmei Wang Zed Rengel 《Biology and Fertility of Soils》2007,44(1):143-153
The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and
canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum
and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg−1 P as Ca3(PO4)2 (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in
the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity
in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH
and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis,
of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the
bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat
genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the
genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for
P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role
in the better growth of the canola compared to the wheat genotypes. 相似文献
995.
采用培养试验研究了磷缺乏与正常供磷条件下,CO2浓度由350μL/L升高至800μL/L苗期番茄的生物量、根系特征和不同器官N、P、K养分含量的变化。结果表明,无论缺磷与否,CO2浓度升高均能显著增加番茄地上部及根系的干物质积累量,提高根冠比。在磷缺乏条件下,CO2浓度升高对番茄根系生长的促进主要表现为增加根系的体积和表面积;而在磷正常供应条件下主要表现为同时增加根体积和分根数,有利于形成强壮的根系。在两种供磷水平下,CO2浓度升高对番茄各器官的N、P、K含量产生不同的稀释效应,但N、P、K总积累量却随CO2浓度升高而显著增加;而且CO2浓度与供P水平对番茄植株的N、P、K积累量具有极显著的正交互效应。 相似文献
996.
沼液一体化综合处理与循环利用工艺 总被引:2,自引:1,他引:2
沼液过滤膜浓缩作为沼液处理的一个新方向,得到了广泛关注。该文将可堆肥处理的生物质材料秸秆作为滤料引入沼液的预处理,并对沼液过滤、膜浓缩及秸秆滤料堆肥效果进行了整体研究。研究结果表明,经过秸秆、火山岩、石英砂依次过滤后,可以去除沼液中95%以上的悬浮物,过滤后的秸秆经过简单晾晒,在含水率为75%的情况下,可以正常升温堆肥,高温期可以持续10 d,经过28 d堆置后基本达到腐熟要求,堆肥产品总养分含量接近5%,但含水率下降不明显。经过预处理的沼液进行超滤和纳滤之后,营养物质含量得到不同程度的提升,和未处理的原液相比,氮磷钾含量分别提升了2.6倍、1.9倍和4.5倍,Mg和Fe的浓缩倍数均达到10以上,氨基酸浓度提高了8.8倍,而且纳滤产生的透过液基本可以达到农田灌溉水水质标准。该研究结果为沼液的处理利用提供了参考。 相似文献
997.
Yuyou Zhi Peng Li Jiachun Shi Lingzao Zeng Laosheng Wu 《Journal of Soils and Sediments》2016,16(2):467-475
Purpose
Cadmium (Cd) is regarded as one of the most toxic heavy metals in the environment and can undermine the ecosystem function and human health at trace level due to its high toxicity. In order to reduce the anthropogenic Cd input into agricultural soil, it is of utmost importance to pinpoint the sources of Cd in soils and apportion their respective contributions.Materials and methods
One hundred twenty-seven topsoil samples and 21 subsoil samples were collected from croplands of Meishan Basin, Changxing County, Zhejiang Province, Eastern China, and analyzed for concentrations of Cd and other heavy metals. Finite mixture distribution model (FMDM) was employed to fit the data to obtain the local soil Cd threshold value, a critical indicator to assess soil heavy metal contamination. Then, principal component analysis (PCA) and geographic information system (GIS) were used to identify the potential sources of Cd. Finally, positive matrix factorization (PMF) was applied to apportion the source contributions.Results and discussion
Among the 127 topsoil samples, 71 were subject to Cd contamination with a mean concentration of 0.66 mg kg?1 while the others were considered as background with a lower mean concentration of 0.145 mg kg?1, close to the local background concentration of 0.142 mg kg?1. Further, three components were extracted by PCA and interpreted as natural background, lead-acid battery manufacturing plants, and construction material associated activities, respectively. Additionally, most of the topsoil samples around the lead-acid battery manufacturing plants, construction material plants, and limestone/marble quarries were classified as Cd contaminated. However, PMF failed to get a successful portioning.Conclusions
Lead-acid battery manufacturing plants and construction material associated activities were the main anthropogenic sources of soil Cd contamination. With the help of FMDM, it is possible to distinguish the contaminated soil and estimate the contribution of anthropogenic sources to soil Cd. The apportionment by PMF was not successful in this paper due to the high skewness or outliers of Cd concentration in sampling sites and violation of the assumption that all samples have the same sources.998.
Yalong Liu Ping Wang Genxing Pan David Crowley Lianqing Li Jinwei Zheng Xuhui Zhang Jufeng Zheng 《Journal of Soils and Sediments》2016,16(5):1460-1471
Purpose
Rice paddy soils undergo pedogenesis driven by periodic flooding and drainage cycles that lead to accumulation of organic matter and the stratification of nutrients and oxygen in the soil profile. Here, we examined the effects of continuous rice cultivation on microbial community structures, enzyme activities, and chemical properties for paddy soils along a chronosequence representing 0–700 years of rice cropping in China.Materials and methods
Changes in the abundance and composition of bacterial and fungal communities were characterized at three depths (0–5, 5–10, and 10–20 cm) in relation to organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon/nitrogen, and activities of acid phosphatase, invertase, and urease.Results and discussion
Both soil organic carbon and total nitrogen increased over time at all three depths, while pH generally decreased. Microbial abundance (bacteria and fungi) and invertase and urease activity significantly increased with the duration of rice cultivation, especially in the surface layer. Fungal abundance and acid phosphatase activity declined with depth, whereas bacterial abundance was highest at the 5–10-cm soil depth. Profiles of the microbial community based on PCR-DGGE of 16S rRNA indicated that the composition of fungal communities was strongly influenced by soil depth, whereas soil bacterial community structures were similar throughout the profile.Conclusions
Soil bioactivity (microbial abundance and soil enzymes) gradually increased with organic carbon and total nitrogen accumulation under prolonged rice cultivation. Microbial activity decreased with depth, and soil microbial communities were stratified with soil depth. The fungal community was more sensitive than the bacterial community to cultivation age and soil depth. However, the mechanism of fungal community succession with rice cultivation needs further research.999.
Yong Wang Shuqing Gao Cuilan Li Jinjing Zhang Lichun Wang 《Journal of Soils and Sediments》2016,16(7):1849-1857
Purpose
Under a global warming scenario, understanding the response of soil organic carbon fractions and aggregate stability to temperature increases is important not only for better understanding and maintaining relevant ecosystem services like soil fertility and crop productivity, but also for understanding key environmental processes intimately related with the maintenance of other regulatory ecosystem services like global climate change mitigation through carbon sequestration. An increase in temperature would accelerate the mineralization of soil organic carbon. However, the properties of organic carbon remained in soil after mineralization is not well known.Materials and methods
Mollisol was collected at 0–20-cm depth from maize (Zea mays L.) field in Northeast China. A 180-day incubation experiment was conducted at three different temperatures (10, 30, and 50 °C) under constant soil moisture (60 % water holding capacity). Soil samples were assayed for total organic carbon (TOC), water-soluble organic carbon (WSOC), easily oxidizable organic carbon (EOC), humic fractions carbon, aggregate-associated carbon, and water stability of aggregates. Elemental analysis and solid-state 13C nuclear magnetic resonance spectroscopy were used to characterize humic acid and humin fractions.Results and discussion
The contents of soil TOC, EOC, humic fractions carbon, and aggregate-associated carbon decreased with the increase in temperature. The proportion of 2–0.25-mm macroaggregate and the mean weight diameter (MWD) of aggregates also decreased. The C, H, N, S, alkyl C, and O-alkyl C contents of humic acid and humin decreased, whereas the O, aromatic C, and carbonyl C contents increased. The H/C, aliphatic C/aromatic C, and O-alkyl C/aromatic C ratios in humic acid and humin fractions decreased.Conclusions
The increase in temperature has a negative impact on soil organic carbon content, soil aggregation, and aggregate stability. Moreover, humic acid and humin molecules become less aliphatic and more decomposed with the increase in temperature.1000.
NH4+对K+在土壤肥际微域中迁移和转化的影响 总被引:3,自引:0,他引:3
NH4^+和K^+共施在农田施肥中是很普遍的现象,研究它们共施后二者在土壤中的交互作用对指导施肥具有重要意义。采用室内土柱实验研究了共施条件下NH4^+对K^+在红壤和潮土肥际微域中的迁移和形态转化的影响,供试肥料为NH4Cl和KCl。试验结果表明,与单施K^+相比,共施NH4^+没有改变肥料钾在红壤和潮土中的迁移距离,但提高了肥际微域中的水溶性钾含量;在靠近施肥点附近,NH4^+的施用减少了土壤交换性钾含量,这种作用在潮土中的表现不如在红壤中明显;与单施KCl相比,共施NH4Cl明显降低了施肥点附近土壤微域内的非交换性钾含量。研究结果表明,共施NH4Cl减少了土壤晶格对钾离子的固定,增加了钾的淋溶风险。 相似文献