全文获取类型
| 收费全文 | 40599篇 |
| 免费 | 14918篇 |
| 国内免费 | 542篇 |
专业分类
| 林业 | 3286篇 |
| 农学 | 2379篇 |
| 基础科学 | 2620篇 |
| 24377篇 | |
| 综合类 | 16492篇 |
| 农作物 | 1136篇 |
| 水产渔业 | 59篇 |
| 畜牧兽医 | 1826篇 |
| 园艺 | 547篇 |
| 植物保护 | 3337篇 |
出版年
| 2025年 | 812篇 |
| 2024年 | 1808篇 |
| 2023年 | 2050篇 |
| 2022年 | 1899篇 |
| 2021年 | 1894篇 |
| 2020年 | 2009篇 |
| 2019年 | 2222篇 |
| 2018年 | 1824篇 |
| 2017年 | 2493篇 |
| 2016年 | 2881篇 |
| 2015年 | 1996篇 |
| 2014年 | 2262篇 |
| 2013年 | 3197篇 |
| 2012年 | 4010篇 |
| 2011年 | 2917篇 |
| 2010年 | 2295篇 |
| 2009年 | 2349篇 |
| 2008年 | 2116篇 |
| 2007年 | 2301篇 |
| 2006年 | 2000篇 |
| 2005年 | 1688篇 |
| 2004年 | 1295篇 |
| 2003年 | 1125篇 |
| 2002年 | 881篇 |
| 2001年 | 850篇 |
| 2000年 | 731篇 |
| 1999年 | 535篇 |
| 1998年 | 470篇 |
| 1997年 | 489篇 |
| 1996年 | 415篇 |
| 1995年 | 438篇 |
| 1994年 | 406篇 |
| 1993年 | 305篇 |
| 1992年 | 258篇 |
| 1991年 | 253篇 |
| 1990年 | 178篇 |
| 1989年 | 155篇 |
| 1988年 | 95篇 |
| 1987年 | 76篇 |
| 1986年 | 41篇 |
| 1985年 | 13篇 |
| 1984年 | 6篇 |
| 1983年 | 2篇 |
| 1982年 | 2篇 |
| 1981年 | 2篇 |
| 1980年 | 4篇 |
| 1976年 | 1篇 |
| 1963年 | 1篇 |
| 1962年 | 5篇 |
| 1955年 | 1篇 |
排序方式： 共有10000条查询结果，搜索用时 20 毫秒
51.
The study examines vegetation – environment relationships. Vegetation measurements included species frequency, density, diameter and tree height, while environmental measurements were soil particle size distribution, acid properties (pH, Al, SO4), nutrient cations (Ca, Na, Mg, K), organic carbon, nitrogen, phosphorus and chloride content. Nypa fruticans was the dominant species in the A stratum (> 3 m tall) while Rhizophora mangle was dominant in the B stratum (1–3 m tall). The C stratum (< 1 m tall) was dominated by mangrove, Nypa and Raphia saplings. Silt was dominant and the most variable particle size fraction. A principal components analysis of the soil data indicated the first three dominant components influencing the vegetation were salinity, nutrient and soil texture. Tree height and density correlated highly with the salinity and soil texture gradients (P < 0.01), while basal area correlated with salinity and nutrient gradients (P < 0.01). While Avicannia africana in the A stratum was influenced largely by the salinity and soil texture gradients. Nypa fruticans in the B stratum was influenced by salinity and nutrients. 相似文献
52.
孙殿芝 《内蒙古林业调查设计》2007,30(2):15-16
文章在分析牙克石市自然条件及水土流失特点的基础上,提出牙克石市水土保持生态修复的可行性及生态修复的措施,以及近年来牙克石市生态修复所取得的可喜成果。 相似文献
53.
Hongjun Yang Jiangbao Xia Wenjun Xie Shoucai Wei Qian Cui Pengshuai Shao Jingkuan Sun Kaikai Dong Xingchao Qi 《Land Degradation u0026amp; Development》2023,34(7):2061-2072
The effects of different straw returning and nitrogen addition levels on soil quality are important for proper coastal saline soil remediation. However, relatively little is understood concerning the remediation effects, especially, on long-term effects and mineral composition and microstructures of coastal saline soil. Two maize/wheat straw returning levels [1.0 × 104 kg ha−1 (2S) and 5.0 × 103 kg ha−1 (S)] and three inorganic nitrogen addition levels [300 kg ha−1 (N2), 150 kg ha−1 (N) and 75 kg ha−1 (N1/2)]—were studied, with 150 kg ha−1 inorganic nitrogen and without straw addition treatment as the control (CK), to elucidate the response of soil physical and chemical properties to the two factors. After four consecutive wheat-maize cycles, different straw and N fertilizer treatments obviously decreased the salinity contents, increased the total nutrient contents, and optimized the soil structure of the saline soil. The saline soil reclamation effects showed significant distinctions among the different straw and N fertilizer treatments. The 2SN2 treatment displayed the greatest effects in regard to reducing salinity, increasing the total soil nutrient contents and optimizing the soil structure, which resulted in the best remediation effect. Straw returning plays a major role in decreasing soil salinity and enhancing saline soil aggregate formation. N fertilizer addition supplies rich nutrients for straw decomposition, and promotes soil microbial growth and reproduction, which brought about C sequestration in coastal saline soil. During the coastal saline soil remediation process in the Yellow River Delta, it is suggested to prioritize straw returning and moderate N fertilizer addition, and live together with moderate P fertilizer application. 相似文献
54.
P. J. O'Connor M. B. Lynch E. Cahalan M. O'Donovan D. Hennessy 《Grass and Forage Science》2013,68(4):564-576
A cut plot experiment was undertaken at two sites in Ireland, one a free‐draining acid brown earth at Moorepark (MPK) and the other a fine loam soil with imperfect drainage at Johnstown Castle (JC). The effect of applying the nitrification inhibitor dicyandiamide (DCD) at 10 kg ha?1 in July, August and September or not applying DCD to plots receiving synthetic urine or zero urine on spring and annual herbage production was examined. In the experiment, each site received 350 kg nitrogen (N) fertilizer ha?1 year?1. The application of DCD in August at a rate of 10 kg ha?1 significantly increased spring and annual herbage production by 14 and 15%, respectively, at MPK, when applied following urine application in year 1. There was no effect of DCD applied in year 1 on herbage production at JC. The application of DCD in August resulted in lower soil total oxidized N (TON) content up to sampling day 56 post‐urine application, at MPK in year 1, retaining higher N content in the soil. There was no effect of DCD on any of the parameters measured in year 2 at MPK or at JC. Urine application did not increase spring herbage production at either site. Urine application significantly increased annual herbage production at MPK only in year 1. Urine application increased annual herbage N uptake, herbage crude protein (CP) content and soil mineral N at both sites in both years. 相似文献
55.
沧州滨海盐碱地造林绿化技术 总被引：5，自引：0，他引：5
根据沧州滨海地区的自然状况,提出了滨海盐碱地区造林绿化的基本观点,及采取的造林绿化技术措施,并推介了一批该地区的适生树种。 相似文献
56.
Soil carbon release along a gradient of physical disturbance in a harvested northern hardwood forest
Changes in soil respiration associated with forest harvest could increase net loss of CO2 to the atmosphere relative to pre-harvest values. By excavating quantitative soil pits across a gradient of physical disturbance in a harvested northern hardwood forest, this study examines C release from mineral soil. Mineral soil samples were analyzed for pH, percent organic matter (%OM), C and N concentration, δ13C, and total C per unit area. Results show a relationship between degree of disturbance and C concentration in soil 10-30 cm beneath the O-horizon. Highly disturbed sites show C depletion, with horizons from disturbed sites containing 25% less total C than the least disturbed sites. δ13C signatures of soil profiles at these sites show vertical mixing of plant-derived material into deeper mineral horizons. Mixing, as a result of physical disturbance, could have led to the observed C depletion by physical or chemical destabilization, or through the promotion of microbial respiration in deep mineral soil. Regardless of the mechanism, these results suggest elevated CO2 emissions from soil following harvest, and, thus, have implications for the validity of wood biomass as a carbon neutral energy source. 相似文献
57.
Hideo Kubotera 《Soil Science and Plant Nutrition》2020,66(1):15-20
ABSTRACT Soils in the Kyushu Okinawa region with problematic properties for agriculture were studied and the following findings were obtained. 1) Mahji-soils in Nansei Islands became very hard with desiccation under high-pH conditions; therefore, soil treatments that cause alkalinization such as overliming should be avoided. 2) In contrast to the generally favorable effects of organic matter on the physical properties of soil, the application of manure compost at 100 Mg ha?1 to Mahji-soils increased the degree of hardening with desiccation. 3) Application of clinker ash (coarse fraction of coal ash) at 10% (wt/wt) mitigated the hardening and shrinkage of Jagaru, a heavy-textured gray soil found on Nansei Island. 4) Non-allophanic surface horizons with strong acidity were widely distributed in the Andosol area of central Kyushu, covering at least 340 km2. These horizons showed a relatively large ratio of carbon per Alp (pyrophosphate extractable aluminum) and high content of allophane, and began forming around 2500 years ago. 5)Some Andosols in Kyushu showed remarkable hardness. They are divided into two types, according to the degree of weathering and hardening characteristics, and the different measures required to improve each type. 6) An experimental method for determining nitrate retention by soils was developed, and factors influencing the nitrate retention of Andosols were elucidated using this method. 7) A method for micromorphological observation of pelletized manure compost (PC) grains was developed. This method revealed that the internal microstructure of PC did not change in 18 months after compost application, whereas the volume of the grains decreased with the formation of voids around them. 8) As an indicator of soil acidity concerning the countermeasure for common scab, pH(KCl) can easily be used instead of exchangeable acidity y1, which is laborious to determine. The maximally simplified pH(KCl) measurement method can be carried out by farmers in the field. 相似文献
58.
Effects of soil freezing on nitrogen (N) mineralization have been the subject of increased attention in the ecological literature, though fewer studies have examined N mineralization responses to successive mild freezing, severe freezing and cyclic freeze–thaw events. Even less is known about relationships of responses to soil N status. This study measured soil N mineralization and nitrification in the field along an experimental N gradient in a grassland of northern China during the dormant season (October 2005–April 2006), a period in which freezing naturally occurs. Net N mineralization exhibited great temporal variability, with nitrification being the predominant N transformation process. Soil microbial biomass C and N and extractable NH4 + pools declined by 40, 52, and 56%, respectively, in April 2006, compared with their initial concentrations in October 2005; soil NO3– pools increased by 84%. Temporal patterns of N mineralization were correlated with soil microbial biomass C and N. N mineralization and nitrification increased linearly with added N. Microbial biomass C in treated soils increased by 10% relative to controls, whereas microbial N declined by 9%. Results further suggest that freezing events greatly alter soil N dynamics in the dormant season at this site, with considerable available N accumulating during this period. 相似文献
59.
Improving the soil environment for seedling emergence and plant growth is possible using a good tillage system. From March 1993 until June 1994, the effect of tillage system and plant residue incorporation practices on the bulk density and the soil strength of the surface layer of a vertisol (very fine, smectitic, thermic, chromic Haploxerert), on a less than 1 % slope, was investigated. The two tillage methods were moldboard (T1) and chisel (T2). The two plant residue incorporation dates were in August (R1) and in October (R2). Results indicated that R1 gave significantly higher bulk density than R2, while the different tillage treatments had no significant effect. Soil strength was significantly lower under T1 and R2 than under the other treatments. 相似文献
60.