首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1793篇
  免费   112篇
  国内免费   165篇
林业   250篇
农学   106篇
基础科学   77篇
  793篇
综合类   495篇
农作物   67篇
水产渔业   18篇
畜牧兽医   28篇
园艺   27篇
植物保护   209篇
  2024年   13篇
  2023年   46篇
  2022年   45篇
  2021年   80篇
  2020年   65篇
  2019年   53篇
  2018年   64篇
  2017年   73篇
  2016年   86篇
  2015年   86篇
  2014年   82篇
  2013年   121篇
  2012年   180篇
  2011年   106篇
  2010年   96篇
  2009年   90篇
  2008年   73篇
  2007年   111篇
  2006年   76篇
  2005年   66篇
  2004年   53篇
  2003年   54篇
  2002年   44篇
  2001年   44篇
  2000年   55篇
  1999年   34篇
  1998年   39篇
  1997年   19篇
  1996年   25篇
  1995年   22篇
  1994年   8篇
  1993年   11篇
  1992年   9篇
  1991年   15篇
  1990年   6篇
  1989年   6篇
  1988年   4篇
  1987年   4篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1982年   1篇
  1979年   1篇
排序方式: 共有2070条查询结果,搜索用时 171 毫秒
1.
The use of cattle manure (CM) for fertilization presents challenges for optimizing nitrogen (N) use. Our work aimed to assess N efficiencies, in a 6‐year experiment with three biennial rotations of four crops: oat–sorghum (first year) and ryegrass–maize (second year) in a rainfed humid Mediterranean area of Spain. Fertilization treatments included the following: control (no N), 250 kg mineral N ha?1 year?1 (250MN), three CM rates (supplying 170, 250 and 500 kg N ha?1 year?1) and four treatments where the two lowest CM rates were complemented with either 80 or 160 kg mineral N ha?1 year?1. Treatments were distributed randomly in each of three blocks. Maximum dry‐matter yield (~44–49 t ha?1 rotation?1) was achieved in the third rotation, and only the control and the 170CM yielded significantly less. Within the limitations of the EU Nitrate Directive, the N steady state supply of 170CM always requires a complement of mineral N (80 kg N ha?1) to maximize N agronomic efficiency. The maximum N‐fertilizer replacement value (250CM vs. 250MN) was 0·67, without significant differences between the two treatments in other N‐related efficiency indexes, which indicates that plants took advantage of residual‐N effects. Nitrogen losses by leaching in the 250CM treatment were around 5–7% of the N applied. This reinforces the sustainability of manure recycling in long cropping seasons.  相似文献   
2.
Abstract. The repeated application of pig slurry to agricultural soils may result in an accumulation of salts and a risk of aquifer pollution due to nitrate leaching and salinization. Under Mediterranean conditions, a field experiment on a sandy loam soil (Typic Xerofluvent) was performed with maize (Zea mays) in 1998, 1999 and 2001 to study the effects of applying optimal (P1) and excessive rates (P3) of pig slurry on soil salinization, nitrate leaching and groundwater pollution. The rate of pig slurry was established considering the optimal N rate for maize in this soil (170, 162 and 176 kg N ha?1 for 1998, 1999 and 2001, respectively). Pig slurry treatments were compared to an optimal N rate supplied as urea (U) and a control treatment without N fertilizer (P0). The composition of the slurries showed great variability between years. Mean NO3? leaching losses from 1998 to 2001 were 329, 215, 173 and 78 kg N ha?1 for P3, P1, U and P0 treatments, respectively. The amount of total dissolved salts (TDS) added to the soil in slurry application between 1998 and 2001 was 2019 kg TDS ha?1 for the P1 treatment and 6058 kg TDS ha?1 for the P3 treatment. As a consequence, the electrical conductivity (EC) of the slurry‐treated soils was greater than that of the control soil. The EC correlated significantly with the sodium concentration of the soil solution. Over the entire experimental period, 2653, 2202 and 2110 kg Na ha?1 entered the aquifer from the P3, P1 and P0 treatments, respectively. The P3 treatment did not significantly increase grain production in 1999 and 2001 compared with that achieved with the optimal N rate treatment (P1). This behaviour shows the importance of establishing application guidelines for pig slurry that will reduce the risk of soil and groundwater pollution.  相似文献   
3.
本文对天山云杉人工幼林树冠结构以及树冠结构与林木之间的关系进行了研究,并基于生物学原理,在确定有效树冠的法础上,通过决定林木能力的树冠特征对其人林分间伐中的应用进行了初步研究。  相似文献   
4.
5.
Crown rust (caused by Puccinia coronata f. sp. lolii) is a serious foliar disease of the pasture and turfgrass perennial ryegrass (Lolium perenne). Previous genetic studies have detected both qualitative and quantitative resistance mechanisms, and interpretation of the genetic system is complicated by variation within the sexually reproducing pathogen. Resistant and susceptible parental genotypes of ryegrass were identified using a composite urediniospore population collected from three geographically distinct locations. A two-way pseudo-testcross mapping population was obtained as the F1 progeny of the pair-cross between ryegrass parental genotypes Vedette6 and Victorian9. Both parents showed intermediate resistance against a pathogen population collected in a single geographical zone (Hamilton, Victoria), but in the F1 population, significant variation for a range of resistance-associated characters was detected. Statistical analysis of phenotypic data suggested a major gene effect, hence bulked segregant analysis with map-assigned simple sequence repeat (SSR) markers was used to scan the genome. A marker showing strong association with resistance was assigned to linkage group (LG) 2 of perennial ryegrass. Analysis of 11 LG2 SSR markers defined an interval between loci xlpssrh03f03 and xlpssrk02e02 as containing the gene or genes (LpPc1) conferring crown rust resistance. Resistance gene determinants were inherited from both parents, with up to 80% of the total phenotypic variation explained by markers segregating from Vedette6 and up to 26% of the variation explained by markers segregating from Victorian9. The two contributions together resulted in an additive increase in effect, with fully resistant individuals requiring determinants from both parents. A conserved syntenic relationship was observed with linkage group B of Avena strigosa, which is the location of a cluster of resistance genes to the oat form of crown rust. The implications of this study for marker-assisted selection of disease resistance in perennial ryegrass are discussed.  相似文献   
6.
苜蓿根和根颈腐烂病病原及防治研究进展   总被引:8,自引:2,他引:8  
孟嫣  李敏权 《草业科学》2005,22(5):89-92
综述了国内外苜蓿Medicago sativa根和根颈腐烂病的病原研究、品种抗病性鉴定及其防治方法的研究进展,为该病害的进一步研究提供理论依据.  相似文献   
7.
为研究浙江红花油茶高接换冠成活率影响因素,解决嫁接成活率低的问题,2013年6月从中国林业科学研究院亚热带林业研究所引进浙江红花油茶优树穗条,在青田县大垟山林场开展浙江红花油茶高接换冠试验,分析砧木林郁闭度、坡位、砧木生长势及接穗保存时间等因素对嫁接成活率的影响。结果表明,林分郁闭度在0.3~0.7之间,随着郁闭度增加嫁接平均成活率提高;郁闭度在0.7~0.9之间,随着郁闭度增加,嫁接成活率降低;当郁闭度为0.7时,嫁接成活率最高为88.8%。山腰的嫁接成活率高达86%,显著高于山顶和山脚。浙江红花油茶砧木生长势越强嫁接成活率越高。接穗于采集当天嫁接效果最好成活率达89.1%,接穗保存1、2、3、4、5、6天嫁接,成活率分别为85.2%、75.2%、51.6%、28.1%、12.2%、9.3%。偏相关分析表明,各因素对嫁接成活率影响由大到小依次为接穗保存时间(-0.97)、郁闭度(0.85)、生长势(0.81)、坡位(-0.65)。  相似文献   
8.
为明确改良剂在苏打盐化草甸土不同深度土层的改良效果,采用室内淋溶土柱的试验方法,设置S0、S1、S2、S3、S4和S5(每个土柱横截面积0.0113 m~2)分别施用改良剂0.00、0.01、0.05、0.10、0.50 g和1.00 g,折合成公顷用量分别为0.0、8.9、44.3、88.5、442.5 kg·hm~(-2)和885.1 kg·hm~(-2))6个处理,分析淹水条件下不同改良剂用量对苏打盐化草甸土的洗盐效应。结果表明,与未施改良剂处理(S0)相比,S3、S4和S5处理显著降低不同土层(0~30cm)土壤容重(降幅3.88%~8.87%)、增加土壤孔隙度(增幅3.82%~9.38%)、降低土壤pH值(降幅2.36%~8.05%);S2、S3、S4和S5处理显著提高20~40 cm土壤水分入渗量,增幅55.9%~294.6%(P0.05);施用改良剂处理交换性Na~+含量在0~10 cm和10~20 cm土层显著降低,降幅4.84%~56.5%(P0.05),20~30 cm和30~40 cm土层中增幅1.09%~17.1%;S3、S4和S5处理0~10 cm土层电导率显著降低,降幅49.0%~60.4%(P0.05),20~40 cm土层显著增加,增幅3.68%~19.8%(P0.05);S2、S3、S4、S5处理10~20 cm土层碳酸根含量显著降低,降幅5.98%~23.4%(P0.05),S3、S4、S5处理0~30 cm土层碳酸氢根含量降幅1.49%~18.0%;S2、S3、S4和S5处理显著提高0~30 cm≥0.25 mm水稳性团聚体,增幅达16.4%~161.7%(P0.05);S3、S4和S5处理0~30 cm土层MWD显著增加,增幅达5.78%~161.7%(P0.05)。针对本次供试土壤盐渍化程度,综合改良效果及改良剂施用量,改良剂适宜用量为每个土柱0.10 g(88.5 kg·hm~(-2))(S3处理),可达到最优淋洗效果。  相似文献   
9.
[目的]研究四川牡丹岩土环境背景以及微生物在元素迁移过程中的作用,为了解营养元素的表生地球化学行为规律及微生物在元素迁移体系中的作用提供理论依据。[方法]利用ICP-OES测定四川牡丹生境母岩、土壤以及种子的大量(P,S,K,Ca,Na,Mg,Al)、微量(Fe,Mn,Cu)营养元素含量,同时利用Illumina高通量测序技术测定根际土壤中细菌、真菌的组成和结构。[结果](1)四川牡丹生长土壤以弱碱富钙为主要特征,元素组成基本保留了母岩特征;(2)元素从母岩到土壤中的迁移富集系数排序为:CaNaSPMgKAlFeMnCu,植物吸收系数排序为:PSKCuMgCaNaMnFeAl;(3)碱土金属淋溶率和铝铁率对土壤细菌的多样性(Alpha)影响显著,但对真菌的影响不显著;(4)P,Mn的迁移主要受母岩控制,而Ca,Mg,Fe,S等元素的迁移富集受到了微生物的影响。[结论]细菌和真菌在元素迁移体系中的作用相似,主要参与Fe,S,Mg,Ca的相关反应过程。  相似文献   
10.
为了研究磷酸铵镁堆肥产品的氮磷养分释放特性及其肥效,分别采用土柱淋溶方法及盆栽试验研究了磷酸铵镁堆肥(MAPC)产品的氮磷养分释放特性及对油菜的肥效,并与普通化肥(CCH)、普通堆肥(CCO)、磷酸铵镁肥(MAP)3种肥料进行对比。结果表明:在整个淋溶过程中,CCH和MAPC处理淋溶液的总氮及总磷浓度均较其他处理高;MAP肥具有缓释性,且堆肥对其养分释放具有促进作用;CCO处理的养分释放最慢。在最后一次淋洗(即第5次)后,总氮及总磷养分浓度排序均为:CCHMAPCMAPCCO。盆栽试验18 d后油菜快速生长,且与对照(不施肥)处理相比,MAP和MAPC处理能显著提高油菜的株高及生物量。在45 d盆栽试验结束时,CCH、CCO、MAP和MAPC处理的生物量分别是对照处理的14.8、4.8、20.5倍和16.0倍。MAP处理能显著提高油菜的养分含量,TN和TP含量分别比对照提高了2.24 g·kg~(-1)和1.54 g·kg~(-1)。研究表明,4种肥料均可改善油菜的生长发育情况,且MAP和MAPC的肥效优于CCH和CCO,在堆肥中利用鸟粪石结晶原理可固定部分氮磷元素,提高堆肥产品肥效。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号