共查询到20条相似文献，搜索用时 515 毫秒
1.
Continued conversion of woodlands into grazing and farmland is seriously undermining the natural ecosystem of the dry and fragile Rift Valley areas of Ethiopia. This study investigated the effects of land‐use change on soil organic carbon (SOC), total nitrogen (N), pH, exchangeable bases, cation exchange capacity (CEC) and base saturation (per cent) in three adjacent land‐use types: controlled grazing, open‐grazing and farmland. A total of 81 soil samples were collected and analysed. Contents of SOC and total N decreased drastically in open‐grazing and farmland (p < 0·001), and were significantly higher in the top 0·2 m than in the subsurface soil layer. Compared with the controlled grazing, reductions in the contents of SOC and total N in the top 1 m soil layer were 22–30 and 19 per cent, respectively, due possibly to the decrease in plant biomass input into the soil and the fast decomposition of organic materials. Long‐term cultivation had significantly increased the concentration of exchangeable K. Exchangeable Na was high in the lower layers, while Mg was higher in the top surface soil. CEC also varied with soil depth (p = 0·016); it was higher in the topsoil than in the subsurface soil, which may be, among others, due to the differences in soil organic matter distribution with depth. Although these semi‐arid soils are known to have low organic carbon and CEC levels, the values from the current study area are critically low, and may indicate the further impoverishment of the soils under high agricultural and grazing pressures. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
2.
黄土高原半干旱区不同土地利用下的土壤有机碳、碳组分和土壤营养 总被引：5，自引：0，他引：5
Cropland (CP), native grassland (NG) and two shrub land treatments which were converted from cropland in 1985:seabuckthorn (Hippophae rhamnoides L. ) (ST), and branchytamarisk (Tamarix ramosissima) (BT) were investigated to evaluate effects of land use conversion on soil organic carbon (SOC) and soil nutrients in the semi-arid region of the Loess Plateau of China. Total organic carbon (TOC), light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), total N (TN), nitrate nitrogen (NO3--N) and nitrite nitrogen (NO2--N), ammonium nitrogen (NH4+-N), total P, and available P (AP) were measured. The results showed that SOC in NG, ST and BT were 12.7%, 27.7% and 34.8% higher than that of the cropland, respectively. LFOC, light fraction (LF) dry matter, ratio of TOC to TN (C/N) and the ratio of TOC to AP (C/P) were higher in the shrub land or native grassland than in the cropland. Cropland had the highest TN, the sum of NO3--N and NO2--N, TP and AP due to the use of chemical fertilizers. TOC significantly correlated with LFOC, HFOC and C/N. LFOC significantly correlated with dry matter of the LF and C/N. TN, the sum of NO3--N and NO2--N and AP were significantly negatively correlated with TOC and LFOC. Therefore, land use conversion from cropland to shrub land, or maybe grassland, contributed to SOC sequestration and improved soil nutrients stabilization. 相似文献
3.
喀斯特石漠化区不同土地利用方式下土壤有机碳分布特征 总被引：12，自引：3，他引：9
为探究喀斯特土壤有机碳分布特征及其对人为干扰的响应,挖掘了2 854个土壤剖面,采集了22 786个土壤样品,分析了贵州省不同土地利用方式下土壤有机碳分布规律;并结合贵州省石漠化防治规划,初步估算了石漠化防治工程的土壤碳增汇贡献。结果表明:贵州省土壤有机碳呈现含量高、密度小的特征。表层土壤(0-20cm)有机碳平均含量25.07g/kg,平均密度仅为4.27kg/m~2。不同用地类型土壤表层有机碳含量大小为灌木林地乔灌木林地灌草地乔木林地弃耕地与荒地草地水田园地旱地与坡耕地;表层碳密度大小为水田灌木林地乔木林地乔灌木林地弃耕地与荒地灌草地旱地与坡耕地草地园地。0-60cm土层土壤有机碳含量对人为干扰较为敏感,60-100cm土层土壤有机碳含量差异较小。实施退耕还林,人工种草及人工造林等石漠化防治工程会明显促进土壤有机碳的积累,到2050年,贵州省0-10,0-20,0-30,0-100cm土层土壤有机碳将增加1.99×10~(13),3.37×10~(13),4.45×10~(13),6.29×10~(13) g。可见,喀斯特地区土壤有机碳具有含量高、密度低的显著特征,石漠化治理能有效增加喀斯特地区土壤碳汇。 相似文献
4.
晋西黄土丘陵区不同土地利用类型对土壤碳氮储量的影响 总被引：2，自引：0，他引：2
通过对比晋西黄土区蔡家川流域人工刺槐林地、人工油松林地、退耕荒草地和农地土壤有机碳和总氮含量的差异性,探讨了土地利用变化对该流域土壤碳氮储量的影响。结果表明:（1）4种土地利用类型下,土壤有机碳和总氮均表现出表聚效应,且荒草地垂向降幅最大,各样地土壤有机碳和总氮均呈极显著线性正相关;（2）油松林地0—48 cm土层有机碳含量高于其他样地,刺槐林地0—48 cm土层总氮含量显著高于其他样地,而4种土地利用类型在48—96 cm土层和96—160 cm土层中有机碳和总氮含量差异不显著;（3）从蔡家川流域退耕土壤固碳和固氮效应整体而言,退耕林地具有显著的土壤碳氮固存效应,随着人工林地面积占比的增加,流域土壤总有机碳和总氮储量相应增加。 相似文献
5.
不同土地利用方式对土壤有机无机碳比例的影响 总被引：1，自引：0，他引：1
6.
Better understanding of how the loess soils respond to topography and land use under catchment‐scale vegetation restoration is needed to enable science‐based land management interventions for the policy‐driven “Grain‐for‐Green” eco‐restoration program in the Loess Plateau of China. The objective of this study was to characterize the relationships of four selected soil quality indicators to land use under vegetation restoration and topography for a small catchment (0·58 km2) in the Loess Plateau. The major land uses established in the catchment are cropland, fallow (i.e., natural revegetation), grassland, and jujube orchard. The four soil quality indicators were soil organic carbon (SOC), soil total nitrogen (STN), soil total phosphorus (STP), and mean root zone soil water content during the wet season (MRZSWwet). SOC, STN, and MRZSWwet were significantly different (p < 0·05) for different land uses. Grassland showed the highest values for these three properties, whereas cropland had relatively low values for SOC and STN. Land use had no effect on STP, although the lowest value was observed in grassland. Spatial analysis showed that various relations between soil quality indicators and topography (slope and elevation) were observed. These relations were generally weak for most of them, and they varied with land uses. Further analyses indicated that land uses, slope, and elevation had significant effects on the relations between different soil quality indicators. The results here should provide useful information for the further development of “Grain‐for‐Green” program. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
7.
滇中尖山河小流域不同土地利用类型土壤活性有机碳分布特征 总被引：2，自引：0，他引：2
土壤活性有机碳对土地利用方式最为敏感,定量分析不同土地利用方式对土壤活性有机碳分布特征的影响对流域的土壤碳循环研究具有重要意义。从滇中尖山河小流域坡耕地、荒草地、林地、园地4种不同土地利用类型角度,系统地分析了0—10,10—20,20—30 cm土层土壤有机碳（SOC）、微生物有机碳（MBC）、易氧化有机碳（EOC）及可溶性有机碳（DOC）的分布特征及其相关性。结果表明:不同土地利用类型下土壤SOC,MBC,EOC,DOC整体均表现为园地 > 林地 > 坡耕地 > 荒草地;4种土地利用类型MBC,EOC,DOC整体上随着土层深度的增加而逐渐降低,且主要分布在0—20 cm土层,在20—30 cm土层含量较低（低于30%）;4种土地利用类型下SOC和MBC,EOC,DOC呈极显著正相关关系,MBC,EOC,DOC两两之间也表现出极显著正相关。综上,退耕还林以及在荒草地种植人工林可作为提高土壤有机碳及活性有机碳含量的有效措施,并将在减少流域水土流失和面源污染、改善土壤质量、恢复土壤肥力等方面起到重要作用。 相似文献
8.
黄土丘陵区不同退耕还林地土壤颗粒结合态碳库分异特征 总被引：3，自引：0，他引：3
为揭示黄土丘陵区不同退耕还林土壤有机碳库差异及变化机制,比较分析了15 a生刺槐、山杏、杨树、沙棘、柠条5种退耕还林地土壤砂粒(≥53~2000μm)、粉粒(≥2~53μm)、黏粒(2μm)结合碳的质量分数与分布变化状况。结果表明,与坡耕地比较,不同退耕林地从表层0~10cm到40~60 cm土层土壤总有机碳质量分数增加了1.0~1.9 g/kg,砂粒碳、粉粒碳、黏粒碳质量分数分别增加了0.5~0.1、1.0~0.6、0.4~0.3 g/kg。同时,各林地0~20 cm土层土壤总有机碳和粉粒碳密度差异为杨树=柠条沙棘山杏刺槐,两种碳库增幅分别为2.4~5.8和1.2~3.5 Mg/hm2;但不同林地该土层黏粒碳和砂粒碳基本无显著差异,平均增幅分别为0.7和0.9 Mg/hm2。土层深度为0~60 cm时,土壤总有机碳、粉粒碳、黏粒碳密度均表现为沙棘=杨树=柠条山杏=刺槐,3种碳库增幅分别7.1~12.1、3.8~6.8、1.8~3.2 Mg/hm2;该土层砂粒碳密度在不同林地间仍无显著差异,平均提高了1.5 Mg/hm2。不同林地土壤颗粒碳组分占全有机碳比例均以粉粒碳最高(56.8%)、黏粒碳次之(29.3%),砂粒碳最低(13.8%)。综上,不同退耕林地均以粉粒碳为土壤碳库变化和累积的主要形式,但以刺槐和山杏林提升退耕土壤总有机碳及颗粒碳组分库效应最明显,可作为该区域优选的退耕还林生态固碳技术。 相似文献
9.
Influence of land use change on soil nutrients in an intensive agricultural region of North China 总被引：7，自引：0，他引：7
Xiangbin Kong Fengrong Zhang Qi Wei Yan Xu Jiangguang Hui 《Soil & Tillage Research》2006,88(1-2):85-94
The pressures of development and major shifts in land use have increased the need to assess the impact of land use change on soil nutrients. This study was conducted in an area with intensive agricultural land use in north China to assess effects of land use change on soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP) and available potassium (AK). Soil samples were collected from rainfed land, irrigated land, virgin grassland and vegetable land in 1980 and 1999. SOC, TN and AP were higher in 1999 than in 1980, and the increases followed the following order: vegetable land > irrigated land > rainfed land, except for AK. Soil nutrient contents were increased when land uses underwent the following changes (except for AK): virgin grassland → rainfed land → irrigated farmland → vegetable land. The SOC, TN and AP fluxes (rate of change in C, N, P and K as kg(ha y)−1) in the transition systems from rainfed land to vegetable land were 518.46, 69 and 3.9 kg(ha y)−1, respectively, which were considerably higher than the transition systems from rainfed land to irrigated land. The SOC, TN and AP fluxes in the transition systems from virgin grassland to irrigated land were 297.83, 56.3, 2.1 kg(ha y)−1, respectively, which were higher than the transition systems from virgin grassland to rainfed land. The study shows that land use intensification can increase soil nutrient contents if accompanied by increased fertilizer application. 相似文献
10.
《Communications in Soil Science and Plant Analysis》2012,43(2):181-189
Soil organic carbon (SOC) density was researched in southern NingXia with five different land uses: shrubland, farmland, grazing grassland, orchard, and artificial grassland. The results have shown that (1) content of SOC in soil 0–20 cm deep was greatest for grazing grassland and less for bush forestland, farmland, artificial grassland, and orchard; (2) content of SOC of bush forestland and orchard decreased slowly as depth of profile increased, whereas SOC of farmland, grazing grassland, and artificial grassland decreased fast; (3) SOC density of grazing grassland and farmland was greater than the bush forestland, artificial grassland, and orchard; and (4) the establishment and development of C. korshinski on eroded soil resulted in significant improvement of SOC density. 相似文献
11.
Purpose
Soil nutrients, elemental stoichiometry, and their associated environmental control play important roles in nutrient cycling. The objectives of this study were (1) to investigate soil nutrients and elemental stoichiometry, especially potassium and its associative elemental stoichiometry with other nutrients under different land uses in terrestrial ecosystems; (2) to discuss the impacts of climate factors, soil texture, and soil physicochemical properties; and (3) to identify the key factors on soil nutrient levels and elemental stoichiometry.
Materials and methodsSoil data, including pH, bulk density (BD), cation exchange capacity (CEC), volumetric water content (VMC), clay, silt and sand contents, total carbon (TC), nitrogen (TN), phosphorous (TP) and potassium (TK), available nitrogen (AN), phosphorus (AP), potassium (AK), and soil organic matter (SOM) under different land-use types, were collected, and their elemental stoichiometry ratios were calculated. Climate data including temperature, precipitation, relative humidity, wind speed, and evapotranspiration were collected. The least significant difference test and one-way analysis of variance were applied to investigate the variability of soil nutrients and elemental stoichiometry among land-use types; the ordinary least squares method and the general linear model were used to illustrate the correlations between soil nutrients, elemental stoichiometry, and soil properties or climate factors and to identify the key influencing factors.
Results and discussionWoodlands had the highest SOM, TN, AN, and AK contents, followed by grasslands, croplands, and shrublands, while the TP and TK contents only varied slightly among land-use types. SOM, TN, AN, N/P, and N/K were strongly negatively correlated to soil pH (p <?0.05) and were strongly positively correlated to soil CEC (p <?0.05). For soil texture, only C/N was moderately negatively correlated to silt content but moderately positively correlated to sand content (p <?0.05). For climate factors, SOM, TN, AN, N/P, and N/K were significantly negatively correlated to evapotranspiration and temperature (p <?0.05), and the correlations were usually moderate. Soil pH explained most of the total variation in soil nutrients, and climate factors explained 5.64–28.16% of soil nutrients and elemental stoichiometry (except for AP (0.0%) and TK (68.35%)).
ConclusionsThe results suggest that climate factors and soil properties both affect soil nutrients and elemental stoichiometry, and soil properties generally contribute more than climate factors to soil nutrient levels. The findings will help to improve our knowledge of nutrient flux responses to climate change while also assisting in developing management measures related to soil nutrients under conditions of climate change.
相似文献12.
《Land Degradation \u0026amp; Development》2017,28(4):1363-1375
Overgrazing contributes to rangeland degradation altering plant community composition, erosion and biodiversity. Little unanimity in the literature exists on the effects of livestock grazing on soil carbon and biodiversity, in part, due to uncontrolled grazing pressure from native and feral animals. Paired paddock contrasts at three, long‐term (>8 years) study locations in the southern Australian rangelands were used to examine the effects of managing grazing intensity through the use of exclusion fencing and rotational grazing on soil organic carbon (SOC), soil nitrogen (TN), ground cover and biodiversity (flora and invertebrates). Grazing management had no effect on SOC or TN on grey soils (Vertisols), but for red soils (Lixisols), significantly higher levels of SOC were found for both the 0 to 5 and 5 to 10‐cm soil depths (0·3% and 0·27% respectively) and associated with increased TN. We found strong and consistent relationships among SOC and higher perennial (p < 0·001), higher litter (p < 0·05) cover and close proximity to trees (p < 0·05). Managing grazing intensity resulted in significantly higher perennial ground cover (p < 0·001) on Vertisols (8·9 to 11%) and Lixisols (12·5 to 15%) and higher plant diversity (both native and exotic) but negatively impacted invertebrate diversity, indicating trade‐offs between production and resources. We provide evidence that the effects of grazing management on SOC are mediated by ground cover and increased organic matter supply and/or reduced soil carbon redistribution (erosion), which indicates that the management of grazing intensity may provide a tool to avoid soil carbon loss in rangelands. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
13.
作为土壤质量的重要指标,土壤活性有机碳（SLOC）在土壤许多物理、化学和生物特性中发挥着重要作用。通过对不同土地利用方式土壤进行采样和分析,系统地研究和比较了四川省岷江流域不同土地利用方式（次生林、人工林、灌草丛和坡耕地）SLOC分布特征及其影响因子。结果表明:岷江流域不同土地利用方式土壤机械组成不尽一致,土壤总孔隙度与土壤容重变化趋势相反。不同土地利用方式对土壤养分具有较大影响,土壤有机碳、全氮、全磷和全钾均呈现出一致性规律,大致表现为次生林和灌草丛高于人工林和坡耕地,而不同土地利用方式土壤全磷差异并不显著（p > 0.05）。不同土地利用方式SLOC均呈现出一致性规律,大致表现为次生林和灌草丛高于人工林和坡耕地。不同土地利用方式SLOC垂直方向随土层深度的增加表现出递增趋势,并且降低幅度逐渐减小,坡耕地、灌草丛、次生林和人工林SLOC从表层至底层逐渐递减,呈“T”形分布,“表聚性”较为明显。人工林、灌草丛和坡耕地在10—20 cm SLOC/SOC比例最高,而次生林0—10 cm SLOC/SOC比例最高,不同土地利用方式SLOC/SOC比例均在30—40 cm最低。相关性分析表明,不同土地利用方式SLOC与SOC和全氮均呈极显著正相关性（p < 0.01）,与土壤容重均呈负相关。表明SLOC受土壤养分和机械组成影响较大,其中SOC和全氮是不同土地利用方式SLOC变化的重要影响因素。SLOC与土壤有机碳之间呈极显著正相关关系（R2=0.9961）,说明SLOC能够作为研究不同土地利用方式SOC动态变化的一个敏感性指标而提前反映土壤碳库的动态变化,其含量在很大程度上依赖于所处生境下SOC含量。 相似文献
14.
封育与放牧对黄土高原天然草地土壤化学计量特征的影响 总被引：1，自引：1，他引：0
为揭示封育与放牧2种草地利用方式下土壤化学计量特征及其驱动因素,以宁夏固原云雾山封育草地和放牧草地为研究对象,定量分析了0—30 cm土壤化学计量特征及其影响因子。结果表明:(1)封育降低了土壤紧实度、容重和含水率,土壤孔隙度略有上升。(2)封育草地0—10 cm土层以0.25 mm水稳性团聚体为主,其中5 mm团聚体达到了46%,平均重量直径(MWD)和几何平均直径(GWD)分别为3.78,1.70,显著高于放牧草地(P0.05),但两者分形维数并无差异,封育草地土壤团聚体稳定性有所提高。(3)封育草地0—10 cm土层土壤有机碳(SOC)、全氮(TN)、全磷(TP)分别为17.714,2.018,0.659 g/kg,均高于放牧草地,其中土壤TN含量达到显著水平(P0.05)。(4)封育草地0—20 cm土层碳氮比(C/N)、碳磷比(C/P)、氮磷比(N/P)均高于放牧草地。(5)封育草地土壤SOC、TN、C/P和N/P与土壤容重和分形维数呈显著性负相关(P0.05),并且SOC和TN与MWD和GWD呈显著性正相关(P0.05),放牧草地土壤化学计量特征与物理特性相关性低于封育草地。封育与放牧相比,改善了土壤物理特性和团聚体结构,土壤养分有所提升,土壤化学计量特征与土壤物理特性密切相关,土壤物理结构可能是影响土壤化学计量特征的主控因子。 相似文献
15.
Soil organic carbon distribution in relation to land use and its storage in a small watershed of the Loess Plateau, China 总被引：2，自引：0，他引：2
Soil organic carbon (SOC) is an important component in agricultural soil, and its stock is a major part of global carbon stocks. Estimating the SOC distribution and storage is important for improving soil quality and SOC sequestration. This study evaluated the SOC distribution different land uses and estimated the SOC storage by classifying the study area by land use in a small watershed on the Loess Plateau. The results showed that the SOC content and density were affected by land use. The SOC content for shrubland and natural grassland was significantly higher than for other land uses, and cropland had the lowest SOC content. The effect of land use on the SOC content was more significant in the 0-10 cm soil layer than in other soil layers. For every type of land use, the SOC content decreased with soil depth. The highest SOC density (0-60 cm) in the study area was found in shrublandII (Hippophae rhamnoides), and the other land uses decreased in the SOC density as follows: natural grassland > shrublandI (Caragana korshinskii) > abandoned cropland > orchard > level ground cropland > terrace cropland > artificial grassland. Shrubland and natural grassland were the most efficient types for SOC sequestration, followed by abandoned cropland. The SOC stock (0-60 cm) in this study was 23,584.77 t with a mean SOC density of 4.64 (0-60 cm). 相似文献
16.
通过比较荒地和玉米地土壤养分、胡敏酸热解特性和红外光谱特征,研究了灰漠土农田生态条件对土壤养分和有机质特性的影响。结果表明,种植玉米增加土壤有机质、全N、全P、全K、缓效钾和碱解氮的含量,但使速效磷和速效钾含量下降,土壤pH略有下降。荒地土壤胡敏酸的脂肪族侧链较多,胡敏酸较年轻,而玉米地土壤胡敏酸侧链较少,胡敏酸相对较老。荒地土壤胡敏酸的脂族性强于玉米地。农田生态条件使土壤胡敏酸老化。 相似文献
17.
雾灵山低山区土地利用类型对土壤理化性质的影响 总被引：2，自引：0，他引：2
针对雾灵山低山区不同土地利用类型的土壤进行采样和分析,研究农田、果园、绿化林、果农间作和未利用地5种土地利用类型对土壤理化性质的影响,以期为雾灵山地区的土地合理利用和水土保持等提供科学依据.结果表明:5种土地利用类型中,绿化林的有机质、全氮、碱解氮、速效钾含量均为最高,而含水量最低;果农间作的有机质、全氮、碱解氮含量最低;未利用地的含水量和容重最高,而速效磷和速效钾含量最低.不同土地利用类型土壤有机质含量随土层深度的增加而减少,而其他土壤养分含量随土层深度的增加其减少量有明显差异.同全国第二次土壤普查养分分级标准相比,各土地利用类型的土壤速效磷和速效钾含量较为丰富,全氮和有机质含量较为缺乏,其中绿化林的各养分均较为丰富. 相似文献
18.
西北风积沙区采煤扰动下土壤侵蚀与养分演变特征 总被引：3，自引：2，他引：1
为更好地理解西北风积沙矿区生态环境演变规律,以神东哈拉沟与上湾矿区为例,利用~(137)Cs示踪法分析了未采区、自恢复沉陷区(1、2、4、8 a沉陷区)与植被修复区(13 a沉陷区)的土壤侵蚀与养分特征,研究了矿区土壤侵蚀与养分的演变规律。结果表明:采煤扰动可以导致未采区土壤侵蚀强度增大与有机碳、微生物量碳、全氮、全磷、碱解氮养分明显损失。开采沉陷后,沉陷区土壤侵蚀强度随着时间的推移呈现出先增加后降低的趋势;其中,地表沉陷后的最初2 a是土壤侵蚀急剧增大的时段。采煤扰动下,土壤有机碳、微生物量碳、全氮、全磷、碱解氮养分的演变规律与土壤侵蚀演变密切相关。植被修复可以有效降低沉陷区土壤侵蚀强度与提高土壤养分含量。西北风积沙区采煤沉陷地表的生态恢复应该及时开展地表沉陷后的早起侵蚀防治,兼顾考虑植被修复与土壤微生物联合修复,以此促进土壤、植被正向演替。 相似文献
19.
喀斯特峰丛洼地不同植被类型土壤微生物量碳氮磷和养分特征 总被引：1，自引：0，他引：1
20.
Abstract. Natural secondary succession, forest planting and agricultural practices after deforestation affect soil properties in many ways. During the last 50 years, land uses have greatly changed in the mountainous areas in southwestern China as the result of deforestation and cultivation. A study was initiated in Wolong Nature Reserve, Sichuan province to elucidate the complex relationships in a humid mountainous region. Soil properties under six typical land use types (natural forest, grassland, shrub, secondary forest, cultivated land and reforested land) were compared. Significant differences between land uses were found for soil bulk density (BD), total nitrogen (TN), soil organic carbon (SOC), available phosphorus (AP) and available potassium (AK). Cultivated land had the lowest levels for most soil properties compared to other land uses and shrubland had a higher SOC, TN and available nitrogen (AN) than other land uses. Soils under grassland and shrub contained the greatest carbon mass (TC). Further studies on reforested land indicated that soil properties could be changed by length of reforestation. The SOC and TN in particular showed a linear relationship with years since reforestation. The results suggested that in an area of China where the climate favours secondary succession, 'leave nature as it is' is a better choice than the policy 'change farmland to forest land ', especially for the mountainous regions where there is lack of labour and financial support. 相似文献