绿洲边缘夹粘沙丘持水特性

夹粘沙丘是指剖面中分布有一层红褐色粘化层的沙丘,其夹粘层埋藏深度常在120—500cm左右,厚度在40—100cm之间,粘粉粒含量达到90%以上。夹粘沙丘人工梭梭固沙植被郁闭度在0.5以上,明显高于相同水热条件下的植被生产力,这可能与夹粘层通过改变土壤持水性及水分状况,从而提高了土壤水分承载力有关。以河西走廊绿洲边缘夹粘沙丘为研究对象,在测定0—8.0m剖面土壤物理性质和土壤水分特征曲线的基础上,计算了土壤孔隙分布和持水特性,研究了剖面中土壤持水性的变化规律及其影响因素。结果表明:(1)夹粘层土壤含水量最高,上覆沙土土壤含水量最少且随深度增加而增加,下伏沙土受地下水毛管作用影响水分含量较高;(2)土壤机械组成决定了不同土层的持水特性,土壤中粘粉粒含量越高,田间持水量和饱和含水量就越大,夹粘层的土壤持水性远高于沙土层;(3)夹粘层是沙丘土壤水分暂时贮存的重要土层,可以为植被提供深层土壤水分,这对绿洲边缘人工固沙植被的生存和繁衍有重要影响,而土壤持水性和毛管作用的强弱决定了夹粘层对沙丘水分调节能力的强弱。本研究为深入理解绿洲边缘雨养固沙植被斑块状分布的机理提供了一定的理论依据。     

宁夏中宁枸杞林地粉壤土阳离子交换量空间分布预测方法对比

土壤阳离子交换量(CEC)是土壤施肥、改良的主要依据和土壤质量的评价指标,研究土壤CEC的空间分布及模型预测可为土壤养分监测、管理及精准农业实施提供科学依据。本研究以中宁枸杞林地粉壤土为对象,在自相关、交互相关等分析基础上,采用协同克里格(CoKriging)、普通最小二乘法(OLS)、地理加权回归(GWR)和随机森林(RF)模型对土壤CEC进行回归分析,比较了制图效果及模型预测精度。结果表明:中宁枸杞林地粉壤土CEC平均值为13.12 cmol·kg^-1,属中等肥力;土壤CEC的空间分布具有自相关性,并与土壤pH、有机质、黏粒和电导率在不同滞后距离上存在不同的空间相互关系;RF模型预测图避免了CoKriging、OLS和GWR模型预测图中土壤CEC图斑边界两侧破碎程度大、突变明显的缺陷,使土壤CEC在空间变化上表现为自然、平缓的过渡;RF模型RMSE值分别比CoKriging、OLS和GWR模型减少33.82%、20.55%和19.81%,R^2分别提高8.84%、51.92%和7.69%。RF模型考虑了样点空间位置,明显提高了插值精度且制图效果更加平缓。     

不同尺度下土壤粒径分布特征及其影响因子——以黄土丘陵沟壑区为例

土地利用方式和环境因子是影响土壤粒径的重要因素,尺度不同其影响效应差异明显。研究不同尺度表层土壤粒径与土地利用、环境因子的关系对评价黄土丘陵沟壑区土壤物理性质具有积极意义。选择黄土丘陵沟壑区安塞集水区和沐浴小流域作为研究区,探讨两个尺度上表层土壤粒径含量、分布特征及其与土地利用类型和环境因子的关系。研究结果表明:(1)研究区域内表层土壤颗粒主要为砂粒和粉粒,在小流域和集水区尺度上,各粒径百分含量属于中等变异,D值为弱变异,但随着研究区由沐浴小流域变化到安塞集水区,粒径和D值的空间变异性均有所提高;(2)尺度不同,土地利用对土壤粒径的影响效应不同,在沐浴小流域不同土地利用类型的砂粒含量从高到低依次为荒草地农用地林地灌木林地园地,在集水区的变化顺序依次为荒草地灌木林地林地农用地园地,粉粒含量的次序均与砂粒相反,小流域土壤粒径分形维数D依次为灌木林地荒草地林地园地农用地,在安塞集水区为灌木林地荒草地农用地林地园地;(3)在沐浴小流域和安塞集水区,土壤颗粒分形维数与土壤砂粒、黏粒百分含量呈极显著正相关关系,与土壤粉粒百分含量呈极显著负相关关系,且砂质壤土D值高于壤土和粉壤土;(4)在沐浴小流域,植被盖度、相对海拔和坡位相对于土地利用类型、坡向和坡度对土壤粒径影响较大;而在安塞集水区植被盖度、土地利用类型和坡度对土壤粒径影响较大。     

Population Increase of Pratylenchus hexincisus on Corn as Related to Soil Temperature and Type

Population increase of Pratylenchus hexincisus on corn was tested over 3 months at 15, 20, 25, and 30 C in Marshall silt loam, Clarion silt loam, Buckner coarse sand, and Haig silty clay loam soils. The optimum temperature for increase was 30 C in all soils. The nematode population was significantly larger in Buckner coarse sand than in other soil types at 50 C. The recovered P. hexincisus populations equaled or exceeded initial inoculum levels at the two higher temperatures in Marshall silt loam and Haig silty clay loam and at 30 C in Clarion silt loam and Buckner coarse sand. P. hexincisus required 32,400 heat units in Haig silty clay loam and more than 40,000 heat units in the three other soil types to reach a level that is known to cause significant height and biomass reduction in corn under controlled condition.     

Desert soil fungi isolated from Saudi Arabia: cultivable fungal community and biochemical production

Desert soils harbor fungi that have survived under highly stressed conditions of high temperature and little available moisture. This study was designed to survey the communities of cultivable fungi in the desert soils of the Arabian Peninsula and to screen the fungi for the potentially valuable antioxidants (flavonoids, phenols, saponins, steroids, tannins, terpenoids, and alkaloids) and enzymes (cellulase, laccase, lipase, protease, amylase, and chitinase). Desert soil was sampled at 30 localities representing different areas of Saudi Arabia and studied for physico-chemical soil properties. Five types of soil texture (sand, loamy sand, sandy loam, silty loam, and sandy clay loam) were observed. A total of 25 saprotrophic species was identified molecularly from 68 isolates. Our survey revealed 13 culturable fungal species that have not been reported previously from Arabian desert soils and six more species not reported from Saudi Arabian desert soils. The most commonly recorded genera were Aspergillus (isolated from 20 localities) and Penicillium (6 localities). The measurements of biochemicals revealed that antioxidants were produced by 49 and enzymes by 52 isolates; only six isolates did not produce any biochemicals. The highest biochemical activity was observed for the isolates Fusarium brachygibbosum and A. phoenicis. Other active isolates were A. proliferans and P. chrysogenum. The same species, for instance, A. niger had isolates of both high and low biochemical activities. Principal component analysis gave a tentative indication of a relationship between the biochemical activity of fungi isolated from soil and soil texture variables namely the content of silt, clay and sand. However, any generalizable relation between soil properties and fungal biochemical activities cannot be suggested. Each fungal isolate is probable to produce several antioxidants and enzymes, as shown by the correlation within the compound groups. Desert soil warrants further research as a promising source of biochemicals.     

Soil types with different texture affects development of Rhizoctonia root rot of wheat seedlings

The effect of different soil textures, sandy (97.5% sand, 1.6% silt, 0.9% clay), loamy sand (77% sand, 11% silt, 12% clay) and a sandy clay loam (69% sand, 7% silt, 24% clay), on root rot of wheat caused by Rhizoctonia solani Kühn Anastomosis Group (AG) 8 was studied under glasshouse conditions. The reduction in root and shoot biomass following inoculation with AG-8 was greater in sand than in loamy sand or sandy clay loam. Dry root weight of wheat in the sand, loamy sand and sandy clay loam soils infested with AG-8 was 91%, 55% and 28% less than in control uninfested soils. There was greater moisture retention in the loamy sand and sandy clay loam soils as compared to the sand in the upper 10–20 cm. Root penetration resistance was greater in loamy sand and sandy clay loam than in sand. Root growth in the uninfested soil column was faster in the sand than in the loamy sand and sandy clay loam soils, the roots in the sandy soil being thinner than in the other two soils. Radial spread of the pathogen in these soils in seedling trays was twice as fast in the sand in comparison to the loamy sand which in turn was more than twice that in the sandy clay loam soil. There was no evidence that differences among soils in pathogenicity or soil spread of the pathogen was related to their nutrient status. This behaviour may be related to the severity of the disease in fields with sandy soils as compared to those with loam or clay soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil and forest structure in the Colombian Amazon

Forests structural differences could result of environmental variations at different scales. Because soils are an important component of plant's environment, it is possible that edaphic and structural variables are associated and that, in consequence, spatial autocorrelation occurs. This paper aims to answer two questions: (1) are structural and edaphic variables associated at local scale in a terra firme forest of Colombian Amazonia? and (2) are these variables regionalized at the scale of work? To answer these questions we analyzed the data of a 6ha plot established in a terra firme forest of the Amacayacu National Park. Structural variables included basal area and density of large trees (diameter > or = 10cm) (Gdos and Ndos), basal area and density of understory individuals (diameter < 10cm) (Gsot and Nsot) and number of species of large trees (sp). Edaphic variables included were pH, organic matter, P, Mg, Ca, K, Al, sand, silt and clay. Structural and edaphic variables were reduced through a principal component analysis (PCA); then, the association between edaphic and structural components from PCA was evaluated by multiple regressions. The existence of regionalization of these variables was studied through isotropic variograms, and autocorrelated variables were spatially mapped. PCA found two significant components for structure, corresponding to the structure of large trees (G, Gdos, Ndos and sp) and of small trees (N, Nsot and Gsot), which explained 43.9% and 36.2% of total variance, respectively. Four components were identified for edaphic variables, which globally explained 81.9% of total variance and basically represent drainage and soil fertility. Regression analyses were significant (p < 0.05) and showed that the structure of both large and small trees is associated with greater sand contents and low soil fertility, though they explained a low proportion of total variability (R2 was 4.9% and 16.5% for the structure of large trees and small tress, respectively). Variables with spatial autocorrelation were the structure of small trees, Al, silt, and sand. Among them, Nsot and sand content showed similar patterns of spatial distribution inside the plot.     

A comparison of fine root distribution and water consumption of mature Caragana korshinkii Kom grown in two soils in a semiarid region, China

Water is a key limiting factor for vegetation restoration in the semi-arid areas of China. Caragana korshinkii Kom is a shrub that is widely planted in this region to control soil erosion and land desertification. The objective of this study was to investigate the fine root distribution of mature C. korshinkii and its water consumption, when grown in either silt loam or sandy soils, in order to understand differences between the water cycles of two such soils found in the transition zone between fertile loess hills and desert of the Northern Loess Plateau. Fine root distributions were measured using the trench-profile method. Soil water dynamics were monitored with a neutron probe during two growing seasons. The results showed that fine root area density (FRAD) declined with increasing soil depth in both soils, with 70.7% and 96.6% of the total fine roots being concentrated in the upper 1-m layer of the silt loam and sandy soils, respectively. Water consumption by C. korshinkii in the silt loam was close to that in the sandy soil. Most water consumption in both soil types was from the upper 1-m layer. Little variation in plant available water (PAW) occurred in the 3–6 m soil layer during the whole study period. However, in this layer, the PAW was significantly lower in the silt loam soil than in the sandy soil. Total actual evapotranspiration (ET_a) was slightly higher from the sandy soil plots than from those of the silt loam soil during both growing seasons. Our study indicated that mature C. korshinkii effectively uses about the same amount of water from either the silt loam or sandy soils, but that more soil water at depth was extracted from silt loam soil than from sandy soil.     

Vegetation communities and plant distributions and their relationships with parent materials in the forest-tundra of northwestern Canada

Vegetation data from 95 sites and chemical and textural data for parent materials from 98 sites in subarctic northwestern Canada were analyzed by ordination, classification, and statistical techniques A discontinuity in plant species occurrence, vegetation communities, and parent materials takes place near the southeast/northwest boundary north of Great Slave Lake Till parent materials in the forest-tundra show regional differences in nutrient concentration and texture that correlate with the local bedrock Northwestern tills, as a group, are markedly richer in nutrients and finer-textured than Shield tills, with much higher levels of cation exchange capacity, nitrogen, potassium, calcium, magnesium, hydrogen, and organic carbon, and more silt and clay than Shield tills Plant species occurrence and vegetation community composition show strong relationships with soil pH, moisture, texture, and latitude The northwest is characterized by calciphilic white spruce. Dryas , legume, and bryophyte communities, and high species diversity, on basic loamy Cryosols The southeast is characterized by acidophiles and widespread generalist black spruce, ericad, and lichen communities, and low species diversity, on acidic loamy sand and sandy loam Brunisols     

Effect of Substrate Type and Moisture on Pupation and Emergence of <Emphasis Type="Italic">Heortia vitessoides</Emphasis> (Lepidoptera: Crambidae): Choice and No-Choice Studies

Heortia vitessoides Moore is the most destructive defoliator of Aquilaria sinensis (Loureiro) Sprenger, an endangered and economically important plant that produces the high-prized agarwood. After maturing, H. vitessoides larvae will leave the A. sinensis and pupate in soil, however, factors affecting this behavior have not been previously addressed. In the present study, choice and no-choice tests were conducted to study the effects of substrate type (sand, sandy loam A, sandy loam B, and silt loam) and moisture condition (0 %, 20 %, 40 %, 60 %, 80 %, and 100 % water saturation) on burrowing, pupation and emergence behaviors of H. vitessoides under laboratory conditions. Six-choice tests showed that significantly fewer larvae burrowed and pupated in each substrate under extremely dry (0 % saturation) or wet (80 % and 100 % saturation) conditions. Four-choice tests showed that at 20 % saturation a significantly lower percentage of larvae burrowed and pupated in silt loam as compared to other substrates, whereas at 40 % saturation sand was least preferred. At 60 % saturation, however, there was no significant difference in burrowing and pupation among the four substrates. No-choice test showed that either or both 0 % and 80 % water saturation suppressed burrowing behavior and emergence success compared with the intermediate saturation levels in sandy loam (A and B) and silt loam, but not in sand. In addition, the effect of substrate type varied with saturation level. Our study enhance the understanding of pupation ecology of H. vitessoides, and applied implications are discussed.     

Quantification and correlation of vesicular-arbuscular mycorrhizal propagules with soil properties of some mollisols of northern India

The populations of vesicular-arbuscular mycorrhizae (VAM) propagules by the most probable number method in some mollisols and their correlations with some important soil properties were determined. On average, the six soils, Phoolbagh clay loam, Beni silty clay loam, Haldi loam, Nagla loam, Khamia sandy loam and Patherchatta sandy loam contained 4.9, 4.0, 7.9, 7.9, 3.3 and 13.0 propagules/g soil, respectively, i.e. none of the soils was found to be high in VAM. The size of the VAM population was compared to soil properties such as pH, organic carbon, sand content, available phosphorus and available potassium, cation-exchange capacity, silt and clay contents. A significant positive correlation (r=0.586) was only found with available soil phosphorus (P<0.05) and a significant negative correlation (r=-0.555) with soil clay content (P<0.05).Directorate research paper series No. 7862     

Spatial ordination of vegetation data using a generalization of Wartenberg's multivariate spatial correlation

Question: Are there spatial structures in the composition of plant communities? Methods: Identification and measurement of spatial structures is a topic of great interest in plant ecology. Univariate measurements of spatial autocorrelation such as Moran's I and Geary's c are widely used, but extensions to the multivariate case (i.e. multi‐species) are rare. Here, we propose a multivariate spatial analysis based on Moran's I (MULTISPATI) by introducing a row‐sum standardized spatial weight matrix in the statistical triplet notation. This analysis, which is a generalization of Wartenberg's approach to multivariate spatial correlation, would imply a compromise between the relations among many variables (multivariate analysis) and their spatial structure (autocorrelation). MULTISPATI approach is very flexible and can handle various kinds of data (quantitative and/or qualitative data, contingency tables). A study is presented to illustrate the method using a spatial version of Correspondence Analysis. Location: Territoire d'Etude et d'Expérimentation de Trois‐Fontaines (eastern France). Results: Ordination of vegetation plots by this spatial analysis is quite robust with reference to rare species and highlights spatial patterns related to soil properties.     

Effect of soil type on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei, Steinernema glaseri, Heterorhabditis zealandica, and Heterorhabditis bacteriophora

We tested the effect of soil type on the performance of the entomopathogenic pathogenic nematodes Steinernema scarabaei, Steinernema glaseri, Heterorhabditis zealandica, and Heterorhabditis bacteriophora. Soil types used were loamy sand, sandy loam, loam, silt loam, clay loam, acidic sand, and a highly organic potting mix. Infectivity was tested by exposing third-instar Anomala orientalis or Popillia japonica to nematodes in laboratory and greenhouse experiments and determining nematode establishment in the larvae and larval mortality. Infectivity of H. bacteriophora and H. zealandica was the highest in potting mix, did not differ among loamy sand and the loams, and was the lowest in acidic sand. Infectivity of S. glaseri was significantly lower in acidic sand than in loamy sand in a laboratory experiment but not in a greenhouse experiment, and did not differ among the other soils. Infectivity of S. scarabaei was lower in silt loam and clay loam than in loamy sand in a greenhouse experiment but not in a laboratory experiment, but was the lowest in acidic sand and potting mix. Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with Galleria mellonella larvae. Persistence of both Heterorhabditis spp. and S. glaseri was the shortest in potting mix and showed no clear differences among the other substrates. Persistence of S. scarabaei was high in all substrates and its recovery declined significantly over time only in clay loam. In conclusion, generalizations on nematode performance in different soil types have to be done carefully as the effect of soil parameters including soil texture, pH, and organic matter may vary with nematode species.     

Soil organic matter dynamics in tiger bush (Niamey,Niger). Preliminary results

Some typical features of soil organic matter dynamics and soil texture were studied to discuss the particular spatial pattern of tiger bush in Niger and its dynamics. The soil texture through silt and clay contents showed a high variability in the vegetation arc as well as in the bare area. These variations were clearly linked to water/wind erosion and termite activity. Tiger bush soils showed a high capacity to store soil organic matter despite a moderate primary production, even in the bare area where the input of plant debris has been nil for many decades. The carbon content was higher within the vegetation arc (0.93 %) than within the bare area (0.45 %). Additionally, potential carbon mineralisation significantly varied in relation to the total carbon content and thus to primary production. Then, the vegetation arcs can be viewed as `fertility islands' as in many arid ecosystems. The measurements of δ¹³C showed a dominant contribution of C₃ plants to the soil organic matter pool. Nevertheless, the contribution of C₄ plants was not negligible. Two hypotheses could be proposed: a different mineralisation rate between C₃ and C₄ plants; or (ii) a better physical protection of C₄ compounds against biodegradation. The soil variables depending totally or partly on biological factors, such as carbon and nitrogen contents, carbon isotopic composition, carbon potential mineralisation, did not show any symmetry in their variations along the studied transects. It was expected in the vegetation arc because the vegetation cover does not show symmetry in its specific composition and spatial structure. In the bare area, a clear asymmetry was observed on some of the variables: carbon content, fine material content and natural abundance of ¹³C. This supports the hypothesis that the vegetation arcs move upslope, and weakens the hypothesis of the alternance of contraction and spreading periods of the vegetation cover.     

The consequences of spatial structure for the design and analysis of ecological field surveys

In ecological field surveys, observations are gathered at different spatial locations. The purpose may be to relate biological response variables (e.g., species abundances) to explanatory environmental variables (e.g., soil characteristics). In the absence of prior knowledge, ecologists have been taught to rely on systematic or random sampling designs. If there is prior knowledge about the spatial patterning of the explanatory variables, obtained from either previous surveys or a pilot study, can we use this information to optimize the sampling design in order to maximize our ability to detect the relationships between the response and explanatory variables?
The specific questions addressed in this paper are: a) What is the effect (type I error) of spatial autocorrelation on the statistical tests commonly used by ecologists to analyse field survey data? b) Can we eliminate, or at least minimize, the effect of spatial autocorrelation by the design of the survey? Are there designs that provide greater power for surveys, at least under certain circumstances? c) Can we eliminate or control for the effect of spatial autocorrelation during the analysis? To answer the last question, we compared regular regression analysis to a modified t‐test developed by Dutilleul for correlation coefficients in the presence of spatial autocorrelation.
Replicated surfaces (typically, 1000 of them) were simulated using different spatial parameters, and these surfaces were subjected to different sampling designs and methods of statistical analysis. The simulated surfaces may represent, for example, vegetation response to underlying environmental variation. This allowed us 1) to measure the frequency of type I error (the failure to reject the null hypothesis when in fact there is no effect of the environment on the response variable) and 2) to estimate the power of the different combinations of sampling designs and methods of statistical analysis (power is measured by the rate of rejection of the null hypothesis when an effect of the environment on the response variable has been created).
Our results indicate that: 1) Spatial autocorrelation in both the response and environmental variables affects the classical tests of significance of correlation or regression coefficients. Spatial autocorrelation in only one of the two variables does not affect the test of significance. 2) A broad‐scale spatial structure present in data has the same effect on the tests as spatial autocorrelation. When such a structure is present in one of the variables and autocorrelation is found in the other, or in both, the tests of significance have inflated rates of type I error. 3) Dutilleul's modified t‐test for the correlation coefficient, corrected for spatial autocorrelation, effectively corrects for spatial autocorrelation in the data. It also effectively corrects for the presence of deterministic structures, with or without spatial autocorrelation.
The presence of a broad‐scale deterministic structure may, in some cases, reduce the power of the modified t‐test.     

Classification and ordination of plant communities in Abune Yosef mountain range,Ethiopia

Understanding how topography-soil-disturbance drives spatial distribution of vegetation is the interest of ecologists. This study was, conducted to investigate the topography-soil-disturbance and vegetation relationships in Abune Yosef mountain range, Ethiopia. A total of 85 nested sample plots measuring 400 m², 25 m² and 1 m² were established for trees, shrubs and herbs respectively. Topographic, soil and disturbance variables were also assessed from each plot. Plant community classification was described and identified by agglomerative hierarchical clustering using Ward's minimum variance clustering methods. Shannon diversity index was employed to determine community diversity. After detecting the length of the first Detrended Correspondence Analysis (DCA) axis, Canonical Correspondence Analysis (CCA) forward and backward stepwise selection of environmental variables was performed based on their p-value by running permutation tests. The first axis explained 43.63% of the overall inertia and is correlated with Elevation, pH, slope aspect, total Nitrogen, soil organic Carbon & Clay. On the other hand, the second axis explained 32.06% of the total inertia and is correlated with bulk density, slope, logging, & available Phosphorus. The present study revealed that topographic variables have a profound influence on vegetation spatial distribution than soil and disturbance factors.     

甘肃敦煌西湖荒漠-湿地生态系统土壤水分空间异质性及其影响因子研究

土壤水分是内陆荒漠区湿地生态系统中重要的限制因子,为了揭示该区域土壤水分空间分布特征,采用传统统计学和地统计学相结合的方法,对甘肃敦煌西湖国家级自然保护区0~200cm内各层土壤水分的空间变异性及海拔、土壤质地和植被对其的影响进行了研究,旨在为极干旱区湿地生态系统植被修复和保育提供科学依据。结果表明:(1)本研究所得各变量的变异系数、块金方差、基台值、变程和结构比分别为36.51%~88.65%、0.007~0.098、0.112~0.549、116~453和76.6%~97.6%,各变量均为中等变异,存在高度异质性,具有较强空间自相关。(2)深层(60~200cm)土壤水分含量较浅层(0~60cm)变异大,且不同层次土壤水分含量的空间异质性差别也较大,空间变异主要发生在较小尺度上(分维数D在1.902~1.989之间)。(3)海拔是影响该区域深层土壤水分空间变异的主导因子。(4)土壤质地与浅层(0~60cm)土壤水分含量的相关性大于与深层(60~200cm)土壤含水量的相关性,它们与海拔相关性表现相反;草本植被盖度与浅层土壤水分含量呈较高的正关联关系,灌木根量与深层土壤水分含量呈较高的负关联关系。     

Impact of rapid urbanization on the floral diversity and agriculture land of district Dir,Pakistan

In the World urbanization is a serious problem especially in developing countries which creates serious environmental problems like climatic and ecological changes in the ecosystem. The present paper aims to explain urbanization that causes loss of agriculture lands, biodiversity, soil erosions and grazing in District Dir. Urbanization decreased species richness such as Salix alba and Populus alba in the last few years in the local area. Soil of local area was divided into three different zones and was tested for soil texture and mineral percentage. Zone I soil showed sandy loamy texture with a pH of 8.3, Nitrogen 0.012%, Phosphorus 5.0% and organic matter was 0.74 (ppm). Zone II soil was loamy sand in texture with pH 8.1, Nitrogen 0.011%, Phosphorus 6.2%, and organic matter was 0.24 (ppm) while Zone III soil texture was silty clay loam with a pH of 8.1, Nitrogen 0.032%, Phosphorus 11.3%, and organic matter was 0.60 (ppm). The current work concludes that urbanizations affect natural biodiversity and agriculture lands, and that soil erosion and watering-points trampled by livestock is one of the significant problems in district Dir, and that the main degrading factor is the overexploitation of vegetation for fuel-wood and livestock grazing.     

Effect of Soil Texture on the Distribution and Infectivity of Neoaplectana carpocapsae (Nematoda: Steinernematidae)

The vertical migration of N. carpocapsae infective juveniles applied to the soil surface or introduced 14 cm below the soil surface was studied in four different soil types (pure silica sand, coarse sandy loam, silty clay loam, and clay). The percentage of juveniles able to migrate and infect wax moth pupae placed in the soil decreased as the percentage of clay and silt increased. Most nematodes placed on the soil surface remained within 2 cm of the surface, but some penetrated to a depth of 10 cm in pure silica sand and coarse sandy loam to infect pupae. Some pupae at the same depth were also infected with nematodes in silty clay loam soil. In pure silica sand and coarse sandy loam, nematodes introduced 14 cm below the soil surface were able to infect wax moth pupae located between 4 and 24 cm. Movement was least in clay soil and limited in silty clay loam. Nematodes showed a tendency to disperse upwards from the point of application. In all cases the number of migrating nematodes was greatest when wax moth pupae were present.     

Effect of long-term application of animal manure on physical properties of three soils

Surface soil samples to 15 cm depth were taken from replicated plots in an ongoing long-term field experiment involving application of animal manure on three soils in Virginia. The sampled plots had received either no manure or the equivalent of 289,000 kg ha^–1 of manure as dry weight. The manure was applied annually at the beginning of each spring for 15 years from 1978 through 1992. The plots were cropped similarly since 1978. Soil textures were a fine sandy loam at Holland in the Atlantic Coastal Plain region, a silt loam at Blacksburg in the Appalachian region, and a clay loam at Orange in the Piedmont region of Virginia. The following measurements were made on subsamples: liquid and plastic limits, wet aggregate stability, aggregate size distribution, dispersible clay percentage, water retention at 0. 03, 0.1, 0.3, 0.5, 1.0, and 1.5 MPa tension, and modulus of rupture of moulded briquettes at a water content corresponding to 0.1 MPa tension. Organic matter content by the Walkley-Black method was significantly higher in the manure-treated soils at all three locations. Increases were 3% for the sandy loam and 25% for the silt loam and clay loam. From these values it was estimated that at least 95% of the total applied manure had been degraded over the 15 years. Results showed that the liquid and plastic limits for all three soils were higher (p<0.05) for the manure-treated samples. However, the differences in the limits were only 2 to 3%. The modulus of rupture values were lowered by addition of the animal manure. Decreases (p<0.05) occurred for the silt loam and clay loam samples. The wet aggregate stability increased and the dispersible clay decreased in the manure-treated soils. Increases (p<0.05) in wet aggregate stability occurred for the sandy loam and silt loam samples. Decreases (p<0.05) in dispersible clay were measured for the sandy loam and clay loam samples. Water retention was consistently, but only slightly, increased by manure addition. The increases, in the order of sample texture, were clay loam > sandy loam silt loam. Increases tended to be higher at the lower values of tension. Manure addition consistently increased the weight percentages of aggregates passing a given mesh size. Increases, in order of sample texture, were silt loam > clay loam > sandy loam. In their entirety, these results show that the manure produced measurable changes in the soil physical properties. The magnitude of the changes, in most cases, were small and depended on the soil texture. Given the high total amount of manure applied, the results indicate that manure-induced physical changes in the soil were small and evidently did not accumulate over time. Rapid microbial degradation of the manure could be responsible for the lack of marked changes in the soil physical properties.