Different Degrees Of Erosion Research Articles

Physical property responses of soils subjected to different degrees of erosion and seasonal freeze-thaw cycles in Northeast China

Changes in soil pores and aggregate stability due to freeze-thaw cycles (FTCs) are important causes of increased soil erosion during snowmelt. Soil erosion causes spatial redistribution of soils, enhancing soil heterogeneity and potentially impacting soil responses to FTCs. Nonetheless, there is minimal knowledge of the responses of soils subjected to different degrees of erosion to seasonal FTCs. To reveal the impact of seasonal FTCs, the dynamic variations of pore characteristics and aggregates of soils with four different degrees of erosion (original, degraded, deposited and parent soil) were measured, and the connections between influencing factors and soil properties were analyzed. The results showed that FTCs altered the structure of the soils and weakened their resistance to erosion and that soils with different degrees of erosion responded differently to FTCs. After seasonal FTCs, soil porosity increased (0.4 %-11.9 %) to some extent in all soils, with greater changes observed in the more eroded soils. Notably, capillary porosity exhibited a complex changing trend compared to total porosity. Degraded and parent soils showed a stable bulk density, while the original soil showed a decrease (2.1 %) in bulk density and the deposited soil showed an increase (18.4 %) in bulk density. With the increase of FTCs, the field capacity of original, degraded, and deposited soils exhibited a gradual decrease (15.1 %-18.5 %), while that of the parent soil slightly increased (0.9 %). After seasonal FTCs, the saturated hydraulic conductivity decreased for original and deposited soils (19.5 %-41.5 %), while it increased for degraded and parent soils (29.2 %-41.6 %). Throughout the FTCs, the proportion of the large aggregates decreased and the small aggregates increased, and the transformation was greater on the less eroded soils. The mean weight diameter and geometric mean diameter of the soils gradually decreased with increasing FTCs, while the change was smaller for the more eroded soils. After seasonal FTCs, the less eroded soils were at greater risk of erosion. Our results demonstrated that the number of FTCs had a more significant impact on soil physical properties compared to the temperature difference and soil water content. Overall, freeze-thaw action reinforced the spatial heterogeneity of soil properties, potentially intensifying soil erosion. These findings help reveal the effects of FTCs on the physical properties of soils with different degrees of erosion and deepen the understanding of the mechanism of FTCs on soil erosion processes.

Water-Extractable Organic Matter of the Soils with Different Degrees of Erosion and Sedimentation in a Small Catchment in the Central Forest-Steppe of the Central Russian Upland: Soil Sediments on the Dry Valley Bottom

Water-Extractable Organic Matter of the Soils with Different Degrees of Erosion and Sedimentation in a Small Catchment in the Central Forest-Steppe of the Central Russian Upland: Soil Sediments on the Dry Valley Bottom

Effect of erosive conditions on different sealant materials used in paediatric dentistry.

To evaluate the effect of acidic challenge on erosion depth and topographic characteristics of different materials used as occlusal sealants. Two hundred specimens of five sealant materials (Fuji IX, Ketac Molar, Fuji II, Equia and Clinpro) and forty bovine teeth enamel samples (control) were prepared and exposed to acidic challenge. The specimens were immersed in four different solutions (orange juice, coke drink, citric acid or distilled water) under mildly shaken conditions for 3 days. The erosion depth profiles were measured using a profilometer and Scanning Electron Microscope (SEM). Two-way ANOVA with Tukey post-hoc test was performed to evaluate the interactions. Sealant material and acidic challenge had significant effects on erosion depth. Among the materials, Fuji II presented the highest mean of erosion depth after immersion in orange juice, coke drink, and citric acid. All materials groups presented higher erosion depth values after immersion in the citric acid solution, except Clinpro. Bovine enamel presented higher erosion depth values compared to all materials when submitted to erosive challenge. Sealant materials submitted to the acidic challenge presented different degrees of erosion and topographic modification; however, they are less susceptible to erosion than bovine teeth enamel.

Evaluation of the spatial characteristics of slope erosion using field electrical resistivity measurements

Slope erosion due to rainfall, being a complex and serious problem worldwide, is related to natural hazards and the environment. However, scientific evaluations of slope stability are lacking, and research on the spatial characteristics of slope erosion is scarce. As such, in this study, we have constructed a method of evaluating the spatial characteristics of slope erosion that we combined with electrical measurements; subsequently, we have conducted an in situ model test. We observe a step-type growth trend in the change in resistivity, which was divided into different degrees of erosion using the K-means clustering method. Moreover, we have selected excavated slopes with supporting structures and landfill slopes without supporting structures to evaluate the spatial characteristics of engineered slopes after several months; finally, we have applied the proposed evaluation method. We use seasonal and trend decomposition using loess to remove the noise and randomness in the resistivity data, and we determine the change in resistivity based on the threshold calculated through probability density statistics. We categorize the erosion of slopes into different states and identify a spatial step-type growth trend in slope erosion, which was in good agreement with the spatial distribution characteristics of cracks on the slope. We observe serious erosion at the bottom and shallow parts of both slopes, which indicated a high risk of local collapse according to the spatial characteristics of slope erosion and our slope stability analysis.

Impacts of Soil Erosion on Soil Quality and Agricultural Sustainability in the North-Western Himalayan Region of India

Erosion by water reduces soil quality and hence crop yield. Understanding the response of crop yields to soil erosion is vital in assessing agriculture’s vulnerability to erosion. However, these effects are difficult to quantify. The study presents a quantitative relationship between soil erosion and soil quality and productivity of rainfed wheat (Triticum aestivum) by comparing field plots with different degrees of erosion in some sub-tropical alfisols in the Doon Valley region of India. By comparing the topsoil depth with the reference site as the control, erosion severity was classified into different phases such as slight, moderate, severe, and very severe. A quantitative, weighted additive model was used to evaluate soil quality for different phases of erosion using soil clay content, water holding capacity, soil aggregate, soil organic carbon, pH, CEC, total N, available P, and available K. The synthesis of long-term experimental data revealed that the mean soil erosion rate varied from 5.5 Mg ha−1 yr−1 in slightly eroded plots to 33.4 Mg ha−1 yr−1 in very severely eroded plots. Compared with the reference forest, the soil organic carbon (SOC) declined by 81.4% and water holding capacity by 31% in severely eroded soils. A substantial loss of total N, extractable P, and available K was also observed. Water stable aggregates (WSA) decreased from 86% to 12.6%, and the cation exchange capacity (CEC) from 25 to 12.6 c mol(+) kg−1. The soil quality index was 0.7 in slightly eroded compared with 0.4 in severely eroded soil. Similarly, the sustainable yield index for wheat was 0.9 and 0.6 for slightly and severely eroded soils, respectively. Thus, there is a strong need to identify land management systems that reduce erosion risks, restore eroded soils, and enhance soil quality.

Characteristics of humus content on different soil types of Zdolbuniv district of Rivne Region

The dynamics of changes in the humus state of the soils of Zdolbuniv district during the V–XI rounds of agrochemical survey were analyzed. The state of biologization of agriculture, in particular the plowing of siderates, the introduction of straw and manure and the use of peat over the last 8 years in the studied area, is highlighted. Mandatory biologization of agriculture, taking into account the biological features of each crop, in particular, plant needs for nutrients, indicators of potential and effective soil fertility, its physical and chemical properties, assortment and chemistry of fertilizers, climatic conditions in a zonal section — comprehensive implementation of these measures will contribute to a significant increase soil fertility of the district’s agricultural lands, in particular improving its humus condition. Since an important reason for the lack of accumulation of humus reserves in the studied soils is the unsatisfactory state of biologicalization of agriculture in the area. Conclusions and suggestions on improving the humus condition of the soils of the studied area have been prepared. It is established that according to the results of agrochemical studies of agricultural lands of Zdolbuniv district of Rivne region for the XI round of agrochemical certification of agricultural lands, the weighted average humus content is 2.6% and corresponds to the average content. It was found that the studied soils of the district are characterized by very low and low (less than 2.1%) — 0.86 thousand hectares (10.0%), medium (from 2.1 to 3.0%) — 6.14 thousand hectare (71.7%), elevated (from 3.1 to 4.0%) — 1.5 thousand hectare (17.5%), high and very high (> 4%) — 0.06 thousand hectare (0.7%) humus content. In terms of the main types of soils, the highest humus content in meadow soils, meadow and chernozem soils, chernozem and meadow soils with content of 3.3% and on chernozems typical and chernozems strongly degraded of different degrees of erosion with an average humus content of 2.8%. It should be noted that the dynamics of humus content in the soils of Zdolbuniv district of Rivne region for the last 10 years of research shows the process of its stabilization and increase from 2.2% in 2007 to 2.6% in 2017. Comparing only the area of land plots, which were surveyed in three rounds — 2007, 2012 and 2017, the humus content is 2.5; 2.5 and 2.6%, respectively, which confirms the stabilization of the humus content over the past ten years of agrochemical research. An important reason for the lack of accumulation of humus reserves in the studied soils is the unsatisfactory state of biologization of agriculture in the area.

Temporal and Spatial Distribution of Surface Sediment and Its Implications in the Yangtze Subaqueous Delta

Characteristics of sediments vary significantly temporally and spatially in the Yangtze subaqueous delta and adjacent East China Sea shelf due to complicated dynamic conditions. Magnetic properties of sediments are sensitive to hydrodynamics and provenance, and therefore can indicate sediment transportation and deposition. High-resolution surface sediments collected in different seasons and years from the Yangtze subaqueous delta and neighboring East China Sea shelf were subjected to environmental magnetic analyses. In combination with granulometric analysis, this paper discusses temporal and spatial variations of magnetic properties and their implications for indicating sediment transportation, sediment source identification, and hydrodynamic response. The results show that magnetic parameters SIRM and XARM/SIRM suggest sediment transportation path from the Yangtze River into the East China Sea, with majority migrating to the south and southeast and then deposited to the west of 123°E, while a little fine sediment delivered to the northeast in summer. Cluster analysis demonstrates three sedimentation districts according to magnetic properties and particle size in the study area, which represent modern Yangtze sediment, late Pleistocene relict sand, and their mixture, respectively. Temporal-spatial changes in particle size of sediments suggests erosion outside the North Branch in the past few years, while opposite trend of changes on particle size of surface sediment in the adjacent area might reflect different degrees of erosion.

Evaluation of the quality of cultivated-layer soil based on different degrees of erosion in sloping farmland with purple soil in China

The assessment of cultivated-layer soil quality (CLSQ) is important for improving crop growth capacity and is the premise for taking effective remedial measures. The cultivated-layer soil quality index (CLSQI) is a significant tool for evaluating CLSQ. This study aims to select the most effective indicators to construct a minimum data set (MDS) for soil in erosion-impacted hilly areas with purple soil in China and to analyse the association between CLSQ and the erosional environment. Taking the erosion of purple soil of sloping farmland in China as the focus of this study, different combinations of scoring methods, indicators, and data set methods were compared. In this study, the MDS is constructed by principal component analysis (PCA) in conjunction with the norm value, and the accuracy of the MDS is verified by the Nash effective coefficient. Furthermore, the nonlinear scoring (NLS) method is used to normalize the indicator data and then assess the CLSQ of soils with different degrees of erosion using the CLSQI. The results showed the following. 1) The soil indicators significantly varied under the different erosion degrees: the soil productivity showed a decreasing trend with increasing erosion, while the soil anti-erosion ability and tillage performance showed increasing trends with increasing erosion. 2) For the study area, the generated MDS included saturated hydraulic conductivity, bulk density, shear strength, penetration resistance and available phosphorus. 3) The CLSQI-NLS evaluation method was most suitable for this research topic, and combined with the MDS indicators, it could be used to accurately evaluate the CLSQ. 4) As the erosion degree intensified, the CLSQI showed an obvious decreasing trend, and the CLSQI under soil erosion of 20 cm (S-20, 0.393) was significantly lower than that under other erosion degrees, indicating that soil erosion gradually reduced the CLSQ. After the cultivated-layer soil was eroded and the subsoil was converted to cultivated-layer soil, the soil quality obviously decreased. The indexing approach used in this study provides a practical and time- and cost-effective method for the quantitative evaluation of CLSQ for erosion-prone soil in hilly areas with purple soil worldwide.

Water Use Efficiency of Soybean under Water Stress in Different Eroded Soils

Soil erosion could change the effective storage of soil moisture and affected crop water use efficiency (WUE). To quantitative study differences in the WUE of soybean and the crop’s response to water stress for soils with different degrees of erosion in northeastern China, three erosion degrees—(1) lightly, (2) moderately and (3) severely—eroded black undisturbed soils and four years (from 2013 to 2016) of soybean pot experiments were used to control soil water content (100%, 80%, 60%, and 40% field capacity (FC)) and observe the crop growth processes. To study the relationships between erosion–water use–productivity, the following results were achieved: (1) the optimal water content was 80% FC for lightly eroded soil (L) and 100% FC for both moderately (M) and severely (S) eroded soil. Yield (Y) was best in M with the value of 3.12 t ha−1, which was 4.6% and 85.5% higher than L and S, respectively. Under the conditions of adequate water supply, there was no significant change in Land M, but the values were significantly different for the S ( p < 0.05). (2) Y and biomass (B) were sensitive to water stress except in the branching stage. (3) The values of WUEY and WUEB for the three eroded soils were the best at 80% FC. The stress coefficient (SF) values of the three eroded soils were not significantly different. In the flowering and pod formation stage, the SF reached the maximum under waterlogging stress. While the water shortage stress reached the maximum in the seed filling stage, the soil water content decreased by 10%, and the WUEB decreased by 15%, which was 2.5 times more powerful than the waterlogging stress. This study indicated the change in soybean growth with respect to the water response caused by soil erosion, and provided a scientific basis and data for the reasonable utilization of black soil with different erosion intensities. The results also provided important parameters for the growth of simulated crops.

Spatial analysis of Samara region land degradation and differentiation of antierosion territory organization

In the research, a spatial analysis of the degradation of Samara region agricultural land with the assessment of economic losses due to water erosion is carried out. A map chart of the distribution of districts with different degrees of erosion has also been developed. The values of the degradation factor coefficient and economic losses due to the influence of erosion processes are calculated. The key points of antierosion territory organization and land regulation depending on landscape nature and kind of damage are represented. The plan of action for the antierosion territory organization of a farm in Kinelsky district is proposed and its effectiveness to stop and prevent erosion processes, as well as for rational use of land and increase soil fertility is shown.

Effect of erosion and methods for its control on the surface roughness of composite resin

Objective: This study evaluated the surface roughness of a nanofilled composite resin submitted to different degrees of erosion and methods of control.Methods: 120 cylindrical composite specimens (6×1.5 mm) were randomly divided into four groups, according to the surface protection against the erosive challenge [negative control, topical application of fluoride, glass-ionomer sealant, resin sealant]. After the application of the respective method, specimens were divided into three subgroups (n=10): a) absence of erosive challenge; b) 9 cycles of DES-RE; c) 18 cycles of DES-RE. Surface roughness (Ra, μm) was recorded and statistically analyzed (2-way Anova/Tukey).Results: The glass-ionomer sealant exhibited less rough surfaces under all the conditions. However, the average roughness was significantly higher after 18 DES-RE cycles.Conclusion: Despite the potential shown by glass-ionomer sealant, it was concluded that no material prevented an increase in surface roughness of the nanofilled composite after an intense erosive challenge.

Temporal changes of eroded soils depending on their agricultural use in the southern Cis-Ural region

Temporal changes of eroded soils in the southern Cis-Ural region (Republic of Bashkortostan) depending on their agricultural use during the period from 1975 to 2011 were studied. In the northern foreststeppe zone, the development of erosion processes was retarded upon the use of soil-saving management practices and grain-fallow-grass crop rotations. In slightly eroded light gray forest soils (Eutric Retisols (Cutanic)), the thickness of humus-accumulative horizons and the content of humus increased; the conversion of cropland into permanent fallow was found to be the most efficient measure to control soil erosion. In podzolized chernozems (Luvic Greyzemic Chernic Phaeozems) and typical chernozems (Haplic Chernozems) of the Cis-Ural steppe, the content of humus in the plow layer under grain-row crop rotation and classical soil management decreased, especially in moderately eroded soils. The development of water and wind erosion on slopes depended on the slope shape: the texture of soils at different degrees of erosion on slopes with free runoff became coarser by one gradation after 35 years; in the presence of linear obstacles in the lower part of slopes, the content of fine fractions in moderately and strongly eroded soils increased.

Spectral response of different eroded soils in subtropical china: A case study in Changting County, China

Hyper-spectral data is widely used to determine soil properties. However, few studies have explored the soil spectral characteristics as response to soil erosion. This study analysed the spectral response of different eroded soils in subtropical China, and then identify the spectral characteristics and soil properties that better discriminate soils with different erosion degrees. Two methods were compared: direct identification by inherent spectral characteristics and indirect identification by predictions of critical soil properties. Results showed that the spectral curves for different degrees of erosion were similar in morphology, while overall reflectance and characteristics of specific absorption peaks were different. When the first method is applied, some differences among different eroded groups were found by integration of associated indicators. However, the index of such indicators showed apparent mixing and crossover among different groups, which reduced the accuracy of identification. For the second method, the correlation between critical soil properties, such as soil organic matter (SOM), iron and aluminium oxides and reflectance spectra, was analysed. The correlation coefficients for the moderate eroded group were primarily between −0.3 to −0.5, which were worse than the other two groups. However, the maximum value of R2 was obtained as 0.86 and 0.94 for the non-apparent eroded and the severe group. Furthermore, these two groups also showed some differences in the spectral response of iron complex state (Fep), Aluminium amorphous state (Alo) and the modelling results for soil organic matter (SOM). The study proved that it is feasible to identify different degrees of soil erosion by hyper-spectral data, and that indirect identification by modelling critical soil properties and reflectance spectra is much better than direct identification. These results indicate that hyper-spectral data may represent a promising tool in monitoring and modelling soil erosion.

The volcanoes of an oceanic arc from origin to destruction: A case from the northern Luzon Arc

Volcanoes were created, grew, uplifted, became dormant or extinct, and were accreted as part of continents during continuous arc–continent collision. Volcanic rocks in Eastern Taiwan’s Coastal Range (CR) are part of the northern Luzon Arc, an oceanic island arc produced by the subduction of the South China Sea Plate beneath the Philippine Sea Plate. Igneous rocks are characterized by intrusive bodies, lava and pyroclastic flows, and volcaniclastic rocks with minor tephra deposits. Based on volcanic facies associations, Sr–Nd isotopic geochemistry, and the geography of the region, four volcanoes were identified in the CR: Yuemei, Chimei, Chengkuangao, and Tuluanshan. Near-vent facies associations show different degrees of erosion in the volcanic edifices for Chimei, Chengkuangao, and Tuluanshan. Yuemei lacks near-vent rocks, implying that Yuemei’s main volcanic body may have been subducted at the Ryukyu Trench with the northward motion of the Philippine Sea Plate. These data suggest a hypothesis for the evolution of volcanism and geomorphology during arc growth and ensuing arc–continent collision in the northern Luzon Arc, which suggests that these volcanoes were formed from the seafloor, emerging as islands during arc volcanism. They then became dormant or extinct during collision, and finally, were uplifted and accreted by additional collision. The oldest volcano, Yuemei, may have already been subducted into the Ryukyu Trench.

FUERO: foundations of a fuzzy exploratory model for soil erosion hazard prediction

This paper describes the foundations of FUERO, a FUzzy exploratory model for soil EROsion hazard prediction, which explores cause–effect relationships on the basis of general knowledge about causes and specific relations between processes and indicators of soil erosion. The model was designed to investigate the susceptibility of specific areas of a landscape to erosion by incorporating expert knowledge, in the sense that information on soil properties and/or landscape elements assumed to control accelerated soil erosion could be incorporated into the modelling process. Fuzzification of the landscape elements used in modelling the likelihood of an area to be affected by different degrees of erosion was done using a Fuzzy Semantic Import Modelling approach. Fuzzy min–max operators were used within a GI System for determining the likelihood an area to low, moderate or high erosion hazard. Although the model provides qualitative estimations, it showed very useful to explore indicators–causes–processes relationships. In addition, it allowed testing the importance of individual landscape elements related to soil erosion and selection of those that best predict soil erosion over a particular area.

Catclaw ( Mimosa buincifiera ): a pest or a means to restore soil fertility in heavily eroded soil from the central highlands of Mexico?

In the central highlands of Mexico, heavily eroded soils are often colonized by catclaw (Mimosa buincifiera): an N2-fixing shrub. An experiment was carried out to investigate how this shrub affected characteristics of the soil and its biological functioning. Soil was sampled from outside and under the canopy of catclaw at three sites characterized by different degrees of erosion and an increase in plant density. The soil microbial biomass C, total amounts of bacteria, fungi, actinomycetes and free-living N2-fixing micro-organisms were measured, while production of CO2 and dynamics of nitrate (NO3 –), nitrite (NO2 –) and ammonium (NH4 +) were monitored in an aerobic incubation at 22±1 °C for 35 days. The C content was 1.6 times greater in the area with the largest density of plants and the least erosion (RECUP) compared with the site with the lowest density and greatest erosion (DEGR), while it was 1.2 times greater under the canopy of the catclaw than outside it (average of the three sites). The incorporation of N into the soil organic matter was greater under the canopy of the catclaw than outside it as the C:N ratio was on average 8.4 and 9. 1, respectively. The microbial biomass C, as a percentage of soil organic matter, was 1.5 times greater in the RECUP than in the DEGR site. Greatest total number of colony-forming bacteria and fungi (mean of organisms found under and outside the canopy) were found in the RECUP treatment and lowest in the DEGR treatment. Free-living N2-fixing organisms and actinomycetes showed opposite trends. Greater total numbers of colony-forming bacteria, fungi, actinomycetes and free-living N2-fixing organisms (mean of the three treatments) were found under the canopy of catclaw than outside of it, Production of CO2 was 1.8 times greater in the RECUP than in the DEGR and 1.6 times greater under the canopy of catclaw than outside. Production of NO3 – was 1.3 times greater in the RECUP than in the DEGR and 3.5 times greater under the canopy of catclaw than outside. There was no significant effect of location or canopy on NO2 – and NH4 + concentrations. It is concluded that the natural vegetation of catclaw increased microbial biomass and soil organic matter content under, but also outside its canopy, and preserved N better, releasing greater amounts of inorganic N upon mineralization. Catclaw can serve as a first colonizer of heavily eroded soil and be replaced by other vegetation, natural or crops, when fertility is restored.

Soil quality effects of accelerated erosion and management systems in three eco-regions of Tanzania

Soil erosion can adversely influence soil quality, especially in tropical soils. Thus, a multi-location field experiment was conducted on eight major agricultural soils with different degrees of erosion, in three eco-regions in Tanzania. The objective was to assess the impact of topsoil depth (TSD) and management on soil properties. Three eco-regions comprising of humid at Kilimanjaro, sub-humid at Tanga and sub-humid/semi-arid at Morogoro were selected. There were a total of eight locations within three eco-regions comprising two at Kilimanjaro (e.g., Kirima Boro and Xeno Helena), two at Tanga (Mlingano 1 and Mlingano 2) and four at Morogoro (Misufini 1, Misufini 2, Misufini 3, and Mindu). The soil management treatments consisted of farmyard manure (FYM), N and P fertilizer, tie-ridging and farmers’ practice. Plant nutrient content was generally lowest on severely eroded and the highest on least eroded soil classes. Soil pH decreased with increasing severity of erosion on soils with higher content of Ca +2 in the sub-surface. In general, there occurred a decline in soil organic carbon (SOC) and P with the decrease in TSD. The SOC content decreased on severely eroded soil class by 0.16%, 0.39% and 0.13% at Misufini 1, Mlingano 1 and Kirima Boro, respectively, compared to slightly or least eroded soil class. Corresponding decline in available P at these sites was 41%, 62% and 61%, respectively. Application of FYM significantly increased soil pH at some sites. Soil content of SOC, N, P, K and Mg were significantly increased by FYM application. Significant effects of N and P fertilizers on SOC and P were observed at most sites. In comparison with farmer’s practice, FYM application increased SOC by 0.55%, N by 0.03%, P by six-fold and K by two-fold. Nitrogen and phosphorus fertilizers had comparable effects for SOC and P only at some sites. The results indicate that FYM is a better soil input than N and P fertilizers in improving soil quality. The data show that SOC, N and P are most adversely affected with accelerated erosion and that FYM fertilizer applications have the potential to improve fertility of eroded soils.

The Dynamics of the Plant Lactuca sibirica and the Frequency of Its Rust Puccinia minussensis in River Valleys in Northern Sweden

The population dynamics of the rust Puccinia minussensis and its host plant Lactuca sibirica were studied in four localities. Three of the localities were subjected to different degrees of erosion and/or deposition (Kukkola, Aspliden and Hummelholm) during flooding, while one locality (La˙nged) was situated above the flood level. At Kukkola and Hummelholm both the densities of the host plant L. sibirica and the frequency of the rust P. minussensis varied markedly within transects. At the other two localities, La˙nged which is not disturbed and Aspliden which is dry and subjected to erosion, densities and disease frequencies remained constantly low between the years. There was a positive correlation between host densities and frequency of aecia-producing shoots at Hummelholm. However, it is suggested that this correlation is caused by similar responses to variation in environmental conditions and not by a response of the pathogen to host plant densities. Furthermore we found that systemically diseased shoots (aecia) at all localities died before August, while uredial and telial stages did not cause death of shoots. It is suggested that (1) the host densities are primarily regulated by stochastic disturbances and (2) the direct and close fit in the response of the fungus is due to resource limitation

Redistribution of Bromide by Rainfall Infiltration into a Cecil Sandy Loam Landscape

AbstractThe redistribution of bromide applied to the soil surface at selected positions in a sample Cecil sandy loam (clayey, kaolinitic, thermic Typic Hapludults) landscape was measured in relation to rainfall and pedon characteristics. Three bromide application sites were located in drainageway positions and three on slope positions exhibiting different degrees of erosion. Observations of nearly 4 yr measured 4.7 m of rainfall to show that rainfall was more effective in transporting Br through drainageway pedons than those on the slopes. After 1000 mm of rainfall, half of the applied Br was above 0.6 m on the slope sites and above 1.0 m on the drainageway sites. This difference seems to relate closely to the higher hydraulic conductivity in the Bt horizon above 1.2 m in the drainageway pedons than in the slope pedons. Also, the maximum clay zones in the Bt horizons of the drainageway sites were significantly lower than those on the slope sites and occurred about 0.5 m deeper. At the completion of this study, an elevated Br level was measured in a drainageway site between 1.5 and 2.4 m with no evidence of Br below 3.8 m. These data provide a basis for assigning solute transport characteristics to the identified variability in this and similar landscapes.

Erosion structures on cathodes arced in vacuum

An analysis is presented of the structural changes which occur on cathode surfaces arced in vacuum. Cadmium, copper, molybdenum and tungsten cathodes carrying currents up to a few hundred amps showed a variety of erosion patterns which have been interpreted as representing a number of stages with different degrees of erosion. Cathode spots moving either randomly or under the influence of an external magnetic field produce basically the same erosion pattern. This pattern consists of craters varying in size from less than one micrometre up to several micrometres. A three-dimensional analysis of a crater-structure is given and quantitative information is obtained on crater size distributions and current densities for different metals. The influence of surface contamination on cathode tracking is pronounced.