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1.
We compared seed banks between two contrasting anthropogenic surface disturbances (compacted, trenched) and adjacent undisturbed controls to determine whether site condition influences viable seed densities of perennial and annual Mojave Desert species. Viable seeds of perennials were rare in undisturbed areas (3–4 seeds/m2) and declined to <1 seed/m2 within disturbed sites. Annual seed densities were an order of magnitude greater than those of perennials, were one-third the undisturbed seed densities on compacted sites, but doubled on trenched sites relative to controls. On trenched sites, greater litter cover comprising the infructescences of the dominant spring annuals, and low gravel content, enhanced seed densities of both annuals and perennials. Litter cover and surface ruggedness were the best explanations for viable perennial seed densities on compacted sites, but litter cover and the presence of a common harvester ant explained annual seed densities better than any other surface characteristics that were examined. Surface disturbances can have a varied impact on the condition of the soil surface in arid lands. Nevertheless, the consistently positive relationship between ground cover of litter and viable seed density emphasizes the importance of litter as an indicator of site degradation and recovery potential in arid lands.  相似文献   

2.
Patterns of rock fragment cover generated by tillage erosion   总被引：6，自引：0，他引：6  
Intensively cultivated areas in the upper part of the Guadalentin catchment (southeast Spain) show a systematic spatial pattern of surface rock fragment cover (Rc). The objective of this paper is to quantify and to explain this spatial rock fragment cover pattern. Therefore, a map of an intensively cultivated area of 5 km2 was digitised, and for each pixel total topographic curvature was calculated. Next, rock fragment cover was determined photographically at 35 sites with a range of total slope curvatures. A linear relation between total curvature and rock fragment cover was found, except for narrow concavities. It was hypothesised that this pattern can be explained by a significant net downslope movement of rock fragments and fine earth by tillage. The displacement distances of rock fragments by tillage with a duckfoot chisel were measured by monitoring the displacement of tracers (painted rock fragments and aluminium cubes) on 5 sites having different slopes. The rare of tillage erosion for one tillage pass with a duckfoot chisel, expressed by the diffusion constant (k), equals 282 kg/m for up and downslope tillage and only 139 kg/m for contour tillage. Nomograms indicate that mean denudation rates in almond groves due to tillage erosion (3 to 5 tillage passes per year) can easily amount to 1.5–2.6 mm/year for contour tillage and up to 3.6–5.9 mm/year for up- and downslope tillage for a field, 50 m long and having a slope of 20%. These figures are at least one order of magnitude larger than reported denudation rates caused by water erosion in similar environments. Hence tillage erosion contributes significantly to land degradation. The downslope soil flux induced by tillage not only causes considerable denudation on topographic convexities (hill tops and spurs) and upper field boundaries but also an important sediment accumulation in topographic concavities (hollows and valley bottoms) and at lower field boundaries. Kinetic sieving (i.e. the upward migration of rock fragments) by the tines of the duckfoot chisel also concentrates the largest rock fragments in the topsoil in such a way that a rock fragment mulch develops in narrow valleys and at the foot of the slopes. These results clearly indicate that tillage erosion is the main process responsible for the observed rock fragment cover pattern in the study area. Since the study area is representative for many parts of southern Spain where almond groves have expanded since 1970, the results have a wider application. They show to what extent intensive tillage of steep slopes has contributed to the increase in soil degradation, to changes in hillslope morphology (i.e. strong denudation of convexities, development of lynchets and rapid infilling of narrow valley bottoms) and to the development of rock fragment cover patterns which control the spatial variability of the hydrological and water erosion response within such landscapes.  相似文献   

3.
In the western United States, overgrazing, weed invasion and wildfire have resulted in the conversion of shrub-steppe to annual grasslands, with substantial effects on ecosystem function. In these landscapes, badgers disturb large areas of soil while foraging for fossorial animals. Mounds created by badgers contained the lowest concentrations of total carbon, nitrogen and sulphur, mineral nitrogen and mineralizable nitrogen, inter-mound soils had the highest concentrations, and excavation pits had intermediate levels. Soil C:N ratio and pH were greater, and electrical conductivity and soluble Ca2+, Mg2+ and K+ were lower on mound soils compared with either pit or inter-mound soils. Larger pits generally trapped more litter, and increased litter mass equated with greater concentrations of active carbon, but only at the burned sites. Older mounds supported more vascular plants and cryptogamic crusts. Our results demonstrate reduced levels of nutrients and a higher C:N ratio on the mounds compared with either the pits or inter-mounds. Alteration to the homogeneous post-fire landscape by badgers contributes to patchiness in soils and vegetation, which is critical to the functioning of arid systems. Given their effect on soil C:N ratios, mounds may be important sites for recovery of indigenous shrub-steppe plant species.  相似文献   

4.
It has been suggested that lower rates of granivory in South American warm deserts compared to their North American counterparts arose from a reduction of seed reserves in the former owing to the extinction of argyrolagid marsupials. We measured seed reserves in two habitats of the central Monte Desert in an attempt to detect such seed decline, but to no avail. After moderate rainfall, maximum seed standing crops reached 16,000 and 23,000 seeds m−2in shrublands and open forests, respectively. Under the canopy of trees and shrubs there were 19,000 and 37,000 seeds m−2; whereas in exposed areas there werec. 10,000 seeds m−2in both habitats. Seed banks in other South American semi-arid areas showed similar values. Total grass seeds as well as those presumably preferred by ants also seem to be similar in both continents. Hence, granivory in South America, as has been already reported for Australia, is lower than in North America in spite of the great similarity of seed bank sizes. Moreover, argyrolagids were unlikely seed-hoarding granivores, therefore some other reason than argyrolagid extinction should be sought to explain the lack of specialized seed-eating mammals, and the smaller overall seed consumption rates in South American deserts.  相似文献   

5.
The objectives of this study were to determine the composition, density and spatial distribution of the soil seed bank of woody species, as well as their regeneration pattern in two different land use systems, controlled (ranch) and open grazing, in an Acacia woodland of the Rift Valley in Ethiopia. We also compared the species composition of the soil seed bank and the above-ground vegetation to find out if differences exist in the soil seed bank and advance regeneration between the two land use systems. The germination requirements of seeds of the woody species were also investigated under laboratory conditions. Acacia senegal, Acacia seyal, Acacia tortilis, Dichrostacys cinerea and Balanites aegyptiaca were encountered in the above-ground vegetation in both systems. Seeds of only A. tortilis (90±32 seeds m−2) from the ranch, and A. senegal (5±3 seeds m−2) and A. tortilis (72±34 seeds m−2) from the open system were found in the soil seed bank along transects with patchy horizontal pattern. The two systems were not significantly different in density of soil seed banks of A. senegal andA. tortilis . Jaccard's similarity index showed that only a few woody species were common in the soil seed flora and above ground vegetation. However, all of the species accumulated seeds (58±43–331±130 seeds m−2) in/on the soil under the canopy. Very large numbers of seeds of A. tortilis (19382±9722 seeds m−2) and D. cinerea (1278±494 seeds m−2) were also found in barns. Most of the seeds recovered from the soil samples (60–80%) were found in the litter layer. Acid and mechanical scarification improved legume germination (36–99% and 60–99%, respectively) over boiling water (0–48%). Treatment means differed significantly for all the legumes (p<0·001) but not for Balanites. Height and diameter class distribution of regeneration of A. tortilis and D. cinerea in both systems and A. senegal in the open system had a negative exponential correlation (rs=−0·5, −0·25 and −0·86, respectively). A. seyal and B. aegyptiaca showed poor regeneration. Horizontal distribution of advance regeneration of all the species was patchy. Advance regeneration of A. seyal, A. tortilis and B. aegyptiaca were not significantly different, while that of A. senegal and D. cinerea were significantly different between the two systems. Poor representation of species in the soil seed bank along transects and in the different height and diameter classes may be attributed to the low density of mature trees as well as the mode and strategy of seed dispersal. Ungulate and wind dispersed species (e.g. A. senegal, A. tortilis and D. cinerea) were highly favoured. Patchiness in the distribution of seeds and advance regeneration was also a result of endozoochory. Dispersal of non-ungulate dispersed seeds (A. senegal, A. seyal and B. aegyptiaca) was restricted to the canopy zone. Piles of seeds ofA. tortilis and D. cinerea that were found in barns were a result of consumption of their pods by cattle. High concentration of seeds in the litter layer may be due to low soil disturbance and larger size of seeds. The height and diameter class distribution of A. senegal (in the open system), A. tortilis and D. cinerea also indicated that the species have good regeneration. Results from the germination tests indicated that seeds of the legumes require pre-sowing treatments to give a rapid, uniform and improved germination. Intervention through artificial regeneration should be employed to improve the density and regeneration capacity of those species with hampered regeneration at both systems.  相似文献   

6.
Tree uprooting plays an important role in hillslope evolution. The geomorphological impact of tree uprooting after a foehn wind occurrence, in December 2013 in the Tatra Mountains, was investigated. Geomorphological mapping was conducted in three watersheds. Additionally, in one of the watersheds, 459 windthrow pits were measured, in an area of 6.4 ha. The mean volume of a pit was 2.41 m3, and the mean surface area was 5.47 m2. 3.9% of the area was affected by windthrow pits, however locally the magnitude of changes was significantly higher, reaching up to 14.5% of the surface area. Slope inclination weakly influenced the effects of uprooting, and a decrease in the average depth of pits on steep slopes was observed. Individual windthrow pits (five cases) initiated the activity of geomorphological processes, and two cases of periodic springs were noted. Changes in the relief of small landforms caused by tree uprooting were documented. Windthrow creation facilitated the delivery of the soil material from the slopes into the channels.  相似文献   

7.
This study examines the development of a conceptual model of sediment processes in the upper Yuba River watershed; and we hypothesize how components of the conceptual model may be spatially distributed using a geographical information system (GIS). The conceptual model illustrates key processes controlling sediment dynamics in the upper Yuba River watershed and was tested and revised using field measurements, aerial photography, and low elevation videography. Field reconnaissance included mass wasting and channel storage inventories, assessment of annual channel change in upland tributaries, and evaluation of the relative importance of sediment sources and transport processes. Hillslope erosion rates throughout the study area are relatively low when compared to more rapidly eroding landscapes such as the Pacific Northwest and notable hillslope sediment sources include highly erodible andesitic mudflows, serpentinized ultramafics, and unvegetated hydraulic mine pits. Mass wasting dominates surface erosion on the hillslopes; however, erosion of stored channel sediment is the primary contributor to annual sediment yield. We used GIS to spatially distribute the components of the conceptual model and created hillslope erosion potential and channel storage models. The GIS models exemplify the conceptual model in that landscapes with low potential evapotranspiration, sparse vegetation, steep slopes, erodible geology and soils, and high road densities display the greatest hillslope erosion potential and channel storage increases with increasing stream order. In-channel storage in upland tributaries impacted by hydraulic mining is an exception. Reworking of stored hydraulic mining sediment in low-order tributaries continues to elevate upper Yuba River sediment yields. Finally, we propose that spatially distributing the components of a conceptual model in a GIS framework provides a guide for developing more detailed sediment budgets or numerical models making it an inexpensive way to develop a roadmap for understanding sediment dynamics at a watershed scale.  相似文献   

8.
Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m− 1 y− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m− 1 y− 1 on rangeland but only 3.9 kg m− 1 y− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.  相似文献   

9.
开封市黄河滩区土地资源规避洪水风险的安全利用   总被引：2，自引：1，他引：1  
基于ArcGIS与ERDAS遥感影像制图软件,利用2007年SPOT2.5m分辨率遥感影像,结合实地调查,详细编绘了开封市段黄河河道边界、开封段黄河滩区土地资源的利用现状。采用1992~2007年最大洪峰时期的TM影像与沿河水文站点的观测资料,提取1992~2007年7个典型日期的河道行洪边缘线,与本底数据叠加,编制了开封段不同常遇洪水流量下滩区淹没范围。按照黄河下游二维水沙数学模型,采用2004年汛后河道大断面资料,计算生成不同流量级大洪水在滩区的可能淹没范围图(淹没区边界)。在此基础上,结合土地资源管理与河道行洪安全性的要求,总结当前黄河下游滩区利用开发的现状与存在问题,制订土地安全利用规避洪险的原则,规划土地安全利用分区,分为临河风险缓冲带、近河宜耕地带、相对稳定利用带(中风险带)和稳定利用带。提出了黄河滩区土地资源合理安全利用的对策。  相似文献   

10.
中国南方山地的土地荒漠化初探   总被引：4，自引：0，他引：4  
吴薇 《中国沙漠》1989,9(3):36-43
我国的土地沙漠化已不仅仅在北方干旱、半干旱、包括部分半湿润地区, 而且在我国不少湿润和半湿润地带的东部平原(如黄淮海平原等)、沿海地区以及南方山地区也有发生, 其环境退化过程所包括的内容已不局限于"风沙化", 还包括了"砂石化"的类似砾质荒漠景观和山地丘陵斜坡的流水侵蚀造成的劣地景观, 内容较为丰富, 因此对我国南方山区的环境趋向于类似荒漠条件的退化过程, 我们采用了较广义的土地荒漠化的含义。  相似文献   

11.
Domestic grazing animals that congregate around watering points in arid rangelands create clearly-defined trampling-induced grazing gradients. Grazing and trampling alter soil and vegetation condition, often leading to substantial reductions in ecological function. We measured foraging pits and mounds created by native soil foraging animals over 12 months at three watering points in a Chihuahuan Desert grassland, and hypothesized that the density and cover of their disturbances would increase with increasing distance from water. We recorded an average of 3756 disturbances ha−1 and cover of 34.18 m2 ha−1 across the grazing gradients, which comprised mainly pits (43%) and mounds (25%) of heteromyid rodents, ants and spiders. Soil turnover was estimated at 1.43 m3 ha−1. We detected no differences in density, cover, soil volume or composition of disturbances in relation to distance from water, but there were significant, though ill-defined, differences across the five sampling periods, with generally more activity in the warm–wet months. Small animal-created mounds and pits are important sources of soil and sinks for litter within grazing gradients, and may represent the only sites where plants can establish given a relaxation in grazing pressure.  相似文献   

12.
The Waite–Nicolson rangeland management method for semi-arid chenopod shrublands predicts that smaller paddocks with medium to moderate stocking rates help to preserve the native vegetation. Vegetation cover around waterpoints in three small paddocks (<2000 ha) from Middleback Station, South Australia was studied using multivariate analysis. Data from quadrats sampled along radiating transects were tested for correlations with a number of site features and grazing history factors. Two significant associations were detected: quadrats with an abundance of Rhagodia parabolica and less palatable species such as Maireana pyramidata, and Atriplex stipitata were correlated positively with proximity to water points, paddock age and stocking rate, and negatively with paddock size. In contrast, quadrats with species such as Rhagodia ulicina and the more palatable M. sedifolia were correlated with increasing distance from the water points and paddock size, but negatively with age and stocking rates. Transect direction was not correlated with either group. Twelve of the 20 species examined, including the important forage species A. vesicaria, also were not correlated with those paddock and grazing features included here. These results suggest that the distribution of some chenopod shrub species in fenced paddocks is still possibly affected by a combination of these factors in the long term by grazing at densities of 6 ha sheep−1 and that the method, although maintaining the fodder species, may not be preserving biodiversity at these grazing levels, although further study is needed.  相似文献   

13.
The Petexbatún region of Guatemala's Petén tropical forest lowlands has been largely deforested since the early 1980s as landless peasants have turned even national parks into milpas. The soils of these tropical forest, karst landscapes are known mainly from small-scale maps and archaeological investigations. This paper investigates soil catenas and natural slope processes from the region's last virgin tropical forests and compares these with catenas and processes on slashed and burned slopes. Soils were sampled and analyzed for physical, fertility, and taxonomic measures. This primary forest toposequence forms a typical downward translocation catena, with the thickest soils (>200 cm) in depressions, the thinnest soils on shoulder slopes, and moderately thin soils on the crests and backslopes. Most crest and sloping soils are Lithic Rendolls and contain simple O-A-C horizon sequences, and most soils have little sign of either eluviation or illuviation. A few deeper soils on the slopes are Vertic Rendolls and have moderate slick-ensides, whereas footslopes and seasonally drained depressions are well developed Vertisols. The primary forest soil catena was compared with two slash-and-burn milpas, one with comparable slopes and one with steeper slopes. The comparably sloped milpa had soil thicknesses that were 7.9 to 13.8 cm thinner, truncated horizons, and physical evidence of erosion. The steeper milpa had soil thicknesses that were 11.1 to 18.2 cm thinner and also had truncated horizons and physical evidence of erosion. Where ancient Maya terraces (Late Classic, ca. 550 to 830 A.D.) are present in the slash-and-burn landscape, an average of 25 to 46.5 cm soil remain upslope and 9.3 to 16.1 cm remain downslope. The findings show high soil truncation rates and erosion to bedrock in a decade in one place, though ancient Maya terraces are still holding 2.7 to 3.6 times more soil than the surrounding hillslopes. Despite high modern erosion, lake sediments show surprisingly low soil losses in the Maya Late Classic during the period of the most intensive land use. In contrast, studies at Copán, Honduras and in the northern Petén, where terrace evidence is scant, show high rates of erosion during this period. [Key words: soil catenas, tropical deforestation, milpa, soil erosion, Maya Lowlands.]  相似文献   

14.
Anthropogenic, ecological, and land‐surface processes interact in landscapes at multiple spatial and temporal scales to create characteristic patterns. The relationships between temporally and spatially varying processes and patterns are poorly understood because of the lack of spatiotemporal observations of real landscapes over significant stretches of time. We report a new method for observing joint spatiotemporal landscape variation over large areas by analyzing multitemporal Landsat data. We calculate the spatiotemporal variation of the Normalized Difference Vegetation Index (NDVI) in the area covered by one Landsat scene footprint in north central Florida, over spatial windows of 104–108 m2 and time steps of two to sixteen years. The correlations, slopes, and intercepts of spatial versus temporal regressions in the real landscape all differ significantly from results obtained using a null model of a randomized landscape. Spatial variances calculated within windows of 105–107 m2 had the strongest relationships with temporal variances (regressions with both larger and smaller windows had lower coefficients of determination), and the relationships were stronger with longer time steps. Slopes and y‐intercepts increased with window size and decreased with increased time step. The spatial and temporal scales at which NDVI signals are most strongly related may be the characteristic scales of the processes that most strongly determine landscape patterns. For example, the important time and space windows correspond with areas and timing of fires and tree plantation harvests. Observations of landscape dynamics will be most effective if conducted at the characteristic scales of the processes, and our approach may provide a tool for determining those scales.  相似文献   

15.
In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha−1), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha−1) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h−1 for 30 min applied on 1 -m2 plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h−1, followed by recent pasture (8.2 mm h−1), long-established pasture (5.9 mm h−1) and open woodland (4.2 mm h−1). Total sediment production in recent pasture was 11.6 g m−2, followed by woody encroachment (9.0 g m−2), long-established pasture (7.3 g m−2) and open woodland (4.3 g m−2). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h−1 and 1.3 g m−2) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h−1and 6.5 g m−2, respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.  相似文献   

16.
Populations of two ornamental exotic species, Kalanchoe daigremontiana and Stapelia gigantea, have been recently detected inside a protected area containing arid ecosystems in Venezuela. We indirectly tested their invasive potential by characterizing their reproductive biology and recruitment patterns and comparing our estimates against Baker's Law and reproductive profiles reported for invasive plants. K. daigremontiana is autogamous, produces >16,000 seeds per plant and also reproduces clonally. Despite low seed viability (17.9%) and germination rates (11.9%), seeds were present in the seed bank. Plantlets of asexual origin showed high survival (75–100%) compared to seedlings of sexual origin (10%). S. gigantea is self-incompatible, xenogamous and produces close to 1500 seeds per m2 of plant tissue. Seed viability (77%) and germination rates (62%) were high, but this species is not represented in the seed bank. It has a vegetative growth. A combination of reproductive and recruitment attributes, which match those considered in Baker's Law and others reported for invasive plants, confer K. daigremontiana and S. gigantea the potential to invade Neotropical arid zones; the former mainly through selfing and production of numerous asexual plantlets, and the latter through an association with a locally abundant pollinator and production of wind dispersed seeds.  相似文献   

17.
The spatial distribution of Calanus species was examined near Franz Josef Land archipelago in August 2006 and 2007. Surface and bottom water temperatures exceeded the average multiannual values. Calanus species dominated the total mesozooplankton abundance and biomass, accounting for 818 ± 178 individuals m−3 (mean ± SE) and 803 ± 163 mg wet weight (WW) m−3, respectively, in 2006. In 2007, the values were much lower (153 ± 29 individuals m−3 and 192 ± 17 mg WW m−3, respectively), reflecting the weaker influence of warm Atlantic water that year. Calanus glacialis dominated the Calanus populations, contributing 95% and 60% of the biomass in 2006 and 2007, respectively. Older copepodite stages (CIV–CV) predominated in the C. finmarchicus (69% and 76%, respectively) and C. hyperboreus populations (80% and 77%, respectively), whereas young copepodites (CI–CIII) were predominant in C. glacialis (45% and 59%, respectively). A clear negative relationship between the average water temperature and the total Calanus biomass was observed in 2006, whereas a positive correlation between these parameters was observed in 2007. The distributions of three Calanus species off Franz Josef Land were mainly associated with hydrological conditions and circulation patterns.  相似文献   

18.
Wind erodibility of major soils in the farming-pastoral ecotone of China   总被引：2，自引：0，他引：2  
Wind erosion and desertification are severe problems in China's farming-pastoral ecotone. In this study, wind erodibility of five major soils in both uncultivated and simulated cultivated conditions, were determined through wind tunnel tests at nine wind speeds ranging from 10 to 26 m s−1. The average wind erosion rate (g m−2 min−1) under the uncultivated condition (q0) for the five soils could be set in the order: chestnut soil (28.5)>brown soil (24.8)>sierozem (21.8)>chernozem (19.9)>fixed sandy soil (11.4). The highest natural wind erosion might take place in the semi-arid steppe zone where the Chestnut soils predominate. Cultivation can significantly accelerate wind erosion, the mean wind erosion rate under the cultivated condition (qc) for all five soils was 743.7 g m−2 min−1 in the following order: sandy soil (3313.2)>brown soil (227.2)>chernozem (221.8)>sierozem (85.1)>chestnut soil (81.2). For both the uncultivated and cultivated soil samples, the relationship between wind erosion rate (q) and wind speed (U) could be expressed in general as q=A eBU (A and B are constant coefficients). There was a critical wind speed for each soil type except for the sandy soil. Below the critical wind speed, cultivation reduced wind erosion rate possibly due to soil clodiness and roughness effects. Above the critical speed, cultivation greatly intensified wind erosion rates due to the break down of the original soil structure. The critical wind speed measured at 20 cm above the soil surface was 20 m s−1 for the brown soil, 14 m s−1 for chernozem and the chestnut soils, and 10 m s−1 for the sierozem. Among the five tested soils, the high wind erosion rate of the cultivated sandy soil showed its extreme sensitivity to cultivation, possibly because of the structureless nature of the loose sand. The “effect of cultivation on wind erosion” index, η (=qc/q0), increased exponentially with the increase of wind speed, indicating that under higher wind speed conditions, cultivation could result in more severe wind erosion.  相似文献   

19.
In field trials conducted at Uzbekistan, inoculation of mungbean (Phaseolus aureusRoxb.) with commercial strain CIAM1901 ofBradyrhizobiumsp. (Phaseolus) increased (on average for two cultivars) the herbage mass by 46·6±6·0%, seed mass by 39·2 ± 3·6%, mass of 1000 seeds by 16·0 ± 0·8%, nitrogen content in seeds by 58·3 ± 8·9%, starch content in seeds by 30·0 ± 5·5% and number of nodules by 254%. Inoculation with this strain produced the same herbage (seed) mass as NH4NO3application (120 kg ha−1of N), while the combined treatment of rhizobia and 60 kg ha−1of N produced significantly higher yields than rhizobia inoculation alone or application of 120 kg ha−1of N. The rhizobia strain M11 was isolated from Uzbekistan soils and significantly exceeded the commercial strain CIAM1901 in its influence on herbage mass by 3·9–10·6%. Two-factor analysis of variance demonstrated that the herbage mass and number of nodules are controlled mainly by the rhizobia strain genotypes, while seed yield, number of pods, mass of 1000 seeds and N and starch content in seeds are influenced by the plant cultivar genotypes.  相似文献   

20.
Application of210Pb in soils   总被引：2，自引：0，他引：2  
The spectroscopic measurement of soil samples is described.210Pb data from a soil survey in western-Europe are briefly reviewed. The average210Pb flux from the atmosphere, as determined from210Pb exc soil inventories, is 90 Bq m–2a–1. A simple one-dimensional box-chain model is described. The model simulates the vertical, post depositional transport of natural and fall-out radionuclides in the soil. Simulation of measured210Pb exc ,134Cs,137Cs, and241Am soil profiles shows that mixing (bioturbation) is a very efficient transport mechanism. Lead seems to be strongly fixed to organic and clay particles. It is transported by the displacement of the organic and clay carrier substances. The mean residence time of lead, caesium, plutonium and americium in organic rich forest soils is in the order of 250–1000 years. An applicability study in investigate the use of210Pb in erosion problems showed erosion rates from 60–180 g m–2a–1 on organic rich forest and meadow sites with 10°–25° slopes.This is the eighth of a series of papers to be published by this journal following the 20 th anniversary of the first application of210Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.  相似文献   

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