رابطه بين توپوگرافي و برخي ويژگي هاي خاك

Relationship between Topography and Some Soil Properties

توپوگرافی یكی از ویژگی¬های مهم و تأثیرگذار بر كیفیت خاك در هر منطقه است. در این پژوهش رابطه درجه و جهت شیب با برخی از ویژگی-های فیزیكی و شیمیایی خاك مانند بعد فركتال ذرات، درصد شن، رس و سیلت، درصد كربنات¬كلسیم معادل، درصد كربن آلی، میانگین وزنی قطر خاكدانه و شاخص پایداری ساختمان خاكبررسی شد. برای انجام تجزیه¬های آزمایشگاهی یك ردیف پستی و بلندی4با طول شیب، مواد مادری یكسان انتخاب و30 نمونه از خاك در شیب¬های كمتر از 5، 15-5، 30-15، 50-30 و بیشتر از 50 درصد، با سه تكرار و در دو جهت شمالی و جنوبی در قالب طرح آشیانه ای نمونه¬برداری شد. بر اساس نتایج به¬دست آمده، مقادیر كربنات¬كلسیم و كربن آلی در شیب¬های مختلف دارای اختلاف معنی¬داری در سطح پنج درصد بود. مقدار كمینه كربنات¬كلسیم در شیب كمتر از 5 درصد جنوبی و مقدار بیشینه آن¬ در شیب بیشتر از 50 درصد جنوبی مشاهده شد، درحالی كه مقدار كمینه كربن آلی در شیب بیشتر از 50 درصد جنوبی و بیشینه آن در شیب كمتر از 5 درصد جنوبی وجود داشت. بین میانگین درصد كربنات كلسیم و كربن آلی در دو جهت شیب اختلاف معنی¬داری وجود داشت. همچنین مقدار شاخص پایداری ساختمان خاك و میانگین وزنی قطر خاكدانه¬ها در جهت شمالی به طور معنی¬داری بیشتر از جنوبی بود. با وجود اینكه كلاس بافت خاك تقریبا در همه درجه-های شیب یكسان بود، درصد شن، رس و سیلت در طول شیب تغییرات زیادی داشت. به طوری كه با افزایش شیب درصد ذرات درشت¬تر افزایش و درصد ذرات ریزتر كاهش یافت. بعد فركتال ذرات نیز با كاهش درصد شیب، افزایش یافت. با توجه به این كه در هنگام گزارش كلاس بافتی خاك درصد شن، رس و سیلت آن بیان نمی¬شود، می¬توان گفت كه بعد فركتال ذرات نسبت به بافت خاك، تعریف بهتر و روشن¬تری از چگونگی توزیع اندازه ذرات خاك ارایه می¬دهد.

Introduction: Topography is an important and effective property affecting the soil quality. Some researchers demonstrated that degree and aspect of land slope may influence the particle size distribution and gravel. Slope degree affects the surface and subsurface run-off, drainage, soil temperature, stability of soil aggregates and soil erosion. This research was carried out to determine the spatial variation of soil properties in different slope degrees of northern and southern slopes in Khorasan Razavei province, Iran. Material and Methods: This study was performed in Sanganeh research station (longitude 60o 15ʹ60ʺ and latitude 36o 41ʹ 36ʺ), of north-eastern, Khorasan Razavi province of Iran. In order to study the effects of topography on some soil physical and chemical properties, a topo-sequence with the same slope length, parent materials and cover crops was selected. 30 soil samples (0-30 cm depth) were collected from different slopes of less than 5, 5-15, 15-30, 30-50 and more than 50 percent of both southern and northern aspects. In this study, the soil particle size distribution (texture) was measured by hydrometer method, organic carbon and calcium carbonate were determined by wet oxidation and titration with HCl 6 M, respectively and soil structural stability index, aggregates mean weight diameter and particles fractal dimension were calculated by related equations. Finally, the studied soil properties of 5 slopes (less than 5, 5-15, 15-30, 30-50, and more than 50%) and 2 aspects (north and south) with 3 replicates were compared by nested experimental design and Tuky test in JMP statistical software. Results and Discussion: The maximum and minimum clay contents as well as fractal dimension and organic carbon contents were found in less than 5% and more than 50% of south slopes, respectively. Clay content and fractal dimension in north aspect were also significantly (P <0. 001) more than south one. In reverse, the sand content in south aspect was significantly (P <0.05) more than north one. There was no significant difference between silt content in north and south aspects, but the silt content was maximum value in toe slope, which reflects the transportation of silt due to soil erosion from steeper parts of slope to toe slope. The differences of calcium carbonate and organic matter contents in different slope degrees were significant (P < 0.05). The minimum and maximum calcium carbonate contents were noted in less than 5% and more than 50% of south slopes, respectively and the average of calcium carbonate in south aspect was significantly (P < 0.001) more than the north one, while the organic carbon contents in north aspect was significantly (P <0.001) more than the south one. In both southern and northern slopes, the OC as a result of high radiation and less soil moisture content and then poor crop cover has been decreased by increasing the slope degree. The highest and least values of MWD in studied area were found in less than 5% and 30-50% degree of south slope, respectively and the average of MWD in north aspect was significantly (P < 0.01) more than south slope, too. The value of structural stability index in north aspect also was more than the south one and it was decreased by increasing the slope degree. Conclusion: The results showed that, in spite of the same soil texture class in different slope degrees of both north and south aspects, particle size distribution in different parts of slope was varied. By decreasing the degree of slope, the fractal dimension of solid particles increased and vice versa. It seems that the fractal dimension of solid particles and structural stability index of soil can be replaced by soil texture and MWD, respectively, because they can represent the best and explicit description of particle size distribution and the effect of soil forming process in sloped regions, and soil structure.