تأثير موقعيت شيب و تغيير كاربري اراضي بر ويژگي‌هاي خاك و پذيرفتاري مغناطيسي در اراضي تپه ماهوري ياسوج

Slope and Land Use Changing Effects on Soil Properties and Magnetic Susceptibility in Hilly Lands, Yasouj Region

این مطالعه به منظور بررسی اثر تغییر كاربری اراضی و موقعیت زمین‌نما بر برخی از ویژگی‌های خاك در بخشی از اراضی تپه ماهوری شهرستان یاسوج انجام گرفت. در این منطقه از سه كاربری جنگل طبیعی، جنگل تخریب شده و زراعت دیم و در هر كاربری از سه كلاس شیب 10-0، 20-10 و 30-20 درصد و در هر كلاس از 5 نقطه به صورت تصادفی، جمعاً در 45 نقطه از عمق 10-0 سانتی متری انجام گرفت. نتایج نشان داد كه میانگین ماده آلی در كاربری جنگل طبیعی (8/5 درصد) به طور معنی‌داری (01/0 p <) بیشتر از كاربری‌های جنگل تخریب شده (12/2) و زراعت دیم (7/1) بود. ماده آلی و درصد رس با تغییر كاربری جنگل طبیعی به كشاورزی كاهش معنی‌دار نشان دادند. درصد كربنات كلسیم معادل، چگالی ظاهری، درصد سیلت و شن با تغییر كاربری از جنگل طبیعی به كشاورزی افزایش یافته‌اند. تغییر كاربری باعث كاهش معنی‌دار پذیرفتاری مغناطیسی در كاربری‌های تحت كشت و جنگل تخریب ‌شده نسبت به جنگل طبیعی شده است. كاهش پذیرفتاری مغناطیسی در این دو كاربری احتمالاً به دلیل هدر رفت بیشتر رس و در نتیجه ذرات ریز مغناطیسی همراه آن می باشد. تغییرات اكثر ویژگی‌های اندازه‌گیری شده خاك با تغییر موقعیت اشكال اراضی نیز اغلب معنی‌دار شده است كه می‌تواند به دلیل وجود فرسایش آبی شدید حاكم بر منطقه، وجود سازندهای حساس به فرسایش و مدیریت نامناسب باشد كه باعث فرسایش خاك سطحی غنی از ماده آلی، رس و كانی‌های مغناطیسی از موقعیت‌های بالای شیب و رسوب آنها در موقعیت‌های پایین شیب باشد.

Introduction: Land use changes are the most reasons which affect natural ecosystem protection. Forest soils have high organic matter and suitable structure, but their land use management change usually affects soil properties and decreases soil quality. There are several outcomes of such land use changes and intensification: accelerated soil erosion and decline of soil nutrient conditions, change of hydrological regimes and sedimentation and loss of primary forests and their biodiversity. Establishing effects of land use and land cover changes on soil properties have implications for devising management strategies for sustainable use. Forest land use change in Yasouj caused soil losses and decreased soil quality. The objectives of this study were to assess some soil physical and chemical properties and soil magnetic susceptibility changes in different land uses and slope position. Materials and Methods: Soil samples were taken from natural forest, degraded forest and dryland farm from different slops (0-10, 10-20 and 20-30 percent) in sout east of Yasouj. They were from 0–10 cm depth in a completely randomized design with five replications. Soil moisture and temperature regimes in the study area are xeric and mesic, respectively. Particle size distribution was determined by the hydrometer method and soil organic matter, CaCO3 equivalent and bulk density was determined using standard procedures described in Methods of Soil Analysis book. Magnetic susceptibility was measured at low and high frequency of 0.46 kHz (χlf) and 4.6 kHz (χHf) respectively with a Bartington MS2D meter using approximately 20 g of soil held in a four-dram clear plastic vial. Frequency dependent susceptibility (χfd) is expressed as the difference between the high and the low frequency measurements as a percentage of χ at low frequency. Results and Discussion: Soil texture was affected by land use change from silty clay loam in forest to silty loam in dry land farm. Declining of organic matter and aggregate stability caused soil surface loss by erosion. The bulk density increased from 1.12 to 1.54 gcm-3 when forest changed to dry land farms. Soil compaction by tillage and lower amount of organic matter in farm lands are some of the reasons for increasing bulk density. Another possible reason could be decreasing of biological activity and parent material with greater calcite mixed with soil surface layer during land use change. Thus, the maximum and minimum amount of calcite was observed in dry land farm in 20-30 % slopes (57.46 %) and forest in 0-10 % slopes (13.37 %), respectively. In addition during soil formation calcite was translocated to lower horizons in natural forest. The greatest organic matter was 7.45 % and related to natural forest in 0-10 % slopes. Overall, the organic matter content was greater in all forest slopes than all other land use. In mineral soil, total organic carbon is not a proper factor in soil physical behavior. Complex and noncomplex organic carbon influence the soil physical behavior. Organic carbon in degraded forest and dry land farming was in complex form but in forest land use it was observed in two complex and noncomplex forms. Noncomplex organic matter was 53% and complex organic matter was 47%. It means that forest soil have better quality than degraded forest and dry land farm, respectively. Sedimentary rocks have rather low concentration of magnetic minerals with magnetic susceptibility from 0.1 (10-8 m3 kg-1) in the limestone to approximately 20 (10-8 m3 kg-1) in the siltstone. Low magnetite susceptibility in natural forest was more than degraded forest and dry land farm. Mean magnetite susceptibility values were 61.8, 48.6 and 42.4 10-8 m-3 kg-1, respectively which probably related to magnetic minerals formation during pedogenesis. Frequency magnetite susceptibility (χfd) was more than 3% in the most soils, significantly in forest soil (from 4.63-5 percent). Greater frequency magnetite susceptibility (χfd) values are suggested to be indicative of the dominance of super-paramagnetic grains and fiug single domain particles. χfd in soils reflects significant pedogenic magnetic minerals which formed during soil formation from calcitic parent materials.