تغییر شاخص‌های مغناطیسی و اکسیدهای آهن خاک در پی تغییر کاربری (مطالعه موردی: دشت مختار استان کهگیلویه و بویراحمد)

پژوهش حاضر به منظور بررسی اثرات تغییر کاربری اراضی و شیب بر برخی ویژگی‌های خاک با تمرکز بر شاخص های مغناطیسی و اکسیدهای آهن خاک در منطقه مختار یاسوج انجام گرفت. در این منطقه از چهار کاربری جنگل متراکم، جنگل تُنُک، اراضی فرسوده و کشت دیم و در هر کاربری از دو کلاس شیب 150 و 3015 درصد و در هر کاربری از 10 نقطه سطحی (عمق صفر تا 15 سانتی متر) به صورت تصادفی (مجموعا40 نمونه) نمونه برداری انجام گرفت.  میانگین ماده آلی در کاربری جنگل متراکم (24/5%) به طور معنی‌داری بیشتر از سایر کاربری‌ها بوده است. جنگل‌تراشی و عملیات زراعی بر روی اراضی شیب دار، موجب سبک تر شدن بافت خاک و کاهش معنی دار مقدار رس در کاربری زراعی شده است. همچنین چگالی ظاهری و کربنات کلسیم معادل با تغییر کاربری از جنگل متراکم به کاربری زراعی افزایش یافته‌اند. پذیرفتاری مغناطیسی (χlf) خاک به میزان زیادی تحت تاثیر تغییر کاربری و به میزان کمتر تحت تاثیر موقعیت شیب بوده است. تغییرات کاربری از جنگل متراکم به سایر کاربری ها کاهش معنی داری (25 تا 100 درصد) بر χlf داشته است. با توجه به مقادیر 9/1 تا 2/7 درصدی پذیرفتاری مغناطیسی وابسته به فرکانس (χfd)، این خاک ها دارای مقادیر کم تا متوسط ذرات پدوژنیک سوپرپارامگنتیک بوده اند. همبستگی نسبتاً بالایی بین χlf با برخی ویژگی های از جمله شکل های آهن، ماده آلی، بافت خاک و میزان کربنات کلسیم مشاهده شد. با اندازه گیری χlf که امکان اندازه گیری سریع و ارزانی دارد، می توان مناطق حساسی را که در معرض تخریب زیادی بوده، شناسایی و اقدامات مدیریتی مناسب را اتخاذ نمود.

Changes in Soil Magnetic Properties and Iron Oxides Following Land Use Change (Case Study: Mokhtar Plain, Kohgilouye Province)

IntroductionGlobal observations have confirmed that in recent decades, forests have been converted into agricultural land at a swift pace; this is a major global concern. Forests around the world have also experienced severe disturbances due to other anthropogenic activities. The conversion of forests to cropland often results in soil degradation. Slope gradient and land use change are known to influence soil quality; therefore, the assessment of soil quality is important in determining sustainable landuse and soilmanagement practices. Magnetic susceptibility (χlf) measurements are widely used to study soilforming processes. Many efforts have been made to correlate soil magnetic susceptibility with different soil properties, such as topography, parent material, Fe oxide forms, etc. The Yasouj area of Kohgilouye Province is one of the most densely forested areas in Zagros mountainous region. Parts of the area have been cultivated to feed the growing population, which has led to forest degradation. The objectives of this study were to assess some soil properties focusing on soil χlf and Fe oxides forms in different land uses and slope positions.Materials and Methods Forty soil samples were taken from dense forest, sparse forest, eroded lands and dryland farming from different slops (015 and 1530 percent) in Mokhtar Plain, west of Yasouj city. Soil samples were taken from the depth of 0–15 cm in a completely randomized design with five replications. Soil moisture and temperature regimes in the study area are xeric and thermic, respectively. Particle size distribution was determined by the hydrometer method and soil organic matter, CaCO3 equivalent and bulk density were determined using standard procedures. Fe (Feo) were extracted by acid ammonium oxalate, using a single 4h extraction at pH 3 in the dark. Total free iron (Fed) was extracted with the CBD method. The total Fe contents (Fet) in the soil samples were determined after extraction with 5 mol L1 HNO3. Magnetic susceptibility of the soils was measured at low (0.46 kHz; χlf) and high (4.6 kHz; χhf) frequencies, respectively; using a Bartington MS2 dualfrequency sensor, with approximately 10 g of airdry soil in polyethylene vials. The percentage of frequencydependent magnetic susceptibility (χfd%) was calculated to study the size of magnetic crystals in soils and the abundance of pedogenic ferrimagnetic in SPSSD (~0.03 μm) boundary.Results and DiscussionThe results of this study showed that the land use and slope positions were among the important factors affecting the change of soil properties in this area. Land use change along with the reduction of organic matter reduced the stability of aggregates and increased land erosion. This process caused the loss of clay particles and magnetic minerals and affected many soil properties. Organic matter as an important indicator of soil quality, showed a decrease of about 3 times as a result of land use change from dense forest to eroded lands following by an increase in bulk density and a decrease in soil permeability and other soil quality indicators. Longterm afforestation and agricultural activities on sloping lands changed the soil texture from a class of silty loam in the forest to a lighter class of silty loam in agricultural use. Soil magnetic susceptibility, which is a function of soil magnetic particles was greatly affected by land use change and to a lesser extent by slope position. Due to the fact that magnetic susceptibility is influenced by factors such as soil texture, drainage class, erosion conditions, magnetic mineral contents, soil evolution conditions, land use changes from forest to other uses had significant effects (about 2 times) on χlf. Significant decrease in the amount of calcium carbonate in low slope positions was another reason for the increase in magnetic susceptibility in these positions. According to the measured values ​​of χfd (ranged from 1.9 to 7.2%), the magnetic particles of the soils had low to moderate amounts of superparamagnetic (SP) particles, which indicates the combined effect of pedogenic superparamagnetic ultrafine particles and  lithogenic (inherited) magnetic particles in χlf of the soils. The effect of slope on Fe forms (Feo, Fed and Fet) has been significant (p < 0.01) in almost all land uses. Due to the relatively high correlation of χlf with some soil properties such as Fe forms, soil clay, the amount of diamagnetic compounds including calcium carbonate in the studied soils, it is possible to estimate the value of these soil properties using χlf, which is a quick and costeffective approach. Overall, it seems that magnetic susceptibility could be applied successfully to estimate some soil properties in hilly regions of Zagros Mountains of southwestern Iran, especially for monitoring the consequences of land use changes. It should also be noted that any change in the use of the area should be defined in accordance with the potential of the land in the long term to prevent a reduction in soil quality.