پديد آورندگان :
اوليايي، حميدرضا دانشگاه ياسوج - دانشكده كشاورزي - گروه علوم خاك، ياسوج، ايران , ادهمي، ابراهيم دانشگاه ياسوج - دانشكده كشاورزي - گروه علوم خاك، ياسوج، ايران , نجفي قيري، مهدي دانشگاه شيراز - دانشكده كشاورزي و منابع طبيعي داراب
كليدواژه :
اكسيد آهن , پذيرفتاري مغناطيسي , تغيير كاربري اراضي , كيفيت خاك , موقعيت شيب
چكيده فارسي :
پژوهش حاضر به منظور بررسي اثرات تغيير كاربري اراضي و شيب بر برخي ويژگيهاي خاك با تمركز بر شاخصهاي مغناطيسي و اكسيدهاي آهن خاك در منطقه مختار ياسوج انجام گرفت. در اين منطقه از چهار كاربري جنگل متراكم، جنگل تُنُك، اراضي فرسوده و كشت ديم و در هر كاربري از دو كلاس شيب 15-0 و 30-15 درصد و در هر كاربري از 10 نقطه سطحي (عمق صفر تا 15 سانتيمتر) به صورت تصادفي (مجموعا40 نمونه) نمونهبرداري انجام گرفت. ميانگين ماده آلي در كاربري جنگل متراكم (24/5%) به طور معنيداري بيشتر از ساير كاربريها بوده است. جنگل‌تراشي و عمليات زراعي بر روي اراضي شيبدار، موجب سبكتر شدن بافت خاك و كاهش معنيدار مقدار رس در كاربري زراعي شده است. همچنين چگالي ظاهري و كربنات كلسيم معادل با تغيير كاربري از جنگل متراكم به كاربري زراعي افزايش يافته‌اند. پذيرفتاري مغناطيسي (χlf) خاك به ميزان زيادي تحت تأثير تغيير كاربري و به ميزان كمتر تحت تأثير موقعيت شيب بوده است. تغييرات كاربري از جنگل متراكم به ساير كاربريها كاهش معنيداري (25 تا 100 درصد) بر χlf داشته است. با توجه به مقادير 9/1 تا 2/7 درصدي پذيرفتاري مغناطيسي وابسته به فركانس (χfd)، اين خاكها داراي مقادير كم تا متوسط ذرات پدوژنيك سوپرپارامگنتيك بودهاند. همبستگي نسبتاً بالايي بين χlf با برخي ويژگيهاي از جمله شكلهاي آهن، ماده آلي، بافت خاك و ميزان كربنات كلسيم مشاهده شد. با اندازهگيري χlf كه امكان اندازهگيري سريع و ارزاني دارد، ميتوان مناطق حساسي را كه در معرض تخريب زيادي بوده، شناسايي و اقدامات مديريتي مناسب را اتخاذ نمود.
چكيده لاتين :
Introduction
Global 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 land-use and soil-management practices. Magnetic susceptibility (χlf) measurements are widely used to study soil-forming 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 (0-15 and 15-30 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 4-h 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 L-1 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 dual-frequency sensor, with approximately 10 g of air-dry soil in polyethylene vials. The percentage of frequency-dependent magnetic susceptibility (χfd%) was calculated to study the size of magnetic crystals in soils and the abundance of pedogenic ferrimagnetic in SP-SSD (~0.03 μm) boundary.
Results and Discussion
The 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. Long-term 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 cost-effective 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.
