بررسي اثر تغيير كاربري اراضي بر برخي خصوصيات فيزيكي و شيميايي خاك و فرسايش‌ در مناطق خشك (مطالعه موردي: منطقه‌ي هيرمند سيستان)

(Investigating of land use change effect on some soil physicalchemistry characteristics and Erosion in arid areas (case study: Hirmand of sistan

هدف از اجراي اين تحقيق، بررسي خصوصيات فيزيكي ـ شيميايي و فرسايش‌پذيري خاك در اراضي تغيير كاربري يافته­ ي منطقه­ ي سيستان است. براي اجراي اين پژوهش تغييرات مكاني كاربري‌ اراضي براساس تصاوير ماهواره‌‌اي بررسي شد. با استفاده از تصاوير سنجنده‌هاي ETM و OLI ماهواره­ ي لندست مربوط به سال‌هاي 2001 و 2019، نقشه­ ي كاربري اراضي تهيه و نقشه تغيير كاربري يافته از مرتع به كشاورزي سيلابي و باير مشخص و صحت‌سنجي شد. 10 نمونه خاك از هر كاربري‌ در قالب طرح بلوك تصادفي و از دو عمق 25 - 0 و 50 - 25 سانتي‌متر برداشت شد. يافته‌ها نشان داد كه درصد رس، سيلت و شن در اراضي تغيير كاربري يافته از مرتع به كاربري‌هاي كشاورزي سيلابي و باير تفاوت معني‌دار داشته­ است (0/05 >P). در بررسي شاخص پايداري خاكدانه‌ها (DSA) نيز يافته‌ها نشان داد كه مقادير اين شاخص در كاربر‌هاي كشاورزي سيلابي و باير به ­ترتيب 34 و 45 درصد كاهش يافت. همچنين نتايج بيانگر كاهش به­ ترتيب 18 و 43 درصدي رطوبت خاك و افزايش معني‌دار مقادير وزن مخصوص ظاهري نسبت به كاربري مرتع بوده­ است. در بررسي خصوصيات شيميايي خاك نيز يافته‌ها نشان داد كه مقدار اسديته، هدايت الكتريكي، نسبت جذب سديم، سديم و درصد آهك خاك در كاربري‌هاي كشاورزي سيلابي و باير به ­طور معني‌دار نسبت به كاربري مرتع، افزايش و در مقابل درصد مواد آلي و پتاسيم به­ طور محسوسي كاهش يافته ­است (0/05>P). بر اين اساس با توجه به نتايج به دست آمده از اين تحقيق، تغيير كاربري اراضي به ايجاد محدوديت­ هايي در خصوصيات فيزيكي و شيمايي خاك منجر شده كه چنين تغييراتي تخريب خاكدانه‌ها و افزايش فرسايش‌پذيري خاك را در اين اراضي به ­همراه داشته است. با توجه به بحران­هاي زيست محيطي موجود، اتخاذ برنامه‌هاي مديريتي مناسب در بهره‌برداري اصولي از اراضي منطقه ي سيستان ضروري به نظر مي ­رسد.

1- Introduction The land-use change is one of the critical worldwide issues that has caused a problem in natural resource sustainability. The dynamics of land-use and land-cover change (LULC) affected by the anthropogenic activities and climate change in arid areas, were caused environmental degradation of these areas. This process has the negative effects on soil physicochemical properties. Soil degradation is the key component of land degradation that affected by land-use changes. So that it causes organic waste carbon, decreases the soil aggregate stability and structure and, consequently the raises of soil erodibility. So to understand the impacts related to these changes in land-use (LU), assessment of the effects of these changes on soil characteristics is important. Therefore, the current study aim was to investigate the effect of land-use change on physico-chemical characteristics of soil and erosion in Sistan area. 2- Methodology This research has done in Sistan area. This area laid southeastern border Iran and Afghanistan. The Sistan region forms a part of the Dasht-e-Lut and hence an exhibits extreme arid climatic regime. The region is nearly flat and featureless arid plain with an average height of 465–500m ASL in its eastern parts. In this study, at first spatial land-use changes in Sistan area determined. To achieve this purpose, used satellite images. By using the Landsat satellite images related to the years 2001 and 2019, the land use maps prepared, and by comparison approach in Idrisi Selva software, the map of land-use change was extraction. After that, the areas that converted from rangeland to flood-agri and bare land determined and that accuracy were assessed. Infield operation, 60 soil samples were collected from tow depths 0-25 and 25-50 cm in different land-use such as rangeland, flood-agri, and bare land. The Samples were collected in a completely randomized block design. To study the physical properties of soil, by hydrometer method, by the soil texture, and by measuring the mean weight diameter (MWD), the geometric mean diameter (GMD) and dry stable aggregates (DSA), the aggregate stability was determined. Also, the soil moisture and bulk density were measured. To investigate the chemical characteristics, the soil properties such as organic carbon, acidity (PH), electrical conductivity (EC), sodium absorption ratio (SAR), CaCO3, Na, and available K and P measured in the soil laboratory. The soil erodibility index calculated five characteristics, including sand, silt with fine sand, organic matter, soil structure class, and the infiltration rate class of the soil profile. To analysis the mean of parameters among different land uses were used the Duncan test. 3- Results The result of the study of physical properties showed that the percentages of sand, silt, and clay particles among the land-use that under study were significantly different. So that by the land-use change from rangeland to flood-agri and bare land, the distribution of soil particle size differently, that consequently the soil texture has changed from loam-silt-clay class in rangeland to lighter class loamsilt and loam-sand in flood-agri and bare land respectively. According to this change, the aggregate stability of soil inland converted from rangeland to flood-agri, and bare land 34 and 45 percent have decreased. Also by land-use change, the soil moisture 18% and 43 % has decreased respectively. The results also showed that the bulk density of soil, acidity, electrical conductivity, Na, CaCO3 and absorption ratio (SAR), increased in converted land-use whereas, the organic carbon and available K soil, have decreased significantly. In addition, after land-use change, the erodibility index has increased significantly (P < 0.05). So that the amount of this index in flood-agri and bare land compared to the rangeland land-use, more than equal 4 and 7 respectively. 4- Discussion & Conclusions In this study, for the first time, has investigated the physico-chemistry characteristics of soil and erodibility in land converted from rangeland in Sistan area. The results revealed that the conversion of natural rangeland to flood-agri and bare land had negative effects on the measured soil properties. So that the percent of sand has increased, that changed the distribution of soil particle size, and according to that, the soil texture has changed also. This caused the aggregate stability soil to reduce. In addition, by decreasing the soil moisture, the plant growth has limited in this lands and given the high evaporation potential in Sistan area, the evaporation from land surface increased and according to that the electrical conductivity, acidity, Na and absorption ratio (SAR) in converted lands have increased, whereas, the organic carbon and available K soil, have decreased. This process has caused the Soil degradation, that affected by land-use changing. Therefore, the land-use change by destroying the soil structure and land-cover in converted lands, cause increases the potential of erodibility, that given exist environmental crises in Sistan area, has required the suitable planning to principle productivity of lands in Sistan area.