تغييرات ويژگيهاي فيزيکي خاک در اثر خاکورزي سنتي در کشتزارهاي ديم در منطقه نيمهخشک
محورهای موضوعی : مدیریت بهینه منابع آب و خاکعلی رضا واعظی 1 * , خدیجه سهندی 2 , فرشته حق شناس 3
1 - استاد گروه علوم و مهندسي خاک، دانشکده کشاورزي، دانشگاه زنجان.
2 - دانش‌آموخته کارشناسي ارشد گروه علوم و مهندسي خاک دانشکده کشاورزي، دانشگاه زنجان.
3 - دانشجوي دکتري گروه علوم و مهندسي خاک، دانشکده کشاورزي، دانشگاه زنجان.
کلید واژه: پايداري خاکدانه, خاکورزي حفاظتي, خاک هاي ميان بافت, ماده آلي, مرتع,
چکیده مقاله :
زمينه و هدف: ساختمان خاک از ويژگيهاي مهم فيزيکي خاک است که بر باروري خاک و مقاومت به فرسايش آبي اثر ميگذارد. تغيير کاربري زمين از جمله فعاليتهاي انسان در طبيعت است که بر ويژگيهاي مختلف خاک و در نتيجه باروري خاک اثر ميگذارد. تغيير کاربري مراتع به زمينهاي کشاورزي در مناطق نيمهخشک در حال افزايش است. بسياري از کشتزارهاي ديم در اين مناطق با روشهاي سنتي تحت شخم و کشت هستند. چنين روشي بسياري از ويژگيهاي شيميايي فيزيکي مختلف خاک را ميتواند تحت تأثير قرار دهد. اطلاعات اندکي در مورد اثرات تغيير کاربري زمين و خاکورزي سنتي بر ويژگيهاي فيزيکي خاک در مناطق نيمهخشک وجود دارد. از اين رو اين مطالعه با هدف بررسي اثرات خاکورزي سنتي بر ويژگيهاي فيزيکي خاک در منطقه نيمهخشک انجام گرفت.
روش پژوهش: اين آزمايش در هفت منطقه تحت پوشش دو کاربري زمين (مرتع و ديم) در منطقه نيمهخشک زنجان واقع در شمال غرب ايران انجام شد. ميانگين بارندگي 340 ميلي‌متر و ميانگين دماي سالانه C° 7/11 مي‌باشد. اين منطقه حدود 320000 هکتار اراضي ديم دارد که حدود 90 درصد آن براي توليد گندم زمستانه با ميانگين عملکرد دانه 100 کيلوگرم در هکتار کشت مي‌شود. در هر منطقه، توزيع اندازه ذرات خاک در دو کاربري زمين يکسان بوده اما آن بين نواحي مختلف بود. در هر کاربري زمين سه نمونه خاک از عمق 30-0 سانتي‌متري خاک نمونه‌برداري شد. در مجموع از هفت نقطه در هر منطقه و چهل و دو نقطه در کل منطقه مورد مطالعه نمونه‌هاي خاک جمع‌آوري شد. ويژگيهاي فيزيکي خاک شامل حجم درشت منافذ، تخلخل، چگالي ظاهري، ظرفيت نگهداري آب، نقطه اشباع، متوسط اندازه خاکدانه، پايداري خاکدانه و هدايت هيدروليکي اشباع در نمونه‌هاي خاک به روشهاي آزمايشگاهي تعيين شدند. مقايسه بين مناطق مختلف از نظر ويژگيهاي خاک با روش مقايسه ميانگين‌ها با استفاده از آزمون توکي انجام شد. از آزمون t زوجي براي مقايسه هر ويژگي خاک بين دو کاربري زمين (مرتع و ديم) استفاده شد.
يافته‌ها: بر اساس نتايج، بافت خاک در هفت منطقه مورد بررسي عبارت از لوم رسي، لوم رسي سيلتي، لوم رسي شني، لوم سيلتي، لوم، لوم شني و شن لومي بودند. همچنين در ساير ويژگيهاي خاک (ساختار، تخلخل، حفظ آب و هدايت هيدروليکي) تفاوت معني‌داري بين مناطق وجود داشت. اثر خاکورزي بر تخريب ويژگيهاي فيزيکي خاک قابل توجه بود، به طوري که براي هر ويژگي فيزيکي خاک بين دو کاربري تفاوت معني‌داري مشاهده شد. کاهش قابل توجهي در حجم تخلخل درشت (41 درصد)، تخلخل کل (22 درصد)، اندازه خاکدانه (60 درصد)، پايداري خاکدانه (71 درصد) و هدايت هيدروليکي اشباع (71 درصد) در زمينهاي ديم تحت خاکورزي معمولي مشاهده شد، در حالي که چگالي ظاهري (28 درصد) و ظرفيت نگهداري آب (11 درصد) با خاکورزي معمولي نسبت به خاک مراتع افزايش يافت. کاهش رطوبت اشباع خاک (حدود 4 درصد) در بيشتر مناطق معني‌دار نبود. ميانگين تخريب ويژگيهاي فيزيکي خاک محاسبه شد و از 34 درصد در لوم رسي سيلتي تا 50 درصد در لوم متغير بود که با محتواي بالاتر ماده آلي خاک و پايداري خاکدانه‌ها در اين خاک همراه بود.
نتايج: ويژگيهاي فيزيکي خاک به شدت تحت تأثير خاکورزي در اراضي ديم گندم در مناطق نيمهخشک قرار دارد. ميزان تخريب فيزيکي در بين ويژگيهاي خاک متفاوت است. پايداري خاکدانه‌ها و هدايت هيدروليکي اشباع حساس‌ترين ويژگيهاي فيزيکي خاک با خاکورزي معمولي هستند. حساسيت خاک‌هاي منطقه نيمهخشک به تخريب فيزيکي توسط خاکورزي معمولي متفاوت است. خاک‌هاي با بافت متوسط مانند لوم که نسبتاً خوب انباشته شده‌اند، حساس‌ترين خاک‌ها به تخريب فيزيکي در منطقه هستند. پيشگيري از تغيير کاربري اراضي در مناطق داراي خاک‌هاي با بافت متوسط اولين گام و انجام خاکورزي حفاظتي دومين راهکار براي کنترل تخريب فيزيکي خاک‌هاي نيمهخشک است.
Introduction: Soil structure is an important soil physical property that influences soil productivity and resistance to water erosion. Land use change is the most human activity in nature that can affect various soil properties, as well as soil productivity. The change of pastures to agricultural lands has been increasing rapidly in the semi-arid regions. Most of wheat farms are tilled and cultivated using conventional method. This method can destruct different soil physicochemical properties. Limited information is available on the effects of land use change and performing conventional tillage on soil physical properties in semi-arid regions. Therefore this study was conducted to find effects of conventional tillage on soil physical properties in rainfed lands of a semi-arid region in Iran..
Materials and Methods: The experiment was done in seven areas covering the two land uses (pasture and rainfed lands) in a semi-arid region in Zanjan located,north west of Iran. Mean annual precipitation and temperature are 340 mm and 11.7 °C, respectively. The area covers about 320000 hectare rainfed lands which about 90% from it is cultivated for winter wheat production with an average yield of 100 kg per hectare. In each area, soil particle size distribution was the same in the two land uses but it was different among the areas. Three Soil samples were taken from 0-30 cm depth in each land uses. In total, soil samples were collected from seven points in each area and forty two points in the study area. Soil physical properties consist of macropores volume, porosity, bulk density, water-holding capacity, saturated point, aggregate size, aggregate stability and saturated hydraulic conductivity were determined in the soil samples by laboratory methods. Comparisons among different areas in soil properties were done with means comparison procedure using the Tukey’s test. In order to effects of conventional tillage on soil physical properties, a paired t-test was used for comparing each soil property between the two land uses (pasture and rainfed lands).
Results and Discussion: Based on the results, soil texture of the areas were clay loam, silty clay loam, sandy clay loam, silt loam, loam, sandy loam and loamy sand. Moreover there were significant differences among the areas in other soil properties (structure, porosity, water retention and hydraulic conductivity). Effect of conventional tillage on deterioration of soil physical properties was considerable, so that significant difference was found for each soil physical property between the two land uses. A considerable decline was found in macropores volume (41%), porosity (22%), mean aggregate size (60%), aggregate stability (71%) and saturated hydraulic conductivity (71%) in rainfed lands under conventional tillage, while bulk density (28%) and water-holding capacity (11%) increased by conventional tillage as compared to pastures soil. Decreases in saturated soil moisture (about 4%) weren’t significant in most areas. Mean deterioration of soil physical properties was computed and varied from 34% in silty clay loam to 50% in loam which was associated with higher soil organic matter content and aggregate stability in this soil.
Conclusions: Soil physical properties are strongly affected by conventional tillage in wheat rainfed lands in semi-arid region. The rate of physical destruction is different among the soil properties. Aggregate stability and saturated hydraulic conductivity are the most susceptible soil physical properties by conventional tillage. The sensitivity of soils of the semi-arid region to physical destruction by conventional tillage is different. Medium-textured soils such as loam which relatively well aggregated are the most sensitive soils to physical destruction in the area. Prevention of land use change in areas with Medium-textured soils is the first step and performing conservation tillage is the second strategy to control physical destruction of semi-arid soils.
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