اثر مديريت اراضي بر فرسايش پذيري خاك-مطالعه موردي بخشي از حوزهي آبخيز زاينده رود

Effects of land management on soil erodibility-A case study in part of Zayandeh-Rood watershed

هدف از اين مطالعه بررسي اثرات تغيير كاربري اراضي بر فرسايشپذيري خاك در بخشي از حوزه آبخيز زاينده رود بوده است. بدين منظور، برخي از ويژگيهاي خاك شامل توزيع اندازه ذرات خاك، درصد سنگريزه، جرم ويژه ظاهري، نفوذپذيري نيمرخ خاك، كد ساختمان و كد نفوذپذيري، ماده آلي، كربنات كلسيم، ميانگين وزني قطر خاكدانه و مقاومت برشي لايه سطحي خاك اندازهگيري گرديد. فرسايشپذيري خاك با استفاده از يك دستگاه شبيهساز باران جهت اعمال بارندگي با شدت (mm h-1) 30 به مدت 30 دقيقه در پلاتي با مساحت m225/0 و شيب 9% در دو كاربري مرتع و مرتع تخريب شده اندازهگيري گرديد. تجزيه آماري در قالب طرح كاملا تصادفي انجام گرديد كه در آن تاثير بافت و تغيير كاربري اراضي مورد تجزيه و تحليل قرار گرفتند. نتايج نشان داد كلاس بافت خاك تاثير معنيداري بر فرسايشپذيري خاك در سطح احتمال 5 درصد نداشته و تغييرات فرسايشپذيري منطقه تحت تاثير كاربري اراضي بوده است. ميانگين فرسايشپذيري خاك در كاربري مرتع 05/0 (ton h MJ-1 mm-1) و در مرتع تخريب شده 09/0 (ton h MJ-1 mm-1) به دست آمد. براساس نتايج به دست آمده از توابع انتقالي مربوط به هر كاربري مشاهده شد كه ذرات رس و شن درشت (86/0R2=) در كاربري مرتع و مقاومت برشي لايه سطحي و كد نفوذپذيري (90/0R2=) در كاربري مرتع تخريب شده در سطح 5 درصد نسبت به ساير ويژگيهاي خاك براي توجيه رفتار فرسايشپذيري خاك مناسبتر ميباشند.

Introduction Soil erodibility can be viewed as the integral result of the processes determining the infiltration of rain into the soil and of the processes determining the soil’s resistance to the detachment of its particles and their subsequent transport (Lal, 1988). It is generally considered as an inherent soil property with a constant value for a given soil type and widely adopted as an important factor in soil erosion prediction models, such as the Universal Soil Loss Equation (USLE), and the Revised USLE (RUSLE). The erodibility factor, commonly known as the K factor, in the USLE was defined as the average rate of soil loss per unit of rainfall erosivity index from a cultivated continuous fallow plot, on a 9% slope 22.1 m long. Soil erodibility factor K is not only an internal factor indicating the amount of soil loss, but also the basis for the quantitative study of soil erosion. Soil erodibility is closely related to the basic physicochemical characteristics of soils. The total aggregate content, 1–10 mm aggregate content, aggregation degree, aggregate dispersion coefficient and erosion rate are indexes for the analysis of soil anti-erosion capability. Not only may soil erosion be different for various types of soils, but also it is different for the same type of soil under different climate conditions or land use management. Different land use systems might alter several soil properties and processes. Kosmas et al. (2000) reported that land use change could impact soil physical, chemical, and biological properties. Studies by Duiker et al. (2001) showed that land use changes from natural and semi-natural vegetation to cultivated and grazed lands affect soil bulk density, porosity and water storage, water infiltration and water flow characteristics and surface runoff. Abbaszadeh Afshar et al. (2010) found that organic matter content and bulk density were greater in pasture soils than in dry farm soils. Kay (2000) showed that large aggregate sizes and high organic matter content protect soils against splash detachment. Although there are many studies on land use impacts on soil erodibility, to the best of the authors’ knowledge, no study has of yet been reported on the effects of land use change on soil erodibility in Zayandeh-Rood watershed. Therefore, the objective of this study was to investigate the impacts of different land uses on soil erodibility in a part of Zayandeh-Rood watershed. Materials and Methods For this purpose, soil properties including particle size distribution, gravel percent, bulk density, permeability of soil profile, Structure code and permeability code, organic matter, calcium carbonate equivalent, mean weight diameter of aggregates and surface shear strength were measured. Soil erodibility was measured with a rainfall simulation device with rainfall intensity of 30 mm h-1 and 30 min duration in a plot with 0.25 m2 area and 9% slop in two land uses, namely pasture and degraded pasture. A completely randomized design was used in which soil texture and the land use changes were analyzed. The multiple-linear regression analysis was used to relate soil erodibility factor to different soil parameters. Results and Discussion The influence of land use change on soil erodibility was investigated based on simulated rainfall in field conditions. The findings of this study demonstrated that a considerable amount of soil erodibility occurred in the study region characterized by low organic matter and mismanagement. On average, soil erodibility was significantly affected in the Zayandeh-Rood watershed, Iran, by the land use system (i.e., soil structure and management practices) rather than by the soil textural class. Average soil erodibility was obtained in pasture land use 0.05 (ton h MJ-1 mm-1) and in degraded pasture 0.09 (ton h MJ-1 mm-1). Low soil organic matter content in the degraded pasture land is probably caused by livestock overgrazing and ultimately grazing. Based on the results obtained of the transfer functions in each land uses, it was observed that clay and coarse sand particles (R2=0.86) in pasture land uses and surface shear strength and permeability code (R2=0.90) in degraded-rangeland at the 5% level compared to other soil properties were more suitable parameters for estimating of soil erodibility. Thus, vegetation cover protection was recommended to soil conservation in this region.