بررسی نقش تغییر کاربری اراضی بر برخی از خصوصیات فیزیکی، شیمیایی و فرسایش‌پذیری خاک

Introduction
Different land-uses are being changed every day by the man, all over the world. Nowadays the increase of population and request for agricultural products causes natural vegetation such as transferring forests to farmlands. Rapid population growth in northern Iran requires additional farmlands for food production leading to clear cutting of forests and converting to the agricultural land. Land-use changes, mainly through conversion of natural vegetation may influence many natural phenomena and ecological processes and leading to a remarkable change in soil properties. There is a considerable concern that land-use changes can lead to an alteration of the soil organic carbon. Conversion of natural vegetation to agricultural land uses leads to a significant decrease of soil organic matter (SOM) content and conversely, reversion of cultivated land to natural vegetation can reliably replenish SOM and return lost soil carbon via increased soil carbon storage. Understanding the effects of land use and land cover changes on soil properties include many implications for devising land management strategies for identifying sustainable practices in each region with the intention of land degradation, sustainable development of agriculture and environmental protection. Therefore this study has been conducted to assess the impact of conversion of native forests into cultivated land (Wheat) and garden (Orange) on some soil properties and soil erodibility and investigate the effects of some soil properties on soil erodibility in northern Iran.


Materials and Methods
The study has been conducted in the Berenjestanak watershed of Mazandaran. The altitudinal range of the study site is between 153 and 777.2 m above sea level with a mean annual precipitation of 905 mm. Most precipitations are during the winter and spring seasons (November–May), and means annual temperature ranges from 16°C.
Soil samples were taken at two depth of 0-10 and 10- 20 cm to characterize the physical and chemical properties, in particular soil texture, soil organic carbon, total N, bulk density, PH and EC. Soil erodibility measured by Wischmeier method. One-way Analysis of Variance (ANOVA) (SPSS 15) was used to compare the effects of land- use changes to selected physical and chemical properties and soil erodibility. The DUNCAN procedure was conducted to compare soil properties mean at P < 0.05. Pearson correlation coefficients were performed for response and explanatory relationships between soil erodibility with selected soil properties in three given land-uses.

Results and Discussion
The Conclusions show that land-use changes lead to significant changes in soil properties and its erodibility. Soils under three land-use types in the north of Iran have been shown to differ markedly in their contents of soil organic carbon, total N, bulk density, sand, and PH and soil erodibility. The organic carbon and total nitrogen content of the natural forest situation were significantly higher than the organic carbon and total nitrogen content of the garden and cultivated land. These results confirm the conclusions from several other experiments that have also shown a decrease in soil organic carbon and total nitrogen after cultivation. The bulk density of the soils was at the lowest level in areas under natural forest and the highest in cultivated areas of wheat. The result showed that land-use affected the measured content of sand in 0-10 cm of soil, but did not have a significant interactive effect on clay and silt contents. Also reduction of the permanent vegetation, loss of SOM, destruction of soil structure and the soil stability in the process of these alterations on the forest areas were caused increasing the soil erodibility 2 times more than the forest conditions soil. The correlation matrix revealed that soil organic carbon, total N and clay showed negative correlation with soil erodiblity, while silt and fine sand showed positive correlation whit soil erodibility.

Conclusion
Overall results indicated that conversion of natural forest and cultivation in the given area caused a degraded the soil properties, great decrease in soil quality and made the surface too sensitive for soil erosion. In conclusion, when these systems are converted for agriculture without proper practices of securing organic matter and soil stability, they are easily threatened. Therefore, conservation and good management after natural forest removal and reforesting can go some way to provide a solution to the problems of soil degradation, nutrient depletion as well as improving the soil quality