Evaluation of the Effects of Land Use Change from Forest Areas into Agricultural Lands on Some Chemical Properties of Soil (Case Study: Zarin Abad, Sari, Iran)

Introduction
Land use change and its effects on soils is one of the important factors that decrease soil quality. Nowadays because of rapid growth of population, vast forest areas are changed into agricultural lands but chemical properties of the soils are commonly affected by changes in land usage.  Converting the forest areas into agricultural lands is one of the prevalent issues in Iran, especially in north Iran. Nevertheless, relevant studies about the effects of land use change are very limited and lack of enough and suitable studies due to quantifying of these effects is so significant. Because of that, this study was carried out to investigate the impact of changes in usage of forest lands into the dry farming lands (canola) and paddy lands on some chemical properties of soil such as: pH, organic carbon, total nitrogen, available phosphorous and potassium, electrical conductivity and cation exchanging capacity in Zarin Abad area of Sari, in northern Iran.
 
Methodology
In order to study the effects of changes in usage of forest lands (Hyrcanian forest) into the dry farm lands (canola) and paddy lands on some chemical properties of soil in Zarin Abad area of Sari, this research was conducted in factorial design in complete randomized block. The samples of soil from each land use were collected in four replications from 2 depths of 0-20 and 20-50 centimeter. The samples were dried and passed from the 2mm sieve, and then the organic carbon was determined by the method of wet oxidation. Total nitrogen of the soil was measured by Kejeldal method. To calculate the soil available phosphorous, the samples were extracted in two ways: Olson (for measuring available phosphorous in soil samples with pH=7) and Bray-Kortz (for measuring available phosphorous in soil samples with pH=7). Then, the amount of phosphorous in each extraction was determined with Expectrophotometer device. Available potassium was measured by extraction with Ammonium acetate with pH=7 and Flame photometer device. Cation Exchanging Capacity was also determined by Bawer method. To measure the reaction and the electrical conductivity of the soil samples, after making saturation mud, the amount of reaction (pH) was measured by pH meter in each sample. The amount of electrical conductivity after extracting from saturation mud was determined in extractions from each sample by Ec meter. Then, the analysis of data was done after performing normalization test with Colomogrof-Smirnof test in SPSS application. Comparison of the averages of data was done with Duncan test.
 
Results and Discussion
Results showed that land use change from forest lands (Hyrcanian forest) into paddy lands, increased soil reaction from 6.43 to 7.52 but change to dry farming lands (canola) had no significant effect on soil reaction. Land use change from forest lands into dry farm lands caused decrease in the amount of organic carbon about 46.5 percent. This was about 38 percent in paddy lands. The amount of total nitrogen was decreased in land use change from forest usage into dry farm usage but this decrease was not significant in forest lands into paddy lands. Although land use change caused increase about 4 times in the amount of available phosphorus in dry farming lands and increase about 2 times in the areas converted into paddy lands, it had no effects on the amount of available potassium and cation exchanging capacity. The lowest amount of electrical conductivity was observed in dry farming lands. Correlation coefficients between the parameters showed positive and significant correlations between organic carbon with available potassium and electrical conductivity. There were positive correlations between electrical conductivity with the amount of available potassium and soil reaction. The amount of available phosphorus also showed a positive relationship with cation exchanging capacity.
 
Conclusion
Land use change caused transformations in some soil chemical properties and each property affected the values of other properties. Given the ecological importance of  northern forests of Iran, the results of this study showed the necessity of more accuracy and attention to adjust the unfavorable effects of land use change by applying efficient methods of land management in these areas.