تأثیر تبدیل اراضی طبیعی به زراعی بر برخی خصوصیات فیزیکی و شیمیایی خاک (مطالعه موردی: واوسر – کیاسر، استان مازندران)

Abstract
The aim of this study was to investigate the effect of converting natural lands to agricultural lands in Vavsar village of Kiasar city. For this purpose, a rangeland use, along with three other uses, including lentil, wheat, and potato cultivation, were selected as the study area. Soil sampling was performed at a depth of 30-0 cm from each soil surface. A total of 40 soil samples were collected. Then the physical and chemical factors of the soil were measured including soil texture, aggregate stability, bulk density, total organic matter, total nitrogen, potassium, absorbable phosphorus, calcium carbonate, pH and EC. The results of variance analysis showed that the conversion of rangeland to the three mentioned uses had no effect on the overall soil texture and the soil texture in all four uses was Lummi. However, in the comparison of the average of the three factors that make up the soil texture (silt, clay and sand), it was found that there is a significant difference between the percentage of sand and silt at the level of 0.05. The results also showed that other factors such as aggregate stability, total organic matter, total nitrogen, calcium carbonate were significantly reduced as a result of this conversion. So that in the use of lentils, potatoes and wheat, the stability of the soil is 50.95, 20.44 and 8.86 percent, respectively, the total organic matter is 41.94, 24.73 and 22.44 percent, respectively, the total nitrogen is 42.86, respectively. 21.43% and 7.14% and calcium carbonate levels decreased by 72%, 62% and 88%, respectively. While phosphorus increased by 55.91, 86.39 and 24.39, respectively, and potassium increased by 44.05, 71.88 and 8.45%, respectively. But this conversion did not have a significant effect on pH, EC and bulk density. The results of this study showed that land use change can have different effects on soil quality characteristics. Therefore, it is suggested that continuous monitoring and evaluation of the fertility status and quality of the cultivated lands be done.

Introduction
In many developing countries, human populations depend on rangelands for livelihoods, and therefore have many effects on ecosystems by converting rangeland lands to other uses. Land use change is one of the most important threats in natural ecosystems, which poses the protection of these ecosystems to challenges (Zhao, et al. 2012). Land use change and management methods (such as plowing and fertilizing) directly affect the physical, chemical, microbiological properties and the ability of soil aggregates and organic carbon (Helfrich, et al. 2006). In Rangeland ecosystems, especially in the north of the country, is seen a lot the phenomenon of land use change such as rangeland conversion into agricultural and residential lands. Then the aim of this study was to investigate the effect of converting rangelands to agricultural lands in Vavsar village of Kiasar city functions.
Study area
Kiasar city is located in Mazandaran province and in the southern part of Sari city. The geographical coordinates of the study area are 53°-00´-27" to 53°-24´-18" longitude and 36°-09´-56" to 36°-29´-24" latitude. The area of the study is 800 kilometers2, altitude is 97 meters above sea level in the lowlands and 1670 meters in the mountains, the average rainfall is 1000 mm.
Methods
For this purpose, one rangeland use, along with three other uses, including lentil, wheat, and potato cultivation (At least 10 years have passed since their conversion) and had almost the same slope, direction and height, were selected as the study area. Soil sampling was performed at a depth of 30-0 cm from each soil surface. A total of 40 soil samples (10 replicates from each land use) were collected. Then the physical and chemical factors of the soil including soil texture, aggregate stability, bulk density, total organic matter, total nitrogen, potassium, absorbable phosphorus, calcium carbonate, pH and EC were measured. Finally, to investigate the effect of land use change on each of the soil quality characteristics, one-way ANOVA test and Duncan test were used to compare the means using SPSS software version 22.
Results
The results of variance analysis showed that the conversion of rangeland to the three mentioned uses had no effect on the overall soil texture and the soil texture in all four uses was Lummi. However, in the comparison of the average of the three factors that make up the soil texture (silt, clay and sand), it was found that there is a significant difference between the percentage of sand and silt at the level of 0.05. The results also showed that other factors such as aggregate stability, total organic matter, total nitrogen, calcium carbonate were significantly reduced as a result of this conversion. So that in the use of lentils, potatoes and wheat, the stability of the soil is 50.95, 20.44 and 8.86 percent, respectively, the total organic matter is 41.94, 24.73 and 22.44 percent, respectively, the total nitrogen is 42.86, respectively. 21.43% and 7.14% and calcium carbonate levels decreased by 72%, 62% and 88%, respectively. While phosphorus increased by 55.91, 86.39 and 24.39, respectively, and potassium increased by 44.05, 71.88 and 8.45%, respectively. But this conversion did not have a significant effect on pH, EC and bulk density. The results of multiple comparisons of means (Duncan test) showed that although the amount of total organic carbon, total nitrogen and calcium carbonate in three uses of lentils, potatoes and wheat were significantly reduced, but the two elements of absorbable potassium and phosphorus and the highest amount in potato cultivation. There was no significant difference in acidity (soil reaction) and electrical conductivity. However, the highest soil acidity was related to potato cultivation and the lowest was related to wheat cultivation. Regarding electrical conductivity, the highest amount was related to the use of wheat and the lowest amount was related to the lentils cultivation.
Discussion
Due to the conversion of the rangeland into three uses of lentils, potatoes and wheat, decreased the amount of aggregate stability. Reduction of soil organic matter and microbial activity as well as the use of agricultural machinery are the main factors to reduce aggregate stability in agricultural lands. The general trend of changes in the percentage of clay, sand and silt during land use change showed that in rangeland use was less the amount of clay and silt was and was more the amount of sand. One of the reasons for the higher amount of sand in the rangeland use can be attributed to the lower amount of clay in this use. Also, one of the reasons for the low amount of clay in the pasture can be attributed to the excessive entry of livestock into the field, which causes the soil particles to be kicked and crushed, resulting in soil erosion and leaching and the transfer of fine particles downstream. Tillage and plowing accelerate the decomposition of organic matter and increase soil erosion and thus reduce the amount of soil organic matter. The amount of total nitrogen of the study area decreased due to the conversion of pasture to three crop uses, the reasons for this decrease can be attributed to the reduction of crop residues for production of humus and the reduction of soil organic carbon due to cultivation. One of the reasons for the high amount of lime in rangeland use can be attributed to its bedrock. Also, the reason for the low calcium carbonate in arable lands can be attributed to its non-return after harvest. The reason for the high amount of absorbed phosphorus and soil potassium in agricultural use compared to pasture use was the use of phosphate and potash fertilizers as well as animal fertilizers. The use of phosphorus and potash fertilizers in the surface layers of agricultural lands as well as the return of plant residues to the soil, causes the accumulation and increase of these elements. The results of this study showed that land use change can have different effects on soil quality characteristics. Therefore, it is suggested that continuous monitoring and evaluation of the fertility status and quality of the cultivated lands be done.