عنوان مقاله [English]
Introduction Soil degradation is a widespread environmental problem that occurs as a result of land use change and destruction of vegetation cover that may lead to changes in soil structure and porosity. Land use change and land management have significant effects on physical and chemical properties and biological capabilities of soil. The investigating of undisturbed and natural soil structure using microscopic and ultramicroscopic techniques provides invaluable information about the physicochemical, mineralogical, morphological properties and soil genesis and calcification. Image analysis is an advanced method for quantifying soil properties and increasing the precision of morphological and micromorphological studies.
Materials and Methods In this study, in order to investigate the impact of different land uses on porous and microstructure of surface soil horizons, 9 profiles in different land uses, including natural forests, artificial forest, abandoned land, orchard and cropland were extracted and described. Then one sample was taken from each horizon for physical and chemical analysis as well as a few undisturbed samples for micromorphological studies. Physical and chemical parameters such as texture, bulk density (BD), calcium carbonate equivalent (CCE), organic carbon (OC) and mean weight diameter (MWD) were measured. After preparation of thin sections of soil, micromorphological studies were conducted by polarizing microscope. Then from each thin section, 20 photos were taken randomly in plane polarized light (PPL) and cross polarized light (XPL) and transferred to image tool software. The percentage of total porosity of soil, feret diameter and area pores parameters were studied quantitatively. Three classes of feret diameter in micrometer and five classes of area in square micrometers were considered for pores in the soil thin sections. After importing photos to the software and performing calibration, grayscale and subtracting two images, the range of pores was identified by the software. Then in the classification section of software, the highest level of classes in each part was determined and the percentage of pores in each class was calculated and data obtained were analyzed by SPSS 16.0 software.
Results and Discussion Micromorphological observations showed that in natural and artificial forests, a significant amount of organic matter in the soil has resulted in the formation of granular and subangular blocky dominant microstructure. While in cropland land use the type of microstructure is mainly massive and angular blocky, due to deforestation and agricultural practices, which resulted in the degradation of soil microstructure. Appropriate environmental conditions and dense vegetation in natural and artificial forests land use lead to significant biological features in comparison to other land uses that were subjected to deforestation. In natural and artificial forests land uses, the percentage of channel and large vughs pores is more than other land uses mentioned above. Tillage results in degradation of soil structure in cropland land use, the majority of pores observed in thin section are vugh and plane. Also, the results of image analysis showed that in natural forests and orchard land uses, pores with diameters ranging from 2 to greater than 10 micrometer and areas ranging from 500 to greater than 1000 square micrometers had the highest frequency in terms of percentage of soil pores. Hence, these soils are considered as quite porous class, while in cropland land use, tillage results in the degradation of large pores showed that pores with diameters less than 2 to 10 micrometers and areas smaller than 5 to 50 square micrometers comprised and the highest percentage of soil pores.
Conclusion Asignificant amount of organic matter and low bulk density, and the highest percentage of total porosity are found in natural forest and orchard land uses, while deforestation and cultivation in cropland land use has led to compression and destruction of soil structure. This fact reflects itself in the increased bulk density and decreased total porosity. Agricultural practice has a significant effect on destruction of surface soil structure. Microstructure and voids of cropland land use are mainly massive and angular blocky and plane and vughs, respectively. With changes of land use from forest to cropland, and consequently incorrect land management causes decrease in organic matter. Shortage of organic matter causes decreasing biological activity in surface soils. The best way to prevent degradation of the soil in this area is to preserve natural forests and change cropland land use to orchard and artificial forest land uses.