Spatial variability of some soil fertility characteristics as affected by land use change, Yasouj region

Introduction: In recent decades, site-specific management (SSM) has been specifically considered to achieve to increased input efficiency, improved economic margins of crop production and reduced environmental risks. Short-scale spatial variability of soil properties caused more necessity of SSM techniques. Natural variability of soil results from complex interactions between geology, topography and climatic factors, as well as land use change and land management strategies. Deforesting and vast land use changes are considered as the important land management strategies that have been extensively used in recent decades. Therefore, determining the effects of land use change on soil properties in conjunction with local environmental conditions and on the spatial variability of soil properties may drastically help the land use planners. Therefore, the present study was done aimed to explore the effects of oak trees deforesting in Shah-Mokhtar region in Yasouj and land use changes to dry farming on the spatial variability of nutritional elements using geostatistical techniques.
Materials and Methods: The present study was conducted in a Shah-Mokhtar, north, and northwest of Yasouj, in Kohgiluyeh Province, southern Iran. Spatial variability of seven soil fertility properties, including N, P, K, Fe, Cu, Zn, and Mn concentration in soil, were examined in three land uses, including dense forest, degraded (semi-dense) forest and rain-fed lands. A total of 100 surface (0-30 cm) soil samples were collected and analyzed for the nutritional elements after preparing in the laboratory. Data were analyzed statistically and their normal distribution pattern was examined using the Kolmogrov-Smirnov test. In the geostatistical analyses step, the spatial structure of studied variables was analyzed by fitting the suitable authorized models on calculated experimental semi-variograms. The concentration of studied elements was interpolated using ordinary kriging (OK) and inverse distance weighting (IDW) estimators and finally, the spatial distribution map of each soil nutrient was prepared using ArcGIS 10.3.
Results: The results showed that the highest mean soil concentration of almost all of selected nutrients belonged to the dense forest lands and due to the land use changes to the degraded forest and then dry farming, average values of selected elements significantly decreased. Among the studied properties, soil N concentration with an average of 0.34%, 0.17% and 0.08% in dense forest, degraded forest, and rain-fed soil samples, had the most decrease caused by deforesting. Semi-variogram analyses showed that spherical model had the best performance. For interpolating the soil K and Mn concentration, IDW method and for other studied elements, OK method was efficiently used. The spatial correlation class was strong for N, Cu, and Fe; whereas a moderate class was calculated for soil P and Zn concentration. The spatial distribution maps of selected nutrients revealed that the dense forest and dry farming soils had the highest and lowest contents of soil nutritional elements.
Conclusion: According to the findings of the present study, it can be stated that degradation of oak forest may lead to the significant decrease of soil nutritional elements, specifically soil N concentration as one of the main soil fertility quality. Therefore, it seems that not only the significant soil quality decline but also the extensive degradation of whole ecosystem and unfavorable climatic consequences will be the unavoidable results of deforesting in Shah-Mokhtar region.