The Effect of Land Use Change of Rangeland to Agriculture and Forest Park Land Use on Different Potassium Forms and Clay Mineralogy

Land use change is a factor that affects the loss of soil fertility such as macronutrient. Potassium (K) is an essential macro-nutrient for the growth of plant that its distribution in soil depends on clay minerals, manure, degree of weathering, land use type, climate condition and leaching.  The purpose of this research was an investigation of different K forms in three agricultures, rangeland and forest park land uses in surface (0-15 cm) and subsurface (15-30 cm) depths in Oshtorninan region (Lorestan province). Soil sampling was done in 45 points in three land use and two depths. Some soil physicochemical and different K forms were determined in laboratory. Mineralogical study was carried out with X-ray diffraction. Results showed that the mean contents of soluble (15.9 mgkg-1), non-exchangeable (627.5 mgkg-1) and structural K (3563.2 mg kg-1) in the forest park and the content of exchangeable K (484 mgkg-1) in the rangeland was higher than other land uses. All K forms (except structural K) were higher in the surface soils compared to the subsurface soils. Land use had significant effects (p < 0.01) on all K forms, but depth had no significant effect on them. Soluble K only had a positively significant correlation with exchangeable K. Positively significant correlations between structural K with exchangeable and non-exchangeable K were observed. Also, clay and cation exchange capacity had positively significant relationships with exchangeable, non-exchangeable and structural K. Mineralogical studies showed that vermiculite and smectite control the content of exchangeable K in the rangeland and the smectite and illite are important minerals that control the non-exchangeable and structural K in the forest park. The change in land use from rangeland to agricultural land may induces the long-term reduction of all K forms, while the change to forest park increases soluble, non-exchangeable and structural K forms.