The clay mineralogy of soils and of the main calcareous sedimentary parent rocks of southern Iran were investigated to determine their origin and factors controlling their distribution pattern in soils. The results revealed that the soil-available moisture plays the major role in the distribution pattern of palygorskite and smectite clay minerals in the arid and semi-arid areas studied. There is an inverse correlation between palygorskite and smectite with regard to the soil-available moisture as expressed by P/ETº (ratio of mean annual precipitation to mean annual reference crop evapotranspiration). At P/ETº values >0.4 palygorskite transforms to smectite. Smectite is thought to be mainly of ‘transformed’ origin. It is detected in trace amounts in soils of more arid areas and increases in soils having greater available moisture. The general decrease in illite content with depth is related mainly to its transformation to smectite under favourable moisture conditions of the deeper horizons. Palygorskite is considered to be inherited in plateau soils of the arid regions whereas in saline and alkaline soils and soils with high gypsum, it is mainly of authigenic origin. The P/ETº and gypsum content show a significant correlation with the palygorskite content. The occurrence of kaolinite in some soils is due to its inheritance from the surrounding kaolinite-bearing Cretaceous rocks. Illite and chlorite abundance in soils is also largely related to their presence in parent rocks. The rare occurrence of vermiculite in the studied calcareous soils is mainly related to its lower stability under high pH, low Al activity and the presence of large amounts of Si and Mg in soils.

Soil quality is a concept that integrates soil biological, chemical, and physical factors into a framework for soil resource evaluation (Karlen et al., 1997). Larson and Pierce (1991) define soil quality as the capacity of a soil to function within the ecosystem boundaries and to interact positively with surrounding ecosystems. Conversion of forest and grasslands into agricultural land is one of the main concerns worldwide in the context of environmental degradation and global climate change (Wali et al., 1999). Conversion of natural land resources to crop production as the largest source of anthropogenic carbon emissions after fossil fuel burning, has resulted in the release of about 200 Pg C over the past 250 years, globally (Scholes and Noble, 2001; Fitzsimmons et al., 2004). It is also very well known that cultivation of the natural land resources brings about the loss of OM and this directly affects the soil chemical, physical, and biological properties resulting in loss of

No abstract

No abstract

No abstract