Investigation of Changes in Clay Minerals and Soil Manganese Status in the Surface and Subsurface of Paddy Soils with Long-term Rice Cultivation in Fars Province

Waterlogging is an important factor, affecting soil properties. This research was conducted to evaluate the effect of waterlogging on secondary soil clay minerals, as well as manganese (Mn) of paddy soils with long-term rice cultivation in Fars province. In each region, two soil profiles in paddy and non-paddy fields were digged on calcareous parent materials and the same landform in a pairwise manner. Analysis of the clay mineralogy indicated that the long-term rice cultivation seems to have an influence on the quantity of clay minerals, as indicated by higher smectite in paddy soils. But, higher chlorite, illite and palygorskite was found in non-paddy soils. Clay minerals were probably affected more by parent materials and less by the aquatic condition. Chlorite and illite were observed in both paddy and non-paddy soils and increased with depth due to their presence in parent rocks. Transformation of illite to smectite in the A horizon increased relative abundance of smectite but it decreased with depth. The results showed that the paddy soils have more available Fe and Mn (extracted by DTPA), total Fe and Mn (extracted by HNO3), and poorly crystalline Fe and Mn oxides (extracted by Ammonium oxalate), compared to the non-paddy soils and the surface horizons of paddy soils showed the highest rates. Also, the long-term cultivation of rice decreased the content of pedogenic (extracted by Citrate-Bicarbonate-Dithionite) and crystalline Mn oxides.