Micromorphological studies of long-term paddy rice cultivation: Emphasis on Fe-Mn sesquioxide pedofeatures

Rice is a major agricultural crop cultivated under conditions of prolonged flooding and water saturation. This study aims to investigate the morphology and micromorphology of paddy field soils and to examine the effects of waterlogging on soil characteristics, with particular emphasis on Fe–Mn sesquioxide features. The study area, covering approximately ۷۵,۰۰۰ hectares, is the Silakhor Plain located in the northeastern part of Lorestan Province, Iran. Landforms in the region include hills, piedmont plains, fans, floodplains, alluvial plains, and lowlands. The primary land uses are pasture and agriculture. Rice is the dominant crop, followed by alfalfa as the second most prevalent crop. Morphological, physicochemical, and micromorphological properties were investigated in seven representative pedons cultivated with rice and located in the floodplain and alluvial plain. After description and sampling, the soils were classified as Inceptisols according to Soil Taxonomy. Physical and chemical analyses showed that long-term rice cultivation increased the clay content of the surface horizons in all soil pedons and reduced soil cation exchange capacity (CEC). The dominant pedofeatures consisted of Fe–Mn sesquioxide coatings and hypocoatings, which increased in abundance with depth and covered much of the soil matrix. Other sesquioxide pedofeatures included loose discontinuous, dense complete, and dense incomplete infillings within voids and along channel walls. Redox pedofeatures were identified where oxidized Fe and/or Mn accumulated in the matrix as nodules or as Fe/Mn oxide hypocoatings or coatings along voids or coarse mineral grains, resulting from changes in the oxidation state of these elements. Calcite nodules of micrite size (< ۵ μm), exhibiting typical and geodic fabrics, were also observed and were predominantly impregnated with Fe oxides. The results showed that most voids were planar, followed by channel and chamber voids, respectively, and that the quantity of voids decreased with increasing depth. Continuous rice cultivation was found to negatively affect soil structure and reduce porosity. Therefore, an alternating cultivation system is recommended to improve soil quality and maintain sustainable productivity.

کلیدواژه ها:

B-Fabric microstructure ، Rice ، Voids