Hydropedological Implications of Ferromanganiferous Nodules in Rice‐Growing Plinthitic Ultisols under Different Moisture Regimes

Abstract

Rice ( Oryza sativa L.)‐growing soils with plinthite and abundant ferromanganiferous nodules (≥60 g kg −1 ) were selected to evaluate the formation mechanisms of ferromanganiferous nodules and to determine the relationships among the characteristics of ferromanganiferous nodules under different moisture regimes in soils. Depth to water table, water potential, and redox potential (Eh) were monitored at different soil depths during 2004 and 2005. The results show that the greatest quantity of ferromanganiferous nodules (91–492 g kg −1 ) were found in plinthic horizons, which had reducing conditions for a moderate amount of time (about 47% of the year). This was especially true for nodules >20 mm in diameter. Micromorphological observation of ferromanganiferous nodules revealed that co‐presentation of secondary Fe and Mn oxides and oriented clay is more apparent in coarser nodules than finer nodules. Based on the micromorphological observation, we deduced that nodules were formed when Mn(II) first precipitated within micropores or coated the coarse nuclear grain surface. Later, reduced Fe, Mn, and illuvial clay continued reoxidizing and precipitating on the initial Mn nodules, which then grew to a larger size. The Fe activity ratio (oxalate‐ to dithionite‐extractable Fe) is suggested as a good index for prediction of the annual duration of reducing conditions in the soils examined in this study. Furthermore, the dithionite‐extractable Mn/Fe ratio in nodules is a good indicator for prediction of the soil depths where the water table fluctuated, because Mn accumulated and was well crystallized at these depths.