Pedogenic evolution of paddy soils in different soil landscapes

Paddy soils are Hydragric Anthrosols and are an important soil resource for food production. They are widely distributed in China and Asia. Soils that can be used for paddy cultivation vary considerably but basically are grouped into three kinds of landscape, i.e., well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained polder areas with a near-surface water table. Typical soil chronosequences were studied to clarify the pedogenic changes after wetland rice cultivation, especially in relation to management practices. All of them are in important rice production regions of China. The study focussed on redox condition and iron oxides dynamics. On alluvial plains, the soil conditions range from continuously reducing controlled by high groundwater to alternating reduction and oxidation because of artificial submergence and groundwater fluctuations. Artificially induced leaching losses, previously prevented by a high groundwater table, are partly compensated during paddy cultivation by the rejuvenation process of alluvial deposition. Soils on an older delta, which no longer receives river-sediments, are more strongly developed and showed a decrease of DCB-extractable iron in surface horizon of young paddy soils and more profoundly throughout the profile in old paddy soils. In the polder area, the apparent lowering of groundwater table by deposition of sediment has changed the soil hydrology and increased redox potential. With the better soil aeration, crystalline iron oxides and their ratio to total iron increase with cultivation age. On well-drained terraces, the change of soil moisture regime is overwhelming. Previously well-aerated upland soils are periodically reduced, which accelerates leaching and loss of iron oxides from surface horizons, and probably clay decrease by lateral movement and ferrolysis processes superimposed on the difference of parent soils. An iron-enriched subsurface horizon has formed rapidly and it is a diagnostic feature of this type of paddy soil. (C) 2003 Elsevier Science B.V. All rights reserved.