Simulation of silicon leaching from flooded rice paddy soils in the Red River Delta, Vietnam

Leaching of silicon (Si) has been recognized as an important mechanism for diminution of soil plant-available Si budgets. However, loss of dissolved Si via transport in flooded paddy fields is poorly studied. Here Si transport in a 100-? deep soil profile of a typical rice-based cropping system of the Red River Delta(Vietnam) is modelled to provide estimates of fluxes of Si under waterlogged conditions and to suggest a staregy for Si fertilization. By measuring Si content of the rice straw and rice yield, it was predicted that 1 ton ha-1 of Si can be annually cycled and contribute to amorphous Si (ASi) pools. A low ratio of the soil ASi and total Si content(1/90) provides evidence for Si leaching under waterlogged condition in the paddy soil. Infiltration rates of Si between top- and bottom layers were found to decrease from 2.5 to 0.6 cm day-1, depending on soil hydraulic properties. Arrival of Si at lower boundary of a 100-cm soil domain can be observed after 75 days. Approximately 10 ton ha-1, which is about one order-of-magnitude higher than that potentially taken up by rice was predicted to be annually lost by leaching. It can be concluded that a strong deficiency of Si might occur near the end of the flooding period, and a peak application of Si at the same time is necessary to insure optimum supply of Si for rice.