Stable isotope analysis of water extraction from subsoil in upland rice (Oryza sativa L.) as affected by drought and soil compaction

Water extraction from subsoil in upland rice (Oryza sativa L.) was examined as related to topsoil desiccation and subsoil compaction. The water extraction was observed by measurements of heavy water concentrations in transpiring plants. The plants were grown in pots that were filled with sandy soil and vertically compartmented into two columns. Heavy water was applied to the subsoil. Plants exposed to mild topsoil desiccation (-120 kPa in water potential) eventually increased water extraction from the subsoil and maintained photosynthetic rate and stomatal conductance at the wet condition level. The rates of the plants subjected to severely droughted topsoil (-190 kPa) were significantly lowered due to less water uptake from the subsoil. Subsoil compaction at bulk densities of 1.45 and 1.50 Mg m(-3) inhibited increase of root length densities. Limited water extraction from the subsoil was insufficient to maintain plant productivity under drought conditions. Daily water uptake per unit of root length in the lower tube did not apparently increase even if water demand on the unit root length increased. When water to topsoil was completely withheld, water extraction from the subsoil gradually increased as the topsoil dried out. Plants that were watered and rewatered took up very little water from the subsoil. The extraction from the subsoil occurred only when water potential of the topsoil was below about -190 kPa.