Soil Respiration Controls Ionic Nutrient Concentration In Percolating Water In Rice Fields

Soil water in the plow layer in rice fields contains various kinds of cations and anions, and they are lost from the plow layer by water percolation. Some portions of CO₂ produced by respirations of rice roots and soil microorganisms are also leached by water percolation to the subsoil layer as HCO₃^-. As the electrical neutrality of inorganic substances in percolating water is maintained when they are assumed to be in the form of simple cations and anions, soil respiration accelerates the leaching of ionic nutrients from the plow layer by water percolation. The proportion of inorganic carbon (Σ CO₂) originated from photosynthates in the total Σ CO₂ in soil solution in the plow layer was from 28 to 36 % in the rice straw amended soil and from 16 to 31 % in the soil without rice straw amendment in a soil pot experiment with rice plant after the maximum tillering stage. Most of Σ CO₂ in percolating water from the plow layer accumulates in the subsoil layer. Periodical measurement of Σ CO₂ in percolating water at 13 and 40 cm soil depths indicated that 10 % of total soil organic C in the plow layer was leached down from the plow layer (13 cm), and that about 90 % of it was retained in the subsoil layer to the depth of 40 cm. Water soluble organic materials are also leached from the plow layer by water percolation, and the leaching is accelerated by soil reduction. Soil reduction decreased the content of organic materials that were bound with ferric iron in soil (extractable by 0.1M Na₄P₂O₇ + NaBH₄) and increased the content of organic materials that were extractable by the neutral chelating solution (0.1M Na₄P₂O₇). In addition, water percolation transformed the latter organic materials to those that were extractable by water and a neutral salt. Considerable portions of organic materials in percolating water are adsorbed in the subsoil layer, and then partially decomposed and polymerized to specific soil organic materials in the subsoil. Organic materials that were leached from the plow layer by percolating water amounted to 170 kgC ha^-1 in a Japanese rice field, among which 120 kgC of organic materials were adsorbed in the subsoil layer between 13 and 40 cm depth.