Mineral weathering and silicon uptake by rice plants promote carbon storage in paddy fields

Abstract

ABSTRACT Increasing carbon storage in soil is a potential measure against global warming, but the mechanisms of carbon accumulation in soil are not well understood. Clarifying the mechanism would help in the development of new methods for soil carbon storage, such as in agricultural systems. We grew high-biomass forage rice cultivars for 11 years under conditions of potassium and silicic acid deficiency. Rice cultivars were healthy, without signs of potassium or silicic acid deficiency. The quantities of potassium and silicic acid absorbed by the rice plants were greater than those of available forms in the soil, fertilizer, and irrigation water, indicating that mineral weathering promoted the release of potassium, silicic acid, and aluminum from primary minerals. Furthermore, the carbon bound to active aluminum increased by 69.4 kg C ha−1 year−1 during 11 years of paddy rice cultivation. Although organic acids secreted by rice roots have been expected to be a factor in the strong mineral weathering effect in the rhizosphere, an only extremely small amount of organic acids were detected from rice roots. Therefore, organic acids are not a factor in promoting mineral weathering. On the other hand, leaching of potassium and silicic acid is observed in the contact reaction between the mineral powder and the cell wall prepared from rice root. These findings show that the primary minerals could be reacted with chelating sites on the cell wall of the rice root surface to dissolve potassium, aluminum, and silicic acid. The rice plants absorb potassium and a large amount of silicic acid under potassium and silicic acid deficient conditions. Then, the highly active aluminum remained in the rhizosphere, where it bound to organic matter, producing persistent soil carbon. Plants that can absorb silicic acid vigorously have a role of increasing active aluminum through mineral weathering, resulting contribute to carbon accumulation in soil.