Effects of weathering and silicic acid uptake by rice plants on weathered products -Quantitative changes in potassium, silica, and aluminum in granitic Fluvisol-

Abstract

ABSTRACT Potassium (K) fertilizers have been widely used to promise agricultural production, yet rice (Oryza sativa L.) showed healthy growth for long periods under K-deficient conditions. These findings imply that rice could take up minerals-bound K as non-exchangeable K. When the weathering of primary silicate minerals occurs in the rhizosphere of rice, the rice takes up not only dissolved K but also dissolved silicic acid (SiO2). We grew three rice cultivars with different Si-uptake abilities (Hokuriku 193, Oochikara, and Lsi-1) in granitic Fluvisol under K- and Si-deficient conditions to quantitatively reveal the significance of the Si-uptake by rice for the mineral weathering in the rhizosphere. As a result, the Si-uptake quantity by rice was positive linearly correlated with the amount of Al accumulation in soil, suggesting a substance containing a certain ratio of Si and Al was decomposed by rice. The K:Al:Si molar ratio of the decomposed substance by rice was 0.2:1:2.6, and the ratio is suggestive of silicate minerals such as K-feldspar. Because silicate minerals such as K-feldspar (e.g., KAlSi3O8) contain a large amount of aluminum (Al), Al release alongside K ions and silicic acid in their weathering. Dissolved Al is highly active and can be stabilized by combining with organic matter (OM) or silicic acid in the rhizosphere. The Si-uptake levels and mineral decomposing abilities of rice affected the variety of the accumulated substances (i.e., Al hydroxy oxide with OM, clay minerals) in the soil after cultivation. In particular, the Al hydroxy oxide with OM accumulates as persistent soil organic carbon. Thus, it appears that the Si-uptake abilities of plants have a great influence on the weathering of minerals and their products.