Plant silicon uptake increases active aluminum minerals in root-zone soil: Implications for plant influence on soil carbon

Abstract

Aluminum (Al) minerals form a strong complex with organic matter (OM) in soil, affecting the stability and degradability of OM and carbon (C) dynamics in soil. However, plant effects on the accumulation of Al minerals (particularly Al hydroxide minerals) that are not crystallized and actually or potentially associated with OM (active Al minerals) in soil have received little attention. Al also interacts with silicon (Si) in soil to form aluminosilicate minerals, and plants remove Si from soil through Si uptake. Plant Si uptake may thus lead to an increase in active Al minerals in soil because of a reduced influence of Si in soil and the resulting reduced formation of aluminosilicate minerals. To test whether plant Si uptake changes active Al minerals in root-zone soil, we performed two cultivation experiments using plants varying in Si uptake and compared changes in soil mineral indices during cultivation, including soil extractable Al concentrations in root-zone soils. In the first experiment, using five plant species varying in Si uptake, plant species with greater Si uptake, in particular grass species, showed higher pH (NaF) and CuCl2-extractable Al in their root-zone soils, suggesting that plant Si uptake increases the amount of active Al in soil. This suggestion was supported by the second experiment, using two types of rice (Oryza sativa), a wild type and a mutant defective for a Si transporter. The results showed that pH (NaF) and extractable Al in root-zone soil decreased under the low-Si-uptake mutant, especially in the vicinity of their roots. These observations suggest that Si uptake by plant roots leads to an increase in active Al minerals, possibly enhancing OM retention and C accumulation in root-zone soil and may partly explain why grassland soils are often rich in C.