Changes of microbial functional capacities in the rhizosphere contribute to aluminum tolerance by genotype-specific soybeans in acid soils

Changes in root exudates and rhizospheric microbial functional capacities involved in carbon (C) decomposition and nitrogen (N) transformation of aluminum (Al)-tolerant and Al-sensitive soybean genotypes were investigated under Al stress. Rhizosphere soils were collected from rhizobox systems with two soybean genotypes. A wide range of microbial functional capacities were determined by functional gene arrays. Microbial functional capacities of rhizospheres were distinct between Al-tolerant and Al-sensitive genotypes, which were related to exchangeable Al concentrations. Functional capacities associated with decomposition of chemically recalcitrant C were lower in Al-tolerant than Al-sensitive genotypes, which was negatively correlated with the secretion rate of succinic acid. Microbial functional capacities were higher for nitrification but lower for denitrification in the rhizosphere of Al-tolerant than Al-sensitive genotype, which coincided with higher succinic acid secretion and lower abundance of microbial oxygen stress genes. Soybean biomass increased logarithmically with nitrification capacities. We concluded that exchangeable Al concentrations, root exudation, and microbial C and N cycling capacities were intrinsically linked, which served as a key mechanism for legume genotypes toward enhancing tolerance to Al stress occurring in acid soils.