Abstract

Embothrium coccineum (Proteaceae) is a pioneer endemic species from temperate forests of southern South America. This species have a root adaptation to improve phosphorus (P) acquisition called cluster roots (CR), which are formed mainly in P deficiency soils. These CR exude organic acids (OA) allowing P solubilization and improving its availability by plants. Microorganisms may consume the released OA and therefore, rhizospheric microbial populations could be strongly affected by CR and their exudates. The aims were: i) to study the growth and CR formation of E. coccineum seedlings growing in contrasting nutritional soils and, ii) To study the effect of roots exudates on microbial abundance. We conducted a pot experiment where E. coccineum seedlings were randomly assigned to growth in contrasting nutritional soils such as young volcanic (pumicite) or organic soil for two years. We evaluate growth rate (RGR), biomass distribution, and OA exuded by roots. Adittionally, the abundance of microorganisms (the amount of colony-forming unit per g of soil: CFU) able to utilize the main OA released by CR were determined in rhizospheric and non-rhizospheric (bulk) soil by plate counting method using the Angle medium amended with the suitable carbon source (glucose, malate, citrate or oxalate). The results showed that plants growing in volcanic soils have lower RGR, but higher CR formation, and high oxalic acid exudation than plants grown in organic soils. The microbial abundance was higher in the organic than in volcanic soil but in both volcanic and organic soils the CFU were higher in the rhizosphere than in bulk soil. The microorganisms utilizing oxalate were the most abundant in the rhizospheric soil, which is coincident with the highest oxalate exudation rate detected in the CR of E. coccineum plants. We conclude that E. coccineum adjusts its growth and functioning (root exudation rate) depending on each type of soil, intensifying their strategies for nutrient uptake in the poorest soil. Additionally, we observed that of E. coccineum shape the soil microbial abundance in their rhizosphere through their OA exudation