Effects of tree species on root exudation and mineralization of organic acids in a tropical forest

Habitat differentiation of diverse tropical tree species is supported by different root strategies of nutrient acquisition from the highly weathered soils. In the rhizosphere (soil affected by roots), tree species modify carbon (C) and nutrient cycles directly through root exudation and indirectly through increased microbial activity. We test whether root exudation and rhizosphere C fluxes of organic acids and sugars differ between dominant dipterocarp trees and pioneer trees (Macaranga gigantea). To quantify the C fluxes of organic acids in the rhizosphere soils, we measured in situ root exudation from mature trees, concentrations of organic acids (acetate, oxalate, malate, and citrate) and monosaccharides in the rhizosphere and bulk soil fractions, and mineralization kinetics of 14C-radiolabelled substrates. Dipterocarp roots release greater amounts of malate than pioneer tree roots, while monosaccharides are dominant exudates of pioneer trees. Organic acid exudation increases with in...