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
increasing root surface area and with decreasing soil pH. Microbial
activities of malate mineralization are enhanced in the rhizosphere both
under Dipterocarp and Macaranga trees, but the C fluxes of malate
mineralization by rhizosphere microbes exceeded root exudation due to
microbial malate production in rhizosphere of Dipterocarp trees. Tree
species affects both root exudation composition and rhizosphere microbe
activities that increase malate production at lower soil pH, likely for
phosphorus solubilization, aluminum detoxification, and lignin
degradation.