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

Tropical forests can develop by roots foraging nutrients in the highly weathered soils. In rhizosphere, soil volume 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 monosaccharides and organic acids (acetate, oxalate, malate, and citrate) in the rhizosphere and bulk soil fractions, and mineralization kinetics of ¹⁴C-radiolabelled substrates. Organic acid exudation increases with increasing root surface area. Dipterocarp roots release greater amounts of malate, while monosaccharides are dominant exudates of pioneer trees. Microbial activities of malate mineralization increase in the rhizosphere soil both under dipterocarp and pioneer trees. The greater C fluxes of malate mineralization, compared to root exudation, suggests rhizosphere microbes are another malate producer under dipterocarp trees. Both root exudation composition and rhizosphere microbes increase malate production with increasing phosphorus demands and with increasing soil acidity.