Root exudation and biodegradation of organic acids in a tropical forest soil under dipterocarp and pioneer trees

Root exudation of organic acids is one of strategies for tropical trees to facilitate nutrient uptake from the highly weathered soils. However, paradoxical relationship remains that root exudation also stimulates microbial activities to consume organic acids in the rhizosphere (root-soil interface). Plant-specific root exudation might shape different rhizosphere carbon (C) cycles in soils under different tree species. We test whether root exudation and rhizosphere C fluxes of organic acids and sugars differ between soils under dominant dipterocarp trees (Dipterocapus cornutus and Shorea laevis) and pioneer trees (Macaranga spp.). We measured (1) root exudation from mature trees, (2) soil solution concentrations of organic acids and monosaccharides, and (3) mineralization kinetics of 14C-radiolabelled substrates in the rhizosphere and bulk soils of the Dipterocarp and Macaranga trees. Malate was a dominant organic acid exuded from Dipterocarp roots, while monosaccharides were dominant exudates of pioneer Macaranga trees. Malate exudation rates by Dipterocarp roots were greater compared to Macaranga roots. Organic acid exudation increased with increasing root surface area and with decreasing soil pH. Microbial activities of malate mineralization were enhanced in the rhizosphere both under Dipterocarp and Macaranga trees, but the C fluxes of malate mineralization far exceeded root exudation of malate in the rhizosphere of Dipterocarp trees. Tree species develop different strategies to increase malate concentration in rhizosphere soil directly through root exudation or indirectly through rhizosphere microbial activities to increase malate production, which might be favorable for phosphorus solubilization, aluminum detoxification, and lignin degradation in acidic soils.