We compared exudation and rhizosphere microbial activity of two macrophytes growing in tropical marshes. Eleocharis spp. are adapted to low nutrient level in phosphorus limited conditions, while Typha domingensis is a strong competitor in nutrient enriched areas. In situ measurements of carbon fluxes from roots to interstitial water and C-13 partitioning after pulse-labelling of the plants in a mesocosm experiment were used to estimate root-derived C fluxes to rhizosphere under P limited and enriched conditions. Root-released compounds collected in the field were analysed for dissolved organic C, dissolved nitrogen and their biodegradability was characterized through microbial respiration, N mineralization and phosphatase activity. Independent of P loading, Eleocharis released more C from roots than T. domingensis, and the released compounds were more biodegradable. The two species responded to P enrichment differently. While Eleocharis invested more assimilated C-13 to the belowground (roots, rhizomes and rhizodepositions) after P fertilization, in T. domingensis the belowground investment decreased. The effect of plant species on belowground C allocation was larger than the effect of P enrichment. Low nutrients adapted Eleocharis invested more carbon into exudation and promotion of its rhizosphere microbial community while competitive T. domingensis spent more fixed carbon on its own growth and metabolism.