Abstract The effect of P and Si additions on P fluxes and microbial activity in sediment were examined using 33P-labeled surface sediment samples which were enriched with 32P or 32P+Si. The distribution of both P-isotopes between the interstitial water and operationally defined solid-phase P fractions was followed for 15 days. The bulk of the 33P sediment label was adsorbed to particles, since only 0.02% of the activity was recovered in the interstitial water after the 4-day pre-equilibration before the enrichment treatments. The Si addition promoted the desorption of 33P to the interstitial water and, accordingly, retarded the adsorption of 32P from solution to solid-phase. The increase in labile P, measured with anion exchange resin-impregnated membranes, indicated that the availability of P was enhanced immediately after the Si enrichment as well as at the end of the experiment. As a result of the Si addition, the production of organic P compounds by bacteria increased (mainly bound to bacterial cells) and the phosphatase activity decreased, demonstrating increased bioavailability of P. The chemical competition for sorption sites between P and Si, observed for soils, seems to be valid also for sediments.