Background and aims Due to global warming, it is to be expected that mangroves will move north- and southwards and enter salt marshes, which will then be enriched with mangrove-derived organic compounds. Little is known about the effects that tannins, which are abundantly present in leaf litter of Rhizophora mangle, will have on the biogeochemical cycles in salt marsh soils. Methods In microcosms, soil from a Distichlis spicata salt marsh was enriched with R. mangle-derived leaf powder (LP), crude condensed tannins (CCT) and purified condensed tannins (PCT), or with commercially available tannic acid (TA). Each treatment received a fixed amount of tannins, i.e. 10 mg /g dry soil. Due to the enrichment with tannins in each fraction, the amount of carbon added increased from C < TA = PCT < CCT < LP. At Day 7, 21, 35 and 42 of the incubation period, CO2 emission rates were determined after closing the microcosms for 24 h. Net nitrogen and phosphorous mineralization rates were also determined for the entire incubation period. Experiments were done at Practical Salinity Units of 20, 28 and 35 to cover the range in salinities observed in the field (20 and 35 PSU). Results For the whole incubation period, the CO2 emission increased significantly (p < 0.05) in the order of C < PCT < CCT = TA < LP microcosms. The microcosms with LP showed immobilization of mineral nitrogen and phosphorus, while the other microcosms revealed net nitrogen and phosphorus mineralization in the order of CCT < TA < PCT < C for N mineralization, and CCT = TA < C < PCT for P mineralization. The C and N mineralization rates measured at a salinity of 20 PSU were significantly larger than at 28 and 35 PSU. Conclusion The results showed that mangrove leaf litter stimulated net C mineralization and N and P immobilization. Purified condensed tannins of senescing R. mangle leaves enhanced C and P, and suppressed net N mineralization.