© 2015, Springer-Verlag Berlin Heidelberg. Dead fine roots are the major component of organic carbon (C) stored in mangrove forests. We measured the mass and decomposition of fine root detritus in three mangrove forests along an intertidal gradient in tropical Australia to provide the first integrated estimates of the rate of turnover of fine root detritus. The grand mean dry masses of dead fine roots in the forests decreased in the order mid-intertidal Rhizophora (mean 28.4 kg m−2), low-intertidal Rhizophora (16.3 kg m−2) and high-intertidal Ceriops (mean 8.9 kg m−2), and were some of the highest on record. The first-order decay coefficients (day−1) for dead fine roots in the low Rhizophora, mid Rhizophora and high Ceriops forest sites were 0.0014, 0.0017 and 0.0007, respectively, and were the lowest on record. The estimated mean fluxes of C via decomposition of dead fine roots were very high in all forests, decreasing in the order mid Rhizophora (18.8 g C m−2 day−1), low Rhizophora (8.4 g C m−2 day−1) and high Ceriops (2.5 g C m−2 day−1). There were relatively low levels of uncertainty in these estimates when all sources of error were considered. The fluxes of C for the two Rhizophora sites integrate all losses from saprophytic decay and leaching of dissolved C and were 50–200 % higher than the estimated total annual loss of C derived by summing rates of bacterial metabolism and export via groundwater and surface waters in these forests. The significant difference reflects both the very high dead root masses and the incorporation of the impact of fungi in our estimates.