Microbial degradation is the major mechanism for removal of organic carbon from wood-waste leachate in surface flow constructed wetlands. To explore the relations of microbial biomass and heterotrophic production with hydraulic and organic loading rates as well as mass reduction rates, this study examined microbial ATP concentration and leucine incorporation rate in water, epiphytic biofilm and sediment of four surface flow mesocosm wetlands fed at different hydraulic loading rates during two periods with 10x and 3x diluted woodwaste leachate. The hydraulic loading rates from 13 to 49 mm d(-1) had no effects on microbial biomass and heterotrophic production. The organic loading rates between 32 and 396 gCOD d(-1) m(-3) had significant, negative correlations with heterotrophic production rate and microbial biomass of sediment, and positive correlations with heterotrophic production rates of water and epiphytic biofilm. Higher organic loading yields greater substrate availability for planktonic and epiphytic bacteria, and may induce inhibitory effect on sedimentary bacteria. Microbial attachment played an important role in the establishment of epiphytic and sedimentary communities. The reduction rates for chemical oxygen demand (COD), tannin and lignin (T&L), and volatile fatty acids (VFAs) were significantly correlated to the heterotrophic production rates of water, epiphytic biofilm and sediment together. (C) 2007 Published by Elsevier B.V.