Rice-wheat rotation and poplar afforestation are two typical land use types in the coastal reclaimed flatlands of eastern China.This study investigated two rice-wheat rotation lands(one reclaimed from 1995 to 2004 and cultivated since 2005, RW1, and the other reclaimed from 1975 to 1995 and cultivated since 1996, RW2) and a poplar woodland(reclaimed from 1995 to 2004 and planted in2004, PW1) to determine the effects of land use types and years of cultivation on soil microbial biomass and mineralizable carbon(C) in this coastal salt-affected region. The results showed that the soil in PW1 remained highly salinized, whereas desalinization was observed in RW1. The total organic C(TOC) in the top soil of PW1 and RW1 did not show significant differences, whereas at a soil depth of 20–30 cm, the TOC of RW1 was approximately 40%–67% higher than that of PW1. The TOC of 0–30-cm soil in RW2 was approximately 37% higher than that in RW1. Microbial biomass C(MBC) and mineralizable C(MNC) exhibited the trend of RW2 > RW1 > PW1. Sufficient nutrition with more abundant C substrates resulted in higher MBC and MNC, and soil respiration rates were negatively correlated with C/N in RW1 and RW2. Nutrient deficiency and high salinity played key roles in limiting MBC in PW1. These suggested that rice-wheat rotation was more beneficial than poplar afforestation for C accumulation and microbial biomass growth in the coastal salt-affected soils. Rice-wheat rotation and poplar afforestation are two typical land use types in the coastal reclaimed flatlands of eastern China. This study investigated two rice-wheat rotation lands (one reclaimed from 1995 to 2004 and cultivated since 2005, RW1, and the other reclaimed from 1975 to 1995 and cultivated since 1996, RW2) and a poplar woodland (reclaimed from 1995 to 2004 and planted in 2004, PW1) to determine the effects of land use types and years of cultivation on soil microbial biomass and mineralizable carbon (C) in this The total organic C (TOC) in the top soil of PW1 and RW1 did not show significant differences, but at at soil The TOC of RW1 was approximately 40% -67% higher than that of PW1. The TOC of 0-30-cm soil in RW2 was approximately 37% higher than that in RW1. Microbial biomass C (MBC ) and mineralizable C (MNC) exh ibited the trend of RW2> RW1> PW1. Sufficient nutrition with more abundant seed substrates in higher MBC and MNC, and soil respiration rates were negatively correlated with C / N in RW1 and RW2. Nutrient deficiency and high salinity played key roles in limiting MBC in PW1. These suggested that rice-wheat rotation was more beneficial than poplar afforestation for C accumulation and microbial biomass growth in the coastal salt-affected soils.