Quantifying the net carbon (C) storage of forest plantations is required to assess their potential to offset fossil fuel emissions. In this study, a biometric approach was used to estimate net ecosystem productivity (NEP) for two monoculture plantations in South China: Acacia crassicarpa and Eucalyptus urophylla . This approach was based on stand-level net primary productivity (NPP, based on direct biometric inventory) and heterotrophic respiration ( R h ). In comparisons of R h determination based on trenching vs. tree girdling, both trenching and tree girdling changed soil temperature and soil moisture relative to undisturbed control plots, and we assess the effects of corrections for disturbances of soil moisture and soil moisture on the estimation of soil CO 2 efflux partitioning. Soil microbial biomass and dissolved organic carbon were significantly lower in trenched plots than in tree girdled plots for both plantations. Annual soil CO 2 flux in trenched plots ( R h-t ) was significantly lower than in tree-girdled plots ( R h-g ) in both plantations. The estimates of R h-t and R h-g , expressed as a percentage of total soil respiration, were 58<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>4% and 74<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>6%, respectively, for A. crassicarpa , and 64<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>3% and 78<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>5%, respectively, for E. urophylla . By the end of experiment, the difference in soil CO 2 efflux between the trenched plots and tree-girdled plots had become small for both plantations. Annual R h (mean of the annual R h-t and R h-g ) and net primary production (NPP) were 470<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>25 and 800<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>118<ce:hsp sp="0.25"/>g<ce:hsp sp="0.25"/>C<ce:hsp sp="0.25"/>m −2 <ce:hsp sp="0.25"/>yr −1 , respectively, for A. crassicarpa , and 420<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>35 and 2380<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>187<ce:hsp sp="0.25"/>g<ce:hsp sp="0.25"/>C<ce:hsp sp="0.25"/>m −2 <ce:hsp sp="0.25"/>yr −2 , respectively, for E. urophylla . The two plantations in the developmental stage were large carbon sinks: NEP was 330<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>76<ce:hsp sp="0.25"/>C<ce:hsp sp="0.25"/>m −2 <ce:hsp sp="0.25"/>yr −1 for A. crassicarpa and 1960<ce:hsp sp="0.25"/>±<ce:hsp sp="0.25"/>178<ce:hsp sp="0.25"/>g<ce:hsp sp="0.25"/>C<ce:hsp sp="0.25"/>m −2 <ce:hsp sp="0.25"/>yr −1 for E. urophylla .