Transpiration alters the contribution of autotrophic and heterotrophic components of soil CO2 efflux.

• An unbiased partitioning of autotrophic and heterotrophic components of soil CO(2) efflux is important to estimate forest carbon budgets and soil carbon sequestration. The contribution of autotrophic sources to soil CO(2) efflux (F(A)) may be underestimated during the daytime as a result of internal transport of CO(2) produced by root respiration through the transpiration stream. • Here, we tested the hypothesis that carbon isotope composition of soil CO(2) efflux (δ(FS)) in a Eucalyptus plantation grown on a C(4) soil is enriched during the daytime, which will indicate a decrease in F(A) during the periods of high transpiration. • Mean δ(FS) of soil CO(2) efflux decreased to -25.7‰ during the night and increased to -24.7‰ between 11:00 and 15:00 h when the xylem sap flux density was at its maximum. • Our results indicate a decrease in the contribution of root respiration to soil CO(2) efflux during the day that may be interpreted as a departure of root-produced CO(2) in the transpiration stream, leading to a 17% underestimation of autotrophic contribution to soil CO(2) efflux on a daily timescale.