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Abstract: On sites where C4-plants have replaced C3-plants, changes in soil δ13C allow the turnover of C3- and C4-derived C to be separated. Studies of decadal scale turnover of soil C following conversion to C4-plants generally lack δ13C values for previous C4-residue inputs and assume that estimates of C4-derived soil C to be based on a fixed δ13C value. Further assumptions are that changes in the initial (time-zero) soil δ13C values are insignificant following conversion to C4-plants. We tested these assumptions by measuring: 1) the δ13C of annual samples of silage maize biomass (C4-plant) and winter wheat grains (C3-plant) grown during 1988 to 2006, and 2) the δ13C of soil kept under bare fallow during 1956 to 1983. The δ13C of plants was related to climate variables, and the impact of maize δ13C was based on estimates of maize-derived soil C using different approaches to establish the δ13C in maize inputs. The δ13C of both maize and wheat decreased with time, but the rate of change and annual variations were considerably larger for wheat than for maize. Maize as well as wheat δ13C was best related to year (probably reflecting a decrease in atmospheric δ13C) and the water balance during the active growth period. Using the smallest (−12.44‰) and the largest (−11.26‰) δ13C measured during 1988 and 2006, estimates of maize-derived C in soil after 18 years ranged from 13.2% to 14.2% of the soil total C. Despite a loss of 31% of the soil C pool under bare fallow, the increase in soil δ13C was significant only at P < 0.10. We conclude that annual variations in maize δ13C values and changes in the δ13C of the soil C fraction derived from the pre-conversion C3-vegetation have only little impact on estimates of maize-derived soil C that cover a few decades. For estimates covering several decades to centuries, the subtle but consistent changes in plant and soil δ13C need to be accounted for. The variability in δ13C in wheat grains suggest that the use of any fixed δ13C value for C3-residues in estimates of C turnover in soils on which C4-plants have been replaced by C3-plants can be associated with considerable uncertainty. [Copyright &y& Elsevier]