While radiocarbon (14C) abundances in standing stocks of soil carbon have been used to evaluate rates of soil carbon turnover on timescales of several years to centuries, soil-respired 14CO2 measurements are an important tool for identifying more immediate responses to disturbance and climate change. Soil Δ14CO2 data, however, are often temporally sparse and could be interpreted better with more context for typical seasonal ranges and trends. We report on a semi-high-frequency sampling campaign to distinguish physical and biological drivers of soil Δ14CO2 at a temperate forest site in northern Wisconsin, USA. We sampled 14CO2 profiles every three weeks during snow-free months through 2012 in three intact plots and one trenched plot that excluded roots. Respired Δ14CO2 declined through the summer in intact plots, shifting from an older C composition that contained more bomb 14C to a younger composition more closely resembling present 14C levels in the atmosphere. In the trenched plot, respired Δ14CO2 was variable but remained comparatively higher than in intact plots, reflecting older bomb-enriched 14C sources. Although respired Δ14CO2 from intact plots correlated with soil moisture, related analyses did not support a clear cause-and-effect relationship with moisture. The initial decrease in Δ14CO2 from spring to midsummer could be explained by increases in 14C-deplete root respiration; however, Δ14CO2 continued to decline in late summer after root activity decreased. We also investigated whether soil moisture impacted vertical partitioning of CO2 production, but found this had little effect on respired Δ14CO2 because CO2 contained modern bomb C at depth, even in the trenched plot. This surprising result contrasted with decades to centuries-old pre-bomb CO2 produced in lab incubations of the same soils. Our results suggest that root-derived C and other recent C sources had dominant impacts on respired Δ14CO2 in situ, even at depth. We propose that Δ14CO2 may have declined through late summer in intact plots because of continued microbial turnover of root-derived C, following declines in root respiration. Our results agree with other studies showing declines in the 14C content of soil respiration over the growing season, and suggest inputs of new photosynthates through roots are an important driver.