Biological And Physical Influences On Soil 14CO2 Seasonal Dynamics In A Temperate Hardwood Forest

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.