Biological and physical influences on soil 14CO 2 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 14CO 2 measurements are an important tool for identifying more immediate responses to disturbance and climate change. Soil Δ 14CO 2 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 Δ 14CO 2 at a temperate forest site in northern Wisconsin, USA. We sampled 14CO 2 profiles every three weeks during snow-free months through 2012 in three intact plots and one trenched plot that excluded roots. Respired Δ 14CO 2 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 Δ 14CO 2 was variable but remained comparatively higher than in intact plots, reflecting older bomb-enriched 14C sources. Although respired Δ 14CO 2 from intact plots correlated with soil moisture, related analyses did not support a clear cause-and-effect relationship with moisture. The initial decrease in Δ 14CO 2 from spring to midsummer could be explained by increases in 14C-deplete root respiration; however, Δ 14CO 2 continued to decline in late summer after root activity decreased. We also investigated whether soil moisture impacted vertical partitioning of CO 2 production, but found this had little effect on respired Δ 14CO 2 because CO 2 contained modern bomb C at depth, even in the trenched plot. This surprising result contrasted with decades to centuries-old pre-bomb CO 2 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 Δ 14CO 2 in situ, even at depth. We propose that Δ 14CO 2 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.