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Biological and physical influences on soil ¹⁴CO₂ seasonal dynamics in a temperate hardwood forest

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  • 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.
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  • Phillips, C. L., McFarlane, K. J., Risk, D., and Desai, A. R.: Biological and physical influences on soil ¹⁴CO₂ seasonal dynamics in a temperate hardwood forest, Biogeosciences, 10, 7999-8012. doi:10.5194/bg-10-7999-2013, 2013.
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  • 10
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  • 12
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  • This work wasperformed under the auspices of the US Department of Energyby Lawrence Livermore National Laboratory under contractDE-AC52-07NA27344, with support from Lawrence LivermoreNational Laboratory (LDRD 11-ERD-053) and the WisconsinFocus on Energy Environmental and Economic Research andDevelopment (EERD) grant # 10-06. LLNL-JRNL-637140.
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