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

Other literature type, Article English OPEN
C. L. Phillips ; K. J. McFarlane ; D. Risk ; A. R. Desai (2013)
  • Publisher: Copernicus Publications
  • Journal: Biogeosciences (issn: 1726-4170, eissn: 1726-4189)
  • Related identifiers: doi: 10.5194/bg-10-7999-2013
  • Subject: Ecology | QH540-549.5 | QE1-996.5 | QH501-531 | Geology | Life

While radiocarbon (<sup>14</sup>C) 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 <sup>14</sup>CO<sub>2</sub> measurements are an important tool for identifying more immediate responses to disturbance and climate change. Soil Δ<sup>14</sup>CO<sub>2</sub> 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<sup>14</sup>CO<sub>2</sub> at a temperate forest site in northern Wisconsin, USA. We sampled <sup>14</sup>CO<sub>2</sub> profiles every three weeks during snow-free months through 2012 in three intact plots and one trenched plot that excluded roots. Respired Δ<sup>14</sup>CO<sub>2</sub> declined through the summer in intact plots, shifting from an older C composition that contained more bomb <sup>14</sup>C to a younger composition more closely resembling present <sup>14</sup>C levels in the atmosphere. In the trenched plot, respired Δ<sup>14</sup>CO<sub>2</sub> was variable but remained comparatively higher than in intact plots, reflecting older bomb-enriched <sup>14</sup>C sources. Although respired Δ<sup>14</sup>CO<sub>2</sub> 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<sup>14</sup>CO<sub>2</sub> from spring to midsummer could be explained by increases in <sup>14</sup>C-deplete root respiration; however, Δ<sup>14</sup>CO<sub>2</sub> continued to decline in late summer after root activity decreased. We also investigated whether soil moisture impacted vertical partitioning of CO<sub>2</sub> production, but found this had little effect on respired Δ<sup>14</sup>CO<sub>2</sub> because CO<sub>2</sub> contained modern bomb C at depth, even in the trenched plot. This surprising result contrasted with decades to centuries-old pre-bomb CO<sub>2</sub> 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<sup>14</sup>CO<sub>2</sub> in situ, even at depth. We propose that Δ<sup>14</sup>CO<sub>2</sub> 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 <sup>14</sup>C content of soil respiration over the growing season, and suggest inputs of new photosynthates through roots are an important driver.
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