Biological and physical influences on soil <sup>14</sup>CO<sub>2</sub> seasonal dynamics in a temperate hardwood forest

Abstract

Abstract. While radiocarbon ( 14 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 14 CO 2 measurements are an important tool for identifying more immediate responses to disturbance and climate change. Soil 14 CO 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 14 CO 2 at a temperate forest site in northern Wisconsin, USA. We sampled 14 CO 2 profiles every three weeks during snow-free months through 2012 in three intact plots and one trenched plot that excluded roots. Respired 14 CO 2 declined through the summer in intact plots, shifting from an older C composition that contained more bomb 14 C to a younger composition more closely resembling present 14 C levels in the atmosphere. In the trenched plot, respired 14 CO 2 was variable but remained comparatively higher than in intact plots, reflecting older bomb-enriched 14 C sources. Although respired 14 CO 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 14 CO 2 from spring to midsummer could be explained by increases in 14 Cdeplete root respiration; however, 14 CO 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 14 CO 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 14 CO 2 in situ, even at depth. We propose that 14 CO 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 14 C content of soil respiration over the growing season, and suggest inputs of new photosynthates through roots are an important driver.

Distinct fungal and bacterial δ13C signatures as potential drivers of increasing δ13C of soil organic matter with depth

“…A mobile SOC fraction percolating through horizons also could provide such a substrate source, and our field sites are characterized by relatively high precipitation and exhibit significant vertical DOC fluxes from the organic to the mineral horizons (10-30 g C m -2 year -1 ; Edwards unpublished data). This is congruent with reports that in situ heterotrophic soil respiration can be derived from modern C even in deep mineral soil horizons containing centuries old SOC and even when root inputs were excluded (Phillips et al 2013). Substrate transport across soil horizons can also occur via fungal mycelia (Strickland and Rousk 2010) and perhaps via actinobacterial mycelia, but this mechanism is unlikely in the B horizon where we detected only marginal amounts of fungal or actinobacterial PLFA.…”

“…Such stable isotope probing experiments typically require severe manipulations of the soils, including the removal of the soils from their in situ location, homogenization, addition of substrates often above ambient concentrations, and the exclusion of many C sources available to soil microorganisms in situ, like litter leachates or root exudates. The patterns of C cycling found in such experiments therefore can differ drastically from in situ measurements of the same soil (Phillips et al 2013).…”

Observations of 14CO2 in ecosystem respiration from a temperate deciduous forest in Northern Wisconsin

“…These results and the associated methods were reported in detail by Phillips et al . [], and relevant comparisons to atmospheric measurements and the CASA model are presented here. Soil respiration was measured using forced diffusion chambers (FD, Forerunner Research, Dartmouth, NS, Canada) coupled with Vaisala GMP343 CO 2 sensors (Vaisala Corp, Helsinki, Finland), a technique described in Risk et al .…”

“…[]. Direct field measurements of respired Δ 14 CO 2 from terrestrial ecosystems are few, however, and have focused primarily on soil respiration [ Czimczik et al ., ; Gaudinski et al ., ; Phillips et al ., ; Randerson et al ., ; Schuur and Trumbore , ].…”

“…We argue, however, that there are at least two problems focusing on heterotrophic respiration to constrain terrestrial ecosystem C turnover. The first is that the Δ 14 CO 2 of heterotrophic respiration can only be isolated through disturbance, as in laboratory soil incubations [ Czimczik et al ., ; Gaudinski et al ., ] or in trenched field plots to exclude roots [ Phillips et al ., ], or isolated at times of year when plants are less active [ Phillips et al ., ], and these constraints almost certainly introduce observational biases. For instance, Ewing et al .…”