Karis J. McFarlane

Fine-root growth in a forested bog is seasonally dynamic, but shallowly distributed in nutrient-poor peat

Background and aimsFine roots contribute to ecosystem carbon, water, and nutrient fluxes through resource acquisition, respiration, exudation, and turnover, but are understudied in peatlands. We aimed to determine how the amount and timing of fine-root growth in a forested, ombrotrophic bog varied across gradients of vegetation density, peat microtopography, and changes in environmental conditions across the growing season and throughout the peat profile.MethodsWe quantified fine-root peak standing crop and growth using non-destructive minirhizotron technology over a two-year period, focusing on the dominant woody species in the bog: Picea mariana, Larix laricina, Rhododendron groenlandicum, and Chamaedaphne calyculata.ResultsThe fine roots of trees and shrubs were concentrated in raised hummock microtopography, with more tree roots associated with greater tree densities and a unimodal peak in shrub roots at intermediate tree densities. Fine-root growth tended to be seasonally dynamic, but shallowly distributed, in a thin layer of nutrient-poor, aerobic peat above the growing season water table level.ConclusionsThe dynamics and distribution of fine roots in this forested ombrotrophic bog varied across space and time in response to biological, edaphic, and climatic conditions, and we expect these relationships to be sensitive to projected environmental changes in northern peatlands.

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

The 14CO2 composition of plant and soil respiration can be used to determine the residence time of photosynthetically fixed carbon before it is released back to the atmosphere. To estimate the residence time of actively cycled carbon in a temperate forest, we employed two approaches for estimating the Δ14CO2 of ecosystem respiration (Δ14C-Reco) at the Willow Creek AmeriFlux site in Northern Wisconsin, USA. Our first approach was to construct nighttime Keeling plots from subcanopy profiles of Δ14CO2 and CO2, providing estimates of Δ14C-Reco of 121.7‰ in June and 42.0‰ in August 2012. These measurements are likely dominated by soil fluxes due to proximity to the ground level. Our second approach utilized samples taken over 20 months within the forest canopy and from 396 m above ground level at the nearby LEF NOAA tall tower site (Park Falls, WI). In this canopy-minus-background approach we employed a mixing model described by Miller and Tans (2003) for estimating isotopic sources by subtracting time-varying background conditions. For the period from May 2011 to December 2012 the estimated Δ14C-Reco using the Miller-Tans model was 76.8‰. Together, these Δ14C-Reco values represent mean Reco carbon ages of approximately 1–19 years. We also found that heterotrophic soil-respired Δ 14C at Willow Creek was 5–38‰ higher (i.e., 1–10 years older) than predicted by the Carnegie-Ames-Stanford Approach global biosphere carbon model for the 1 × 1 pixel nearest to the site. This study provides much needed observational constraints of ecosystem carbon residence times, which are a major source of uncertainty in terrestrial carbon cycle models.

Strong regional atmospheric 14C signature of respired CO2 observed from a tall tower over the midwestern United States

Radiocarbon in CO2 ((CO2)-C-14) measurements can aid in discriminating between fast ( 5-10 years) cycling of C between the atmosphere and the terrestrial biosphere due to the 14C disequilibrium between atmospheric and terrestrial C. However, (CO2)-C-14 in the atmosphere is typically much more strongly impacted by fossil fuel emissions of CO2, and, thus, observations often provide little additional constraints on respiratory flux estimates at regional scales. Here we describe a data set of (CO2)-C-14 observations from a tall tower in northern Wisconsin (USA) where fossil fuel influence is far enough removed that during the summer months, the biospheric component of the (CO2)-C-14 budget dominates. We find that the terrestrial biosphere is responsible for a significant contribution to (CO2)-C-14 that is 2-3 times higher than predicted by the Carnegie-Ames-Stanford approach terrestrial ecosystem model for observations made in 2010. This likely includes a substantial contribution from the North American boreal ecoregion, but transported biospheric emissions from outside the model domain cannot be ruled out. The (CO2)-C-14 enhancement also appears somewhat decreased in observations made over subsequent years, suggesting that 2010 may be anomalous. With these caveats acknowledged, we discuss the implications of the observation/ model comparison in terms of possible systematic biases in the model versus short-term anomalies in the observations. Going forward, this isotopic signal could be exploited as an important indicator to better constrain both the long-term carbon balance of terrestrial ecosystems and the short-term impact of disturbance-based loss of carbon to the atmosphere.

Erratum to Decadal cycling within long-lived carbon pools revealed by dual isotopic analysis of mineral-associated soil organic matter (Biogeochemistry, 10.1007/s10533-011-9673-0)

The authors regret errors arising from ambiguity in C-based turnover time calculations. Corrected values are reported in a revised Table 5. These corrections necessitate the following modifications to the text. In the last paragraph of the Results, hydrolysable silt and clay had turnover times that were generally \100 years (not \5 years), and the DC of brome total silt should be -30% (not -33%). The authors ask that readers disregard the section in the first paragraph of the Conclusions that states, ‘Using our fractionation scheme and the C-based turnover times, the hydrolysable fraction would be deemed ‘‘active’’. Given that about one quarter of the soil C was hydrolysable, these findings are inconsistent with a prevailing paradigm in some models that the active pool is relatively small (\10 %) and largely attributable to microbial biomass and readily decomposable microbial products (Di Tizio and Grego 2008; Parton et al. 1988). Furthermore, only the POM fraction would be considered to be an intermediate turnover pool in most soil C models.’ The online version of the original article can be found under doi:10.1007/s10533-011-9673-0.