The age of root and soil respired CO2 in a Pacific Northwest old-growth forest: Implications of seasonality and drought effects on carbon source use
Abstract
Abstract Climate change has the potential to impact the carbon (C) cycle in unknown ways. Factors such as temperature, light, and moisture can strongly influence whether forest ecosystems are net sources or sinks of CO2. In this study, we used radiocarbon (14C) to determine the age and source of soil- and root-respired CO2 using a combination of soil chamber and biomass incubation measurements in an old-growth forest at the Wind River Field Station, WA. We had two main goals for this study. The first was to determine if the contribution of recent photosynthate to root respiration changed between spring and summer seasons. 14C measurements were used to determine the average age of respired CO2, since respired CO2 fueled by recent photosynthates have Δ14C values similar to that of the current atmosphere (~ 25‰ in 2012), whereas C stored by trees from prior years would be 30‰ or higher. This study occurred over two growing seasons, examining the effects of seasonality and water stress on root/soil respiration. Because of the summer drought conditions consistently experienced by this old-growth forest, this study provides a new dataset to test the hypothesis that plants allocate their C resources in response to stress. Initial results showed soil organic matter components had Δ14C values 80-120‰ greater than that of the background atmosphere, suggesting turnover times on the order of years to decades. In contrast, root respiration was much lower in Δ14C (~40‰), but still elevated with respect to current atmospheric Δ14C values, suggesting that root respiration was at least partially composed of C stored for > 1 year. The second goal was to partition the contribution of autotrophic to heterotrophic respiration and to determine how this ratio differs on diurnal and seasonal timescales. We used the difference between autotrophic and heterotrophic Δ14C values to partition total soil respiration. Preliminary results for April 2013 showed that ~1/3 of soil respiration originated from autotrophic sources. We did not observe differences in the relative contribution of root-derived C from morning to afternoon. Additional soil respiration measurements to be made in August of 2013 will allow us to examine the seasonality component as well as the contribution of mineral soils to total soil respiration.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B21B0484T
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0454 BIOGEOSCIENCES Isotopic composition and chemistry;
- 0438 BIOGEOSCIENCES Diel;
- seasonal;
- and annual cycles