Old soil carbon decomposition is underestimated when plant productivity and surface soil decomposition are high
Abstract
Arctic warming and permafrost thaw threaten to expose large amounts of old soil organic carbon (SOC) to microbial decomposition. Increased decomposition of old SOC from deep soil layers could contribute to rising atmospheric CO2 and amplify global warming. The Δ14C signature of ecosystem respiration (Reco) can be used to detect old SOC losses, but high levels of autotrophic plant respiration may dilute the old soil signal making old SOC loss harder to detect. Arctic warming and permafrost thaw stimulate plant productivity, which can lead to an even greater plant dilution effect and mask the signal of old SOC decomposition with thaw. To determine how plant respiration influences estimates of old SOC loss in response to thaw, we measured Reco Δ14C during maximum active layer thickness (ALT) in an experimental soil warming manipulation which increased ALT from 60 to 110cm and from nearby vegetation removal plots, with 50cm ALT. We captured changes in plant productivity through time by measuring Reco Δ14C when tundra was a net ecosystem CO2 sink (August) and again after transition to a net source (September). Using a Hierarchical Bayesian model framework, we partitioned plant, young soil, and old soil contributions to Reco, while simultaneously accounting for uncertainty in isotopic end-members and effects of thaw, soil temperature, water table depth, and gross plant productivity (GPP) on old SOC loss. We found that accounting for variation in GPP substantially improved model fit and allowed better resolution of old SOC contributions than could be obtained with isotope information alone. Old SOC loss increased with decreasing GPP, decreasing surface soil temperature, and with rising water table, indicating that the proportion of old SOC loss was highest when plant respiration and decomposition of young surface soil declined. As GPP declined, the proportion of old SOC respiration increased from 10% to 25% and in vegetation removal plots old SOC contributions increased from 10% to 60% as surface soils froze. From August to September ALT and deep soil temperatures remained similar suggesting that SOC decomposition rates should not change over this time. Thus, if we do not account for the dilution effect of plant respiration on Reco, the magnitude of old SOC flux rates could be underestimated 4-5-fold.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B31E2496M
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0475 Permafrost;
- cryosphere;
- and high-latitude processes;
- BIOGEOSCIENCESDE: 0702 Permafrost;
- CRYOSPHEREDE: 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE