Elevated CO₂ differentiates ecosystem carbon processes: Deconvolution analysis of duke forest face data

Quantification of the flux of carbon (C) through different pathways is critical to predict the impact of global change on terrestrial ecosystems. Past research has encountered considerable difficulty in separating root exudation, root turnover rate, and other belowground C fluxes as affected by elevated CO₂. In this study we adopted a deconvolution analysis to differentiate C flux pathways in forest soils and to quantify the flux through those pathways. We first conducted forward analysis using a terrestrial-C sequestration (TCS) model to generate four alternative patterns of convolved responses of soil surface respiration to a step increase in atmospheric CO₂. The model was then validated against measured soil respiration at ambient CO₂ before it was used to deconvolve the CO₂ stimulation of soil respiration. Deconvolved data from the Duke Forest free-air CO₂ enrichment (FACE) experiment suggest that fast C transfer processes, e.g., root exudation, are of minor importance in the ecosystem C cycling in the Duke Forest and were not affected by elevated CO₂. The analysis indicates that the fine-root turnover is a major process adding C to the rhizosphere. This C has a residence time of several months to ∼2 yr and increases significantly with increased CO₂. In addition, the observed phase shift in soil respiration caused by elevated CO₂ can be only reproduced by incorporation of a partial time delay function in C fluxes into the model. This paper also provides a detailed explanation of deconvolution analysis, since it is a relatively new research technique in ecology.