Dynamics and pathways of autotrophic and heterotrophic soil CO2 efflux revealed by forest girdling

Abstract

1. Quantifying pathways and temporal dynamics of carbon (C) flux between plants and soil is critical to our understanding of the long-term fate of C stored in soils. The potential priming of old organic matter decomposition by fresh C input from plants means that the impact of environmental changes on the interactions between plant C allocation and soil C storage need to be better understood. We used forest girdling to investigate the partitioning of total soil CO(2) efflux (R(S)) into autotrophic (R(A)) and heterotrophic (R(H)) flux components and their interaction with litter decomposition. 2. The reduction in R(S) in girdled plots stabilized within two weeks at 65% of control plot values, indicating that R(S) is dominated by R(H), and that only a small pool of available non-structural C remains in roots in late summer to sustain rhizosphere metabolic processes. R(A) contributions declined from 35% late in the growing season to about 25% in winter. 3. Our results indicate that actual root respiration (R(R)) and respiration by ectyomycorrhizas and other rhizospheric organisms (R(M)) contribute c. 50% each to R(A) between September and early November. During winter, R(A) remained significantly greater than zero despite frequent sub-zero air temperatures, with R(M) being a dominant component of R(A) during this period. 4. Forest girdling significantly increased the age of C in soil-respired CO(2), consistent with the removal of contemporary C derived from R(A). Partitioning of soil CO(2) efflux on the basis of 14C results shows good agreement with the flux reduction observed between girdled and control plots. 5. Litter bag incubations indicate a promoting influence of an intact C supply to the rhizosphere on decomposition, indicating a positive rhizosphere priming effect. 6. Synthesis: Our results demonstrate significant contribution of mycorrhizas and other rhizosphere organisms to R(S), and suggest a direct link between an intact rhizosphere and litter decomposition dynamics. These results highlight the tight coupling between autotroph activity and soil decomposition processes in forest soils, and add to the growing body of evidence that plant and soil processes cannot be treated separately.