Soil CO 2 dynamics and fluxes as affected by tree harvest in an experimental sand ecosystem

• Published in: Journal of Geophysical Research: Biogeosciences Vol. 111; no. G3
• Main Authors: Keller, C. K., White, T. M., O'Brien, R., Smith, J. L.
• Format: Journal Article
• Language: English
• Published: 01-09-2006
• ISSN: 0148-0227
• DOI: 10.1029/2005JG000157

Soil CO
2
production is a key process in ecosystem C exchange, and global change predictions require understanding of how ecosystem disturbance affects this process. We monitored CO
2
levels in soil gas and as bicarbonate in drainage from an experimental red pine ecosystem, for 1 year before and 3 years after its aboveground biomass was removed. Lack of physical disturbance, strict prevention of plant regrowth, and a comparison ecosystem without rooted plants facilitated isolation of the microclimatic and biochemical effects of instantaneous canopy removal and cessation of photosynthesis. Preharvest gas‐phase CO
2
levels fluctuated with growing‐season soil temperature but reached their greatest levels (up to 10,000 ppmV) during late winter beneath snow and ice cover. This pattern, and the annual CO
2
efflux of ∼500 g C m
−2
yr
−1
, continued for 2 years following harvest; the efflux declined by half in the third year. The surprising continuity of preharvest and postharvest rates of soil CO
2
production reflects the replacement of root respiration with microbial respiration of root and litter substrates of declining lability, but boosted by soil temperature increases. Mass balance is consistent with a bulk root+litter exponential decay time (−1/k) of 4–6 years, such that most of the subsurface biomass accumulated over 15 years of tree growth would be lost in a decade after the harvest. The preharvest bicarbonate C efflux, which was less than 0.1% of the gas‐phase efflux, trebled after the harvest owing to elimination of evapotranspiration and consequent increases in drainage while soil CO
2
levels remained high. A large fraction of this “hydrospheric” sink for atmospheric CO
2
is attributed to weathering under high soil CO
2
levels before spring snowmelt and soil‐water flushing. These observations suggest that disturbance may enhance long‐term chemical‐weathering CO
2
sinks.