Soil CO₂ dynamics and fluxes as affected by tree harvest in an experimental sand ecosystem.

Abstract

Soil CO₂ production is a key process in ecosystem C exchange, and global change predictions require understanding of how ecosystem disturbance affects this process. We monitored CO₂ 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 CO₂ 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 CO₂ 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 CO₂ production reflects the replacement of root respiration with microbial respiration of root and litter substrates of declining liability, but boosted by soil temperature increases. Mass balance is consistent with a bulk root+litter exponential decay time (-l/k) of 4 to 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 percent of the gas-phase efflux, trebled after the harvest owing to elimination of evapotranspiration and consequent increases in drainage while soil CO₂ levels remained high. A large fraction of this "hydrospheric" sink for atmospheric CO₂ is attributed to weathering under high soil CO₂ levels before spring snowmelt and soil-water flushing. These observations suggest that disturbance may enhance long-term chemical-weathering CO₂ sinks.

Citation

Keller, C.K.; White, T.M.; O''Brien, R.; Smith, J.L. 2006. Soil CO₂ dynamics and fluxes as affected by tree harvest in an experimental sand ecosystem. Journal of Geophysical Research. 111. G03011. doi: 10.1029/2005JG000157. 10 p