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

Abstract

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.