Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2006
Publication Date: 9/7/2006
Citation: Keller, C.K., White, T.M., O'Brien, R.O., Smith, J.L. 2006. Soil Co2 dynamics and fluxes as affected by tree harvest in an experimental sand ecosystem. Journal of Geophysical Research-Biogeosciences. 111:124-134. Interpretive Summary: Globally, the destruction of forest is a significant activity occurring on every continent. The destruction of forest is hypothesized to be at least a partial cause of increased CO2 in the atmosphere. However, this process of losing CO2 after clearing land has not been quantified, and was the objective of our study. We found that after clearing land of trees, CO2 released from soil over time did not decrease as rapidly as we had expected. We also found a complex interaction of processes was responsible for the continued increase in CO2 released from soil. In addition, we found that CO2 dissolved in soil water and drained from the system may be an important pathway for CO2 being removed from the atmosphere, though CO2 gas released to the atmosphere from soil was greater. This is some of the first research to show that CO2 released from soil after tree removal will not necessarily decrease rapidly over time and that there may be more CO2 released from cleared land than previously calculated. This has relevance to scientists studying the effects of land use changes on increases in atmospheric CO2 concentrations and subsequent global warming.
Technical 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. Our objective was to determine the effects of disturbance on soil C cycling and loss in a forested ecosystem. We monitored CO2 levels in soil atmosphere and as bicarbonate in drainage from an experimental red-pine mesocosm, for one year before and three years after its aboveground biomass was removed. Lack of physical disturbance, prevention of regrowth, and a comparison mesocosm 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 estimated cumulative annual CO2 efflux of about 500 g C m-2 yr-1 continued for two years following the harvest then declined to about half in the third year. The surprising continuation of post-harvest soil CO2 production reflects the replacement of root respiration by microbial respiration of root and litter substrates, stimulated by soil temperature increases. Mass balance suggests a bulk root+litter decay time of 4-6 yr, such that most of the subsurface biomass accumulated over 15 years of tree growth would be lost in a decade. The preharvest bicarbonate C efflux, which was less than 0.1% of the gas-phase efflux, trebled after the harvest due 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.