Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2005
Publication Date: 3/2/2006
Citation: Polley, H.W., Mielnick, P.C., Dugas, W.A., Johnson, H.B., Sanabria, J. 2006. Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation. Global Change Biology. 12:1390-1399. Interpretive Summary: The concentration of carbon dioxide (CO2) in the atmosphere is increasing. Because CO2 is a ‘greenhouse’ gas that traps heat in Earth’s atmosphere, air temperatures are expected to rise as CO2 accumulates with enormous economic and social consequences. Plants remove CO2 from air via photosynthesis, a process that is stimulated by higher CO2 concentrations. Plants and the microbes in soil that decompose dead plant material, however, return CO2 to air via the process of respiration. We measured CO2 uptake and release on grassland in central Texas that was exposed to CO2 levels that spanned pre-Industrial (subambient) to elevated concentrations to determine how atmospheric CO2 affects respiration and its sensitivity to seasonal changes in temperature and photosynthesis. Respiration rates of the grassland we studied were greater at elevated than subambient CO2 levels because both C input (net photosynthesis) and respiration per unit of C input increased with CO2 concentration. Conversely, CO2 treatment did not affect the response of grassland respiration to seasonal changes in temperature. By increasing respiration rates, CO2 enrichment may reduce the amount of additional carbon captured via photosynthesis that remains in terrestrial ecosystems.
Technical Abstract: Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO2 enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO2, but also may be affected indirectly by CO2-mediated changes in the sensitivity of respiration to temperature or C uptake. We measured CO2 fluxes continuously during the final 2 years of a 4-year experiment on a C3/C4 grassland that was exposed to a 200-560 umol mol-1 CO2 gradient. Flux measurements were used to determine whether CO2 treatment affected night-time respiration rates and the sensitivity of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO2 from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing both the sensitivity of C efflux to daily changes in C fixation and the respiration rate in the absence of C uptake (basal ecosystem respiration rate). Together, these changes contributed to a 29-47% increase in the ratio of night-time respiration to daytime net C influx as CO2 increased from 225 to 550 umol mol-1. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically-controlling for collinearity between temperature and C uptake reduced the sensitivity of respiration to C influx, but respiration generally remained more responsive to changes in C uptake over elevated than subambient CO2 concentrations. Conversely, CO2 treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO2 concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems.