Increasing CO sub(2) from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation

摘要

Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO sub(2) enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO sub(2), but also may be affected indirectly by a CO sub(2)-mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO sub(2) fluxes continuously during the final 2 years of a 4-year experiment on C sub(3)-C sub(4) grassland that was exposed to a 200-560 mu mol mol super(-1) CO sub(2) gradient. Flux measurements were used to determine whether CO sub(2) treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO sub(2) from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30-47% increase in the ratio of nighttime respiration to daytime net C influx as CO sub(2) increased from subamient to elevated concentrations. 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 effect of a given change in C influx on respiration. Conversely, CO sub(2) treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO sub(2) 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.