Biotic and Abiotic Contribution to Diurnal Soil CO(2) Fluxes from Saline/Alkaline Soils

As the second largest carbon flux in terrestrial ecosystems, the soil CO(2) flux is closely related to the atmospheric CO(2) concentration. The soil CO(2) flux is the sum of biotic respiration and abiotic geochemical CO(2) exchange; however, little is known about abiotic CO(2) fluxes in arid areas. To investigate the relative contribution of abiotic and biotic soil CO(2) fluxes over a diurnal course, the abiotic CO(2) flux was distinguished by autoclaving sterilization in both saline and alkaline soils at an arid site in northwestern China. The results demonstrated that: (1) Over the diurnal course, the abiotic CO(2) was a significant component of the soil CO(2) flux in both saline and alkaline soil, which accounted for more than 56% of the diurnal soil CO(2) flux. (2) There was a dramatic difference in the temperature response between biotic and abiotic CO(2) fluxes: the response curves of biotic respiration were exponential in the saline soil and quadratic in the alkaline soil, while the abiotic CO(2) flux was linearly correlated with soil temperature. They were of similar magnitude but with opposite signs: resulting in almost neutral carbon emissions on daily average. (3) Due to this covering up effect of the abiotic CO(2) flux, biotic respiration was severely underestimated (directly measured soil CO(2) flux was only one-seventh of the biotic CO(2) flux in saline soil, and even an order of magnitude lower in alkaline soil). In addition, the soil CO(2) flux masked the temperature-inhibition of biotic respiration in the alkaline soil, and veiled the differences in soil biological respiration between the saline and alkaline soils. Hence, the soil CO(2) flux may not be an ideal representative of soil respiration in arid soil. Our study calls for a reappraisal of the definition of the soil CO(2) flux and its temperature dependence in arid or saline/alkaline land. Further investigations of abiotic CO(2) fluxes are needed to improve our understanding of arid land responses to global warming and to assist in identifying the underlying abiotic mechanisms.