Biotic and Abiotic Contribution to Diurnal Soil CO2 Fluxes from Saline/Alkaline Soils
Zhongyuan Wang, Jiangbo Xie, Yugang Wang, Yan Li
Abstract
Abstract 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.