Abiotic contribution to total soil CO2 flux across a broad range of land-cover types in a desert region

As an important component of ecosystem carbon（C） budgets, soil carbon dioxide（CO2） flux is determined by a combination of a series of biotic and abiotic processes. Although there is evidence showing that the abiotic component can be important in total soil CO2 flux(Rtotal), its relative importance has never been systematically assessed. In this study, after comparative measurements of CO2 fluxes on sterilized and natural soils, the Rtotal was partitioned into biotic flux(Rbiotic) and abiotic flux(Rabiotic) across a broad range of land-cover types（including eight sampling sites: cotton field, hops field, halophyte garden, alkaline land, reservoir edge, native saline desert, dune crest and interdune lowland） in Gurbantunggut Desert, Xinjiang, China. The relative contribution of Rabiotic to Rtotal, as well as the temperature dependency and predominant factors for Rtotal, Rbiotic and Rabiotic, were analyzed. Results showed that Rabiotic always contributed to Rtotal for all of the eight sampling sites, but the degree or magnitude of contribution varied greatly. Specifically, the ratio of Rabiotic to Rtotal was very low in cotton field and hops field and very high in alkaline land and dune crest. Statistically, the ratio of Rabiotic to Rtotal logarithmically increased with decreasing Rbiotic, suggesting that Rabiotic strongly affected Rtotal when Rbiotic was low. This pattern confirms that soil CO2 flux is predominated by biotic processes in most soils, but abiotic processes can also be dominant when biotic processes are weak. On a diurnal basis, Rabiotic cannot result in net gain or net loss of CO2, but its effect on transient CO2 flux was significant. Temperature dependency of Rtotal varied among the eight sampling sites and was determined by the predominant processes（abiotic or biotic） of CO2 flux. Specifically, Rbiotic was driven by soil temperature while Rabiotic was regulated by the change in soil temperature（ΔT）. Namely, declining temperature（ΔT<0） resulted in negative Rabiotic（i.e., CO2 went into soil） while rising temperature（ΔT>0） resulted in positive Rabiotic（i.e., CO2 released from soil）. Without recognition of Rabiotic, Rbiotic would be overestimated for the daytime and underestimated for the nighttime. Although Rabiotic may not change the sum or the net value of daily soil CO2 exchange and may not directly constitute a C sink, it can significantly alter the transient apparent soil CO2 flux, either in magnitude or in temperature dependency. Th