The abiotic contribution to total CO₂ flux for soils in arid zone

Abstract. As an important component of ecosystem carbon budgets, soil carbon dioxide (CO₂) flux is determined by a combination of a series of biotic and abiotic processes. Although there is evidence that the abiotic component can be important in total soil CO₂ flux, its relative importance has never been systematically assessed. In this study, the total soil CO₂ flux (R_total) was partitioned into biotic (R_biotic) and abiotic (R_abiotic) components over eight typical landscapes in a desert–oasis ecotone, including cotton field, hops field, halophyte garden, reservoir edge, native saline desert, alkaline soil, dune crest and interdune lowland in the Gurbantunggut Desert, and the relative importance of these two components was analyzed. Results showed that R_abiotic always contributed to R_total for the eight landscapes, but the degree of contribution varied greatly. In the cotton and hops fields, the ratio of R_abiotic to R_total was extremely low (< 10 %); whereas R_abiotic was dominant in the alkaline soil and dune crest. Statistically, R_abiotic/R_total decreased logarithmically with rising R_biotic, suggesting that R_abiotic strongly affected R_total when R_biotic was low. This pattern confirms that soil CO₂ flux is predominantly biological in most ecosystems, but R_abiotic can dominate when biological processes are weak. On a diurnal basis, R_abiotic resulted in no net gain or loss of carbon but its effect on instantaneous CO₂ flux was significant. Temperature dependence of R_total varied among the eight landscapes, determined by the predominant components of CO₂ flux: with R_biotic driven by soil temperature and R_abiotic regulated by the rate of change in temperature. Namely, declining temperature resulted in negative R_abiotic (CO₂ went into soil), while rising temperature resulted in a positive R_abiotic (CO₂ released from soil). Furthermore, without recognition of R_abiotic, R_biotic would have been either overestimated (for daytime) or underestimated (for nighttime). Thus, recognition that abiotic component in total soil CO₂ flux is ubiquitous in soils has widespread consequences for the understanding of carbon cycling. While the abiotic flux will not change net daily soil CO₂ exchange and not likely directly constitute a carbon sink, it can alter transient soil CO₂ flux significantly, either in magnitude or in its temperature dependency.