A physico-chemical framework for explaining the generation mechanism of Nocturnal Soil CO₂ Uptake in Deserts

Abstract. Nocturnal soil CO₂ influx (Fs) has been widely documented in global deserts, while the underlying mechanisms remain unclear. Based on 563 observations from 30 published studies across five major desert regions in the world, we employed multiple linear mixed-effects models (LMMs), random forest (RF) regression, polynomial regression, and structural equation modeling (SEM) to quantify the effects of air‑soil temperature difference (ΔT = Ta - Ts), soil water content (θv), soil pH, and ln(clay/sand) on soil nocturnal CO₂ influx (Fs), and to establish a physico-chemical framework for explaining the generation mechanism of Fs. The results displayed a global mean Fs of −0.195 μmol m^-2 s^-1, with the strongest CO₂ uptake occurring in the Central Asian Desert Area and the weakest in the North American Desert Area. Fs showed a significantly positive correlation with ΔT and negative correlations with θv, pH, and ln(sand/clay). SEM revealed that ln(sand/clay) and θv influenced Fs both directly and indirectly. The relationships of ΔT, θv, pH, and ln(clay/sand) with Fs suggested exerted pronounced effects on nocturnal CO₂ influx in desert soils. The correlation analysis demonstrated that these environmental factors may serve as important drivers in the generation of nocturnal soil CO₂ influx. Integrating our results ad previous studies, we proposed a comprehensive physico‑chemical mechanism framework. In this framework, thermal convection driven by the temperature difference served as the physical driving force, continuously transporting atmospheric CO₂ into soil. One fraction is temporarily stored as gas in soil pores, and the other fraction enters chemical reactions under alkaline conditions. The above physico‑chemical processes generated the nocturnal CO₂ influx process in desert soils.