PLANT-CANOPY EFFECTS ON ARIDLAND INORGANIC CARBON DYNAMICS: EVIDENCE FROM DRY SEASON SURFACE AND SUB-SURFACE CO2 DYNAMICS

The distribution and accumulation of inorganic carbon is a critical feature in the pedogenesis and hydrological behavior of desert soils. Long-lived woody desert plants serve as focal points for material accumulation, and facilitate and sustain long-term biotic activity that modifies soil physicochemical characteristics. However, little effort has been made to quantify the effects of plant canopies on inorganic carbon dynamics in aridland systems. We measured surface soil respiration (R_soil), soil moisture and temperature in inter- and under-canopy soils, subsurface intercanopy soil [CO₂], and net ecosystem CO₂ exchange (NEE) and its constituent fluxes over a three-month pre-monsoon dry period in an Arizona Chihuahuan Desert shrubland. Nocturnal soil carbon uptake was frequently observed, a phenomenon previously observed only in hyper-arid Antarctic and highly alkaline Asian deserts, and never with concurrent belowground and ecosystem-level carbon measurements. Soil CO₂ uptake depended on the strength of air-soil temperature gradients, and these were more readily established in intercanopy soils. Diurnally integrated R_soil in both locations was always positive, but significantly lower in the intercanopy due to nighttime uptake and limited positive response to rain. Soil uptake always preceded sub-surface downward CO₂ movement, showing uptake is likely a surface process that follows downward propagation of thermal gradients that affect inorganic carbon dynamics. The ecosystem-level effects of these soil processes led us to believe that aridland vegetative structure mediates inorganic carbon dynamics across prolonged dry periods, and may more directly affect soil carbonate distribution than previously thought.